IMPLICACIONES TECTONICAS Y PLEOGEOGRAFICAS

Universidad Nacional Autónoma de México Centro de Geociencias Programa de Posgrado en Ciencias de la Tierra

Estudio de la Geoquímica, la Estructura y el Metamorfismo en el Este del Complejo Acatlán: Implicaciones Tectonicas y Paleogeograficas —— Continental arc development along the periphery of Pangea: Late Paleozoic pluton emplacement and basin evolution in a transtensional setting, eastern Acatlán Complex, Mexico

Moritz Kirsch

Tesis sometida en cumplimiento parcial de los requisitos para el grado de Doctor en Ciencias de la Tierra

ASESORES: Dr. J. Duncan Keppie Dr. J. Brendan Murphy

Juriquilla, Qro, México

JURADO EXAMINADOR: Dra. Elena Centeno-García Dr. Peter Schaaf Dr. J. Duncan Keppie Dr. Luca Ferrari Dr. Michelangelo Martini

Agosto, 2012

Moritz Kirsch: Estudio de la GeoquĂ­mica, la Estructura y el Metamorfismo en el este del Complejo AcatlĂĄn: Implicaciones Tectonicas y Paleogeograficas, Tesis c Agosto, 2012 Doctoral

En memoria cariĂąosa de mis abuelos Otto y Ruth Kirsch, y mi abuelo Siegfried SchrĂśder.

RESUMEN

En el sector este del Complejo Acatlån, sur de MÊxico, se encuentra un conjunto de rocas de edad Paleozoico tardío, compuesto por un cuerpo intrusivo de la asamblea gabro-diorita-tonalita-trondhjemita (plutón Totoltepec) y una secuencia clåstica-calcårea de bajo grado de metamorfismo (formación Tecomate). Estas rocas fueron emplazadas y depositadas despuÊs de la orogenia colisional asociada a la formación de Pangea. Por lo tanto, el årea de estudio ofrece la oportunidad de investigar procesos geológicos en diferentes niveles corticales de un arco magmåtico en la periferia de Pangea durante el tiempo crucial entre amalgación y rotura del supercontinente. El plutón Totoltepec con una superficie de afloramiento de 15 × 5 km estå limitado por dos fallas PÊrmicas dextrales con orientación N–S, al sur por una cabalgadura E–W con buzamiento norte, y al norte por una falla normal E–W. El plutón estå compuesto por rocas måficas–ultramåficas subordinadas (306 ¹ 2 Ma; anålisis concordante de U-Pb en circón) que afloran en el margen de la intrusión måfica–felsica principal (289 ¹ 2 Ma). La geoquímica de las rocas marginales muestra una afinidad toleítica a calco-alcalina con alto LILE/HFSE (elementos litófilos de radio iónico grande/elementos de alto potencial iónico), tierras raras de espectro plano y valores iniciales de Nd entre +1.3 y +3.3. Los elementos traza de la fase plutónica mås joven describen una afinidad calco-alkalina con espectros de tierras raras moderadamente fraccionados y valores iniciales de Nd entre -0.8 a +2.6, lo cual tambiÊn sugiere un ambiente de arco para su formación. Datos termobaromÊtricos indican que el cuerpo principal del plutón fue emplazado a 620 km de profundidad y una temperatura de >700 ◌ C, y fue exhumado a 11 km y 400 ◌ C en 4 ¹ 2 Ma. Se ha documentado la siguiente secuencia intrusiva: (i) la fase måfica del margen norte de 306 Ma, (ii) la fase principal trondhjemítica de 287 Ma, y (iii) diques subverticales de approx. 289–283 Ma que varian desde (a) N39◌ E, no-deformados con crecimiento de cristales perpendiculares a las mårgenes, a (b) approx. N50–73◌ E, foliados y plegados con indicadores cinemåticos sinistrales, hasta (c) N73–140◌ E con estructuras tipo boudinage. La obliquidad del límite entre los diques plegados y estirados en relación a fallas dextrales de rumbo N-S sugiere un emplazamiento secuencial en un ambiente transtensional con 20 % de extensión con dirección E-W, pasando por un campo de acortamiento bajo diferentes grados de rotación en sentido horario, acompaùado por cizallamiento lateral izquierdo, a un campo de extensión. La intrusión de approx. 289–287 Ma contiene una foliación subvertical de rumbo ENE y un lineamiento que varia de subhorizontal a muy inclinada, probablemente debido al emplazamiento en un ambiente de deformación triclínica. Se infiere que el magmatismo cesó cuando un componente de movimiento fue transferido de la falla del límite occidental a la falla del límite oriental, resultando en

iv

un cabalgamiento a lo largo del lĂ­mite sur del plutĂłn. Este mecanismo puede explicar el rĂĄpido levantamiento y exhumaciĂłn del plutĂłn entre approx. 287 y 283 Ma. La formaciĂłn Tecomate se define actualmente como un compuesto de unidades de litologĂ­a similar, depositadas desde el CarbonĂ­fero hasta el PĂŠrmico en mĂşltiples cuencas limitadas por fallas. Al sur del plutĂłn Totoltepec, la edad de depositaciĂłn de la formaciĂłn Tecomate estĂĄ bien definida en âˆź300 Ma en una secciĂłn, y entre âˆź288 y âˆź263 Ma en otra. Las rocas de la formaciĂłn Tecomate estĂĄn interpretadas como derivados de un arco magmĂĄtico del Paleozoico tardĂ­o, basandose en (i) su geoquĂ­mica de afinidad de arco, (ii) valores Nd(t) entre -5.6 a +0.3 (t = 288 Ma) que traslapan los del plutĂłn Totoltepec, y (iii) una poblaciĂłn dominante de circones con edades que varian de CarbonĂ­fero a PĂŠrmico. Las unidades de Totoltepec y Tecomate en el ĂĄrea de estudio forman parte de un arco continental que se extiende desde Guatemala hasta California, lo cual implica subducciĂłn del paleoPacĂ­fico bajo el margen occidental en una configuraciĂłn paleogeogrĂĄfica de Pangea-A.

ABSTRACT

In the eastern Acatlån Complex of southern Mexico, a Late Paleozoic assemblage comprising a gabbro-diorite-tonalite-trondhjemite suite (Totoltepec pluton) and clastic-calcareous metasedimentary rocks (Tecomate Formation), post-dates collisional orogeny that resulted in the amalgamation of Pangea. This region offers a rare opportunity to examine assemblages developed at different crustal levels of a magmatic arc along the periphery of Pangea at the critical stage between amalgamation and breakup. The 15 x 5 km Totoltepec pluton is bounded by two N–S Permian dextral faults, an E–W thrust to the south, and an E–W normal fault to the north. The pluton consists of minor mafic–ultramafic rocks (306 ¹ 2 Ma; concordant U-Pb zircon analysis) that are marginal to the main mafic–felsic intrusion (289 ¹ 2 Ma). Geochemistry of the marginal rocks indicates an arc tholeiitic to calc-alkaline character with high LILE/HFS, flat REE patterns and initial Nd values of +1.3 to +3.3. The younger Totoltepec phase exhibits a calc-alkaline trace element geochemistry with flat to moderately fractionated LREE enriched patterns, and initial Nd values of -0.8 to +2.6, which are also consistent with an arc environment. Thermobarometric data indicate that the main, ca. 289–287 Ma part of the pluton was emplaced at 620 km depth and >700 ◌ C, and was exhumed to 11 km and 400 ◌ C in 4 ¹ 2 Ma. The following intrusive sequence is documented: (i) the 306 Ma northern marginal mafic phase; (ii) the 287 Ma main trondhjemitic phase; and (iii) ca. 289–283 Ma subvertical dikes that vary from (a) N39◌ E, undeformed with crystal growth perpendicular to the margins, through (b) ca. N50–73◌ E, foliated and folded with sinistral shear indicators, to (c) N73– 140◌ E and boudinaged. The obliquity of the boundary between the folded

v

and stretched dikes relative to the N-S dextral faults suggests sequential emplacement in a transtensional regime (with 20 % E–W extension), followed by different degrees of clockwise rotation passing through a shortening field accompanied by sinistral shear into an extensional field. The ca. 289–287 Ma intrusion also contains a steep ENE-striking foliation, and hornblende lineations varying from subhorizontal to steeply plunging, probably the result of emplacement in a triclinic strain regime. We infer that magmatism ceased when some of the dextral motion was transferred from the western to the eastern bounding fault, causing thrusting to take place along the southern boundary of the pluton. This mechanism is also invoked for the rapid uplift and exhumation of the pluton between ca. 287 and 283 Ma. The Tecomate Formation, as currently defined, is a composite of lithologically similar strata deposited in several fault-bounded basins ranging from Carboniferous to Early Permian in age. To the south of the Totoltepec pluton, the depositional age of the Tecomate Formation is tightly constrained in one section to âˆź300 Ma but in another section it is between âˆź288 and âˆź263 Ma. The Tecomate Formation rocks are interpreted to have been derived from a Late Paleozoic arc based on (i) its arc-related geochemistry, (ii) Nd(t) values ranging from -5.6 to +0.3 (t = 288 Ma) that overlap those of the Totoltepec pluton, and (iii) detrital zircons with predominantly Carboniferous– Permian ages. The Totoltepec and Tecomate units in the study area form part of a continental arc extending from Guatemala to California, which necessitates subduction of the paleo-Pacific oceanic lithosphere beneath the western margin of a Pangea-A configuration.

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Oh du schönes, o du wunderschönes, uraltes, sagen- und liederreiches Land Mexiko! Desgleichen gibt es nicht wieder auf dieser Erde. — B. Traven

AGRADECIMIENTOS

Me gustaría reconocer a J. Duncan Keppie y J. Brendan Murphy por su supervisión, paciencia, motivación y financiación. Me siento tremendamente afortunado de haber tenido la oportunidad de venir a México y trabajar con ustedes. Además de la geología espectacular que me tocó estudiar, realmente fue una experiencia cultural, tal como se había prometido. He aprendido mucho de ustedes dos en estos cuatro años—Brendan, me enseñaste la importancia de principios y procesos en la redacción científica, no perder nunca vista del panorama completo, hacer mil cosas al mismo tiempo, mantener impulso, estar pendiente de muestras y atar cabos sueltos, ser articulado y organizado. Duncan, tu experiencia geológica y sentido de la orientación en el campo, tu humor, tu parsimonia y eficiencia en todas las cuestiones científicas y burocráticas eran una verdadera inspiración. Estoy convencido de que me han preparado bien para una carrera como geólogo hard-rock. Además de los asesores de mi tesis, quisiera dar las gracias a todas las demás personas que han contribuido de una manera u otra a este trabajo. Maria Helbig, Mario A. Ramos-Arias, Gonzalo Galaz, y Domingo Schievenini donaron su tiempo y esfuerzo para asistirme en el campo y ayudarme con la preparación de muestras. Luigi Solari, Consuelo Macías Romo, Carlos Linares, Carlos Ortega-Obregón, Ofelia Pérez-Arvizu, Aldo Izaguirre, Alex Iriondo, y Harald N. Böhnel brindaron asistencia invaluable en el laboratorio. He beneficiado mucho de la colaboración y las conversaciones inspiradoras con Maria Helbig, Luigi Solari, James K. W. Lee, Fraser Keppie, Uwe Kroner, Fernando Ortega-Gutiérrez, R. Damien Nance, Cecilio Quesada, Roberto S. Molina-Garza, Barbara M. Martiny, James Sears, Chris Smith, y Axel Renno. También me gustaría agradecer a los miembros de mi comité tutoral: Luigi Solari, Fernando Ortega-Gutiérrez, Duncan Keppie, y Brendan Murphy, así como el comité de mi examen predoctoral: Elena Centeno-García, Peter Schaaf, Gustavo Tolson, y Dante J. Morán Zenteno por su tiempo y el asesoramiento profesional. Peter Schaaf, Bodo Weber, Luca Ferrari, W. Gary Ernst, y dos revisores anónimos son reconocidos por proporcionar revisiones constructivas de los manuscritos de los artículos que forman parte de esta tesis. Gracias a Roberto S. Molina-Garza y María Clara Zuluaga Velez por tomarse el tiempo para corregir la versión española de la tesis. Deseo extender un agradecimiento especial a la secretaria académica Marta Pereda Miranda y la abogada Lic. Ana Paola González Cruz del Centro

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de Geociencias. Sin su ayuda competente me hubiera perdido en la jungla de la burocracia académica. Agradezco al Consejo Nacional de Ciencia y Tecnología y a la Dirección General de Estudios de Posgrado de la UNAM por las becas otorgadas para la realización de mis estudios de doctorado. Además agradezco la Coordinación de Posgrado por el apoyo financiero brindado en la impresión de la tesis. Por último, deseo expresar mi profundo y sincero agradecimiento a mis padres, Bettina y Frank-Michael, y sus respectivas parejas Michael Freitag y Heike Kirsch, mi hermana Steffi, mis abuelos Ruth y Otto, y Brigitta y Siegfried, que me han aconsejado y apoyado a lo largo de mi educación. Gracias también a los padres de María, Martina y Johannes Helbig, por su generosidad y su perspectiva fresca. A mis amigos, tanto aquí en México – Domingo, María de la O, Oscar, Lina, Lariza, Gianluca, Daniele, Matteo, Ramón, Alma, Mario, Fabián, y el equipo de básquet INDEREQ – como los amigos de mi tierra: Martin, Matt, Bob, Mathias, Mel, Paul, Jo, y Nadja: les debo demasiado. Gracias por todos los buenos momentos! Estoy especialmente agradecido a mi novia Maria, por su amor, aliento y su compañía en esta aventura mexicana. Tú eres la luz de mi vida.

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ÍNDICE GENERAL

1

introducción 1.1 Marco geológico . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Motivación, objetivos y metodología . . . . . . . . . . . . . . .

1 1 5

2

geoquímica y geocronología de las unidades del carbonífero–pérmico 10

3

historia estructural del plutón totoltepec

4

eventos del paleozoico tardío hasta el mesozoico temprano en la periferia de pangea 58

5

resumen y conclusión

33

76

a métodos analíticos a.1 Geocronología U-Pb . . . . . . . . . . . . . . . . . . . . . . . . a.2 Geoquímica . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a.3 Geocronología 40 Ar-39 Ar . . . . . . . . . . . . . . . . . . . . .

80 80 81 81

b

83

tablas geocronología u-pb

c tablas geoquímica

117

d tablas análisis de microsonda

138

e

tablas geocronología

40 ar/ 39 ar

bibliografía

146

148

ix

1

INTRODUCCIÓN

Esta tesis se concentra en el estudio del plutón Totoltepec de edad Carbonífero–Pérmico y sus rocas encajonantes (parte del Complejo Acatlán), con énfasis en la geoquímica y el control estructural en el emplazamiento del plutón. A pesar de que la geoquímica publicada (basada en 6 muestras: Malone et al., 2002) muestra una afinidad de arco, estos datos no fueron suficientes para distinguir entre magmas formados en una zona de subducción y magmas contaminados por la corteza continental (Pearce y Peate, 1995; Turner et al., 1996; Kuscu et al., 2010). Un muestreo más amplio y un conjunto más exhaustivo de los elementos y los isótopos analizados constituyen la base para este estudio. Se obtuvieron datos geocronológicos adicionales de U-Pb para determinar la edad y duración del evento de intrusión, complementándose con edades de 40 Ar/39 Ar para conocer también la historia tectono-térmica. El estudio además incluye análisis geocronológicos y geoquímicos de las rocas sedimentarias contemporáneas a la intrusión para los cuales no existían ningunos datos de este tipo. Aunque estudios anteriores han sugerido que el emplazamiento del plutón Totoltepec fue sintectónico, los controles estructurales no eran conocidos. La base de datos mejorada que aporta este estudio permite el desarrollo de un modelo estructural para el emplazamiento del plutón. Estas conclusiones se utilizan para documentar el desarrollo de un arco regional y para diferenciar entre los diferentes modelos paleogeográficos para el Complejo Acatlán con respecto a Pangea (Keppie et al., 2010; Vega-Granillo et al., 2009; Böhnel, 1999). 1.1

marco geológico

Con una extensión superficial que supera los 10,000 km2 , el Complejo Acatlán de edad Ordovícico al Pérmico Medio constituye el basamento del terreno Mixteca y el mayor afloramiento de rocas paleozoicas en México (Ortega-Gutiérrez, 1978; Campa y Coney, 1983; Sedlock et al., 1993; Keppie, 2004). Las rocas expuestas en la región de Acatlán registran una historia paleozoica tectonotermal muy compleja que refleja la apertura y el cierre de una o más cuencas oceánicas y sus consiguientes interacciones continentales que culminaron en la amalgamación de Pangea (por ejemplo, Nance et al., 2006). Estos eventos fueron acompañados por subducción durante el Devónico hasta el Pérmico a lo largo del sur de México (Keppie et al., 2008). El Complejo Acatlán está limitado al este por la falla de Caltepec, una zona de cizalla con dirección N–S y mecanismo dextral, que lo separa de los gneises en facies de granulita de ∼1.0 Ga del Complejo Oaxaqueño (ElíasHerrera y Ortega-Gutiérrez, 2002). Al sur, está limitado por la falla Cenozoica La Venta-Chacalapa (Tolson, 2007; Solari et al., 2007), que lo yuxtapone

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1.1 marco geológico

contra las rocas plutónicas y metamórficas de alto grado del Complejo Xolapa (Pérez-Gutiérrez et al., 2009). Hacia el oeste, sobreyace en forma de cabalgadura sobre carbonatos cretácicos de la plataforma Guerrero-Morelos, que están expuestos entre el Complejo Acatlán y el terreno compuesto de Guerrero (Centeno-García et al., 2008; Ramos-Arias y Keppie, 2011). Hacia el norte, se encuentra cubierto discordantemente por rocas sedimentarias de origen continental y marino de edad Pérmico Superior–Jurásico Medio (Morán-Zenteno et al., 1993; Centeno-García et al., 2009), así como por rocas volcánicas y volcaniclásticas del Mioceno Medio y Tardío de la Faja Volcánica Transmexicana (Ferrari et al., 1999). El área de estudio se encuentra en la parte oriental del Complejo Acatlán, a unos 30 km al este de Acatlán de Osorio (estado de Puebla). Está limitado de forma aproximada por los pueblos Xayacatlán de Bravo al oeste, Santo Domingo Tianguistengo al este y San José Chichihualtepec al sur. Las rocas estudiadas ocurren en el bloque tectónico Tonahuixtla (Morales-Gámez et al., 2009), que está limitado en ambos lados por fallas normales-dextrales de rumbo N–S. En base a los mapas geológicos publicados (Ortega-Gutiérrez, 1978; Malone et al., 2002; Keppie et al., 2004a), la estratigrafía del área de estudio está conformada por las siguientes unidades litotectónicas: el plutón Totoltepec, la Formación Tecomate y la Formación Cosoltepec. A continuación se resumen los datos publicados sobre cada una de estas unidades y se identifican las problemáticas científicas que se tratan de resolver en este estudio. El plutón Totoltepec, con una superficie de afloramiento de 15 × 5 km, constituye la parte central del área de estudio. El cuerpo intrusivo fue nombrado por Fries et al. (1970) y primero cartografiado por Ortega-Gutiérrez (1975), como parte de su trabajo pionero en el Complejo Acatlán. De acuerdo con Ortega-Gutiérrez (1978), el plutón está en contacto intrusivo con rocas del Subgrupo Acateco y la Formación Cosoltepec (Grupo Petlalcingo, Fig. 1). Por otro lado, Malone et al. (2002) y Keppie et al. (2004a) ubican el plutón Totoltepec en el bloque cabalgante de una gran falla, estructuralmente sobreyaciendo las formaciones Tecomate y Cosoltepec. Hacia el norte, el plutón está sobreyacido discordantemente por capas rojas deformadas, pero sin metamorfismo, de edad inferida jurásica (Malone et al., 2002). El plutón Totoltepec está conformado principalmente por diorita de hornblenda, trondhjemita y tonalita (Malone et al., 2002). Las fracciones de circón de una fase félsica han dado una edad concordante U-Pb TIMS de 287 ± 2 Ma (Yañez et al., 1991), mientras que una fase máfica de la parte sur del plutón dio una edad U-Pb TIMS de 289 ± 1 Ma (Keppie et al., 2004a). Ortega-Gutiérrez (1975) ha documentado cuerpos de gneises máficos de estructura migmática y bandeada en el margen norte del plutón Totoltepec, que Calderón-García (1956) sospecha pertenecen al basamento de la zona. La edad de estas rocas máficas marginales y su significado geodinámico es desconocido. Una cantidad limitada de datos estructurales del plutón (Malone et al., 2002; Morales-Gámez et al., 2009) sugieren la presencia de una foliación de

2

1.1 marco geolĂłgico

P C

235 245 251 260 271

Formación Patlanoaya (Dev. tardío – PÊrmico medio)

318

385

S

416 428 423 444

PlutĂłn La Noria (337Âą34 Ma)

472 488 501 510 521 542

granitoides Esperanza (440Âą14 Ma)

Fm. XayacatlĂĄn

PlutĂłn Totoltepec

Unidad Salada , Cosoltepec

ExumaciĂłn de rocas de alta presiĂłn (facies eclogita)

Orogenia Mixteca (facies esquisto verde)

398

461

Grupo Patlanoaya & Fm. Tecomate

Fm. Tecomate Orogenia Acateca (facies eclogita)

Fm. Cosoltepec Fm. Chazumba Mig. Magdalena

Ortega-GutiĂŠrrez et al. (1999)

Grupo Petlalcingo

D

_

Evento OrogĂŠnico (facies esquisto verde)

299

359

O

PlutĂłn Totoltepec (287Âą2 Ma)

Grupo Piaxtla

^

Huerta, Amate, Las Minas

magmatismo bimodal (480–440 Ma)

Nance et al. (2006) Keppie et al. (2008)

Figura 1: Diagrama de relaciones espaciales y temporales que muestra la tectonoestratigrafĂ­a tradicional (izquierda) y revisada (derecha) del Complejo AcatlĂĄn. Figura modificada de Ortega-GutiĂŠrrez et al. (1999); Nance et al. (2006); Keppie et al. (2008).

rumbo norte y buzamiento de alto ångulo, así como pliegues de dirección N–S, por cual la foliación se encuentra plegada a nivel local. Ademås, Malone et al. (2002) sugieren que el emplazamiento del plutón puede haber sido sintectónico con respecto a la deformación regional. Datos geoquímicos de un estudio de reconocimiento de rocas del putón Totoltepec reflejan una afinidad calco-alcalina (Malone et al., 2002). Isotópicamente, el plutón Totoltepec ha dado valores de Nd(t) mås positivos y edades modelo TDM mås jóvenes (Yaùez et al., 1991) en comparación a plutones contemporåneos en el sur de MÊxico, lo que indica que proviene de una fuente de caråcter mås juvenil. La intrusión ha sido interpretada como parte de un arco continental PÊrmico–Triåsico que se extiende a lo largo de MÊxico centro-oriental (Torres et al., 1999; Malone et al., 2002; Keppie et al., 2004a). Alternativamente, de acuerdo con su modelo paleogeogråfico, VegaGranillo et al. (2009) consideran el plutón como un producto de la colisión continental y una expresión local de la orogenia Ouachita-Alleganiana. La Formación Tecomate, originalmente definida por Rodríguez-Torres (1970), es una unidad clåstica ligeramente metamorfoseada, pero intensamente deformada que consiste en alternancias de rocas psammiticas-pelíticas, mårmoles y conglomerados de cantos rodados, así como rocas volcånicas que principalmente estån formadas por flujos y tobas con escasas unidades fÊlsicas (Ortega-GutiÊrrez, 1993; Sånchez-Zavala et al., 2000). En su årea tipo, la Formación Tecomate ocurre en una zona de cizalla subvertical de orientación N–S situado entre la ciudad de Acatlån de Osorio y el pueblo de El Tecomate (Ortega-GutiÊrrez, 1975), pero rocas correlacionables con la

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1.1 marco geológico

Formación Tecomate también afloran localmente en los sectores norte y este (por ejemplo, Ortega-Gutiérrez et al., 1999), así como el sector oeste del Complejo Acatlán (Talavera-Mendoza et al., 2005; Vega-Granillo et al., 2009). Según mapas geológicos publicados de la zona de estudio, la Formación Tecomate está en contacto con el plutón Totoltepec en su margen sur y este (Ortega-Gutiérrez et al., 1999), así como en sus bordes suroriente y oeste (Keppie et al., 2004a). La edad de depositación de la Formación Tecomate es sujeto de controversia. Originalmente, se infirió como Devónica (Fig. 1) basada principalmente en la presencia de equinodermos, crinoides, blastoideos y micromoluscos de edad pre-Carbonífero obtenidos en esta unidad (Ortega-Gutiérrez, 1993) y en la interpretación de que la formación está intruida por el granito La Noria (Ortega-Gutiérrez et al., 1999), de los cuales datos U-Pb (circón) indicaban una edad Devónico Tardío. Sin embargo, más recientemente, la fauna recuperada de tres horizontes diferentes de mármol ha permitido precisar una edad pérmica temprana a media para la depositación de la Formación Tecomate en el área tipo (Keppie et al., 2004b). Estas restricciones paleontológicos han sido corroboradas por edades U-Pb SHRIMP de aproximadamente 320–264 Ma de circones separados de cantos de granito en los metaconglomerados de la Formación Tecomate en la parte oriental del Complejo Acatlán. Sin embargo, datos geocronológicos publicados de la Formación Tecomate (Keppie et al., 2004b; Sánchez-Zavala et al., 2004; Talavera-Mendoza et al., 2005) sugieren que la unidad, como se define actualmente, puede ser de diferentes edades en diferentes lugares. La Formación Tecomate ha sido interpretada como un sedimento sinorogénico depositado posterior al emplazamiento de una capa cabalgante (Ortega-Gutiérrez, 1993; Weber et al., 1997), una secuencia turbidítica relacionada a un arco volcánico depositado en la zona frontal de una colisión de arco-continente (Sánchez-Zavala et al., 2000), un depósito de arco y de extensión intracontinental (Talavera-Mendoza et al., 2005) y un sedimento marino somero de ante-arco (Keppie et al., 2004b). Basado en el traslape de las edades de depósito y similitudes faunísticas, la Formación Tecomate se ha correlacionado con la Formación San Salvador Patlanoaya de edad Devónico Superior a Pérmico Inferior en la parte norte del Complejo Acatlán (Keppie et al., 2004b). Sin embargo, a diferencia de la Formación Tecomate que ha sido deformada en forma penetrativa y afectada por metamorfismo en facies del esquisto verde, la Formación Patlanoaya no está metamorfoseada y casi no ha sido deformada. Mediciones de la forma de clastos en metaconglomerados de la Formación Tecomate en el área de estudio cerca de San José Chichihualtepec han dado esferoides alargados, con dimensiones típicos de la deformación transtensional (Morales-Gámez et al., 2009). Una edad 40 Ar/39 Ar de 263 ± 3 Ma para una filita sericítica de la Formación Tecomate adyacente a la zona de estudio ((Morales-Gámez et al., 2009) define el límite de edad de esta deformación. Adicionalmente, se ha documentado cizallamiento lateral N–S entre aproximadamente 307 y 269 Ma a lo largo de la falla Caltepec (Elías-

4

1.2 motivación, objetivos y metodología

Herrera y Ortega-Gutiérrez, 2002; Elías-Herrera et al., 2005). El significado del mecanismo y los límites temporales de la deformación con respecto a la configuración paleogeográfica regional y su papel en el emplazamiento del plutón Totoltepec permanecen inexplorados. Se ha reportado que la Formación Cosoltepec aflora en la parte sur de la zona de estudio, donde está en contacto con el plutón Totoltepec en sus márgenes sur y oeste (Ortega-Gutiérrez, 1978; Ortega-Gutiérrez et al., 1999; Malone et al., 2002). En el mapa geológico de Keppie et al. (2004a), las rocas de la Formación Cosoltepec sólo afloran en una zona estrecha a lo largo del límite suroeste del plutón Totoltepec. La Formación Cosoltepec está definida como una secuencia monótona de metapelitas y metapsamitas con escasas intercalaciones de anfibolita. Originalmente la unidad fue incluida en el Grupo Petlalcingo de edad Cámbrico–Ordovícico (Ortega-Gutiérrez, 1978), pero trabajos geocronológicos recientes han identificado grupos de circones detríticos más jóvenes de edad Ordovícico (∼455 Ma: Keppie et al., 2004a, 2006), Devónico-Carbonífero (∼410 y/o ∼374 Ma: Talavera-Mendoza et al., 2005), o de edad Carbonífero (∼352 Ma: Morales-Gámez et al., 2008) en las unidades asignadas originalmente a la Formación Cosoltepec; esto indica que está compuesta por diferentes unidades. El ambiente tectónico para la depositación de las rocas de la Formación Cosoltepec sigue siendo parte de la discusión en curso, ya que la unidad ha sido interpretada como un prisma de acreción (Ortega-Gutiérrez et al., 1999), una secuencia del margen pasivo (Talavera-Mendoza et al., 2005; Vega-Granillo et al., 2007) o un depósito de la eminencia continental (Keppie et al., 2006). 1.2

motivación, objetivos y metodología

Basado en una serie de estudios de reconocimiento del área (por ejemplo, Yañez et al., 1991; Malone et al., 2002; Keppie et al., 2004a,b), el plutón Totoltepec y la Formación Tecomate son propuestos como pertenecientes a un arco magmático continental del Pérmico–Triásico que se extiende desde el suroeste de los Estados Unidos y continua a lo largo de México centrooriental (Centeno-García y Silva-Romo, 1997; Torres et al., 1999; Dickinson y Lawton, 2001). Sin embargo, la existencia de este arco se basa en (i) una cantidad limitada de datos geocronológicos, la mayoría de los cuales se han obtenido utilizando métodos isotópicos K-Ar y Rb-Sr (por ejemplo, Torres et al., 1999; Schaaf et al., 2002; Yañez et al., 1991) que son conocidos por ser indicadores menos confiables para establecer la edad de cristalización de un plutón en comparación a la geocronología U-Pb de circones (por ejemplo, Steiner y Walker, 1996), y (ii) escasos datos geoquímicos de rocas ígneas del Paleozoico tardío en México (Torres et al., 1999; Solari et al., 2001; Malone et al., 2002; Rosales-Lagarde et al., 2005; Arvizu et al., 2009). Por otra parte, una firma geoquímica de arco en las rocas ígneas de composición félsica e intermedia suele interpretarse como una evidencia directa de magmatismo de arco contemporáneo. Sin embargo, modelos para la generación de magma intermedio y silícico incluyen tanto la diferenciación de magmas

5

1.2 motivación, objetivos y metodología

máficos derivados del manto por cristalización fraccionada dentro de la corteza o el manto superior (por ejemplo, Gill, 1981), como la fusión parcial de rocas de la corteza pre-existente (por ejemplo, Thompson, 1982). Por lo tanto, una firma geoquímica de arco en rocas ígneas de composición félsica o intermedia puede ser adquirido como resultado de la subducción de litosfera oceánica (por ejemplo, Pearce y Peate, 1995), o por la fusión de la corteza continental que ha sido generada por procesos de subducción (por ejemplo, Turner et al., 1996; Kuscu et al., 2010). La composición isotópica y/o la abundancia de xenolitos de granulita en las rocas ígneas que se utilizaron para definir el arco magmático continental del Pérmico–Triásico indican que están significativamente contaminados por la corteza continental. El terreno Oaxaquia, que incluye rocas de aproximadamente 1.0 Ga del núcleo cristalino de México, registra un episodio de magmatismo de arco entre aproximadamente 1300 y 1200 Ma (Keppie y Ortega-Gutiérrez, 2010). Por lo tanto, las abundancias de los elementos exhibidos por las rocas de arco principalmente félsicas, derivados de la corteza o contaminados que intruyen el basamiento de tipo Oaxaquia, puede reflejar una herencia en lugar de una firma de arco original. Antes de poder utilizar estas rocas para reconstruir la evolución de un arco magmático, el origen de la firma geoquímica con respecto a la generación de magmas félsicos tiene que ser evaluado críticamente. Se necesitan más datos y una evaluación rigurosa de éstos para demostrar de manera incuestionable la existencia de un arco regional del Paleozoico tardío. En este estudio, datos geocronológicos, geoquímicos y estructurales del plutón Totoltepec son combinados con datos geoquímicos y geocronológicos de rocas volcaniclásticas contemporáneas de la Formación Tecomate, para proporcionar un modelo de los procesos relacionados con la subducción en diferentes niveles de la corteza y además refinar las características temporales y espaciales, así que la evolución del arco propuesto. En cinturones orogénicos el plutonismo granitoide se presenta con frecuencia asociado espacial y temporalmente con sitios de deformación activa (Hutton, 1988), donde el ascenso y el emplazamiento de magma pueden ser controlados por zonas de cizallamiento de niveles corticales profundos (Brown y Solar, 1998). En un estudio de reconocimiento, Malone et al. (2002) observaron que los diques que cortan la foliación de forma oblicua en el plutón Totoltepec contienen una foliación interna paralela a sus márgenes del dique, lo que sugiere un posible emplazamiento sintectónico con respecto a la deformación regional. Sin embargo, no existen datos estructurales definitivos que establezcan los plazos de la deformación en relación al estado de cristalización del plutón (siguiendo los criterios de, por ejemplo, Blumenfeld y Bouchez, 1988; Paterson et al., 1989, 1991; Miller y Paterson, 1994) para sustanciar la naturaleza sin-cinemática de intrusión. Mostrar que el emplazamiento del plutón fue acompañado por deformación regional es de suma importancia, ya que los plutones sintectónicos bien datados se pueden utilizar para evaluar la temporalidad, el mecanismo y la historia térmica de la deformación regional (por ejemplo, Ingram y Hutton, 1994;

6

1.2 motivación, objetivos y metodología

Tribe y D’Lemos, 1996). Teniendo en cuenta los trabajos anteriores, que han relacionado el plutón Totoltepec con un arco magmático regional, un plutón emplazado sintectónicamente marcaría un lugar excepcionalmente adecuado para investigar la cinemática y el desarollo geodinámico de un sistema de falla en un arco continental antiguo y explorar la relación entre el magmatismo granitoide y la deformación en un margen de placa convergente. Este estudio demuestra que el emplazamiento fue contemporáneo con la deformación, proporciona límites en cuanto a profundidad de emplazamiento, tasa de exhumación e historia de enfriamiento del plutón, empleando una combinación de geocronología U-Pb y 40 Ar/39 Ar, el análisis de meso y microfábrica y de termobarometría de aluminio en hornblenda. Además, el estudio identifica el desarrollo de las fábricas, la secuencia temporal y los mecanismos de emplazamiento de las diversas fases intrusivas. Por otra parte, se desarrolla un modelo que pretende explicar el emplazamiento del plutón en el contexto de fallamiento regional de orientación N–S y mecanismo transcurrente-dextral. Estos datos ayudan a entender la cinemática del sistema de fallas que permitió el emplazamiento y la exhumación del plutón como un medio para reconstruir la evolución geodinámica del arco magmático del Paleozoico tardío. Existen dos modelos en competencia concernientes a la posición paleogeográfica del terreno Mixteco en relación a la configuración de Pangea en el Paleozoico. Keppie et al. (2010) y Weber et al. (2007) ubican el terreno Mixteca en el margen activo occidental de Pangea, mientras que Vega-Granillo et al. (2009) consideran que el terreno se ubicó en la zona de colisión entre Gondwana y Laurentia. Un tercer modelo, que se basa en un modelo alternativo de Pangea (Pangea-B, Irving, 1977; Morel y Irving, 1981) invocando un mega-sistema de cizallamiento dextral, coloca el terreno Mixteca frente al nordeste de Canadá en el Jurásico (Böhnel, 1999). Por lo tanto, la ubicación del terreno Mixteca en el sur de México en las reconstrucciones paleogeográficas del Paleozoico tardío tiene implicaciones profundas para las hipótesis Pangea-A y -B. En este estudio, se evalúan los diferentes escenarios; la evaluación se basa en determinar la fuente de magma y el mecanismo de emplazamiento del plutón Totoltepec, así como el ambiente tectónico y la procedencia de la Formación Tecomate. A su vez, esto permite evaluar el significado geodinámico de estas rocas en relación con la amalgamación y la desintegración de Pangea. Dependiendo de si la ubicación paleogeográfica del terreno Mixteca en el Paleozoico tardío era periférica o interna con respecto a Pangea, el magmatismo y los procesos de formación de cuenca caracterizados por el plutón Totoltepec y la Formación Tecomate representan eventos relacionados a la subducción en un orógeno periférico del tipo andino, o eventos colisionales parecidos a las de la orogenia Ouachita-Alleganiana en el sur de los Apalaches. Estos procesos del Paleozoico tardío en cualquier de los dos ambientes tectónicos posibles son pertinentes a un proceso importante de escala global que involucra la transferencia de las zonas de subducción desde el interior de Pangea a la periferia (por ejemplo, Murphy y Nance, 2008; Murphy et al., 2009).

7

1.2 motivación, objetivos y metodología

En la primera sección de este trabajo se presentan los datos geológicos de campo, petrografía, geocronología U-Pb de circones y geoquímica de elementos mayores y trazas, así como geoquímica isotópica de Sm-Nd para el plutón Totoltepec y la Formación Tecomate de la zona de estudio. Las descripciones detalladas de las metodologías están incluidas en el Apéndice A.1. Las edades de cristalización de las fases plutónicas y las edades detríticas de las rocas metasedimentarias fueron obtenidas por medio de la ablación láser (LA-ICP-MS) en el Laboratorio de Estudios Isotópicos (LEI), Centro de Geociencias, UNAM. Los granos de circón fueron separados utilizando diferentes protocolos analíticos con el fin de maximizar la pureza del concentrado obtenido, así como minimizar cualquier sesgo. Se realizaron observaciones por catodoluminiscencia (CL) antes de los análisis LA-ICPMS para ayudar a la selección de puntos y para aumentar la interpretabilidad geológica de los resultados. Los datos de edad son utilizados para establecer la secuencia de intrusión del plutón Totoltepec, la edad máxima de sedimentación y la procedencia de la Formación Tecomate en el área de estudio. Los datos geoquímicos (véase el Apéndice A.2 para detalles metodológicos), obtenidos del Regional Geochemical Centre de Saint Mary’s University en Nueva Escocia, Canadá, se utilizan para evaluar el ambiente tectónico del plutón Totoltepec y la Formación Tecomate. Datos isotópicos de Sm-Nd, adquiridos del Atlantic Universities Regional Isotopic Facility (AURIF), Memorial University en Terranova, Canadá, se emplean como trazador tectónico para investigar la fuente de magma del plutón Totoltepec y para proporcionar información sobre la procedencia de las rocas de la Formación Tecomate; éstos complementan los datos geocronológicos. También se incluye una revisión de los datos geocronológicos, geoquímicos e isotópicos de los sistemas magmáticos aproximadamente coetáneos en México y Guatemala así como propios datos geoquímicos e isotópicos de los plutones Cozahuico y La Carbonera en el Complejo Oaxaqueño. La segunda sección de la tesis contiene datos meso- y micro-estructurales, petrográficos, de microsonda, termobarométricos y geocronológicos del plutón Totoltepec; éstos se utilizan para investigar la historia de emplazamiento del plutón y su significado en el desarrollo geodinámico del arco continental del Paleozoico tardío en el sur de México. Se llevó a cabo un extenso trabajo de campo para documentar las relaciones de contacto internos y externos al plutón, recabar datos estructurales, así como muestrear para secciones delgadas, análisis de microsonda y análisis geocronológicos. Las observaciones petrográficas de láminas delgadas y los análisis de microsonda se utilizan para examinar la asociación de fases y para determinar la composición de algunos minerales. La historia de la deformación del plutón está reconstruida sobre la base de microestructuras distintivas que se desarrollan por diferentes mecanismos de recristalización dinámica. Con el fin de obtener una estimación de la profundidad del emplazamiento y la tasa de exhumación, se emplea una combinación de termobarometría Alen-hornblenda y dataciones por el método 40 Ar/39 Ar. Los datos químicos de plagioclasa y hornblenda coexistente fueron obtenidos mediante micro-

8

1.2 motivación, objetivos y metodología

sonda electrónica y espectrometría de dispersión por longitud de onda en el Laboratorio Universitario de Petrología (LUP) del Instituto de Geofísica (UNAM) en la Ciudad de México. Los fechamientos de moscovita mediante 40 Ar/39 Ar se llevaron a cabo por un procedimiento de calentamiento en pasos con láser en el Geochronology Research Laboratory de Queen’s University en Kingston, Canadá (véase el Apéndice A.3 para las especificaciones técnicas y detalles del método analítico). En conjunto, estos datos se utilizan para explicar la intrusión, deformación y exhumación del plutón en el contexto del marco estructural regional. La tercera sección está constituida por una guía de una excursión geológica, publicada como parte del Programa Internacional de Correlación Geológica Proyecto 597 (IGCP—amalgamación y ruptura de Pangea) y la 108a Reunión Anual de la Sección Cordillerana del GSA en Querétaro, México (28 a 31 marzo 2012). La guía da una visión general de los eventos del Pensilvánico–Jurásico en la periferia de Pangea. El capítulo correspondiente a la zona de estudio describe las relaciones de campo en una serie de afloramientos considerados principales y resume datos publicados. Además, contiene datos nuevos de geocronología U-Pb y 40 Ar/39 Ar, geoquímica y geoquímica isotópica Sm-Nd.

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GEOQUÍMICA Y GEOCRONOLOGÍA DE LAS UNIDADES DEL CARBONÍFERO–PÉRMICO

Artículo: Kirsch, M., Keppie, J.D., Murphy, J.B., y Solari, L.A., 2012, Permian–Carboniferous arc magmatism and basin evolution along the western margin of Pangea: geochemical and geochronological evidence from the eastern Acatlán Complex, southern Mexico: Geological Society of America Bulletin, en prensa, doi: 10.1130/B30649.1. Contribuciones individuales de los autores: Moritz Kirsch: concepción y diseño del estudio; trabajo de campo el cual incluye mapeo, selección de puntos de muestreo y toma de muestras para análisis de geoquímica y geocronología U-Pb; adquisición de los datos LA-ICP-MS, incluyendo la separación de circones y catodoluminiscencia; revisión de literatura; análisis e interpretación de datos; redacción del artículo. J. Duncan Keppie: contribución a la concepción y el diseño; supervisión de las actividades de campo; participación en la interpretación de los datos y en la revisión del artículo remitido; adquisición de fondos. J. Brendan Murphy: contribución a la concepción y el diseño; supervisión de las actividades de campo; participación en la interpretación de los datos y en la revisión del artículo remitido; adquisición de fondos. Luigi A. Solari: participación en la interpretación de datos y en la revisión del artículo remitido; responsable de las instalaciones de análisis LA-ICP-MS.

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Permian–Carboniferous arc magmatism and basin evolution along the western margin of Pangea: Geochemical and geochronological evidence from the eastern Acatlán Complex, southern Mexico Moritz Kirsch1,†, J. Duncan Keppie2, J. Brendan Murphy3, and Luigi A. Solari1 1

Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, 76230 Querétaro, QRO, Mexico Departamento de Geología Regional, Instituto de Geología, Universidad Nacional Autónoma de México, 04510 México D.F., Mexico 3 Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada 2

ABSTRACT In the Acatlán Complex of southern Mexico, a late Paleozoic assemblage, consisting of a gabbro-diorite-tonalite-trondhjemite suite (Totoltepec pluton) and clastic-calcareous metasedimentary rocks (Tecomate Formation), postdates collisional orogeny that resulted in the amalgamation of Pangea. This region offers a rare opportunity to examine assemblages developed at different crustal levels along the periphery of Pangea at the critical stage between amalgamation and breakup. The Totoltepec pluton consists of minor mafic-ultramafic rocks (306 ± 2 Ma; concordant U-Pb zircon analysis) that are marginal to the main mafic-felsic intrusion (289 ± 2 Ma). Geochemistry of the marginal rocks indicates an arc tholeiitic to calc-alkaline character with high large ion lithophile elements (LILEs)/high field strength elements (HFSEs), flat rare earth element (REE) patterns, and initial εNd values of +1.3 to +3.3. The younger Totoltepec phase exhibits a calc-alkaline trace-element geochemistry with flat to moderately fractionated light (L) REE–enriched patterns and initial εNd values of –0.8 to +2.6, which are also consistent with an arc environment. The Sm-Nd isotopic signature is more primitive compared to contemporaneous arc-related igneous rocks in southern Mexico, suggesting the pluton was emplaced in a less mature, outboard part of the arc, and/or along a fault conduit. The Tecomate Formation, as currently defined, is a composite of lithologically similar strata deposited in several faultbounded basins ranging from Carboniferous to Early Permian in age. To the south of the † E-mails: moritz@geociencias.unam.mx; moritz .kirsch@gmail.com

Totoltepec pluton, the depositional age of the Tecomate Formation is tightly constrained in one section to ca. 300 Ma, but in another section, it is between ca. 288 and ca. 263 Ma. The Tecomate Formation rocks are interpreted to have been derived from a late Paleozoic arc based on (1) arc-related geochemistry, (2) εNd (t) values ranging from –5.6 to +0.3 (t = 288 Ma) that overlap those of the Totoltepec pluton, and (3) detrital zircons with predominantly Carboniferous–Permian ages. The Totoltepec and Tecomate units in the study area form part of a continental arc extending from Guatemala to California, which necessitates subduction of the paleo-Pacific oceanic lithosphere beneath the western margin of a Pangea-A configuration. INTRODUCTION Although it is accepted that Pangea had largely been assembled by the Carboniferous– Permian, two competing models have been proposed for the late Paleozoic configuration of the supercontinent: Pangea-A, essentially the “Wegenerian” fit (Bullard et al., 1965; Smith and Hallam, 1970), and Pangea-B (Irving, 1977; Morel and Irving, 1981; Muttoni et al., 2003), which is based on the paleomagnetic data in which Gondwana is positioned ~3000 km farther east relative to Laurasia. In paleogeographic reconstructions of Pangea-A, southern Mexico occupies a position similar to its present location relative to North America (Figs. 1A and 1B; e.g., Fang et al., 1989; Alva-Valdivia et al., 2002), whereas in reconstructions of Pangea-B, southern Mexico is placed off eastern Canada during the Jurassic (Fig. 1C; Böhnel, 1999). There are also variants of the Pangea-A reconstruction, in which southern Mexico is either peripheral (Keppie, 2004; Keppie et al., 2008a, 2010; Fig. 1A) or internal to Pangea, between

the Maya terrane and the southern United States (Talavera-Mendoza et al., 2005; Vega-Granillo et al., 2007, 2009; Fig. 1B). Based on reconnaissance studies (e.g., Keppie et al., 2004a), the Totoltepec pluton and the Tecomate Formation in the eastern Acatlán Complex (Mixteca terrane) of southern Mexico are inferred to be part of a late Paleozoic continental arc assemblage that extended from the southern United States through Mexico to the northern Andes (Torres et al., 1999; Dickinson and Lawton, 2001). Alternatively, in accordance with their hypothesized within-Pangea location, Vega-Granillo et al. (2009) attributed late Paleozoic tectonothermal events in southern Mexico (including the eastern Acatlán Complex) to be related to continental collision (Alleghanian orogeny). In order to test the validity of these contrasting models, we investigated the tectonic setting of the Totoltepec pluton and Tecomate Formation using a combination of new geochemical, isotopic, and geochronological data. Examining magmatic systems in conjunction with sedimentary rocks enables the expression of tectonic events at different crustal levels to be documented. Almost all of the crystallization ages of plutons used by Torres et al. (1999) to constrain the age of the hypothesized magmatic arc were obtained using K-Ar or Rb-Sr isotopic methods, which are known to be susceptible to postcrystallization processes and hence may be less precise than U-Pb zircon geochronology in obtaining ages of magmatic crystallization. The central phase in the Totoltepec pluton has been investigated by reconnaissance U-Pb geochronology (Yañez et al., 1991; Keppie et al., 2004a). However, mafic igneous rocks at the margin of the Totoltepec pluton have not been dated, so the age range of the pluton is not constrained, and its regional significance is unclear. Although the existence of a Permian–Triassic

GSA Bulletin; September/October 2012; v. 124; no. 9/10; p. 1607–1628; doi:10.1130/B30649.1; 15 figures; 1 table; Data Repository item 2012220.

For permission to copy, contact editing@geosociety.org © 2012 Geological Society of America

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Figure 1. Paleogeographic reconstructions showing the location of the Mixteca terrane (Mx) in different configurations: (A) at the western margin of Pangea-A (modified after Weber et al., 2007), (B) within Pangea-A (modified after Vega-Granillo et al., 2009), or (C) off eastern Canada in the Jurassic (Pangea-B; modified after Böhnel, 1999). Oax—Oaxaquia terrane; Coa—Coahuila terrane; CM—Chiapas Massif; CA—Colombian Andes.

arc in Mexico has been proposed (e.g., Torres et al., 1999; Dickinson and Lawton, 2001; Centeno-García, 2005), geochemical data of late Paleozoic igneous rocks that would test this proposal are scarce (Torres et al., 1999; Solari et al., 2001; Malone et al., 2002; Rosales-Lagarde et al., 2005; Arvizu et al., 2009). Thus, neither the age nor the geochemistry of the magmatism, which are both crucial in assessing its potential geodynamic connection to the evolution of Pangea, is precisely constrained. We present U-Pb geochronology coupled with geochemical and Sm-Nd isotopic data to refine the age range of the hypothesized late Paleozoic arc in southern Mexico and to assess its geodynamic significance relative to the amalgamation and breakup of Pangea. Sedimentary sequences containing detritus from an orogenic source provide complementary data that can be used to constrain the role of basin formation as well as uplift and exhumation of the crust during orogenesis. Conglomerates in the Tecomate Formation in the study area contain granitic pebbles (Keppie et al., 2004b), suggesting a potential linkage between magmatism and basin evolution. However, based on the available data, it is unclear whether the Tecomate Formation, which has been mapped on the basis of lithologic comparison, is the same age in different locations. In this paper, we investigate this possibility by providing U-Pb laserablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) age data of single detrital zircon grains to help constrain the depositional age of the Tecomate Formation in the study area as well as enable a comparison with the equivalent data from the type area in the central Acatlán Complex. In addition, we combine these data with petrographic, geochemical, and Sm-Nd isotopic evidence to assess the prov-

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enance and tectonic setting of these metasedimentary rocks. Taken together, the data from the Totoltepec pluton and the Tecomate Formation constrain processes operating at different crustal levels at a critical time in the evolution of Pangea. These data also bear on the Pangea-A versus Pangea-B controversy and on the location of southern Mexico in reconstructions of Pangea. If indeed southern Mexico was in a peripheral position with respect to Pangea in the late Paleozoic (Pangea-A configuration), then this region offers a rare opportunity to examine the subduction-related magmatic and basinforming events after continental collision. If, on the other hand, the magmatism and basin formation reflect collisional orogenesis (Pangea B configuration), this region provides a record of these processes that can be compared with the Alleghanian orogeny in the Southern Appalachians. Our results indicate that the Totoltepec pluton and Tecomate Formation were both situated on the outboard part of a regionally extensive Pennsylvanian–Permian continental arc, consistent with subduction of paleo-Pacific oceanic lithosphere beneath the western margin of North America in a Pangea-A configuration. GEOLOGICAL SETTING The Acatlán Complex in southern Mexico is tectonically bounded to the east by the Permian Caltepec fault zone, which separates it from the ca. 1 Ga Oaxacan Complex (Elías-Herrera and Ortega-Gutiérrez, 2002), and to the south by the Cenozoic La Venta and Chacalapa faults (Tolson, 2007; Solari et al., 2007), juxtaposing it against the Xolapa Complex (Fig. 2). To the west, the Acatlán Complex is thrust over Cretaceous platformal carbonates, located between the exposed Acatlán Complex and the emplaced

Guerrero terrane (Centeno-García et al., 2008; Ramos-Arias and Keppie, 2011). To the north, the complex is unconformably overlain by Mesozoic rocks and the Cenozoic Trans-Mexican volcanic belt (Ferrari et al., 1999). The geological history of the Acatlán Complex was recently summarized by Keppie et al. (2008a) and Vega-Granillo et al. (2009) and is not repeated here. Despite differences in the interpretation of this history, all authors agree that the late Paleozoic events involved subduction-related tectonothermal events; however, the polarity of subduction is debated, either eastward beneath Pangea (Keppie et al., 2008a) or northward beneath Laurentia (Vega-Granillo et al., 2009). The Totoltepec pluton and the Tecomate Formation both occur within the Tonahuixtla fault block (Morales-Gámez et al., 2009), which is bounded in the west by the N-S–trending dextral San Jerónimo fault (Fig. 3; Morales-Gámez et al., 2008), where the Tecomate Formation is tectonically juxtaposed against the Carboniferous Salada unit along N-striking, dextralnormal faults and above N-dipping shear zones (Morales-Gámez et al., 2008). In the east, the Totoltepec pluton and Tecomate Formation are delimited by the Tianguistengo normal fault (Fig. 3; Servicio Geológico Mexicano, 2001). Along its southern margin, the Totoltepec pluton is thrust over metasedimentary rocks of the Tecomate Formation (Malone et al., 2002). The southern limit of the Tecomate Formation is not exposed in the study area, but the unit is inferred to structurally overlie rocks of the Cosoltepec Formation further south (Malone et al., 2002). The Cosoltepec Formation was originally thought to have been deposited in the Cambrian–Ordovician (Ortega-Gutiérrez, 1978), but recent geochronological data indicate that it is a composite of both Cambrian–Ordovician and

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Totoltepec pluton

Downloaded from gsabulletin.gsapubs.org on September 13, 2012 Permian–Carboniferous arc magmatism and basin evolution along the western margin of Pangea Devonian–Carboniferous units (Talavera-Mendoza et al., 2005; Keppie et al., 2006, 2008b; Morales-Gámez et al., 2008; Ortega-Obregón et al., 2009). To the northeast, an unnamed amphibolites-facies unit (consisting of garnet schist and quartzite with rare amphibolite dikes) is in tectonic contact with the Totoltepec pluton and the Tecomate Formation (Fig. 3). An unconformity, locally modified by normal faulting, marks the northern contact between the pluton and overlying red beds of inferred Jurassic age (Malone et al., 2002). The Totoltepec pluton is compositionally diverse, ranging from hornblendite and hornblende gabbro through diorite to tonalite, trondhjemite, granodiorite, monzogranite, and quartz-rich granitoid. The petrography of these rocks is described in detail in Kirsch et al. (2012). Plagioclase-rich cumulates are found locally in the central part of the pluton. The hornblendite and hornblende gabbro occur only in three 0.2–0.6 km2 lens-shaped bodies along the northeastern margin of the pluton that coincide with relatively high-amplitude magnetic anomalies (Servicio Geológico Mexicano, 2004a, 2004b). Although the exposed contacts between the main and marginal phases are faults, the marginal bodies are cut by trondhjemitic dikes identical to those in the main phase, implying that the faults have limited displacement. Only the trondhjemite and diorite were dated, and they yielded ages of 287 ± 2 Ma and 289 ± 1 Ma, respectively (U-Pb thermal ionization mass spectrometry [TIMS] zircon ages; Yañez et al., 1991; Keppie et al., 2004a). Within the pluton, a locally developed, subvertical fabric is defined by flattened quartz and feldspar grains as well as by aligned hornblende. Al-in-hornblende thermobarometric data from the main phase (Kirsch et al., 2012) indicate that the pluton was emplaced into midcrustal levels (~20 km). The Tecomate Formation adjacent to the Totoltepec pluton consists of greenschist-facies metapelite, feldspar-bearing metapsammite with local intercalations of metaconglomerate, unfossiliferous marble horizons, and rare very fine-grained, green, tuffaceous layers. The metapsammites are made up of quartz, plagioclase, and K-feldspar, phyllosilicates (white mica, biotite partially altered to chlorite), and opaque minerals, as well as secondary carbonate and epidote. Relict feldspar porphyroclasts in the metapsammites are angular to subrounded and display a wide range of grain sizes. Pebbleto cobble-sized clasts in the metaconglomerate are composed of trondhjemite, vein quartz, and metapsammite. Marble horizons, a distinctive feature of the Tecomate Formation, occur as intensely deformed, 1–2-m-thick tabular bodies that are occasionally boudinaged. Apart from

abundant quartz veins, thin granitoid dikes are localized to an area south of Santo Domingo Tonahuixtla. Though lithologically identical to the Tecomate Formation type area in the central Acatlán Complex (Ortega-Gutiérrez, 1978), reconnaissance geochronological analyses of Tecomate Formation metasedimentary rocks from the field and the type area, respectively (Keppie et al., 2004b; Sánchez-Zavala et al., 2004), have yielded distinct detrital zircon age populations, suggesting contrasting sources for the two units. U-Pb GEOCHRONOLOGY Analytical Methods Seven samples (see Table A1a1 and Fig. 3 for locations) were collected for U-Pb zircon dating by LA-ICP-MS at the Laboratorio de Estudios Isotópicos (LEI), Centro de Geociencias, Universidad Nacional Autónoma de México, Mexico. Zircons were extracted using standard mineral separation techniques, as described by Solari et al. (2007). For details on the analytical procedure, see GSA Data Repository file 1 (see footnote 1). In figures, tables, and results, 206Pb/238U ages are quoted for zircons younger than 1.0 Ga, whereas older grains are quoted using their 207 Pb/206Pb ages (e.g., Gehrels et al., 2006). The latter ages become increasingly imprecise younger than 1.0 Ga due to small amounts of 207 Pb. Zircon analyses with <10% normal and <5% reverse discordance are considered to be geologically meaningful (e.g., Harris et al., 2004; Dickinson and Gehrels, 2008; Gehrels, 2012) and are used to date the time of intrusion in igneous rocks or the maximum age of deposition in metasedimentary rocks. The latter is considered robust if it belongs to a cluster of three of more zircons with similar ages (e.g., Gehrels et al., 2006). Results Totoltepec Pluton A sample of hornblende gabbro from one of the lens-shaped bodies at the northeastern margin of the Totoltepec pluton (TT-72) is composed of hornblende, plagioclase, epidote, and chlorite, as well as accessory zircon, apatite, and opaque minerals (Table A1a [see footnote 1]). Zircons from the marginal mafic phase are ≤370 µm in length and exhibit uniform igneous oscillatory- and sector-zoning patterns. The 1 GSA Data Repository item 2012220, analytical methods and tables of LA-ICP-MS geochronological and geochemical data, is available at http://www .geosociety.org/pubs/ft2012.htm or by request to editing@geosociety.org.

analyses yielded 34 concordant 206Pb/238U ages (Table A1b [see footnote 1]; Figs. 4A and 4B) ranging from 299 ± 4 Ma to 311 ± 6 Ma. The TuffZirc (Ludwig and Mundil, 2002) 206Pb/238U age calculated from a coherent group of 25 zircon analyses is 306 ± 2 Ma. The quartz diorite sample from the central part of the Totoltepec pluton (TT-76B) consists of oligoclase, quartz, muscovite, and chlorite, with accessory apatite, zircon, and magnetite (Table A1a [see footnote 1]). Zircons separated from the dioritic phase are relatively small (≤200 µm in length) and possess a complex internal texture with partially resorbed cores and zircon overgrowths, as revealed by cathodoluminescence (CL) imaging. Zircon data (Table A1c [see footnote 1]; Figs. 4C and 4D) range from 278 ± 2 Ma to 310 ± 4 Ma, exhibiting a slightly right-skewed distribution. The TuffZirc algorithm yields a 206Pb/238U age of 289 ± 2 Ma for a coherent group of 22 analyses. Interpretation. The TuffZirc age of 306 ± 2 Ma is interpreted as the time of intrusion of the Totoltepec hornblende gabbro. The other two marginal bodies (Fig. 3), which are spatially proximal to the one dated and have similar dimensions and petrologic characteristics, are inferred to be coeval. The TuffZirc age of 289 ± 2 Ma is interpreted as the crystallization age of the quartz diorite, corroborating earlier U-Pb dating by Yañez et al. (1991) and Keppie et al. (2004a), who reported concordant U-Pb zircon ages of 287 ± 2 Ma and 289 ± 1 Ma for the intrusion of the Totoltepec pluton near Tonahuixtla, respectively. Tecomate Formation Three metasedimentary samples from the Tecomate Formation (TT-486A, TT-81, TT-82), one sample from metasedimentary rocks previously mapped as the Cosoltepec Formation by Ortega-Gutiérrez (1978) (TT-612), and a sample of a thin granitoid dike (TT-615) intruding these metasedimentary rocks were collected for geochronological analysis (Table A1a [see footnote 1]; Fig. 3). Zircons from the psammitic sample TT-486A (consisting of quartz, muscovite, K-feldspar, and opaque minerals) from the Tecomate Formation in the northwestern part of the study area, in the hanging wall just above the fault contact with the Salada Unit (Fig. 3), yielded only Proterozoic ages (Figs. 5A–5B; Table A1d [see footnote 1]). Results show that 75% of the 99 concordant zircon analyses fall in the age range between ca. 1014 and 1368 Ma. The second-largest population consists of 17 zircons of early Mesoproterozoic age between ca. 1407 and 1629 Ma. The weighted mean age (incorporating both internal analytical and external systematic error) of the youngest cluster over-

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Figure 4. Histograms (A, C) as well as Tera-Wasserburg diagrams (B, D) for U-Pb laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon analyses of Totoltepec pluton rocks; mean 206Pb/ 238U age calculated by TuffZirc age algorithm of Ludwig and Mundil (2002). Black error bars are for the arguably syngenetic zircons, gray error bars for zircons likely to be xenocrystic, and white error bars indicate analyses ignored due to anomalously high errors. Also displayed are cathodoluminescence images of representative zircon crystals from dated rock samples.

lapping in age at 2σ, calculated using the DZ Age Pick program developed at the LaserChron Center of the University of Arizona (www.geo .arizona.edu/alc), is 1005 ± 17 Ma (three grains). A metapsammite assigned to the Tecomate Formation in the eastern part of the field area (TT-81) is composed mainly of quartz, plagioclase, muscovite, and opaque minerals. Zircons separated from this sample yielded 79 concordant analyses ranging from 273 ± 10 Ma to 1796 ± 34 Ma (Figs. 5C–5D; Table A1e [see footnote 1]). The most prominent population is defined by 50 grains between the ages of ca. 277 and ca. 332 Ma. Three smaller populations are defined by ages of ca. 400–570 Ma, ca. 780– 845 Ma, and ca. 925–1240 Ma, respectively. The youngest cluster overlapping in age at 2σ

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error yields a weighted mean value of 288 ± 3 Ma (eight grains). A metapelite sample (TT-82) from the same area as TT-81 contains quartz, chlorite, and muscovite, as well as accessory minerals. Ninety-seven concordant zircons from this sample display an age span of 282 ± 2 Ma to 2621 ± 42 Ma (Figs. 5E–5F; Table A1f [see footnote 1]), where a group of 16 grains with ages between ca. 293 and ca. 313 Ma defines the largest probability peak at ca. 303 Ma. Ages between ca. 905 Ma and 1230 Ma define another significant age cluster, whereas age populations of ca. 470–570 Ma and ca. 655–720 Ma are represented by 10 and 4 zircons, respectively. Two zircon analyses with ages between ca. 362 and ca. 385 Ma indicate a subordinate Devonian

source. The youngest cluster overlapping in age at 2σ yields a weighted mean age of 299 ± 3 Ma (six grains). The metapsammite (TT-612) collected south of Santo Domingo Tonahuixtla from a unit originally mapped as the Cosoltepec Formation is primarily made up of quartz, plagioclase, K-feldspar, and muscovite. Zircon analyses from this sample (Figs. 5G–5H; Table A1g [see footnote 1]) yielded 90 concordant ages ranging from 289 ± 2 Ma to 2708 ± 22 Ma. The most dominant zircon population is made up of ages between ca. 299 and ca. 326 Ma, yielding a probability peak at ca. 309 Ma. Another major age population is defined by ages of ca. 950–1340 Ma. A smaller population has ages between ca. 420 and ca. 605 Ma, and includes

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Figure 5. Relative age probability and histogram plots (A, C, E, G, I) as well as Tera-Wasserburg concordia diagrams (B, D, F, H, J) for U-Pb laser ablation– inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon analyses of Chichihualtepec Tecomate Formation metasedimentary rocks and a granitoid dike. Black error ellipses in amplified concordia plot were used for weighted mean age calculation of the youngest age group. Histograms indicate number of analyses within 100 m.y. interval; histograms of the youngest age group have a 10 m.y. bin width.

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Downloaded from gsabulletin.gsapubs.org on September 13, 2012 Kirsch et al. a single Ordovician zircon of 469 ± 4 Ma. The weighted mean of the youngest age cluster overlapping at 2σ error is 303 ± 3 Ma (five grains). A sample of a thin granitoid dike (TT-615) intruding the TT-612 Tecomate metapsammites is principally composed of quartz, plagioclase, and muscovite. In total, 57 concordant zircon analyses exhibit an age range from 290 ± 2 Ma to 2614 ± 22 Ma (Figs. 5I–5J; Table A1h [see footnote 1]). The two most prominent probability peaks are defined by a group of 16 grains with ages of ca. 294–316 Ma and by a group of 11 zircons with ages of ca. 318–335 Ma, followed by smaller age populations between ca. 505 and 635 Ma and between ca. 985 and 1310 Ma. The youngest zircons that overlap within 2σ error give a weighted mean age of 298 ± 3 Ma (five grains). Interpretation. The ca. 1005 Ma age for the youngest detrital zircons in sample TT-486A, located near the stratigraphic base of the Tecomate Formation, suggests that this part of the unit was deposited at a time when late Paleozoic igneous sources were not exposed. Similarly, the type Tecomate Formation yielded no zircons younger than ca. 1.0 Ga (SánchezZavala et al., 2004). U-Pb ages of detrital zircon grains in the other samples are used to constrain the maximum depositional age of the Tecomate Formation in the study area (e.g., Dickinson and Gehrels, 2009). Of the three metasedimentary samples from the Tecomate Formation south of the Totoltepec pluton, TT-81 yields the youngest weighted mean age (288 ± 3 Ma, Lower Permian), which is taken to represent the maximum depositional age in that locality. A 40Ar/39Ar whole-rock age of 263 ± 3 Ma (Morales-Gámez et al., 2009) from a Tecomate sericitic phyllite northwest of the Totoltepec pluton provides a younger age limit for the deposition of the Tecomate Formation as well as the age of metamorphism. In another locality within the study area, the depositional age of the Tecomate Formation is more tightly constrained to ca. 300 Ma. Sample TT-612 contains detrital zircons of Permian age, so it is assigned to the Tecomate Formation rather than the Devonian–Carboniferous Cosoltepec Formation, with which it was originally associated (Ortega-Gutiérrez, 1978). This conclusion is consistent with field observations. The weighted mean of the youngest age cluster in sample TT-612 is 303 ± 3 Ma, which is similar within error to the weighted mean age of the youngest zircon cluster from the granitoid dike (298 ± 3 Ma) at the same locality, suggesting that the host metapsammite in this locality was deposited at ca. 300 Ma and intruded very soon afterward. This depositional age is older than the maximum depositional age obtained from

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sample TT-81. The ca. 298 Ma age falls between the 306 ± 2 Ma age of the marginal gabbro and the 289 ± 2 Ma of the central Totoltepec pluton, suggesting that the dike is either a late phase of the marginal gabbro or an early phase of the main Totoltepec intrusion. Whereas deposition of the Tecomate Formation south of the Totoltepec pluton occurred at ca. 300 Ma in one location and between ca. 288 and ca. 263 Ma in another, fossiliferous limestone horizons in the type Tecomate Formation in the central Acatlán Complex range from latest Pennsylvanian to early Middle Permian (Keppie et al., 2004b) and middle Pennsylvanian (Kazimovian = 306–304 Ma) to Early Permian. Thus, the Tecomate Formation may be a composite unit, collectively spanning the middle Pennsylvanian–Early Permian, but of different ages in different locations. Nevertheless, these data suggest that some of the Tecomate Formation in the type area as well as in the study area was deposited before intrusion and exhumation of the Totoltepec pluton. To avoid confusion with rocks in the type area, in this paper, the Tecomate Formation south of the Totoltepec pluton is informally designated Chichihualtepec Tecomate Formation (CTF; Fig. 3). Provenance of the Chichihualtepec Tecomate Formation. Taken together, there are 144 zircon grains in the age range between ca. 344 and ca. 273 Ma in analyzed samples from the Chichihualtepec Tecomate Formation. A compilation of these ages (Fig. 6), including sensitive high-resolution ion microprobe (SHRIMP) data of a sample from granite cobbles in Chichihualtepec Tecomate Formation metaconglomerates (Keppie et al., 2004b), shows that (1) the distribution and range of ages are more or less continuous, and (2) there is a significant overlap between the detrital zircon age spectra and ages obtained from samples of the Totoltepec pluton. The measured Th/U ratios (Table A1 [see footnote 1]), which are >0.01, support a magmatic origin of these zircons (e.g., Rubatto, 2002). However, the Totoltepec pluton cannot be a source of these zircons, as thermobarometric data suggest that the pluton was at a depth of ~20 km at ca. 289 Ma, and 40Ar/39Ar data indicate it did not cool through the muscovite closure temperature until 283 ± 1 Ma (Kirsch et al., 2012). Assuming this uplift rate of ~1.4 mm/yr was maintained, the Totoltepec pluton was not exposed until ca. 275 Ma. Zircons of Carboniferous–Permian age in parts of the Chichihualtepec Tecomate Formation that were deposited before ca. 275 Ma therefore cannot have been derived from the Totoltepec pluton, and are interpreted to have been derived from the regional arc edifice and from epizonal plutons exposed during the Pennsylvanian and Early Permian.

All of the U-Pb samples from the Chichihualtepec Tecomate Formation contain major detrital zircon age peaks between ca. 920 and 1250 Ma, which are within the range of ages documented from the adjacent Oaxacan Complex (Keppie et al., 2001, 2003; Solari et al., 2003). Whereas ca. 600 Ma, 1500–1600 Ma, 1750–1900 Ma, and 2100–2500 Ma zircons could have been derived from Amazonia, Oaxaquia, and/or Laurentia, those with 800–950 Ma ages can come only from Amazonia (Keppie et al., 2008a) or Oaxaquia (e.g., the ca. 917 Ma Etla pluton; Ortega-Obregón et al., 2003). The source for the five zircons with ages between 454 and 476 Ma may be the rift-related granitoid plutons within the Acatlán Complex, which have yielded ages between 440 and 480 Ma (Keppie et al., 2008b). Five detrital zircons in the samples from the Chichihualtepec Tecomate Formation are Devonian–Mississippian, spanning ages of 357–402 Ma. A postulated arc on the western margin of the Mixteca terrane, most of which was subsequently removed by subduction erosion (Keppie et al., 2008a, 2010), may have been the source for these zircons. GEOCHEMISTRY Analytical Methods In order to determine the tectonic setting for the igneous and metasedimentary rocks in the Totoltepec area, 34 samples from the Totoltepec pluton and 41 metasedimentary rocks of the Tecomate Formation were analyzed for major and selected trace elements (Fig. 3) by X-ray fluorescence at the Regional Geochemical Centre, St. Mary’s University, Canada (for details of analytical methods, see Dostal et al., 1994). Of these, 15 representative samples from the Totoltepec pluton and 7 from the Tecomate Formation were selected for analysis of additional trace elements (rare-earth elements [REEs], Y, Zr, Nb, Ba, Hf, Ta, and Th) by ICPMS according to methods described in Jenner et al. (1990). Sm-Nd isotopic compositions of these 15 samples were determined in order to characterize the source regions and tectonic history of the respective geological units according to the method described in Kerr et al. (1995). For details on the analytical procedures, see GSA Data Repository file 1 (see footnote 1). Results Results of the geochemical analyses are presented in Table A2 (see footnote 1). The samples are affected to varying degrees by secondary processes, including low-grade metamorphism and deuteric alteration. These secondary processes have modified their primary chemical

Geological Society of America Bulletin, September/October 2012

Downloaded from gsabulletin.gsapubs.org on September 13, 2012 Permian–Carboniferous arc magmatism and basin evolution along the western margin of Pangea Frequency 50

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TT-82 Metapelite

TT-612 Metapsammite

TT-615 Granitoid dike

350

TEC-10 Granite cobble metaconglomerate (Keppie et al., 2004b)

Rel. prob

Figure 6. A 2σ error bar plot showing concordant 206Pb/ 238U ages of detrital zircons from the Chichihualtepec Tecomate Formation metasedimentary rocks. Data also include inherited zircons extracted from a granitoid dike intruding Chichihualtepec Tecomate Formation metapsammites as well as U-Pb sensitive high-resolution ion microprobe (SHRIMP) analyses of zircons separated from Chichihualtepec Tecomate Formation metaconglomerate granitoid cobbles (Keppie et al., 2004b). Diagonally hatched regions and histograms represent U-Pb age data from Totoltepec pluton rocks. Gray shaded histogram on the right-hand side shows the age distribution of all detrital zircon data featured in this diagram. Qz—quartz; Hbl—hornblende.

composition, resulting in a scatter on diagrams featuring alkali and alkaline earth elements and elevated loss on ignition (LOI) values. Hence, inferences about the petrogenesis of the rocks are largely based on high field strength elements (HFSEs) and rare earth elements (REEs), which are considered to be relatively “immobile” during alteration processes (e.g., Winchester and Floyd, 1977), and should reflect original magma chemistry for the igneous rocks as well as provenance compositions of the sedimentary rocks (Taylor and McLennan, 1985). Totoltepec Pluton Geochronological data indicate that the Totoltepec pluton was formed in two distinct intrusive events. Hence, geochemical data for rocks from the ca. 306 Ma marginal bodies and for rocks from the ca. 289 Ma main body of the pluton are presented separately. Older (ca. 306 Ma) Totoltepec rocks. Samples from the older marginal bodies of the Totoltepec pluton range from hornblende gabbro to hornblendite with SiO2 (LOI-free) between 41.9 and 51.1 wt%. These mafic to ultramafic rocks have relatively wide ranges in TiO2 (0.19–0.99 wt%), Fe2O3 (2.89–12.6 wt%), MgO (2.45–11.3 wt%), Cr (38–313 ppm), V (66–434 ppm), Co (11–

47 ppm), and Ni (21–163 ppm) (Fig. 7; Table A2a [see footnote 1]). The samples have low Nb/Y (0.03–0.15) and Zr/Ti (0.002–0.009), which is typical of subalkaline basaltic rocks (Fig. 8). The rocks are characterized by low Th/Yb and Ta/Yb and highly variable Th/Hf ratios. On discrimination diagrams using these parameters, the data straddle the boundary between calc-alkaline and island-arc tholeiitic fields (Figs. 9A and 9B). The hornblende gabbros exhibit relatively flat chondrite-normalized REE patterns (Fig. 10A; average [La/Yb]n = 1.8) with ΣREE of 3–13 times chondrite, and positive Eu anomalies (up to Eu/Eu* = 2.6) that decrease with increasing SiO2. The presence of pronounced positive Eu anomalies in some gabbro samples suggests plagioclase accumulation. The hornblendite sample is characterized by a concave-upward REE pattern ([La/Sm]n = 0.4; [Gd/Yb]n = 1.7), indicative of the dominance of cumulus amphibole. The mid-ocean-ridge basalt (MORB)–normalized multi-element plot of the ca. 306 Ma marginal rocks (Fig. 10B) shows enrichment in large ion lithophile elements (LILEs; Cs, Rb, Ba, U, K, Pb, and Sr), moderate depletion in the less incompatible elements (Zr, Hf, Ti, middle to heavy REEs), and strong depletion in the HFSEs Nb and Ta. This signature reflects

derivation from a mantle wedge affected by slab fluxing processes, i.e., patterns that are typical of subduction-related magmas (e.g., Saunders et al., 1988; McCulloch and Gamble, 1991). Sm-Nd isotope analyses on the older marginal Totoltepec rocks yield initial εNd ranging from +1.3 to +3.3 and 147Sm/144Nd ratios from 0.15 to 0.24 with a TDM model age (147Sm/144Nd < 0.165; Stern, 2002) of 0.84 Ga (Table 1; Fig. 11). The 306 Ma sample (TT-72) with the lowest εNd (i) plots well above the mantle depletion-enrichment array in Figure 9A and toward the Th apex in Figure 9B, indicating contamination by a crustal or a subduction component. Accordingly, on the εNd (t) versus 147Sm/144Nd diagram (Fig. 11B), this sample lies on a curve representing assimilation and fractional crystallization (DePaolo, 1981) between one of the more juvenile 306 Ma samples and the average composition of the Oaxacan Complex (Ruiz et al., 1988). By contrast, the Sm-Nd isotopic signature of the other hornblende gabbro samples as well as the hornblendite is similar to Ordovician mafic rocks within the Mixteca terrane (Murphy et al., 2006; Ortega-Obregón et al., 2010), which are interpreted to have been derived from a ca. 1.0 Ga subcontinental lithospheric mantle. The 306 Ma gabbro and hornblendite may hence

Geological Society of America Bulletin, September/October 2012

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TiO2 (wt%)

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Figure 7. Variation diagrams for selected major elements, high field strength trace elements, and ratios of Totoltepec pluton rocks and correlative Carboniferous–Permian igneous suites (labeled in part F). Division lines in K2O plot are from Le Maitre et al. (2002). Included in the comparison (from north to south) are geochemical data from (1) andesitic to basaltic lava flows from the 290–260 Ma Tuzancoa Formation in the Sierra Madre terrane (Rosales-Lagarde et al., 2005); (2) the 270 ± 3 Ma Cozahuico granite (this paper), which intrudes the N-S dextral transpressive Caltepec fault zone (CFZ); (3) the 275 ± 4 Ma La Carbonera stock, which intrudes the northern Oaxacan Complex (Solari et al., 2001; this paper); (4) ca. 272–251 Ma orthogneisses of the Chiapas Massif (Maya block; Weber et al., 2005); and (5) ca. 318– 313 Ma plutons in the Altos Cuchumatanes Range, Guatemala (Maya block; Solari et al., 2010; Solari, 2012, personal commun.). be derived from the same subcontinental lithospheric mantle as the Ordovician mafic rocks. Mafic rocks with similar isotopic compositions also occur along other parts of the Gondwanan margin (Avalonia: e.g., Murphy and Dostal, 2007; Iberia-western Europe: e.g., Murphy et al., 2008; Keppie et al., 2011), suggesting the

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subcontinental lithospheric mantle that underlay the area in the late Paleozoic may have been regionally widespread. Main-phase (ca. 289 Ma) Totoltepec rocks. Totoltepec pluton samples of ca. 289 Ma age consist of hornblende diorite, tonalite, quartz diorite, trondhjemite, quartz-rich granitoid, and

plagioclase-rich cumulates from the main body of the pluton. Trondhjemite dikes that intrude the ultramafic and mafic marginal bodies of the pluton are included in the ca. 289 Ma Totoltepec rocks based on matching petrographic and geochemical characteristics. The samples display a wide range in chemistry, with an SiO2 content

Geological Society of America Bulletin, September/October 2012

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Figure 9. Tectonic discrimination diagrams for rocks of the Totoltepec pluton and comparative igneous suites (see caption of Fig. 7 for references). (A) Th/Yb versus Ta/Yb diagram identifying mantle source and subduction components (modified after Pearce, 1982, 1996). Compositional fields: TH—tholeiitic; TR—transitional; ALK—alkaline; CA—calcalkaline; SHO—shoshonitic. Compositions of normal mid-ocean-ridge basalt (N-MORB), enriched (E) MORB, and ocean-island basalt (OIB) are after Sun and McDonough (1989); (B) Th-Hf/3-Ta discrimination diagram after Wood et al. (1979). MM—mantle source; UC—upper crust; LC—lower crust; SZ—subduction component. (C) Yb versus Ta diagram for felsic rocks (after Pearce et al., 1984). VAG—volcanic arc granites; syn-COLG—syncollision granites; WPG—within-plate granites; ORG—ocean-ridge granites.

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Ultraalkaline

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Figure 8. Geochemical rock classification of samples from the Totoltepec pluton and other igneous suites of similar age (see caption of Fig. 7 for references). (A) Zr/TiO2-SiO2 diagram and (B) bivariate Nb/Y versus Zr/Ti diagram (after Winchester and Floyd, 1977; Pearce, 1996).

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Geological Society of America Bulletin, September/October 2012 306 Ma

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Figure 10. Chondrite-normalized rare earth element (REE) patterns (A, C, E) and normal mid-ocean-ridge basalt (N-MORB)–normalized multi-element plots (B, D, F) for Totoltepec pluton rocks and comparative igneous suites (see caption of Fig. 7 for references). Normalizing values are from Sun and McDonough (1989).

(LOI-free) spanning 51.6–77.7 wt% and Mg# (100 × Mg/[Mg + Fe] molar) of 21–63. With increasing silica, the samples show a decrease in TiO2, Al2O3, CaO (Fig. 7), and Ni, suggesting that these rocks may represent a comagmatic series with fractionating plagioclase and hornblende. When plotted against SiO2, P2O5, Zr, Nb, and Ce display convex-upward patterns,

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indicating fractionation of apatite, zircon, and other accessory phases. The 289 Ma Totoltepec rocks are characterized by low abundances of Cr (≤27 ppm), V (≤288 ppm), Co (≤29 ppm), and Ni (≤19 ppm). On the Zr/TiO2-SiO2 diagram (Fig. 8A; Winchester and Floyd, 1977; Pearce, 1996), the samples are classified as mafic to felsic in composition, and their low Zr/Ti ratios

are typical of subalkaline mafic to intermediate rocks (Fig. 8B). Their subalkaline character is also indicated by their low Nb/Y (0.02–0.49) values as well as their position above the mantle array in the Ta/Yb versus Th/Yb plot (Fig. 9A). A volcanic arc origin is indicated on the Th-Hf-Ta and Yb versus Ta diagrams (Figs. 9B and 9C), and by Zr/Nb (~30), Ce/Yb (~12), and

Geological Society of America Bulletin, September/October 2012

Downloaded from gsabulletin.gsapubs.org on September 13, 2012 Permian–Carboniferous arc magmatism and basin evolution along the western margin of Pangea TABLE 1. Sm-Nd ISOTOPIC DATA FOR SAMPLES FROM THE TOTOLTEPEC AREA, ACATLÁN COMPLEX, MEXICO Nd Sm T(i) 147 143 Rock type (ppm) (ppm) Sm/Nd Sm/144Nd Nd/144Nd 2σ (Ma) εNd (0)* εNd (i)

Sample Totoltepec pluton Ca. 306 Ma marginal rocks TT-24 Hornblende gabbro TT-26A Hornblende gabbro TT-26B Hornblende gabbro TT-28 Hornblendite TT-72 Hornblende gabbro

TDM† (Ga)

1.77 1.58 2.07 4.44 6.45

0.60 0.40 0.71 1.74 1.86

0.340 0.256 0.344 0.391 0.288

0.2054 0.1548 0.2077 0.2365 0.1743

0.512811 0.512722 0.512813 0.512858 0.512659

10 20 10 6 7

306 306 306 306 306

3.4 1.6 3.4 4.3 0.4

3.0 3.3 3.0 2.7 1.3

– 0.84 – – (1.47)

Ca. 289 Ma rocks Quartz-rich granitoid TT-12§ TT-13A Tonalite TT-13B Tonalite TT-14 Hornblende diorite TT-16 Trondhjemite TT-22 Trondhjemite TT-27 Trondhjemite TT-52 Plagioclase-rich cumulate TT-74 Trondhjemite TT-78 Tonalite

1.06 7.58 10.23 10.05 2.03 7.51 1.23 1.90 3.07 9.22

0.04 1.98 3.08 2.60 0.47 1.64 0.40 0.44 0.61 2.37

0.035 0.261 0.301 0.259 0.233 0.219 0.327 0.234 0.199 0.257

0.0213 0.1578 0.1821 0.1562 0.1405 0.1322 0.1973 0.1416 0.1202 0.1556

0.512523 0.512642 0.512677 0.512690 0.512561 0.512528 0.512747 0.512492 0.512458 0.512691

8 5 7 7 7 6 8 7 4 7

289 289 289 289 289 289 289 289 289 289

-2.2 0.1 0.8 1.0 –1.5 –2.1 2.1 –2.8 –3.5 1.0

4.2 1.5 1.3 2.5 0.6 0.2 2.1 –0.8 –0.7 2.6

(0.42) 1.09 (1.74) 0.94 1.00 0.97 (2.89) 1.16 0.96 0.93

Chichihualtepec Tecomate Formation (CTF) TT-5A Metapsammite TT-6 Metapelite TT-7B Metapsammite TT-36 Metapsammite TT-39 Metapsammite TT-61A Metapsammite TT-67 Meta-arkose

13.62 22.03 23.25 16.81 26.36 39.88 16.91

3.13 5.14 4.96 4.06 4.80 7.74 3.51

0.230 0.233 0.213 0.241 0.182 0.194 0.208

0.139 0.1411 0.129 0.1459 0.0988 0.1173 0.1255

0.512520 0.512396 0.512299 0.512558 0.512168 0.512308 0.512319

7 8 6 7 4 5 8

288 288 288 288 288 288 288

–2.3 –4.7 –6.6 –1.6 –9.2 –6.4 –6.2

–0.2 –2.7 –4.1 0.3 –5.6 –3.5 –3.6

1.07 1.35 1.33 1.09 1.16 1.16 1.25

Cozahuico granite TT-560 Granite TT-563 Granite TT-564 Granite

5.08 7.01 10.82

1.02 1.64 1.79

0.200 0.233 0.166

0.1206 0.1411 0.1002

0.512313 0.512384 0.512287

7 7 7

270 270 270

–6.3 –5.0 –6.8

–3.7 –3.0 –3.5

1.19 1.37 1.02

La Carbonera Stock TT-565A Diorite 17.71 3.34 0.188 0.1139 0.512303 7 275 –6.5 –3.6 1.13 TT-565B Diorite 23.72 4.88 0.206 0.1243 0.512336 7 275 –5.9 –3.4 1.20 Gabbro 71.64 18.70 0.261 0.1578 0.512338 7 275 –5.9 –4.5 1.91 TT-568 TT-569 Granodiorite 22.29 3.782 0.170 0.1026 0.512311 6 275 –6.4 –3.1 1.00 Note: Analyses were performed at the Atlantic Universities Regional Isotopic Facility, Memorial University of Newfoundland. For details on analytical procedures, see GSA Data Repository File 1 (see text footnote 1). *εNd values are relative to 143Nd/144Nd = 0.512638 and 147Sm/144Nd = 0.196593 for present-day chondrite uniform reservoir (CHUR; Jacobsen and Wasserburg, 1980) and λ147Sm = 6.54 × 10–12/yr (Steiger and Jäger, 1977). † Depleted mantle model ages (TDM ) were calculated using the depleted mantle model of DePaolo (1981). Values in parentheses denote model ages that may be unreliable due to high 147Sm/144Nd (>0.165; Stern, 2002). § Sample has anomalously low Sm and Nd concentrations and is thus excluded from further consideration.

La/Nb (~3) ratios, which are typical of modern calc-alkaline suites (Gill, 1981; Pearce, 1982; Cabanis and Lecolle, 1989). However, some of the samples exhibit a more primitive tholeiitic character due to lower Th/Yb (Fig. 9A) and higher Hf/Th (Fig. 9B) ratios. The chondrite-normalized REE patterns of the ca. 289 Ma diorite and tonalite samples (Fig. 10C) are characterized by flat light (L) REEs (average [La/Sm]n = 1.5), flat to moderately fractionated heavy (H) REEs ([Gd/Yb]n = 1.1–3.3), small negative to small positive Eu anomalies (Eu/Eu* = 0.7–1.4), and ΣREE abundances of 12–21 times chondrite. Chondrite-normalized REE patterns of the trondhjemites, and samples of quartz-rich granitoid and plagioclase-rich cumulate (Fig. 10E) are typified by an LREE enrichment (average [La/Sm]n = 2.7; average [La/Yb]n = 4.6), flat HREE patterns (average [Gd/Yb]n = 1.2), negative to positive Eu anomalies (Eu/Eu* = 0.5–2.9), and low ΣREE. The general trend for these samples is a decrease in ΣREE with increasing SiO2,

which may be an effect of accessory phase fractionation (Miller and Mittlefehldt, 1982). Multi-element patterns for diorite and tonalite normalized to N-MORB (Fig. 10D) show enrichment in LILEs (Cs, Ba, K, Pb, and Sr) and moderate depletion in the HFSEs Ta and Ti. Abundances of Nb, Zr, and Hf as well as the middle REEs are similar to N-MORB. The heavy REEs are depleted in all but one tonalite sample, which exhibits HREE values identical to N-MORB. Trace-element profiles (Fig. 10F) for the Totoltepec trondhjemites, and samples of quartz-rich granitoid and plagioclase-rich cumulate are enriched in strongly incompatible elements (Cs, Rb, Ba, Th, U), moderately depleted in the less incompatible elements (middle to heavy REEs, Ti, Y), and show strong Nb and Ta negative anomalies. These patterns are a characteristic feature of arc magmas (e.g., Pearce and Peate, 1995). The Sm-Nd isotopic data for the ca. 289 Ma Totoltepec rocks yield initial εNd values between

–0.8 and +2.6 and 147Sm/144Nd ratios from 0.12 to 0.20 (Table 1; Fig. 11). Rocks with 147Sm/144Nd < 0.165 yield TDM ages of 0.93–1.16 Ga. Although considerably less radiogenic than the contemporary depleted mantle, one of the ca. 289 Ma tonalite samples (εNd [i] = 2.6) and the hornblende diorite (εNd [i] = 2.5) have similar isotopic compositions to those of the ca. 306 Ma mafic rocks, suggesting derivation from the same subcontinental lithospheric mantle. The other ca. 289 Ma Totoltepec rocks exhibit lower εNd (i) and higher TDM values. These rocks could not have originated by simple differentiation of a Totoltepec pluton mafic parent, because simple fractional crystallization should not affect the 143Nd/144Nd ratio. Their position along assimilation and fractional crystallization (AFC) trajectories in the εNd (t) versus 147Sm/144Nd diagram (Fig. 11B) suggests that their isotopic signature may be an effect of mixing between a basaltic magma (P) and crustal melts (C) derived from Oaxacan Complex basement.

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Downloaded from gsabulletin.gsapubs.org on September 13, 2012 Kirsch et al. Figure 11. (A) ε Nd (t) versus Deplete d time plot comparing Sm-Nd mantle isotopic data of the Totoltepec pluton (vertically hatched) and 5 the Chichihualtepec Tecomate Formation metasedimentary rocks (diagonally hatched) with metasedimentary rocks from the Tecomate Formation type 0 area (Yañez et al., 1991), rocks Totoltepec pluton from the Oaxacan Complex (Ruiz et al., 1988), and OrdoChichihualtepec Tec. Fm. vician amphibolites from the Metapelite Asis area (Murphy et al., 2006). Metapsammite Modern depleted mantle com5 Meta-arkose position is from DePaolo (1988). 147 144 Asis amphibolites (B) Sm/ Nd versus ε Nd (t) diagram for Totoltepec pluton Tecomate Fm. type area rocks as a means to evaluate Oaxacan Complex crustal contamination. Fields correspond to Sm-Nd data from the 10 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Cozahuico granite (this paper; Elías-Herrera et al., 2005; t (Ga) Trondhjemite Torres et al., 1999), the La CarPlag-rich cumulate bonera stock (this paper), the 8 Tonalite Depleted Altos Cuchumatanes granitoids mantle Quartz diorite (Solari, 2012, personal comHornblende diorite 6 mun.), the Tuzancoa Formation volcanic rocks (Rosales-Lagarde Hornblende gabbro Plag Ol, Px, Hbl et al., 2005), and Ordovician Hornblendite 4 amphibolites from the Asis lithodeme (Murphy et al., 2006) P 2 and the Olinalá area (Ortegar = 0.25 Obregón et al., 2010). For comparison, εNd (t) data for all 0 samples are shown at t = 289 Ma. The black curves show trends Cozahuico granite 270 Ma r = 0.75 -2 for assimilation and fractional La Carbonera 275 Ma crystallization (AFC; DePaolo, Cuchumatanes 318–313 Ma r = 10 1981) in which crust (C—aver-4 Ord. amphibolites Olinalá age composition of the Oaxacan (Ortega-Obregon et al., 2010) r=2 Complex calculated from Ruiz Ord. amphibolites Asis -6 Oaxacan (Murphy et al., 2006) et al., 1988) is assimilated by Complex Totoltepec pluton trondhjemite a basaltic parent magma (P— (Martiny-Kramer, 2008) Ol, Px, Ap, Zrc C K-fsp, Plag average composition of four -8 most juvenile 306 Ma marginal 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 ultramafic to mafic rocks of the 147 Sm/144Nd Totoltepec pluton). Values for r (rate of assimilation relative to fractional crystallization) are indicated adjacent to AFC lines. For r ≥ 1, curves extend to values of F (fraction of remaining liquid) = 5; for r < 1, curves end at F = 0.1. Partition coefficients are from Arth (1976). Composition of depleted mantle is from DePaolo (1988). Gray arrows indicate trends for pure fractional crystallization of olivine (Ol), pyroxene (Px), hornblende (Hbl), plagioclase (Plag), apatite (Ap), zircon (Zrc), and K-feldspar (K-fsp).

εNd(t = 289 Ma)

306 Ma

B

289 Ma

εNd(t)

A

Comparative Geochemistry In order to evaluate their regional significance, we compare the Totoltepec rocks to other Permian to Carboniferous igneous suites along the North American Cordillera for which ages have been determined by U-Pb geochronology

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(Figs. 2 and 7). Some of these suites form part of the putative late Paleozoic continental magmatic arc extending along the length of Mexico (e.g., Torres et al., 1999; Centeno-García, 2005). Harker diagrams display considerable overlap between the Totoltepec pluton and the com-

parative suites for major elements TiO2, Al2O3, Fe2O3, and CaO (Fig. 7), but the Cozahuico granite, La Carbonera stock, and igneous rocks from the Chiapas Massif and the Altos Cuchumatanes exhibit consistently higher SiO2 and K2O. The more felsic and more alkalic character of these

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Downloaded from gsabulletin.gsapubs.org on September 13, 2012 Permian–Carboniferous arc magmatism and basin evolution along the western margin of Pangea exhibit an εNd (t) value of –1.3 (t = 275) and a TDM model age of 1.4 Ga (recalculated from Rosales-Lagarde et al., 2005). These values, which are less radiogenic than Totoltepec pluton rocks with similar SiO2 content, suggest derivation by melting of older continental crust. This conclusion is consistent with an abundance of inherited zircons and/or crustal xenoliths documented in these rocks (Elías-Herrera et al., 2005; Solari et al., 2001, 2010). Other plutonic rocks, for which their late Paleozoic ages are based on K-Ar or Rb-Sr dating (not shown), as well as sedimentary rocks of Permian–Carboniferous age exhibit εNd (i) values similar to the Cozahuico and La Carbonera stocks (Torres et al., 1999; Schaaf et al., 2002; Yañez et al., 1991) and therefore are in broad agreement with this interpretation.

suites is also apparent in the rock classification diagrams (Fig. 8), where they plot mainly in the fields of (trachy-)andesite, rhyolite, and trachyte. Lavas from the Tuzancoa Formation overlap the composition of Totoltepec pluton tonalite and quartz diorite. Tectonic discrimination diagrams (Fig. 9) classify the majority of the samples from the comparative igneous suites as calc-alkaline arc rocks, but they show higher Ta and Yb abundances as well as higher Ta/Yb and Th/Yb ratios than rocks of the Totoltepec pluton. Chondritenormalized REE patterns of comparative igneous suites are more fractionated (higher[La/Yb]n ratios; Figs. 7F, 10A, 10C, and 10E), and they display higher total REE abundances than most samples of the Totoltepec pluton. The MORBnormalized spidergrams of comparative igneous suites (Figs. 10B, 10D, and 10F) are similar to those of rocks from the Totoltepec pluton, showing a jagged pattern with positive LILE enrichment and negative HFSE anomalies, typical of arc-derived rocks. On average, however, the Totoltepec rocks have lower abundances of Nb, Ta, Zr, and LREE, and are less enriched in LILEs (Rb, Th, K) than comparative Carboniferous–Permian igneous rocks. The Sm-Nd isotopic signature of the Cozahuico, La Carbonera, and the Altos Cuchumatanes rocks is less radiogenic than that of the Totoltepec pluton, displaying initial εNd values between –4.9 and –3.0, TDM model ages from 1.1 to 2.2 Ga, and 147 Sm/144Nd ratios between 0.10 and 0.16 (Fig. 11; this paper: Table 1; Torres et al., 1999; ElíasHerrera et al., 2005; Solari, 2012, personal commun.). The Tuzancoa Formation volcanic rocks

Andesitic arc source

La/Th

10 8

Mixed felsic-basic source

6 Acid arc source

4 Passive margin source

2

0.35

B

0.30 OIA

Al2O3 / SiO2 (wt%)

Tholeiitic oceanic arc source

12

Chichihualtepec Tecomate Formation Samples from the Chichihualtepec Tecomate Formation collected for geochemistry include 16 metapsammites, 11 metapelites, 12 metaarkoses, and 2 metaconglomerates. The majorelement abundances of these metasedimentary rocks lie in the range of typical shales, sandstones, and graywackes, with their SiO2 content ranging from 56.4 to 76.9 wt% and Al2O3 values from 11 to 21 wt% (LOI-free basis). SiO2 displays negative correlations with Al2O3 (correlation coefficient r = –0.84), Fe2O3 (r = –0.92), Co (r = –0.84), and V (r = –0.80), respectively, which reflect the different proportions of clay/mud-rich and quartz-rich components, as documented in other sedimentary sequences (e.g., Bhatia, 1983).

Metapelite Metapsammite Meta-arkose Metaconglomerate Upper continental crust North American Shale Composite Post Archean Australian Shale

A

14

The range of Al2O3/TiO2, Cr/Th, Th/Co, Cr/V, and V/Ni ratios in the Chichihualtepec Tecomate Formation rocks suggests that the majority of samples are derived from felsic sources (Taylor and McLennan, 1985; Cullers, 1994; Girty et al., 1996), which is consistent with the low average MgO, Fe2O3, Cr, Ni, and Co abundances. Incompatible elements such as Zr, Nb, Hf, Ta, Y, Th, and U have higher abundances in the Chichihualtepec Tecomate Formation rocks than they display in sedimentary suites derived from mafic sources (Feng and Kerrich, 1990). Similarly, Hf and La/Th characteristics of the Chichihualtepec Tecomate Formation rocks (Floyd and Leveridge, 1987; Fig. 12A) indicate an acid arc to mixed felsic-basic source with a minor influence of older sedimentary components. Chondrite-normalized REE patterns of the Chichihualtepec Tecomate Formation (Fig. 13A) are characterized by a moderate enrichment in LREE ([La/Yb]n = 3.0–7.1), flat HREE ([Gd/Yb]n = 1.0–1.5), and negative Eu anomalies (Eu/Eu* = 0.62–0.82). These features suggest that the source of the clastic rocks was fractionated with respect to plagioclase (Slack and Stevens, 1994). One metapsammite sample exhibits a greater REE fractionation (LREE/HREE = 16.9, [La/Yb]n = 25.0) and a slightly steeper HREE slope ([Gd/Yb]n = 2.5). Total REE abundances for all samples range between 20 and 150 times chondrite. The Sm-Nd isotopic compositions for Chichihualtepec Tecomate Formation samples are highly variable (Fig. 11A), with εNd (t) values

0.25 CA 0.20 ACM

0.15 0.10 PM

OIA – Oceanic island arc CA – Continental island arc ACM – Active continental margin PM – Passive margin

0.05

Increasing old sediment component

0

0 0

2

4

6

8

10

12

14

0

2

4

Hf (ppm)

6

8

10

12

14

16

(Fe2O3+MgO) (wt%)

Figure 12. Discrimination diagrams for the metasedimentary rocks of the Chichihualtepec Tecomate Formation. (A) Hf versus La/Th diagram after Floyd and Leveridge (1987); (B) (Fe2O3 + MgO) versus (Al2O3/SiO2) diagram after Bhatia (1983). Post-Archean Australian Shale, upper continental crust (Taylor and McLennan, 1985), and North American Shale Composite (Gromet et al., 1984) are shown for comparison.

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Sample / chondrite

Tecomate Formation (Fig. 13B) rocks are characterized by positive Cs, Ba, and U anomalies for some samples and a strong depletion in HFSEs, particularly Nb and Ta. This signature suggests that the Chichihualtepec Tecomate Formation was deposited in a sedimentary basin that formed in an arc environment. An arc-related provenance is also indicated on the Hf versus La/Th plot (Floyd and Leveridge, 1987; Fig. 12A) and the (Fe2O3 + MgO) versus Al2O3/SiO2 ratio diagram (Bhatia, 1983; Fig. 12B). A limited range in Ti/Zr ratios and the petrographic observations, such as the high modal abundance of feldspar, a wide range of grain sizes, and the angularity of relict porphyroclasts, point to a compositionally immature, poorly sorted sediment that was only transported over a short distance (Garcia et al., 1994).

Metapsammite Meta-arkose Metapelite

100

UC

(Taylor and McLennan,1985)

10

A 1

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

SUMMARY AND DISCUSSION

10

Samples / upper continental crust

Our results indicate that the late Paleozoic Totoltepec pluton and the metasedimentary Chichihualtepec Tecomate Formation postdate collisional orogenesis and developed at different crustal levels along the periphery of Pangea. The Totoltepec pluton consists of minor maficultramafic rocks (306 ± 2 Ma) that are marginal to the main felsic-mafic intrusion (289 ± 2 Ma). Both intrusive phases have an arc geochemistry but are more primitive than contemporaneous arc complexes in southern Mexico. The Chichihualtepec Tecomate Formation was derived from a late Paleozoic arc.

1

0.1

B Cs Rb Ba Th U Nb Ta K La Ce Pr Sr Nd Zr Sm Eu Ti Dy Y Yb Lu Hf Tb Tm Gd Ho Er

Figure 13. (A) Chondrite-normalized rare earth element (REE) plot (normalizing values from Sun and McDonough, 1989); and (B) upper continental (UC) crust–normalized traceelement diagram (normalizing values from Taylor and McLennan, 1995) of Chichihualtepec Tecomate Formation metasedimentary rocks. ranging from –5.6 to +0.3 (t = 288 Ma) and depleted mantle model ages (TDM) between 1.07 and 1.35 Ga. These data represent the weighted average of Sm-Nd isotopic compositions for all the detrital contributions from the source area (Arndt and Goldstein, 1987; Murphy and Nance, 2002). The εNd (t) evolution lines of the Chichihualtepec Tecomate Formation rocks lie between, and partially intersect, the Sm-Nd envelopes of both the Oaxacan Complex rocks and the Totoltepec pluton, suggesting components of both these sources in the clastic rocks. This interpretation is consistent with detrital zircon geochronological data, which indicate that the Oaxacan Complex and the regional

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Carboniferous–Permian arc are the two main contributing source areas. In contrast to the data presented herein, the Tecomate Formation in the type area (Yañez et al., 1991) lacks a contribution from the regional Carboniferous–Permian arc, as their εNd (t) values closely correspond with the Sm-Nd isotopic composition of the Oaxacan Complex (Fig. 11A). This inference is supported by U-Pb geochronological data of metasedimentary rocks from the Tecomate type area (Sánchez-Zavala et al., 2004), which yielded detrital zircon populations of Ordovician and Mesoproterozoic age. Upper continental crust–normalized traceelement patterns of the Chichihualtepec

Along-Arc Variation Whereas Torres et al. (1999) advocated the presence of a Permian–Triassic arc, the ca. 306 Ma age and the arc geochemistry of the gabbroic component of the Totoltepec pluton provide firm evidence of magmatic arc activity in the Pennsylvanian. Other igneous rocks of a similar age in the southern part of the North American Cordillera (Fig. 2) include the Cuananá plutonic complex (Vega-Carrillo et al., 1998), which yielded a SHRIMP U-Pb age of 307 ± 2 Ma (Elías Herrera et al., 2005), and ca. 313–318 Ma granitic to dioritic intrusions in the Altos Cuchumatanes, Guatemala (Solari et al., 2010). Further north, the Aserradero rhyolite in the Sierra Madre terrane yielded a U-Pb TIMS age of 334 ± 39 Ma (Stewart et al., 1999), and in the Coahuila terrane, the La Pezuña rhyolite was dated at 331 ± 4 Ma (Lopez et al., 1996). However, due to the lack of geochemical data, an arc association of these Carboniferous rocks can only be substantiated for the Totoltepec pluton and the Altos Cuchumatanes granitoids. The overall spatial, geochemical, and isotopic similarity of the ca. 289 Ma main body of the

Geological Society of America Bulletin, September/October 2012

Downloaded from gsabulletin.gsapubs.org on September 13, 2012 Permian–Carboniferous arc magmatism and basin evolution along the western margin of Pangea Totoltepec pluton with the precursor 306 Ma gabbroic rocks suggests that regional arc activity continued into the Early Permian. Other evidence for Early Permian arc magmatism in southern Mexico (Figs. 2 and 14) includes the 270 ± 3 Ma Cozahuico granite (Elías-Herrera and Ortega-Gutiérrez, 2002; Elías-Herrera et al., 2005), the 275 ± 4 Ma La Carbonera stock (Solari et al., 2001), a 272 ± 10 Ma tonalitic gneiss in the Xolapa Complex (Ducea et al., 2004), and an arc-related orthogneiss in the Chiapas Massif that yielded a U-Pb SHRIMP age of 272 ± 3 Ma (Weber et al., 2007). Arc-related ca. 270– 280 Ma granitoid plutons intruding Caborca terrane basement in northwestern Mexico (Arvizu et al., 2009; Riggs et al., 2009, 2010) indicate that Early Permian arc magmatism extended into the North American craton. Arc magmatism in southern Mexico is likely to have continued into the Middle to Early Permian, as suggested by an arc-related orthogneiss of 258 ± 2 Ma age in the Chiapas Massif (Weber et al., 2005) and by a crystallization age of 254 ± 7 Ma for the Mixtequita stock in the Maya terrane (Murillo-Muñeton, 1994). The northern extension of the Early to Middle Permian con-

Mixteca

tinental arc into terranes of the North American craton is represented by ca. 258 Ma to ca. 266 Ma arc-related granites and granodiorites in the Sierra Pinta (Arvizu et al., 2009). This fragmentary record of late Paleozoic arc magmatism is in broad agreement with a number of Permian K-Ar and Rb-Sr ages of igneous rocks in Mexico (Fig. 2; Ruiz-Castellanos, 1979; Damon et al., 1981; Torres et al., 1999; Grajales-Nishimura et al., 1999). The Chichihualtepec Tecomate Formation strata, containing interstratified arc-derived volcanic and clastic rocks, provide complementary data on the nature of the late Paleozoic evolution of the shallow crust. Much of the Chichihualtepec Tecomate Formation was deposited before the Totoltepec pluton was exposed. Detrital zircon, geochemical and Sm-Nd isotopic data, together with the presence of plutoniclastic conglomerate, indicate that the Chichihualtepec Tecomate Formation was largely derived from a regional arc. The local abundance of thin granitoid dikes and very fine-grained, green, tuffaceous strata in the Chichihualtepec Tecomate Formation suggests that arc activity was contemporaneous with Chichihualtepec Tecomate

Oaxaquia

Maya Mixtequita

251

Sierra Madre

Formation deposition. Evidence of arc activity in southern Mexico (Figs. 2 and 14) may also be preserved in (1) the latest Pennsylvanian to Middle Permian Tecomate Formation type area, which contains mafic flows, tuffs, and rare felsic units (Keppie et al., 2004b; Sánchez-Zavala, 2008), (2) the Middle Permian Los Hornos Formation (Ramírez et al., 2000; Vachard et al., 2004), and (3) the uppermost Devonian to Lower Permian Patlanoaya Group, which contains intercalations of bentonite horizons representing ash-fall deposits (Vachard et al., 2000; Vachard and de Dios, 2002). The undeformed Olinalá Formation of Middle to Upper Permian age (Buitrón et al., 2005) is potentially also partially correlative with the Chichihualtepec Tecomate Formation strata, although volcanic rocks have not been described in this unit. Further north, the magmatic arc is characterized by (1) the Early to Middle Permian Tuzancoa Formation (Sierra Madre terrane), which contains andesitic to basaltic lava flows and felsic tuffs (Rosales-Lagarde et al., 2005), (2) the Early Permian bentonite-bearing Guacamaya Formation in the Sierra Madre terrane (Gursky and Michalzik, 1989), (3) the late Mississippian to

Coahuila

Xolapa

Chihuahua Caborca

Chiapas Massif orthogneiss

La Carbonera

Chiapas Massif orthogneiss

299 PENNSYLVANIAN

318

MISSISSIPPIAN

Tecomate Fm./ CTF

E

Tonalitic gneiss

Sierra Pinta Los Tanques

Cuanana Patlanoaya Group

Totoltepec

271

Sonoyta

Rara Fm.

Cozahuico

Las Delicias Fm.

M

Guacamaya Fm. Tuzancoa Fm.

260

Totoltepec

CARBONIFEROUS

Age (Ma)

PERMIAN

L

Altos Cuchumatanes Aserradero

La Pezuña

Igneous arc-related rocks dated by U-Pb geochronology Sedimentary arc-assemblages containing intercalated arcrelated volcanic rocks

359

Figure 14. Correlation chart of various arc-related igneous and sedimentary suites of Carboniferous to Permian age sorted according to their location in the various tectonostratigraphic terranes of Mexico. For references see text.

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Downloaded from gsabulletin.gsapubs.org on September 13, 2012 Kirsch et al. Middle Permian Las Delicias Formation in the Coahuila terrane (McKee et al., 1999), which contains interstratified rhyolites, and (4) the Early Permian, bentonite-bearing Rara Formation of the Sierra del Cuervo in the southern extension of the North American craton (Handschy and Dyer, 1987). As one of the samples from near the stratigraphic base of the Chichihualtepec Tecomate Formation does not contain any Carboniferous– Permian zircons, this regional magmatic arc in southern Mexico is inferred to have started developing during Chichihualtepec Tecomate Formation deposition (Figs. 15A and 15B). The Totoltepec pluton did not become a source for the Chichihualtepec Tecomate Formation until ca. 275 Ma, by which time a significant amount of the arc crust had been removed to expose the pluton (Fig. 15C). Although the population peak of the late Paleozoic detrital zircon record of the Chichihualtepec Tecomate Formation occurs at an age of ca. 307 Ma (Fig. 6), the detrital zircon record extends back to ca. 344 Ma without any significant gaps, suggesting that regional magmatic arc activity may have initiated in the Mississippian. Taken together, the data suggest that arc magmatism had commenced in some of the southern to central Mexican continental blocks by Mississippian times, whereas it probably did not become established in the Laurentian (northern) part of Mexico until the Early Permian (Fig. 14). Across-Arc Variation There are subtle differences in the geochemical and isotopic compositions between the various magmatic arc suites of southern Mexico and Guatemala. For rocks with the same SiO2 content, the Totoltepec pluton exhibits lower HFSE (Nb, Ta, LREEs, Zr) and LILE (Rb, Th, K) abundances and more radiogenic Sm-Nd isotopic compositions relative to the intermediate to felsic suites in both Oaxaquia and the Maya block, which also contain evidence of substantial crustal contamination. As certain southern Mexican arc suites of different age show similar geochemical characteristics and certain arc suites of roughly the same age show different geochemical characteristics, the compositional differences cannot be ascribed to temporal variations in arc magmatism. Instead, observed contrasts in composition between the individual arc suites considered in this paper are attributed to spatial intra-arc variation. In general, arc rock compositions vary both in time and with distance from the active trench, reflecting increasing degrees of AFC as magmas pass through thicker crust, and a change from subduction-enriched to within-plate man-

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sea level

OAX

MX

MAYA

pa

A

Pre-arc ca. 330 Ma (?)

ctf/ tf

Tt—Totoltepec Cz—Cozahuico Cu—Cuananá Ca—La Carbonera Tz—Tuzancoa Ch—Chiapas Massif Ac—Altos Cuchumatanes

ctf/tf—Chichihualtepec Tecomate/ Tecomate pa—Patlanoaya gu/dm—Guacamaya/ Del Monte

50 km

?

sea level exposed basement

Arc development ca. 300 Ma

pa

ctf/ tf

B

++ ++

++ ++

++ ++ +

Cu

Tt

MX—Mixteca terrane OAX—Oaxaquia terrane MAYA—Maya block

Ac

?

++ +

Plutons

v vv

Lava flow Fault Fault, inferred

sea level

gu/dm

Late arc: Tt exhumation ca. 270 Ma

pa v vv

ctf/ tf

C

++

+ ++ + +

Tt

Tz

Cu

++ + +

++ +

Cz

++ +

Ch

Ca

Sediment transport

Basement Arc crust

Ac

++ +

?

Figure 15. Generalized sections across the active western margin of Pangea in the late Paleozoic, showing arc development and the relative locations of the various magmatic arc assemblages in southern Mexico and Guatemala.

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Downloaded from gsabulletin.gsapubs.org on September 13, 2012 Permian–Carboniferous arc magmatism and basin evolution along the western margin of Pangea tle sources (e.g., Brown et al., 1984). Typical transverse geochemical variations include a systematic increase in LILEs, HFSEs, and alkalis and a decrease in LILE/HFSE ratios and εNd values from the front to the rear of the arc (Kimura et al., 2010, and references therein). Hence, in order to account for the more juvenile composition of the Totoltepec pluton in comparison to other Carboniferous–Permian arc suites in Mexico and Guatemala, the Totoltepec pluton is inferred to have been emplaced into a more primitive, less mature location within the Carboniferous–Permian arc. In this model, the pluton would constitute a more trenchward part of the arc, lying to the west (modern coordinates) of the more mature arc suites inferred to represent a more inboard location (Figs. 15B and 15C). Assuming that the southern Mexican crustal blocks, in which the Mixteca terrane occupies the western, most outboard position relative to Oaxaquia and the Maya block (Fig. 1A), were only involved in lateral translation relative to each other along transcurrent faults trending along strike of the arc (e.g., Dickinson and Lawton, 2001), the position of the Totoltepec pluton relative to the other southern Mexican arc suites should not have changed substantially since the late Paleozoic. Hence, the increased arc maturity is consistent with models that advocate an eastward polarity of subduction (e.g., Centeno-García, 2005; Keppie et al., 2008a). An alternative (but not necessarily mutually exclusive) model to explain the geochemical differences between the Totoltepec pluton and contemporaneous arc-related plutonic rocks in southern Mexico involves the emplacement of the Totoltepec pluton along a fault in the arc that facilitated its ascent and made it less prone to contamination. This model is consistent with transtensional kinematics associated with strike-slip faulting documented in Chichihualtepec Tecomate Formation metaconglomerates (Morales-Gámez et al., 2009) and evidence for the syntectonic emplacement of the Totoltepec pluton (Kirsch et al., 2012). Pangea Implications Late Carboniferous continental collision in Mexico was a key event in the amalgamation of Pangea and is expressed by a southerly source for flysch deposits in the Ouachitan orogeny in the Mississippian (Arbenz, 1989) and by the appearance of early Mississippian fossils in Oaxaquia with Midcontinent (U.S.) faunal affinities (Navarro-Santillán et al., 2002). Because Carboniferous to Permian continental arc magmatism recorded by the Totoltepec pluton, the Chichihualtepec Tecomate Formation, and correlative rocks elsewhere in the belt postdates the

amalgamation of Pangea, a location of the Mixteca terrane adjacent to a subducting part of the Panthalassa Ocean on the periphery of Pangea-A seems most likely (Fig. 1A). Such a location is preferable to models that assign the Mixteca terrane to a position within Pangea, either off northeastern Canada (Fig. 1C; Böhnel, 1999) or in the Gulf of Mexico, ~2000 km inland from the western margin of Pangea (Vega-Granillo et al., 2009; Fig. 1B), because these locations lie too far from any potential subducting ocean. ACKNOWLEDGMENTS

We acknowledge the Consejo Nacional de Ciencia y Tecnología (CONACyT; Project CB-2005-1: 24894), Programa de Apoyo a Proyectos de Investigación e Innovación Tecnológica (PAPIIT: IN100108-3), and a Natural Sciences and Engineering Research Council of Canada Discovery grant to Murphy for funds to support the field work and geochemical and isotopic analyses. Carlos OrtegaObregón and Ofelia Pérez-Arvizu provided technical assistance in the Laboratorio de Estudios Isotópicos, Centro de Geociencias. Kirsch is grateful to Maria Helbig for help in the field and with figure preparation. We thank Associate Editor Luca Ferrari, and reviewers Peter Schaaf and Bodo Weber, as well as two anonymous reviewers, for constructive comments on this and a previous version of the manuscript. This is a contribution to International Geological Correlation Project 597. REFERENCES CITED Alva-Valdivia, L.M., Goguitchaichvili, A., Grajales, M., de Dios, A.F., Urrutia-Fucugauchi, J., Rosales, C., and Morales, J., 2002, Further constraints for PermoCarboniferous magnetostratigraphy: Case study of the sedimentary sequence from San Salvador–Patlanoaya (Mexico): Comptes Rendus Geoscience, v. 334, no. 11, p. 811–817, doi:10.1016/S1631-0713(02)01821-7. Arbenz, J.K., 1989, The Ouachita system, in Bally, A.W., and Palmer, A.R., eds., The Geology of North America: An Overview: Boulder, Colorado, Geological Society of America, The Geology of North America, v. A, p. 371–398. Arndt, N.T., and Goldstein, S.L., 1987, Use and abuse of crust-formation ages: Geology, v. 15, p. 893–895, doi: 10.1130/0091-7613(1987)15<893:UAAOCA>2.0.CO;2. Arth, J.G., 1976, Behaviour of trace elements during magmatic processes—A summary of theoretical models and their application: Journal of Research of the U.S. Geological Survey, v. 4, no. 1, p. 41–47. Arvizu, H.E., Iriondo, A., Izaguirre, A., Chávez-Cabello, G., Kamenov, G.D., Solís-Pichardo, G., Foster, D.A., and Cruz, R.L.-S., 2009, Rocas graníticas pérmicas en la Sierra Pinta, NW de Sonora, México: Magmatismo de subducción asociado al inicio del margen continental activo del SW de Norteamérica: Revista Mexicana de Ciencias Geológicas, v. 26, no. 3, p. 709–728. Bhatia, M.R., 1983, Plate tectonics and geochemical composition of sandstones: The Journal of Geology, v. 91, no. 6, p. 611–627, doi:10.1086/628815. Böhnel, H., 1999, Paleomagnetic study of Jurassic and Cretaceous rocks from the Mixteca terrane (Mexico): Journal of South American Earth Sciences, v. 12, p. 545–556, doi:10.1016/S0895-9811(99)00038-3. Brown, G.C., Thorpe, R.S., and Webb, P.C., 1984, The geochemical characteristics of granitoids in contrasting arcs and comments on magma sources: Journal of the Geological Society of London, v. 141, no. 3, p. 413– 426, doi:10.1144/gsjgs.141.3.0413. Buitrón, B.E., Pineda, S., de Dios, A., and Vachard, D., 2005, New Permian macrofauna and macroflora from the

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tings: Earth and Planetary Science Letters, v. 45, no. 2, p. 326–336, doi:10.1016/0012-821X(79)90133-X. Yañez, P., Patchett, P.J., Ortega-Gutiérrez, F., and Gehrels, G.E., 1991, Isotopic studies of the Acatlán Complex, southern Mexico: Implications for Paleozoic North American tectonics: Geological Society of America Bulletin, v. 103, no. 6, p. 817–828, doi:10.1130/0016-7606 (1991)103<0817:ISOTAC>2.3.CO;2. SCIENCE EDITOR: NANCY RIGGS ASSOCIATE EDITOR: LUCA FERRARI MANUSCRIPT RECEIVED 23 NOVEMBER 2011 REVISED MANUSCRIPT RECEIVED 13 FEBRUARY 2012 MANUSCRIPT ACCEPTED 18 MARCH 2012 Printed in the USA

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Artículo: Kirsch, M., Keppie, J.D., Murphy, J.B., y Lee, J.K.W., Arc plutonism in a transtensional regime: the Late Palaeozoic Totoltepec pluton, Acatlán Complex, southern Mexico: International Geology Review, en prensa, doi: 10.1080/00206814.2012.693247. Contribuciones individuales de los autores: Moritz Kirsch: concepción y el diseño del estudio; trabajo de campo el cual incluye mapeo, obtención de datos estructurales, selección de puntos de muestreo y toma de muestras para el análisis de petrografía y de microsonda, así como la geocronología 40 Ar/39 Ar; adquisición de datos de microsonda; revisión de literatura; análisis y interpretación de datos; redacción del artículo. J. Duncan Keppie: contribución a la concepción y el diseño; supervisión de las actividades de campo; participación en la interpretación de los datos y en la revisión del artículo remitido; adquisición de fondos. J. Brendan Murphy: contribución a la concepción y el diseño; supervisión de las actividades de campo; participación en la interpretación de los datos y en la revisión del artículo remitido; adquisición de fondos. James K.W. Lee: participación en la interpretación de datos y en la revisión del artículo sometido; encargado de las instalaciones de análisis 40 Ar/39 Ar.

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Arc plutonism in a transtensional regime: the late Palaeozoic Totoltepec pluton, Acatlán Complex, southern Mexico Moritz Kirscha*, J. Duncan Keppieb , J. Brendan Murphyc and James K.W. Leed a

Centro de Geociencias, Universidad Nacional Autónoma de México, 76230 Querétaro, Mexico; b Departamento de Geología Regional, Instituto de Geología, Universidad Nacional Autónoma de México, 04510 México D.F., Mexico; c Department of Earth Sciences, St. Francis Xavier University, Antigonish, NS, Canada; d Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, ON K7L 3N6, Canada

Downloaded by [Moritz Kirsch] at 06:52 18 June 2012

(Accepted 9 May 2012) The ENE-trending, ca. 306–287 Ma, Totoltepec pluton is part of a Carboniferous–Permian continental magmatic arc on the western Pangaean margin. The 15 km × 5 km pluton is bounded by two N–S Permian dextral faults, an E–W thrust to the south, and an E–W normal fault to the north. Thermobarometric data indicate that the main, ca. 289–287 Ma, part of the pluton was emplaced at ≤20 km depth and ≥700◦ C and was exhumed to 11 km and 400◦ C in 4 ± 2 million years. We have documented the following intrusive sequence: (1) the 306 Ma northern marginal mafic phase; (2) the 287 Ma main trondhjemitic phase; and (3) ca. 289–283 Ma sub-vertical dikes that vary from (a) N39E, undeformed with crystal growth perpendicular to the margins, through (b) ca. N50–73E, foliated and folded with sinistral shear indicators, to (c) N73–140E and boudinaged. The obliquity of the boundary between the folded and stretched dikes relative to the N–S dextral faults suggests sequential emplacement in a transtensional regime (with 20% E–W extension), followed by different degrees of clockwise rotation passing through a shortening field accompanied by sinistral shear into an extensional field. The ca. 289–287 Ma intrusion also contains a steep ENE-striking foliation and hornblende lineations varying from sub-horizontal to steeply plunging, probably the result of emplacement in a triclinic strain regime. We infer that magmatism ceased when some of the dextral motion was transferred from the western to the eastern bounding fault, causing thrusting to take place along the southern boundary of the pluton. This mechanism is also invoked for the rapid uplift and exhumation of the pluton between ca. 287 Ma and 283 Ma. The distinctive characteristics of the Totoltepec pluton should prove useful in identifying similar tectonic settings within continental arcs. Keywords: emplacement; syntectonic pluton emplacement; magmatic arc; transtension; Acatlán Complex; Mexico; Pangaea

Introduction Calc-alkaline magmatism at convergent plate margins is commonly associated with strike–slip faulting (e.g. Fitch 1972; Jarrard 1986; Glazner 1991; Tobisch and Cruden 1995; Gibbons and Moreno 2002), which provides conduits for magma ascent, accommodates pluton emplacement (Tikoff and Teyssier 1992; Grocott et al. 1994; Grocott and Taylor 2002), and facilitates the exhumation of deeper crustal sections (Crawford et al. 1999; Žák et al. 2005). Due to the complex interaction between thermal and structural effects of plutonism and regional-scale deformation, the mechanisms responsible for pluton emplacement in continental magmatic arcs have attracted much attention. We present a case study of the mechanisms controlling the emplacement of the ca. 306–287 Ma, suprasubduction zone Totoltepec pluton in the eastern Acatlán Complex, southern Mexico. This pluton is representative of a Pennsylvanian to Early Permian arc assemblage *Corresponding author. Email: moritz@geociencias.unam.mx ISSN 0020-6814 print/ISSN 1938-2839 online © 2012 Taylor & Francis http://dx.doi.org/10.1080/00206814.2012.693247 http://www.tandfonline.com

along the western margin of Pangaea that developed soon after Pangaea formed (e.g. Torres et al. 1999; Dickinson and Lawton 2001; Centeno-García 2005). A previous contribution (Kirsch et al. 2012) documented the geochemical/isotopic characteristics and age of the pluton, and indicates that the body is a composite intrusion with mantle and crustal sources, emplaced along an immature, trenchward part of the late Palaeozoic continental arc. In this article, we use a combination of meso- and microfabric analyses, Al-in-hornblende thermobarometry and 40 Ar/39 Ar geochronology, which provide evidence for (1) the incremental assembly of the pluton (e.g. Coleman et al. 2004; Glazner et al. 2004; de Saint Blanquat et al. 2006; Pignotta et al. 2010), (2) sequential injection of sheets (Miller and Paterson 2001; Mahan et al. 2003), (3) progressive fabric development during crystallization of the pluton in a strain field (e.g. Paterson et al. 1989, 1998; Tribe and D’Lemos 1996; Barros et al. 2001), and

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(4) intrusion in a transtensional environment (Petford and Atherton 1992; Paterson and Fowler 1993; Hanson and Glazner 1995; Kratinová et al. 2007). Given the pluton location, our data also provide insights into the geodynamic evolution of the late Palaeozoic magmatic arc that developed along the periphery of Pangaea.

Geological setting The Totoltepec pluton is well-exposed in the eastern part of the Palaeozoic Acatlán Complex (Figure 1A; Mixteca terrane) and is one of several Carboniferous to Permian intrusions associated with a continental magmatic arc that formed as a consequence of subduction along the palaeo-Pacific margin of Pangaea (Torres et al. 1999; Keppie et al. 2004a; Kirsch et al. 2012). The Acatlán Complex is tectonically bound to the south by the Cenozoic La Venta/Chacalapa Fault (Solari et al. 2007; Tolson 2007), juxtaposing it against the Xolapa Complex (Figure 1A). To the west, the Acatlán Complex is thrust over Cretaceous platformal carbonates, located between the exposed Acatlán Complex and the accreted Guerrero terrane (Centeno-García et al. 2008; Ramos-Arias and Keppie 2011). To the north, the complex is unconformably overlain by Mesozoic rocks and the Cenozoic Trans-Mexican Volcanic Belt (Ferrari et al. 1999). To the east, the Acatlán Complex is bounded by the >150 kmlong, N–S-striking, dextral Caltepec Fault Zone (CFZ), which separates it from the ∼1 Ga Oaxacan Complex (Elías-Herrera and Ortega-Gutiérrez 2002). White mica from a mylonitic mica schist in the CFZ yielded a 40 Ar/39 Ar age of ca. 269 million years (Elías-Herrera et al. 2005). However, two syntectonic plutons – the ca. 307 Ma Cuananá plutonic complex and the ca. 270 Ma Cozahuico granite – attest to tectonomagmatic activity along this fault during late Pennsylvanian to Early Permian times. The Totoltepec pluton is approximately elliptical in map view; its long axis (15 km) trends roughly WNW– ENE (Figure 1B) and it crops out over an area of 68 km2 with a relief of 490 m. External contacts between the Totoltepec pluton and the surrounding strata are either non-conformable or tectonic (Malone et al. 2002), i.e. none of the original contact relationships are preserved. Along its southern margin, the Totoltepec pluton is thrust over intensely deformed, lower greenschist-facies metasedimentary rocks of the Pennsylvanian to Middle Permian Tecomate Formation (Keppie et al. 2004b; Kirsch et al. 2012). To the east, an unnamed, medium-grade metamorphic unit consisting of garnet schist and quartzite with rare amphibolite dikes is faulted against the Totoltepec pluton (Kirsch et al. 2012). To the north, the granitoid body is unconformably overlain by and faulted against redbeds of inferred Jurassic age (Malone et al. 2002). The N–S trending, dextral San Jerónimo fault (Morales-Gámez et al.

2009) separates the pluton from the Tecomate Formation and Jurassic redbeds along its western margin. Field relationships and geochronological data indicate that the Totoltepec pluton was emplaced over a ca. 19 million year period involving at least two discrete intrusions, i.e. (1) 306 ± 2 Ma, mafic–ultramafic intrusive bodies of hornblende-rich gabbros and hornblendites that occur as three minor (0.2–0.6 km2 ), elongate, fault-bounded bodies distributed along the northern and northeastern margin of the pluton (Kirsch et al. 2012), and (2) the main body, making up approximately 98% of the exposed area, dated at 287 ± 2 Ma (trondhjemite: Yañez et al. 1991), 289 ± 1 Ma (diorite: Keppie et al. 2004a), and 289 ± 2 Ma (quartz diorite: Kirsch et al. 2012), ranging in composition (in order of decreasing proportion) from trondhjemite (hornblende-rich) tonalite, and diorite to quartz granitoid, granodiorite, monzogranite, and rare plagioclase-rich (cumulate?) layers. Geochemistry of the older marginal Totoltepec mafic– ultramafic rocks indicates an arc tholeiitic to calc-alkaline affinity characterized by high LILE/HFSE ratios, flat REE patterns, and initial εNd values of +1.3 to +3.3 (t = 306 Ma). The younger Totoltepec main body exhibits a calc-alkaline trace-element geochemistry with flat to moderately fractionated LREE-enriched patterns, and initial εNd values of –0.8 to +2.6 (t = 289 Ma; Kirsch et al. 2012). Although minor degrees of crustal assimilation and fractionation processes are detected by simple isotopic modelling, the isotopic data plot within the evolutionary envelope defined by Ordovician mafic rocks of the Mixteca terrane interpreted to have been derived from a ca. 1.0 Ga subcontinental lithospheric mantle (Murphy et al. 2006; Ortega-Obregón et al. 2010). Lithological units and internal contacts The oldest, ca. 306 Ma gabbros and minor hornblendites occur in three fault-bounded, lenticular bodies along the northern and northeastern margin of the pluton. These mafic to ultramafic rocks are massive to weakly foliated and, in places, possess a compositional banding. Locally, they are intruded by steep, <1 m-wide, intensely deformed, locally disrupted felsic dikes (Figure 2A) of inferred 289–287 Ma age (Kirsch et al. 2012). The ca. 289–287 Ma rocks from the main body of the pluton have two modes of occurrence: (1) hornblende-bearing mafic to intermediate rocks, which occur as sheets consisting of compositionally banded, foliated, locally mylonitic, fine grained to megacrystic hornblende diorite, quartz diorite, and tonalite and (2) felsic rocks, forming the interior and largest proportion of the Totoltepec pluton, chiefly composed of equigranular, weakly to moderately foliated trondhjemite. The mafic–intermediate sheeted domain occurs as smaller bodies along the southern margin of the pluton

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and in a larger, margin-parallel, lenticular, sigmoidal zone (Figures 1B and 3). These zones are composed of steeply dipping to sub-vertical, aplitic to coarse-grained sheets,

or dikes of centimetre to several tens(?) of metres width displaying variable degrees of pinch-and-swell undulations (Figure 2D). Locally, dikes occur as swarms of

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Figure 2. Diking in the Totoltepec pluton. (A) Disaggregated felsic dike in marginal hornblende gabbro. (B) Swarm of parallel, narrow, interconnected mafic dikes that locally display tapering terminations. (C) Boudinaged composite dike with a felsic interior and a mafic margin. (D) Sequence of pinching and swelling dikes composing the mafic–intermediate sheeted domain in the southern part of the pluton. (E) Gently folded pegmatite dike, and (F) fabric-discordant late felsic dike in the sheeted zone. (G) Close-up of Figure 2E, showing elongate, sigmoidal quartz grains growing perpendicular to the dike margin. (H) Sigmoidal internal fabric of a felsic dike in the mafic–intermediate sheeted zone. (I) Deflection of amphibole into the plane of cross-cutting dikes. (J) Aplitic dike cross-cutting the fabric of megacrystic diorite. Note the dike-internal foliation parallel to the dike margin. (K) Trondhjemite dike in the felsic interior of the pluton.

many parallel, narrow (2–15 cm wide), steep, anastomosing sheets that locally show tapering terminations (Figure 2B), or as 10–15 cm-wide composite dikes with a

felsic, pegmatitic interior and a mafic margin (Figure 2C). Dikes of trondhjemitic or quartz-rich granitoid composition, which are inferred to be co-magmatic with the

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International Geology Review felsic pluton interior based on matching petrographic and geochemical characteristics (Kirsch et al. 2012), intrude the mafic–intermediate sheeted domain and are commonly laterally traceable for several metres along strike. These felsic dikes are generally steep and either (1) fabricdiscordant, around 10 cm wide, and undeformed to gently folded (Figures 2E and 2F) or (2) fabric-concordant, 5–40 cm wide, and exhibiting pinch-and-swell as well as boudinage structures (e.g. Figure 3D). The felsic dikes vary from fine grained to pegmatitic. The transition between the mafic–intermediate sheeted zone and the more felsic pluton interior is characterized by the gradual decrease in the occurrence of mafic dikes. Compared with the mafic–intermediate sheeted domain, the felsic interior part of the pluton is compositionally and texturally more homogeneous. In certain locations, however, the felsic interior exhibits elevated modal plagioclase or biotite, with subordinate granodiorite and monzogranite occurring near the northern boundary of the pluton. Moreover, although less conspicuous due to their compositional similarity, felsic dikes intrude trondhjemite in the main body of the pluton (Figure 2K). Enclaves in the Totoltepec pluton are very rare and heterogeneously distributed (Figure 4). In the mafic–intermediate sheeted domain, these include centimetre- to decimetre-sized, rounded to elongate, foliated microgranular enclaves (autoliths) of dioritic composition (Figure 4A), whose contacts with the igneous host are defined by chilled margins or dark, hornblenderich reaction rims (Figure 4B). Elongate enclaves are commonly oriented parallel to the foliation in the host rock (Figure 4C). Locally, within the mafic–intermediate sheeted domain, 5–15 cm-long, partly disaggregated clots of hornblende can be observed (Figure 4D). The interior felsic domain locally contains (1) isolated, centimetresized, ovoid globules made up of coarse-grained biotite (Figure 4E) and (2) strongly sheared, fault-bounded microdioritic enclaves of 25 cm length (Figure 4F). No xenoliths derived from the surrounding country rocks have been recognized in the Totoltepec pluton, consistent with the absence of xenocrystic zircons (Kirsch et al. 2012). Locally within the mafic–intermediate sheeted domain, a steeply dipping textural and compositional banding is developed, made up of alternating coarse-grained and finegrained bands that coincide with subtle differences in the relative proportions of hornblende and plagioclase. This banding is most conspicuous in an outcrop east of Santo Domingo Tonahuixtla (Figures 1B and 3), where individual layers are continuous for several metres along strike. In diorite, the banding is irregularly spaced, consisting of a few millimetres to about 1.5 cm-wide leucocratic and relatively coarse-grained bands, and approximately 0.5–3.5 cm, locally bifurcating, dark (fine-grained) bands (Figure 3H). In tonalite, dark and light bands have a similar average width of about 3 cm (Figure 3I). In both lithologies, light bands exhibit a porphyritic grain-size

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distribution, containing hornblende (diorite) or plagioclase (tonalite) phenocrysts of up to 0.5–1 cm length in a finegrained matrix. Dark bands are composed of small grains with roughly equal grain size. The transition between light and dark layers is generally sharp and the rocks tend to split along this anisotropy plane. Locally, within banded tonalite, however, plagioclase phenocrysts are observed to grow across the boundaries between fine-grained and coarse-grained domains.

Petrography The ca. 306 Ma marginal mafic–ultramafic bodies of the Totoltepec pluton are dominated by hornblende gabbro, with averages of 53% modal plagioclase (labradorite), 37% amphibole, and 10% other phases including magnetite, ilmenite, chalcopyrite, muscovite, titanite, and zircon, as well as secondary minerals epidote, chlorite, sericite, and antigorite. Locally, the gabbro grades into hornblendite, which is characterized by approximately 90% modal amphibole. The ca. 289–287 Ma main body of the pluton predominantly consists of trondhjemite, whose average modal composition is 54% plagioclase (oligoclase), 35% quartz, and 11% other constituents, including primary muscovite, biotite, apatite, magnetite, titaniferous magnetite, ilmenite, and zircon, as well as rare K-feldspar and titanite. Secondary minerals include albite (after oligoclase), sericite, chlorite, epidote, antigorite, haematite, and calcite. Mafic–intermediate rocks in the southern part of the pluton are composed of hornblende-rich tonalite (32% andesine, 38% amphibole, 23% quartz, and 7% of other phases), hornblende-rich diorite (40% andesine, 53% amphibole, and 7% other), and quartz-diorite (80% andesine, 15% quartz, and 5% other). Rare leucocratic felsic dikes (see above) intruding this mafic–intermediate domain have a composition corresponding to either trondhjemite or quartz-rich granitoid (68% quartz, 30% albite, and 2% other). East of the town of Totoltepec de Guerrero (Figure 1B), plagioclase-rich rocks (93% albite, 3% quartz, and 4% other) and biotite trondhjemite (55% oligoclase, 35% quartz, 7% biotite, and 3% other) are the predominant lithologies. Towards the northern margin of the pluton, the felsic rocks locally contain abundant potassium feldspar (up to 40%) and are classified as granodiorite and monzogranite according to the nomenclature of Streckeisen (1976). Plagioclase is the predominant mineral of the Totoltepec pluton, occurring as subhedral to euhedral grains varying in size from 2 mm to 6 mm. Depending on lithology, plagioclase varies in composition from albite, through oligoclase to labradorite. In tonalite, diorite, and gabbro, plagioclase exhibits normal compositional zoning (Table DR-1; see supplementary material at http://dx.doi. org/10.1080/00206814.2012.693247). Biotite occurs as isolated, tabular grains or as inclusions in plagioclase.

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Figure 4. Magmatic enclaves in the Totoltepec pluton. (A) Foliated, rounded microgranular enclaves in hornblende diorite. (B) Hornblende-rich reaction rim marking the contact between a microgranular enclave and diorite host. (C) Elongate microgranular enclave aligned parallel to foliation of host diorite. (D) Disaggregated clot of hornblende in diorite. (E) Ovoid biotite globule in trondhjemite. (F) Strongly sheared, fault-bounded microdioritic enclave in trondhjemite.

Amphibole is by far the most abundant mafic mineral in the Totoltepec pluton. It occurs as subhedral to euhedral, prismatic grains, 1–3 mm in length, and locally as megacrysts up to 4 cm in length. It commonly exhibits simple {100} twinning and commonly

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hornblendite, amphiboles are calcic (i.e. have (Ca + Na)B ≥ 1.00 and NaB < 0.50 atoms per formula unit) (Leake et al. 2003; Table DR-2; see supplementary material at http://dx.doi.org/10.1080/00206814.2012.693247). On the Mg/(Mg + Fe2+ ) vs. Si classification diagram (Figure 5A),

they range from (ferrian- to ferri-) tschermakite to tschermakitic hornblende and magnesio-hornblende in composition. A few amphiboles in the tonalite and hornblendite have (Na + K)A ≥ 0.50, corresponding to hastingsite to magnesio-hastingsite. On the (Na + K)A vs.

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International Geology Review Si classification diagram (Figure 5B), the samples fall in the field of hornblende, tschermakitic hornblende, pargasitic hornblende, and tschermakite. Aluminium preferentially resides in the tetrahedral position (Figure 5C), which suggests the dominance of high-T edenite-type ((Na, K)A AlIV = [ ]A Si) substitution. Si in all investigated amphiboles varies between 6.0 and 6.8. These overall compositional characteristics are typical of igneous amphiboles (Leake 1971; Leake et al. 2003). Although optically continuous, most investigated amphiboles are zoned, and most cores contain higher AlIV and (Na + K)A and lower Si and Mg/(Mg + Fe2+ ) than rims. Quartz generally occurs as interstitial, anhedral aggregates of variable grain size. The shapes of individual quartz clusters range from subspherical to lenticular. In places, quartz is present as hexagonal, equigranular, polygonized grains. It is also found as a vermicular intergrowth in plagioclase (myrmekite). Biotite occurs as small euhedral inclusions in plagioclase and more rarely as subhedral grains in the matrix. It is generally, partially, or entirely replaced by chlorite. In the area east of Totoltepec de Guerrero (Figure 1B), modal biotite is as high as 7%. Muscovite is ubiquitous in trondhjemite, occurring as large, discrete grains up to 1.5 mm wide or as foliated aggregates wrapping around hornblende or plagioclase phenocrysts. Si contents of investigated muscovite grains reach values of 3.17 per formula unit (Table DR-3; see supplementary material at http://dx.doi.org/10.1080/00206814.2012.693247), indicating the presence of a minor phengitic component (e.g. Massonne and Schreyer 1987). Potassium feldspar occurs as rare interstitial, fine- to medium-grained crystals of microcline. In granodiorite and monzogranite near the northern margin, orthoclase forms subhedral phenocrysts up to 3 mm in diameter containing flame-shaped albite lamellae. Opaque phases include (titaniferous) magnetite, ilmenite, and minor secondary pyrite and chalcopyrite (Table DR-4; see supplementary material at http://dx.doi. org/10.1080/00206814.2012.693247). Magnetite is spatially related to the main mafic minerals. It is dominantly subhedral or euhedral with a diameter of up to 0.5 mm, containing lamellae of ilmenite, which are interpreted as oxidation–exsolution intergrowths. Two diorite samples from the main body of the pluton contain ovoid interstitial intergrowths of ilmenite with euhedral apatite. Ilmenite is also observed to form broad lamellae in sandwich-like intergrowths with an impure Ti-Fe-Al bearing silicate phase. Meso- and microstructures Marginal, ultramafic–mafic plutonic phase (ca. 306 Ma) Mesoscopic structures in the older, ultramafic–mafic rocks of the pluton are preserved in one of the gabbroic

9

bodies along the northern margin. Here, a weak magmatic foliation, defined by the preferred orientation of prismatic to tabular amphibole, is steeply dipping to sub-vertical and appears to be folded about a steeply, westerly plunging fold axis (Figure 6F). An associated moderately plunging to sub-horizontal mineral lineation is locally defined by the alignment of sub- to euhedral, elongate amphibole. Dike orientations in this part of the marginal plutonic phase are highly variable and can be explained by folding about an axis that coincides with the fold axis derived from the foliation plane distribution. This suggests that the foliation and the dikes had similar pre-folding orientations. Microstructures in the older, marginal phase of the Totoltepec pluton record magmatic through incipient solidstate deformation at high temperature. Magmatic textures are typified by large, euhedral to subhedral, unstrained, and evenly distributed amphibole and plagioclase set in a feldspathic matrix that lacks evidence of plastic deformation. Plagioclase has a typically igneous composition (An54–57 ; Table DR-1) and is characterized by a normal growth zoning. The amphibole also has an igneous composition (Figure 5) and occurs as independent, stubby to lath-shaped crystals (Figure 7A) or as undeformed poikilitic grains around plagioclase. Some thin sections from two of the ultramafic–mafic bodies at the northeastern margin (Figure 8) contain textures indicative of minor subsolidus deformation at high temperature, such as (1) albite twins within plagioclase sub-grains that are misoriented with respect to the host grain (Figure 7E), suggesting progressive sub-grain rotation recrystallization that occurred at temperatures above upper greenschist-facies conditions (Fitz Gerald and Stünitz 1993; Rosenberg and Stünitz 2003), and (2) evidence of myrmekite replacement at plagioclase grain peripheries, which has been interpreted to reflect deformation temperatures in excess of 500◦ C (Menegon et al. 2006, and references therein). Main, mafic–felsic plutonic phase (ca. 289–287 Ma) The main body of the Totoltepec pluton contains a mesoscopic foliation and lineation of variable intensity, which formed under a variety of temperature conditions. Foliations that are interpreted to reflect deformation in the magmatic state or in a high-temperature solid state are defined by the preferred orientation of elongate, undeformed, magmatic amphibole in mafic–intermediate lithologies. Commonly, amphibole is randomly distributed on the foliation plane (Figure 3C). Locally, however, a sub-horizontal to sub-vertical lineation is defined by the alignment of sub- to euhedral amphibole laths within the foliation plane. In the more leucocratic rocks, deformation at magmatic to high-temperature, solid-state conditions is indicated by sub- to euhedral plagioclase and interstitial quartz that define a weak planar grain-shape preferred orientation as well as a poorly developed

10

M. Kirsch et al.

High-temperature solid-state domain

(B)

Magmatic domain

MAIN BODY

(A)

n(S) = 174 n(L) = 34

Low-temperature solid-state domain

Moderate-temperature solid-state domain

Poles to foliation Mineral lineation

(C)

n(S) = 51 n(L) = 13

(D)

n(S) = 26 n(L) = 8

39°

Dikes

Calculated fold axis

Undeformed/folded

(E)

Extended/boudinaged Unclassified

n = 58

(F)

n(S) = 8 n(L) = 3 n(D) = 6

MARGINAL BODIES

73° Simple shear

Dike orientations

Foliation sion Transpre s nst en sio Tr a

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n(S) = 13 n(L) = 1

Figure 6. Mesoscopic structural data from the Totoltepec pluton. (A–D) Foliation and lineation data for the magmatic as well as hightemperature, moderate-temperature, and low-temperature solid-state domains in the main body of the pluton. (E) Lower hemisphere, equal angle projection of dike orientations in the main phase of the pluton. 39◦ corresponds to the minimum clockwise (interpreted as initial) dike angle; 78◦ marks the clockwise angle of transition between folded dikes, and dikes that show pinch-and-swell and/or boudinage. Grey lines and arrows indicate the theoretical transitions between the field of finite shortening, and the field of finite shortening followed by extension based on forward modelling along a vertical, dextral N–S-striking shear zone boundary (modified after Kuiper and Jiang (2010)). See text for details. (F) Orientation of structural elements in the marginal mafic–ultramafic bodies of the Totoltepec pluton. All stereograms (except Figure 6E) are equal-area, lower-hemisphere projections. Contours were drawn according to the method of Kamb (1959) using a 3σ significance level and a 2σ contour interval.

International Geology Review

(B)

(A)

(C)

0.1 mm

0.25 mm

0.75 mm

(D)

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11

(F)

(E) (D)

0.75 mm

0.25 mm

(G)

(I)

(H)

0.1 mm

0.25 mm

0.75 mm

0.75 mm

Figure 7. Photomicrographs of characteristic textures in the magmatic domain (m), as well as high-T (hss), medium-T (mss), and low-T (lss) solid-state domains of the Totoltepec pluton. (A) Euhedral, randomly distributed amphibole (m). (B) Quartz with chess-board subgrain pattern (hss). (C) Rectangular, mosaic-like contours in quartz (hss). (D) Equigranular, polygonal quartz grains (hss). (E) Sub-grain rotation in plagioclase (hss). (F) Myrmekite formation in plagioclase at the boundary with K-feldspar (hss). (G) Glide twins in plagioclase (mss). (H) Quartz aggregate recrystallized by sub-grain rotation (mss). (I) Fractured plagioclase grain with development of new grains by microcracking and bulging-recrystallization (lss).

1

2

3

4

5 km

Magmatic High-temperature solid state Moderate-temperature solid state Low-temperature solid state

Figure 8. Spatial distribution of microstructural types within the Totoltepec pluton based on thin section analyses using criteria outlined by Blumenfeld and Bouchez (1988), Paterson et al. (1989), Miller and Paterson (1994), and B端ttner (1999).

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M. Kirsch et al.

lineation. Locally, in the mafic–intermediate sheeted domain, the foliation is defined by primary igneous compositional banding (Figures 3H and 3I). Mesoscopic fabrics of tectonic origin occur within a high-strain zone in the southern part of the pluton (Figure 3) and show pervasive recrystallization under subsolidus conditions. In this zone, the foliation is defined by the preferred orientation of amphibole and plagioclase, as well as mica. An associated linear fabric is defined by the alignment of stretched amphibole and plagioclase (Figure 3G), which show evidence of straining (both ductile recrystallization and fracturing) in a section perpendicular to the foliation and parallel to the lineation. The compositions of recrystallized amphibole and plagioclase are similar to those of igneous grains. In agreement with the range of observed mesoscopic fabrics, the younger, main phase of the Totoltepec pluton shows a continuum from primary igneous (i.e. pre-full crystallization) microstructures to those corresponding to deformation in the solid state (i.e. subsolidus). Criteria outlined by Blumenfeld and Bouchez (1988), Paterson et al. (1989), Miller and Paterson (1994), and Büttner (1999) allow the distinction of four microstructural types: (1) magmatic, (2) high-temperature (>500◦ C) solid state, (3) moderate-temperature (450–500◦ C) solid state, and (4) low-temperature solid state. Magmatic microstructures in the main body of the pluton are predominantly encountered in the northeastern part and locally within the mafic–intermediate sheeted domain near the southern margin of the pluton (Figure 8). They are characterized by coarse-grained plagioclase and amphibole laths with angular outlines surrounded by a largely isotropic matrix composed of ovoid, interstitial, optically continuous quartz, smaller grains of randomly distributed plagioclase, and decussate, undeformed muscovite, and/or biotite. The plagioclase has a typically igneous composition (An30–45 ; Table DR-1), shows normal compositional zoning, and commonly displays grain aggregation (synneusis) textures. Igneous amphibole (Figure 5) occurs as single subhedral poikilitic grains or as euhedral inclusions in plagioclase. The majority of samples in the main ca. 289–287 Ma body of the pluton exhibit microstructures consistent with high-temperature subsolidus deformation (Figure 8). Quartz shows (1) basal and prismatic (chess-board) sub-grain patterns (Figure 7B), indicating deformation at temperatures of about 650–750◦ C (Mainprice et al. 1986; Kruhl 1996), (2) lobate grain boundaries typical of recrystallization by grain boundary migration (Hirth and Tullis 1992), (3) rectangular, mosaic-like contours (Figure 7C), suggesting strong crystallographic control on grain boundary orientations under high-temperature deformation (Gapais and Barbarin 1986), and (4) a strain-free, equigranular, polygonal texture, characteristic of recovery and recrystallization processes above

epidote-amphibolite facies conditions (Figure 7D; Simpson 1985). Plagioclase grains locally show evidence of subgrain rotation recrystallization and myrmekitic intergrowth (Figure 7F). Microstructural features diagnostic of moderatetemperature solid-state deformation mostly occur in samples of high-strain zones in the southern part of the pluton (Figure 8) and include plagioclase showing a sweeping undulatory extinction, bent or tapering twin lamellae (Figure 7G), as well as internal fracturing (Fitz Gerald and Stünitz 1993). Locally, plagioclase is recrystallized along its margins, forming core-and-mantle structures. Muscovite occurs as kinked or bent grains, and as aligned fine-grained anastomosing laths that enclose relict phenocrysts. K-feldspar exhibits abundant perthite flames (Pryer 1993). Quartz is characterized by large relict grains exhibiting patchy, undulose extinction passing laterally into polycrystalline quartz aggregates with irregular grain boundaries developed predominantly by sub-grain rotation recrystallization (Figure 7H). Low-temperature, solid-state microstructures in mainphase rocks of the Totoltepec pluton are also locally present in high-strain zones near the southern margin of the pluton (Figure 8) and are characterized by a pervasive cataclastic texture (Figure 7I). The presence of angular plagioclase grains and a wide range of grain sizes suggest that grainsize reduction in feldspar is achieved by microcracking and comminution (Tullis and Yund 1987). Quartz exhibits deformation lamellae transected by bands of small, new grains formed by bulging recrystallization (Hirth and Tullis 1992). Feldspar and amphibole are almost entirely replaced by chlorite, sericite, epidote, and antigorite, indicating fluid-enhanced deformation under lower greenschist-facies conditions. Overall, the orientations of the foliation in the main part of the pluton vary from (1) moderately northerly dipping in the southern part of the pluton, (2) sub-vertical, E–W striking in the centre, to (3) steeply southerly dipping in the northern part, defining a fan-like pattern in N–S cross section (Figure 1C). There is a close agreement in the orientation of foliations over the entire range of deformation temperatures (Figures 6A–6D). Moderateto low-temperature foliations tend to be less steeply dipping, because these occur mainly in the southern part of the pluton. The foliation within the mafic–intermediate, sheeted zone is deformed into intrafolial, mesoscopic, recumbent, tight to isoclinal, predominantly S-shaped folds (Figure 9). Regional variations in the trend of foliation planes throughout the pluton are attributed to mapscale, open, upright, gently northerly plunging folds, about which the southern thrust contact is also folded (Figure 1B). Lineations show a variation in orientation from sub-horizontal to down-dip (Figures 3G and 6A–6D).

International Geology Review

13

(A)

N

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(B)

N Figure 9. Photographs of tight to isoclinal intrafolial folds in the mafic–intermediate sheeted domain of the Totoltepec pluton.

Sigma-type hornblende porphyroclasts in mylonitic tonalite and diorite indicate sinistral to top-to-SSE kinematics (Figure 3B). Further evidence for sinistral shear is provided by C’-type shear bands (Figure 3A), asymmetrically boudinaged mafic and felsic dikes (Figures 3D and 3F), and foliation deflection patterns (Figures 2G–2I and 3E). The predominant orientation of dikes in the main body of the pluton is concordant with respect to the foliation (Figure 6E). However, the overall range in the orientations of individual dikes and the variations in the amount of strain they display indicate the occurrence of several generations of dikes. Undeformed or gently folded dikes are steeply dipping and have typical strikes of 36◦ –67◦ (measured clockwise from north), typically cutting the fabric developed in the earlier intrusive phases (Figures 2E, 2F and 2J), whereas dikes exhibiting pinch-and-swell structures or boudinage commonly occur at a low angle to or in the foliation plane and have strikes of 71◦ –136◦ (Figures 2D, 2H, and 6E). Many dikes in the mafic– intermediate sheeted domain contain a foliation that is either sigmoidal or parallel with respect to dike margins, irrespective of the orientation of the dikes relative to the

foliation of the surrounding rocks (Figures 2G, 2H, and 2J). In a section perpendicular to the foliation and parallel to the local sub-horizontal lineation, amphibole is locally observed to be deflected into the plane of cross-cutting dikes (Figure 2I). Some areas in the Totoltepec pluton, particularly near the contacts, exhibit brittle features (Figure 10), including tension gashes, faults, and associated brecciation zones, jointing, small-scale horst-and-graben structures, and quartz-carbonate veins that are attributed to hydraulic fracturing and fluid mobilization late in the cooling history of the pluton, and to episode(s) of regional post-emplacement deformation. N-dipping fault planes and associated N-plunging slickensides within the pluton are consistent with top-to-the-S thrusting of the pluton over metasedimentary rocks of the Tecomate Formation as implied by the regional map (Figure 1B; Malone et al. 2002). Muscovite from the low-angle, brittle–ductile thrust contact between the Totoltepec pluton and the Tecomate Formation yields a mid-Triassic age (Kirsch et al. 2012). Locally, this shear zone is associated with a Fe-P-REE deposit containing the mineral association magnetite, apatite, barite, chlorite, quartz, chalcopyrite, and

14

M. Kirsch et al.

(A)

(B) Gabbro

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Trondhjemite

(C)

(D)

(E)

(F)

N

N

Figure 10. Late- to post-emplacement brittle features in the Totoltepec pluton. (A) Fault contact between marginal gabbro and main body trondhjemite. (B) Small-scale, horst-and-graben structure in diorite. (C) Pegmatitic quartz-carbonate vein. (D) Jointing. (E) Mylonitic thrust contact between the Totoltepec pluton and Tecomate Formation. (F) S-C fabrics in Tecomate Formation metasedimentary rocks indicating top-to-S thrusting.

a cerium mineral (Table DR-4; see supplementary material at http://dx.doi.org/10.1080/00206814.2012.693247). The mineralization is confined to two discrete, elongated bodies of about 100 m length coinciding with strong aeromagnetic anomalies (Servicio Geol贸gico Mexicano 2004a,b).

Al-in-hornblende thermobarometry Five samples of the Totoltepec pluton (one from the older, marginal bodies and four from the younger, main body) were selected for hornblende thermobarometry in order to obtain an estimate of the emplacement temperatures and pressures. For this purpose, coexisting hornblende and

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International Geology Review plagioclase were analysed by electron microprobe wavelength dispersive spectrometry (WDS) at the Laboratorio Universitario de Petrología (LUP), Instituto de Geofísica, UNAM, Mexico City, Mexico. Representative analytical data are presented in Tables 1, DR-1, and DR-2. As one of the only available means of calculating emplacement temperatures in calc-alkaline igneous rocks, Blundy and Holland (1990) suggested that amphiboleplagioclase mineral pairs in equilibrium could be used as a geothermometer. Accounting for non-ideal amphibole solid-solutions, the geothermometer was later extended by Holland and Blundy (1994), using two different exchange reactions: (A) edenite + 4 quartz = tremolite + albite, and (B) edenite + albite = richterite + anorthite. Whereas thermometer A is only applicable to quartz-bearing rocks, thermometer B can also be used for silica undersaturated assemblages, but is restricted to temperatures in the range of 500–900◦ C and plagioclase with XAn between 0.1 and 0.9 as well as hornblende with XNa(M4) > 0.03, AlIV < 1.8 atoms per formula unit (pfu), and Si in the range of 6.0–7.7 pfu. The precision of both thermometers is ±40◦ C at 1–15 kbar (Holland and Blundy 1994). The selected samples satisfy all compositional constraints with respect to temperature and oxygen fugacity, so the geothermometers of Holland and Blundy (1994) are applicable. Because independent pressure data are not available for the Totoltepec pluton, emplacement temperatures were calculated at pressures ranging from 0 kbar to 15 kbar using the HB-PLAG program developed by Holland and Powell (http://www.esc.cam.ac. uk/research/research-groups/holland/hb-plag). Minimum and maximum pressures determined by the different non-temperature-corrected Al-in-hornblende barometers (Table 1) were used to define a narrow pressure interval, from which average temperature values were calculated. For sample TT-14 (gabbro from the older, marginal body), which does not contain quartz and thus does not fulfil the prerequisites of an Al-in-hornblende barometer, pressure limits were adopted from the quartz-bearing tonalite samples. Geothermometric data yield essentially magmatic temperatures of mineral equilibration in all samples, ranging from 716◦ C to 788◦ C for thermometer A and 719◦ C to 843◦ C for thermometer B. Using thermometer B, which according to Anderson (1996) yields more accurate results, the calculated median value of samples from the main body of the pluton is 762 ± 40◦ C, whereas the hornblende gabbro from the northern marginal body yields a slightly higher median temperature of 807 ± 40◦ C. These temperatures are broadly consistent with temperatures calculated from the Ti-in-zircon geothermometer of Watson et al. (2006), yielding 713 ± 76◦ C (1σ error) for zircons of a quartz diorite from the main body of the pluton and 731 ± 49◦ C for zircons from a marginal hornblende

15

gabbro sample (Table DR-6; see supplementary material at http://dx.doi.org/10.1080/00206814.2012.693247). Hammarstrom and Zen (1986) and Hollister et al. (1987) were the first to conduct empirical studies that suggested a relationship between the total Al-content of calcic amphiboles and the confining pressure. Subsequent experimental studies (Johnson and Rutherford 1989; Thomas and Ernst 1990; Schmidt 1992) confirmed this correlation. Based on these experiments, a number of calibrations for Al-in-hornblende barometry have been developed, with which an intrusion depth can be calculated from microprobe measurements of amphiboles in granitoids. These Al-in-hornblende barometers only consider the pressuredependent Tschermak substitution as an influence on the Al-content of hornblende. Their application, therefore, requires the presence of an appropriate buffer assemblage (Qz-Kfs-Pl-Hbl-Bt-Tit/Mag and fluid melt) to limit the thermodynamic degrees of freedom (Hammarstrom and Zen 1986). Another important prerequisite is that anorthite compositions of coexisting plagioclase should range between 25% and 35% (Hollister et al. 1987). Anderson and Smith (1995) recognized that temperature also strongly influences the Al-content in hornblende (edenite substitution), developing a new formula that is based on calibrations of Johnson and Rutherford (1989) and Schmidt (1992), but introducing a temperaturecorrection term to the pressure estimates. Apart from the limitations mentioned above, the application of the Al-inhornblende barometer of Anderson and Smith (1995) is furthermore restricted to amphiboles that crystallized at high f O2 , i.e. have Fe# ≤ 0.65 and Fe3+ /(Fe3+ +Fe2+ ) ≥ 0.25. Hornblende crystallization pressures of the Totoltepec pluton were calculated with the calibration of Anderson and Smith (1995) and compared to the pressures derived from Al-in-hornblende barometer calibrations of Hammarstrom and Zen (1986), Hollister et al. (1987), Johnson and Rutherford (1989), and Schmidt (1992). The precision of these barometers is estimated at ±0.5 to ±0.6 kbar (2σ ). All samples exhibit Fe# and Fe3+ /(Fe3+ +Fe2+ ) ratios that indicate crystallization conditions under high oxygen fugacity, conforming to the requirements of the method. However, all of the selected samples from the Totoltepec pluton lack potassium feldspar, and most of them do not contain biotite or titanite (Table 1), so the calculated pressures should be considered maximum values (Anderson and Smith 1995). Three samples contain plagioclase with a higher An content than recommended, which may lead to lower Al content in hornblende and thus yield pressures that are lower than their true value (Anderson and Smith 1995). Temperature corrections were applied using median temperature values calculated by the amphibole-plagioclase thermometer of Holland and Blundy (1994) (see above).

18.214033 −97.88385

18.215866 −97.88085

18.220783 −97.87857

18.258100 −97.85163

289 ± 2

289 ± 2

289 ± 2

306 ± 2

TT-14

TT-13a

TT-55

TT-54

TT-17

Hbl gabbro HblPIQzMagIIm

Tonalite HblPIMsQzIIm

Tonalite HblPIQzTiMag

Tonalite HblPIQzBtIIm

Hbl diorite HblPIMsMaglim

Rock type assemblagea A1-P1 A2-P3 A3-P2 A1-P1 A2-P3 A3-P2 A1-P2 A2-P3 A3-P1 A1-P1 A2-P2 A3-P3 A1-P1 A2-P1 A3-P2

44.1 45.2 42.7 31.7 32.5 34.9 40.7 41.0 40.7 29.6 34.1 29.3 56.5 56.5 53.5

1.721 1.900 1.833 1.992 2.129 2.113 2.119 2.379 2.104 2.327 2.251 2.183 1.398 1.394 1.453

4.66 5.58 5.24 6.06 6.76 6.66 6.66 7.97 6.60 7.74 7.34 7.01 3.07 3.05 3.35

4.87 5.89 5.51 6.43 7.21 7.10 7.10 8.57 7.04 8.31 7.87 7.50 3.08 3.05 3.40 3.76 4.53 4.24 4.93 5.52 5.44 5.44 6.54 5.39 6.34 6.01 5.73 2.42 2.40 2.66

5.18 6.03 5.72 6.47 7.12 7.05 7.08 8.31 7.01 8.07 7.70 7.38 3.64 3.63 3.90

3.79 4.54 4.26 4.93 5.51 5.44 5.47 6.56 5.40 6.35 6.02 5.74 1.41 1.39 1.62

4.18 ± 0.39 5.29 ± 0.31 5.79 ± 0.65 6.03 ± 0.31 2.49 ± 0.14

13.7 16.5 15.4 18.0 20.1 19.8 19.8 23.8 19.6 23.1 21.9 20.9 8.8 8.7 9.7

15.2 ± 1.4 19.3 ± 1.1 21.1 ± 2.4 22.0 ± 1.1 9.1 ± 0.5

807 828 803 788 716 765 743 741 750 757 788 754 774 787 786

PHol PJR PSch PAS c P avg Depthd Depth T (ed tr)e Pair PI An Amp AI PHz b Amp-PI (%) (total) (kbar) (kbar) (kbar) (kbar) (kbar) (kbar) (km) avg (km) (◦ C)

836 843 822 780 719 766 755 752 757 752 788 746 802 815 807

833 ± 11 755 ± 32 755 ± 3 762 ± 23 808 ± 7

T (ed-tr)f T avg (◦ C) (◦ C)

Notes: Preferred values indicated in bold font; a mineral abbreviations after Whitney and Evans (2010); b HZ – Hammerstrom and Zen (1986); Hol – Hollister et al. (1987); JR – Johnson and Rutherford (1989); Sch – Schmidt (1992); c temperature correction in P(AS) based on average temperatures calculated from ed-ri temperatures (Holland and Blundy 1994) limited by PHZ , PHol , PJR , and PSch ; d average crustal density is assumed as 2.8 g cm–3 ; e temperature calculated using plagioclase-hornblende geothermometer A (edenite-tremolite) of Holland and Blundy (1994); f temperature calculated using plagioclase-hornblende geothermometer B (edenite-richterite) of Holland and Blundy (1994). Values in grey are unreliable due to lack of full mineralogical assemblage required for the thermobarometer.

18.208433 −97.89087

289 ± 2

Sample

Lat/Lon (decimal degrees)

Results of Al-in-hornblende geothermobarometry.

Age (Ma)

Table 1.

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16 M. Kirsch et al.

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International Geology Review

40 Ar/39 Ar

geochronology

Foliation-parallel muscovite between 180 and 250 μm in size was separated from a Totoltepec pluton trondhjemite sample E of Santo Domingo Tonahuixtla (TT-57: 18◦ 12 30 N, 97◦ 52 50 W). The mineral concentrate was loaded into Al-foil packets and irradiated together with the hb3gr hornblende standard (1072 ± 11 Ma) as a neutron flux monitor at the McMaster University research reactor in Hamilton, Ontario, Canada. 40 Ar/39 Ar analyses were performed at the 40 Ar/39 Ar Geochronology Research Laboratory at Queen’s University in Kingston by a laser step-heating procedure using a a 30W New Wave Research MIR 10–30 CO2 laser and a MAP 216 mass spectrometer. The data, corrected for blanks, mass discrimination, and neutron-induced interferences, are presented in Table DR-5; see supplementary material at http://dx.doi.org/10.1080/00206814.2012.693247 and in

300 Apparent age (Ma)

Hornblende diorite and marginal hornblende gabbro apparently yield much lower pressures than the other samples of the Totoltepec pluton, because both of these samples have plagioclase compositions well outside the recommended range and, in addition, the diorite lacks quartz. They are thus omitted from further consideration. The average hornblende Altot -content in tonalite from the main body of the pluton is 2.177 ± 0.122. Resulting pressures calculated with calibrations without a temperature correction term range from 5.3 to 7.9 kbar (Table 1). The lowest values are obtained by the Al-in-hornblende barometer of Johnson and Rutherford (1989), whereas the formula of Hollister et al. (1987) yields the highest values. Because the amphibole-plagioclase thermometer of Holland and Blundy (1994) generates temperatures well above the solidus of wet tonalite (Schmidt 1993), a temperature correction according to Anderson and Smith (1995) is reasonable. For samples within the recommended compositional range of plagioclase as well as tonalite sample TT-55 with a slightly higher average XAn of 0.41, an average pressure of 5.7 ± 0.6 kbar is obtained. This pressure value is equivalent to the average pressure calculated with the Johnson and Rutherford (1989) barometer, which is attributable to the fact that the experiments conducted by Johnson and Rutherford (1989) were calibrated at temperatures between 720◦ C and 780◦ C, corresponding to the crystallization temperatures of hornblende from the Totoltepec pluton. The calculated emplacement pressure of 5.7 ± 0.6 kbar for tonalite from the main body of the pluton translates into a maximum emplacement depth of 20.7 ± 2.2 km, assuming an average crustal density of 2.8 g/cm3 . In summary, the main, ca. 289–287 Ma, body of the Totoltepec pluton was emplaced at moderate temperatures (762 ± 40◦ C) and middle to high pressures (≤5.7 ± 0.6 kbar) into middle crustal levels (around 20 km).

17

Plateau age 283 ± 1 Ma

280

2σ errors

260 240 AOR-1107 /TT-57 Totoltepec pluton trondhjemite Muscovite

220 0

10

20

30

40

50

60

70

80

90

100

Fraction 39Ar (%)

Figure 11. 40 Ar/39 Ar age spectrum for foliation-parallel muscovite from Totoltepec pluton trondhjemite. For sample location see Figure 1B.

Figure 11. The plateau age and mean square of the weighted deviates (MSWD) are obtained based on the following criteria, i.e. when the apparent ages of at least three consecutive steps, comprising a minimum of 50% of the total 39 Ar released, agree within 2σ error with the integrated age of the plateau segment (e.g. McDougall and Harrison 1999; Baksi 2006). All age errors are quoted at the 2σ level. Muscovite from sample TT-57 yields an excellent plateau age of 283 ± 1 million years (MSWD = 0.248), defined by 11 fractions and representing 99.2% of the total 39 Ar released (Figure 11). The first step, comprising 31% of atmospheric argon, is associated with a small amount of contaminating phases as indicated by the corresponding Ca/K ratio (Table DR-6). Assuming a relatively high cooling rate of 50◦ C/million years, consistent with the plateau age spectrum, the closure temperature for muscovite from this sample is calculated to be 390–400◦ C, using the equation developed by Dodson (1973). These data suggest that by 283 ± 1 Ma, the main body of the Totoltepec pluton had cooled through 390–400◦ C. Assuming a geothermal gradient of 35◦ C/km, which is consistent with modelled values for active portions of continental magmatic arcs (e.g. Rothstein and Manning 2003), the calculated closure temperature for muscovite corresponds to a depth of 11.1–11.4 km. Given that barometric data indicate that the main phase of the pluton was emplaced at around 20 km, the 40 Ar/39 Ar data require a substantial and rapid uplift and exhumation of the pluton between ca. 287 and 283 Ma (around 2.25 km/million years).

Discussion Structural context The Totoltepec pluton is a component of a regional Carboniferous–Permian continental arc extending from

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Guatemala to the southern USA (Kirsch et al. 2012). In the southern Mexican portion of this arc, ca. 307–269 Ma dextral shear along the >150 km-long, N–S-striking CFZ that separates the Acatlán Complex from the Oaxacan Complex (Figure 1A) has been well-documented (ElíasHerrera and Ortega-Gutiérrez 2002; Elías-Herrera et al. 2005). The significance of dextral shear along N–S-striking faults within the Mexican Carboniferous–Permian continental arc is further supported by the presence of a S-directed, dextral, sub-vertical, ca. 330–300 Ma fault (Dowe et al. 2005) that separates the Palaeozoic Granjeno Schist (a correlative of the Acatlán Complex; e.g. Nance et al. 2007) from the ca. 1 Ga Novillo Gneiss (a correlative of the Oaxacan Complex; e.g. Ortega-Gutiérrez et al. 1995) in northeastern Mexico. In addition, the Totoltepec pluton is bounded to the W by the N–S dextral San Jerónimo fault. Muscovite from the N–S Las Ollas fault lying to the west yielded a 40 Ar/39 Ar age of 278 ± 2 million years (Morales-Gámez et al. 2009). The emplacement history of the Totoltepec pluton can be explained within this regional context. The pluton occurs in a crustal block bounded by two N–S-striking dextral faults – the San Jerónimo fault to the west and the Caltepec fault to the east (Figure 12). Dextral shear along these boundary faults is inferred to have led to

(A)

SJF

CFZ

(B)

SJF

CFZ

the development of an intervening, sub-vertical, SW–NE extensional fault (Figure 12A), which may have controlled the emplacement of the Totoltepec pluton. Progressive dextral movement on the bounding faults would have led to clockwise rotation of NE–SW lines and objects in the intervening block. Interpretation of dike orientations The systematic variation in dike orientation with progressive strain (Figure 6E) is consistent with the hypothesis that the Totoltepec pluton was emplaced during deformation. The younger dikes, as identified by cross-cutting relationships, are steeply dipping to vertical, undeformed to gently folded, and are discordant with respect to the foliation. These dikes exhibit a minimum clockwise angle of 39◦ with respect to the N–S boundary faults, which is inferred to represent the initial dike orientation. The perpendicular orientation of plagioclase and quartz adjacent to the margins of late SW–NE pegmatitic dikes in the main phase of the pluton (Figure 2G) attests to the orthogonal dilation accompanying initial intrusion. A permissive mechanism of pluton emplacement along extensional fractures is also supported by the relatively minor crustal contamination of the mantle-derived, ultramafic–intermediate

(C)

SJF

A’

B’

C’

A

B

C

CFZ

(D)

SJF

CFZ

D’

thrust

39°

?

ca. 306 Ma

ca. 289 Ma

ca. 287 Ma

ca. 283 Ma

entrained blocks

gabbro/ hornblendite

thrust

diorite tonalite

A

D

A’

B

trondhjemite

B’

C

C’

D

D’

Figure 12. Plan-view structural models and hypothetical cross-sections illustrating the emplacement of the Totoltepec pluton. (A) Intrusion of early mafic to ultramafic magma along a lineament in the transfer zone between the dextral, N–S-striking San Jerónimo Fault (SJF) and the Caltepec Fault Zone (CFZ). (B) Ascent of several sheet-like mafic to intermediate magma batches during regional transtension along a vertical, initially extensional SW–NE fault that became a WSW–ENE, sinistral cross-fault, rotating clockwise due to dextral displacement on N–S boundary faults. (C) Synkinematic emplacement of a larger, more felsic batch of melt during continued clockwise rotation of a cross-fault in the regional transtension zone. (D) Transference of dextral motion on the SJF to the CFZ, resulting in south-southeastward thrusting of the pluton and rapid uplift/exhumation.

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International Geology Review rocks and the lack of xenoliths or xenocrystic zircon in marginal and main phases of the pluton (Kirsch et al. 2012). In contrast, older, sub-vertical dikes are folded, variably sheared, foliation-concordant bodies that appear to have been reoriented during progressive dextral shear. With progressive dextral shear on the bounding faults, these NE-striking dikes would have initially rotated clockwise into a sector involving sinistral shear (Figure 12B), which is consistent with the orientation of the sigmoidal internal fabric of some dikes (Figures 2G and 2H) and the locally observed deflection of amphibole into dike planes (Figure 2I). Further clockwise rotation of the dikes led to the development of extensional structures, such as boudinage and pinch-and-swell. In order to assess the significance of the dike array in terms of the strain regime that accompanied its emplacement, we compare the dike patterns in the Totoltepec pluton to theoretical finite strain geometries and predicted material line sectors for simple shear, transpression, and transtension (Figure 6E). The present surface of the Totoltepec pluton limits the strain analysis to 2D rather than a comprehensive 3D analysis (Kuiper and Jiang 2010). The ca. 307–269 Ma dextral shear along bounding vertical N–S faults was synchronous with emplacement of the Totoltepec pluton. If the movement on the N–S faults was purely strike–slip, the transition from the shortening to the extensional field of the finite strain ellipse, i.e. between the orientation of folded dikes, and dikes that have been folded and subsequently boudinaged, should occur at angles of 90◦ to the N–S Caltepec and San Jerónimo faults. However, in the Totoltepec pluton, this transition occurs at angles of 73◦ , i.e. before the clockwise rotation reaches 90◦ relative to the N–S-striking shear zone boundaries. This geometrical distribution pattern of material line sectors is indicative of transtensional deformation (Figure 6E; Kuiper and Jiang 2010). Transtensional strain within the crustal block that contains the Totoltepec pluton is consistent with the prolate spheroid shapes in pebbles of metaconglomerates in the Pennsylvanian–Middle Permian Tecomate Formation north and south of the Totoltepec pluton (Morales-Gámez et al. 2009). The long axes of these clasts are oriented parallel to the shallow NNE-plunging stretching lineation in the metasedimentary rocks of the Tecomate Formation. A sericitic phyllite from the Tecomate Formation northwest of the Totoltepec pluton yielded a 40 Ar/39 Ar whole-rock age of 263 ± 3 million years (Morales-Gámez et al. 2009) and may record syntectonic growth of sericite. However, earlier transtensional deformation is indicated by the ca. 306 million year age of the mafic part of the Totoltepec pluton, suggesting that dextral movement along N–S striking faults in the regional arc was long-lived. Three components of strain are documented in the Totoltepec pluton: (1) NW–SE

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extension, as indicated by the orientation of late pegmatitic dikes; (2) sub-vertical emplacement, as indicated by down-dip hornblende mineral elongation lineations; and (3) sub-horizontal, WSW–ENE-directed sinistral shear, as indicated by along-strike mineral lineations and a range of different kinematic indicators. As there is no evidence for one set of lineations overprinting another, and as both sets of lineations are defined by magmatic hornblende, i.e. formed during the early stages of pluton crystallization, these three components of shear are inferred to belong to a single episode of deformation. Although only 2D data are available for the Totoltepec pluton, the vorticity axis was probably oblique to these axes, indicating triclinic deformation (e.g. Jiang and Williams 1998; Lin et al. 1999). In this context, the relative predominance of lineations with shallow and steep plunges in different parts of the mafic– intermediate sheeted domain may be attributed to either deformation-path partitioning into simple shear-dominated and pure shear-dominated movement components across the shear zone (Lin and Jiang 2001), or superimposition of the sinistral shear component on vertical emplacement. The latter is consistent with the clockwise rotation of the pluton, and the lack of significant lateral displacement in the 2D outcrop shape of the pluton, suggesting that the sinistral component during emplacement was minor compared with the amount of vertical extension. The amount of vertical emplacement is constrained by thermobarometric and geochronological data, which indicate a rapid (around 2.25 km/million years) exhumation of the pluton between ca. 287 Ma and ca. 283 Ma. Using the present horizontal width of the main plutonic phase (around 4 km) as a measure for NW–SE extension yields around 20% of E–W extension across the zone. More structural data are required to more rigorously quantify the strain in the pluton.

Ascent/emplacement mechanism Geochronological data indicate that the Totoltepec pluton was assembled by at least two magmatic episodes separated by around 17 million years. According to thermal modelling results (e.g. Stimac et al. 2001), this time span of pluton construction exceeds the thermal lifetime of a large magma reservoir, indicating that the compositional variability between the main and marginal phase of the pluton is not due to in situ differentiation of a steady-state magma chamber, but represents at least two compositionally distinct magma pulses. Field evidence indicates that the younger intrusion was also generated by a series of different magma batches ranging from felsic to mafic in composition. Physical interaction between magmatic increments of this main plutonic phase is indicated by the occurrence of autoliths, microgranular enclaves, and composite dikes. The crystallization ages obtained for various phases of the main plutonic body are the same within error (Yañez et al. 1991;

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Keppie et al. 2004a; Kirsch et al. 2012). However, contacts between undated individual sheets, dikes, and enclaves are usually sharp and locally show chilled margins or reaction rims, suggesting that magmatic injections were sufficiently spaced in time for preceding increments to cool and solidify. On the other hand, within the compositionally banded zone, which on the basis of its field relationships, steepness of banding, and lack of ‘sedimentary’ structures (e.g. Barbey 2009), is interpreted as having originated by multiple dike injections, feldspar phenocrysts grow across dike margins, suggesting that some magmatic increments were intruded before the igneous host was completely crystallized. These relationships suggest that diking was an important emplacement mechanism, at least for the highly heterogeneous mafic–intermediate sheeted zone in the southern part of the pluton, where individual narrow, sub-vertical sheets can be traced for tens of metres along strike and locally show tapering terminations (e.g. Paterson and Miller 1998; Petford et al. 2000). An emplacement mechanism involving magma migration through propagating dike conduits is consistent with the regional structural context indicating extension oblique to strike–slip faults, which favours the emplacement of plutons as multiple injections with thin, dike-like geometry (e.g. Pitcher and Berger 1972). Trondhjemitic rocks in the interior of the pluton are more voluminous and more homogeneous in composition with the marginal mafic–intermediate sheeted zone. This outcrop pattern agrees with thermal models that predict a transitory sheeted-dike phase followed by the formation of an ephemeral, central magma chamber (Hanson and Glazner 1995; Coleman et al. 2004). However, the identification of rare felsic dikes and the presence of different microstructural types within this domain suggest that the trondhjemitic part of the pluton may also have an episodic emplacement history. More generally, Bartley et al. (2008) point out that intrusive contacts between magmatic increments may be more numerous than is apparent in the field because internal contacts may have become cryptic due to recrystallization processes related to the extended periods of high temperatures that accompany slow incremental growth of a pluton.

Synthesis/intrusive sequence Geochronological (Kirsch et al. 2012), combined with thermobarometric, structural, and kinematic data, lead us to propose the following sequence of intrusive events. Early mafic–ultramafic rocks were emplaced at ca. 306 Ma along a crustal lineament (Figure 12A). At ca. 289 Ma, renewed magmatic activity in a transtensional regime within the regional magmatic arc led to several successive sheet-like intrusions of dike-fed, mafic–intermediate magmas (Figure 12B). Heat provided by mafic–intermediate

magma and regional arc activity (Solari et al. 2001; ElíasHerrera et al. 2005; Rosales-Lagarde et al. 2005; Solari et al. 2010) may have led to crustal melting and the formation of felsic magma at ca. 287 Ma. The stabilization of partially molten pathways (e.g. Miller and Paterson 2001) potentially allowed for these voluminous, more felsic batches of melt to become wedged between the older mafic–intermediate rocks (Figure 12C). Parts of the older mafic–ultramafic phase may have become entrained in the rising felsic melts, physically disaggregated by diking, rotated and dispersed along the margin of the pluton. Entrainment of the mafic–ultramafic phase is consistent with (1) the presence of inherited zircons of ca. 306 Ma age in ca. 289 Ma rocks from the pluton interior (Kirsch et al. 2012), suggesting that these older mafic–ultramafic blocks were partly assimilated; (2) the scattered spatial distribution of the marginal bodies; and (3) the visible evidence that the marginal bodies are intruded by felsic dikes. The occurrence of undated boundinaged folded dikes, folded dikes, and undeformed dikes suggests that dike intrusion started with, and continued after, intrusion of the main phase. Immediately following intrusion, clockwise rotation of the main phase of the pluton into its current WSW–ENE orientation produced a vertical foliation and horizontal lineation as well as intrafolial folds in the pluton. Fabric development occurred synchronously with deformation over a large temperature range from magmatic to low-temperature conditions and was spatially diachronous. This is consistent with the continuum from magmatic to solid-state foliations and the parallelism between these respective fabrics (e.g. Paterson et al. 1989; Vernon et al. 1989; Miller and Paterson 1994; Tribe and D’Lemos 1996). With decreasing temperature, the plutonic body may have become increasingly coupled to the country rocks (e.g. Tribe and D’Lemos 1996; Barros et al. 2001), which led to the overprinting of igneous structures by solid-state fabrics. Deformation was concentrated in zones of competency contrast, i.e. the mafic–intermediate sheeted domain near the pluton margin, where it produced discrete mylonite zones (Figure 3). Structural evidence within these mylonite zones, such as the local development of tectonic down-dip lineations with kinematic indicators of top-to-SSE transport, suggests that in the last stages of the emplacement history, thrusting may have developed locally within the regional transtensional environment. Thrusting may be associated with a decrease of slip along the San Jerónimo fault and resulting transfer of dextral displacement onto the Caltepec fault (Figure 12C). This transfer may have led to (1) termination of magma supply by truncating the magma conduit, although regional magmatism occurred elsewhere in the arc and (2) substantial (around 2.25 km/million years) uplift of the pluton between ca. 287 Ma and ca. 283 Ma as inferred by Al-in-hornblende thermobarometry combined with 40 Ar/39 Ar geochronology.

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International Geology Review Regional significance Transtensional deformation is a common feature of magmatic arcs, resulting from oblique convergence and strain partitioning at plate margins (e.g. Teyssier et al. 1995; Dewey 2002). The southern Mexican portion of the extensive Carboniferous–Permian continental arc exhibits a well-pronounced arc-parallel structure of N–S oriented, dextral strike–slip faulting and synkinematic plutons emplaced along these faults. The Totoltepec pluton is interpreted to have been emplaced along a SW–NE extensional fault that was synchronous with the dextral shear along N–S fault zones, and is thus an example of arc-transverse strike–slip tectonics in the southern Mexican arc. The geochemistry of the Totoltepec pluton, which is isotopically more primitive than coeval igneous rocks elsewhere in the regional magmatic arc (Kirsch et al. 2012), and the intrusive history involving the incremental injection of several sheet-like magma batches into mid-crustal levels during transtensional deformation, followed by local thrusting resulting in uplift and exhumation, may be characteristic of pluton emplacement in such a specialized tectonic environment and may be utilized in identifying similar tectonic settings within continental arcs.

Acknowledgements MK thanks Maria Helbig for invaluable assistance in the field and support throughout the writing process. MK also acknowledges the helpful discussions with Uwe Kroner, Luigi Solari, Fernando Ortega-Gutiérrez, Harald Böhnel, Axel Renno, and Ángel F. Nieto Samaniego. Carlos Linares provided technical assistance during microprobe work at the Laboratorio Universitario de Petrología, UNAM. This study was funded by CONACyT and PAPIIT grants to JDK, and by NSERC discovery grants to JBM and JKWL.

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Guía de la excursión geológica: Keppie, J.D., Galaz-Escanilla, G., Helbig, M., y Kirsch, M. (2012). Late Paleozoic–Early Mesozoic of the Acatlán and Ayú complexes, southern Mexico: events on the periphery of Pangæa synchronous with amalgamation and breakup. GSA Cordilleran Section, 108th Annual Meeting, Field Trip 1, 31 March – 4 April, Geological Society of America, IGCP Project 597, 17 p. Contribuciones individuales de los autores: J. Duncan Keppie: líder y organizador de la excursión; concepción y diseño de la guía de excursión. Gonzalo Galaz-Escanilla: co-líder de la excursión; descripción de las paradas 3-1 a 3-7; redacción de las figuras 7 y 8. Maria Helbig: co-líder de la excursión; descripción de las paradas 1-1 a 1-6; redacción de las figuras 3, 4 y 5. Moritz Kirsch: co-líder de la excursión; descripción de las paradas 21 a 2-8; elaboración de la figura 6; con respecto a los nuevos datos de una unidad metamórfica del Misisipiense en la parte oriental del área de estudio: trabajo de campo incluyendo mapeo y muestreo para la geocronología 40 Ar/39 Ar; análisis geoquímico e isotópico; adquisición de datos de los análisis 40 Ar/39 Ar incluyendo la separación de minerales, el análisis y la interpretación de datos.

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Amalgamation and Breakup of Pangæa: the type example of the supercontinent Cycle International Geological Correlation Program Project #597: Late Paleozoic–Early Mesozoic of the Acatlán and Ayu complexes, southern Mexico: events on the periphery of Pangæa synchronous with amalgamation and breakup J. Duncan Keppie Gonzalo Galaz-Escanilla Departamento de Geología Regional, Insituto de Geología, Universidad Nacional Autónoma de México, 04510 Mexico, D.F. Maria Helbig Mortiz Kirsch Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, 76230 Querétaro, QRO, Mexico

INTRODUCTION There is widespread acceptance that between 300 and 200 million years ago, all of the Earth’s continental land masses were assembled into a giant supercontinent, Pangæa, surrounded by a superocean, Panthalassa. However, different configurations have been proposed, e.g., Pangæa A1, A2, B, and C (Fig. 1A). Reconstructions based on Mexican paleomagnetic data have been used to support both A and B models: (a) PANGEA-A. A Permo-Triassic Pangea-A reconstruction where southern Mexico lies approximately in its present location relative to North America (Fang et al., 1989, Alva-Valdivia et al., 2002); (b) PANGEA-B. A Pangea-B reconstruction placing southern Mexico off eastern Canada during the Jurassic (Fig. 1B: Böhnel, 1999). There are also Middle American variants of the Pangea-A reconstruction: (i) southwestern Mexico is placed either along the western margin of Pangea (Fig. 1C and 1D: Keppie, 2004, Keppie et al., 2008, 2010), or within Pangea between the Maya terrane and southern USA (Fig. 1E: TalaveraMendoza et al.,2005, Vega-Granillo et al., 2007, 2009); (ii) the Yucatan block is placed either within Pangea along the southern margin of USA (Fig. 1F: Pindell and Dewey, 1982), or on the western margin of Pangea during the mid-late Permian migrating into the Gulf of Mexico by the Middle Jurassic (Steiner, 2005); (iii) the Chortis block has generally been placed off southwestern Mexico on the western marin of Pangea (Fig. 1E)(e.g., Pindell and Dewey, 1982), or within the within Pangea along the eastern margin of Mexico (Fig. 1G: Keppie and Keppie, in review). On this field trip we will examine the evidence for subduction-related tectonics during the Pennsylvanian-Jurassic in the Ayu and Acatlán complexes, which suggests proximity to an ocean that is more consistent with the Pangea-A model (Fig. 2).

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A

C

B

D 350–330 Ma

E

300–270 Ma

G F

Figure 1. Reconstructions of Pangea by various authors.

Amalgamation and Breakup of Pangรฆa: the type example of the supercontinent Cycle

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Figure 2. (A) Terranes of Middle America (after Keppie, 2004); (B) Ages od units in the Acatlรกn Complex; (C) Map of the Acatlรกn Complex (modified after Keppie et al., 2010) showing the field trip route.

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DAY 1 Maria Helbig and J. Duncan Keppie The Triassic-Jurassic Ayú Complex Southern Mexico: Evidence for Deposition on the Proximal Margin of a Backarc Basin, Underthrusting and Extrusion into the Acatlán Complex during the Breakup of Pangea-A Helbig, M, Keppie, J.D., Murphy, J.B., and Solari, L.A., in press. U-Pb geochonological constraints on the Triassic–Jurassic Ayú Complex southern Mexico: derivation from the western margin of Pangea: Gondwana Research. ABSTRACT Rocks of the newly designated Ayú Complex are located in the eastern Mixteca terrane (southern Mexico), and comprise polyphase-deformed turbiditic rocks (Chazumba Lithodeme) that are intercalated with boudinaged ortho-amphibolites. In the south, the metasedimentary sequence is affected by partial melting and grades into the ~171 Ma Magdalena Migmatite. Migmatitzation was accompanied by 171–168 Ma granitoid minor intrusions and pegmatites with inherited zircon populations of ca. 260–290, 320–360, 420–480, 880–990, and 1080– 1250 Ma that are also found in the Chazumba Lithodeme. Detrital U/Pb zircon ages from the migmatized and unmigmatized Chazumba Lithodeme yielded clusters of ca. 297, 266, 250, 214, 198, and 192 Ma, suggesting Upper Triassic—Lower Jurassic deposition. The MORB tholeiitic geochemistry of the amphibolites within the Chazumba Lithodeme indicates a back-arc environment with sedimentation occurring along the inboard rifted passive margin, the Upper Triassic–Lower Jurassic detrital zircons being derived from a contemporaneous, outboard magmatic arc. These characteristics suggest correlation with the lens-shaped Central terrane typified by the Potosi turbiditic fan in the rift-passive margin of Pangea that is absent west of the Mixteca terrane. The presence of this arc requires deposition adjacent to a subducting ocean and thus supports a Pangea-A reconstruction. Early Jurassic flattening of the subduction zone is inferred to have led telescoping of the Triassic–Early Jurassic back arc basin, during which the Chazumba Lithodeme was thrust beneath the Pangean margin where it was metamorphosed under amphibolite facies metamorphic conditions. It is further inferred that Middle-Upper Jurassic steepening of the subducting zone led to tectonic exhumation of the Chazumba Lithodeme by normal faulting along the reactivated Providencia Shear Zone. Deposition, underthrusting and exhumation of the Chazumba Lithodeme are synchronous with the breakup of Pangea and the opening of the Gulf of Mexico. STOP 1-1 (W97.78834, N17.9387426: Fig. 3) Location: Road between Sta. María Ayú and Ahuehuetitlán, riverbed of Río La Peña.

Micaceous schists and garnet-biotite gneisses are intercalated with boudinaged amphibolites, that underwent migmatization at ~171 Ma (leucosome dated by Keppie et al., 2004) and formed a mappable unit, called the Magdalena Migmatite. This tectonothermal event was accompanied by syntectonic intrusion of granitic, granodioritic and dioritic dikes and sheets (Yañez et al., 1991). A granite dike that cuts the paleosome yielded only one igneous zircon of 171 ± 4 (Middle Jurassic), whereas the rest of the dated grains are inherited zircons. Two paleosome samples of the Magdalena Migmatite yielded youngest detrital zircons of ~198 Ma (Early Jurassic) and 214 Ma (Late Triassic), respectively. Amphibolites were previously dated by Keppie et al. (2004) and showed 40Ar/39Ar cooling ages of 150 ± 2 Ma for biotite and 136 ± 2 for hornblende, suggesting rapid exhumation. Geochemically, amphibolites sampled across the Ayú Complex are MORB-like, rift-related tholeiites (Helbig et al., 2010). The majority of the ortho-amphibolites have jagged NMORBnormalized REE patterns that imply contamination either by a crustal and/or subduction component and suggest a formation in a back-arc basin. STOP 1-2 (W97.807052°°, N17.9987634°°: Fig. 3) Location: short road stop east of Tetaltepec. Structural relationships in the Magdalena Migmatite: Large scale, close to open, upright to gently inclined parasitic fold (F4) that folds the leucosome and boudinaged S3-parallel granite sheets. STOP 1-3 (W97.830789°°, N18.036058°°: Fig. 3) Location: Riverbed, south of San Miguel Ixtápan. Partially molten, and strongly deformed metasedimentary rocks that are intruded by granodiorites and pegmatites. Xenoliths are probably the metasedimentary host rock and show internal foliation as well as partial melting of the fertile domains. 40 Ar/39Ar dating of a pegmatite and a granitic sheet yielded 167 ± 2 Ma for muscovite, and 155 ± 5 Ma for biotite (Keppie et al., 2004). STOP 1-4 (W97.832239°°,N18.0421378°°: Fig. 3) Location: Road section, south of San Miguel Ixtapan. Outcrop exhibits a dike that cuts across the micaceous schists and feeds a granite sheet. The emplacement of the granite sheet is parallel to the main foliation, inflating the surrounding metapelitic host rock. The dike continues its way into the hanging metasedimentary host rock. Where in the contact with the host rock, the dike is overprinted by the same fabric as in the metasedimentary rocks. The rock is affected by later brittle normal block faulting. The fabric-parallel granite sheets show sharp contacts with the metapelites and exhibit minor pinch-and-swell structures and a distinct tectono-magmatic foliation at their margins suggesting stress-related emplacement.

Amalgamation and Breakup of Pangæa: the type example of the supercontinent Cycle

–97°48′

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Figure 3. Geological map and section of the Totoltepec-Ayu area, southern Mexico showing field trip stops (modified after Helbig et al., in press).

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STOP 1-5 (W97.828639°°, N18.0526397°°: Fig. 4) Location: Foothills of the Cerro de La Peña (Cenozoic volcanic plug), north of Tejepillo; San Miguel Ixtapan road exit to Tultitlán. Micaceous schists intercalated with minor quartzites are intruded by granites, leucogranitic and aplitic dikes. A granite dike cuts a tight, recumbent E-trending F3 fold in the metasedimentary host rock. Small leucogranite veins that probably originate from the dike are parallel to the folded S2 fabric. These relationships suggest that the intrusion was syn- to late-tectonic with respect to F3. The granite is characterized by zircon inheritance and the crystallization age is inferred from the youngest grain with an age of 168 Ma. U-Pb detrital zircon analyses of a psammitic and a pelitic mica schist yielded maximum depositional ages of ~269 Ma and ~263 Ma (Middle Permian), respectively. In the hanging wall, the mafic-ultramafic Tepejillo lens lies structurally as a nappe above the Chazumba Lithodeme (Keppie et al., 2004) and consists of four bodies that crop out along the foothills of Cenozoic volcanic plug (C. La Peña). The Tepejillo lens comprises coarse crystalline ultramafic (mainly dunite) to gabbroic rocks that are cut by diabase dikes. Geochemically, they are interpreted as part of a cumulatic body intruded into the lower continental crust (Keppie et al., 2004). The contact between the metasedimentary rocks of the Chazumba Lithodeme and the Tepejillo lens has been mapped as a folded thrust (Keppie et al., 2004). The Tultitán lens, 4 km to the northeast of the Tepejillo lens, consists of massive amphibolite and a core of metamorphosed norite. One concordant U-Pb LA-ICP-MS analysis of a prismatic tip of a euhedral zircon from a metanorite yielded an age of 174 ± 1 Ma, which is interpreted as age of intrusion for both lens (Keppie et al., 2004). Biotite from a gabbroic dike of the Tepejillo lens yielded a 40Ar/39Ar cooling age of 166 ± 2 Ma, whereas muscovite from a granite dike yielded a 40Ar/39Ar age of 161 ± 2 Ma (Keppie et al., 2004), suggesting excess argon in the biotite. Lower power increments of Late Cretaceous to Tertiary age can be observed in almost all 40Ar/39Ar analyses, implying that the Ayú Complex was affected by a later deformational event. STOP 1-6 (W97.899842°°, N18.113004°°: Fig. 5) Location: road section near the town La Providencia, on the road between Petlalcingo and Tonhuixtla. Reactivation of a Triassic S-vergent thrust fault as a listric normal fault in the Middle-Late Jurassic. The Providencia shear zone forms a major structural feature between rocks of the Acatlán Complex (Tecomate Formation and Cosoltepec Formation) and the Ayú Complex and comprises weathered mylonites. A micaceous metapsammite just south of the shear zone yielded only seventeen concordant analyses. Relatively narrow age spectra ranging from 194 to 339 Ma were obtained with the two youngest grains (190 ± 4, 193 ± 4 Ma) forming a mean of 192 ± 19 Ma (Early Jurassic). To the north of the shear zone, a

mylonitic phyllite yielded a youngest detrital zircon age of 314 ± 4 Ma, which lies within the error of the mean of the three youngest grains with an age of 321 ± 30 Ma (Late Mississippian/Early Pennsylvanian). A graphite- and feldspar-bearing mylonitic metasedimentary rock, yielded two youngest detrital zircon ages of 281 ± 4 Ma and 295 ± 8 Ma with a mean age of 284 ± 71 Ma (Early Permian). The presence of a major shear zone (Providencia Shear Zone) that separates the Acatlán Complex from the Ayú Complex was previously mapped as a thrust based on s-c fabrics in the hanging block (Malone et al., 2002; Keppie et al., 2004). However, 40 Ar/39Ar cooling ages for amphibole of an amphibolite lens and muscovite from micaceous schists, north of the shearzone yielded cooling ages of ~214 Ma and ~224 Ma, respectively (Keppie et al., 2004). These fabrics are Late Triassic, and thus developed before or during the deposition of the Chazumba Lithodeme. It is envisaged that this Triassic shear zone was reactivated during or after the Middle Jurassic as a listric normal fault and formed the upper boundary of the exhuming Chazumba Lithodeme. DAY 2 Moritz Kirsch and J. Duncan Keppie Lower Permo-Carboniferous Arc Magmatism and Sedimentation on the Margin of Pangea-A Kirsch, M., Keppie, J.D., Murphy, J.B., and Solari, L.A. in press. Permian-Carboniferous arc magmatism and basin evolution along the western margin of Pangea: geochemical and geochronological evidence from the eastern Acatlán Complex, southern Mexico: GSA Bulletin. ABSTRACT The Late Paleozoic evolution of Mexico records part of a continental arc that extends along the western margin of Pangea from western USA to the northern Andes. In the Acatlán Complex of southern Mexico, an arc assemblage consisting of a Permo-Carboniferous intrusion (Totoltepec pluton) and Permian sedimentary rocks (Tecomate Formation) offers a rare opportunity to examine events along the periphery of Pangea at the critical stage of final amalgamation. The Totoltepec pluton ranges in composition from hornblendite and hornblende gabbro through diorite to tonalite, trondhjemite, granodiorite and monzo-granite. U-Pb LA-ICPMS zircon analyses yield concordant ages of 306 ± 2 Ma in minor marginal mafic to ultramafic rocks and 289 ± 2 Ma for the main, more voluminous mafic to felsic intrusion. Major and trace element geochemistry of the Totoltepec rocks exhibit a tholeiitic to calc-alkaline character, high LILE/HFSE and flat REE patterns, which is typical of arc-related magmas. The precursor gabbroic rocks display εNd(t) values ranging from +1.3 to +3.3 (t = 306 Ma), whereas rocks from the main body of the pluton have εNd(t) values between −0.8 and +2.6 (t = 289 Ma).

Amalgamation and Breakup of Pangæa: the type example of the supercontinent Cycle

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All of the samples are variably affected by wall rock assimilation, mixing and fractionation processes, but are more juvenile compared to contemporaneous arc-related igneous rocks in southern Mexico, suggesting the pluton was emplaced into thinner crust in a less mature part of the arc or along a fault that acted as a conduit for mantle-derived melts. The Tecomate Formation consists of low-grade, poorly sorted, compositionally immature and largely unweathered metapsammites and metapelites. Several factors indicate derivation from the Permo-Carboniferous arc: (i) an arc-related geochemistry, (ii) εNd(t) values ranging from −5.7 to +0.3 (t = 280 Ma) that overlap those of the Totoltepec pluton, and (iii) detrital zircons with predominantly Permo-Carboniferous ages. The depositional age of the Tecomate Formation is constrained between the youngest detrital zircon population (ca. 280 Ma) and a published Ar/Ar age of 263 ± 3 Ma from the Tecomate Formation in the adjacent area. However, a metapsammite sample from the base of the Tecomate Formation yielded only Proterozoic zircons, indicating that deposition may have initiated earlier. Possible correlative sequences that may have been deposited in a similar peri-arc setting include the latest Pennsylvanian to Middle Permian Tecomate Formation type area, the latest Devonian to Lower Permian Patlanoaya Group, the Early to Middle Permian Tuzancoa Formation, the Middle Permian Los Hornos Formation, and the Olinalá Formation of Middle to Upper Permian age.

ment along a transpressional fault. Hornblende-bearing diorites and tonalites within the low to medium-temperature solid-state domain in the southern part of the pluton exhibit a compositional and textural banding that is interpreted to have formed by a combination of steep igneous layering, layer-parallel dike injection and melt-enhanced deformation. Although we were unable to document any regional-scale structures that may have controlled its intrusion, the timing and emplacement mechanism of the Totoltepec pluton is similar to that reported for syn-tectonic Late Carboniferous to Early Permian plutons along the Caltepec Fault zone that separates the Mixteca terrane from the Oaxacan Complex. Strike-slip tectonism along this fault may be associated with oblique subduction of the paleo-Pacific beneath the western margin of Pangea.

Kirsch, M., Keppie, J.D., Murphy, J.B. and Lee, J.K.W., in preparation. Structural history of the arc-related Totoltepec pluton, Acatlán Complex, southern Mexico: Syntectonic emplacement along a mid-crustal transpressional shear zone.

STOP 2-2 (W97.890711°°, N18.208455°°: Fig. 6)

ABSTRACT The 306–289 Ma tholeiitic to calc-alkaline Totoltepec pluton in the eastern Acatlán Complex, southern Mexico, is part of a Permo-Carboniferous continental magmatic arc along the western margin of Pangea. The pluton is a well-exposed, composite, felsic to ultramafic intrusive suite containing a conspicuous mesoscopic fabric, making it an ideal place to study the relationship between tectonic processes in magmatic arcs and pluton emplacement. We use an integrated approach combining field observations, structural measurements, analysis of micro-fabrics, as well as Al-in-hornblende thermobarometry and 40Ar/39Ar thermochronology to decipher the structural evolution of the Totoltepec pluton. The data suggest that the pluton was emplaced in ~20 km depth and rapidly uplifted to allow it to cool to ~400 °C within 6 ± 2 Ma. The elongate pluton shape, parallel, decreasing temperature fabrics, similar crystallization and deformation ages and the rapid exhumation of the pluton speak for a syntectonic emplacement. A subvertical, fanning foliation and subhorizontal to subvertical lineations as well as the presence of internal, margin-parallel sinistral shear zones suggest emplace-

STOP 2-1. (W97.88385°°, N18.214033°°: Fig. 6) Transpressional shear zone within the Totoltepec pluton near Santo Domingo Tonahuixtla. Here, strongly banded and foliated hornblende-bearing diorite and tonalite is intruded by felsic and mafic dikes at low angles to the WSW-striking planar fabric. Hornblende fish and asymmetrically boudinaged dikes consistently display sinistral kinematics. The crystallization age of the mafic rocks give an age of 289 ± 2 Ma (Keppie et al., 2004), whereas foliation-parallel muscovite in trondhjemite (1 km due SE) yield a 40Ar/39Ar age of 283 ± 1 Ma.

Aplitic dikes intruding megacrystic hornblende diorite/ tonalite north of Santo Domingo Tonahuixtla. Dikes are mostly foliation-parallel, but are locally observed to cut the foliation at low angles. Some dikes contain an internal tectono-magmatic fabric parallel to the dike wall and dike-host contacts are sharp to irregular suggesting dike emplacement was syntectonic and occurred prior to complete crystallization of the host. The megacrystic hornblende-bearing rocks are laterally traceable. Whereas at this location, lineations are weakly developed or subhorizontal with sinistral kinematics, further east, between the villages of Tonahuixtla and Totoltepec, the rocks possess a strong down-dip mineral lineation and sigma-shaped tails on hornblende porphyroblasts suggest thrusting toward the south. STOP 2-3 (W97.874964°°, N18.207481°°: Fig. 6) Compositional/ textural banding in hornblende-bearing tonalites east of Tonahuixtla. Rocks at this stop have a mylonitic fabric and contain a conspicuous banding defined by a more or less rhythmic variation in grain size and modal proportions of feldspar and hornblende. Locally, these rocks exhibit gentle, ca. 2 m wavelength, fold-like structures resembling trough-banding characteristic of layered intrusions. These features indicate that the banding may have a complex, multi-stage history of development, involving magma chamber, injection as well as tectonic processes.

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Keppie et al. 97°54′0″W 320

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Figure 6. Stops 2-1 to 2-8: geological map of the Totoltepec pluton (after Kirsch et al., in press) showing field trip stops.

Amalgamation and Breakup of Pangæa: the type example of the supercontinent Cycle STOP 2-4 (W97.86816°°, N18.218361°°: Fig. 6) Hornblende gabbro at the northern margin of the Totoltepec pluton in contact with Jurassic redbeds. This outcrop is located in one of three ca. 0.2–0.6 km2, fault-bounded, precursor (306 ± 2 Ma) gabbroic phase, which is distributed along the northern and north-eastern margin of the pluton. Locally, these rocks are intruded by intensely deformed felsic dikes. To the north, the pluton is unconformably overlain by redbeds of inferred Jurassic age, which sit steeply against the pluton buttress due to a subsequent period of normal faulting.

sional deformation. Rotated pebbles with asymmetric tails show top-to-the-south shear, which is consistent with other kinematic indicators in this area. DAY 3 Gonzalo Galaz-Escanilla and J. Duncan Keppie A High Pressure Zone within the Acatlán Complex: Uppermost Devonian: Lower Carboniferous Subduction and Extrusion under Extension during the Initial Stages of Pangea Amalgamation

STOP 2-5 (W97.851633°°, N18.2581°°: Fig. 6)

Center of High Pressure Zone

Amarillo Unit (new name), SE of Santo Domingo Tianguistengo. This unit is characterized by medium- to high-grade metasedimentary rocks locally intruded by amphibolite dikes. Youngest detrital zircons from a garnet schist sample indicate a maximum depositional age of 337 ± 4 Ma (Mississippian). The amphibolite dikes exhibit a MORB-like geochemistry with εNd (i) values of +5.2 to +7.6 and TDM model ages between 333 and 433 Ma. These features are very similar to those documented in the Salada Unit (Morales-Gámez et al., 2008), on the western side of the Totoltepec pluton.

Keppie, J.D., Nance, R.D., Dostal, J., Lee, J.K.W., and Ortega-Rivera, A. 2011 Constraints on the subduction erosion/extrusion cycle in the Paleozoic Acatlán Complex of southern Mexico: geochemistry and geochronology of the type Piaxtla Suite. Gondwana Research, doi:10.1016/j.gr.2011.07.020

STOP 2-6 (W97.776016°°, N18.257016°°: Fig. 6) Thrust contact between the Totoltepec pluton and the Tecomate Formation metasedimentary rocks. The exposed contact is a low-angle brittle-ductile thrust. At another location, this thrust is mylonitic and yielded a Middle Triassic 40Ar/39Ar age on muscovite. The contact is furthermore associated with a Fe-P-REE deposit containing the mineral association magnetite, apatite, barite, chlorite, quartz, chalcopyrite, and a cerium mineral. The mineralization is confined to two discrete, elongated bodies of ~100 m length coinciding with strong aeromagnetic anomalies. STOP 2-7 (W97.794857°°, N18.262697°°: Fig. 6) S-C fabrics in the Tecomate Formation ~1 km south of the margin of the Totoltepec pluton, indicating top-to-the-south thrusting. Thermochronological data from this area as well as other samples from the Tecomate Formation and Amarillo Unit reveal a regionally significant tectonothermal event of midTriassic age. STOP 2-8 (W97.892266°°, N18.190066°°: Fig. 6) Pebble metaconglomerates of the Tecomate Formation near Chichihualtepec. The pebbles from this outcrop, which are petrographically similar to the Totoltepec pluton trondhjemite, yielded zircons with ages between 320 and 264 Ma (Keppie et al., 2004). Morales-Gámez et al. (2009) conducted strain measurements in these rocks, documenting prolate spheroids typical of transten-

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ABSTRACT The type high-pressure (HP) Piaxtla Suite in the Acatlán Complex of southern Mexico consists of retrogressed eclogite (amphibolite), megacrystic granitoids and high-grade metasedimentary rocks. Exhumation of these HP rocks has recently been interpreted as the result of extrusion into the upper plate, rather than by return flow up the subduction zone. Geochemical analyses of the retrograde eclogites indicate that they have a rift tholeiitic-transitional alkalic composition. These are closely associated with a megacrystic meta-granitoid that has yielded an intrusive age of 452 ± 6 Ma (concordant U-Pb zircon analyses) with inherited zircon populations at ca. 800–950 Ma and 1000–1200 Ma derived from the underlying basement, probably the Oaxacan Complex which borders the Acatlán Complex to the east. The bimodal nature of these igneous rocks and their close association with continentally-derived sedimentary rocks is similar to most HP rocks in the Acatlán Complex derived from a rifted passive margin. The youngest detrital zircon population in a metapsammite sample yielded an U-Pb age of 365 ± 15 Ma with older analyses distributed along a chord with an upper intercept of 1287 ± 29 Ma. The ca. 365 Ma age provides a maximum age for the time of deposition of this sample. 40 Ar/39Ar ages from the retrogressed eclogites provided hornblende plateau ages of 342 ± 2 Ma and 344 ± 2 Ma, whereas muscovite from the granitoid and metapsammite yielded 334 ± 2 Ma plateau ages. These data constrain the subduction erosionextrusion cycle to ≤35 my during which the rocks were taken to a depth of ca. 40 km at a rate of 2.7 km/my and back to the surface at 2.4 km/my. Such exhumation rates are slower than those in continent-continent collision zones, but similar to those in the Iberia-Czech Variscan belt where tectonic interpretation also suggests extrusion into the upper plate.

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Western Boundary of High Pressure Zone: A HighPressure Folded Klippe Explaced during the Lower Carboniferous at Tehuitzingo Galaz E., Gonzalo, et al., in press. A high-pressure folded klippe at Tehuizingo on the western margin of an extrusion zone, Acatlán Complex, southern Mexico ABSTRACT The Acatlán Complex is divided into two blocks of lowgrade metamorphic rocks by a central belt of high-pressure (HP) rocks, which at Tehuitzingo is composed of metabasites, serpentinite, granite and mica schist. 580–430 Ma detrital zircon ages indicate that these rocks were deposited adjacent or very close to the Gondwana supercontinent during the Early Paleozoic and are more consistent with a development on the southern margin of the Rheic Ocean rather than the Iapetus Ocean. These rocks were then removed by a subduction-erosion to depths of ~50km, reaching a metamorphic peak of ~16 kbar and 750 °C (eclogite facies). The HP rocks underwent rapid extrusion during a major Late Devonian-Pennsylvanian tectonothermal event indicated by 40 Ar/39Ar analyses, which yielded ages of ~373 Ma (hornblende in metabasite) and of 328–317 Ma (muscovite in granite, mica schist and metabasite) that indicate cooling through ~570 °C and ~350 °C respectively, indicating a very high cooling rate of ~4.9–3.9 °C/m.y. During the extrusion process these rocks were affected by retrogression to amphibolite-epidote and green schist facies, and finally emplaced as a klippe on a greenschist facies psammite-pelite unit that constitutes the western block of the Acatlán Complex. Petrologic, deformational and geothermobarometric data suggest that west and east blocks belong to the same terrane, indicating that a subduction-erosion process and subsequent extrusion is more consistent with the genesis of the HP central belt than a collisional event as has been proposed. The P-T-t pattern of these HP rocks is consistent with subduction environments reported elsewhere in the world and suggests a serpentinite extrusion channel on the western margin of Pangea. Eastern Boundary of High Pressure Zone: A Listric Normal Shear Zone Synchronous with Deposition of the Uppermost Devonian–Lower Permian Patlanoaya Group Keppie, J.D., Nance, R.D., Ramos-Arias, M.A., Lee, J.K.W., Dostal, J., Ortega-Rivera, AQ., and Murphy, J.B. 2010. Late Paleozoic subduction and exhumation of CambroOrdovician passive margin and arc rocks in the northern Acatlán Complex, southern Mexico: geochronological constraints. Tectonophysics, v. 495, p. 213–229. ABSTRACT The origin and age of high pressure (HP) rocks is crucial for paleogeographic reconstruction because they either mark an oceanic suture or an extrusion zone within the upper plate. HP

rocks in the San Miguel Las Minas area in the northern part of the complex has been inferred to be of early Paleozoic age and to mark oceanic sutures. However, blueschists in the northern part of the Acatlán Complex in southern Mexico have yielded Mississippian 40Ar/39Ar plateau ages of 344 ± 5 Ma for glaucophane and 338 ± 3 Ma and 337 ± 2 Ma for muscovite. These ages are slightly younger than recently published ages: a U-Pb zircon age of 353 ± 1 Ma from associated eclogite, and a 347 ± 3 Ma muscovite age from the tectonically overlying, greenschist facies Las Minas Unit. Taken together, these data indicate rapid cooling between 700° and 340°C in ca. 17 Myr. On the other hand, associated Ordovician Anacahuite Amphibolite cooled through ca. 500°C at 299 ± 6 Ma (40Ar/39Ar on hornblende) suggesting a second, Permian period of exhumation. Protoliths of the high grade rocks include Cambrian-Ordovician, rift-passive margin, psammites, pelites, and tholeiitic dykes, an Ordovician mafic intrusion (Anacahuite Amphibolite dated at 470 ± 10 Ma: U-Pb zircon) and megacrystic granite (dated at 492 ± 12 Ma: U-Pb zircon), and arc-related mafic rocks of unknown age. These upper plate rocks are inferred to have been removed by subduction erosion and taken to depths between 35 and 55 km where they underwent blueschist-eclogite facies metamorphism. This was followed by rapid extrusion along a channel bounded by an easterly dipping, Mississippian, listric normal shear zone, and a thrust modified by a Permian dextral fault. Rocks above and below the extrusion zone are mainly Cambro-Ordovician rift-passive margin units, but a small vestige of the arc preserved as dikes cutting rocks lying unconformably beneath the fossiliferous latest DevonianLower Permian Patlanoaya Group. Since faunal data indicate that Pangea had amalgamated by the Mississippian, at which time the Acatlán Complex lay 1500–2000 km south of the Ouachita collisional orogen between Gondwana and Laurentia, it is inferred that subduction and extrusion of the high pressure rocks occurred on the active western margin of Pangea. Ramos-Arias, M., Keppie, J.D., Ortega-Rivera, A., and Lee, J.W.K. 2008. Extensional late Paleozoic deformation on the western margin of Pangea, Patlanoaya area, Acatlán Complex, southern Mexico. Tectonophysics, v. 448, p. 60–76. ABSTRACT New mapping in the northern part of the Paleozoic Acatlán Complex (Patlanoaya area) records several ductile shear zones and brittle faults with normal kinematics (previously thought to be thrusts). These movement zones separate a variety of units that pass structurally upwards from: (i) blueschist-eclogitic metamorphic rocks (Piaxtla Suite) and mylonitic megacrystic granites (Columpio del Diablo granite ≡ Ordovician granites elsewhere in the complex); (ii) a gently E-dipping, listric, normal shear zone with top to the east kinematic indicators that formed under upper greenschist to lower amphibolite conditions; (iii) the MiddleUpper Ordovician Las Minas quartzite (upper greenschist facies psammites with minor interbedded pelites intruded by mafic dikes

Amalgamation and Breakup of Pangæa: the type example of the supercontinent Cycle and a leucogranite dike from the Columpio del Diablo granite) unconformably overlain by the Otate meta-arenite (lower greenschist facies psammites and pelites): roughly temporal equivalents are the Middle-Upper Ordovician Mal Paso unit and prelatest Devonian Ojo de Agua unit (interbedded metasandstone and slate, and metapelite and mafic minor intrusions, respectively)— the Otate and Mal Paso units are intruded by the massive, 461 ± 2 Ma, Palo Liso megacrystic granite: decussate, contact metamorphic muscovite yielded a 40Ar/39Ar plateau age of 440 ± 4 Ma; (iv) a steeply-moderately, E-dipping normal fault; (v) uppermost Devonian-Lower Permian sedimentary rocks (Patlanoaya Group: here elevated from formation status). The upward decrease in metamorphic grade is paralleled by a decrease in the number of penetrative fabrics, which varies from (i) three in the Piaxtla Suite, through (ii) two in the Las Minas unit (E-trending sheath folds deformed by NE-trending, subhorizontal folds with top to the southeast asymmetry, both associated with a solution cleavage), (iii) one in the Otate, Mal Paso, and Ojo de Agua units (steeply SE-dipping, NE-SW plunging, open-close folds), to (iv) none in the Patlanoaya Group. 40Ar/39Ar analyses of muscovite from the earliest cleavage in the Las Minas unit yielded a plateau age of 347 ± 3 Ma and show low temperature ages of ~260 Ma. Post-dating all of these structures and the Patlanoaya Group are NE-plunging, subvertical folds and kink bands. An E-W, vertical normal fault juxtaposes the low-grade rocks against the Anacahuite amphibolite that is cut by megacrystic granite sheets, both of which were deformed by two penetrative fabrics. Amphibole from this unit has yielded a 40Ar/39Ar plateau age of 299 ± 6 Ma, which records cooling through ~490 °C and is probably related to a Permo-Carboniferous reheating event during exhumation. The extensional deformation is inferred to have started in the latest Devonian (~360 Ma) during deposition of the basal Patlanoaya Group, lasting through the rapid exhumation of the Piaxtla Suite at ~350–340 Ma synchronous with cleavage development in the Las Minas unit, deposition of the Patlanoaya Group with active fault-related exhumation suggested by Mississippian and Early Permian conglomerates (~340 and 300 Ma, respectively), and continuing at least into the Middle Permian (≡ 260 Ma muscovite ages). The continuity of Mid-Continent Mississippian fauna from the USA to southern Mexico suggests that this extensional deformation occurred on the western margin of Pangea after closure of the Rheic Ocean. STOP 3-1 (N18°° 11.728′, W98°° 14.690′ to N18°° 11.652′, W98°° 15.065′: Fig. 2) Contact between a deformed, Ordovician megacrystic granitoid, a Tertiary dike, and the HP Piaxtla Suite at Piaxtla. STOP 3-2 (UTM: 1405110/2023872: Fig. 7) Thrust contact between Tehuitzingo serpentinite and polydeformed psammitic-pelitic rocks at Solozuchitl near Atopoltitlan.

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The Piaxtla serpentinites are composed almost entirely of secondary minerals. Decussate, acicular and fibrous crystals serpentine aggregates make up 95% of the rock, magnetite, calcite, white mica and talc, and accessory chromite, clinochlore, undulose quartz, amphibole and epidote. This serpentinite are thrust over the low grade psammite-pelite unit along a gently NW-dipping thrust (320/15°), on which there are striae that plunge westwards (290/12°): associated recumbent folds, S-C fabrics and thrust horses indicate thrusting toward the west. Cutting across this thrust zone are several N-S vertical faults with subhorizontal striae. The low grade psammite-pelite unit (498 ± 2 Ma, U-Pb detrital zircon; Galaz-Escanilla et al., in press) is composed mainly of primary minerals such as quartz, feldspar and zircon (accessory), which suggests a medium-grained quartz-arenitic (0.25– 0.5 mm) and shaly (<0.06 mm grain size) protoliths respectively. The equilibrium secondary mineralogy is composed of quartz, albite (Ab99–100), Mg-Fe-chlorite (ripidolite type), phengite, epidote, calcite and leucoxene, whose geothermobarometry indicated P-T conditions of ~2.7 kbar and ~350 °C (greenschist facies; GalazEscanilla and Keppie, in press). STOP 3-3 (UTM: 140571085/2023811: Fig. 7) Serpentinite, amphibolite, metabasite, Ordovician granitic and psammitic-pelitic rocks along the eastern margin of the Tehuitzingo serpentinite at Tecolutla. In Tecolutla area outcrop a HP unit (eclogitic facies) that mainly consists of serpentinized harzburgite with small marginal fault blocks of metabasite, metagranitoid (485 ± 3 Ma, U-Pb zircon age: Galaz-Escanilla et al., in press) and mica schist (433 ± 3 Ma, U-Pb detrital zircon: Galaz-Escanilla et al., in press) juxtaposed by N-S structures. The serpentinized harzburgites contain elliptical metabasite lenses up to several meters in size, which have fine grained margins that may reflect an original intrusive relationship. The long axes of the elliptical lenses are parallel to the foliation indicating ductile deformation. On the other hand, the high-grade metasediments have a composite foliation where S1 is parallel to a second S-C foliation with the S2 planes subparallel to the border of the block and oriented ~128/27° (dip direction/dip angle), and C2 planes oriented ~147/58°. The HP unit is tectonically juxtaposed against a low grade psammite-pelitic unit along N-S structures. This unit has a planar fabric composed mainly of white mica and chlorite evidencing a low-temperature (greenschist) ductile deformation. The geothermobarometry suggests a common prograde metamorphic history for the Tehuitzingo HP rocks: (a) a metamorphic peak eclogite facies of zoisite-amphibole, with a temperature of ~750 °C and a pressure of ~16 kbar; (b) retrogression to amphibolite-epidote facies, with a temperature of ~472 °C and variable pressures between ~7.1–3.4 kbar; (c) retrogression to greenschist facies with a temperature of ~360 °C and whose pressures were not obtained (Galaz-Escanilla et al., in press). The Piaxtla serpentinite contains three types of serpentine group minerals:

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Figure 7. Stops 3-2 and 3-3: geological map, structural data, and section (after Galaz-Escanilla et al., in press).

Figure 8. Stops 3-4, 3-5, 3-6 and 3-7: geological map and section (after Keppie et al., 2010).

Amalgamation and Breakup of PangĂŚa: the type example of the supercontinent Cycle 15

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chrysotile, lizardite and antigorite. Based on the stability field of the latter was estimated a P-T peak of ~550 °C and ~9 kbar (González-Mancera, 2001), however, has been reported in subduction zones antigorite reaching ~720 °C and high pressures of ~20 kbar (Ulmer and Trommsdorff, 1995). The geochemistry data of the eclogitic mafic rocks indicate that these rocks have an arc affinity (author´s unpublished data). The P-T-t pattern of these rocks is consistent with subduction environments and serpentinite subduction channel exhumation (e.g., Guillot et al. 2009), where the driving forces for exhumation are a combination of buoyancy and channel flow coupled with underplating of slabs. STOP 3-4 (N18°° 31.051′, W98°° 19.733′: Fig. 8) Listric normal shear zone between megacrystic, Columpio del Diablo granitoid and Ordovician Las Minas unit. The Columpio del Diablo megacrystic granite (492 ± 12 Ma, U-Pb zircon age: Keppie et al., 2010) consists of blastomylonitic granite containing quartz, K-feldspar (perthitic orthoclase), white mica, chlorite, epidote, and accessory opaque minerals. The megacrystic granite is cut by thin leucogranite sheets that consist mainly of quartz and potassium feldspar and are inferred to be a late differentiates of the granite. Structurally, the granite varies from an L-tectonite to an L-S tectonite with kinematic indicators, such as σ fabrics associated with the feldspars and generally vertical, extensional, quartz-filled fractures within the feldspars that indicate top-to-east movement along the contact with the Las Minas Unit: a minor, brittle fault has been superimposed on the contact. The Las Minas unit consists predominantly of polydeformed, low-grade psammites interbedded with thin pelitic phyllites, and intruded by many tholeiitic mafic dikes and sills (Keppie et al., 2008). The psammites consist mainly of quartz with minor muscovite, chlorite, and K-feldspar, and accessory zircon, whereas the phyllites are composed of muscovite, chlorite, quartz, and opaque minerals. The youngest concordant detrital zircon is dated at 496 ± 25 Ma (Keppie et al., 2008) The mafic intrusions contain amphibole (tremolite-actinolite), chlorite, epidote, quartz, plagioclase, muscovite, and accessory calcite, and opaque minerals. 40Ar/39Ar analyses of muscovite from the earliest cleavage in the Las Minas unit yielded a plateau age of 347 ± 3 Ma (Mississippian) and show low temperature ages of ~260 Ma. STOP 3-5 (N18°° 30.351′, W98°° 17.57′: Fig. 8) The Cerro Puntiagudo Formation of Strunian age (latest Devonian) is 63 m thick and consists of shale, sandstone, and limestone. It is overlain by conglomerates of the Potrerillo Formation (124 m thick) that consists of red sandstone with Oseagean fossils and conglomerate with large K-feldspar clasts: these clasts are inferred to have been derived from the nearby megacrystic granitoids. The Cerro Puntiagudo Formation rests unconformably upon the Ojo de Agua unit, which consists of

finely bedded, black pelitic rocks intruded by green, fine grained, mafic dikes with an arc-related chemistry. These latter rocks are deformed by isoclinal, upright-steeply inclined, NE- and SEtrending, subhorizontal folds. The youngest detrital zircons in this unit are 466 ± 25 Ma (Keppie et al., 2008), abd 471 ± 9 Ma (Keppie et al., 2010). STOP 3-6 (N18°° 30.66′ W98°° 17.78′: Fig. 8) The La Junta Formation is a 126 m thick shale unit containing Missourian fossils; the Tepazulco Formation (193 m thick) is made up of interbedded limestone, shale, and sandstone and contains Virgilian-Missourian fossils. An Ordovician plug is faulted against the Patlanoaya Group at this locality. STOP 3-7 (N18°°31′, W°°16.79′: Fig. 8) The Lower Permian La Mesa, La Cuesta and La Cueva Formations consist of a conglomerate (45 m thick) and calcareous sandstone unit containing Wolfcampian fossils; interbedded shale and limestone with mid-Wolfcampian to middle Leonardian fossils; and sandstone (>280 m thick) containing late Leonardian fossils at its base. REFERENCES CITED Alva-Valdivia, L.M., Goguitchaichvli, A., Grajales, M., Flores de Dios, A., Urrutia-Fucugauchi, J., Rosales, C., and Morales, J., 2002, Further constraints for Permo-Carboniferous magnetostratigraphy: case study of the sedimentary sequence from San Salvador-Patlanoaya (Mexico): Comptes Rendus Geoscience, v. 334, p. 811–817, doi:10.1016/S1631-0713 (02)01821-7. Böhnel, H., 1999, Paleomagnetic study of Jurassic and Cretaceous rocks from the Mixteca terrane (Mexico): Journal of South American Earth Sciences, v. 12, no. 6, p. 545–556, doi:10.1016/S0895-9811(99)00038-3. Bullard, E.C., et al., 1965, A symposium on continental drift-IV. The fit of the continents around the Atlantic: Philosophical Transactions of the Royal Society, v. 258, p. 41–51, doi:10.1098/rsta.1965.0020. Fang, W., Van der Voo, R., Molina-Garza, R., Moran-Zenteno, D.J., and UrrutiaFucugauchi, J., 1989, Paleomagnetism of the Acatlan terrane, southern Mexico: evidence for terrane rotation: Earth and Planetary Science Letters, v. 94, no. 1-2, p. 131–142, doi:10.1016/0012-821X(89)90089-7. Galaz-Escanilla, G., Keppie, J.D., Lee, J.K.W., and Ortega-Rivera, A., in press, A high-pressure folded klippe at Tehuitzingo on the western margin of an extrusion zone, Acatlán Complex, southern Mexico: Gondwana Research. González-Mancera, G., 2001, Mineralogía y petrología de las serpentinitas del cuerpo ultramáfico de Tehuitzingo, Estado de Puebla: Tesis de Maestría, Universidad Nacional Autónoma de México, 103p. Guillot, S., Hattori, K., Agard, P., Schwartz, S., and Vidal, O., 2009, Exhumation Processes in Oceanic and Continental Subduction Contexts: A Review, in Lallemand, S., Funiciello, F., eds., Subduction Zone Geodynamics. Springer-Verlag Berlin Heidelberg, p. 175–205. Helbig, M., Keppie, J.D., Murphy, B., and Solari, L., 2010, Jurassic Amphibolites of the Eastern Acatlan Complex (Southern Mexico) Related to Both Back-Arc Rifting and the Opening of the Gulf of Mexico?: Geological Society of America Abstracts with Programs, v. 42, no. 5, p. 679. Irving, E., 1977, Drift of the major continents since the Devonian: Nature, v. 270, p. 304–309, doi:10.1038/270304a0. Keppie, J.D., 2004, Terranes of Mexico revisited: A 1.3 billion year odyssey: International Geology Review, v. 46, no. 9, p. 765–794, doi:10.2747 /0020-6814.46.9.765. Keppie, D.F., and Keppie, J.D., in review, An alternative Pangean reconstruction for Middle America with the Chortis and Yucatan blocks in the Gulf of Mexico: implications for Mesozoic and Cenozoic tectonics: International Geology Review.

Amalgamation and Breakup of Pangæa: the type example of the supercontinent Cycle Keppie, J.D., Nance, R.D., Dostal, J., Ortega-Rivera, A., Miller, B.V., Fox, D., Powell, J., Mumma, S., and Lee, J.W.K., 2004, Mid-Jurassic Tectonothermal Event Superposed on a Paleozoic Geological Record in the Acatlán Complex of Southern Mexico: Hotspot Activity During the Breakup of Pangea: Gondwana Research, v. 7, p. 238–260, doi:10.1016/S1342-937X (05)70323-3. Keppie, J.D., Dostal, J., Murphy, J.B., and Nance, R.D., 2008, Synthesis and tectonic interpretation of the westernmost Paleozoic Variscan orogen in southern Mexico: From rifted Rheic margin to active Pacific margin: Tectonophysics, v. 461, no. 1-4, p. 277–290, doi:10.1016/j.tecto.2008.01.012. Keppie, J.D., Nance, R.D., Ramos-Arias, M.A., Lee, J.K.W., Dostal, J., OrtegaRivera, A., and Murphy, J.B., 2010, Late Paleozoic subduction and exhumation of Cambro-Ordovician passive margin and arc rocks in the northern Acatlán Complex, southern Mexico: Geochronological constraints: Tectonophysics, v. 495, no. 3-4, p. 213–229, doi:10.1016/j.tecto .2010.09.019. Kirsch, M., Keppie, J.D., Murphy, J.B., and Solari, L.A., in press, Permian– Carboniferous arc magmatism and basin evolution along the western margin of Pangea: geochemical and geochronological evidence from the eastern Acatlán Complex, southern Mexico: GSA Bulletin. Malone, J., Nance, R.D., Keppie, J.D., and Dostal, J., 2002, Deformational history of part of the Acatlan Complex: Late Ordovician-early Silurian and early Permian orogenesis in southern Mexico: Journal of South American Earth Sciences, v. 15, no. 5, p. 511–524, doi:10.1016/S0895-9811 (02)00080-9. Morales-Gámez, M., Keppie, J.D., and Norman, M.D., 2008, Ordovician-Silurian rift-passive margin on the Mexican margin of the Rheic Ocean overlain by Carboniferous-Permian periarc rocks: Evidence from the eastern Acatlán Complex, southern Mexico: Tectonophysics, v. 461, p. 291–310, doi:10.1016/j.tecto.2008.01.014. Morales-Gámez, M., Keppie, J.D., and Dostal, J., 2009, Carboniferous tholeiitic dikes in the Salada unit, Acatlán Complex, southern Mexico: a record of extension on the western margin of Pangea: Revista Mexicana De Ciencias Geológicas, v. 26, p. 133–142. Pindell, J.L., and Dewey, J.F., 1982, Permo-Triassic reconstruction of western Pangaea and the evolution of the Gulf of Mexico/Caribbean region: Tectonics, v. 1, p. 179–211, doi:10.1029/TC001i002p00179.

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Pindell, J.L., and Dewey, J.F., 1982, Permo-Triassic reconstruction of western Pangaea and the evolution of the Gulf of Mexico/Caribbean region: Tectonics, v. 1, p. 179–211, doi:10.1029/TC001i002p00179. Smith, A.G., et al., 1981, Phanerozoic paleocontinental world maps. Cambridge University Press, Cambridge, 102 p. Steiner, M.B., 2005, Pangean reconstruction of the Yucatan Block: Its Permian, Triassic, and Jurassic geologic and tectonic history, in Anderson, T.H., Nourse, J.A., McKee, J.W., and Steiner, M.B., eds. The Mojave-Sonora megashear hypothesis: Development, assessment, and alternatives. Geological Society of America Special Paper 393, p. 457–480. doi: 10.1130/2005.2393(17). Talavera-Mendoza, O., Ruiz, J., Gehrels, G.E., Meza-Figueroa, D., VegaGranillo, R., and Campa-Uranga, M., 2005, U-Pb geochronology of the Acatlán Complex and implications for the Paleozoic paleogeography and tectonic evolution of southern Mexico: Earth and Planetary Science Letters, v. 235, p. 682–699, doi:10.1016/j.epsl.2005.04.013. Ulmer, P., and Trommsdorff, V., 1995, Serpentine Stability to Mantle Depths and Subduction-Related Magmatism: Science, v. 268, no. 5212, p. 858– 861, doi:10.1126/science.268.5212.858. Van der Voo, R., and French, R.B., 1974, Apparent polar wandering for the Atlantic-bordering continents: Late Carboniferous to Eocene: Earth-Science Reviews, v. 10, p. 99–119, doi:10.1016/0012-8252(74)90082-8. Vega-Granillo, R., Meza-Figueroa, D., Ruiz, J., Talavera-Mendoza, O., and López-Martínez, M., 2009, Structural and tectonic evolution of the Acatlán Complex, southern Mexico: Its role in the collisional history of Laurentia and Gondwana: Tectonics, v. 28, no. 4, p. TC4008, doi:10.1029/2007TC002159. Vega-Granillo, R., Talavera-Mendoza, O., Meza-Figueroa, D., Ruiz, J., Gehrels, G.E., and López-Martínez, M., 2007, Pressure-temperature-time evolution of Paleozoic high-pressure rocks of the Acatlán Complex (southern Mexico): Implications for the evolution of the Iapetus and Rheic Oceans: Geological Society of America Bulletin, v. 119, no. 9/10, p. 1249–1264, doi:10.1130/B226031.1. Yañez, P., Patchett, P.J., Ortega-Gutierrez, F., and Gehrels, G.E., 1991, Isotopic studies of the Acatlán Complex, southern Mexico: Implications for Paleozoic North American Tectonics: Geological Society of America Bulletin, v. 103, no. 6, p. 817–828, doi:10.1130/0016-7606(1991)103<0817: ISOTAC>2.3.CO;2.

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RESUMEN Y CONCLUSIÓN

Este trabajo documenta el desarrollo de un sistema de arco CarboníferoPérmico a lo largo del margen occidental de Pangea junto a una zona de subducción del océano Paleo-Pacífico. Se proporciona información detallada sobre la relación entre el plutonismo, la formación de cuencas y la deformación a escala regional en un orógeno periférico. Las principales conclusiones son las siguientes: a. La cartografía geológica detallada de la zona en combinación con los datos de la geocronología U-Pb de circones da como resultado la modificación de la distribución espacial y las relaciones de contacto de las unidades litotectónicas previamente mapeadas en el área de estudio. Los contactos externos entre el plutón Totoltepec y las rocas circundantes son o no conformables, o son tectónicos (es decir, ninguna de las relaciones de contacto originales son preservadas). Sin embargo, la edad de diques graníticos delgados que ocurren dentro de la Formación Tecomate sugieren que pueden ser comagmáticos con el plutón Totoltepec, lo que implica una relación originalmente intrusiva entre el plutón y la Formación Tecomate. b. Rocas clásticas al suroeste del plutón Totoltepec que fueron asignados originalmente a la Formación Cosoltepec contienen circones detríticos de edad Pérmico y por lo tanto se consideran equivalentes a la Formación Tecomate. Esta interpretación es coherente con la presencia de metaconglomerados y mármoles en esta parte de la zona de campo. Además, en la parte oriental del área de estudio se identifica una unidad metamórfica de grado medio y edad Misisipiense (artículo en preparación), que consiste en cuarcitas y esquistos de granate con escasos diques de anfibolita; esto limita la distribución espacial de la unidad previamente mapeada como la Formación Tecomate. La unidad Misisipiense está en contacto de falla con el plutón Totoltepec, cabalgando en dirección sur sobre la Formación Tecomate y está sobreyacida por capas rojas del Jurásico hacia el norte. Con base en datos isotópicos y geoquímicos, las rocas de esta unidad Misisipiense se pueden correlacionar con las de la Unidad Salada del Carbonífero (Morales-Gámez et al., 2008) que afloran en el lado occidental del plutón Totoltepec. c. Circones detríticos extraídos de rocas de la Formación Tecomate en el área de estudio en combinación con datos publicados de 40 Ar/39 Ar proporcionan límites temporales de depositación a alrededor de 300 Ma en un nivel estratigrafico, y a entre 288 ± 3 Ma y 263 ± 3 Ma en

76

resumen y conclusión

otro. Estos datos coinciden con la edad bioestratigráficamente determinada en el área tipo de la Formación Tecomate (Keppie et al., 2004b) e indican que la formación, como se define actualmente de manera colectiva se extiende desde el Pensilvánico medio hasta el Pérmico Inferior, pero puede haber sido depositada en distintas sub-cuencas de diferentes edades. Una de las muestras analizadas, que proviene de cerca de la base estratigráfica de la Formación Tecomate, produjo solamente circones de edad Proterozoica, lo cual sugiere que esta parte de la unidad fue depositada cuando fuentes ígneas paleozoicas no estaban expuestas o no fueron muestreadas por el sistema de drenaje local. La última explicación también podría aclarar la discrepancia entre las edades de las poblaciones más jóvenes de circones detríticos de la zona de estudio y el área tipo de la Formación Tecomate (Sánchez-Zavala et al., 2004), respectivamente. d. La secuencia intrusiva del plutón Totoltepec se establece mediante la documentación del rango composicional, los contactos internos y la edad de las fases magmáticas; esto se basa en el análisis petrográfico y trabajo de campo detallado, complementado por la geocronología de U-Pb y 40 Ar/39 Ar. Estos datos sugieren que el plutón es una intrusión compuesta, formado por (i) tres cuerpos discretos, alargados e intensamente fallados de 306 ± 2 Ma, que consisten en gabro hornblendico y hornblendita, aflorando a lo largo del margen norte del plutón; (ii) trondhjemita de 287 ± 2 Ma, la litología predominante del plutón, localmente mostrando una mayor abundancia de biotita o plagioclasa y transformándose a una composición granodiorítica y monzogranítica cerca del margen norte; y, (iii) cuerpos intrusivos de tonalita y diorita hornblendica junto con diques félsicos que se presentan en el parte sur del plutón, los cuales fueron emplazados secuencialmente entre 289 ± 2 y 283 ± 1 Ma. e. Tanto la fase marginal (gabróica) como la fase principal (trondhjemitatonalita-diorítica) del plutón Totoltepec muestran una afinidad geoquímica toleítica a calco-alcalina, típico de magmas asociados con subducción. Estos datos proporcionan evidencia de un manto litosférico subcontinental hidratado por fluidos de subducción ya en 306 Ma aproximadamente, mucho antes de lo que varios estudios proponen. Datos isotópicos de Sm-Nd indican una relación genética de asimilación-cristalización fraccionada (AFM) por cantidades menores entre la fase marginal y principal del plutón. Plutones coetáneos del arco Carbonífero–Pérmico en el sur de México y Guatemala son más félsicas y más alcalinas en composición, muestran patrones de tierras raras más diferenciados y tienen una firma isotópica de Sm-Nd menos radiogénico; esto indica una mayor contaminación cortical en comparación con el plutón Totoltepec y sugiere que el plutón fue emplazado en una parte más primitiva, más cercana a la trinchera del arco y/o a lo largo de una falla que facilitó su ascenso.

77

resumen y conclusión

f. Las rocas de la Formación Tecomate en el área de estudio son derivadas del edificio del arco regional y de plutones epizonales expuestos durante el Carbonífero y el Pérmico Inferior, lo cual es indicado por: (i) la ocurrencia de estratos intercalados de rocas volcánicas y clásticas que son derivados de un arco; (ii) la inmadurez composicional y textural de los sedimentos; (iii) la firma geoquímica de arco de las rocas clásticas; (iv) composiciones isotópicas de Sm-Nd relativamente radiogénicos que sugieren un componente de procedencia juvenil; y, (v) el predominio de circones detríticos del Carbonífero–Pérmico procedentes de una fuente ígnea. Por lo tanto, circones detríticos de la Formación Tecomate complementan el registro detrítico fragmentado de la actividad del arco magmático regional en el sur de México. En conjunto con otras rocas ígneas y sedimentarias relacionadas con el arco en México y Guatemala, cuya edad también está bien definida por bioestratigrafía o por geocronología U-Pb, los datos sugieren que la actividad de arco ya había iniciado en el Misisípico en los bloques más al sur y probablemente no fue establecido en los terrenos del norte de México hasta el Pérmico Inferior. g. La historia estructural para la fase principal del plutón Totoltepec de aproximadamente 289–287 Ma, como se infiere a partir de la termobarometría Al-en-hornblenda y la geocronología 40 Ar/39 Ar involucró el emplazamiento de magma en niveles medianos de la corteza (unos 20 km de profundidad) y un levantamiento rápido hasta aproximadamente 11 km en 4 ± 2 Ma. La forma superficial elíptica del plutón, una progresión de una fábrica de flujo magmático a una fábrica de estado sólido de baja temperatura, así como el paralelismo entre las foliaciones de temperaturas distintas indican que el emplazamiento de la fase principal del plutón fue controlada tectónicamente. La intrusión principal contiene una foliación subvertical paralela al eje largo del plutón, y una lineación mineral que varia desde subhorizontal con cinemática sinistral hasta muy inclinada con indicadores de cabalgamiento de vergencia sur. La variación en la orientación y el grado de deformación manifestado por las diferentes poblaciones de diques sugieren que los diques fueron emplazados secuencialmente y fueron sometidos a diferentes grados de rotación en sentido horario. h. En el marco del arco regional que está dominado por cizallamiento dextral a lo largo de fallas N–S, el emplazamiento del plutón Totoltepec se infiere haber tenido lugar a lo largo de un sistema de fallas transversales al arco de extensión oblicua y dirección NE en un régimen general de transtensión. El magmatismo puede haber cesado en la zona cuando una parte del cizallamiento dextral de la falla delimitante del oeste fue trasladado a la falla delimitante del este, que culminó en cabalgamiento cerca del margen sur del plutón. Por último, como parte de un evento importante de deformación regional en el Triásico Medio a Tardío, que se registra en las rocas tanto de

78

resumen y conclusión

la Formación Tecomate como de la unidad Misisipiense sin nombre (artículo en preparación), el plutón fue cabalgado sobre las rocas metasedimentarias de la Formación Tecomate. i. Este estudio documenta la evolución geodinámica de un arco continental del Paleozoico tardío en el Complejo Acatlán. Los datos sugieren subducción oblicua de litosfera oceánica hacia el este, por debajo del terreno Mixteco, que produjo el desarrollo de fallas laterales N–S paralelas a la trinchera y fallas antitéticas de orientación NE que promovió el plutonismo y dio lugar a la formación de múltiples cuencas pull-apart. Esta situación tectónica es más compatible con una ubicación paleogeográfica del terreno Mixteco en el margen de Pangea que una posición en el Golfo de México, varios miles de kilómetros hacia el interior, o incluso una ubicación frente al noreste de Canadá.

79

A

MÉTODOS ANALÍTICOS

A.1 y A.2: Material suplementario publicado en línea como parte del artículo: Kirsch, M., Keppie, J.D., Murphy, J.B., y Solari, L.A., 2012, Permian– Carboniferous arc magmatism and basin evolution along the western margin of Pangea: geochemical and geochronological evidence from the eastern Acatlán Complex, southern Mexico: Geological Society of America Bulletin, en prensa, doi: 10.1130/B30649.1. a.1

geocronología u-pb

About 70 grains for igneous analyses and 150 grains in case of detrital zircon analyses were handpicked and mounted on double-sided adhesive tape. To avoid introducing bias into sample preparation, no selection was made on the basis of optical and physical characteristics. The mount was then cast in epoxy resin, ground with sandpaper to expose the crystals and polished. Cathodoluminescence imaging was performed using an ELM-3R luminoscope, to reveal internal zoning of the zircons, helping with the spot selection and aiding the geological age interpretation. The isotopic analyses were performed with a Resolution LPX220 ArF Excimer laser ablation system coupled to a Thermo Xii series quadrupole ICP-MS (Solari et al., 2010) installed in the Laboratorio de Estudios Isotópicos (LEI), Centro de Geociencias, UNAM. A 34 µm spot was used for all the analyses performed during the current work. Repeated standard measurements of the Plešovice standard zircon, (Sláma et al., 2008) enabled mass-bias correction, as well as downhole and drift fractionation corrections. The analytical routine includes a standard glass (normally NIST 610 is analyzed), 5 standard zircons, 5 unknown zircons, and then 1 standard zircon every 5 unknowns, ending with 2 standard zircons. The same analytical protocol is employed (timing, energy density, laser frequency, spot size) for all the analyses, both standard and unknowns. NIST standard glass analyses are used to recalculate the zircons trace element concentrations. Time-resolved analyses are then reduced off-line using an in-house developed sofware written in R (Solari y Tanner, 2011), and the output is then imported into Excel, where the concordia as well as age-error calculations are obtained using Isoplot v. 3.70 (Ludwig, 2008), while the probability density distribution and histogram plots are produced using AgeDisplay (Sircombe, 2004). During the analytical sessions in which the data presented in this paper were measured at LEI, UNAM, the observed uncertainties (1σ relative standard deviation) on the 206 Pb/238 U, 207 Pb/206 Pb and 208 Pb/232 Th ratios measured on the Plešovice standard zircon were 0.6, 0.9 and 1.1 % respectively. Those errors are quadratically added to the quoted uncertainties observed on the measured

80

A.2 geoquĂ­mica

isotopic ratios of the unknown zircons. This last factor takes into account the heterogeneities of the natural standard zircons. 204 Pb, which would be used to correct for initial common Pb, is not measured because its tiny signal is swamped by 204 Hg, normally present in the He carrier gas. Common Pb is thus evaluated using the 207 Pb/206 Pb ratio, carefully graphing all the analyses on Tera y Wasserburg (1972) diagrams. Correction, if needed, is then performed with the algebraic method of Andersen (2002). a.2

geoquĂ­mica

Major and certain trace elements (V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Ba, La, Pb, Th, U, Ce, Nd, Cs) were determined by X-ray fluorescence spectrometry (XRF) at the Regional Geochemical Centre at Saint Mary’s University, Nova Scotia. Precision and accuracy are generally within 5 % for most major elements, and within 5–10 % for minor and trace elements. Details of the analytical procedures are given in Dostal et al. (1986, 1994). REEs and selected trace elements were analyzed by inductively coupled plasma mass spectrometry (ICP-MS) at Memorial University, Newfoundland. The accuracy and precision of these data are better than 10 %; the analytical procedure is detailed in Longerich et al. (1990). Sm-Nd isotopic analyses were performed at the Atlantic Universities Regional Isotopic Facility (AURIF), Memorial University, Newfoundland. Sm and Nd concentrations as well as isotopic compositions and ratios were measured by isotope dilution thermal ionization mass spectrometry (ID-TIMS) after chemical separation of Nd and Sm by ion exchange chromatography (see Kerr et al., 1995). Instrumental mass fractionation of Nd isotopes is corrected relative to 146 Nd/144 Nd = 0.7219 (O’Nions et al., 1977) using a Raleigh fractionation law. External precision is assessed by replicate analyses of the JNdi-1 standard (Tanaka et al., 2000). The difference between the certified value (143 Nd/144 Nd = 0.512115) and the mean (0.512101 Âą 0.000008 [n = 45]) is added to the measured value for 143 Nd/144 Nd after it has been spike-corrected. Errors quoted in Table 1 represent standard errors of individual 143Nd/144Nd measurements at the 95 % confidence level. Nd parameters were calculated relative to 143 Nd/144 Nd = 0.512638 for CHUR (Goldstein et al., 1984). For initial values, 147 Sm/144 Nd = 0.1967 (Jacobsen y Wasserburg, 1980) and Îť147 Sm = 6.54 x 10−12 yr−1 (Steiger y Jäger, 1977) were used. Depleted mantle model ages (TDM) are calculated in two ways: TDM(1) using the depleted mantle model of DePaolo (1981, 1988), and TDM(2) which assumes a linear evolution of the depleted mantle between a value of +10 (present day) and 0 at 4.5 Ga (Goldstein et al., 1984). TDM(1) values are quoted in the text. a.3

geocronologĂ­a

40 ar- 39 ar

Los separados de minerales y los monitores de flujo (estĂĄndares) fueron envueltos en papel de aluminio y apilados verticalmente en una cĂĄpsula de irradiaciĂłn 8.5 cm de largo y 2.0 cm de diĂĄmetro. Esta se irradiĂł con neutro-

81

A.3 geocronología

40 ar- 39 ar

nes rápidos en la posición 5C del Reactor Nuclear de McMaster (Hamilton, Ontario) para una duración de 24 h (a 2.5 MWh). Los paquetes de monitores de flujo se encuentran a intervalos de aprox. 1 cm a lo largo del contenedor de irradiación y valores de J para las muestras individuales se determinaron por interpolación polinómica de segundo orden entre los análisis repetidos para cada posición del monitor en la cápsula. Típicamente, valores de J varían menos que 10 % a lo largo de la cápsula. No se controlan gradientes horizontales de flujo ya que se consideran ser de menor importancia en el núcleo del reactor. Para la fusión total de los monitores y el calentamiento por pasos utilizando un láser, las muestras se colocan en un portamuestras de cobre, por debajo del viewport ZnS de una celda de acero inoxidable conectado a un sistema de la purificación del vacío ultra-alto. Para el calentamiento por pasos se utilizó un láser New Wave Research MIR 10-30 de CO2 con potencias de hasta 30W y una lente de facetas. Los periodos de calefacción son aprox. 3 minutos por cada incremento de energía (2 % a 20 %; diámetro del haz 3.8 mm). El gas liberado, después de la purificación mediante un getter SAES C50 (unos 5 minutos), es conducido a un espectrómetro de masas MAP 216 con una fuente Bäur Signer y un multiplicador de electrones (ajustado a una ganancia de 100 por el detector de copa de Faraday). Posteriormente, los análisis rutinarios del blanco se restan de las fracciones de gas de las muestras. Los blancos de extracción son típicamente <10 × 10−13 , <0.5 × 10−13 , <0.5 × 10−13 , y <0.5 × 10−13 cm−3 STP para las masas 40, 39, 37, y 36, respectivamente. Mediciones de los picos de los isótopos de argón son extrapolados al tiempo cero, normalizados a la relación 40 Ar/36 Ar atmosférica (295.5) utilizando los valores obtenidos para el argón atmosférico, y corregidos a 40 Ar producido por potasio, 39 Ar y 36 Ar por calcio, y a 36 Ar producido por cloro Roddick (1983). Las fechas y los errores se calcularon utilizando el procedimiento de Dalrymple et al. (1981) y las constantes de Steiger y Jäger (1977). La meseta e la inversa correlación de las fechas isotópicas se calcularon utilizando ISOPLOT v. 3.60 (Ludwig, 2008). Una meseta se define aquí como 3 o más etapas contiguas que contienen >50 % del 39 Ar liberado, con una probabilidad de ajuste >0.01 y un promedio ponderado de las desviaciones cuadráticas <2. Si los pasos contiguos contienen <50 % del 39 Ar liberado, se conoce como un segmento de meseta. Los errores citados en la tabla y en los espectros de edad representan la precisión analítica a 2σ, suponiendo que los errores en las edades de los monitores de flujo son cero. Esto es adecuado para comparar la variación dentro del espectro y determinar cuáles pasos forman una meseta (por ejemplo, McDougall y Harrison (1988), p. 89). Una estimación conservadora de este error en el valor de J es de 0.5 %; este se puede agregar para la comparación entre muestras. Las fechas están referenciadas a hornblenda Hb3Gr de 1072 Ma (Turner et al., 1971; Roddick, 1983).

82

B

TA B L A S G E O C R O N O L O G Í A U - P B

Material suplementario publicado en línea como parte del artículo: Kirsch, M., Keppie, J.D., Murphy, J.B., y Solari, L.A., 2012, Permian–Carboniferous arc magmatism and basin evolution along the western margin of Pangea: geochemical and geochronological evidence from the eastern Acatlán Complex, southern Mexico: Geological Society of America Bulletin, en prensa, doi: 10.1130/B30649.1.

Tabla 1: Description of samples for LA-ICP-MS U-Pb geochronology from the Totoltepec area, Acatlán Complex. Sample

Latitude

Longitude

(◦ N)

(◦ W)

Rocktype

Mineralogy Primary

Secondary

Age Accessory

(Ma)*

Permian granitoids (Totoltepec pluton) TT-72

18.2581000

97.85163333

Hbl gabbro

Hbl+Pl

Ep+Chl

Ap+Zrn+Op

306±2

TT-76b

18.2286500

97.86343333

Qz diorite

Pl+Qz+Ms

Chl

Ap+Zrn+Op

289±2

Permian low-grade metasedimentary rocks TT-81

18.25146667

97.78271667

Metaps.

Qz+Kfs+Ms

Chl

Zrn+Op

288±3

TT-82

18.25058333

97.78281667

Metapel.

Qz+Ms

Chl+Ser

Zrn+Op

299±3

TT-612

18.1912838

97.898541

Metaps.

Qz+Kfs+Ms

Chl

Zrn+Op

303±3

TT-486A

18.2840884

97.9111441

Metaps.

Qz+Kfs+Ms

Chl

Zrn+Op

1005±17

Zrn+Op

298±3

Thin dikes intruding Carboniferous–Permian low-grade metasedimentary rocks TT-615

18.1908117

-97.8990905

Granitoid

Qz+Pl+Ms

Abbreviations: Qz–quartz, Kfs–K-feldspar, Pl–plagioclase, Hbl–hornblende, Bt–biotite, Ms– muscovite, Ser–sericite, Ep–epidote, Chl–chlorite, Ap–apatite, Grt-garnet, Tnt–titanite, Zrn– zircon, Op–opaque minerals * LA-ICP-MS U-Pb zircon ages representing the age of crystallization in igneous rocks and an average of the youngest detrital zircon cluster in metasedimentary rocks, respectively (see text for details).

83

0.49 0.54 0.39 0.48 0.61 0.60 0.42 0.42 0.54 0.52 0.55 0.51 0.55 0.53 0.55 0.45 0.53 0.43

Zrc_31_043

Zrc_19_029

Zrc_21_032

Zrc_24_035

Zrc_02_009

Zrc_09_017

Zrc_05_012

Zrc_10_018

Zrc_17_027

Zrc_01_008

Zrc_03_010

Zrc_38_050

Zrc_04_011

Zrc_35_046

Zrc_32_044

Zrc_34_045

Zrc_07_015

Zrc_08_016

Spot name Th/U

0.05333

0.05365

0.05209

0.05425

0.05213

0.05386

0.05173

0.05494

0.05345

0.05326

0.05588

0.05769

0.05493

0.05220

0.05304

0.05306

0.05332

0.05891

207 Pb/206 Pb

207 Pb/235 U

1σ 206 Pb/238 U

1σ Rho

2.91 0.35595 2.98 0.04856 0.68 0.22

2.11 0.35874 2.23 0.04863 0.74 0.33

2.30 0.34687 2.41 0.04841 0.70 0.29

2.19 0.36186 2.30 0.04846 0.68 0.31

2.09 0.34737 2.19 0.04836 0.62 0.30

2.10 0.35898 2.22 0.04831 0.72 0.33

2.20 0.34235 2.26 0.04810 0.52 0.22

2.29 0.36274 2.41 0.04806 0.73 0.31

2.51 0.35438 2.60 0.04806 0.71 0.26

2.29 0.35147 2.39 0.04796 0.65 0.29

2.70 0.36909 2.79 0.04795 0.71 0.25

2.89 0.38116 3.01 0.04804 0.81 0.28

2.60 0.36184 2.69 0.04783 0.71 0.26

2.30 0.34437 2.41 0.04788 0.73 0.30

2.39 0.34732 2.48 0.04757 0.61 0.26

3.20 0.34550 3.45 0.04744 1.31 0.37

2.10 0.34853 2.18 0.04748 0.59 0.27

2.21 0.34265 2.34 0.04224 0.80 0.33

1σ

Isotopic ratios (errors in %)

306

306

305

305

304

304

303

303

303

302

302

302

301

301

300

299

299

267

206 Pb/238 U

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

4

2

2

1σ

309

311

302

314

303

311

299

314

308

306

319

328

314

300

303

301

304

299

207 Pb/235 U

8

6

6

6

6

6

6

7

7

6

8

8

7

6

6

9

6

6

1σ

64

46

51

48

47

46

49

50

54

50

58

62

56

50

53

71

46

47

306

306

305

305

304

304

303

303

303

302

302

302

301

301

300

299

299

267

2

2

2

2

2

2

2

2

2

2

2

2

2

2

2

4

2

2

1σ Best age 1σ

Continued on next page...

343

356

289

381

291

365

273

410

348

340

448

518

409

294

331

331

342

564

207 Pb/206 U

Ages (Ma)

Tabla 2: LA-ICP-MS U-Pb isotopic data for Totoltepec pluton hornblende gabbro.

tablas geocronología u-pb 84

0.50 0.46 0.40 0.61 0.55 0.47 0.40 0.49 0.37 0.41 0.42 0.49 0.30 0.30 0.48 0.59 0.34

Zrc_12_021

Zrc_16_026

Zrc_22_033

Zrc_27_038

Zrc_13_022

Zrc_28_039

Zrc_29_040

Zrc_37_049

Zrc_39_051

Zrc_20_030

Zrc_30_041

Zrc_36_048

Zrc_06_014

Zrc_26_037

Zrc_41_053

Zrc_11_020

Zrc_15_024

Spot name Th/U

0.05529

0.05456

0.05327

0.05695

0.05841

0.05287

0.05283

0.05441

0.05341

0.05334

0.05121

0.05196

0.05418

0.05430

0.05211

0.05491

0.05411

207 Pb/206 Pb

207 Pb/235 U

1σ 206 Pb/238 U

1σ Rho

3.60 0.37543 3.70 0.04944 0.87 0.23

2.20 0.37171 2.30 0.04950 0.69 0.30

2.59 0.35962 2.72 0.04925 0.79 0.30

3.11 0.38595 3.19 0.04931 0.73 0.22

3.41 0.39474 3.52 0.04920 0.91 0.25

2.10 0.35770 2.22 0.04917 0.71 0.32

2.31 0.35707 2.38 0.04914 0.61 0.24

3.11 0.36635 3.19 0.04910 0.75 0.23

3.30 0.35617 3.86 0.04891 2.00 0.52

2.10 0.35955 2.21 0.04892 0.67 0.31

2.89 0.34582 2.98 0.04894 0.65 0.24

3.00 0.35012 3.11 0.04889 0.82 0.26

2.49 0.36420 2.59 0.04872 0.70 0.28

2.39 0.36295 2.48 0.04854 0.64 0.26

2.71 0.34804 2.78 0.04856 0.66 0.23

2.60 0.36824 2.70 0.04864 0.74 0.26

2.20 0.36172 2.29 0.04861 0.62 0.27

1σ

Isotopic ratios (errors in %)

311

311

310

310

310

309

309

309

308

308

308

308

307

306

306

306

306

206 Pb/238 U

3

2

2

2

3

2

2

2

6

2

2

2

2

2

2

2

2

1σ

324

321

312

331

338

310

310

317

309

312

302

305

315

314

303

318

313

207 Pb/235 U

10

6

7

9

10

6

6

9

10

6

8

8

7

7

7

7

6

1σ

424

394

340

490

545

323

322

388

346

343

250

284

379

384

290

409

376

207 Pb/206 U

Ages (Ma)

Tabla 2: LA-ICP-MS U-Pb isotopic data for Totoltepec pluton hornblende gabbro.

78

48

57

67

72

47

51

68

73

46

65

67

54

53

60

56

48

311

311

310

310

310

309

309

309

308

308

308

308

307

306

306

306

306

3

2

2

2

3

2

2

2

6

2

2

2

2

2

2

2

2

1σ Best age 1σ

tablas geocronología u-pb 85

0.29 0.32 0.53 0.39 0.35

Zrc_69_089

Zrc_78_100

Zrc_72_093

0.28

Zrc_46_062

Zrc_49_065

0.72

Zrc_42_057

Zrc_64_083

0.35 0.62

Zrc_70_090

0.52

Zrc_66_086

Zrc_74_095

0.29 0.42

0.27

Zrc_76_098

Zrc_79_101

0.63

Zrc_47_063

Zrc_59_077

1.19 0.27

Zrc_52_069

0.65

Zrc_81_104

Zrc_57_075

0.01

Zrc_56_074

Spot name Th/U

0.05170

0.05412

0.05337

0.05426

0.04966

0.05258

0.05444

0.05159

0.05270

0.05334

0.05376

0.05452

0.05301

0.05201

0.05294

0.05239

0.05271

0.05190

207 Pb/206 Pb

207 Pb/235 U

1σ 206 Pb/238 U

1σ Rho

1.80 0.32825 1.86 0.04599 0.50 0.26

1.90 0.34121 2.00 0.04583 0.63 0.31

1.50 0.33761 1.61 0.04587 0.59 0.36

1.81 0.34365 1.91 0.04591 0.63 0.32

2.50 0.31351 2.60 0.04582 0.70 0.27

2.21 0.33114 2.30 0.04581 0.68 0.29

1.60 0.34365 1.69 0.04584 0.55 0.33

1.61 0.32509 1.69 0.04572 0.52 0.30

1.71 0.33238 1.78 0.04570 0.50 0.27

1.59 0.33561 1.70 0.04560 0.57 0.35

1.90 0.33778 2.00 0.04556 0.61 0.31

2.49 0.34052 2.57 0.04532 0.60 0.24

2.21 0.33200 2.28 0.04542 0.62 0.26

1.50 0.32588 1.57 0.04543 0.46 0.30

1.91 0.32939 1.99 0.04517 0.60 0.28

1.20 0.32621 1.33 0.04515 0.58 0.42

1.90 0.32368 2.15 0.04461 1.01 0.47

1.43 0.31578 1.59 0.04413 0.48 0.40

1σ

Isotopic ratios (errors in %)

290

289

289

289

289

289

289

288

288

287

287

286

286

286

285

285

281

278

206 Pb/238 U

1

2

2

2

2

2

2

1

1

2

2

2

2

1

2

2

3

1

1σ

288

298

295

300

277

290

300

286

291

294

295

298

291

286

289

287

285

279

207 Pb/235 U

5

5

4

5

6

6

4

4

4

4

5

7

6

4

5

3

5

4

1σ

41

43

34

38

54

47

33

37

39

34

40

56

50

32

40

26

43

33

290

289

289

289

289

289

289

288

288

287

287

286

286

286

285

285

281

278

1

2

2

2

2

2

2

1

1

2

2

2

2

1

2

2

3

1

1σ Best age 1σ

Continued on next page...

272

376

345

382

179

311

389

267

316

343

361

393

329

286

326

302

316

281

207 Pb/206 U

Ages (Ma)

Tabla 3: LA-ICP-MS U-Pb isotopic data for Totoltepec pluton quartz diorite.

tablas geocronología u-pb 86

0.27 0.40 0.33 0.27 0.25

Zrc_71_092

Zrc_51_068

Zrc_62_081

0.40

Zrc_53_070

Zrc_54_071

0.35

Zrc_50_066

Zrc_43_058

0.52 0.26

Zrc_77_099

0.38

Zrc_75_096

Zrc_80_102

0.61 0.40

0.33

Zrc_67_087

Zrc_44_059

0.62

Zrc_58_076

Zrc_68_088

0.34 0.56

Zrc_55_072

0.32

Zrc_48_064

Zrc_65_084

0.59

Zrc_82_105

Spot name Th/U

0.05176

0.05225

0.05291

0.05167

0.05076

0.05174

0.05305

0.05278

0.05205

0.05314

0.05266

0.05245

0.05296

0.05238

0.05252

0.05302

0.05229

0.05244

207 Pb/206 Pb

207 Pb/235 U

1σ 206 Pb/238 U

1σ Rho

6.39 0.34109 6.70 0.04780 0.90 0.24

2.79 0.34302 2.90 0.04764 0.76 0.27

2.10 0.34411 2.24 0.04720 0.76 0.35

1.99 0.33458 2.10 0.04702 0.64 0.32

2.01 0.32714 2.12 0.04680 0.68 0.31

2.20 0.33470 2.28 0.04685 0.60 0.26

2.51 0.34103 2.61 0.04681 0.75 0.28

2.10 0.33938 2.20 0.04665 0.64 0.29

1.50 0.33415 1.57 0.04653 0.47 0.31

1.90 0.34070 1.97 0.04651 0.52 0.26

1.90 0.33861 2.00 0.04658 0.62 0.32

1.51 0.33597 1.60 0.04644 0.56 0.34

1.70 0.33881 1.79 0.04636 0.56 0.31

1.39 0.33411 1.51 0.04627 0.56 0.39

1.81 0.33404 1.93 0.04620 0.69 0.35

1.79 0.33711 1.90 0.04610 0.61 0.34

1.80 0.33269 1.93 0.04616 0.69 0.37

1.39 0.33359 1.48 0.04606 0.50 0.35

1σ

Isotopic ratios (errors in %)

301

300

297

296

295

295

295

294

293

293

293

293

292

292

291

291

291

290

206 Pb/238 U

3

2

2

2

2

2

2

2

1

1

2

2

2

2

2

2

2

1

1σ

298

299

300

293

287

293

298

297

293

298

296

294

296

293

293

295

292

292

207 Pb/235 U

17

8

6

5

5

6

7

6

4

5

5

4

5

4

5

5

5

4

1σ

136

60

47

43

43

47

53

47

34

43

43

32

38

30

38

38

38

31

301

300

297

296

295

295

295

294

293

293

293

293

292

292

291

291

291

290

3

2

2

2

2

2

2

2

1

1

2

2

2

2

2

2

2

1

1σ Best age 1σ

Continued on next page...

275

296

325

271

230

274

331

319

288

335

314

305

327

302

308

330

298

305

207 Pb/206 U

Ages (Ma)

Tabla 3: LA-ICP-MS U-Pb isotopic data for Totoltepec pluton quartz diorite.

tablas geocronología u-pb 87

0.02

0.36 0.18 0.43 0.48

Zrc_84_107

Zrc_39_053

Zrc_21_032

Zrc_08_016

0.38

Zrc_57_075 0.38

0.55

Zrc_06_014

Zrc_13_022

Th/U

Spot name

Zrc_81_104

0.36 0.58

Zrc_73_094

0.44

Zrc_60_078

Zrc_45_060

0.38

Zrc_63_082

Spot name Th/U 207 Pb/235 U

1σ 206 Pb/238 U

1σ Rho

2.80 0.36256 2.87 0.04923 0.65 0.23

1.91 0.34846 1.98 0.04870 0.55 0.27

1.70 0.34822 1.79 0.04822 0.56 0.31

2.20 0.37221 2.37 0.04794 0.90 0.37

1σ

310

307

304

302

206 Pb/238 U

2

2

2

3

1σ

314

304

303

321

207 Pb/235 U

8

5

5

7

1σ

355

281

303

463

207 Pb/206 U

Ages (Ma)

0.07491

0.07474

0.07408

0.07287

0.07208

0.07165

0.07098

0.06802

207 Pb/206 Pb

207 Pb/235 U

1σ 206 Pb/238 U

1σ Rho

1.29 1.76560 1.38 0.17138 0.46 0.34

0.83 1.87920 0.91 0.18265 0.37 0.41

0.86 1.74320 0.94 0.17084 0.37 0.39

0.84 1.67810 0.91 0.16724 0.36 0.40

1.53 1.66974 1.73 0.16802 0.45 0.37

0.75 1.61810 0.84 0.16398 0.36 0.44

0.85 1.52490 0.92 0.15600 0.34 0.38

1.71 1.06950 1.94 0.11398 0.93 0.47

1σ

Isotopic ratios (errors in %)

1020

1081

1017

997

1001

979

935

696

206 Pb/238 U

4

4

3

3

4

3

3

6

1σ

1033

1074

1025

1000

997

977

940

738

207 Pb/235 U

9

6

6

6

11

5

6

10

1σ

310

307

304

302

2

2

2

3

26

15

16

17

31

14

17

35

1066

1062

1044

1010

1001

979

935

696

26

15

16

17

4

3

3

6

1σ Best age 1σ

59

43

36

46

1σ Best age 1σ

Continued on next page...

1066

1062

1044

1010

988

976

957

869

207 Pb/206 U

Ages (Ma)

Tabla 4: LA-ICP-MS U-Pb isotopic data for Tecomate Formation metapsammite TT-486A.

0.05361

0.05190

0.05240

0.05626

207 Pb/206 Pb

Isotopic ratios (errors in %)

Tabla 3: LA-ICP-MS U-Pb isotopic data for Totoltepec pluton quartz diorite.

tablas geocronología u-pb 88

0.51 0.44 0.31 0.34 0.78

Zrc_99_125

Zrc_07_015

Zrc_63_082

0.28

Zrc_47_063

Zrc_55_072

0.06

Zrc_83_106

Zrc_70_090

0.53 0.47

Zrc_19_029

0.48

Zrc_05_012

Zrc_78_100

0.81 0.22

0.22

Zrc_15_024

Zrc_16_026

0.39

Zrc_25_036

Zrc_72_093

0.26 0.32

0.12

Zrc_67_087

Zrc_97_123

0.38

Zrc_93_118

Zrc_54_071

Th/U

Spot name

0.07801

0.07793

0.07790

0.07790

0.07783

0.07784

0.07781

0.07773

0.07766

0.07748

0.07745

0.07730

0.07730

0.07708

0.07686

0.07651

0.07606

0.07492

207 Pb/206 Pb

207 Pb/235 U

1σ 206 Pb/238 U

1σ Rho

0.79 2.16540 0.87 0.20144 0.35 0.41

0.89 2.13660 1.01 0.19894 0.47 0.48

0.95 2.13890 1.03 0.19927 0.41 0.40

1.00 2.14200 1.07 0.19967 0.38 0.35

1.90 1.88790 1.97 0.17650 0.52 0.26

0.87 2.08620 0.95 0.19451 0.35 0.39

0.87 2.09840 0.93 0.19576 0.34 0.35

0.81 2.02400 0.89 0.18904 0.36 0.40

1.70 2.07240 1.84 0.19409 0.71 0.39

0.83 2.04260 0.90 0.19138 0.34 0.39

0.98 1.97950 1.10 0.18577 0.49 0.44

1.20 1.93710 1.28 0.18209 0.45 0.35

1.20 1.93710 1.28 0.18209 0.45 0.35

1.60 1.74460 1.70 0.16436 0.57 0.34

1.70 1.92070 1.76 0.18136 0.45 0.25

1.01 1.70040 1.89 0.16053 1.60 0.85

1.00 2.25670 1.08 0.21545 0.41 0.38

1.09 1.79300 1.15 0.17388 0.35 0.32

1σ

Isotopic ratios (errors in %)

1183

1170

1171

1174

1048

1146

1153

1116

1143

1129

1098

1078

1078

981

1074

960

1258

1033

206 Pb/238 U

4

5

4

4

5

4

4

4

7

4

5

4

4

5

4

14

5

3

1σ

1170

1161

1161

1162

1077

1144

1148

1124

1140

1130

1109

1094

1094

1025

1088

1009

1199

1043

207 Pb/235 U

6

7

7

7

13

7

6

6

13

6

7

9

9

11

12

12

8

8

1σ

16

17

19

20

35

16

17

16

31

15

19

22

22

29

31

20

20

22

1147

1145

1144

1144

1143

1143

1142

1140

1138

1134

1133

1129

1129

1123

1118

1108

1097

1066

16

17

19

20

35

16

17

16

31

15

19

22

22

29

31

20

20

22

1σ Best age 1σ

Continued on next page...

1147

1145

1144

1144

1143

1143

1142

1140

1138

1134

1133

1129

1129

1123

1118

1108

1097

1066

207 Pb/206 U

Ages (Ma)

Tabla 4: LA-ICP-MS U-Pb isotopic data for Tecomate Formation metapsammite TT-486A.

tablas geocronología u-pb 89

0.31 0.71 0.32 0.18 0.22

Zrc_51_068

Zrc_66_086

Zrc_27_039

0.27

Zrc_82_105

Zrc_80_102

0.38

Zrc_64_083

Zrc_74_095

0.14 0.33

Zrc_04_011

0.23

Zrc_65_084

Zrc_46_062

0.36 0.71

0.51

Zrc_96_122

Zrc_45_060

0.48

Zrc_76_098

Zrc_31_044

0.48 0.44

0.64

Zrc_53_070

Zrc_58_076

0.26

Zrc_94_119

Zrc_98_124

Th/U

Spot name

0.07905

0.07893

0.07893

0.07889

0.07877

0.07865

0.07863

0.07864

0.07862

0.07859

0.07861

0.07854

0.07843

0.07837

0.07829

0.07828

0.07813

0.07806

207 Pb/206 Pb

207 Pb/235 U

1σ 206 Pb/238 U

1σ Rho

0.83 2.18130 0.93 0.20051 0.39 0.43

1.20 2.36360 1.34 0.21732 0.60 0.44

0.95 2.14820 1.04 0.19759 0.43 0.41

0.93 2.11920 1.00 0.19500 0.38 0.39

1.21 2.14520 1.29 0.19785 0.48 0.36

1.28 2.17279 1.47 0.20037 0.46 0.40

0.86 2.15870 0.92 0.19931 0.34 0.35

1.00 2.11060 1.07 0.19493 0.39 0.35

0.83 2.22390 0.90 0.20541 0.35 0.40

0.89 2.13310 0.96 0.19702 0.35 0.36

1.00 2.15620 1.21 0.19921 0.68 0.56

0.78 2.17930 0.85 0.20134 0.33 0.40

0.92 2.13540 1.00 0.19760 0.39 0.40

1.30 1.98910 1.41 0.18436 0.55 0.39

0.88 2.23180 1.08 0.20711 0.63 0.58

1.00 2.12090 1.08 0.19669 0.40 0.38

1.42 2.08883 1.74 0.19390 0.51 0.49

0.99 2.09030 1.08 0.19444 0.42 0.40

1σ

Isotopic ratios (errors in %)

1178

1268

1162

1148

1164

1177

1172

1148

1204

1159

1171

1183

1162

1091

1213

1158

1142

1145

206 Pb/238 U

4

7

5

4

5

5

4

4

4

4

7

4

4

5

7

4

5

4

1σ

1175

1232

1164

1155

1164

1172

1168

1152

1189

1160

1167

1174

1160

1112

1191

1156

1145

1146

207 Pb/235 U

6

10

7

7

9

10

6

7

6

7

8

6

7

10

8

7

12

7

1σ

15

23

17

18

23

25

17

18

15

17

18

14

18

25

17

19

26

19

1173

1170

1170

1169

1166

1163

1163

1163

1163

1162

1162

1161

1158

1156

1154

1154

1150

1148

15

23

17

18

23

25

17

18

15

17

18

14

18

25

17

19

26

19

1σ Best age 1σ

Continued on next page...

1173

1170

1170

1169

1166

1163

1163

1163

1163

1162

1162

1161

1158

1156

1154

1154

1150

1148

207 Pb/206 U

Ages (Ma)

Tabla 4: LA-ICP-MS U-Pb isotopic data for Tecomate Formation metapsammite TT-486A.

tablas geocronología u-pb 90

0.35 0.59 0.36 0.29 0.08

Zrc_23_034

Zrc_60_078

Zrc_44_059

0.28

Zrc_43_058

Zrc_59_077

0.37

Zrc_75_096

Zrc_88_112

0.33 0.51

Zrc_50_066

0.40

Zrc_22_033

Zrc_35_048

0.32 0.48

0.58

Zrc_86_110

Zrc_12_021

0.72

Zrc_52_069

Zrc_09_017

0.69 0.51

0.40

Zrc_24_035

Zrc_29_041

0.39

Zrc_14_023

Zrc_36_050

Th/U

Spot name

0.08259

0.08234

0.08198

0.08119

0.08108

0.08076

0.08071

0.08059

0.08049

0.08044

0.08024

0.08009

0.07990

0.07966

0.07955

0.07955

0.07957

0.07908

207 Pb/206 Pb

207 Pb/235 U

1σ 206 Pb/238 U

1σ Rho

1.30 2.45410 1.46 0.21583 0.67 0.46

1.51 2.28400 1.57 0.20167 0.48 0.29

1.00 2.28860 1.11 0.20279 0.49 0.44

1.31 2.34130 1.42 0.20914 0.57 0.39

1.10 2.33210 1.18 0.20885 0.43 0.37

0.99 2.33120 1.06 0.20962 0.39 0.37

1.40 2.16760 1.50 0.19519 0.53 0.35

1.91 2.31160 2.19 0.20803 0.50 0.34

0.99 2.19340 1.08 0.19800 0.42 0.40

0.92 2.25650 1.00 0.20388 0.40 0.40

1.50 2.17780 1.56 0.19709 0.41 0.27

1.10 2.26130 1.20 0.20505 0.48 0.40

1.20 2.15900 1.28 0.19617 0.44 0.34

1.00 2.22120 1.07 0.20227 0.37 0.34

1.29 2.20740 1.38 0.20154 0.45 0.34

1.11 2.12060 1.15 0.19366 0.34 0.28

1.28 2.17300 1.49 0.19806 0.42 0.35

1.40 2.22390 1.49 0.20451 0.50 0.33

1σ

Isotopic ratios (errors in %)

1260

1184

1190

1224

1223

1227

1149

1218

1165

1196

1160

1202

1155

1188

1184

1141

1165

1200

206 Pb/238 U

8

5

5

6

5

4

6

6

4

4

4

5

5

4

5

4

5

5

1σ

1259

1207

1209

1225

1222

1222

1171

1216

1179

1199

1174

1200

1168

1188

1183

1156

1172

1189

207 Pb/235 U

11

11

8

10

8

8

10

16

8

7

11

8

9

7

10

8

10

10

1σ

23

29

18

25

21

18

27

35

18

17

29

20

23

18

23

20

23

25

1260

1254

1245

1226

1223

1216

1214

1212

1209

1208

1203

1199

1195

1189

1186

1186

1186

1174

23

29

18

25

21

18

27

35

18

17

29

20

23

18

23

20

23

25

1σ Best age 1σ

Continued on next page...

1260

1254

1245

1226

1223

1216

1214

1212

1209

1208

1203

1199

1195

1189

1186

1186

1186

1174

207 Pb/206 U

Ages (Ma)

Tabla 4: LA-ICP-MS U-Pb isotopic data for Tecomate Formation metapsammite TT-486A.

tablas geocronología u-pb 91

0.64 0.20 0.25 0.27 0.50

Zrc_71_092

Zrc_01_008

Zrc_87_111

0.35

Zrc_69_089

Zrc_11_020

1.53

Zrc_48_064

Zrc_37_051

0.53 0.43

Zrc_77_099

0.25

Zrc_10_018

Zrc_26_038

0.03 0.44

0.02

Zrc_38_052

Zrc_56_074

0.54

Zrc_18_028

Zrc_03_010

0.33 0.53

0.36

Zrc_33_046

Zrc_28_040

0.28

Zrc_85_108

Zrc_61_080

Th/U

Spot name

0.08565

0.08543

0.08517

0.08511

0.08511

0.08494

0.08459

0.08460

0.08432

0.08361

0.08349

0.08330

0.08323

0.08318

0.08297

0.08300

0.08296

0.08262

207 Pb/206 Pb

207 Pb/235 U

1σ 206 Pb/238 U

1σ Rho

1.00 2.87220 1.09 0.24341 0.43 0.39

1.53 2.66924 1.66 0.22660 0.45 0.34

0.79 2.68620 0.85 0.22880 0.32 0.39

0.80 2.71160 0.87 0.23117 0.35 0.40

1.10 2.69740 1.20 0.23005 0.48 0.39

0.77 2.59930 0.83 0.22209 0.34 0.39

0.93 2.57470 1.01 0.22100 0.39 0.38

1.50 2.50280 1.56 0.21507 0.44 0.28

1.00 2.60860 1.08 0.22464 0.42 0.40

0.80 1.96430 1.07 0.17031 0.71 0.66

1.40 2.31180 1.54 0.20139 0.63 0.41

1.20 2.25620 1.27 0.19693 0.43 0.34

0.83 2.49260 1.02 0.21737 0.59 0.58

1.00 2.34660 1.09 0.20493 0.43 0.40

1.21 2.29140 1.29 0.20038 0.48 0.36

0.89 2.38370 1.00 0.20866 0.45 0.45

1.10 2.30810 1.17 0.20211 0.41 0.36

0.92 2.34640 0.99 0.20602 0.36 0.36

1σ

Isotopic ratios (errors in %)

1404

1317

1328

1341

1335

1293

1287

1256

1306

1014

1183

1159

1268

1202

1177

1222

1187

1208

206 Pb/238 U

5

5

4

4

6

4

5

5

5

7

7

5

7

5

5

5

4

4

1σ

1375

1320

1325

1332

1328

1300

1293

1273

1303

1103

1216

1199

1270

1227

1210

1238

1215

1226

207 Pb/235 U

8

12

6

6

9

6

7

11

8

7

11

9

7

8

9

7

8

7

1σ

19

27

15

14

19

15

17

27

19

14

25

21

15

18

23

16

20

18

1330

1325

1319

1318

1318

1314

1306

1306

1300

1283

1281

1276

1275

1273

1269

1269

1268

1260

19

27

15

14

19

15

17

27

19

14

25

21

15

18

23

16

20

18

1σ Best age 1σ

Continued on next page...

1330

1325

1319

1318

1318

1314

1306

1306

1300

1283

1281

1276

1275

1273

1269

1269

1268

1260

207 Pb/206 U

Ages (Ma)

Tabla 4: LA-ICP-MS U-Pb isotopic data for Tecomate Formation metapsammite TT-486A.

tablas geocronología u-pb 92

0.21 0.46 0.38 0.62 0.32

Zrc_89_113

Zrc_17_027

Zrc_90_114

0.53

Zrc_92_117

Zrc_49_065

0.22

Zrc_100_126

Zrc_79_101

0.37 0.34

Zrc_73_094

0.17

Zrc_91_116

Zrc_34_047

0.29 0.19

0.20

Zrc_40_054

Zrc_62_081

0.27

Zrc_41_056

Zrc_20_030

0.40 0.40

0.27

Zrc_95_120

Zrc_42_057

0.26

Zrc_02_009

Zrc_30_042

Th/U

Spot name

0.09948

0.09892

0.09864

0.09840

0.09835

0.09672

0.09544

0.09527

0.09363

0.09303

0.09269

0.09249

0.09185

0.09089

0.09067

0.09045

0.08983

0.08751

207 Pb/206 Pb

207 Pb/235 U

1σ 206 Pb/238 U

1σ Rho

1.00 3.91220 1.09 0.28572 0.44 0.41

0.97 3.93380 1.16 0.28860 0.63 0.54

0.81 3.81130 0.88 0.28037 0.35 0.39

0.80 3.67780 0.88 0.27133 0.37 0.41

0.99 3.75920 1.06 0.27768 0.39 0.38

0.78 3.55400 0.85 0.26675 0.33 0.40

1.05 3.57218 1.17 0.27146 0.41 0.45

0.89 3.36210 1.07 0.25626 0.59 0.55

0.85 3.22380 0.96 0.24982 0.45 0.46

0.89 3.43710 0.98 0.26825 0.42 0.42

0.83 3.36360 0.92 0.26348 0.39 0.42

0.99 3.37910 1.05 0.26525 0.36 0.33

0.87 2.94167 1.07 0.23227 0.45 0.48

0.83 3.20960 0.93 0.25643 0.42 0.46

0.92 3.12970 0.99 0.25053 0.37 0.38

0.78 2.65160 0.89 0.21279 0.40 0.46

0.83 2.92800 0.95 0.23655 0.46 0.48

1.10 2.83160 1.20 0.23509 0.48 0.41

1σ

Isotopic ratios (errors in %)

1620

1635

1593

1548

1580

1524

1548

1471

1438

1532

1508

1517

1346

1472

1441

1244

1369

1361

206 Pb/238 U

6

9

5

5

5

4

6

8

6

6

5

5

5

6

5

5

6

6

1σ

1616

1621

1595

1567

1584

1539

1543

1496

1463

1513

1496

1500

1393

1459

1440

1315

1389

1364

207 Pb/235 U

9

9

7

7

9

7

9

8

7

8

7

8

8

7

8

7

7

9

1σ

18

17

15

15

17

14

18

15

16

17

14

19

15

14

16

14

16

19

1614

1604

1599

1594

1593

1562

1537

1533

1501

1488

1482

1477

1464

1444

1440

1435

1422

1372

18

17

15

15

17

14

18

15

16

17

14

19

15

14

16

14

16

19

1σ Best age 1σ

Continued on next page...

1614

1604

1599

1594

1593

1562

1537

1533

1501

1488

1482

1477

1464

1444

1440

1435

1422

1372

207 Pb/206 U

Ages (Ma)

Tabla 4: LA-ICP-MS U-Pb isotopic data for Tecomate Formation metapsammite TT-486A.

tablas geocronología u-pb 93

0.44 0.49 0.63 0.56 0.46 0.79

Zrc_34_047

Zrc_03_010

Zrc_51_068

Zrc_36_050

0.72

Zrc_23_034

Zrc_65_084

0.34

Zrc_67_087

Zrc_05_012

0.44 0.52

Zrc_90_114

1.73

Zrc_44_059

Zrc_06_014

Th/U

207 Pb/235 U

1σ 206 Pb/238 U

1σ Rho

0.75 4.99470 0.93 0.32676 0.55 0.59

1σ

Isotopic ratios (errors in %)

1823

206 Pb/238 U

9

1σ 1818

207 Pb/235 U

8

1σ 1816

207 Pb/206 U

Ages (Ma)

0.05694

0.05492

0.05714

0.04862

0.05360

0.05280

0.05280

0.05410

0.05363

0.05725

0.08570

0.32902

0.34157

0.40647

207 Pb/235 U

2.37

6.14

2.37

1σ

1σ Rho

0.04451 0.58 0.25

0.04327 1.73 0.32

0.03512 0.88 0.37

206 Pb/238 U

0.33552

0.32904

0.32862

2.39

2.39

1.70

0.04530 0.66 0.28

0.04518 0.64 0.28

0.04503 0.58 0.35

2.79

6.99

1.51

0.36390

0.34671

0.36112

2.95

7.35

1.58

0.04640 0.93 0.32

0.04578 0.81 0.21

0.04573 0.50 0.31

10.02 0.30453 10.44 0.04543 0.90 0.10

2.29

2.29

1.59

13.11 0.33403 14.03 0.04478 1.43 0.25

2.29

5.40

2.21

1σ

Isotopic ratios (errors in %)

292

289

288

286

286

285

284

282

281

273

223

206 Pb/238 U

3

2

1

3

2

2

2

4

2

5

2

1σ

315

302

313

270

294

289

289

293

289

298

346

207 Pb/235 U

8

19

4

25

6

6

4

36

6

16

7

1σ

1816

12

61

154

33

210

51

51

36

285

51

118

41

292

289

288

286

286

285

284

282

281

273

223

3

2

1

3

2

2

2

4

2

5

2

1σ Best age 1σ

12

1σ Best age 1σ

Continued on next page...

489

409

497

129

354

320

320

375

356

501

1331

207 Pb/206 U

Ages (Ma)

Tabla 5: LA-ICP-MS U-Pb isotopic data for Tecomate Formation metapsammite TT-81.

0.11098

207 Pb/206 Pb

207 Pb/206 Pb

0.55

Zrc_32_045

Spot name

Th/U

Spot name

Tabla 4: LA-ICP-MS U-Pb isotopic data for Tecomate Formation metapsammite TT-486A.

tablas geocronología u-pb 94

0.30 0.52 0.75 0.47 0.56

Zrc_63_082

Zrc_92_117

Zrc_14_023

0.28

Zrc_85_108

Zrc_95_120

0.59

Zrc_02_009

Zrc_40_054

0.60 0.35

Zrc_04_011

0.60

Zrc_11_020

Zrc_12_021

0.13 0.34

0.11

Zrc_88_112

Zrc_58_076

0.33

Zrc_38_052

Zrc_72_093

0.34 0.50

0.44

Zrc_54_071

Zrc_83_106

0.51

Zrc_52_069

Zrc_09_017

Th/U

Spot name

0.05349

0.05774

0.05395

0.05245

0.05284

0.05994

0.05286

0.05270

0.05307

0.05230

0.05247

0.05330

0.05414

0.05332

0.05139

0.05639

0.05580

0.05631

207 Pb/206 Pb

1.79

1.70

2.30

1.91

1.61

1.80

1.61

2.50

1.51

1.40

1.94

1.80

2.20

2.01

2.80

2.50

2.01

1.90

1σ

0.36143

0.38779

0.36384

0.35339

0.35533

0.40240

0.35513

0.35146

0.35465

0.34653

0.34326

0.34921

0.35308

0.34742

0.33197

0.36381

0.35721

0.36371

207 Pb/235 U

1.90

1.79

2.40

2.01

1.70

1.88

1.69

2.57

1.56

1.48

2.33

1.87

2.29

2.20

2.91

2.58

2.09

2.13

1σ

1σ Rho

0.04891 0.59 0.32

0.04870 0.55 0.31

0.04871 0.68 0.28

0.04874 0.66 0.32

0.04866 0.58 0.32

0.04856 0.54 0.28

0.04860 0.53 0.30

0.04831 0.62 0.23

0.04836 0.43 0.26

0.04802 0.48 0.33

0.04744 0.72 0.50

0.04740 0.51 0.27

0.04725 0.66 0.29

0.04710 0.91 0.41

0.04698 0.79 0.27

0.04681 0.62 0.24

0.04635 0.60 0.28

0.04638 0.97 0.45

206 Pb/238 U

Isotopic ratios (errors in %)

308

307

307

307

306

306

306

304

304

302

299

299

298

297

296

295

292

292

206 Pb/238 U

2

2

2

2

2

2

2

2

1

1

2

1

2

3

2

2

2

3

1σ

313

333

315

307

309

343

309

306

308

302

300

304

307

303

291

315

310

315

207 Pb/235 U

5

5

6

5

5

5

4

7

4

4

6

5

6

6

7

7

6

6

1σ

40

37

50

42

36

38

38

56

34

31

43

39

49

45

64

55

43

41

308

307

307

307

306

306

306

304

304

302

299

299

298

297

296

295

292

292

2

2

2

2

2

2

2

2

1

1

2

1

2

3

2

2

2

3

1σ Best age 1σ

Continued on next page...

350

520

369

305

322

601

323

316

332

299

306

342

377

342

258

468

444

465

207 Pb/206 U

Ages (Ma)

Tabla 5: LA-ICP-MS U-Pb isotopic data for Tecomate Formation metapsammite TT-81.

tablas geocronología u-pb 95

0.44 0.27 0.52 0.43 0.35

Zrc_21_032

Zrc_56_074

Zrc_59_077

0.78

Zrc_71_092

Zrc_01_008

0.39

Zrc_81_104

Zrc_82_105

0.61 0.52

Zrc_26_038

0.50

Zrc_18_028

Zrc_53_070

0.39 0.51

0.55

Zrc_50_066

Zrc_15_024

0.48

Zrc_29_041

Zrc_35_048

0.53 0.36

0.58

Zrc_75_096

Zrc_87_111

0.37

Zrc_27_039

Zrc_20_030

Th/U

Spot name

0.05387

0.05568

0.05233

0.05347

0.05232

0.05197

0.05286

0.05213

0.05401

0.05120

0.05191

0.05273

0.05441

0.05346

0.05199

0.05608

0.05456

0.05148

207 Pb/206 Pb

2.41

1.90

1.80

2.00

1.80

1.50

1.89

1.59

2.50

1.60

1.60

2.60

2.79

2.10

1.50

1.60

3.74

2.10

1σ

0.37536

0.38848

0.36150

0.37195

0.36258

0.35929

0.36516

0.35858

0.36869

0.35085

0.35381

0.35794

0.36785

0.36249

0.35253

0.37935

0.36866

0.34803

207 Pb/235 U

2.78

1.97

3.00

2.09

1.93

1.59

1.99

1.70

2.60

1.70

1.70

2.69

2.89

2.18

1.57

1.72

4.20

2.21

1σ

1σ Rho

0.05054 0.77 0.35

0.05054 0.51 0.26

0.05054 2.39 0.80

0.05036 0.62 0.30

0.05021 0.70 0.36

0.05003 0.54 0.34

0.04990 0.60 0.32

0.04983 0.58 0.35

0.04957 0.73 0.28

0.04960 0.58 0.34

0.04941 0.59 0.34

0.04921 0.67 0.26

0.04905 0.69 0.25

0.04908 0.57 0.28

0.04909 0.47 0.29

0.04894 0.63 0.37

0.04901 0.90 0.29

0.04897 0.69 0.32

206 Pb/238 U

Isotopic ratios (errors in %)

318

318

318

317

316

315

314

313

312

312

311

310

309

309

309

308

308

308

206 Pb/238 U

2

2

7

2

2

2

2

2

2

2

2

2

2

2

1

2

3

2

1σ

324

333

313

321

314

312

316

311

319

305

308

311

318

314

307

327

319

303

207 Pb/235 U

8

6

8

6

5

4

5

5

7

4

5

7

8

6

4

5

12

6

1σ

53

41

40

45

43

34

42

35

56

36

36

59

61

47

34

35

83

48

318

318

318

317

316

315

314

313

312

312

311

310

309

309

309

308

308

308

2

2

7

2

2

2

2

2

2

2

2

2

2

2

1

2

3

2

1σ Best age 1σ

Continued on next page...

366

440

300

349

299

284

323

291

371

250

281

317

388

348

285

456

394

262

207 Pb/206 U

Ages (Ma)

Tabla 5: LA-ICP-MS U-Pb isotopic data for Tecomate Formation metapsammite TT-81.

tablas geocronología u-pb 96

0.26 0.31 0.24 0.27 0.19

Zrc_69_089

Zrc_68_088

Zrc_43_058

0.43

Zrc_89_113

Zrc_45_060

0.33

Zrc_19_029

Zrc_66_086

0.19 1.21

Zrc_74_095

0.44

Zrc_94_119

Zrc_32_045

0.39 0.33

0.72

Zrc_41_056

Zrc_42_057

0.69

Zrc_33_046

Zrc_61_080

0.55 0.33

0.45

Zrc_93_118

Zrc_07_015

0.28

Zrc_70_090

Zrc_91_116

Th/U

Spot name

0.05897

0.06736

0.06360

0.06834

0.05689

0.07007

0.06291

0.05564

0.05788

0.06011

0.05708

0.06044

0.05370

0.05594

0.05403

0.05520

0.05289

0.05384

207 Pb/206 Pb

1.49

2.81

1.79

2.40

1.30

1.80

1.61

1.20

1.83

5.19

2.59

6.63

1.81

2.09

1.70

2.57

1.80

2.15

1σ

0.68338

0.78278

0.73058

0.80177

0.60981

0.77362

0.63835

0.56124

0.51317

0.47211

0.41128

0.43571

0.38559

0.39991

0.38347

0.39132

0.36983

0.37519

207 Pb/235 U

1.57

3.18

2.11

4.00

1.41

3.24

2.41

1.31

2.00

7.14

2.73

6.92

2.00

2.31

1.78

2.92

1.90

2.39

1σ

1σ Rho

0.08391 0.46 0.31

0.08362 1.49 0.47

0.08111 1.10 0.53

0.08010 3.20 0.80

0.07757 0.54 0.38

0.07738 2.70 0.83

0.07330 1.80 0.75

0.07302 0.52 0.39

0.06430 0.58 0.31

0.05697 2.76 0.68

0.05228 0.84 0.32

0.05228 0.71 0.13

0.05198 0.88 0.43

0.05176 0.95 0.42

0.05142 0.53 0.29

0.05141 0.76 0.38

0.05064 0.59 0.32

0.05054 0.65 0.31

206 Pb/238 U

Isotopic ratios (errors in %)

519

518

503

497

482

480

456

454

402

357

329

329

327

325

323

323

318

318

206 Pb/238 U

2

7

5

15

3

13

8

2

2

10

3

2

3

3

2

2

2

2

1σ

529

587

557

598

483

582

501

452

421

393

350

367

331

342

330

335

320

323

207 Pb/235 U

6

14

9

18

5

14

10

5

7

23

8

21

6

7

5

8

5

7

1σ

31

57

37

48

28

36

34

26

39

112

55

141

39

46

38

57

40

48

519

518

503

497

482

480

456

454

402

357

329

329

327

325

323

323

318

318

2

7

5

15

3

13

8

2

2

10

3

2

3

3

2

2

2

2

1σ Best age 1σ

Continued on next page...

566

849

728

879

487

930

705

438

525

607

495

620

358

450

372

420

324

364

207 Pb/206 U

Ages (Ma)

Tabla 5: LA-ICP-MS U-Pb isotopic data for Tecomate Formation metapsammite TT-81.

tablas geocronología u-pb 97

0.32 0.23 0.37 0.10 0.60

Zrc_96_122

Zrc_100_126

Zrc_57_075

0.26

Zrc_39_053

Zrc_49_065

0.35

Zrc_13_022

Zrc_30_042

0.42 0.39

Zrc_55_072

0.35

Zrc_86_110

Zrc_17_027

0.05 0.39

0.22

Zrc_73_094

Zrc_10_018

0.28

Zrc_80_102

Zrc_31_044

0.18 0.57

0.25

Zrc_76_098

Zrc_60_078

0.88

Zrc_16_026

Zrc_47_063

Th/U

Spot name

0.08236

0.08148

0.07810

0.07651

0.07508

0.07416

0.07365

0.07314

0.07151

0.06949

0.06890

0.07258

0.06787

0.07045

0.06674

0.06770

0.06204

0.05909

207 Pb/206 Pb

2.00

0.99

1.20

1.01

1.20

1.29

1.40

1.09

1.96

1.42

1.20

0.99

1.11

1.41

2.08

1.30

2.00

1.69

1σ

2.33510

1.89550

1.94180

1.91090

1.84750

1.75821

1.76040

1.78850

1.60304

1.49016

1.33810

1.35909

1.25740

1.28180

1.18944

1.06170

0.84777

0.75452

207 Pb/235 U

2.15

1.11

1.48

1.08

1.35

1.60

1.52

1.19

2.26

1.69

1.31

1.11

1.70

1.70

2.46

1.59

2.90

1.79

1σ

1σ Rho

0.20546 0.80 0.37

0.16787 0.48 0.44

0.17928 0.87 0.58

0.18069 0.41 0.36

0.17778 0.61 0.46

0.17196 0.62 0.51

0.17311 0.59 0.39

0.17707 0.45 0.39

0.16259 0.62 0.37

0.15554 0.55 0.43

0.14055 0.52 0.39

0.13582 0.52 0.43

0.13323 1.30 0.76

0.13085 0.96 0.56

0.12925 0.67 0.41

0.11301 0.92 0.58

0.09377 2.10 0.72

0.09249 0.55 0.32

206 Pb/238 U

Isotopic ratios (errors in %)

1205

1000

1063

1071

1055

1023

1029

1051

971

932

848

821

806

793

784

690

578

570

206 Pb/238 U

9

4

9

4

6

6

6

4

6

5

4

4

10

7

5

6

12

3

1σ

1223

1080

1096

1085

1063

1030

1031

1041

971

926

862

871

827

838

796

735

623

571

207 Pb/235 U

15

7

10

7

9

10

10

8

14

10

8

6

10

10

14

8

14

8

1σ

38

19

23

20

23

25

28

22

39

29

24

20

22

28

42

26

42

36

1254

1233

1149

1108

1071

1046

1032

1018

971

932

848

821

806

793

784

690

578

570

38

19

23

20

23

25

28

22

6

5

4

4

10

7

5

6

12

3

1σ Best age 1σ

Continued on next page...

1254

1233

1149

1108

1071

1046

1032

1018

972

913

896

1002

865

942

830

859

676

570

207 Pb/206 U

Ages (Ma)

Tabla 5: LA-ICP-MS U-Pb isotopic data for Tecomate Formation metapsammite TT-81.

tablas geocronología u-pb 98

0.45 0.64 1.09 0.71 0.79 0.37

Zrc_33_048

Zrc_97_125

Zrc_37_053

Zrc_93_120

0.89

Zrc_14_023

Zrc_09_017

0.83

Zrc_53_072

Zrc_99_127

0.07

0.92

Zrc_46_062

Zrc_19_029

0.37

Zrc_37_051

Th/U

0.30

Zrc_25_036 0.96

1.00

1.10

1σ

4.84970

2.91770

2.41160

207 Pb/235 U

1.09

1.10

1.18

1σ

1σ Rho

0.31958 0.51 0.48

0.23625 0.45 0.42

0.21017 0.44 0.38

206 Pb/238 U

Isotopic ratios (errors in %)

1788

1367

1230

206 Pb/238 U

8

6

5

1σ

1794

1387

1246

207 Pb/235 U

9

8

9

1σ

0.05420

0.05285

0.05288

0.05314

0.05570

0.05175

0.05308

0.05351

0.05117

207 Pb/206 Pb

207 Pb/235 U

1σ 206 Pb/238 U

1σ Rho

2.90 0.35777 3.00 0.04786 0.75 0.26

2.50 0.34677 2.59 0.04770 0.67 0.26

1.70 0.34672 1.78 0.04752 0.53 0.29

1.39 0.34730 1.45 0.04739 0.38 0.29

3.59 0.36200 3.85 0.04713 0.62 0.29

1.70 0.33431 1.94 0.04692 0.94 0.48

1.81 0.33862 1.88 0.04628 0.56 0.28

2.69 0.33301 2.80 0.04517 0.75 0.28

1.50 0.31544 1.56 0.04470 0.43 0.26

1σ

Isotopic ratios (errors in %)

301

300

299

298

297

296

292

285

282

206 Pb/238 U

2

2

2

1

2

3

2

2

1

1σ

311

302

302

303

314

293

296

292

278

207 Pb/235 U

8

7

5

4

10

5

5

7

4

1σ

1796

1411

1269

17

19

21

63

56

37

31

77

38

40

60

34

301

300

299

298

297

296

292

285

282

2

2

2

1

2

3

2

2

1

1σ Best age 1σ

17

19

21

1σ Best age 1σ

Continued on next page...

379

322

324

335

441

274

332

350

248

207 Pb/206 U

Ages (Ma)

1796

1411

1269

207 Pb/206 U

Ages (Ma)

Tabla 6: LA-ICP-MS U-Pb isotopic data for Tecomate Formation metapelite TT-82.

0.10979

0.08933

0.08299

207 Pb/206 Pb

Spot name

Th/U

Spot name

Tabla 5: LA-ICP-MS U-Pb isotopic data for Tecomate Formation metapsammite TT-81.

tablas geocronología u-pb 99

0.71 0.42 0.87 0.44 0.93

Zrc_70_092

Zrc_80_104

Zrc_98_126

0.49

Zrc_92_119

Zrc_16_026

0.57

Zrc_52_071

Zrc_78_102

0.91 0.62

Zrc_65_086

0.83

Zrc_87_113

Zrc_23_036

0.62 0.60

0.68

Zrc_79_103

Zrc_25_038

0.39

Zrc_68_090

Zrc_73_096

0.44 0.58

0.47

Zrc_40_056

Zrc_13_022

0.77

Zrc_77_101

Zrc_69_091

Th/U

Spot name

0.05158

0.07303

0.05808

0.05371

0.05577

0.05447

0.05614

0.05701

0.05456

0.05283

0.05306

0.05521

0.05298

0.05279

0.05223

0.05281

0.05143

0.05444

207 Pb/206 Pb

207 Pb/235 U

1σ 206 Pb/238 U

1σ Rho

2.00 0.38061 2.07 0.05354 0.52 0.26

2.62 0.53704 2.96 0.05333 0.83 0.41

2.70 0.42251 3.03 0.05276 0.59 0.24

1.99 0.38895 2.07 0.05249 0.53 0.28

4.27 0.40419 4.67 0.05256 0.80 0.24

1.60 0.37827 1.65 0.05037 0.42 0.26

1.91 0.38937 1.96 0.05036 0.48 0.23

4.14 0.39524 4.41 0.05028 0.56 0.26

1.70 0.37160 1.78 0.04934 0.51 0.28

1.50 0.35782 1.56 0.04907 0.43 0.28

2.60 0.35836 2.70 0.04912 0.71 0.26

3.10 0.36981 3.19 0.04876 0.76 0.24

1.70 0.35575 1.76 0.04868 0.45 0.26

2.60 0.35256 2.71 0.04853 0.76 0.29

1.90 0.34731 1.98 0.04821 0.56 0.29

2.40 0.35093 2.47 0.04826 0.58 0.23

2.51 0.34072 2.58 0.04805 0.62 0.23

1.51 0.36050 1.56 0.04798 0.42 0.25

1σ

Isotopic ratios (errors in %)

336

335

331

330

330

317

317

316

310

309

309

307

306

305

304

304

303

302

206 Pb/238 U

2

3

2

2

3

1

1

2

2

1

2

2

1

2

2

2

2

1

1σ

327

436

358

334

345

326

334

338

321

311

311

320

309

307

303

305

298

313

207 Pb/235 U

6

10

9

6

14

5

6

13

5

4

7

9

5

7

5

7

7

4

1σ

44

53

58

45

92

35

42

89

37

34

59

67

39

57

42

53

57

34

336

335

331

330

330

317

317

316

310

309

309

307

306

305

304

304

303

302

2

3

2

2

3

1

1

2

2

1

2

2

1

2

2

2

2

1

1σ Best age 1σ

Continued on next page...

267

1015

533

359

443

391

458

492

394

322

331

421

328

320

295

321

260

389

207 Pb/206 U

Ages (Ma)

Tabla 6: LA-ICP-MS U-Pb isotopic data for Tecomate Formation metapelite TT-82.

tablas geocronología u-pb 100

0.37 0.36 0.41 1.01 0.38

Zrc_36_052

Zrc_18_028

Zrc_34_049

0.41

Zrc_21_034

Zrc_10_018

0.68

Zrc_22_035

Zrc_08_016

0.24 0.31

Zrc_42_059

0.83

Zrc_45_062

Zrc_06_014

0.24 0.35

0.62

Zrc_63_084

Zrc_88_114

0.76

Zrc_27_041

Zrc_57_077

0.08 0.49

0.75

Zrc_75_098

Zrc_47_065

0.60

Zrc_67_089

Zrc_64_085

Th/U

Spot name

0.05990

0.05962

0.06499

0.07085

0.05841

0.05878

0.05801

0.05873

0.05636

0.05603

0.05898

0.05487

0.05649

0.05456

0.05512

0.05253

0.05434

0.05589

207 Pb/206 Pb

207 Pb/235 U

1σ 206 Pb/238 U

1σ Rho

1.90 0.76137 1.95 0.09233 0.43 0.21

1.39 0.74145 1.53 0.08994 0.63 0.42

1.49 0.80320 1.86 0.08811 1.10 0.60

2.10 0.84627 2.39 0.08663 0.70 0.34

1.51 0.69767 1.68 0.08639 0.75 0.44

1.80 0.69653 1.85 0.08597 0.43 0.22

1.40 0.68403 1.47 0.08547 0.44 0.31

1.80 0.64366 1.87 0.07953 0.49 0.25

1.40 0.59644 1.46 0.07670 0.40 0.27

1.61 0.58522 1.67 0.07572 0.48 0.27

2.10 0.58513 3.04 0.06904 2.20 0.72

1.29 0.46724 1.40 0.06160 0.50 0.37

2.80 0.45353 2.88 0.05778 0.67 0.24

2.11 0.42411 2.37 0.05605 1.11 0.46

2.00 0.41858 2.23 0.05485 0.98 0.44

1.41 0.39526 1.45 0.05455 0.38 0.24

2.10 0.40561 2.16 0.05408 0.52 0.24

1.81 0.41577 1.87 0.05369 0.50 0.26

1σ

Isotopic ratios (errors in %)

569

555

544

536

534

532

529

493

476

471

430

385

362

352

344

342

340

337

206 Pb/238 U

2

3

6

4

4

2

2

2

2

2

9

2

2

4

3

1

2

2

1σ

575

563

599

623

537

537

529

505

475

468

468

389

380

359

355

338

346

353

207 Pb/235 U

9

7

8

11

7

8

6

7

6

6

11

5

9

7

7

4

6

6

1σ

41

29

31

42

32

38

30

38

31

35

45

29

60

46

43

32

47

39

569

555

544

536

534

532

529

493

476

471

430

385

362

352

344

342

340

337

2

3

6

4

4

2

2

2

2

2

9

2

2

4

3

1

2

2

1σ Best age 1σ

Continued on next page...

600

590

774

953

545

559

530

557

467

454

566

407

472

394

417

309

385

448

207 Pb/206 U

Ages (Ma)

Tabla 6: LA-ICP-MS U-Pb isotopic data for Tecomate Formation metapelite TT-82.

tablas geocronología u-pb 101

0.57 0.34 0.30 0.24 0.35

Zrc_44_061

Zrc_35_050

Zrc_59_079

0.24

Zrc_72_095

Zrc_30_044

0.41

Zrc_55_074

Zrc_39_055

0.45 0.23

Zrc_94_121

0.27

Zrc_02_009

Zrc_71_094

0.06 0.31

0.33

Zrc_31_046

Zrc_83_108

0.40

Zrc_03_010

Zrc_60_080

0.97 0.67

0.03

Zrc_11_020

Zrc_91_118

0.35

Zrc_05_012

Zrc_43_060

Th/U

Spot name

0.07353

0.07339

0.07338

0.07337

0.07310

0.07305

0.07260

0.07238

0.06990

0.06911

0.06945

0.07280

0.06990

0.06663

0.06229

0.06169

0.06123

0.06038

207 Pb/206 Pb

207 Pb/235 U

1σ 206 Pb/238 U

1σ Rho

1.50 1.80820 1.56 0.17811 0.43 0.29

1.40 1.80310 1.48 0.17837 0.48 0.32

1.70 1.86950 1.77 0.18477 0.48 0.26

1.89 1.77680 2.00 0.17588 0.62 0.32

1.50 1.95000 1.57 0.19336 0.46 0.28

1.40 1.83210 1.47 0.18188 0.45 0.31

1.50 1.80690 1.58 0.18029 0.51 0.32

1.20 1.77700 1.29 0.17772 0.46 0.35

1.40 1.59010 1.45 0.16480 0.38 0.26

1.79 1.50930 1.92 0.15840 0.66 0.35

1.50 1.49530 1.56 0.15605 0.44 0.29

1.30 1.51860 1.36 0.15115 0.40 0.28

2.40 1.45090 2.55 0.15050 0.85 0.33

2.00 1.28230 2.09 0.13942 0.60 0.29

1.61 1.01450 1.68 0.11817 0.51 0.29

1.30 0.95914 1.38 0.11267 0.45 0.33

1.31 0.94427 1.35 0.11180 0.36 0.25

1.71 0.89490 1.77 0.10754 0.51 0.28

1σ

Isotopic ratios (errors in %)

1057

1058

1093

1044

1140

1077

1069

1055

983

948

935

907

904

841

720

688

683

658

206 Pb/238 U

4

5

5

6

5

4

5

4

3

6

4

3

7

5

3

3

2

3

1σ

1048

1047

1070

1037

1098

1057

1048

1037

966

934

928

938

910

838

711

683

675

649

207 Pb/235 U

10

10

12

13

11

10

10

8

9

12

10

8

15

12

9

7

7

9

1σ

30

28

34

38

30

27

30

24

28

36

30

26

49

40

34

27

27

36

1029

1025

1024

1024

1017

1015

1003

997

983

948

935

907

904

841

720

688

683

658

30

28

34

38

30

27

30

24

3

6

4

3

7

5

3

3

2

3

1σ Best age 1σ

Continued on next page...

1029

1025

1024

1024

1017

1015

1003

997

925

902

912

1008

925

826

684

663

647

617

207 Pb/206 U

Ages (Ma)

Tabla 6: LA-ICP-MS U-Pb isotopic data for Tecomate Formation metapelite TT-82.

tablas geocronología u-pb 102

0.38 0.39 0.33 0.43 0.51

Zrc_49_067

Zrc_41_058

Zrc_56_076

0.38

Zrc_24_037

Zrc_32_047

0.64

Zrc_48_066

Zrc_17_027

0.14 0.28

Zrc_15_024

0.48

Zrc_96_124

Zrc_01_008

0.26 0.26

0.18

Zrc_100_128

Zrc_76_100

0.37

Zrc_46_064

Zrc_07_015

0.28 0.37

0.34

Zrc_04_011

Zrc_29_043

0.37

Zrc_54_073

Zrc_85_110

Th/U

Spot name

0.07824

0.07805

0.07782

0.07754

0.07750

0.07740

0.07714

0.07621

0.07503

0.07487

0.07459

0.07446

0.07429

0.07428

0.07402

0.07400

0.07397

0.07374

207 Pb/206 Pb

207 Pb/235 U

1σ 206 Pb/238 U

1σ Rho

1.30 2.16320 1.35 0.20031 0.38 0.27

1.40 2.18960 1.46 0.20343 0.43 0.30

1.40 2.14040 1.45 0.19923 0.39 0.27

1.70 2.11560 1.77 0.19782 0.50 0.28

1.30 2.11210 1.39 0.19759 0.49 0.35

1.78 1.97396 1.98 0.18496 0.49 0.34

1.59 2.02090 1.68 0.18989 0.51 0.31

1.40 2.05560 1.46 0.19565 0.42 0.28

1.31 2.06560 1.37 0.19965 0.44 0.31

1.50 1.95140 1.57 0.18867 0.48 0.31

1.60 1.83180 1.71 0.17814 0.60 0.36

1.40 1.81370 1.51 0.17651 0.57 0.38

1.31 1.82290 1.35 0.17772 0.37 0.26

1.60 1.87250 1.68 0.18253 0.52 0.31

1.50 1.66500 1.58 0.16303 0.50 0.32

1.50 1.82730 1.56 0.17906 0.43 0.28

1.70 1.86690 1.81 0.18275 0.61 0.33

1.51 1.85490 1.56 0.18238 0.44 0.27

1σ

Isotopic ratios (errors in %)

1177

1194

1171

1164

1162

1094

1121

1152

1173

1114

1057

1048

1055

1081

974

1062

1082

1080

206 Pb/238 U

4

5

4

5

5

5

5

4

5

5

6

6

4

5

5

4

6

4

1σ

1169

1178

1162

1154

1153

1107

1123

1134

1137

1099

1057

1050

1054

1071

995

1055

1069

1065

207 Pb/235 U

9

10

10

12

10

13

11

10

9

11

11

10

9

11

10

10

12

10

1σ

26

27

28

33

26

34

32

27

26

29

31

28

25

32

30

29

33

30

1153

1148

1142

1135

1134

1132

1125

1101

1069

1065

1057

1054

1049

1049

1042

1041

1041

1034

26

27

28

33

26

34

32

27

26

29

31

28

25

32

30

29

33

30

1σ Best age 1σ

Continued on next page...

1153

1148

1142

1135

1134

1132

1125

1101

1069

1065

1057

1054

1049

1049

1042

1041

1041

1034

207 Pb/206 U

Ages (Ma)

Tabla 6: LA-ICP-MS U-Pb isotopic data for Tecomate Formation metapelite TT-82.

tablas geocronología u-pb 103

0.35 0.43 0.89 0.17 0.47

Zrc_58_078

Zrc_89_115

Zrc_20_030

0.42

Zrc_95_122

Zrc_81_106

0.52

Zrc_90_116

Zrc_26_040

0.26 0.18

Zrc_74_097

0.26

Zrc_61_082

Zrc_82_107

0.30 0.27

0.33

Zrc_28_042

Zrc_50_068

0.29

Zrc_86_112

Zrc_51_070

0.39 0.31

0.26

Zrc_12_021

Zrc_38_054

0.47

Zrc_62_083

Zrc_66_088

Th/U

Spot name

0.17663

0.13691

0.09615

0.09436

0.08608

0.08504

0.08382

0.08243

0.08229

0.08168

0.08167

0.08071

0.08041

0.07959

0.07870

0.07869

0.07868

0.07833

207 Pb/206 Pb

207 Pb/235 U

1σ 206 Pb/238 U

1σ Rho

1.30 12.36600 1.36 0.50743 0.39 0.28

1.20 7.90740 1.25 0.41835 0.36 0.29

1.30 3.82670 1.36 0.28842 0.41 0.30

1.20 3.59280 1.28 0.27652 0.44 0.35

1.41 2.77100 1.49 0.23334 0.52 0.34

1.51 2.76800 1.57 0.23598 0.48 0.29

1.40 2.38840 1.47 0.20643 0.45 0.31

1.30 2.48270 1.63 0.21778 0.98 0.60

1.40 2.50110 1.46 0.22014 0.41 0.29

1.30 2.53490 1.36 0.22502 0.40 0.30

1.80 2.41270 1.89 0.21420 0.57 0.30

1.30 2.44490 1.39 0.21948 0.49 0.35

1.80 2.29120 1.88 0.20596 0.55 0.29

1.29 2.22040 1.36 0.20187 0.41 0.31

1.30 2.27640 1.35 0.20949 0.36 0.28

1.50 2.17290 1.56 0.20003 0.41 0.26

1.30 2.19530 1.37 0.20232 0.43 0.32

1.40 2.20260 1.47 0.20382 0.44 0.29

1σ

Isotopic ratios (errors in %)

2646

2253

1634

1574

1352

1366

1210

1270

1283

1308

1251

1279

1207

1185

1226

1175

1188

1196

206 Pb/238 U

8

7

6

6

6

6

5

11

5

5

6

6

6

4

4

4

5

5

1σ

2633

2221

1598

1548

1348

1347

1239

1267

1272

1282

1246

1256

1210

1187

1205

1172

1180

1182

207 Pb/235 U

13

11

11

10

11

12

11

12

11

10

14

10

13

10

10

11

10

10

1σ

2621

2188

1551

1515

1340

1316

1288

1256

1252

1238

1238

1214

1207

1187

1165

1164

1164

1155

207 Pb/206 U

Ages (Ma)

Tabla 6: LA-ICP-MS U-Pb isotopic data for Tecomate Formation metapelite TT-82.

21

20

24

22

27

28

26

25

27

25

35

25

35

26

25

29

25

27

2621

2188

1551

1515

1340

1316

1288

1256

1252

1238

1238

1214

1207

1187

1165

1164

1164

1155

21

20

24

22

27

28

26

25

27

25

35

25

35

26

25

29

25

27

1σ Best age 1σ

tablas geocronología u-pb 104

0.58 0.94 0.95 0.30 0.69

Zrc_85_108

Zrc_82_105

Zrc_9_017

0.80

Zrc_18_028

Zrc_55_072

0.84

Zrc_65_084

Zrc_70_090

0.49 0.89

Zrc_71_092

0.64

Zrc_53_070

Zrc_90_114

0.44 0.46

0.50

Zrc_38_052

Zrc_54_071

0.58

Zrc_75_096

Zrc_30_042

0.36 0.28

0.20

Zrc_16_026

Zrc_04_011

0.21

Zrc_72_093

Zrc_46_062

Th/U

Spot name

0.05649

0.05504

0.05606

0.05372

0.05634

0.05617

0.05545

0.05734

0.05718

0.05365

0.05983

0.05899

0.05778

0.05633

0.05429

0.05727

0.05518

0.05389

207 Pb/206 Pb

207 Pb/235 U

2.00 0.38197

1.71 0.37161

1.80 0.37698

1.51 0.36164

2.59 0.37782

2.71 0.37713

1.70 0.37143

2.30 0.38220

3.39 0.37909

1.30 0.35678

5.20 0.39736

3.31 0.38845

4.29 0.38198

2.70 0.37081

1.81 0.35668

4.30 0.36897

2.59 0.35042

1.21 0.33885

1σ

2.05

1.77

1.85

1.58

2.66

2.73

1.78

2.34

3.45

1.36

5.25

3.36

4.34

2.78

1.87

4.33

2.63

1.27

1σ

1σ Rho

0.04932 0.49 0.22

0.04920 0.53 0.25

0.04909 0.45 0.22

0.04910 0.45 0.30

0.04886 0.51 0.23

0.04896 0.47 0.14

0.04885 0.41 0.30

0.04865 0.49 0.19

0.04856 0.68 0.18

0.04845 0.43 0.29

0.04823 1.22 0.14

0.04808 0.62 0.18

0.04809 0.77 0.15

0.04795 0.60 0.24

0.04788 0.52 0.26

0.04685 1.66 0.12

0.04627 1.62 0.18

0.04584 0.48 0.32

206 Pb/238 U

Isotopic ratios (errors in %)

310

310

309

309

308

308

307

306

306

305

304

303

303

302

301

295

292

289

206 Pb/238 U

1

1

1

2

2

1

2

1

2

1

2

2

2

2

1

1

1

1

1σ

328

321

325

313

325

325

321

329

326

310

340

333

328

320

310

319

305

296

207 Pb/235 U

6

5

5

4

7

8

5

7

10

4

15

10

12

8

5

12

7

3

1σ

41

36

37

32

58

59

35

47

70

29

112

72

95

56

41

95

57

25

310

310

309

309

308

308

307

306

306

305

304

303

303

302

301

295

292

289

1

1

1

2

2

1

2

1

2

1

2

2

2

2

1

1

1

1

1σ Best age 1σ

Continued on next page...

472

414

455

359

466

459

430

505

499

356

597

567

521

465

383

502

420

366

207 Pb/206 U

Ages (Ma)

Tabla 7: LA-ICP-MS U-Pb isotopic data for Tecomate Formation metapsammite TT-612.

tablas geocronología u-pb 105

0.48 0.66 0.83 1.08 0.79

Zrc_31_044

Zrc_58_076

Zrc_59_077

0.58

Zrc_66_086

Zrc_57_075

0.54

Zrc_14_023

Zrc_76_098

0.57 0.48

Zrc_48_064

0.47

Zrc_10_018

Zrc_56_074

0.82 0.64

0.41

Zrc_05_012

Zrc_08_016

0.73

Zrc_27_039

Zrc_29_041

0.72 0.41

0.84

Zrc_3_010

Zrc_35_048

0.87

Zrc_96_122

Zrc_17_027

Th/U

Spot name

0.05339

0.05831

0.05524

0.05442

0.05653

0.06054

0.05746

0.06271

0.05872

0.05429

0.05862

0.06046

0.05860

0.05589

0.05818

0.06166

0.05937

0.05427

207 Pb/206 Pb

207 Pb/235 U

2.85

1.38

1σ

1σ Rho

0.04948 1.52 0.20

0.04921 0.39 0.31

206 Pb/238 U

1.59 0.37961

1.70 0.41377

1.50 0.39019

1.40 0.37887

2.00 0.39302

3.04 0.42154

3.10 0.39806

4.10 0.43288

3.00 0.40509

1.60 0.37429

3.51 0.40481

2.50 0.41720

3.60 0.40379

1.50 0.38223

2.70 0.39696

1.67

1.85

1.56

1.46

2.09

3.38

3.14

4.15

3.07

1.69

3.84

2.56

3.67

1.56

2.76

0.05188 0.44 0.30

0.05176 0.66 0.40

0.05153 0.41 0.26

0.05081 0.41 0.30

0.05065 0.67 0.29

0.05050 0.20 0.28

0.05055 0.77 0.17

0.05025 1.23 0.16

0.05021 0.68 0.22

0.05016 0.50 0.32

0.05009 0.20 0.22

0.05010 0.96 0.21

0.05014 1.18 0.19

0.04983 0.42 0.26

0.04973 0.64 0.21

9.78 0.41989 10.40 0.04939 0.34 0.14

2.80 0.40403

1.31 0.36648

1σ

Isotopic ratios (errors in %)

326

325

324

319

319

318

318

316

316

316

315

315

315

313

313

311

311

310

206 Pb/238 U

1

2

1

1

2

2

2

2

2

2

2

2

2

1

2

3

2

1

1σ

327

352

335

326

337

357

340

365

345

323

345

354

344

329

339

356

345

317

207 Pb/235 U

5

6

4

4

6

10

9

13

9

5

11

8

11

4

8

31

8

4

1σ

34

35

34

29

41

61

67

81

66

35

71

50

79

33

58

216

60

27

326

325

324

319

319

318

318

316

316

316

315

315

315

313

313

311

311

310

1

2

1

1

2

2

2

2

2

2

2

2

2

1

2

3

2

1

1σ Best age 1σ

Continued on next page...

345

541

422

388

473

623

509

698

557

383

553

620

552

448

537

662

581

382

207 Pb/206 U

Ages (Ma)

Tabla 7: LA-ICP-MS U-Pb isotopic data for Tecomate Formation metapsammite TT-612.

tablas geocronología u-pb 106

0.14 1.10 0.81 0.16 0.47

Zrc_01_008

Zrc_44_059

Zrc_84_107

0.93

Zrc_19_029

Zrc_39_053

0.86

Zrc_45_060

Zrc_74_095

0.17 0.95

Zrc_40_054

0.86

Zrc_91_116

Zrc_97_123

0.68 0.17