Chronological constraints on the thermal and tilting history of theSierra San Pedro Mártir

Chronological constraints on the thermal and tilting history of the Sierra San Pedro Mártir pluton, Baja California, México, from U/Pb, 40Ar/39Ar, and fission-track geochronology Amabel Ortega-Rivera* E. Farrar J. A. Hanes D. A. Archibald

}

R. G. Gastil D. L. Kimbrough

} Department of Geology, San Diego State University, San Diego, California 92182-1020

Department of Geological Sciences, Queen’s University, Kingston, Ontario K7L 3N6, Canada

M. Zentilli

Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada

M. López-Martínez

División de Ciencias de la Tierra, Centro de Investigación Científica y Educación Superior de Ensenada, Km 107 Carretera Tijuana-Ensenada, 22860, Ensenada, Baja California, México

G. Féraud G. Ruffet

de Géodynamique, Unité de Recherche Associeé, Centre National de la Recherche } Institut Scientifique, 1279, Université de Nice, France

ABSTRACT The tectonothermal history of the four major phases of the Sierra San Pedro Mártir pluton and surrounding metamorphic rocks of the Mesozoic Peninsular Ranges batholith of Baja California is presented on the basis of U/Pb, 40Ar/39Ar step-heating, and fission-track dating, in combination with Al-in-hornblende geobarometry. A previous model proposed up to 90° of east-side-up tilting of the pluton, exposing >20 km of crustal section to account for its crescent shape, asymmetrical zoning, internal structure, the eastward younging of K-Ar dates across the intrusion and eastward increase in the metamorphic grade of the country rocks, from greenschist to amphibolite facies.

The U/Pb data suggest that the different phases of the pluton were emplaced sequentially from west to east between 97.0 +4/–1 Ma and 93.8 +1/–1 Ma. All except one of the 105 40Ar/39Ar age spectra have welldefined plateaus and are interpreted as cooling ages. Samples from the pluton give hornblende and biotite 40Ar/39Ar plateau dates and apatite fission-track dates that young from west to east; thus, hornblende dates decrease from 95 to 91 Ma, biotite dates decrease from 94 to 88 Ma, and apatite dates decrease from 72 to 57 Ma. Muscovite, biotite, and plagioclase from the same rock sample collected at the easternmost phase of the pluton yield concordant 40Ar/39Ar dates of 88 Ma. The exposed part of the pluton underwent rapid cooling (≈40 °C/Ma) down to ≈250 °C in the first 10 m.y. after intrusion. Modeling of track-length distribution in apatite is consistent with monotonic slow cooling from ca. 80 Ma to the present.

The data do not support a history that includes major tilting of the pluton. Eastward younging of 40Ar/39Ar and fission-track dates may be explained by ≈15° of east-side-up tilting of the pluton at or after 88 Ma about a north-south horizontal axis. Furthermore, the fission-track data suggest that part or all of this tilting may have taken place at or after 57 Ma, and therefore may be a consequence of regional-scale crustal extension associated with the opening of the Gulf of California in Neogene time. Such tilting is in agreement with the Al-in-hornblende geobarometry for the hornblende-biotite intrusive phase that yields pressures of 5.2 ± 0.6 kbar. An ≈15° northeast-side-up tilt of the crustal block containing this pluton would explain the apparent paleomagnetic inclination discrepancies with cratonic North America and militates against large-scale northerly transport of Baja California.

INTRODUCTION This study is part of a regional 40Ar/39Ar dating program (Fig. 1) investigating the thermal history of the Peninsular Ranges batholith of Alta and Baja California, and the apparent eastward migration of granitoid emplacement within it (Silver et al., 1969; Krummenacher et al., 1975).

The crescent-shaped Sierra San Pedro Mártir pluton is one of many granitoid bodies that compose the composite Mesozoic Peninsular Ranges batholith of southern California, United States, and Baja California, México (Fig. 1). On the basis of field, geochemical, geochronological, and isotopic studies (Eastman, 1986; McCormick, 1986; Gastil et al., 1990; Walawender et al., 1990), Gastil et al. (1991) proposed that the 400 km2 zoned pluton (Fig. 2) may have been tilted about a horizontal axis parallel to the trend of the batholith to reveal deeper levels to the east, and that the sequence of

*Corresponding author; E-mail: ortegaa@qucdn.queensu.ca

Data Repository item 9727 contains additional material related to this article.

GSA Bulletin; June 1997; v. 109; no. 6; p. 728–745; 11 figures, 5 tables.

728

SIERRA SAN PEDRO MÁRTIR PLUTON, BAJA CALIFORNIA, MEXICO

Figure 1. Map showing the location of the Peninsular Ranges province, regional 40Ar/39Ar study area, and the composite Sierra San Pedro Mártir pluton.

modal zones mapped from west to east and eastward metamorphic gradient of the country rocks from greenschist to amphibolite facies may represent a 20 km crustal section through the pluton. To test the proposal that the pluton may have been substantially tilted, the cooling and exhumation history was determined using U/Pb dating of zircons and monazites, detailed 40Ar/39Ar conventional and laser step-heating analyses on hornblende, muscovite, biotite, and plagioclase, fission-track dating of apatite, and Al-inhornblende geobarometry on samples from the pluton and its host rocks.

Evidence of tilting of the batholith would have a significant bearing on the plausibility of the model of large-scale (100–2500 km) northward tectonic transport of Baja California with respect to the North American craton as proposed, for example, by Hagstrum et al. (1985) on the basis of their paleomagnetic studies. Butler et al. (1991) calculated that a westward 15°–20° tilt of the whole peninsula about its north-northwest longitudinal axis would restore the apparent poles for western Baja to concordance with cratonic North American poles.

REGIONAL GEOLOGIC SETTING The Peninsular Ranges batholith of southern California, United States, and Baja California, México (Fig. 1), is a well-exposed array of granitoid plutons that trends northwest-southeast for 1600 km, has an average width of ≈100 km, and extends southward from lat 34°N in Alta California to lat 28°N in Baja California (Gastil, 1975, 1990; Krummenacher et al., 1975;

Silver, 1979; Silver and Chappell, 1988; Todd and Shaw, 1979). South of lat 28°N, the range is generally considered to extend to the southern tip of the Baja California Peninsula beneath the Cenozoic cover (Gastil et al., 1975).

The plutons are predominantly tonalitic and granodioritic, although compositions range overall from granitic to gabbroic (Gastil et al., 1975).

Available U/Pb zircon ages range from 120 to 90 Ma, indicating that the batholith mainly formed during a continuous and prolonged period of magmatic activity in the late Mesozoic, the locus of intrusion migrating eastward with time (Silver et al., 1969). Krummenacher et al. (1975) showed (1) that K-Ar dates on coexisting hornblende and biotite from the batholith are typically discordant (hornblende dates as much as 15 m.y. older than those for biotite); (2) that these dates are substantially younger (up to 25 m.y.) than the U/Pb zircon dates (Silver et al., 1979); and (3) that K-Ar dates of hornblende and biotite decrease systematically to the northeast, subparallel to the trend of the batholith.

On the basis of geochemical, geophysical, and lithological relationships, the Peninsular Ranges batholith has been divided into western and eastern zones (Gastil et al., 1975). The eastern zone, in which lies the Sierra San Pedro Mártir pluton, is dominated by several large and concentrically zoned (in texture, structure, and composition) plutons ranging in composition from tonalite to granodiorite (the “La Posta-type” plutons of Walawender et al., 1990) that have invaded Phanerozoic metasedimentary, metavolcanic, and metaplutonic rocks as young as Cretaceous (Gastil et al., 1975, 1991; Walawender et al., 1990). The Sierra San Pedro Mártir pluton has a surface

Geological Society of America Bulletin, June 1997

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ORTEGA-RIVERA ET AL.

Figure 2. Simplified geology of the Sierra San Pedro Mártir pluton and environs (after McCormick, 1986; Eastman, 1986) showing sample locations used in this study.

area of 400 km2 and is the third-largest of the La Posta–type intrusions (Walawender et al., 1990).

Sierra San Pedro Mártir Pluton This intrusion (Fig. 2) was emplaced along the boundary between the moderately deformed volcanic and volcaniclastic arc terrane of the AptianAlbian Alisitos Group (Santillán and Barrera, 1930; Eastman, 1986) and earlier plutonic rocks to the west, and migmatitic metasedimentary and garnet- to sillimanite-bearing metaplutonic rocks of unknown age and affin-

730

ity to the east. The metamorphic grade in the country rocks increases from greenschist to upper amphibolite facies from west to east across the 20 km width of the pluton (Gastil et al., 1975).

The Sierra San Pedro Mártir pluton is compositionally zoned, and its component facies are distributed asymmetrically (Fig. 2). Contacts are largely gradational and have been mapped on the basis of modal mineralogy (McCormick, 1986; Eastman, 1986; Gastil, 1990; Gastil et al., 1990, 1991; Walawender et al., 1990). From west to east (Fig. 2), hornblendebiotite tonalite, biotite granodiorite, and muscovite-biotite granodiorite zones are distinguished (McCormick, 1986; Eastman, 1986; Gastil, 1990;

Geological Society of America Bulletin, June 1997

SIERRA SAN PEDRO MÁRTIR PLUTON, BAJA CALIFORNIA, MEXICO

Gastil et al., 1990, 1991; Walawender et al., 1990). The last-named domain has been divided in the field into two parts on the basis of muscovite grain size. The western margin of the pluton exhibits a foliation defined by flattened inclusions, and biotite and hornblende are aligned parallel to the inward-dipping (50° to 90° to the east) contact with the country rocks (Fig. 2). Rocks of the eastern zone are massive and lack pronounced internal structure. Although the mineralogy, texture, and chemistry are in general gradational across the pluton, the muscovite-biotite granodiorite locally truncates the hornblende-biotite tonalite and therefore is younger (McCormick, 1986). Sr, Pb, and O isotopic and rare earth element (REE) data support a model of differentiation from a single parental melt with increasing crustal source rock toward the east (Gastil et al., 1994).

Tonalite from the western contact of the pluton yielded a U/Pb zircon date of 96 ± 1 Ma (McCormick, 1986), and K-Ar dates (Krummenacher et al., 1975) young toward the east (Fig. 3). Walawender et al. (1990) showed that U/Pb zircon data from four separate samples define a chord to concordia, suggesting that these zircons were derived, in part, from a ca. 1.3 Ga source region, and that emplacement of the pluton occurred at 94 Ma (Walawender et al., 1990). Conventional biotite K-Ar dates (McCormick, 1986; Walawender et al., 1990) from the three major facies of the Sierra San Pedro Mártir pluton decrease eastward, from 87 ± 3 Ma in the hornblende-biotite facies, to 83 ± 3 Ma in the biotite facies, to 72 ± 2 Ma in the most easterly, muscovite-biotite facies (Fig. 3). In a fission-track study of the late uplift history of the entire Sierra San Pedro Mártir region, Dorsey and Cerveny (1991, and R. Dorsey, 1993, personal commun.) showed that zircon ages decrease from 105 to 76 Ma and apatites decrease from 76 to 35 Ma over the elevation range of 2800 to 500 m along the Sierra San Pedro Mártir fault escarpment.

SAMPLING AND ANALYTICAL METHODS We selected 26 samples to provide an east-west transect across all phases of the pluton (Fig. 2); 13 samples were also selected from the surrounding country rocks, 3 from the western contact, and 10 from the country rocks at the northeastern contact of the pluton. From these 39 samples, 2 samples were chosen for U/Pb analysis, 35 for 40Ar/39Ar conventional step-heating, 4 for 40Ar/39Ar laser step-heating, 4 for fission-track analysis, and 6 for Al-in-hornblende geobarometry.

U/Pb Methods

Figure 3. Previous geochronological results and sample locations for the Sierra San Pedro Mártir pluton (from McCormick, 1986; Walawender et al., 1990) and sample location of new U/Pb dates.

using a VG Sector 54 multicollector mass spectrometer. Analytical methods, uncertainties, blanks, and common Pb corrections are outlined in Table 1. Zircon fractions were analyzed following mild leaching in hydrofluoric acid (HF) as described by Kimbrough et al. (1992). The HF leaching technique has the effect of removing common Pb from zircon, and by that reducing uncertainties in 207Pb*/206Pb* dates, while also preferentially dissolving more soluble high-U domains most strongly affected by recent Pb loss.

Zircon and monazite U/Pb isotopic analyses (samples 4 and 17, Fig. 2) were carried out at San Diego State University. Isotopic ratios were measured TABLE 1. U-PB DATA FOR THE SIERRA SAN PEDRO MÁRTIR PLUTON Sample name Sample 4 WC-2M Zircon (L)

Size

Weight

Pb

U

fraction

(g)

(ppm)

(ppm)

206/208

(±)

206/207

(±)

206/204

(±)

206*/238

(±)

207*/235

(±)

207*/206*

(±)

206*/238

207*/235 207*/206*

<100 (mesh)

0.0045

5.38 348.4

7.432000

(1)

19.914

(7)

6631

(66)

0.01516

(4)

0.01003

(3)

0.04799

(4)

97.0 ± 1

97.1 ± 2

0.0030

9.65 538.8

6.516000

(1)

13.156

(2)

542

(4)

0.01513

(4)

0.1019

(6)

0.04885

(22)

96.8 ± 1

98.5 ± 2 140.0 ± 11

0.0013 0.0023

195.27 757.0 98.58 792.7

0.055610 0.057102

(3) (6)

14.954 15.040

(7) (4)

712 732

(6) (6)

0.01522 0.01516

(7) (11)

0.0968 0.0968

(6) (8)

0.04614 0.04632

(21) (19)

97.4 ± 1 97.0 ± 1

93.8 ± 2 93.8 ± 2

Sample 17 SSPM-7-25-4 Zircon (L) >200 (mesh) Monazite Bulk a Monazite Bulk b

Lead isotopic comp. corrected for fractionation

Radiogenic ratios

Apparent ages (Ma)

99.0 ± 20

5.0 ± 11 14.0 ± 10

Note: Zircon (L) indicates leaching of fractions with HF on hot plate at ≈100 °C for two days prior to dissolution, following methods outlined by Kimbrough et al. (1992). Separation of U and Pb was done using HCl column chemistry. Concentrations were determined using a mixed 208Pb/235U spike. Lead isotopic compositions corrected for ≈0.10% ± 0.05% per mass unit mass fractionation, based on replicate analyses of NBS981 and NBS983. Errors in 206Pb/204Pb measurements were minimized by use of an ion-counting, Daly-multiplier system, for detection of small 204Pb signal, and are typically <1%. Decay constants used 1.55125E–10 a = 238U and 9.8485E–10 a = 235U. Present-day 238U/235U = 137.88. Corrections for common lead were made using the model of Stacey and Kramers (1975). Total lead blanks average ≈25 pg. Pb* = radiogenic lead. The 2σ value for analytical errors is shown in parentheses behind radiogenic ratios; e.g., 0.02246 (6) means 0.02246 ± 0.00006. Errors were computed using the data reduction program PBDAT of Ludwig (1989). Accuracy of 206*Pb/238U dates is better than ±0.5% (ca. ±1 m.y.) based on long-term reproducibility of a standard zircon sample (OU491271). Uncertainties in the 207*Pb/206*Pb dates are stated at the 2σ level and assumed a ±0.1 uncertainity in the common 207Pb/204Pb.

Geological Society of America Bulletin, June 1997

731

ORTEGA-RIVERA ET AL. TABLE 2. 40AR/39AR DATES FOR THE SIERRA SAN PEDRO MÁRTIR PLUTON AND ITS COUNTRY ROCKS Sample number

Sample name

Mineral

Country rocks Western contact 1 CONT-2-90 hb 1 CONT-2-90 hb 1 CONT-2-90 bt 1 CONT-2-90 bt 2 CONT-1.1 hb 2 CONT-1.1 bt 3 CONT-1.A hb 3 CONT-1.A bt Sierra San Pedro Mártir pluton Hornblende-biotite tonalite zone 4 WC-(2M) *hb 4 WC-(2M) hb 4 WC-(2M) Hb 4 WC-(2M) Hb 4 WC-(2M) hb 4 WC-(2M) bt 4 WC-(2M) bt 4 WC-(2M) bt 4 WC-(2M) bt 4 WC-(2M) bt 5 SSPM-7-26-7-88 hb 5 SSPM-7-26-7-88 bt 35 SSPM-13 +hb 35 SSPM-13 +bt 6 CORONA hb 6 CORONA bt 7 SSPM-7-26-6-88 hb 7 SSPM-7-26-6-88 bt 8 SSPM-H.9-14.5 bt 9 SSPM-7-26-4-88 hb 9 SSPM-7-26-4-88 bt 10 E.8-20.6 *hb 10 E.8-20.6 hb 10 E.8-20.6 *bt 10 E.8-20.6 bt 13 G.4-90-3 hb 13 G.4-90-3 bt 36 SSPM-18 +hb 36 SSPM-18 +bt 16 SSPM-LV-8 *hb 16 SSPM-LV-8 hb 16 SSPM-LV-8 *bt 16 SSPM-LV-8 bt 18 SSPM-G.0-20 bt 18 SSPM-G.0-20 bt 21 SSPM-7-24-1-88 *hb 21 SSPM-7-24-1-88 hb 21 SSPM-7-24-1-88 bt 22 SSPM-7-24-2-88 *hb 22 SSPM-7-24-2-88 hb 22 SSPM-7-24-2-88 bt 22 SSPM-7-24-2-88 bt 23 SSPM-7-24-3-88 hb 23 SSPM-7-24-3-88 hb 23 SSPM-7-24-3-88 bt 23 SSPM-7-24-3-88 bt

40Ar/39Ar

Lab run

Size mesh

C/P

Integrated date (Ma)

AOR-288 AOR-1114 AOR-291 AOR-1106 AOR-746 AOR-760 AOR-757 AOR-766

80/100 80/100 80/100 80/100 80/100 80/100 100/120 100/120

104/44 104/45 104/42 104/43 113/30 113/40 113/10 113/20

97.4 98.7 94.6 94.5 96.0 93.1 95.7 93.1

1.0 2.3 0.9 1.0 1.7 1.2 1.7 1.0

97.4 98.7 95.5 95.6 96.6 94.1 95.7 93.8

1.0 2.3 0.9 1.0 1.4 1.1 1.7 1.0

100.0 100.0 86.1 93.9 87.2 87.7 100.0 90.5

343.4 344.2 374.1 294.0 155.3 366.6 266.4 201.8

67.1 1284.2 128.7 4.6 1804.0 1526.2 353.3 98.9

AOR-10 AOR-181 AOR-709 AOR-1085 AOR-1740 AOR-38 AOR-313 AOR-1157 AOR-1178 AOR-1743 AOR-90 AOR-49 M686-S M714-S AOR-141 AOR-52 AOR-841 AOR-840 AOR-123 AOR-32 AOR-41 AOR-6 AOR-177 AOR-7 AOR-210 AOR-947 AOR-941 M684-S M688-S AOR-8 AOR-178 AOR-4 AOR-234 AOR-1096 AOR-1124 AOR-22 AOR-191 AOR-56 AOR-13 AOR-188 AOR-201 AOR-1173 AOR-691 AOR-1092 AOR-695 AOR-1119

25/40 25/40 25/40 25/40 25/40 25/40 25/40 25/40 25/40 25/40 25/40 25/40 N.D. N.D. 25/40 25/40 25/40 25/40 40/60 25/40 25/40 40/60 40/60 40/60 40/60 25/40 25/40 N.D. N.D. 25/40 25/40 25/40 25/40 40/60 >10 25/40 25/40 25/40 40/60 40/60 40/60 40/60 40/60 40/60 40/60 40/60

79/54 89/16 A104/25A A105/8A0 A143/4A0 88/14 B104/25B B105/8B0 88/13 B143/4B0 88/69 88/35 N.D. N.D. 88/40 88/39 113/24 113/25 87/8 88/21 88/26 79/49 88/21 79/48 89/8 113/26 113/27 N.D. N.D. 79/50 89/19 79/53 89/10 104/80 104/50 79/60 89/24 88/47 79/59 89/25 87/57 87/58 104/13 104/14 104/16 104/15

94.4 97.2 94.7 93.4 92.4 92.8 92.8 94.2 92.9 93.2 92.9 91.4 92.9 92.6 91.5 90.5 94.7 91.5 90.9 94.9 89.9 94.7 92.1 89.5 89.0 91.7 88.4 92.8 87.2 91.0 91.4 88.0 86.1 87.4 88.2 91.9 93.8 92.3 90.4 91.8 86.4 88.7 91.4 90.0 88.1 89.0

1.0 2.0 2.6 1.4 5.5 0.7 1.0 1.2 0.8 2.0 1.6 0.7 4.1 0.7 1.0 0.6 2.8 0.8 0.8 4.4 0.6 5.2 0.7 1.1 0.2 3.8 0.9 1.9 0.3 3.0 0.8 0.6 0.3 1.0 1.3 3.3 0.6 0.5 1.2 0.8 0.7 1.8 2.2 2.0 1.2 0.7

94.7 94.6 94.6 94.4 94.5 93.0 93.5 94.7 93.7 93.2 93.0 92.0 94.1 92.8 92.1 91.0 94.7 92.1 91.1 94.0 89.9 94.0 92.4 90.7 89.3 94.3 89.0 91.7 87.9 92.0 92.2 89.2 87.1 87.7 88.2 92.9 93.8 92.3 90.4 91.8 86.7 88.3 91.4 90.8 87.9 89.0

0.8 0.6 2.7 1.2 4.2 0.7 1.0 1.2 0.8 2.0 1.6 0.7 3.8 0.7 0.8 0.6 2.8 0.8 0.8 1.8 0.6 2.5 0.7 0.5 0.2 1.4 0.8 1.7 0.3 1.8 0.8 0.5 0.3 1.0 1.3 2.3 0.6 0.5 1.0 0.8 0.7 2.2 2.2 1.9 1.2 0.7

97.7 99.7 95.1 94.4 55.3 99.4 93.1 92.4 90.0 100.0 99.3 82.8 94.5 90.9 99.2 97.9 100.0 93.5 98.2 99.6 99.5 90.1 84.6 87.7 95.1 76.8 95.2 96.4 86.9 88.4 94.6 79.3 82.4 91.2 100.0 92.7 89.3 99.7 99.0 100.0 95.2 76.8 100.0 97.4 97.1 100.0

* 296.5 295.3 291.4 297.2 271.4 295.8 292.8 244.3 294.6 265.8 332.5 * * 285.4 301.5 299.4 314.6 295.7 296.8 309.8 * 275.9 * 205.6 238.5 290.3 * * * 279.9 * 179.9 296.4 441.0 * 316.7 287.7 * 317.6 223.2 602.4 290.3 263.3 382.7 233.9

* 18.9 91.0 11.5 161.5 93.3 2.3 7.1 122.6 26.9 217.2 168.7 * * 38.6 18.7 34.7 208.8 30.5 38.0 48.8 * 112.5 * 20.8 404.0 107.5 * * * 89.7 * 92.5 7.6 572.3 * 212.8 28.4 * 76.6 19.8 374.8 276.3 145.6 127.9 160.1

Method

The 99 mineral separates (24 hornblende, 61 biotite, 13 muscovite, and 1 plagioclase) dated at Queen’s University (Table 2, Fig. 2) were purified using a Frantz magnetic separator and heavy organic liquids. Separates, replicates, and flux-monitors (LP-6) and samples of known age (i.e., interlaboratory and intralaboratory monitors) were individually packaged in disc-shaped Al-foil pouches and stacked in an aluminum irradiation cans (11.5 cm long and 2.0 cm in diameter). The positions of samples and monitors were carefully measured before loading into the irradiation cans.

The cans with the samples and monitors were then irradiated with fast

732

2σ error Plateau 2σ error date (Ma)

Volume 39Ar (%)

Initial 2σ error 40/36

neutrons in position 5C of the McMaster University Nuclear Reactor (Hamilton, Ontario) for 14.5 hr. Typically 10 or more monitors and 2 to 4 replicates were used to determine the neutron-flux. J-values for individual samples were determined by a second-order polynomial interpolation. In addition, two or more replicate samples were loaded into each irradiation can to aid in intercan comparison. To accommodate all samples, 10 separate irradiations were required. After irradiation, mineral separates, replicates, and monitors were loaded into niobium crucibles and heated in a pure-silica tube (GE214) within a Lindberg furnace. The bakeable, ultrahigh vacuum, stainless steel, argon-extraction system was operated online in a substantially modified, Associated Electrical Industries MS-10 mass spectrometer

Geological Society of America Bulletin, June 1997

SIERRA SAN PEDRO MÁRTIR PLUTON, BAJA CALIFORNIA, MEXICO TABLE 2. (Continued) Sample number

Sample name

Mineral

Sierra San Pedro Mártir pluton Biotite granodiorite zone 11 SSPM-7-26-3 *bt 11 SSPM-7-26-3 bt 37 SSPM-20 +bt 12 SSPM-7-26-2-88 bt 14 SSPM-G.4-24.3 bt 15 SSPM-G.1.26 bt 19 SSPM-G.0-29 bt 19 SSPM-G.0-29 bt Sierra San Pedro Mártir pluton Muscovite-biotite granodiorite zone 29 SSPM-7-23-4 ms 38 SSPM-21 +bt 17 SSPM-7-25-4(88) ms 17 SSPM-7-25-4(88) bt 17 SSPM-7-25-4(88) bt 17 SSPM-7-25-4(88) bt 17 SSPM-7-25-4(88) bt 17 SSPM-7-25-4(88) bt 17 SSPM-7-25-4(88) bt 17 SSPM-7-25-4(88) bt 17 SSPM-7-25-4(88) bt 17 SSPM-7-25-4(88) bt 20 CT-47 ms 20 CT-47 ms 20 CT-47 bt 20 CT-47 bt 24 CORE ms 24 CORE bt 24 CORE bt 24 CORE bt 24 CORE pg Country rocks Northeastern contact 25 SSPM-VO-9 ms 25 SSPM-VO-9 bt 26 SSPM-VO-9-5 ms 26 SSPM-VO-9-5 bt 26 SSPM-VO-9-5 bt 27 SSPM-VO-15 ms 27 SSPM-VO-15 bt 27 SSPM-VO-15 bt 28 SSPM-VO-15-1 ms 28 SSPM-VO-15-1 bt 28 SSPM-VO-15-1 bt 30 SSPM-VO-15-4 ms 30 SSPM-VO-15-4 bt 31 SSPM-VO-16.1 ms 31 SSPM-VO-16.1 bt 32 SSPM-VO-16A bt 32 SSPM-VO-16A bt 40 SSPM-VO-16.D bt 34 SSPM-VO-17-2 ms 34 SSPM-VO-17-2 bt 33 SSPM-VO-17-A ms 33 SSPM-VO-17-A bt

Lab run

Size mesh

C/P

Integrated date (Ma)

2σ error Plateau 2σ error date (Ma)

Volume 39Ar (%)

Initial 2σ error 40/36

AOR-12 AOR-221 M704-S AOR-46 AOR-45 AOR-127 AOR-176 AOR-1246

25/400 25/400 25/400 25/400 25/400 25/400 40/600 80/100

79/56 89/22 N.D. 88/31 88/37 87/15 87/31 112/18

89.3 89.6 88.6 90.6 90.7 89.9 88.5 90.3

1.9 0.3 0.7 0.6 0.9 0.9 1.1 1.5

89.6 89.9 89.1 90.6 90.7 90.4 90.1 90.8

1.6 0.3 0.6 0.6 0.9 0.9 1.2 1.5

82.7 92.5 85.8 100.0 100.0 93.2 88.0 85.8

* 265.5 * 309.6 289.0 417.4 325.9 189.7

* 31.9 * 54.7 79.0 181.2 168.9 223.9

AOR-98 M706-S AOR-60 AOR-50 AOR-133 AOR-496 AOR-497 AOR-501 AOR-502 AOR-503 AOR-504 AOR-545 AOR-81 AOR-957 AOR-83 AOR-756 AOR-750 AOR-27 AOR-274 AOR-751 AOR-763

25/400 N.D. 60/800 25/400 25/400 25/400 25/400 25/400 25/400 25/400 25/400 25/400 80/100 40/100 40/600 40/600 80/140 25/400 25/400 25/400 25/400

88/68 N.D. 88/58 88/29 87/18 87/19 87/19 87/19 87/19 88/28.1 88/28.1 88/28.2 88/72 113/19 88/66 113/20 113/21 88/24 88/25 113/23 113/22

87.4 88.4 90.0 89.8 89.9 89.5 89.6 90.2 90.0 89.1 89.6 89.0 88.5 90.2 87.3 91.5 93.2 87.8 88.7 87.9 89.4

0.7 0.9 0.5 0.6 1.1 1.0 0.6 0.9 0.5 0.6 0.2 0.5 0.7 1.5 0.7 0.9 1.3 0.6 0.6 0.8 2.2

87.5 89.0 90.0 90.0 90.4 89.5 89.6 90.2 90.0 89.1 89.6 89.0 88.5 90.4 88.3 91.5 88.3 88.3 89.2 88.5 88.2

0.7 0.8 0.5 0.6 1.1 1.0 0.6 0.9 0.5 0.6 0.2 0.5 0.7 1.5 0.9 0.9 1.1 0.6 0.6 0.8 1.1

99.0 91.0 100.0 97.0 94.9 *100.0* *100.0* *100.0* *100.0* *100.0* *100.0* *100.0* 100.0 98.1 74.5 68.8 90.5 99.2 96.5 95.8 73.1

276.3 * 320.9 311.7 304.1 N.D. N.D. N.D. N.D. N.D. N.D. N.D. 327.3 236.2 338.2 186.3 311.6 294.3 191.0 474.0 299.9

29.5 * 18.9 43.9 121.9 N.D. N.D. N.D. N.D. N.D. N.D. N.D. 34.2 44.3 201.1 126.8 225.3 17.5 152.2 491.6 16.2

AOR-34 AOR-40 AOR-218 AOR-131 AOR-271 AOR-1129 AOR-47 AOR-1109 AOR-59 AOR-43 AOR-244 AOR-1103 AOR-724 AOR-96 AOR-213 AOR-57 AOR-269 AOR-1093 AOR-952 AOR-835 AOR-300 AOR-296

80/100 80/100 60/800 60/800 60/800 40/600 40/600 40/600 80/100 60/800 60/800 60/800 60/800 80/100 25/400 80/100 25/400 60/800 60/800 60/800 60/800 60/800

88/22 88/15 87/12 87/6 87/7 104/32 88/19 104/33 88/62 88/17 88/43 104/52 104/46 88/65 89/2 88/48 89/1 104/10 112/39 11/37 104/30 104/31

88.6 88.6 92.4 91.1 91.8 90.1 89.7 90.6 85.6 87.9 87.5 90.5 91.2 85.4 86.3 88.3 88.5 84.1 89.0 85.1 90.4 60.6

0.6 0.8 1.4 0.9 0.9 1.1 0.6 0.9 0.5 0.6 0.5 0.9 0.9 0.6 0.3 0.5 0.8 1.5 4.8 1.2 0.9 0.9

88.7 88.2 91.1 91.4 90.9 90.1 89.6 90.8 85.2 87.9 87.3 89.6 90.8 85.5 86.5 88.3 88.5 84.3 84.2 85.4 88.3 70.6

0.6 0.8 1.2 0.9 0.9 1.1 0.6 0.9 0.5 0.6 0.5 1.0 0.9 0.6 0.3 0.5 0.8 1.5 1.4 1.2 0.9 1.7

98.4 90.5 80.6 98.6 98.3 100.0 97.7 99.2 90.4 100.0 88.4 82.3 80.6 98.6 88.8 100.0 100.0 99.6 95.4 91.7 79.9 36.7

301.3 332.4 547.5 272.3 227.6 329.4 471.8 293.7 315.8 324.9 298.9 294.0 495.2 279.1 277.8 318.9 314.0 293.3 296.8 279.1 452.7 97.2

58.3 150.7 844.7 70.8 1223.9 65.4 143.9 27.1 39.1 51.7 79.2 62.0 497.9 63.9 14.2 41.6 643.8 8.5 30.5 94.5 71.6 54.5

Note: C/P = Can/position; minerals: hb—hornblende, pg—plagioclase, ms—muscovite, bt—biotite, +bt—laser step-heating, *bt—no isotope correlation data. The Ar isotope blanks were not well constrained, and no correlation plots were possible. N.D.—no data. An asterisk in the last two columns means that no isotope correlation data are available.

run in the static mode. For the isochron correlation analyses, step-heating blank runs were measured. The 40Ar blank volume (less than 2% for hornblendes and 1% for biotites) varied between 0.4 × 10–10 (lowest temperature steps) and 0.9 × 10–10 cm3 STP (highest temperature steps), the 37Ar and 39Ar blanks were very small compared to the signals measured in the analyses of the samples, and the 36Ar blanks were at or below the limit of detection of the MS-10.

Six mineral separates were analyzed by single-grain laser step-heating at Nice (two hornblende, four biotite; samples 35–38, Fig. 2). The separates were prepared using a Frantz magnetic separator and handpicking to select

the freshest grains. Five or six grains of the mineral separate were wrapped in 11 × 5 mm aluminum envelopes. Samples and six similarly wrapped flux-monitors were loaded into an irradiation can; the samples were vertically arranged in one level and six samples of monitor MMhb were distributed horizontally within this level. The can was irradiated for 69.5 hr in position 5C of the McMaster University Nuclear Reactor. After irradiation the samples were mounted on a copper sample holder, beneath the Pyrex window of a stainless steel chamber, connected to an ultra-high vacuum purification system, and were uniformly heated with a defocused 5.5W Coherent Innova 70-4 continuous argon-ion laser. The evolved gas, after purification,

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ORTEGA-RIVERA ET AL.

was transferred to an online, 120° mass spectrometer, consisting of an M.A.S.S.E. flight tube, a Bäur Signer source, and a Balzers SEV217 electron multiplier (total gain: 5 × 1012), and analyzed in static mode. Blanks, made routinely each first and third run, were subtracted from the subsequent sample gas fractions. Typical system-blank values were 4.0 × 10–12, 4.0 × 10–14, 1.4 × 10–13, and 6.3 × 10–14 cm3 STP for masses 40, 39, 37, and 36, respectively. Ruffet et al. (1991) documented the laser-fusion technique in detail. All samples were irradiated in the same level. J-uncertainties for each level were less than 0.15%, and the average was used to calculate the ages at that level.

Measured mass spectrometric ratios for both systems were extrapolated to zero time, normalized to the 40Ar/36Ar atmospheric ratio, and corrected for neutron-induced 40Ar from potassium, and 39Ar and 36Ar from calcium.

Dates and errors were calculated using formulae given by Dalrymple et al.

(1981) and the constants recommended by Steiger and Jäger (1977). All errors shown represent the analytical precision at 2σ, and include the analytical uncertainties of the monitor analyses (J-uncertainties), but assume that the error for the age of the monitor is zero. A conservative estimate of 0.5% in the error of the J-value should be added for comparison with samples using different a monitor. The Queen’s University dates are referenced to the LP-6 biotite standard at 128.5 Ma (Roddick, 1983), and the Nice’s dates are referenced to MMhb1 hornblende standard at 520.4 Ma (Samson and Alexander, 1987). Mineral replicates (e.g., samples 4 [hornblende and biotite], and 17 [biotite]; see Table 2 and Data Repository1) irradiated within the same can and in different cans were analyzed to monitor reproducibility. The standard deviations of 0.11, 0.64, and 0.48 Ma, respectively, determined for samples irradiated in different cans supports our estimates of the analytical precision quoted for individual analyses. (For information on size, blanks, and errors in J, see footnote 1.)

Fission-Track Method Apatite separates from samples 2, 4, 10, and 24 (Fig. 2) were obtained using standard heavy liquid and magnetic mineral separation techniques at Queen’s University. The fission-track analyses were done at the Fissiontrack Research Laboratory of Dalhousie University. Mineral separation, grain mounting, polishing, etching, irradiation, and counting were all done by standard techniques using the external detector method and are documented in Ravenhurst and Donelick (1992) and Ravenhurst et al. (1994).

Microprobe Analysis and Aluminum-in-Hornblende Geobarometry Mineral analyses for hornblende barometry (Hammarstrom and Zen, 1986) were carried out at Queen’s University on only six samples from rocks of the hornblende facies (Fig. 2, samples 4, 5, 6, 10, 16, and 22) that contain the required assemblage quartz + plagioclase + K-feldspar + hornblende + biotite + titanite + an oxide phase (magnetite or ilmenite). Five different calibrations were used to estimate pressures (for the empirical calibrations used, see Hammarstrom and Zen, 1986, and Hollister et al., 1987; for the experimental calibrations, see Johnson and Rutherford, 1989, Thomas and Ernst, 1990, and Schmidt, 1992). Mineral analyses were done with an ARL-SEMQ electron microprobe using an energy-dispersive spectrometer (EDS). Operating conditions were maintained at an accelerating voltage of 15 kV and a beam current of 75 nÅ, with a carbon collimator with an effective diameter of 0.32 cm (0.125 in.), so the detector area is less than the normal area suggested by the manufacturer. Kaersutite (Smithsonian Insti1GSA Data Repository item 9727, index table and 40Ar/39Ar data and spectra, is available on request from Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301. E-mail: editing@geosociety.org.

734

tution USNM 143965) was used as a primary standard for the major elements in the hornblende analyses. Mineral compositions were determined from an average of at least three rim analyses (100 s count time) from different points on the same grain or neighboring grains. Structural formulae were computed using MINPROBE software developed by D. M. Carmichael (Queen’s University), with Fe3+/Fe being the average of an upper and lower limit imposed by amphibole stoichiometry.

DISCUSSION OF RESULTS U/Pb Geochronology U/Pb isotopic data (Table 1, Fig. 3) are reported here from two samples of the Sierra San Pedro Mártir pluton. Sample 4, representative of the hornblende-biotite tonalite, was collected at the western margin of the pluton where it is in contact with pervasively mylonitized wall rock. Sample 17, representative of the muscovite-biotite granodiorite, was collected from near the geographic center of the pluton, ≈2 km east of the contact with the biotite granodiorite zone. A single <100 mesh, HF-leached, zircon fraction from sample 4 and two bulk fractions of monazite and a single >200 mesh, HF-leached, zircon fraction from sample 17 were analyzed (Fig. 2). The data and calculated dates are reported in Table 1. The zircon fraction from sample 4 was concordant, with a 206Pb*/238U date of 97.0 Ma and a 207Pb*/206Pb* date of 99 ± 2 Ma. A single >200 mesh, HF-leached, zircon fraction from sample 17 yielded discordant results, with a 206Pb*/238U date of 96.8 Ma and a 207Pb*/206Pb* date of 140 ± 11 Ma. Two U/Pb analyses of monazite from sample 17 plotted above the concordia. Such discordance in relatively young monazites has been attributed to the preferential incorporation of Th relative to Pb in the monazite crystal lattice (Parrish, 1990), leading to an excess of 206Pb (derived from initially incorporated 230Th, an intermediate decay product of 238U). The 207Pb*/235U dates would have been unaffected by the incorporation of 206Pb 207Pb*/235U dates from both fractions are identical at 93.8 Ma (Table 1).

40Ar/39Ar

Geochronology

For almost all the analyses, the age spectra are either completely flat, or have dates that climb over the first ≈10% of 39Ar released to a well-defined plateau for the remaining ≈90% of the spectrum. Integrated and plateau dates for all analyses are presented from west to east in Table 2; representative age spectra for samples from the country rocks and the pluton are illustrated in Figure 4 from west to east. There is excellent agreement between laser and conventional step-heating results. There is also excellent agreement for replicate analyses from samples irradiated within the same can and in different cans (e.g., samples 4 and 17) (Table 2; see also footnote 1). The isotope correlation analyses agree with, but are less reliable than, the plateau dates due to data clustering. The complete 40Ar/39Ar data set, sample localities (in UTM coordinates, Zone 11R), elevations, isotope correlation plots, and age spectra are available from the GSA Data Repository (see footnote 1).

The plateau dates for hornblende, biotite, muscovite, and plagioclase are plotted on the sample location map in Figure 5 (a–c). Replicate results for individual samples have been averaged. These plateau dates are also plotted with respect to east-west locations ion Figure 6 (a–c).

Hornblende. The plateau dates for hornblende range from 98 to 90 Ma. From west to east, the oldest plateau dates (samples 1, 2, and 3, Fig. 2) range from 98 to 95 Ma (Table 2, Fig. 4a) and belong to the metaplutonic rocks of the western contact zone. Hornblende plateau dates of the hornblende-biotite tonalite zone (outer zone) range from west to east from 95 to 90 Ma (Figs. 4, b–d, 5a, and 6a), the oldest hornblende dates (95 Ma) being

Geological Society of America Bulletin, June 1997

SIERRA SAN PEDRO MÁRTIR PLUTON, BAJA CALIFORNIA, MEXICO

Figure 4. Representative 40Ar/39Ar age spectra of mineral pairs from (a) the western country rocks; (b) the western contact of the pluton; (c, d, and e) west to east across the Sierra San Pedro Mártir pluton; (f and g) the eastern country rocks. AOR number—lab number; S number—sample number; PD—plateau date (2σ errors); Hb—hornblende, Bt—biotite, Ms—muscovite. Plateau segment is indicated by arrows. Data Repository (see text footnote 1) has complete data set.

from rocks at the western contact of the pluton (Fig. 4b). The hornblende dates are older in the country rocks to the west of the pluton, supporting the interpretation of postmetamorphic emplacement (Figs. 4a and 5, a and b; Table 2; and Data Repository [see footnote 1]).

Biotite and Muscovite. In general, biotite dates vary smoothly from west to east from 96 to 88 Ma. Biotite plateau dates for the country rocks at the western contact range from 96 to 94 Ma; across the pluton they decrease from 94 Ma to 88 Ma; and those from the country rocks northeast of the pluton decrease from 92 Ma at the contact to 70 Ma toward the north (Figs. 4, 5b, 6b; Table 2; and Data Repository [see footnote 1]). The plateau dates for the three muscovite-biotite pairs from the easternmost phase of the pluton are essentially concordant (Fig. 4e). Most plateau dates for biotitemuscovite mineral pairs from the country rocks northeast of the pluton are also essentially concordant (Fig. 5, b and c; Table 2; and Data Repository [see footnote 1]). Due to their low closure temperatures they would not be expected to retain metamorphic ages corresponding to the probable depth of emplacement, and they do not show older dates toward the north. Among

these eight biotite-muscovite mineral pairs, only two, both from the vicinity of the Sierra San Pedro Mártir fault (Fig. 2), are discordant. The youngest biotite date is from a sample from the country rock at the northernmost end of the north-south transect and yields the only substantially disturbed spectrum with an anomalously young integrated date of 61 ± 1 Ma (Figs. 2, 4g, and 5b; sample 33, Table 2).

Plagioclase. One plagioclase separate from the muscovite-biotite granodiorite zone near the eastern end of the transect yields a well-defined plateau date of 88 Ma (Fig. 2, sample 24). This date is concordant with biotite and muscovite plateau dates for the same sample.

Fission-Track Geochronology Analytical data for the four apatite samples (dates and confined tracklength data) are presented and summarized in Table 3. The dates are plotted with respect to sample location in Figure 7. Dates for three apatite separates from the pluton decrease from 72 ± 8 Ma to 57 ± 15 Ma from west to east.

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ORTEGA-RIVERA ET AL.

Figure 5. Maps showing 40Ar/39Ar plateau dates; (a) hornblende, (b) biotite, and (c) muscovite and plagioclase. Plateau dates for replicate analyses have been averaged. Samples are listed in Table 2 from west to east.

A sample from the metaplutonic country rocks near the western margin of the pluton yields an apatite date of 59 ± 10 Ma. Track lengths average 13.7 µm. There is no sensible correlation between present elevations of the sample localities and uncorrected apatite fission-track ages (Fig. 7), which are very similar within errors. Fission-track annealing in apatite by tracklength reduction is a thermally controlled process. The kinetics of this process have been studied experimentally and several annealing models have been developed (e.g., Laslett et al., 1987; Carlson, 1990). Using the annealing model of Green et al. (1989), modified by Crowley et al. (1991), Willet (1992) designed an algorithm to predict apatite fission-track age and length distributions. The algorithm produces a theoretical age and tracklength distribution for a given thermal history, and compares these to measured age and track-length distributions using a Kolmogorov-Smirnov (K-S) statistic at an acceptable significance level (i.e., 0.95) (for more details see Ravenhurst et al., 1994). The inverse model has been used to model the track-length distributions and calculates apatite fission-track ages for the samples with the most track-length data. Track-length data show that the apatites have undergone moderate annealing.

Sample 2 (Fig. 2, Table 3), which has sufficient track-length measurements to yield satisfactory statistics, has been modeled to elucidate the possible cooling history of the suite using the Willet algorithm. The actual track-length distribution (histogram) and modeled distributions (curve) for the mean are shown in Figure 8, a and b, respectively. A time-temperature

736

envelope (upper and lower curves) was generated by the model for the 250 statistically acceptable solutions (Fig. 8b). The middle curve in Figure 8b is the exponential mean solution. The model was constrained to begin at 95 Ma and 500 °C to match the 40Ar/39Ar hornblende age from the same sample and to be an ambient temperature of 25 °C at present. Because the track-length distribution appears unimodal (Fig. 8a), cooling-only solutions are provided.

Aluminum-in-Hornblende Geobarometry The empirical correlation between the pressure of emplacement of calcalkaline granitic plutons and the total aluminum content of hornblende equilibrated with quartz was investigated by Hammarstrom and Zen (1986) and Hollister et al. (1987). Experimental calibrations were presented by Johnson and Rutherford (1989), Thomas and Ernst (1990), and Schmidt (1992). The geobarometer is only valid when (1) the assemblage quartz + plagioclase + K-feldspar + hornblende + biotite + titanite + an oxide phase (magnetite or ilmenite) is coexistent within the rock; (2) the plagioclase in the sample has a constant rim composition in the range An25–35; (3) the analyses are limited only to the rim composition of hornblende; and (4) the pressure of crystallization is above 2 kbar (Hammarstrom and Zen, 1986; Hollister et al., 1987).

From the 40 samples collected for this study, only 6 samples (Fig. 9) had

Geological Society of America Bulletin, June 1997

SIERRA SAN PEDRO MÁRTIR PLUTON, BAJA CALIFORNIA, MEXICO

Figure 7. Fission-track results for apatite: location, date (2σ error), and elevation in meters.

the correct mineral assemblage (i.e., quartz + K-feldspar + hornblende + biotite + titanite + an oxide phase [magnetite or ilmenite] + primary epidote + plagioclase with a composition constant between An24 and An36). Aluminum contents for the rim of the hornblendes were determined for these six samples. On the basis of these compositions (Table 4), pressures were calculated employing five different calibrations (Table 5). These results show, regardless of the calibration used, that within the typical 2σ error limit of ±1.2 kbar, there is no appreciable difference in pressure at the time of crystallization across the exposed surface of the pluton (Fig. 9). Using the highest and lowest estimates (Table 5), depths between 12 and 20 km are used in the discussion.

INTERPRETATION OF RESULTS Age of Emplacement

Figure 6. 40Ar/39Ar plateau dates, with 2σ errors, projected onto an east-west line. (a) Hornblende, (b) biotite, and (c) muscovite and plagioclase; ellipse—hornblende; triangle—biotite; rectangle—muscovite; white circle—plagioclase; and white squares—laser probe dates. Samples are listed in Table 2 from west to east.

The maximum crystallization age of the hornblende-biotite tonalite is given by the 99 ± 2 Ma 207Pb*/206Pb* zircon date from sample 4. Coupling this with the concordant 206Pb*/238U date of 97.0 Ma, a conservative estimate of the age of emplacement, allowing for the possibility of slight Pb loss, is 97 +4/–1 Ma. The coincident 93.8 +1/–1 Ma 207Pb*/235U monazite dates from sample 17 are interpreted as reflecting the time of closure of the monazite system and most probably representing the crystallization age of the muscovite-biotite granodiorite. This supports the contention of McCormick (1986) from preliminary U/Pb zircon dates that these phases crystallized diachronously and appears to contradict the synchronous emplacement model of Walawender et al. (1990). An alternative interpretation of the younger monazite dates is that the muscovite-biotite granodiorite cooled

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737

ORTEGA-RIVERA ET AL. TABLE 3. APATITE FISSION-TRACK DATES AND TRACK LENGTH DATA FOR THE SIERRA SAN PEDRO MÁRTIR PLUTON AND COUNTRY ROCKS Sample Zone 11R Elevation Sample no. (m) name UTM-E UTM-N 2

631855

3426843

1620

4 10

632349 641676

3426113 3428342

1700 2440

24

650177

3418481

2100

SSPMCONT1.1 WC(2M) SSPME.820.6 CORE

Lab run

Mineral G-C

Ns

Ni

FT92-156

Apatite

12

206

635

FT92-157 FT92-158

Apatite Apatite

11 11

702 178

FT92-159

Apatite

8

82

Rhos Rhoi

Chi-2

Nd

F-T date

± 2 sigma Tracks Tracks ± St-error no. length

32.0

0.5921

5312.0

59.0

±

10.0

27

13.8 ±

0.5

1772 17.0 42.8 512 4.05 11.9

0.2468 0.7148

5312.0 5312.0

72.0 62.0

± ±

7.4 11.2

88 35

13.6 ± 14.0 ±

0.2 0.4

260

0.9182

5312.0

57.0

±

14.8

16

13.4 ±

0.8

10.4

5.36 17.0

Note: A sumary of the track count data. All samples passed the chi-square test at the 95% confidence level and ages were calculated using pooled statistics. All analyses were done by G. Li. A value of 106.9 ± 2.4 was used for the zeta factor. Fission-track date error estimates are at the 95% (2-sigma) confidence level. Abbreviatons are as follows: F-T date = fission-track date in Ma, G-C = grains counted, Chi-2 = chi-square, Ns = number of spontaneous (fossil) tracks, Ni = number of induced tracks, Nd = number of flux dosimeter (CN-1) tracks counted, Rhos = density of spontaneous tracks (10E5/cm2), Rhoi = density of induced tracks (10E5/cm2), Tracks length = mean track lengths (µm), Tracks (number) = number of tracks counted, and St-error = standard error (µm, 2-sigma).

more slowly than the marginal hornblende-biotite tonalite. Thus, synchronous crystallization of all phases of the pluton cannot be completely ruled out. The 207Pb*/206Pb* date of 140 ± 11 Ma for zircon sample 17 from the muscovite-biotite granodiorite indicates an inherited crustal component (similar to results reported in Walawender et al., 1990); therefore, the 206Pb*/238U date from this fraction represents only a maximum crystallization age for the sample, assuming no lead loss for the zircon. U/Pb ages from the two samples reported here suggest a difference in crystallization age of ≈3 m.y. for these two zones of the Sierra San Pedro Mártir pluton.

However, the ages just overlap within the stated uncertainties and therefore, the data do not clearly distinguish two separate intrusive events. Thus, the results from these samples do not resolve the debate.

Tectonothermal History from 40Ar/39Ar Dating

This study was initiated to determine whether the Sierra San Pedro Mártir pluton has undergone significant east-side-up tilting. Because the 40Ar/39Ar results compose the most comprehensive data set, they have been combined with the sparse U/Pb data to provide a framework to develop possible models of the postemplacement tectonothermal history of the Sierra San Pedro Mártir pluton. These models are then evaluated in the light of fission-track and geobarometry studies.

The excellent plateau dates obtained from virtually all hornblende, biotite, muscovite, and plagioclase separates for samples from both the east-west and north-south transects are most readily interpreted as cooling ages (Fig. 4, a to f; Data Repository [see footnote 1]). These 40Ar/39Ar cooling ages are combined with estimated argon closure temperatures (hornblende: 500 °C [Harrison, 1981]; muscovite: 350 °C [Purdy and Jäger, 1976]; biotite: 280 °C [Harrison et al., 1985]; plagioclase: 220 °C [Harrison and

Figure 8. Results of inverse fission-track modeling of sample 4 (FT92-157 [WC-2M]). (a) Measured (histogram) and exponential-mean modeled (smooth curve) apatite confined fission track-length distributions. (b) Time-temperature ranges (upper and lower curves) for 250 statistically acceptable solutions generated by the model. The middle curve is the exponential-mean solution. The model was constrained to begin at 95 Ma and 500 °C, and to provide cooling-only solutions. The measured fission-track date of this sample is 72 ± 8 Ma. The mean closure age for model (i.e., age corrected for thermally induced length shortening) is 77 ± 5 (2σ errors). Samples are listed in Table 3 from west to east.

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Figure 9. Locations of samples used for Al-in-hornblende geobarometry and pressures obtained using the Schmidt (1992) calibration; pressure is in kbar (2σ errors). Samples are listed in Table 5 from west to east.

Clarke, 1979]) and with the U/Pb data to develop four models, initially summarized by Ortega-Rivera et al. (1994) and Gastil et al. (1994) and presented below, that satisfy the data. The first two of these do not require postemplacement tilting of the pluton, whereas the other two do. Any model of the tectonothermal history of the pluton must satisfy the following conditions required by the 40Ar/39Ar and U/Pb data and assumptions about geothermal gradients.

(1) At the western margin (Figs. 2 and 4b), the most precise 40Ar/39Ar plateau date for hornblende of 94.6 ± 0.5 Ma is close to the 97 Ma U/Pb zircon age of intrusion. Hornblende dates from the rest of the pluton are similar (Figs. 4, c and d, 5a, and 6a), ca. 92 Ma, and slightly younger than the 94 Ma U/Pb monazite date from the muscovite-biotite granodiorite. The hornblende dates require that all phases of the pluton must have been emplaced, and the currently exposed surface cooled below 500 °C, by 92 Ma. Assuming a probable minimum paleogeothermal gradient of 30 °C/km, as

TABLE 4. ELECTRON MICROPROBE HORNBLENDE ANALYSES FROM THE SIERRA SAN PEDRO MÁRTIR PLUTON Average analysis (wt%) SiO2 Al2O3 TiO2 Fe2O3 FeO MgO MnO CaO Na2O K2O Cr2O3 H2O Total

Average structural formula Si Al (4) Al (6) Ti Fe3+ Fe2+ Mg Mn CA Na K Cr O OH

Sample no. 4 WC-2M

Sample no. 5 SSPM-7-26-7

44.15 10.05 0.79 3.6 15.43 10.28 0.4 12.13 1.23 1.07 0.05 2.02

43.35 10.08 0.91 4.05 15.21 10.1 0.43 11.85 1.28 1.12 0.07 2

42.48 9.6 0.096 3.36 15.67 9.39 0.48 11.35 1.16 0.98 0 1.94

43.12 10.42 1.05 3.87 13.56 9.26 0.57 11.91 1.25 1.1 0.05 2

44.06 9.23 0.65 3.7 17.42 8.96 0.6 12.08 1.03 0.91 0 1.99

43.42 9.92 0.8 3.59 14.89 10.15 0.34 12.21 1.04 1.06 0.12 1.99

100.39

97.37

101.17

100.63

99.52

Sample no. 5 SSPM-7-26-7

Sample no. 6 CORONA

Sample no. 10 E8-20.6

101.2

Sample no. 4 WC-2M 6.559 1.441 0.319 0.088 0.403 1.917 2.276 0.05 1.931 0.354 0.203 0.006 22 2

6.502 1.498 0.284 0.103 0.457 1.909 2.258 0.055 1.904 0.372 0.214 0.008 22 2

Sample no. 6 CORONA

6.575 1.425 0.327 0.112 0.392 2.029 2.166 0.063 1.882 0.348 0.194 0 22 2

Sample no. 10 E8-20.6

6.462 1.538 0.302 0.118 0.437 2.076 2.068 0.072 1.912 0.363 0.21 0.006 22 2

Sample no. 16 Sample no. 22 SSPM-LV-8 SSPM-7-24-2

Sample no. 16 Sample no. 22 SSPM-LV-8 SSPM-7-24-2 6.643 1.357 0.283 0.074 0.42 2.196 2.014 0.077 1.951 0.301 0.175 0 22 2

6.552 1.448 0.317 0.091 0.408 1.879 2.283 0.043 1.974 0.304 0.204 0.014 22 2

Note: The number of aluminum ions in the structural formula of hornblende decreases slightly with increasing Fe3+. Each hornblende structural formula is the average of a no-glaucophane formula that minimizes Fe3+ by filling 15 sites with all cations except Na and K, and a no-cummingtonite formula that maximizes Fe3+ by filling 13 sites with all cations except Mn, Ca, Na, and K.

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ORTEGA-RIVERA ET AL. TABLE 5. ALUMINUM-IN-HORNBLENDE GEOBAROMETRIC RESULTS FOR THE SIERRA SAN PEDRO MÁRTIR PLUTON Sample number 4 4 5 6 10 10 16 22 22

long

UTM lat

Elevation (m)

632349 632349 636097 637859 641676 641676 640985 649672 649672

3426113 3426113 3428718 3429950 3428342 3428342 3416545 3425197 3425197

1700 1700 2240 2520 2440 2440 2040 2400 2400

Sample name WC-(2M) WC-(2M) SSPM-7-26-7-88 CORONA E.8-20.6 E.8-20.6 SSPM-LV-8 SSPM-7-24-2-88 SSPM-7-24-2-88

Mineral Al (T) hb hb hb hb hb hb hb hb hb

1.84 1.70 1.78 1.75 1.78 1.84 1.64 1.77 1.77

P (1) (kbar)

P (2) P (3) (kbar) (kbar)

5.8 5.1 5.5 5.3 5.5 5.7 4.8 5.4 5.4

4.3 3.7 4.1 4.0 4.1 4.3 3.5 4.0 4.0

3.6 2.9 3.3 3.1 3.3 3.6 2.5 3.2 3.2

P (4) (kbar)

P (5) (kbar)

5.3 4.7 5.0 4.9 5.0 5.3 4.3 5.0 5.0

5.6 4.9 5.3 5.1 5.3 5.6 4.5 5.2 5.2

Note: P (1)—pressure determined using Schmidt (1992) calibration [±1.6], P (2)—pressure determined using Johnson and Rutherford (1989) calibration [±1.0], P (3)—pressure determined using Thomas and Ernst (1990) calibration [±1.6], P (4)—pressure determined using Hammarstrom and Zen (1986) empirical calibration [±6.0], P (5)—pressure determined using Hollister et al. (1987) empirical calibration [±2]. Pressures are in kilobar ± 2-sigma errors. Al (T)—total aluminum.

might be expected in a metamorphic terrane on continental basement, this further requires that the currently exposed surface of the pluton must have been no deeper than ≈16 km at this time.

(2) The observed decrease in the biotite dates from west to east (Figs. 4 and 6b) require that the currently exposed surface of the pluton last passed through 280 °C between 94 Ma and 88 Ma diachronously from west to east.

No comparable diachroneity exists from north to south. Again assuming a probable minimum paleogeothermal gradient of 30 °C/km, the present surface of the pluton was at a depth of 9 km or less by 92 Ma at its western side and by 88 Ma at its eastern side. Furthermore, the plagioclase date (Fig. 5c) constrains the eastern side to have been shallower than 7 km at ca. 88 Ma.

hornblende farther east (Fig. 4d) is somewhat younger than the 94 +1/–1 Ma U/Pb monazite date for the muscovite-biotite granodiorite phase.

In model 1, the decrease of biotite dates from 94 Ma at the western margin to 88 Ma near the eastern margin (Fig. 4) would simply reflect passage through ≈280 °C at progressively younger times from west to east as successively younger phases cooled. If this model is valid, the 92 Ma dates on hornblende and the ≤90 Ma dates on biotite from the interior parts of the hornblende-biotite tonalite (Fig. 5, a and b) must have resulted from somewhat slower cooling of the interior relative to the western margin of this phase, and/or overprinting as a consequence of proximity to the younger magmatic pulses to the east.

MODELS 1 AND 2: NO POSTEMPLACEMENT TILTING REQUIRED

Model 2: Intrusion of All Phases at ca. 97 Ma; Inward Cooling and Faulting

Model 1: Eastward Migration of Plutonism The pattern of eastward-younging of 40Ar/39Ar biotite dates (Figs. 4 and 5b) could be explained by a progressive eastward migration of plutonism over time, with no subsequent tilting of the pluton. McCormick (1986) raised this as a possibility to explain his pattern of conventional K-Ar dates for biotite. The easternmost muscovite-biotite granodiorite phase of the Sierra San Pedro Mártir pluton and the garnet and muscovite-bearing El Diablo granodiorite to the northeast crosscut, and are therefore younger than, the hornblende-biotite tonalite phase (Eastman, 1986; McCormick, 1986).

The U/Pb data from the Sierra San Pedro Mártir pluton indicate that crystallization of the pluton may have progressed from west to east over a ≈3 m.y. time period (from ca. 97 to ca. 94 Ma). This trend is similar to that observed in a sequence of nested plutons in the Sequoia National Park region of the Sierra Nevada batholith (Chen and Moore, 1982). If the 3 m.y. span of crystallization is correct, then the 40Ar/39Ar ages from the Sierra San Pedro Mártir pluton suggest that hornblende-biotite tonalite of the outer zone had cooled below 500 °C (i.e., 94.6 ± 0.5 Ma hornblende age) while the muscovite-biotite granodiorite of the inner zone was still crystallizing (94 +1/–1 Ma, U/Pb monazite date).

40Ar/39Ar hornblende and biotite dates (Figs. 4 and 5, a and b) satisfy a model of such eastward younging of plutonism, in which simple, rapid cooling followed emplacement of each successive phase. The oldest 40Ar/39Ar date of 97.4 ± 1.0 Ma (Fig. 4a) from a hornblende in the country rock, near the western margin of the pluton, agrees with the 97.0 +4/–1 Ma U/Pb age for emplacement of the biotite-hornblende tonalite. The hornblende from the western margin of this tonalite, where cooling would be somewhat slower, yields an age of 94.6 Ma (Fig. 4b). Similarly, the 92 Ma cooling age for the

740

It is possible to explain the pattern of the hornblende and mica dates without requiring tilting of the pluton, even if all phases of the pluton had been emplaced at essentially the same time (probably 97 +4/–1 Ma, the 94 +1/–1 Ma monazite date for the muscovite-biotite granodiorite representing slower cooling of the interior). For this to be the case, it must be argued that the age patterns are a consequence of more rapid cooling of the pluton at its margins, and progressively slower cooling toward its interior. This would explain why the oldest cooling ages for hornblende and biotite are those from samples taken at the western margin of the pluton, and why, in a general way, the mica dates young eastward toward the eastern edge of the pluton.

Model 2 would require the dates for micas from near the northeast margin and from the extreme southeastward part of the pluton to be older than those at the center of the pluton; however, they are not. In model 2, the observed age pattern of the biotites is explained by removal of the east and northeast parts of the original pluton by faulting (Fig. 10), leading to the present asymmetric distribution of the phases and biotite dates (Figs. 5b and 6b).

A significant escarpment at the northeast margin of the pluton marks the location of the still-active Sierra San Pedro Mártir fault. This fault dips northeast at 60° and cannot be responsible for the dismemberment of the pluton, but the possibility of earlier, low-angle faults cannot be ruled out.

There is a minor northeastward younging of micas toward the fault. The only disturbed age spectrum found in this study is from a biotite from the northernmost area, which, if the Sierra San Pedro Mártir fault dips northeastward at a low angle, would have been close to the fault surface. The >20 m.y. discordance between the biotite and muscovite dates from this rock (sample 33, Table 2, and Fig. 4g) may have been caused by hydrothermal fluids from the fault zone.

Geological Society of America Bulletin, June 1997

SIERRA SAN PEDRO MÁRTIR PLUTON, BAJA CALIFORNIA, MEXICO

Figure 10. Geologic map of the Sierra San Pedro Mártir pluton and schematic representation of four possible tectonic cross sections used to explain the tectonothermal history of the Sierra San Pedro Mártir pluton. (c) Modified from Gastil et al. (1991).

MODELS 3 AND 4: POSTEMPLACEMENT TILTING REQUIRED Model 3: Intrusion at 97–94 Ma; Major Rotation by 88 Ma; Minor Rotation Possible After 88 Ma It has been suggested that an east-west transect of the Sierra San Pedro Mártir represents a 20 km crustal section exposed by 90° of rotation about a horizontal axis (Gastil et al., 1991). In model 3, significant tilting of the pluton is invoked to explain the 40Ar/39Ar data, but such tilting (Fig. 10) must have occurred by 88 Ma (i.e., within ≈9 m.y. of emplacement). At least part of the tilting must have occurred by 91 to 92 Ma (the hornblende closure age) in order for both the formerly deepest parts of the pluton (which, at a depth of 20 km, would otherwise have remained too hot for argon retention) and the shallowest parts to have cooled essentially simultaneously through 500 °C. The remainder of this major tilting must have been completed by 88 Ma, in order for the temperature to have fallen below 280 °C (the closure temperature of biotite) and possibly 220 °C (the closure temperature of plagioclase). After major tilting, there could have been later minor (≈15°) tilting after 88 Ma (model 4).

Model 4: Intrusion at 97 Ma; Minor Tilting After 88 Ma The 40Ar/39Ar data can be accommodated if the pluton underwent no tilting before 88 Ma and minor tilting (≈15°) at or any time after 88 Ma (Fig. 9).

In this model, following emplacement of the pluton at 97 +4/–1 Ma, hornblende in the contact zone at the western margin became closed to argon diffusion between 98 and 94 Ma. The rest of the currently exposed surface cooled sufficiently rapidly through 500 °C to close most of the hornblendes to argon loss by 91 Ma. This surface was then tilted (Fig. 10) and cooled diachronously through the closure temperature of biotite (280 °C) from 93 to 88 Ma as the subhorizontal isotherm migrated downward. The 88 Ma cooling age for plagioclase from the eastern part of the pluton shows that the temperatures of the currently exposed surface may have been as low as ≈220 °C by this time. East-side-up tilting at, or any time after, 88 Ma would create the observed biotite age gradient at the currently exposed surface of the pluton.

Assuming a probable minimum paleogeothermal gradient of 30 °C/km at 88 Ma and closure temperatures of 280 °C and 220 °C for Ar loss from biotite and plagioclase, respectively, the difference in paleodepths between the western and eastern ends of the pluton prior to tilting is calculated to have been less than 7 km, and probably less than 5 km. The present-day 20 km east-west exposure of the pluton could therefore have resulted from ≈15° of tilting any time after 88 Ma.

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ORTEGA-RIVERA ET AL.

Depth of Emplacement and Aluminum-in-Hornblende Geobarometry Ideally, the mineralogy of plutons and adjacent metamorphic rocks can be used to estimate the depth of emplacement. For this pluton, the bulk composition of the rocks in the contact zone is unsuitable for geobarometry; therefore, few studies of these metamorphic rocks have been attempted.

Eastman (1986) inferred that this pluton probably crystallized at mesozonal depths of between 7 to 15 km because the pluton contains magmatic epidote and the contact zone has coarse-grained schist and gneisses with sillimanite.

Rothstein and Manning (1994) calculated a minimum pressure and temperature of 5 kbar and 700 °C for mineral assemblages from regionally metamorphosed migmatitic, semipelitic schist east of the pluton on the escarpment of the Sierra San Pedro Mártir fault. Thus, the depth of emplacement of this pluton is not well constrained. However, six of the dated samples from the pluton have the correct mineral assemblage for Al-in-hornblende geobarometry.

Zen and Hammarstrom (1984) stated that primary epidote may form at pressures of ≈8 kbar. Ghent et al. (1991) concluded that the presence of magmatic epidote in calc-alkaline plutons is indicative of a low CO2 activity.

Therefore, because this pluton contains primary epidote (Eastman, 1986), it may be assumed that the coexisting magmatic fluid was H2O dominated.

Schmidt’s (1992) experimental calibration of the Al-in-hornblende geobarometer is based on an H2O-saturated fluid, and is most likely to yield the best pressure estimates for the Sierra San Pedro Mártir pluton. This calibration yields the highest pressure estimate, averaging 5.3 ± 1.2 kbar (Table 5, Fig. 9), whereas that of Thomas and Ernst (1990) yields the lowest, 3.1 ± 1.2 kbar. Thus, if these values are taken as the maximum and minimum, the geobarometry suggests that the currently exposed surface of the pluton was emplaced at a depth between 12 and 20 km (assuming an average overburden density of 2.7 g/cm3). The highest pressure estimate is, moreover, consistent with the Rothstein and Manning (1994) pressure-temperature estimates for the regionally metamorphosed rocks east of the pluton.

Although Al-in-hornblende barometry is still not well understood, the barometer is theoretically sound if equilibrium was attained at the solidus temperature both in the experiment and in the field. It can be seen that, for the purpose of this study and regardless of the calibration used to determine the paleopressures (Table 5), the six analyzed samples equilibrated, within error, at the same pressure. This implies that little if any tilting of this surface has occurred since equilibration, and hence lends support to models 1 and 2, but rules out the major tilting underlying model 3. Errors in the pressure determinations allow differences in paleodepth across the currently exposed surface of 3.5 km, and therefore the data do not preclude minor postemplacement tilting of the pluton (model 4). The geobarometry restricts the maximum tilt to ≈15°, but does not constrain its orientation.

Apatite Fission-Track Analyses The fission-track data (Table 3) are too sparse and the errors in the dates too large to provide definitive evaluation of the tectonic models proposed above.

However, some general inferences are possible. Fission-track dates on apatite (closure temperature of 110 °C; Naeser, 1979) suggest that all parts of the currently exposed surface of the pluton were within ≈3 km of the surface (assuming a paleogeothermal gradient of 30 °C/km) by ca. 60 Ma. The measured fission-track dates support the hypothesis that these rocks cooled through the apatite closure temperature around the Cretaceous-Tertiary boundary. However, the track-length data (Table 3) for sample 2 show that there has been moderate annealing, and that cooling of the plutonic rocks was therefore protracted. The modeling of sample 2 (Fig. 8b) suggests that a simple cooling of the sample after 77 Ma (closure age of the sample when it began to accumulate tracks) represents an adequate model for the Sierra San

742

Pedro Mártir pluton. The measured apatite fission-track date of this sample is 72 ± 8 Ma. The mean closure age from the model (i.e., age corrected for thermally induced length shortening) is 77 ± 5 Ma (2σ errors).

Eastward younging of the fission-track dates (ca. 72 Ma near the western margin and ca. 57 Ma in the east) across the pluton (Fig. 7) suggests diachronous cooling through ≈110 °C, although the large errors in the fissiontrack dates make this conclusion tenuous. The 59 Ma date for apatite from a sample of country rock west of the pluton margin does not fit the apparent pattern of eastward younging shown by samples from the pluton. This may reflect the large uncertainties in the fission-track dates, or alternatively, a fault may have allowed uplift of a region of country rock relative to the pluton (Gastil, 1990).

If the late diachronous cooling implied by the eastward younging of the fission-track ages across the pluton is real, it must be considered in evaluating the models. Such diachroneity is unlikely to be a consequence of the mechanisms proposed in models 1 and 2, because the differential cooling across the pluton called for in those models would not be sufficiently protracted to yield an appreciable age difference in the apatites 25 m.y. later.

Isotherms would have become subhorizontal within 10 m.y. after emplacement of the pluton (e.g., Beck, 1992). However, diachronous cooling is implicit in models 3 and 4, which accommodate (model 3) or require (model 4) minor tilting after 88 Ma. The fission-track data would suggest that part or all of this minor tilting occurred at or after 57 Ma. CONCLUSIONS AND TECTONOTHERMAL IMPLICATIONS Our data support the following history for the Sierra San Pedro Mártir pluton.

(1) The U/Pb ages indicate that crystallization of the pluton probably took place diachronously, from ca. 97 Ma for the hornblende-biotite tonalite in the west to ca. 94 Ma for the muscovite-biotite granodiorite in the east.

(2) The 40Ar/39Ar and U/Pb ages show a rapid rate of cooling (≈40 °C/m.y.) in the first 10 m.y. for all parts of the exposed pluton (Fig. 11). This requires that significant uplift and erosion (≈7 km) must have occurred by 88 Ma to close biotite and plagioclase to argon loss. The apatite fission-track ages suggest an additional 4 km of erosion by 57 Ma (the youngest apatite date). Modeling of track-length distribution supports a history of monotonic slow cooling of the pluton from ca. 80 Ma to the present.

This is in contrast to the history after 80 Ma of the west-central (Snee et al., 1994) and eastern (George and Dokka, 1994) Peninsular Ranges of southern California, which were interpreted to have undergone rapid cooling associated with uplift at ca. 76 Ma for the eastern part and at ca. 62 Ma for the west-central part of the batholith.

(3) Although tilting of the pluton is not required to explain the monotonic decrease in 40Ar/39Ar cooling ages of biotites eastward across the present surface of the pluton, our preferred interpretation is that minor (≈15°) eastside-up tilting of the pluton about a north-south horizontal axis occurred at or after 88 Ma, satisfying the younging in apatite fission-track dates. Furthermore, the fission-track data suggest that part or all of this tilting may have taken place at or after 57 Ma. Similar patterns of southwest to northeast younging of K-Ar dates across other La Posta–type plutons (Krummenacher et al., 1975) may similarly be a consequence of late-stage northeast-side-up tilting about an axis subparallel to the trend of the batholith.

(4) Al-in-hornblende geobarometry suggests that the currently exposed surface of the pluton has not been significantly rotated since its time of formation at depths of between 12 and 20 km. The maximum tilt of the pluton is restricted to ≈15°. This result, coupled with our interpretation of the 40Ar/39Ar data, indicates that the increase in metamorphic grade of the host rocks from west to east across the belt is a preemplacement feature.

(5) Butler et al. (1991) suggested that northeast side-up tilting (15°–20°)

Geological Society of America Bulletin, June 1997

SIERRA SAN PEDRO MÁRTIR PLUTON, BAJA CALIFORNIA, MEXICO

Figure 11. Cooling curves for the currently exposed surface of the Sierra San Pedro Mártir pluton; dates in Ma and 2σ errors, symbols are larger than error bars except those for the apatite data.

of the batholith as opposed to large-scale northward tectonic transport is responsible for the discordant paleomagnetic inclinations obtained from rocks of western California and Baja California. Such tilting is consistent with the upper limit of tilting calculated from our geochronological data and militates against large-scale northerly transport of Baja California, as proposed by, for example, Hagstrum et al. (1985). Moreover, Paleozoic and Mesozoic rocks in Baja California are compatible with a position of peninsular California adjacent to Sonora prior to Cenozoic opening of the Gulf of California (Gastil and Miller, 1984).

If the pattern of northeastward younging of the K-Ar and fission-track

dates in the Sierra San Pedro Mártir pluton is a result of tilting after 57 Ma, the tilting may have been caused by regional-scale crustal extension associated with the opening of the Gulf of California in Neogene time (e.g., Stock and Hodges, 1989). Such effects are not seen in the more northerly areas of the batholith. The protracted cooling after 80 Ma suggested by the annealed apatite fission-tracks is in agreement with a component of late but minor uplift, perhaps associated with such tilting. Local expression of this extension includes the east-dipping, still-active Sierra San Pedro Mártir fault and related listric faults (Gastil et al., 1975) that have rotated crustalscale blocks in the correct sense to produce northeast-side-up tilting.

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ACKNOWLEDGMENTS This paper is part of Ortega-Rivera’s Ph.D. dissertation. Major funding for this research was provided by a grant from the Consejo Nacional de Ciencia y Tecnología (CONACyT) of México to Ortega-Rivera, and grants from the Natural Science and Engineering Research Council of Canada to Farrar, Hanes, and Zentilli (8820). We gratefully acknowledge a minor travel grant from the School of Graduate Studies at Queen’s University and a travel grant from the Department of Geology of San Diego State University, California, to Ortega-Rivera. Logistical support in the field was provided by staff from the Division of Earth Sciences of the Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE), Baja California, and staff from the Department of Geology of San Diego State University. A postdoctoral fellowship from the Commission of European Communities and internal grant from CICESE to López-Martínez allowed use of the laser mass spectrometer system at Nice, France. Technical support to López-Martínez at CICESE was provided by V. Moreno and G. Mora. We thank the staff of the McMaster Nuclear Reactor for assistance with sample irradiation; M. Colpron, H. Sandeman, D. Kempson, and H. Jamieson for their assistance with the Queen’s University microprobe analyses; and D. M. Carmichael for helpful discussions on the metamorphic relationships and aluminum-in-hornblende geobarometry. We thank G. Li and A. Grist of Dalhousie University for assistance with the fission-track analyses and modeling, A. H. Clark and D. M. Carmichael for computer and printer facilities, and A. H. Clark for revision of the final manuscript.

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