Stratigraphy, Geochemistry and Tectonic Significance of The

Tectonophysics 318 (2000) 71–98 www.elsevier.com/locate/tecto

Stratigraphy, geochemistry and tectonic signiﬁcance of the Oligocene magmatic rocks of western Oaxaca, southern Mexico

Barbara Martiny a,*, Raymundo G. Martıńez-Serrano b, Dante J. Morań-Zenteno a, Consuelo Macıás-Romo a, Robert A. Ayuso c a Instituto de Geologıá, Universidad Nacional Autońoma de Me´xico, Apdo. Postal 70-296, Ciudad Universitaria, 04510 Me´xico, Distrito Federal, Mexico b Instituto de Geofı´sica, Universidad Nacional Autońoma de Me´xico, Ciudad Universitaria, 04510 Me´xico, Distrito Federal, Mexico c United States Geological Survey, National Center, Reston, VA20192, USA

Received 26 August 1998; accepted for publication 30 August 1999

Abstract

In western Oaxaca, Tertiary magmatic activity is represented by extensive plutons along the continental margin and volcanic sequences in the inland region. K–Ar age determinations reported previously and in the present work indicate that these rocks correspond to a relatively broad arc in this region that was active mainly during the Oligocene (~35 to ~25 Ma). In the northern sector of western Oaxaca (Huajuapan–Monte Verde–Yanhuitlań), the volcanic suite comprises principally basaltic andesite to andesitic lavas, overlying minor silicic to intermediate volcaniclastic rocks (epiclastic deposits, ash fall tuffs, ignimbrites) that were deposited in a lacustrine-fluvial environment. The southern sector of the volcanic zone includes the Tlaxiaco–Laguna de Guadalupe region and consists of intermediate to silicic pyroclastic and epiclastic deposits, with silicic ash fall tuffs and ignimbrites. In both sectors, numerous andesitic to dacitic hypabyssal intrusions (stocks and dikes) are emplaced at different levels of the sequence. The granitoids of the coastal plutonic belt are generally more differentiated than the volcanic rocks that predominate in the northern sector and vary in composition from granite to granodiorite. The studied rocks show large-ion lithophile element (LILE) enrichment (K, Rb, Ba, Th) relative to high-field-strength (HFS) elements (Nb, Ti, Zr) that is characteristic of subduction-related magmatic rocks. On chondrite-normalized rare earth element diagrams, these samples display light rare earth element enrichment (LREE) and a flat pattern for the heavy rare earth elements (HREE). In spite of the contrasting degree of differentiation between the coastal plutons and inland volcanic rocks, there is a relatively small variation in the isotopic composition of these two suites. Initial 87Sr/86Sr ratios obtained and reported previously for Tertiary plutonic rocks of western Oaxaca range from 0.7042 to 0.7054 and eNd values, from −3.0 to +2.4, and for the volcanic rocks, from 0.7042 to 0.7046 and 0 to +2.6. The range of these isotope ratios and those reported for the basement rocks in this region suggest a relatively low degree of old crustal involvement for most of the studied rocks. The Pb isotopic compositions of the Tertiary magmatic rocks also show a narrow range [(206Pb/204Pb)=18.67–18.75; (207Pb/204Pb)=15.59–15.62; (208Pb/204Pb)=38.44–38.59], suggesting a similar source region for the volcanic and plutonic rocks. Trace elements and isotopic compositions suggest a mantle source in the subcontinental lithosphere that has been enriched by a subduction component. General tectonic features in this region indicate a more active rate of transtensional deformation for the inland volcanic region than along the

* Corresponding author. Fax: +52-5-622-4317. E-mail address: [email protected] (B. Martiny)

0040-1951/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0040-1951 ( 99 ) 00307-8 72 B. Martiny et al. / Tectonophysics 318 (2000) 71–98 coastal margin during the main events of Oligocene magmatism. The lower degree of differentiation of the inland volcanic sequences, particularly the upper unit of the northern sector, compared to the plutons of the coastal margin, suggests that the differentiation of the Tertiary magmas in southern Mexico was controlled to a great extent by the characteristics of the different strain domains. © 2000 Elsevier Science B.V. All rights reserved.

Keywords: arc magmatism; geochemistry; Nd–Sr–Pb isotope ratios; Oaxaca, Mexico; Tertiary; transtension

1. Introduction (Fig. 2) (e.g. McDowell and Clabaugh, 1979; Damon et al., 1981; Ferrari et al., 1994, and Tertiary magmatism of Paleocene to Miocene references therein). The Trans-Mexican Volcanic age in southern Mexico is represented by extensive Belt (TMVB) crosses central Mexico from east to outcrops of plutonic and volcanic rocks that form west at about 19°N and is related to the subduction part of the Sierra Madre del Sur and define two of the Cocos and Rivera plates beneath the North broad belts approximately parallel to the Pacific American plate. Volcanic activity of the TMVB coast: the coastal plutonic belt and the inland initiated at about 16 Ma and continues to this day volcanic sequences (Fig. 1). These rocks, together (Ferrari et al., 1994). These volcanic arc sequences with the latest Cretaceous magmatic rocks, often have an oblique distribution (16°) relative to the represent the highest elevations of the Sierra Madre Acapulco trench. Changes in the Tertiary mag- del Sur (SMS) and extend from the southern part matic activity in this region reflect a major reorga- of the state of Jalisco to the Isthmus of nization of the tectonic plates adjacent to southern Tehuantepec area. The Tertiary magmatism Mexico involving the detachment and lateral dis- roughly displays a decreasing age trend from placement of the Chortis block (Malfait and Paleocene in Colima to Miocene in eastern Oaxaca. Dinkelman, 1972; Ross and Scotese, 1988; The plutonic rocks along the continental margin Ratschbacher et al., 1991; Ferrari et al., 1994; form a chain of intrusive bodies of different scales, Herrmann et al., 1994; Schaaf et al., 1995). The dominated by composite batholiths commonly cut presence of mylonitic shear zones along the coastal by silicic and mafic dike swarms. More discontinu- margin of Guerrero and Oaxaca (Fig. 3), produced ous outcrops of lava flows, pyroclastic deposits during the detachment and subsequent eastward and hypabyssal intrusions make up the inland displacement of the Chortis block, and the unusual volcanic sequences. Between approximately 100°W proximity of the coastal plutonic belt to the and western Oaxaca, the magmatism tends to be Acapulco trench (Fig. 1) support the interpreta- Oligocene in age, whereas to the west, it is predomi- tion of the truncated character of the continental nantly Upper Cretaceous to Eocene. The exposure margin. The study of the distribution, geochronol- of middle crustal plutonic rocks along the conti- ogy and geochemical characteristics of the magma- nental margin of southern Mexico, emplaced at tism in southern Mexico is essential for depths between 13 and 20 km, and the increasing understanding the tectonic evolution of this region abundance of volcanic rocks of similar age in the during the Tertiary. inland region indicate a relatively rapid uplift, Previous studies of the plutonic rocks along the from 30 to 25 Ma before the present time, and Pacific coast of Oaxaca involve the along-the-coast unroofing of the plutonic rocks (Morań-Zenteno variations in geochemistry and geochronology et al., 1998). (Bo¨hnel et al., 1992; Herrmann, 1994; Herrmann In northwestern Mexico, arc volcanism related et al., 1994; Schaaf et al., 1995; Hernańdez-Bernal to Farallon–North American plate convergence and Morań-Zenteno, 1996). There are also a few produced mid-Cretaceous to early Tertiary mag- stratigraphic and geochronologic studies for the matic rocks, including the Oligocene to Miocene inland volcanic rocks (e.g. Salas, 1949; Ruiz- silicic Upper Volcanic sequence of the coast-paral- Castellanos, 1970; Ferrusquıá-Villafranca, 1970, lel NNE-trending Sierra Madre Occidental (SMO) 1976; Ferrusquıá-Villafranca and McDowell, 1991; B. Martiny et al. / Tectonophysics 318 (2000) 71–98 73

Fig. 1. Distribution of volcanic and plutonic rocks in southern Mexico with the study area marked. The inset shows state divisions and geographical locations. J=Jalisco; M=Michoacan; G=Guerrero; O=Oaxaca; C=Chiapas; MC=Mexico City; IT=Isthmus of Tehuantepec (modiﬁed from Mora´n-Zenteno et al., 1999).

Mora´n-Zenteno et al., 1998). Until now, there Mexico during the Tertiary. Major and trace ele- have been no studies of the geochemistry of the ments together with Sr, Nd and Pb isotopes as volcanic rocks of westernmost Oaxaca nor of the well as isotopic dating have been used to address regional geochemical and geochronologic patterns this problem. The details of the petrogenesis of of the magmatic rocks in this region. We therefore these magmatic rocks will be a subject of a separate focused our studies on an area in western Oaxaca paper and will include the determination of the that crosses the Sierra Madre del Sur and includes isotopic compositions of additional samples. both intrusive and extrusive Tertiary rocks in order to detect possible variations in the geochronologi- cal and geochemical patterns perpendicular to the 2. Regional geological setting trench (Figs. 1 and 4). In this paper, we examine the stratigraphy and 2.1. Basement rocks geochemistry of the magmatic rocks in western Oaxaca in order to gain insight into the signiﬁcance The Tertiary magmatic rocks in western and of these rocks in the tectonic evolution of southern central Oaxaca are distributed in a region charac- 74 B. Martiny et al. / Tectonophysics 318 (2000) 71–98

Fig. 2. Tectonic plates and major magmatic provinces of Mexico. IVS=inland volcanic sequences; CPB=coastal plutonic belt; SMO= Sierra Madre Occidental; TMVB=Trans-Mexican Volcanic Belt. The inset shows the distribution of tectonostratigraphic terranes for southern Mexico after Campa and Coney (1983). Abbreviations used in the inset are: G=Guerrero, Mi=Mixteca, O=Oaxaca, X=Xolapa, J=Jua´rez, M=Maya terrane, SM=Sierra Madre, SMO=Sierra Madre Occidental and TMV=Trans-Mexico Volcanic Axis, C=Coahuila. terized by contrasting pre-Cenozoic tectonic and schists, gneisses and amphibolites, including ultra- stratigraphic settings. Three major tectonostrati- mafic and serpentinitic rocks. It has been interpre- graphic units have been recognized on the basis of ted that a major part of this terrane was overthrust the petrotectonic associations and age of their in pre-Pennsylvanian time by the Grenville age basement, namely the Mixteca, Oaxaca and Oaxaca terrane and the Esperanza Granitoids are Xolapa terranes (Campa and Coney, 1983; Sedlock found in the contact between these two terranes et al., 1993) (Fig. 2 inset). The Tertiary volcanic (Sedlock et al., 1993). It is thought that the rocks of western Oaxaca cover metamorphic and Acatlań Complex is underlain by a Precambrian sedimentary units of the Mixteca terrane and prob- basement that is tentatively considered to be ably the westernmost part of the Oaxaca terrane, Grenvillian in age (Ortega-Gutie´rrez et al., 1990). whereas along the coastal margin, Cretaceous and Typical present-day 87Sr/86Sr and eNd values of Tertiary plutons are emplaced in metamorphic the Acatlań Complex range from 0.7153 to 0.7613 rocks of the Xolapa terrane. and −8.5 to −12, respectively, for the metasedi- The basement of the Tertiary volcanic rocks in mentary units and the granitoids of the Esperanza the Mixteca terrane is represented by the Acatlań Formation, whereas mafic components of the Complex of Paleozoic age. This complex is formed eclogitic sequences have values ranging from by a heterogeneous tectonic assemblage of 0.7058 to 0.7094 and +1.7 to +3.1 (Yan˜ez metamorphic units ranging from greenschist- to et al., 1991). eclogite-facies (Ortega-Gutie´rrez, 1978, 1993). It To the east, the Tertiary volcanic rocks of the includes metasedimentary units of phyllites and central part of the state of Oaxaca cover the migmatites, as well as eclogite-facies micaceous Oaxaca terrane, which is characterized by a granu- B. Martiny et al. / Tectonophysics 318 (2000) 71–98 75

metamorphic rocks, for which there are still uncer- tainties concerning the protolith ages. These rocks are distributed along the continental margin of eastern Guerrero and Oaxaca. The Xolapa terrane includes mainly quartz-amphibolites, quartz-feld- spathic gneisses, pelitic paragneisses and schists, as well as some marble lenses and granulite facies relicts (Ortega-Gutie´rrez, 1981; Corona-Cha´vez, 1997; Tolson-Jones, 1998). There is a characteristic occurrence of migmatites throughout most of the Xolapa Complex that indicates different degrees and conditions of anatexis. The present-day 87Sr/86Sr ratios reported up to now for the Xolapa Complex range from 0.706 to 0.724 and eNd values, from −12.4 to +2.5 (Morań-Zenteno, 1992). Undeformed Tertiary plutons of this complex, excluding the Acapulco intrusion that differs in age and geochemistry from other plutons in the region, display low 87Sr/86Sr ratios (0.7038– 0.7051), and positive eNd values (+0.5 to +3.7) (Morań-Zenteno, 1992; Herrmann, 1994). The Guerrero terrane lies farther west and has a younger basement; it is characterized by Late Cretaceous and Paleogene continental deposits that unconformably overlie Mesozoic volcano-sedimentary units, the age and nature of which are subject to controversy. Fig. 3. Map of south-central Mexico showing different Tertiary deformation domains and indicating their age (modified from Morań-Zenteno et al., 1999). 2.2. Tertiary tectonic features

The Tertiary tectonic features of the Oaxaca lite-facies metamorphic basement of Grenvillian region display a contrasting framework that is age (900–1100 Ma) (Ortega-Gutie´rrez, 1981, 1993) suggestive of changing dynamic conditions in both overlain by Paleozoic and Mesozoic sedimentary time and space. Most of the major Cenozoic sequences (Pantoja-Alor, 1970; Schlaepfer, 1970). tectonic features indicate a different tectonic sce- The metamorphic basement is mainly composed nario with respect to that of central and northern of mafic and felsic gneisses, as well as metasedi- Mexico dominated by NNW–SSE extensional mentary rocks and charnockites (Ortega- faults for the Oligocene and Miocene. Although Gutie´rrez, 1981, 1993; Ortega-Gutie´rrez et al., the continuation of the Basin and Range province 1995). The present-day 87Sr/86Sr and eNd isotopic to southern Mexico has been suggested on the values of the Oaxaca Complex generally range basis of the orientation and kinematics of some from close to those of bulk earth to 0.717 (although structures (i.e. Oaxaca fault) (Henry and Aranda- one paragneiss has a 87Sr/86Sr ratio of 0.750) and Gomez, 1992), many other major features indicate from −9to−12, respectively (Patchett and Ruiz, different dynamic conditions from that of central 1987; Ruiz et al., 1988a,b). and northern Mexico and include the deformation To the south, the Tertiary magmatic rocks of associated with the Chortis block displacement western Oaxaca occupy the Xolapa terrane, which (Ratschbacher et al., 1991; Ferrari et al., 1994; is constituted by middle crustal, amphibolite-facies Nieto-Samaniego et al., 1995; Meschede et al., 76 B. Martiny et al. / Tectonophysics 318 (2000) 71–98 B. Martiny et al. / Tectonophysics 318 (2000) 71–98 77

1997; Tolson-Jones, 1998). Some of the normal In the inland region of western Oaxaca, the faults in southern Mexico, such as the Oaxaca distribution of the Tertiary volcanic rocks seems fault, have been reactivated several times. The to be controlled by a group of NNW–SSE-trending Oaxaca fault is a NNW-trending fault system, top faults that bound a series of down-thrown blocks to the west, that delineates the eastern margin of where interlayered volcanic and lacustrine the Valley of Oaxaca (Centeno-Garcıá, 1988; sequences accumulated (Fig. 4). In some cases, the Nieto-Samaniego et al., 1995; Alaniz-A´ lvarez et al., faults cut the Tertiary volcanic units, and in other 1996) (Fig. 3). An early (Triassic?) mylonitization cases, the lava flows and pyroclastics overlap the event along the Oaxaca fault is probably related fault zones. This fact and the occurrence of dikes to the collision of the Mixteco-Oaxaca block emplaced in the faults are indicative of coeval against the more eastern Maya terrane. activity. The faults in this region display lateral, Reactivation occurred for the strike-slip phase of vertical and oblique striae, and, based on this fact this fault during the Jurassic (Alaniz-A´ lvarez et al., and the regional distribution of the Mesozoic and 1996) and as a normal fault zone during the Tertiary units, Silva-Romo (in preparation) inter- Miocene (Ferrusquıá-Villafranca et al., 1988). preted the Oligocene tectonic framework as an en In the coastal region of Oaxaca and eastern echelon left-lateral transfer fault system. Guerrero, a series of shear zones with left lateral To the west of the study area, in the Taxco- and normal kinematics that trend roughly parallel Huautla region, the Tertiary volcanic rocks are to the coast have been recognized. These shear dominantly silicic and range in age from 38 to zones appear to display chronological differences 27 Ma (Morań-Zenteno et al., 1998). The distribu- with later activity to the southeast (Fig. 3). South tion of volcanism in this area does not seem to be of Tierra Colorada, Guerrero, a mylonitic zone controlled, as in Oaxaca, by transtensional tectonic affecting the metamorphic rocks of the Xolapa features. In the Taxco region, an 800 m thick Complex is intruded by a felsic pluton that yielded sequence of rhyolitc ignimbrites and lava flows concordant U–Pb zircon ages ranging from 32.5 overlaps a system of NW-trending subvertical to 34.2 Ma (Herrmann et al., 1994). This mylonitic faults with a complex kinematic history including zone has kinematic indicators of a normal-left- normal and lateral displacements. The lower part lateral oblique shear zone. North of Puerto of the rhyolitic sequence, with K–Ar ages ranging Escondido, in the Juchatengo area, a NW-trending from 38 to 35.5 Ma, is affected by NNE-trending north to northeast-dipping mylonitic zone has been lateral faults, indicating a deformation event con- recognized with shear criteria that indicate normal temporary with the volcanic activity (Morań- fault kinematics (Ratschbacher et al., 1991). North Zenteno et al., 1998). of Huatulco and Puerto Angel, there is a well- Based on the analysis of outcrop-scale fault-slip defined EW trending shear zone, known as the data, Meschede et al. (1997) interpreted that in Chacalapa Fault, that is characterized by a sub- Tertiary times, there was an effective stress trans- vertical anastomosing geometry and left-lateral mission across the plate margin represented by the indicators. According to observations carried out continental lithosphere of southern Mexico. by Tolson-Jones (1998), the Huatulco intrusion According to these authors, prior to 25 Ma, the (U–Pb age of 29 Ma; Herrmann et al., 1994) is s2 axes of the stress field had a sub-vertical orienta- ff a ected by the crystal-plastic deformation of this tion, whereas the s1 was roughly parallel to the shear zone, and the mylonite is truncated by oblique motion vector of the oceanic plate with granodiorite dikes, which yielded a 23.7±1.2 Ma respect to North America. This interpretation does K–Ar age in hornblende. not satisfactorily explain the characteristics of

Fig. 4. Schematic geological map of the study area in western Oaxaca showing Tertiary rock units, general structural features and location of analyzed rocks. Numbers in parentheses refer to isotopic ages obtained in the present work and reported in other studies (Table 1). 78 B. Martiny et al. / Tectonophysics 318 (2000) 71–98 major Tertiary tectonic features of the inland and can be observed to rest directly on Paleozoic region of Oaxaca and Guerrero, and additional or Mesozoic rocks. time-constraint observations seem to be required. The Oligocene volcanic sequence in the northern sector can be divided into two general units. The lower unit consists of pyroclastic (silicic to intermediate lithic and vitric ash fall tuffs) and epiclastic 3. General stratigraphic features deposits that were apparently deposited in a lacustrine fluvial environment. A 31.4±0.8 Ma K–Ar The Tertiary-age inland volcanic sequences in age was determined for biotite of a silicic tuff the westernmost part of the state of Oaxaca extend (sample CON-75, Table 1) north of Huajuapan, 2 over an area of approximately 4000 km in a region but the potassium concentration in the biotite is known as the Mixteca Alta. Plutonic rocks crop anomalously low, and therefore, this radiogenic out to the south along the continental margin and age probably does not represent the exact time of form part of the coastal plutonic belt of southern emplacement. The magmas become more mafic in Mexico. The general stratigraphic characteristics the predominant upper unit, which consists of a of the Tertiary volcanic zone of western Oaxaca thick pile (>400 m in some areas) of up to 14 lava permits its division into the northern sector, where flows and autobreccias of intermediate composi- a thick pile of intermediate composition lavas and tion with interbedded tuffs in the lower part. The ff autobreccias with interbedded tu s are dominant, lavas have a porphyritic or trachytic texture and and the southern sector where the sequence con- contain phenocrysts of clinopyroxene, iddingsit- sists principally of volcaniclastic sequences ized olivine, hornblende or plagioclase. The pres- (Fig. 5). ence of erosional remnants of volcanic vents in the form of volcanic necks that are observed through- 3.1. Volcanic rocks of the northern sector out this region suggests that these lavas were at least partially produced by central volcanic struc- The northern sector includes the areas of tures. Widespread hornblende- or pyroxene-bear- Huajuapan, Zapotitlań, Monte Verde, Chilapa and ing hypabyssal intrusions (dikes and small stocks) Yanhuitlań (Fig. 4). The volcanic sequences in this of intermediate composition that are emplaced at area are mostly Oligocene in age (Table 1) and lie different levels of the Tertiary sequences have been on lower Tertiary age conglomerates or directly recognized throughout western Oaxaca and the on Mesozoic continental and marine sequences or adjacent parts of Puebla (e.g. Ferrusquıá- Paleozoic metamorphic rocks of the Acatlań Villafranca, 1970; Ruiz-Castellanos, 1970). In the Complex. An andesitic hypabyssal intrusion, study area, hornblende concentrates of a stock emplaced in the reddish mudstones, sandstones and dike yielded K–Ar ages of 33.6±1.4 and and tuffaceous beds of the Yanhuitlań Formation 34.2±1.4 Ma, respectively (samples CON-8A and (Fig. 5), yielded a hornblende K–Ar age of CON-91, Table 1). 40.5±1.7 Ma (sample CON-7, Table 1), which is Several K–Ar age determinations for whole rock older than the other ages obtained for volcanic samples of lavas and hypabyssal intrusions in this rocks of this area. This age and a few additional region have also been reported elsewhere. Seven isolated Eocene ages that have been reported for whole-rock ages for the Zapotitlań-Huajuapan this area and the adjacent parts of the state of area range from 32±1to29±1 Ma (Galina- Puebla (Grajales-Nishimura, pers. commun.) seem Hidalgo, 1996). The small variation between these to represent the earliest manifestations of Tertiary whole rock ages and the ages obtained in the magmatic activity in this region. There is no evi- present study for mineral separates is probably dence that Eocene magmatism was widespread or due to the different material dated. Farther east, volumetrically important in this region since, up in the Tamazulapan–Yanhuitlań area, Ferrusquıá- to now, all the other volcanic units in western Villafranca et al. (1974) and Ferrusquıá- Oaxaca have been dated as Oligocene (Table 1) Villafranca and McDowell (1991) report a K–Ar B. Martiny et al. / Tectonophysics 318 (2000) 71–98 79 n, Tamazulapan, ´ ń areas. (b) The southern sector includes the areas of Tlaxiaco, S. M. Cuquila, Laguna de Guadalupe and NW of Chalcatongo. Chilapa, Monte Verde and Yanhuitla Fig. 5. Composite stratigraphic columns for the volcanic sequences of western Oaxaca. (a) The northern sector includes the Huajuapan, Zapotitla 80 B. Martiny et al. / Tectonophysics 318 (2000) 71–98

Table 1 Age determinations of Tertiary magmatic rocks in western Oaxaca1

Sample Site Longitude Latitude Mineral Rock type K (%) 40Ar* Age Age Ref.a W N (ppm) determination (Ma)

Northern sector CON-7 Yanhuitlań97°23∞36◊ 17°34∞05◊ Hornblende Andesitic laccolith 0.360 0.001023 K–Ar 40.5±1.7 a CON-8A Huajuapan 97°47∞16◊ 17°49∞43◊ Hornblende Andesitic stock 0.458 0.001076 K–Ar 33.6±1.4 a CON-75 N of Huajuapan 97°41∞48◊ 18°04∞51◊ Biotite Silicic tuff 5.662 0.012440 K–Ar 31.4±0.8 a CON-91 N of Huajuapan 97°40∞52◊ 18°02∞36◊ Hornblende Andesitic dike 0.496 0.001187 K–Ar 34.2±1.4 a FV69-180 N of Tamazulapan 97°34.8∞ 17°42.8∞ Biotite Silicic tuff- Llano 6.67 0.012624 K–Ar 26.2±0.5 b de Lobos Fm. FV69-182 E of Tamazulapan 97°25∞ 17°34.8∞ Whole rock Yucudaac Andesite 0.934 0.001980 K–Ar 28.9±0.6 b Southern sector CON-59A L. de Guadalupe 97°51∞20◊ 17°11∞17◊ Hornblende Silicic tuff 0.484 0.001180 K–Ar 34.8±1.4 a CON-101 Tlaxiaco 97°36∞45◊ 17°21∞37◊ Biotite Silicic tuff 7.732 0.017810 K–Ar 32.9±0.9 a Coastal plutons CON-53 S. Ma. Zacatepec 97°58∞36◊ 16°53∞27◊ Biotite Granite 7.475 0.013330 K–Ar 25.5±0.7 a G-17 Jamiltepec 97°57∞01◊ 16°10∞21◊ Biotite Granite 7.809 0.0151 K–Ar 27.7±0.7 c MS-28 Progreso 97°45∞55◊ 16°09∞49◊ Biotite Granite 7.793 0.0133 K–Ar 24.4±0.6 c MS-34 Rıó Grande 97°26∞44◊ 16°00∞40◊ Biotite Granite 7.64 0.0125 K–Ar 23.5±0.6 c MS-35 Jamiltepec 97°49∞23◊ 16°16∞38◊ Hornblende Tonalite 0.874 0.0018 K–Ar 29.9±1.1 c MS-42 Progreso 97°47∞24◊ 16°15∞40◊ Hornblende Granodiorite 1.029 0.0019 K–Ar 27.7±1.0 c Mu20 N of S.P. Amuzgos 98°03∞21◊ 16°40∞53◊ Zircon Granodiorite U–Pb 301 d Mx12 NW of Progreso 97°45∞07◊ 16°09∞48◊ Zircon Tonalite U–Pb 281 d Mu9 NW of Pochutla 96°38∞07◊ 15°51∞00◊ Zircon Granodiorite U–Pb 271 d

a Ref. (= reference): a: this work; b: Ferrusquıá-Villafranca et al. (1974) and Ferrusquıá-Villafranca and McDowell (1991); c: Hernańdez-Bernal and Morań-Zenteno (1996); d: Herrmann et al. (1994). 40 1= 40 40 = × −10 + ∞ = × −10 40 = × −4 Ar radiogenic Ar; lb-( K) 4.962 10 /yr; (le l e) 0.581 10 ; K/K 1.193 10 g/g 1 error not reported

whole-rock age of 28.9±0.6 Ma for the lavas of possible causes for this disparity in the ages, includ- the Yucudaac Andesite and a biotite K–Ar age ing the different material dated, possible reheating of 26.2±0.5 Ma for the Llano de Lobos of the tuff during the emplacement of the overlying Formation, a volcaniclastic sequence composed of lavas or a lack of horizontal continuity within the rhyolitic to andesitic tuffs, welded ignimbrites and volcanic sequences and variations in the volcanic epiclastic deposits. stratigraphy so that the tuff dated is actually In the inland volcanic area, an eastward decreas- younger than the lava dated (Ferrusquıá- ing age trend is seen between westernmost Oaxaca Villafranca et al., 1974). We consider that, in order and east-central Oaxaca where Miocene ages are to define the age trend between Huajuapan and reported (Ferrusquıá-Villafranca et al., 1974; Yanhuitlań, it would be necessary to carry out Ferrusquıá-Villafranca and McDowell, 1991). The additional age determinations on mineral concen- younger age obtained for the Llano de Lobos trates in this latter area. Formation might also suggest a decreasing age tendency from Huajuapan to Yanhuitlań, but this 3.2. Volcanic rocks of the southern sector trend is not clear. There is an apparent discrepancy between the two ages from the Tamazulapan- The southern sector includes the areas of Yanhuitlań area since the Llano de Lobos Tlaxiaco, northwest of Chalcatongo, and smaller Formation is shown to underlie the Yucudaac outcrops in the areas of Cuquila and Laguna de Andesite in the stratigraphic column presented by Guadalupe of approximately 5 and 35 km2, respec- Ferrusquıá-Villafranca (1976). There are several tively (Fig. 4). The Tertiary volcanic sequences in B. Martiny et al. / Tectonophysics 318 (2000) 71–98 81 this sector unconformably overlie Mesozoic sedi- along the southern limit, in the area of San Pedro mentary sequences and Tertiary conglomerates Amuzgos, this batholith is in contact with meta- (Fig. 5). The conglomerates in the Tlaxiaco area morphic rocks of the Xolapa Complex. contain lithic fragments of volcanic and calcareous The plutons are medium-grained granodiorites rocks and siltstones and are generally <10 m thick. and granites containing biotite and/or hornblende The sequence in the southern sector is dominated and are more highly differentiated than the inland by intermediate to silicic volcaniclastic deposits of volcanic rocks, particularly the predominant upper epiclastic and pyroclastic origin that include ash- unit lavas of the northern sector. K-feldspar is fall tuffs and reaches a thickness of up to 300 m. generally microcline and commonly displays per- A biotite-bearing silicic ignimbrite caps this thitic intergrowths. Accessory minerals include sequence in Cuquila. The samples dated (K–Ar) sphene, which sometimes occurs in large euhedral from the southern sector are Oligocene in age: to subhedral crystals, apatite, iron-oxides and 34.8±1.4 Ma was obtained for hornblende in a zircon. Abundant swarms of aplitic dikes of NW– volcaniclastic rock and 32.9±0.9 Ma for biotite in SE and NE–SW orientation intrude the plutonic a silicic tuff (samples CON-59b and CON-101, rocks, especially near the southern border of the Table 1). Although extensive lavas were not recog- La Muralla pluton, south of Santa Marıá nized in the southern sector, abundant hypabyssal Zacatepec, as well as in the coastal region. intrusions, similar to those in the northern sector, Isotopic ages of the plutons are only slightly are emplaced in the volcaniclastic sequence. These younger than those of the inland volcanic hypabyssal rocks display a porphyritic texture with sequences. In the present study, a biotite concentration yielded an K–Ar age of 25.5±0.7 Ma pyroxene or hornblende phenocrysts in a microlitic (sample CON-53, Table 1) in a granite north of plagioclase groundmass. The intrusions are dacitic Santa Marıá Zacatepec, and Hernańdez-Bernal to andesitic stocks and dikes of varying dimen- and Morań-Zenteno (1996) report five K–Ar cool- sions. North and northeast of Tlaxiaco, several ing ages in biotite and hornblende (Table 1) of the lava flows extending over a distance of approxi- Jamiltepec and Progreso areas that range from mately 5 km and displaying a general NE–SW 29.9 to 23.5 Ma. U–Pb crystallization ages trend, were recognized. obtained by Herrmann et al. (1994) for undeformed Tertiary age plutonic rocks from Pinotepa Nacional to Huatulco range from 30 to 27 Ma. 3.3. Granitoids of the coastal plutonic belt

In this paper, the intrusive rocks that are exposed in the La Muralla–San Pedro Amuzgos 4. Sample selection and analytical methods region in western Oaxaca (Fig. 4) are referred to as the La Muralla pluton. Similar plutonic rocks Tertiary age volcanic and plutonic rocks as well are also observed throughout the coastal region, as basement rocks of western Oaxaca were col- including the areas of Jamiltepec, Progreso and lected during several work field trips to the area. Rıó Grande (Fig. 4) where they have been named The different stratigraphic units of the volcanic the Rıó Verde batholith by Hernańdez-Bernal and sequences as well as hypabyssal intrusions were Morań-Zenteno (1996). The La Muralla pluton sampled in both the northern and southern areas. appears to be an extension of the Rıó Verde From the coastal plutonic belt, samples were batholith, and together, they form part of one of obtained of the plutonic rocks in the La Muralla– the most extensive composite batholithic structures San Pedro Amuzgos area. Thin sections of more in southern Mexico. The La Muralla pluton is than 150 sampled rocks were studied to classify emplaced between two distinct terranes. At the the rocks and select fresh samples for bulk chemical northern limit, these rocks intrude Paleozoic meta- analyses, K–Ar determinations and other geochem- morphic rocks of the Acaltań Complex, whereas ical studies. 82 B. Martiny et al. / Tectonophysics 318 (2000) 71–98

Table 2 Major and trace elements of volcanic and plutonic rocks from western Oaxaca

Sample: CON-7 CON-9 CON-12 CON-14 CON-18 CON-20 CON-27 CON-28 CON-29a CON-32

Northern sector

Hypabyssal Lava Lava Lava Lava Hypabyssal Lava Lava Lava Lava

SiO2 60.91 54.40 51.28 56.03 58.75 53.96 56.90 54.82 51.54 53.36 Al2O3 17.94 17.02 16.76 17.15 16.84 16.70 16.97 18.24 17.71 16.80 Fe2O3 4.52 8.38 8.33 7.39 6.35 8.34 6.80 6.11 8.85 8.21 MnO 0.02 0.10 0.08 0.07 0.08 0.11 0.07 0.08 0.12 0.11 MgO 1.41 5.00 5.18 4.13 3.30 4.66 3.84 4.03 5.57 5.98 CaO 5.37 7.28 8.43 6.78 5.94 7.32 6.85 7.28 7.87 7.87 Na2O 4.23 4.01 3.92 3.92 3.47 3.90 3.71 3.19 4.02 3.74 K2O 1.31 1.01 1.17 1.25 1.66 1.11 1.55 1.20 0.81 1.00 TiO2 0.80 1.29 1.36 1.24 0.89 1.37 0.87 0.90 1.34 1.24 P2O5 0.29 0.33 0.48 0.34 0.25 0.34 0.24 0.33 0.31 0.32 L.O.I. 2.88 0.90 2.62 1.63 2.05 1.57 1.75 3.78 1.74 1.34 Total 99.68 99.72 99.60 99.93 99.57 99.37 99.55 99.96 99.88 99.96

Sr 855 494 643 593 455 506 463 817 484 459 Rb 23 19 23 23 46 26 31 47 14 21 Ba 412 274 460 381 511 312 436 335 219 309 Th3 14242 321 2 Nb 6 6 13 7 5 7 5 5 5 6 Zr 158 150 165 148 161 146 137 128 130 139 Hf 4 3 3 Y 17161915141613161617 Sc9 141713121414131516 Cr 20 139 188 111 48 114 66 29 214 208 Ni 10 68 78 48 23 50 28 14 101 74 Co 19 31 40 32 38 42 50 23 37 45

La 28 15 30 19 20 17 17 18 12 16 Ce 45 35 61 44 44 40 37 38 30 37 Pr7 58655 554 5 Nd 32 22 31 26 21 23 19 22 19 21 Sm6 57655 445 5 Eu 1.74 1.65 1.97 1.59 1.40 1.61 1.24 1.38 1.47 1.58 Gd5 45434 344 4 Tb 0.64 0.61 0.72 0.61 0.51 0.61 0.47 0.53 0.57 0.61 Dy3 34333 233 3 Ho 0.65 0.57 0.72 0.64 0.52 0.62 0.53 0.63 0.65 0.68 Er 1.49 1.35 1.75 1.41 1.22 1.46 1.21 1.44 1.43 1.52 Tm 0.22 0.20 0.27 0.19 0.19 0.21 0.18 0.21 0.22 0.23 Yb 1.42 1.18 1.59 1.24 1.21 1.32 1.13 1.40 1.35 1.41 Lu 0.20 0.18 0.25 0.20 0.18 0.19 0.18 0.21 0.21 0.23

Major-element and Sc abundances were deter- Pe´trographiques et Ge´ochimiques, Centre mined by inductively coupled plasma emission, National du Recherches Scientiﬁques, in Nancy, and all other trace elements by inductively coupled France. For conventional mineral K–Ar measure- plasma mass spectrometry (ICP-MS) in the analyt- ments, rock was crushed and sieved to retain the ical laboratories of the Centre de Recherches 0.125–0.18 mm size fraction. Biotite was separated B. Martiny et al. / Tectonophysics 318 (2000) 71–98 83

Table 2 (continued)

Sample: CON-35 CON-75 CON-77 CON-88 CON-90 CON-109 CON-141 CON-142 CON-59b CON-60a

Northern sector Southern sector

Lava Tuﬀ Hypabyssal Lava Lava Lava Hypabyssal Hypabyssal Tuﬀ Hypabyssal

SiO2 59.13 67.00 53.03 51.41 52.68 58.55 63.65 65.66 55.59 58.91 Al2O3 16.89 14.70 16.85 17.50 17.43 16.75 16.27 16.33 18.76 16.94 Fe2O3 6.09 1.75 8.33 8.98 9.15 7.37 4.55 4.06 5.54 6.29 MnO 0.09 0.02 0.11 0.07 0.10 0.09 0.07 0.05 0.07 0.06 MgO 3.21 1.09 6.29 4.91 5.57 3.86 2.08 1.84 1.69 2.05 CaO 5.76 2.79 7.50 7.73 7.67 6.45 4.91 4.34 5.69 5.67 Na2O 3.49 3.78 3.79 3.64 3.90 3.35 3.60 3.79 2.96 3.79 K2O 1.93 2.20 0.93 0.83 0.75 1.76 2.26 1.82 1.59 2.28 TiO2 0.91 0.32 1.26 1.32 1.49 1.12 0.68 0.63 0.70 0.91 P2O5 0.26 0.08 0.32 0.31 0.31 0.22 0.14 0.13 0.19 0.26 L.O.I. 1.97 6.14 1.42 3.18 0.82 1.83 2.27 3.19 6.88 2.53 Total 99.73 99.87 99.83 99.88 99.87 101.36 100.47 101.83 99.65 99.68

Sr 467 275 474 441 474 625 464 Rb 40 131 15 15 13 51 53 Ba 575 631 315 213 211 376 511 Th351 11 4 3 Nb665 55 5 6 Zr 173 144 132 115 130 113 151 Hf443 33 Y1411151617 1916 Sc 11 5 15 15 15 10 11 Cr 51 14 202 224 181 28 36 Ni 19 4 83 85 71 20 19 Co 31 24 34 25 33 21 29

La 23 16 16 13 13 17 20 Ce 48 31 36 28 30 29 39 Pr635 44 4 6 Nd 24 12 21 19 20 17 24 Sm525 45 4 5 Eu 1.35 0.89 1.51 1.51 1.69 1.33 1.38 Gd424 44 3 4 Tb 0.53 0.31 0.56 0.57 0.62 0.51 0.59 Dy323 33 3 3 Ho 0.59 0.42 0.59 0.66 0.65 0.63 0.62 Er 1.37 1.09 1.39 1.55 1.54 1.41 1.35 Tm 0.19 0.16 0.20 0.22 0.23 0.21 0.21 Yb 1.24 1.17 1.29 1.43 1.45 1.31 1.18 Lu 0.20 0.18 0.18 0.21 0.21 0.21 0.17

with a shaking table, magnetically and with an to remove other minerals adhered to the horn- electronic mortar to separate the mica sheets and blende. Mineral concentrates of >99.5% purity free possible chlorite. Hornblende was separated were prepared and were analyzed by Geochron magnetically and with heavy liquids. Three of the Laboratory Division of Krueger Enterprises, Inc. hornblende separates were acid-leached at room 87Sr/86Sr and 143Nd/144Nd ratios were measured temperature in an ultrasonic cleaner in 10% HF on a Finnigan MAT 262 mass spectrometer at 84 B. Martiny et al. / Tectonophysics 318 (2000) 71–98

Table 2 (continued)

Sample: CON-61a CON-62 CON-70 CON-72 CON-101 CON-49b CON-52 CON-53 CON-54 CON-56

Southern sector

Hypabyssal Ignimbrite Hypabyssal Hypabyssal Tuﬀ Intrusive Intrusive Intrusive Intrusive Intrusive

SiO2 56.72 67.83 59.26 61.44 68.86 66.82 68.94 69.51 64.90 65.38 Al2O3 17.42 12.24 16.38 16.52 14.70 16.25 15.07 15.11 16.41 16.27 Fe2O3 6.81 1.81 6.01 4.91 2.55 3.60 2.94 2.99 4.42 4.35 MnO 0.10 Traza 0.06 0.05 0.02 0.05 0.05 0.05 0.05 0.05 MgO 3.35 0.75 3.64 1.60 0.82 1.29 0.78 0.81 1.77 1.67 CaO 7.00 1.93 5.85 4.64 1.32 3.37 2.97 2.93 4.12 3.99 Na2O 3.62 0.97 3.60 2.90 1.75 4.04 3.85 3.88 3.97 4.03 K2O 1.58 4.88 1.80 2.48 5.82 2.92 3.29 3.29 2.66 2.65 TiO2 0.89 0.20 0.86 0.62 0.21 0.48 0.36 0.36 0.63 0.59 P2O5 0.24 0.02 0.23 0.20 0.05 0.17 0.13 0.14 0.18 0.18 L.O.I. 1.98 9.09 2.02 4.53 3.90 0.82 0.93 0.74 0.71 0.72 Total 99.71 99.72 99.71 99.89 100.00 99.81 99.31 99.81 99.82 99.88

Sr 593 249 429 448 112 442 315 295 368 357 Rb 37 171 49 66 124 83 86 85 80 75 Ba 411 814 525 484 434 842 671 745 611 646 Th 3 9 4 6 10 6 8 7 5 6 Nb5655677755 Zr 124 111 153 150 100 143 136 138 146 158 Hf 4 3 4 4 4 Y 16 26 14 12 9 12 11 10 11 11 Sc 13 3 12 9 4 6 5 5 7 7 Cr 34 4 110 25 11 7 5 4 11 10 Ni 18 3 40 12 2 3 5 4 7 7 Co 37 10 23 16 21 34 76 74 42 39

La 16 58 22 23 21 23 17 22 18 18 Ce 34 56 46 47 40 47 35 43 41 39 Pr41366464555 Nd 19 49 23 21 16 19 17 19 19 19 Sm4854343344 Eu 1.22 1.37 1.32 1.12 0.49 0.86 0.92 0.92 0.93 0.94 Gd3633233333 Tb 0.53 0.92 0.51 0.45 0.34 0.43 0.39 0.37 0.43 0.43 Dy3532222222 Ho 0.62 0.95 0.52 0.48 0.34 0.45 0.41 0.36 0.43 0.43 Er 1.50 2.31 1.25 1.24 0.79 1.16 0.93 0.89 1.04 1.01 Tm 0.20 0.31 0.18 0.18 0.12 0.17 0.14 0.13 0.13 0.14 Yb 1.32 2.03 1.08 1.18 0.79 1.12 0.99 0.80 0.95 0.97 Lu 0.19 0.30 0.17 0.19 0.13 0.16 0.16 0.14 0.14 0.14

The following errors are reported: <3% per weight per cent for major elements and <10% per ppm for most trace elements. Additional major-element abundances were obtained by X-ray ﬂuorescence ( XRF) at the University Laboratory for Isotope Geochemistry (LUGIS), University of Mexico (UNAM). The precision of XRF is generally better than 1% for major elements. Quality control in the CNRS and LUGIS laboratories is maintained with international standards.

LUGIS (Laboratorio Universitario Geoquı´mico samples were determined on a Finnigan MAT 262 Isoto´pico), UNAM. Lead isotopic compositions mass spectrometer at the United States Geological of HF-leached feldspar separates and whole rock Survey in Reston, Virginia. Procedural Pb blanks B. Martiny et al. / Tectonophysics 318 (2000) 71–98 85 during this study were less than 200 pg and were therefore negligible compared to the values measured in the samples.

5. Geochemical results

Major- and trace-element compositions were determined in the Tertiary magmatic rocks of the study area; major elements were obtained in 31 samples and trace elements in 28 (Table 2). All volcanic and plutonic rocks analyzed were classi- fied on an anhydrous basis. Previous geochemical studies carried out by Hernańdez-Bernal and Morań-Zenteno (1996) on the Rıó Verde batholith include major and trace elements as well as Sr and Nd isotope determinations of the Jamiltepec, Progreso and Rıó Grande intrusions.

5.1. Major- and trace-element geochemistry

Fig. 7. K2O–SiO2 classification diagram after Peccerillo and The lavas are the least differentiated rocks of Taylor (1976). I=arc tholeiitic series; II=calc-alkaline series; % = = the study area and vary from 53 to 61 SiO2 III high-K calc-alkaline series; IV shoshonitic series. (anhydrous basis). Hypabyssal rocks are generally Western Oaxaca samples (this study): crosses=lavas, open cir- = = ff more evolved and range from 54 to 67%. Using cles hypabyssal stocks and dikes, diamonds tu s. Open squares=volcanic rocks from northeastern Guerrero (data the classification system of Le Maitre (1989), the from Morań-Zenteno et al., 1998). lava flows range in composition from basaltic

andesite to andesite and the hypabyssal intrusions, from basaltic andesite to dacite (Fig. 6); most samples from both groups have medium-K contents. The volcanic rocks from western Oaxaca are characterized by being subalkaline (Fig. 6) ffi with a calc-alkaline a nity. The K2O contents of these magmatic rocks are generally typical of the normal calc-alkaline series based on Peccerillo and Taylor (1976), as shown in Fig. 7. Pyroclastic rocks are intermediate to felsic in composition. Oligocene volcanic rocks from the NE Guerrero area (Taxco, Huautla and Buenavista areas) are also more silicic than the intermediate lavas that Fig. 6. Total alkali — SiO2 for Oligocene age volcanic rocks of western Oaxaca for the classification of nonpyroclastic rocks predominate in the northern sector of western after Le Maitre (1989). B=basalt, BA=basaltic andesite, A= Oaxaca. In this part of Guerrero, the volcanic = = = = andesite; D dacite, R rhyolite, TB trachybasalt, TBA sequences consist principally high-K rhyolitic to basaltic trachyandesite, TA=trachyandesite, T=trachyte. Division between alkaline and subalkaline fields from Irvine dacitic ignimbrites and lava flows with no signifi- and Baragar (1971). Crosses=lavas, open circles=hypabyssal cant intermediate components (Morań-Zenteno rocks. et al., 1998) (Fig. 7). 86 B. Martiny et al. / Tectonophysics 318 (2000) 71–98

Fig. 8. Total alkali — SiO2 diagram after Cox et al. (1979) modified by Wilson (1989) for the classification of plutonic rocks. The classification of granitoids of Tertiary age from the coastal plutonic belt of western Oaxaca is shown as: X in shaded area=plutonic rocks of the La Muralla pluton (this study). Circles=Jamiltepec, triangles=Progreso and Rıó Grande intrusions (data from Hernańdez-Bernal and Morań-Zenteno, 1996).

The plutonic rocks analyzed in this study are with enrichment in large-ion lithophile elements from the La Muralla–San Pedro Amuzgos region (LILE) ( K, Rb, Ba, Th) relative to the HFS and plot in the granodiorite and granite fields elements (Nb, Ti±Zr) that are characteristic of = % (SiO2 66–70 ) using the classification of Cox subduction-related magmatism (e.g. Gill, 1981; et al. (1979) modified by Wilson (1989) (Fig. 8, Pearce, 1982, 1983; McCulloch and Gamble, 1991; Table 2). Closer to the Pacific coast, the Rıó Saunders et al., 1991). Compared to the less Grande and Progreso intrusions of the Rıó Verde differentiated inland volcanic sequences, the plu- batholith show a very similar composition, with tons display more negative spikes for Ti and the exception of the Jamiltepec intrusion, which is P2O5, are more enriched in incompatible elements, less differentiated. The data corresponding to the and show a greater depletion in compatible ele- La Muralla pluton and the Rıó Verde batholith ments (Cr, Ni). The patterns for the immobile straddle the boundary between the peraluminous elements (Nb, Zr, Hf, Ti Y and Yb) on variation and metaluminous rocks using Shand’s index diagrams (Fig. 9a and b) show more similarity to (Maniar and Piccoli, 1989). These samples have that of intra-plate basalts than to MORB. This ffi + + an A/CKN coe cient (Al2O3/CaO K2O and the enrichment in LILE suggest an enriched < Na2O) of 1.1 (molar ratio) and relatively high mantle source in the subcontinental lithosphere > % sodium contents (Na2O 3.2 ), corresponding to modified by subduction fluids, which have added I-type granites based on the classification of the more mobile elements (Rb, Ba, K) (Pearce, Chappell and White (1974). 1983; Wilson, 1989). The inland volcanic sequences Variation diagrams for trace elements are shown have Ba/La ratios that vary from 15 to 25 and for the western Oaxaca samples in Fig. 9a and b. La/Nb from 2 to 5 which is within the range that Trace-element patterns of the coastal plutons and is typical of calc-alkaline lavas from other con- the volcanic rocks of the inland region are similar vergent plate boundaries (Gill, 1981). B. Martiny et al. / Tectonophysics 318 (2000) 71–98 87

Fig. 9. Trace-element variation diagrams in Tertiary magmatic rocks of western Oaxaca, MORB normalized using the values of Pearce (1983). (a) Lavas and hypabyssal rocks of the northern and southern sectors. (b) Granitoids of the La Muralla pluton.

The rare earth element (REE) abundances in Sm) and relatively ﬂat patterns for the heavy rare the samples of the inland volcanic sequences and earth elements (HREE; Tb–Lu) (Fig. 10a and b). the coastal plutonic belt also show similar tenden- Although some granitoids display a very modest cies. Chondrite-normalized REE patterns display negative Eu anomaly (Fig. 10b), no signiﬁcant light rare earth element enrichment (LREE; La– anomalies are observed in the volcanic rocks 88 B. Martiny et al. / Tectonophysics 318 (2000) 71–98

Fig. 10. Chondrite-normalized rare earth element data for Tertiary magmatic rocks of western Oaxaca normalized using the values of Nakamura (1974). (a) Lavas and hypabyssal rocks. (b) Granitoids of the La Muralla pluton.

(Fig. 10a), indicating that plagioclase fractionation granitoids. La of the Tertiary rocks varies from 40 was not signiﬁcant. The plutonic rocks have to 90 times chondrite and Lu, four to seven times slightly lower HREE concentrations; (La/Lu)cn chondrite. The LREE correlate positively with ratios range from 6.0 to 13.6 in the lavas and SiO2 in the Tertiary magmatic rocks although this hypabyssal rocks, and from 11 to 16.7 in the tendency is not displayed by the HREE. B. Martiny et al. / Tectonophysics 318 (2000) 71–98 89

5.2. Isotope geochemistry for the volcanic sequences, and from 0.7042 to 0.7044 for the granitoids in the La Muralla–San The results of a ﬁrst group of isotopic analyses Pedro Amuzgos area (Table 3). eNdi values are presented here and consist of 12 Sr determin- obtained for the Tertiary magmatic rocks of this ations, 10 for Nd and 9 for Pb (Tables 3 and 4). study area range from close to 0 to +2.6 (Table 3, Despite the contrast in the degree of diﬀerentia- Fig. 11). Considering the isotopic heterogeneity of tion, the silicic coastal plutons and the intermediate the crust in western Oaxaca, the narrow ranges 87 86 volcanic units of the northern volcanic sector show and generally low Sr/ Sr ratios and eNdi values similar isotopic features within narrow ranges. near and mostly above that of bulk earth suggest Initial 87Sr/86Sr ratios of the Oligocene samples a relatively low degree of crustal contamination. are relatively low and range from 0.7042 to 0.7046 The Eocene age laccolith located in the eastern

Table 3 Sr and Nd isotopic and chemical data: lavas, hypabyssal rocks and coastal plutons of western Oaxaca

87 87 87 143 147 143 Sample Rb Sr Sm Nd ( Sr/ Rb/ ( Sr/ ( Nd/ Sm/ ( Nd/ (eNd)0 (eNd)i 86 86 86 144 144 144 no. (ppm) (ppm) (ppm) (ppm) Sr)m Sr Sr)i Nd)m Nd Nd)i

Lavas and hypabyssal intrusions CON 7 21.7 918 5.75 31.4 0.703727±37 0.068 0.703688 0.512715±19 0.1108 0.512686 1.50 1.95 0.703735±40 0.703688 CON 9 13.2 535 5.16 22.1 0.704371±60 0.071 0.704336 0.512749±33 0.1414 0.512718 2.17 2.41 CON 14 23.5a 646 5.74 27.8 0.704587±47 0.105 0.704536 0.512712±40 0.1248 0.512684 1.44 1.76 22.3 671 6.16 27.2 0.704557±33 0.096 0.704511 CON 18 35.5 508 4.72 23.4 0.704724±41 0.202 0.704626 0.512619±43 0.1219 0.512592 -0.37 -0.05 CON 35 38.4 548 4.96 25.5 0.704715±45 0.203 0.704617 0.512623±20 0.1178 0.512597 -0.29 0.05 CON 70 48.6 490 4.64 23.8 0.704692±36 0.287 0.704553 0.512665±16 0.1179 0.512639 0.53 0.87 CON 77 15.2 556 4.88 22.4 0.704236±36 0.079 0.704198 0.512755±19 0.1316 0.512726 2.28 2.57

La Muralla pluton CON 52 86a 358 3.45 17.8 0.704668±46 0.695 0.704372 0.512726±33 0.1170 0.512703 1.72 2.02 CON 53 85.2a 338 3.69 19.1 0.704677±40 0.730 0.704366 0.512723±45 0.1166 0.512700 1.66 1.96 CON 54 79.3 420 4.03 19.9 0.704423±41 0.546 0.704190 0.512747±24 0.1225 0.512723 2.13 2.41

Rio Verde batholithb Jamiltepec 503 50 660 4 33 0.704339±59 0.263 0.704287 0.512614±29 0.138 0.51258 -0.2 504 47 681 0.704313±46 0.220 0.704270 Progreso 505 75 480 2 24 0.704701±36 0.466 0.704616 0.512617±40 0.118 0.51264 0.9 506 46 450 4 23 0.704735±248 0.355 0.704678 0.512651±33 0.113 0.51263 0.5 507 53 677 0.704271±34 0.284 0.704227 508 94 352 5 18 0.70553±33 0.933 0.705387 0.512513±30 0.112 0.51249 -2.1 Rı´o Grande 509 72 413 0.704997±41 0.601 0.704905 510 62 437 4 22 0.705394±32 0.533 0.705314 0.51247±34 0.090 0.51245 -3.0 511 54 470 2 24 0.705444±41 0.417 0.70538 0.51247±51 0.090 0.51245 -3.0 512 68 651 0.704809±159 0.376 0.704751

Element concentrations obtained by isotope dilution. a Obtained by ICP-MS. Measurements for the La Jolla Nd standard are 143Nd/144Nd=0.511885±27 and for the SRM-987 standard are 87Sr/86Sr=0.710233±16. Initial eNd values and 87Sr/86Sr ratios were calculated at 30 Ma for the plutons, 34 Ma for the lavas and hypabyssal intrusions, with the exception of CON-7, which was calculated at 40.5 Ma, and assuming a present-day 143Nd/144Nd (CHUR)=0.512638. b Rıó Verde batholith data from Hernańdez-Bernal and Morań-Zenteno (1996). 90 B. Martiny et al. / Tectonophysics 318 (2000) 71–98

Table 4 Pb isotopic compositions of Tertiary magmatic rocks and Precambrian basement

Sample number Location Rock type 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb

Lavas and hypabyssal rocks CON-7 WR Yanhuitla´n Laccolith 18.679 15.592 38.457 CON-20 WR Huajuapan Dike 18.720 15.605 38.523 CON-32 WR Huajuapan Lava 18.714 15.608 38.532 CON-32 plag 18.669 15.587 38.442

Plutonic rocks of the La Muralla–San Pedro Amuzgos area CON-52 WR S. M. Zacatepec Granite 18.749 15.618 38.588 CON-52 ksp 18.720 15.623 38.580 CON-53 WR S. M. Zacatepec Granite 18.706 15.587 38.487 CON-53 ksp S. M. Zacatepec 18.696 15.594 38.481 CON-54 ksp La Muralla Granodiorite 18.703 15.615 38.545

Oaxaca Complex CON-215 WR S of Oaxaca City Metagabbro 17.248 15.486 36.578 CON-215 ksp Metagabbro 17.221 15.501 36.602 CON-336 WR Nochixtla´n–Oaxaca Charnockite 17.141 15.499 36.508

Fig. 11. Sr–Nd isotopic initial ratios of Tertiary volcanic and plutonic rocks in southern Mexico and other Tertiary and Quaternary magmatic provinces of Mexico. The shaded field represents the Tertiary age magmatic rocks of western Oaxaca: open circles: plutonic rocks of the La Muralla pluton (this study); filled circles: lavas and hypabyssal rocks of the inland volcanic sequences (this study); triangles: plutonic rocks of the Rıó Verde batholith (data from Hernańdez-Bernal and Morań-Zenteno, 1996); squares: plutonic rocks of the Jamiltepec and San Pedro Amuzgos areas (data from Herrmann, 1994). Other magmatic provinces: SMO: Sierra Madre Occidental (SMO field includes data for the Upper Volcanic sequence of the northern SMO, with dashed lines enclosing typical values, from Lanphere et al., 1980; Verma, 1984; Cameron and Cameron, 1985; Cameron et al., 1986; Smith et al., 1996). TMVB= typical values of the Trans Mexican Volcanic Belt (data from Verma, 1983; Verma and Nelson, 1989). Coastal plutonic belt of southern Mexico: M=Manzanillo, J=Jilotepec, Z=Zihuatanejo, and H=La Huacana (data from Schaaf, 1990); A=Acapulco (data from Schaaf, 1990; Morań-Zenteno, 1992; Herrmann, 1994). B. Martiny et al. / Tectonophysics 318 (2000) 71–98 91

Fig. 12. Plot of 207Pb/204Pb–206Pb/204Pb for feldspars and whole-rock samples of Tertiary magmatic rocks of western Oaxaca and northeastern Guerrero, and Precambrian basement rocks; data for preliminary Paleozoic Acatlań Complex field are from Lopez and Cameron (unpublished data) and Martiny et al. (1997). X=field for undeformed Tertiary plutons of the Xolapa terrane between Acapulco and Huatulco from Herrmann et al. (1994). G=field for Tertiary volcanic rocks from NE Guerrero from Martiny et al. (1997). Additional data for Oaxaca Complex field from Solari et al. (1998), Lopez et al. (in press) and Cameron et al. (submitted for publication). Reference lines are the two-stage terrestial lead evolution curve (Stacey and Kramers, 1975), graduated at 250 Ma intervals (SK), and the Northern Hemisphere Reference Line (NHRL) (Hart, 1984). part of the study area (sample CON-7), northwest Other plutonic rocks from this region reported by of Yanhuitlań, has a lower 87Sr/86Sr ratio of 0.7037 Herrmann (1994) have similar Sr and Nd values. and could reflect less crustal involvement; this Pb isotopic ratios of whole rocks and leached + sample has an eNdi value of 2.0 (Table 3). feldspars of the Tertiary magmatic rocks of western Dacites and rhyolites from northeastern Oaxaca determined in this study display a relatively Guerrero are of a similar age (30.5–38.2 Ma), and restricted range, suggesting that the source of these in Taxco, for example, five samples analyzed have rocks is similar. On Pb isotope diagrams, the ratios higher initial 87Sr/86Sr ratios that range from of the volcanic, hypabyssal and plutonic rocks 0.7052 to 0.7063 (Morań-Zenteno et al., 1998), overlap and plot below the average Pb crust evolu- which could be explained by more crustal contami- tion curve of Stacey and Kramers (1975) (Fig. 12). nation or a more evolved crustal component. The The volcanic rocks of western Oaxaca display volcanic rocks analyzed from Taxco are near the present-day ratios of (206Pb/204Pb)=18.67–18.72, boundary between the Mixteca and Guerrero (207Pb/204Pb)=15.59–15.61, and (208Pb/204Pb)= terranes. 38.44–38.53. The granitoids show similar ratios Sr and Nd ratios obtained by Hernańdez-Bernal of (206Pb/204Pb)=18.70–18.75, (207Pb/204Pb)= and Morań-Zenteno (1996) for the Rıó Verde 15.59–15.62, and (208Pb/204Pb)=38.48–38.59 batholith show more variation than the granitoids (Table 4). The lead isotope range of the Tertiary analyzed in the present study (Table 3, Fig. 11). igneous rocks of the study area resembles that of Tonalitic intrusions of the Jamiltepec area display the orogene reservoir in the plumbotectonics model values similar to those of the La Muralla pluton of Doe and Zartman (1979). located farther inland, whereas the Progreso and In Fig. 12, the Tertiary magmatic rocks of west- Rıó Grande intrusions, located to the east of ern Oaxaca appear to define a steep mixing trend 87 86 207 Jamiltepec, present similar or higher Sr/ Sri between a mantle component and a Pb-rich ratios and similar or lower eNdi values (Table 3). reservoir. Steep trends are typical of some subduc- 92 B. Martiny et al. / Tectonophysics 318 (2000) 71–98 tion-related rocks, such as the Aleutian, Cascades, event commenced in Oligocene times (Table 1). Mariana and Lesser Antilles arcs (Kay et al., 1978; Oligocene magmatism in western Oaxaca and east- Woodhead and Fraser, 1985; White and Dupre, ern Guerrero is coeval with the displacement of the 1986) and have been interpreted as due to the Chortis block along the Pacific margin of southern incorporation of radiogenic Pb from subducted Mexico (Herrmann et al., 1994; Schaaf et al., 1995) sediments (Hawkesworth et al., 1991). The more and the consequent migration of the trench–trench– silicic volcanic rocks from NE Guerrero are slightly transform triple junction that constituted the inter- more radiogenic; in a 207Pb/204Pb vs. 206Pb/204Pb section between the North American, Farrallon and diagram, they plot to the right of the Stacey and Caribbean plates (Pindell et al., 1988; Ross and Kramers curve and fall in a more scattered field Scotese, 1988; Herrmann et al., 1994; Schaaf et al., (Fig. 12) (Martiny et al., 1997). Pb ratios are also 1995; Morań-Zenteno et al., 1996). The relationship reported in leached plagioclase feldspars of five between the ages obtained for the mylonitic zones undeformed Tertiary granitoids of the Xolapa ter- parallel to the coast and the plutons support this rane (Herrmann et al., 1994) from the area that interpretation. extends from Acapulco to Pochutla (Fig. 12). On a regional scale, along-the-coast magmatism Compared to the magmatic rocks of the study in southern Mexico during the Tertiary displays a area, these plutons have similar 206Pb/204Pb ratios rough decreasing age trend from northwest to and similar or slightly lower 207Pb/204Pb and southeast (Schaaf et al., 1995). The gradual extinc- 208Pb/204Pb ratios. tion of magmatism along the coast, at least to the Pb isotopic compositions have been obtained for east of the Zihuatanejo region, is directly related the Precambrian and Paleozoic basement rocks in to the passage of the triple junction (Herrmann the present study and by other workers who are et al., 1994; Schaaf et al., 1995). In the inland addressing problems related to the basement rocks volcanic belt, certain differences are displayed in (Solari et al., 1998; Lopez et al., in press; Cameron this decreasing age trend, particularly by the et al., submitted for publication). Whole rock Miocene ages in the region between the Valley of samples and feldspar separates from the igneous Oaxaca and Nejapa areas (Ferrusquıá-Villafranca units of the Precambrian Oaxaca Complex (meta- and McDowell, 1991) that lie north of the gabbro, metasyenite, charnockite, metagranite and Huatulco area where there are still Oligocene age anorthosite) have present-day Pb isotope ratios that plutonic rocks (Schaaf et al., 1995). are typical of Grenville age rocks [(206Pb/204Pb)= In westernmost Oaxaca, the K–Ar and U–Pb 16.95–17.55; (207Pb/204Pb)=15.47–15.54; (208Pb/ dates reported for plutonic rocks (30–23.5 Ma) 204Pb)=36.40–36.66] (Table 4; Solari et al., 1998; along the coast between Pinotepa Nacional and Lopez et al., in press; Cameron et al., submitted Rıó Grande are slightly younger than those of the for publication). Preliminary Pb isotopic composi- inland volcanic rocks (34.8–31.4 Ma) (Table 1). tions of the Paleozoic Acatlań Complex units are However, the K–Ar ages reported for the plutons very scattered on a 207Pb/204Pb–206Pb/204Pb dia- correspond to mineral cooling ages (biotite and gram (preliminary field shown in Fig. 12) and lie hornblende) and are not directly comparable to above and to the right of the average Pb crust the reported ages of the volcanic rocks. A compari- evolution curve of Stacey and Kramers (1975) son between the U–Pb ages of plutonic rocks (30 (unpublished data from Lopez and Cameron; and 28 Ma) along the coast (Herrmann et al., Martiny et al., 1997). 1994) and K–Ar mineral ages obtained in this study (34–31 Ma) for the volcanic rocks in the Huajuapan–Tlaxiaco area indicates that the extru- 6. Discussion sive rocks are slightly older. None the less, the reports of younger whole rock and mineral ages 6.1. Space–time trends of magmatism present in the inland area (FerrusquıáVillafranca and McDowell, 1991; Galina-Hidalgo, 1996) pre- Stratigraphic and geochronologic evidence indi- vent us from confirming a southward migration of cates that in western Oaxaca, a major magmatic the magmatism. Instead, we consider that the B. Martiny et al. / Tectonophysics 318 (2000) 71–98 93 western Oaxaca magmatic rocks constituted a (Progreso and Rıó Grande areas) also show more broad arc parallel to the coast in Oligocene time variable isotopic compositions and have lower eNd (~35 to ~25 Ma). In western Oaxaca, as well as values and higher 87Sr/86Sr ratios (Table 3). This in northeastern Guerrero, magmatic activity ceased slightly greater range of Sr and Nd ratios in the in the late Oligocene and recommenced to the Tertiary plutons of the Xolapa terrane might be north at about 16 Ma in the Trans-Mexican the result of greater crustal assimilation during Volcanic Belt. The magmatic gap at this longitude magma ascent or assimilation of crust with a more was probably caused by changes in the geometry heterogeneous isotopic composition. of the subducted slab after the passage of the triple An even greater variability is seen in the Nd junction (Morań-Zenteno et al., 1996). In central isotopic composition of the Tertiary coastal plu- and eastern Oaxaca, magmatism continued until tons of the Guerrero terrane. These plutons, with Miocene time (Ferrusquıá-Villafranca and the exception of Puerto Vallarta (Schaaf et al., McDowell, 1991). 1995), have higher eNd values (+1to+6.37) (Schaaf, 1990; Bo¨hnel et al., 1992) than the western 6.2. Geochemical patterns and variations Oaxaca plutons (−3.0 to +2.6) (Fig. 11, Table 3). The reason for this difference is not clear. The There are certain differences in the geochemical Guerrero terrane is part of a relatively young behavior between the extensive inland volcanic crustal segment that was integrated with the North sequences of the predominant upper unit in the American plate during the Mesozoic (Centeno- northern sector and the Oligocene magmatic rocks Garcıá et al., 1993), whereas the Tertiary magmatic of other adjacent regions. The most evident differ- rocks in western Oaxaca have an older basement. ence is the degree of differentiation. In western This difference could be explained by a lithospheric Oaxaca, the SiO2 contents of the magmatic rocks mantle that is more enriched in a subduction increase towards the coast. Basaltic andesite to component in western Oaxaca than in Guerrero andesitic compositions characterize the upper unit or by assimilation of crust with variable isotopic in the northern volcanic sector, andesites and signatures. dacites were identified in the southern volcanic Although the western Oaxaca Tertiary mag- sector, and in the coastal plutonic belt, granites matic rocks were emplaced in Precambrian– and granodiorites are prevalent (Table 2). An Paleozoic basement, the 87Sr/86Sr ratios are low, exception is the Jamiltepec intrusion, the least and eNd values range from −3.0 to +2.6 differentiated pluton within the Rio Verde batho- (Table 3). These eNd values are similar to those lith, which is of tonalitic composition (Fig. 8). displayed by the mid-Tertiary ignimbrites and lavas There is also a significant contrast between the of the Upper Volcanic sequence in the northern degree of differentiation of the inland volcanic Sierra Madre Occidental, where they range from rocks of western Oaxaca and those of northeastern −1.8 to +4.1, although 87Sr/86Sr ratios show a Guerrero; in this latter region, the volcanic rocks larger range (0.7038–0.710) (Lanphere et al., 1980; display rhyolitic to dacitic compositions, and inter- Verma, 1984; Cameron and Cameron, 1985; mediate units are not important (Fig. 7). Cameron et al., 1986; Smith et al., 1996). The Sr and Nd isotopic compositions of the Pb isotopic compositions of the magmatic rocks intermediate lavas of the northern inland volcanic in western Oaxaca, as with Sr and Nd isotopic sequence and the La Muralla pluton in western ratios, show a very narrow range (Table 4, Fig. 12) Oaxaca have a restricted range, with relatively low although these rocks vary from intermediate to 87Sr/86Sr ratios and eNd values from near 0 to acidic compositions. This suggests a similar source +2.6. There is a difference between the 87Sr/86Sr and evolution for these rocks. The distribution of ratios of these western Oaxaca rocks and the more data from Tertiary rocks of the study area on a differentiated rocks of northeastern Guerrero, with 207Pb/204Pb–206Pb/204Pb diagram appears to define slightly higher and more heterogeneous 87Sr/86Sr a steep mixing line with a narrow range. The ratios observed in this latter region (Morań- possible mixing end members cannot have been Zenteno et al., 1999). The plutons along the coast conclusively identified with the data available up 94 B. Martiny et al. / Tectonophysics 318 (2000) 71–98 to now, although the general trend of the data stood but does not appear to be related to the suggests a mantle source contaminated with a presence of a thick crust since this region was 207Pb-rich component. The 207Pb-rich component affected by transtension even before the Oligocene could correspond to the influence in the mantle magmatism. In the Huajuapan–Tlaxiaco region, wedge of fluids derived from the subduction zone thick sequences of lacustrine-fluvial volcaniclastic or assimilation of the Acatlań Complex. It is not deposits and volcanic rocks accumulated in NNW– possible to identify the isotopic composition of a SSE-trending basins at the time of volcanic activity subduction component by establishing an analogy and the presence of oblique, lateral and vertical with the present-day sediments in the Acapulco striae in fault planes reflect the extensional environ- trench. The continental source for the trench sediment for this region. We consider that the greater ments was most likely different in the early differentiation of the coastal plutons is related to Oligocene since the Chortis block must have been a lower extensional deformation at the time of the involved, and extensive exposures of Oligocene magmatism with respect to the inland regions and batholiths were lacking. the greater volume of magma involved in this zone. The preliminary data available up to now for Structural observations in the Chacalapa shear the Paleozoic Acatlań Complex indicate that the zone and the relationship with the Oligocene intru- Sr, Nd and Pb isotopic composition is highly sions of the coastal region suggest that, previous variable (Yan˜ez et al., 1991; Martiny et al., 1997; to the magmatism, extension was the main compo- unpublished data from Lopez and Cameron). It nent of deformation, whereas the strike-slip com- seems that any significant degree of assimilation ponent dominated afterwards (Tolson-Jones, would have resulted in a greater dispersion of the 1998). The peak of magmatism in the coastal data for the Tertiary magmatic rocks, although we region of Oaxaca seems to have occurred during do not completely discard assimilation of the the transition between these two strain regimes. Acatlań Complex. The Pb isotopic composition of We have documented that the volcanic rocks in igneous units of the Oaxaca Complex (Table 4, this region, especially the predominant upper vol- Fig. 12) indicates that these units of the canic unit of the northern sector, are less Precambrian basement were not incorporated into differentiated than the coastal plutons (Table 2). the Tertiary magmatic rocks to any notable degree. The northern sector volcanic sequences are also There are several indications that fractional less differentiated than the volcanic rocks of the crystallization is probably the most important pro- Taxco region, where Oligocene volcanism is associ- cess of magma differentiation for the western ated with NNE-trending strike-slip faults and no Oaxaca magmatic rocks analyzed in this study. significant extensional features have been recog- Assuming a similar source for these rocks and nized (Morań-Zenteno et al., 1998). In central and given the heterogeneity of the basement rocks in southeastern Oaxaca, the occurrence of abundant this region, the narrow range of Sr, Nd and Pb silicic ignimbrites also suggests a lower rate of isotope ratios, particularly for the northern vol- extension. Silicic rocks reported in the Oaxaca canic sector and La Muralla pluton, indicates a fault zone seem to have occurred in an extensional low degree of crustal contamination. Coherent region, but since the Oaxaca fault is an old feature linear trends with little scatter for unaltered with different episodes of reactivation, the exten- samples on variation diagrams of major oxides sion rate at the time of silicic magmatism is and trace elements vs. SiO2 could, thus, be uncertain. explained by fractional crystallization. None the In arc regions, deformation and stress fields less, the probability of a low degree of assimilation influence the generation and the ascent of magmas, cannot be discarded. which, in turn, is regulated by buoyancy and thermal effects (e.g. Singer et al., 1989; Apperson, 6.3. Relationship between tectonics and magmatism 1991; Takada, 1994). Since lithosphere extension in arc regions affects the level to which a magma The cause of the greater differentiation of the body ascends, it will also have an effect on the plutons along the coast is not completely under- degree and type of differentiation (crustal contami- B. Martiny et al. / Tectonophysics 318 (2000) 71–98 95 nation and crystal fractionation) (Burkart and more mafic magmatism. The volcanic sequences in Self, 1985; Glazner and Ussler, 1989). For exam- both sectors are intruded by hypabyssal bodies ple, in the central Aleutian arc, basaltic lavas are that vary in composition from dacite to basaltic associated with the degree of intra-arc extension andesite. The coastal plutonic belt is even more along the volcanic axis that modified the thermal differentiated and is composed principally of gra- and density structure of the lithosphere (Singer nitic to granodioritic plutons. and Myers, 1990). In the currently active exten- (4) The trace-element concentration of the mag- sional Trans-Mexican Volcanic Belt, Alaniz- matic rocks in western Oaxaca is characteristic of A´ lvarez et al. (1998) found a correlation between arc-related magmas. The relatively low Sr ratios the type of volcanism (monogenetic vs. polyge- and eNdi ratios near that of bulk earth as well as netic) and the strain rate. Monogenetic volcanism the general low variability of Sr, Nd and Pb tends to be more mafic and is associated with isotope ratios, especially for the inland volcanic faults having a higher strain rate. In western region and the La Muralla pluton, suggest a low Oaxaca, a correlation also seems to exist between degree of crustal contamination. The narrow range the extensional strain rate and the general degree of isotopic ratios, which are more radiogenic com- of differentiation. pared to depleted mantle, indicate the subcontinental lithospheric mantle contaminated by a subduction component as a probable source. 7. Conclusions (5) The degree of differentiation in the magmatic rocks in western Oaxaca seems to have been (1) K–Ar age determinations of igneous rocks influenced by the different strain domains in the in western Oaxaca indicate that volcanic and plu- region. The higher degree of differentiation of the tonic activity occurred during the Oligocene (~35 plutons along the coastal area and their slightly to ~25 Ma); the volcanic sequences crop out in greater crustal contamination, compared to the the inland region, whereas the plutonic rocks are intermediate volcanic sequences that are dominant found along the coast. These rocks form part of in the northern volcanic sector, seem to be related an extensive magmatic arc in southern Mexico to the lower extensional deformation in the that roughly displays a decreasing age trend from coastal area. Paleocene in Colima to Miocene in eastern Oaxaca. (2) Although age determinations reported in this work for the western Oaxaca region appear Acknowledgements to indicate that the coastal plutonic rocks are slightly younger than the inland volcanic Financial support by the National Council of sequences, other data reported previously give no Science and Technology (CONACyT) (project clear indication of a southward migration for the 3361 T9309) in Mexico is gratefully acknowledged. magmatism and, instead, suggest a broad mag- Pb isotopic determinations were made possible by matic arc parallel to the coast during the student grants received by one of the authors Oligocene. (Barbara Martiny) given by the Program of (3) In general, the SiO2 content of the Tertiary Financial Aid for Graduate Studies (PADEP) at magmas of western Oaxaca increases from the UNAM and the Geological Society of America inland region toward the coast. In the northern (Howard T. Sterns Fellowship Award). The sector of western Oaxaca, magmatism began with authors wish to thank G. Silva-Romo, S.A. Alaniz- volcanic activity of acidic to intermediate composi- A´ lvarez, A´ . F. Nieto-Samaniego and R. Lopez for tion that produced a lower unit of epiclastic depos- discussion and helpful comments; P. Schaaf and its, ash fall tuffs and ignimbrites overlain by a J.J. Morales-Contreras for assistance with the ana- predominant upper unit of basaltic andesite to lytical aspects of the isotopic determinations; R. andesitic lavas and autobreccias. In the southern Lozano-Santacruz for the XRF determinations; volcanic sector dacitic to andesitic compositions M. Reyes-Salas for SEM analyses in the evaluation are predominant with insignificant amounts of of some of the samples for isotopic determinations; 96 B. Martiny et al. / Tectonophysics 318 (2000) 71–98

L. Alba-Aldave and T. Hernańdez-Trevinõ for Ortega-Gutie´rrez, F., 1993. Guerrero terrane of Mexico: Its assistance in the field; A. Victoria-Morales for role in the Southern Cordillera from new geochemical data. Geology 21, 419–422. providing the charnockite of the Oaxaca Complex; Chappell, B.W., White, A.J.R., 1974. Two contrasting granite and J.T. Vazquez for some of the sample prepara- types. Pacific Geol. 8, 173–174. tions. We also thank K. Cameron and F.W. Corona-Cha´vez, P., 1997. Deformazione, metamorfismo e mec- McDowell for their helpful reviews. Additional canismi di segregazione migmatitica nel complesso pluton- unpublished data for the preliminary Acatlań ico-metamorfico del Terreno Xolapa, Messico. Ph.D. thesis, Complex Pb isotope field are from R. Lopez, K. Universita´ Degli Studi di Milano, 78 pp. Cox, K.G., Bell, J.D., Pankhurst, R.J., 1979. The Interpretation Cameron and F. Ortega. of Igneous Rocks. Allen and Unwin, London, 450 pp. Damon, P.E., Shafiqullah, M., Clark, K.F., 1981. Age trends of igneous activity in relation to metallogenesis in the southern Cordillera. In: Dickinson, W.R., Payne, W.D. (Eds.), References Relations of Tectonics to Ore Deposits in the Southern Cor- dillera. Ariz. Geol. Soc. Digest 14, 137–154. Alaniz-A´ lvarez, S.A., van der Heyden, P., Nieto-Samaniego, Doe, B.R., Zartman, R.E., 1979. Plumbotectonics the Phanero- A.F., Ortega-Gutie´rrez, F., 1996. Radiometric and kine- zoic. In: Barnes, H.L. (Ed.), Geochemistry of Hydrothermal matic evidence for Middle Jurassic strike-slip faulting in Ore Deposits. Wiley, New York, pp. 22–70. southern Mexico related to the opening of the Gulf of Ferrari, L., Gardunõ, V.H., Pasquare`, G., Tibaldi, A., 1994. Mexico. Geology 24, 443–446. Volcanic and tectonic evolution of Central Mexico: Oligo- Alaniz-A´ lvarez, S.A., Nieto-Samaniego, A.F., Ferrari, L., 1998. cene to Present. Geofı´s. Int. 33, 91–105. Effect of strain rate in the distribution of monogenetic and Ferrusquıá-Villafranca, I., 1970. Geologıá del a´rea de Tamazu- polygenetic volcanism in the Transmexican Volcanic Belt. lapan–Teposcolula–Yanhuitlań, Mixteca Alta, Estado de Geology 26, 591–594. Oaxaca. In: Segura, L.R., Rodrı´guez-Torres, R. (Eds.), Apperson, K.D., 1991. Stress fields of the overriding plate at Guidebook to the Mexico-Oaxaca Field Trip. Soc. Geol. convergent margins and beneath active volcanic arcs. Sci- Mexicana, pp. 97–119. ence 254, 670–678. Ferrusquıá-Villafranca, I., 1976. Estudios geolo´gico-paleonto- Bo¨hnel, H., Morań-Zenteno, D., Schaaf, P., Urrutia-Fucu- lo´gicos en la regioń Mixteca, Parte 1: Geologıá del a´rea gauchi, J., 1992. Paleomagnetic and isotope data from Tamazulapan-Teposcolula-Yanhuitlań, Mixteca Alta, southern Mexico and the controversy over the pre-neogene Estado de Oaxaca, Me´xico, Boletin 97. Universidad Nacio- position of Baja California. Geofı´s. Int. 31, 253–261. nal Autońoma de Me´xico, Instituto de Geologıá, 160 pp. Burkart, B., Self, S., 1985. Extension and rotation of crustal Ferrusquıá-Villafranca, I., McDowell, F.W., 1991. The Ceno- ff blocks in northern Central America and e ect on the vol- zoic Sequence of Selected Areas in Southeastern Mexico, its canic arc. Geology 13, 22–26. Bearing in Understand Regional Basin Development there: Cameron, K.L., Cameron, M., 1985. Rare earth element, Convencioń sobre la evolucioń geolo´gica de Me´xico y 87Sr/86Sr and 143Nd/144Nd compositions of Cenozoic oro- Primer Congreso Mexicano de Mineralogıá Pachuca, genic dacites from Baja California, northwestern Mexico Hidalgo, Mexico. Memoria, 45–50. and adjacent west Texas: evidence for the predominance of Ferrusquıá-Villafranca, I., Wilson, J.A., Denison, R.E., a subcrustal component. Contrib. Mineral. Petrol. 91, 1–11. McDowell, F.W., Solorio-Munguia, J., 1974. Tres edades Cameron, K.L., Cameron, M., Barreiro, B., 1986. Origin of radiome´tricas oligoceńicas y mioceńicas de rocas volcańicas voluminous mid-Tertiary ignimbrites of the Batopilas region de las regiones Mixteca Alta y Valle de Oaxaca Estado de Chihuahua: implications for the formation of continental Oaxaca. Bol. Asoc. Mex. Geol. Petroleros 26, 249–262. crust beneath the Sierra Madre Occidental. Geofı´s. Int. ´ 25, 39–59. Ferrusquıá-Villafranca, I., McDowell, F.W., Martıńez, E., Bar- Cameron, K.L., Lopez, R., Ortega-Gutie´rrez, F., Keppie, J.D., rios, H., Garcıá, F., 1988. Time constraints on formation of Solari, L.A., Grenvillian terrane correlations based on Pb continental Tertiary basins in the State of Oaxaca, Mexico. isotopes: A perspective from Mexico. Geology. submitted Geol. Soc. Am., Annual Meeting 20, A59, Abstract. for publication. Galina-Hidalgo, S. M., 1996. Geocronologıá y paleomagnet- Campa, M.F., Coney, P.J., 1983. Tectono-stratigraphic terranes ismo de rocas intrusivas y volcańicas del a´rea de Huajuapan and mineral resource distributions in Mexico. Can. J. Earth de Leoń, Oaxaca. Masters thesis. Universidad Nacional Sci. 20, 1040–1051. Autońoma de Me´xico, Facultad de Ingenierıá, Mexico City, Centeno-Garcıá, E., 1988. Evolucioń estructural de la falla de Mexico, 101 pp. Oaxaca durante el Cenozoico. M.Sc. Thesis. Universidad Gill, J.B., 1981. Orogenic Andesites and Plate Tectonics. Nacional Autońoma de Me´xico, Facultad de Ciencias, Springer, Berlin. 390 pp. Mexico City, Mexico, 156 pp. Glazner, A.F., Ussler III, W., 1989. Crustal extension, crustal Centeno-Garcıá, E., Ruiz, J., Coney, P.J., Patchett, P.J., density, and the evolution of Cenozoic magmatism in the B. Martiny et al. / Tectonophysics 318 (2000) 71–98 97

Basin and Range of the western United States. J. Geophys. an example from southern Mexico. Tectonophysics 266, Res. 94, 7952–7960. 81–100. Hart, S.R., 1984. A large-scale isotope anomaly in the Southern Morań-Zenteno, D.J., 1992. Investigaciones isoto´picas de Hemisphere mantle. Nature 309, 753–757. Rb–Sr y Sm–Nd en rocas cristalinas de la regioń de Tierra Hawkesworth, C.J., Hergt, J.M., Ellam, R.M., McDermott, F., Colorada–Acapulco–Cruz Grande, Estado de Guerrero. 1991. Element fluxes associated with subduction related Ph.D. thesis. Universidad Nacional Autońoma de Me´xico, magmatism. Philos. Trans. R. Soc. London. Ser. A 335, UACPyP, Mexico City, Mexico, 186 pp. 393–405. Morań-Zenteno, D.J., Corona-Cha´vez, P., Tolson, G., 1996. Henry, C.D., Aranda-Gomez, J.J., 1992. The real southern Uplift and subduction erosion in southwestern Mexico since Basin and Range: Mid- to late Cenozoic extension in the Oligocene: pluton geobarometry constraints. Earth Mexico. Geology 20, 701–704. Planet. Sci. Lett. 141, 51–65. Hernańdez-Bernal, M.S., Morań-Zenteno, D.J., 1996. Origin of Morań-Zenteno, D.J., Alba-Aldave, L.A., Martıńez-Serrano, the Rıó Verde Batholith, southern Mexico, as inferred from R., Reyes-Salas, M.A., Corona-Esquivel, R., Angeles- its geochemical characteristics. Int. Geol. Rev. 38, 361–373. Garcıá, S., 1998. Stratigraphy Geochemistry and Tectonic Herrmann, U.R., 1994. The origin of a terrane: U/Pb zircon Significance of the Tertiary Volcanic Sequences of the systematics, geochemistry and tectonics of the Xolapa Com- Taxco–Quetzalapa Region, Southern Mexico 15. Universi- plex (southern Mexico). Tu¨binger Geowissenschaftliche dad Nacional Autońoma de Me´xico, Instituto de Geologıá, Arbeiten A 17, 92. Revista Mexicana de Ciencias Geolo´gicas 15, 167–180. Herrmann, U.R., Nelson, B.K., Ratschbacher, L., 1994. The Morań-Zenteno, D.J., Tolson, G., Martıńez-Serrano, R.G., origin of a terrane: U/Pb zircon geochronology and tectonic Martiny, B., Schaaf, P., Silva-Romo, G., Macıás-Romo, C., evolution of the Xolapa complex (southern Mexico). Tec- Alba-Aldave, L., Hernańdez-Bernal, M.S., Solis-Pichardo, tonics 13, 455–474. G.N., 1999. Tertiary arc-magmatism of the Sierra Madre Irvine, T.N., Baragar, W.R. A., 1971. A guide to the chemical del Sur Mexico and its transition to the volcanic activity of classification of the common volcanic rocks. Can. J. Earth the Trans-Mexican Volcanic Belt. J. South Am. Earth Sci. Sci. 8, 523–548. 12, 515–535. Kay, R.W., Sun, S.S., Lee-Hu, C.-N., 1978. Pb and Sr isotopes Nakamura, N., 1974. Determinations of REE, Ba, Fe, Mg, Na in volcanic rocks from the Aleutian Islands and Pribilof and K in carbonaceous and ordinary chondrites. Geochim. Islands Alaska. Geochim. Cosmochim. Acta 42, 263–273. Cosmochim. Acta 38, 757–775. Lanphere, M.A., Cameron, K.L., Cameron, M., 1980. Sr iso- Nieto-Samaniego, A.F., Alaniz-A´ lvarez, S.A., Ortega-Gutie´r- topic geochemistry of voluminous rhyolitic ignimbrites and rez, F., 1995. Estructura interna de la Falla de Oaxaca related rocks, Batopilas area, western Mexico. Nature 286, (Me´xico) e influencia de las anisotropıás litolo´gicas durante 594–596. su actividad Cenozoica. Universidad Nacional Autońoma Le Maitre, R.W., 1989. A Classification of Igneous Rocks and de Me´xico: Instituto de Geologıá. Revista Mexicana de Glossary of Terms. Blackwell, Oxford. 193 pp. Ciencias Geolo´gicas 12, 1–8. Lopez, R.L., Cameron, K.L., Jones, N.W., in press. Evidence Ortega-Gutie´rrez, F., 1978. Estratigrafıá del Complejo Acatlań for Paleoproterozoic, Grenvillian, and Pan-African age en la Mixteca Baja, Estados de Puebla y Oaxaca. Universi- crust beneath northeastern Mexico. Precambrian Res. dad Nacional Autońoma de Me´xico. Instituto de Geologıá McCulloch, M.T., Gamble, J.A., 1991. Geochemical and geody- Revista 2, 112–131. namical constraints on subduction zone magmatism. Earth Ortega-Gutie´rrez, F., 1981. Metamorphic belts of southern Planet. Sci. Lett. 102, 358–374. Mexico and their tectonic significance. Geofı´sica Internacio- McDowell, F.W., Clabaugh, S.E., 1979. Ignimbrites of the nal 20, 177–202. Sierra Madre Occidental and their relation to the tectonic Ortega-Gutie´rrez, F., Mitre-Salazar, L.M., Roldań-Quitana, J., history of western Mexico. Geol. Soc. Am. Spec. Pap. 180, Sańchez-Rubio, G., de la Fuente, M., 1990. H-3 Acapulco 113–124. Trench to the Gulf of Mexico across southern Mexico, Geol. Malfait, B.T., Dinkelman, M.G., 1972. Circum-Caribbean tec- Soc. Am., Centennial Continent/Ocean Transect #13, 1–9, tonic igneous activity and the evolution of the Caribbean Plate with text. Plate. Geol. Soc. Am. Bull. 83, 251–272. Ortega-Gutie´rrez, F., 1993. Tectonostratigraphic analysis and Maniar, P.D., Piccoli, P.M., 1989. Tectonic discrimination of significance of the Paleozoic Acatlań Complex of Southern granitoids. Geol. Soc. Am. Bull. 101, 635–643. Mexico. In: Ortega-Gutie´rrez, F., Centeno-Garcıá, E., Martiny, B., Martıńez-Serrano, R., Ayuso, R.A., Macıás- Morań-Zenteno, D.J., Go´mez-Caballero, A. (Eds.), Terrane Romo, C., Morań-Zenteno, D.J., Alba-Aldave, L., 1997. Pb Geology of Southern Mexico: Guidebook of Field Trip B. isotope geochemistry of Tertiary igneous rocks and conti- First Circum-Pacific and Circum-Atlantic Terrane Confer- nental crustal complexes southern Mexico. In: American ence, Guanajuato, Mexico. Universidad Nacional Auto´- Geophysical Union, Fall Meeting, Transactions, Eos 78, noma de Me´xico, Instituto de Geologıá, pp. 54–60. F844, Abstract. Ortega-Gutie´rrez, F., Ruiz, J., Centeno-Garcıá, E., 1995. Oax- Meschede, M., Frisch, W., Herrmann, U.R., Ratschbacher, L., aquia, a Proterozoic microcontinent accreted to North 1997. Stress transmission across an active plate boundary: America during the late Paleozoic. Geology 23, 1127–1130. 98 B. Martiny et al. / Tectonophysics 318 (2000) 71–98

Pantoja-Alor, J., 1970. Rocas sedimentarias Paleozoicas de la Torres, R. (Eds.), Guidebook to the Mexico-Oaxaca Field regioń centro-septentrional de Oaxaca. In: Segura, L.R., Trip. Soc. Geol. Mexicana, pp. 85–96. Rodrı´guez-Torres, R. (Eds.), Guidebook to the Mexico- Sedlock, R.L., Ortega-Gutie´rrez, F., Speed, R.C., 1993. Tecton- Oaxaca Field Trip. Soc. Geol. Mexicana, pp. 67–84. ostratigraphic terranes and tectonic evolution of Mexico. Patchett, P.J., Ruiz, J., 1987. Nd isotopic ages of crust forma- Geol. Soc. Am. Spec. Pap. 278, 153. tion and metamorphism in the Precambrian of eastern and Singer, B.S., Myers, J.D., Linneman, S.R., Angevine, C.L., southern Mexico. Contrib. Mineral. Petrol. 96, 523–528. 1989. The thermal history of ascending magma diapirs and Pearce, J.A., 1982. Trace element characteristics of lavas from the thermal and physical evolution of magmatic conduits. destructive plate boundaries. In: Thorpe, R.S. (Ed.), Andes- J. Volcanol. Geotherm. Res. 37, 273–289. ites: Orogenic Andesites and Related Rocks. Wiley, Singer, B.S., Myers, J.D., 1990. Intra-arc extension and mag- Chichester, UK, pp. 525–548. matic evolution in the central Aleutian arc Alaska. Geology Pearce, J.A., 1983. Role of the sub-continental lithosphere in 18, 1050–1053. magma genesis at active continental margins. In: Smith, R.D., Cameron, K.L., McDowell, F.W., Niemeyer, S., Hawkesworth, C.J., Norry, M.J. (Eds.), Continental Basalts Sampson, D.E., 1996. Generation of voluminous silicic and Mantle Xenoliths. Shiva, Nantwich, pp. 230–249. magmas and formation of mid-Cenozoic crust beneath Peccerillo, A., Taylor, S.R., 1976. Geochemistry of Eocene calc- north-central Mexico: evidence from ignimbrites, associated alkaline volcanic rocks from the Kastamonu area northern lavas, deep crustal granulites and mantle pyroxenites. Con- Turkey. Contrib. Mineral. Petrol. 58, 63–81. trib. Mineral. Petrol. 123, 375–389. Pindell III, J.L., Cande, S.C., Pitman, W.C., Rowley, D.B., Solari, L.A., Lopez, R., Cameron, K.L., Ortega-Gutie´rrez, F., Dewey, J.F., Labrecque, J., Haxby, W., 1988. A plate kine- Keppie, J.D., 1998. Reconnaissance U/Pb geochronology matic framework for models of Caribbean evolution. Tecto- and common Pb isotopes from the northern portion of the nophysics 155, 121–138. 1Ga Oaxacan Complex, Southern Mexico. Am. Geophys. Ratschbacher, L., Riller, U., Meschede, M., Herrmann, U., Union, Fall meeting, Trans. Eos 79, F931 (Abstr.). Frisch, W., 1991. Second look at suspect terranes in south- Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial ern Mexico. Geology 19, 1233–1236. lead isotope evolution by a two-stage model. Earth Planet. Ross, M.I., Scotese, C.R., 1988. A hierarchical tectonic model Sci. Lett. 26, 207–221. of the Gulf of Mexico and Caribbean region. Tectonophy- Takada, A., 1994. The influence of regional stress and magmatic input on styles of monogenetic and polygenetic volcanism. sics 155, 139–168. J. Geophys. Res. 99, 13563–13573. Ruiz-Castellanos, M., 1970. Reconocimiento geolo´gico en el Tolson-Jones, G., 1998. Deformacioń, exhumacioń y neotectoń- a´rea de Mariscala-Amatitlań, Estado de Oaxaca. In: Segura, ica de la margen continental de Oaxaca: Datos estructurales, L.R., Rodrı´guez-Torres, R. (Eds.), Guidebook to the petrolo´gicos y geotermobarome´tricos. Ph.D. thesis, Uni- Mexico-Oaxaca Field Trip. Soc. Geol. Mexicana, pp. 55–66. versidad Nacional Autońoma de Me´xico, UACPyP, Mexico Ruiz, J., Patchett, P.J., Ortega-Gutie´rrez, F., 1988a. Proterozoic City, Mexico, 98 pp. and Phanerozoic basement terranes of Mexico from Nd iso- Verma, S.P., 1983. Magma genesis and chamber processes at topic studies. Geol. Soc. Am. Bull. 100, 274–281. Los Humeros caldera, Mexico — Nd and Sr isotope data. Ruiz, J., Patchett, P.J., Arculus, R.J., 1988b. Nd–Sr isotope Nature 301, 52–55. composition of lower crustal xenoliths — Evidence for the Verma, S.P., 1984. Sr and Nd isotopic evidence for petrogenesis origin of mid-Tertiary felsic volcanics in Mexico. Contrib. of Mid-Tertiary felsic volcanism in the mineral district of Mineral. Petrol. 99, 36–43. Zacatecas, Zac. (Sierra Madre Occidental ) Mexico. Isotope Salas, G., 1949. Bosquejo geolo´gico de la cuenca sedimentaria Geosci. 2, 37–53. de Oaxaca. Bol. Asoc. Mex. Geo´l. Petroleros 1, 79–156. Verma, S.P., Nelson, S.A., 1989. Isotopic and trace element Saunders, A.D., Norry, M.J., Tarney, J., 1991. Fluid influence constraints on the origin and evolution of alkaline and calc- on the trace element compositions of subduction zone alkaline magmas in the northwestern Mexican Volcanic Belt. magmas. Philos. Trans. R. Soc. London Ser. A 335, J. Geophys. Res. 94, 4531–4544. 377–392. White, W.M., Dupre´, B., 1986. Sediment subduction and Schaaf, P., 1990. Isotopengeochemische Untersuchungen an magma genesis in the Lesser Antilles: isotopic and trace granitoiden Gesteinen eines aktiven Kontinentalrandes: element constraints. J. Geophys. Res. 91, 5927–5941. Alter und Herkunft der Tiefengesteinskomplexe an der Pazi- Wilson, M., 1989. Igneous Petrogenesis: a Global Tectonic fikku¨ste Mexikos zwischen Puerto Vallarta und Acapulco. Approach. Unwin Hyman, London, 466 pp. Ph.D. thesis. Fakulta¨t fur Geowissenschaften de Ludwig- Woodhead, J.D., Fraser, D.G., 1985. Pb, Sr and 10Be isotopic Maximilians Universita¨t. Mu¨nchen, 202 pp. studies of volcanic rocks from the northern Mariana Island. Schaaf, P., Morań-Zenteno, D., Hernańdez-Bernal, M.S., Solis- Implications for magma genesis and crustal recycling in the Pichardo, G., Tolson, G., Ko¨hler, H., 1995. Paleogene conti- Western Pacific. Geochim. Cosmochim. Acta 49, 1925–1930. nental margin truncation in southwestern Mexico: Geochro- Yan˜ez, P., Ruiz, J., Patchett, P.J., Ortega-Gutie´rrez, F., Geh- nological evidence. Tectonics 14, 1339–1350. rels, G.E., 1991. Isotopic studies of the Acatlan Complex, Schlaepfer, C.J., 1970. Geologıá terciaria del a´rea de Yanhuit- southern Mexico: Implications for Paleozoic North Ameri- lań-Nochixtlań, Oaxaca. In: Segura, L.R., Rodrı´guez- can tectonics. Geol. Soc. Am. Bull. 103, 817–828.