stratigraphy, geochemistry and tectonic signiﬁcance of the

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

Stratigraphy, geochemistry and tectonic significanceof the Oligocene magmatic rocks ofwestern Oaxaca, southern Mexico

Barbara Martiny a,*, Raymundo G. Martınez-Serrano b,Dante J. Moran-Zenteno a, Consuelo Macıas-Romo a, Robert A. Ayuso c

a Instituto de Geologıa, Universidad Nacional Autonoma de Mexico, Apdo. Postal 70-296, Ciudad Universitaria,04510 Mexico, Distrito Federal, Mexico

b Instituto de Geofısica, Universidad Nacional Autonoma de Mexico, Ciudad Universitaria, 04510 Mexico, Distrito Federal, Mexicoc United States Geological Survey, National Center, Reston, VA 20192, 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 marginand volcanic sequences in the inland region. K–Ar age determinations reported previously and in the present workindicate 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–Yanhuitlan), the volcanicsuite comprises principally basaltic andesite to andesitic lavas, overlying minor silicic to intermediate volcaniclasticrocks (epiclastic deposits, ash fall tuffs, ignimbrites) that were deposited in a lacustrine-fluvial environment. Thesouthern sector of the volcanic zone includes the Tlaxiaco–Laguna de Guadalupe region and consists of intermediateto silicic pyroclastic and epiclastic deposits, with silicic ash fall tuffs and ignimbrites. In both sectors, numerousandesitic to dacitic hypabyssal intrusions (stocks and dikes) are emplaced at different levels of the sequence. Thegranitoids of the coastal plutonic belt are generally more differentiated than the volcanic rocks that predominate inthe northern sector and vary in composition from granite to granodiorite. The studied rocks show large-ion lithophileelement (LILE) enrichment (K, Rb, Ba, Th) relative to high-field-strength (HFS) elements (Nb, Ti, Zr) that ischaracteristic of subduction-related magmatic rocks. On chondrite-normalized rare earth element diagrams, thesesamples 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 obtainedand 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 isotoperatios and those reported for the basement rocks in this region suggest a relatively low degree of old crustalinvolvement for most of the studied rocks. The Pb isotopic compositions of the Tertiary magmatic rocks also show anarrow range [(206Pb/204Pb)=18.67–18.75; (207Pb/204Pb)=15.59–15.62; (208Pb/204Pb)=38.44–38.59], suggesting a sim-ilar source region for the volcanic and plutonic rocks. Trace elements and isotopic compositions suggest a mantlesource in the subcontinental lithosphere that has been enriched by a subduction component. General tectonic featuresin 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 inlandvolcanic 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 thecharacteristics 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 Volcanicage in southern Mexico is represented by extensive Belt (TMVB) crosses central Mexico from east tooutcrops of plutonic and volcanic rocks that form west at about 19°N and is related to the subductionpart of the Sierra Madre del Sur and define two of the Cocos and Rivera plates beneath the Northbroad belts approximately parallel to the Pacific American plate. Volcanic activity of the TMVBcoast: the coastal plutonic belt and the inland initiated at about 16 Ma and continues to this dayvolcanic sequences (Fig. 1). These rocks, together (Ferrari et al., 1994). These volcanic arc sequenceswith the latest Cretaceous magmatic rocks, often have an oblique distribution (16°) relative to therepresent 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 southernTehuantepec area. The Tertiary magmatism Mexico involving the detachment and lateral dis-roughly displays a decreasing age trend from placement of the Chortis block (Malfait andPaleocene in Colima to Miocene in eastern Oaxaca. Dinkelman, 1972; Ross and Scotese, 1988;

Ratschbacher et al., 1991; Ferrari et al., 1994;The plutonic rocks along the continental marginform a chain of intrusive bodies of different scales, Herrmann et al., 1994; Schaaf et al., 1995). The

presence of mylonitic shear zones along the coastaldominated by composite batholiths commonly cutby silicic and mafic dike swarms. More discontinu- margin of Guerrero and Oaxaca (Fig. 3), produced

during the detachment and subsequent eastwardous outcrops of lava flows, pyroclastic depositsand hypabyssal intrusions make up the inland displacement of the Chortis block, and the unusual

proximity of the coastal plutonic belt to thevolcanic sequences. Between approximately 100°Wand western Oaxaca, the magmatism tends to be Acapulco trench (Fig. 1) support the interpreta-

tion of the truncated character of the continentalOligocene in age, whereas to the west, it is predomi-nantly Upper Cretaceous to Eocene. The exposure margin. The study of the distribution, geochronol-

ogy and geochemical characteristics of the magma-of middle crustal plutonic rocks along the conti-nental margin of southern Mexico, emplaced at tism in southern Mexico is essential for

understanding the tectonic evolution of this regiondepths between 13 and 20 km, and the increasingabundance of volcanic rocks of similar age in the during the Tertiary.

Previous studies of the plutonic rocks along theinland region indicate a relatively rapid uplift,from 30 to 25 Ma before the present time, and Pacific coast of Oaxaca involve the along-the-coast

variations in geochemistry and geochronologyunroofing of the plutonic rocks (Moran-Zentenoet al., 1998). (Bohnel et al., 1992; Herrmann, 1994; Herrmann

et al., 1994; Schaaf et al., 1995; Hernandez-BernalIn northwestern Mexico, arc volcanism relatedto Farallon–North American plate convergence and Moran-Zenteno, 1996). There are also a few

stratigraphic and geochronologic studies for theproduced mid-Cretaceous to early Tertiary mag-matic rocks, including the Oligocene to Miocene inland volcanic rocks (e.g. Salas, 1949; Ruiz-

Castellanos, 1970; Ferrusquıa-Villafranca, 1970,silicic Upper Volcanic sequence of the coast-paral-lel NNE-trending Sierra Madre Occidental (SMO) 1976; Ferrusquıa-Villafranca and McDowell, 1991;

73B. Martiny et al. / Tectonophysics 318 (2000) 71–98

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

Moran-Zenteno et al., 1998). Until now, there Mexico during the Tertiary. Major and trace ele-ments together with Sr, Nd and Pb isotopes ashave been no studies of the geochemistry of the

volcanic rocks of westernmost Oaxaca nor of the well as isotopic dating have been used to addressthis problem. The details of the petrogenesis ofregional geochemical and geochronologic patterns

of the magmatic rocks in this region. We therefore these magmatic rocks will be a subject of a separatepaper and will include the determination of thefocused our studies on an area in western Oaxaca

that crosses the Sierra Madre del Sur and includes isotopic compositions of additional samples.both intrusive and extrusive Tertiary rocks in orderto detect possible variations in the geochronologi-cal and geochemical patterns perpendicular to the 2. Regional geological settingtrench (Figs. 1 and 4).

In this paper, we examine the stratigraphy and 2.1. Basement rocksgeochemistry of the magmatic rocks in westernOaxaca in order to gain insight into the significance The Tertiary magmatic rocks in western and

central Oaxaca are distributed in a region charac-of these rocks in the tectonic evolution of southern


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 terranesfor southern Mexico after Campa and Coney (1983). Abbreviations used in the inset are: G=Guerrero, Mi=Mixteca, O=Oaxaca,X=Xolapa, J=Juarez, M=Maya terrane, SM=Sierra Madre, SMO=Sierra Madre Occidental and TMV=Trans-Mexico VolcanicAxis, C=Coahuila.

terized by contrasting pre-Cenozoic tectonic and schists, gneisses and amphibolites, including ultra-mafic and serpentinitic rocks. It has been interpre-stratigraphic settings. Three major tectonostrati-

graphic units have been recognized on the basis of ted that a major part of this terrane was overthrustin pre-Pennsylvanian time by the Grenville agethe petrotectonic associations and age of their

basement, namely the Mixteca, Oaxaca and Oaxaca terrane and the Esperanza Granitoids arefound in the contact between these two terranesXolapa terranes (Campa and Coney, 1983; Sedlock

et al., 1993) (Fig. 2 inset). The Tertiary volcanic (Sedlock et al., 1993). It is thought that theAcatlan Complex is underlain by a Precambrianrocks of western Oaxaca cover metamorphic and

sedimentary units of the Mixteca terrane and prob- basement that is tentatively considered to beGrenvillian in age (Ortega-Gutierrez et al., 1990).ably the westernmost part of the Oaxaca terrane,

whereas along the coastal margin, Cretaceous and Typical present-day 87Sr/86Sr and eNd values ofthe Acatlan Complex range from 0.7153 to 0.7613Tertiary plutons are emplaced in metamorphic

rocks of the Xolapa terrane. and −8.5 to −12, respectively, for the metasedi-mentary units and the granitoids of the EsperanzaThe basement of the Tertiary volcanic rocks in

the Mixteca terrane is represented by the Acatlan Formation, whereas mafic components of theeclogitic sequences have values ranging fromComplex of Paleozoic age. This complex is formed

by a heterogeneous tectonic assemblage of 0.7058 to 0.7094 and +1.7 to +3.1 (Yanezet al., 1991).metamorphic units ranging from greenschist- to

eclogite-facies (Ortega-Gutierrez, 1978, 1993). It To the east, the Tertiary volcanic rocks of thecentral part of the state of Oaxaca cover theincludes metasedimentary units of phyllites and

migmatites, as well as eclogite-facies micaceous Oaxaca terrane, which is characterized by a granu-


metamorphic rocks, for which there are still uncer-tainties concerning the protolith ages. These rocksare distributed along the continental margin ofeastern Guerrero and Oaxaca. The Xolapa terraneincludes mainly quartz-amphibolites, quartz-feld-spathic gneisses, pelitic paragneisses and schists,as well as some marble lenses and granulite faciesrelicts (Ortega-Gutierrez, 1981; Corona-Chavez,1997; Tolson-Jones, 1998). There is a characteristicoccurrence of migmatites throughout most of theXolapa Complex that indicates different degreesand conditions of anatexis. The present-day87Sr/86Sr ratios reported up to now for the XolapaComplex range from 0.706 to 0.724 and eNdvalues, from −12.4 to +2.5 (Moran-Zenteno,1992). Undeformed Tertiary plutons of this com-plex, excluding the Acapulco intrusion that differsin age and geochemistry from other plutons in theregion, display low 87Sr/86Sr ratios (0.7038–0.7051), and positive eNd values (+0.5 to +3.7)(Moran-Zenteno, 1992; Herrmann, 1994).

The Guerrero terrane lies farther west and hasa younger basement; it is characterized by LateCretaceous and Paleogene continental depositsthat unconformably overlie Mesozoic volcano-sed-imentary units, the age and nature of which aresubject to controversy.

Fig. 3. Map of south-central Mexico showing different Tertiarydeformation domains and indicating their age (modified from

2.2. Tertiary tectonic featuresMoran-Zenteno et al., 1999).

The Tertiary tectonic features of the Oaxacaregion display a contrasting framework that islite-facies metamorphic basement of Grenvillian

age (900–1100 Ma) (Ortega-Gutierrez, 1981, 1993) suggestive of changing dynamic conditions in bothtime and space. Most of the major Cenozoicoverlain by Paleozoic and Mesozoic sedimentary

sequences (Pantoja-Alor, 1970; Schlaepfer, 1970). tectonic features indicate a different tectonic sce-nario with respect to that of central and northernThe metamorphic basement is mainly composed

of mafic and felsic gneisses, as well as metasedi- Mexico dominated by NNW–SSE extensionalfaults for the Oligocene and Miocene. Althoughmentary rocks and charnockites (Ortega-

Gutierrez, 1981, 1993; Ortega-Gutierrez et al., the continuation of the Basin and Range provinceto southern Mexico has been suggested on the1995). The present-day 87Sr/86Sr and eNd isotopic

values of the Oaxaca Complex generally range basis of the orientation and kinematics of somestructures (i.e. Oaxaca fault) (Henry and Aranda-from close to those of bulk earth to 0.717 (although

one paragneiss has a 87Sr/86Sr ratio of 0.750) and Gomez, 1992), many other major features indicatedifferent dynamic conditions from that of centralfrom −9 to −12, respectively (Patchett and Ruiz,

1987; Ruiz et al., 1988a,b). and northern Mexico and include the deformationassociated with the Chortis block displacementTo the south, the Tertiary magmatic rocks of

western Oaxaca occupy the Xolapa terrane, which (Ratschbacher et al., 1991; Ferrari et al., 1994;Nieto-Samaniego et al., 1995; Meschede et al.,is constituted by middle crustal, amphibolite-facies


1997; Tolson-Jones, 1998). Some of the normal In the inland region of western Oaxaca, thedistribution of the Tertiary volcanic rocks seemsfaults in southern Mexico, such as the Oaxaca

fault, have been reactivated several times. The to be controlled by a group of NNW–SSE-trendingfaults that bound a series of down-thrown blocksOaxaca fault is a NNW-trending fault system, top

to the west, that delineates the eastern margin of where interlayered volcanic and lacustrinesequences accumulated (Fig. 4). In some cases, thethe Valley of Oaxaca (Centeno-Garcıa, 1988;

Nieto-Samaniego et al., 1995; Alaniz-Alvarez et al., faults cut the Tertiary volcanic units, and in othercases, the lava flows and pyroclastics overlap the1996) (Fig. 3). An early (Triassic?) mylonitization

event along the Oaxaca fault is probably related fault zones. This fact and the occurrence of dikesemplaced in the faults are indicative of coevalto the collision of the Mixteco-Oaxaca block

against the more eastern Maya terrane. activity. The faults in this region display lateral,vertical and oblique striae, and, based on this factReactivation occurred for the strike-slip phase of

this fault during the Jurassic (Alaniz-Alvarez et al., and the regional distribution of the Mesozoic andTertiary units, Silva-Romo (in preparation) inter-1996) and as a normal fault zone during the

Miocene (Ferrusquıa-Villafranca et al., 1988). preted the Oligocene tectonic framework as an enechelon left-lateral transfer fault system.In the coastal region of Oaxaca and eastern

Guerrero, a series of shear zones with left lateral To the west of the study area, in the Taxco-Huautla region, the Tertiary volcanic rocks areand normal kinematics that trend roughly parallel

to the coast have been recognized. These shear dominantly silicic and range in age from 38 to27 Ma (Moran-Zenteno et al., 1998). The distribu-zones appear to display chronological differences

with later activity to the southeast (Fig. 3). South tion of volcanism in this area does not seem to becontrolled, as in Oaxaca, by transtensional tectonicof Tierra Colorada, Guerrero, a mylonitic zone

affecting the metamorphic rocks of the Xolapa features. In the Taxco region, an 800 m thicksequence of rhyolitc ignimbrites and lava flowsComplex is intruded by a felsic pluton that yielded

concordant U–Pb zircon ages ranging from 32.5 overlaps a system of NW-trending subverticalfaults with a complex kinematic history includingto 34.2 Ma (Herrmann et al., 1994). This mylonitic

zone has kinematic indicators of a normal-left- normal and lateral displacements. The lower partof the rhyolitic sequence, with K–Ar ages ranginglateral oblique shear zone. North of Puerto

Escondido, in the Juchatengo area, a NW-trending from 38 to 35.5 Ma, is affected by NNE-trendinglateral faults, indicating a deformation event con-north to northeast-dipping mylonitic zone has been

recognized with shear criteria that indicate normal temporary with the volcanic activity (Moran-Zenteno et al., 1998).fault kinematics (Ratschbacher et al., 1991). North

of Huatulco and Puerto Angel, there is a well- Based on the analysis of outcrop-scale fault-slipdata, Meschede et al. (1997) interpreted that indefined EW trending shear zone, known as the

Chacalapa Fault, that is characterized by a sub- Tertiary times, there was an effective stress trans-mission across the plate margin represented by thevertical anastomosing geometry and left-lateral

indicators. According to observations carried out continental lithosphere of southern Mexico.According to these authors, prior to 25 Ma, theby Tolson-Jones (1998), the Huatulco intrusion

(U–Pb age of 29 Ma; Herrmann et al., 1994) is s2 axes of the stress field had a sub-vertical orienta-tion, whereas the s1 was roughly parallel to theaffected by the crystal-plastic deformation of this

shear zone, and the mylonite is truncated by oblique motion vector of the oceanic plate withrespect to North America. This interpretation doesgranodiorite dikes, which yielded a 23.7±1.2 Ma

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 andlocation of analyzed rocks. Numbers in parentheses refer to isotopic ages obtained in the present work and reported in otherstudies (Table 1).


major Tertiary tectonic features of the inland and can be observed to rest directly on Paleozoicor Mesozoic rocks.region of Oaxaca and Guerrero, and additional

time-constraint observations seem to be required. The Oligocene volcanic sequence in the northernsector can be divided into two general units. Thelower unit consists of pyroclastic (silicic to interme-diate lithic and vitric ash fall tuffs) and epiclastic

3. General stratigraphic features deposits that were apparently deposited in a lacus-trine 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 tuffthe westernmost part of the state of Oaxaca extend (sample CON-75, Table 1) north of Huajuapan,over an area of approximately 4000 km2 in a region but the potassium concentration in the biotite isknown as the Mixteca Alta. Plutonic rocks crop anomalously low, and therefore, this radiogenicout to the south along the continental margin and age probably does not represent the exact time ofform part of the coastal plutonic belt of southern emplacement. The magmas become more mafic inMexico. The general stratigraphic characteristics the predominant upper unit, which consists of aof the Tertiary volcanic zone of western Oaxaca thick pile (>400 m in some areas) of up to 14 lavapermits 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. Theautobreccias with interbedded tuffs are dominant, lavas have a porphyritic or trachytic texture andand 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-out this region suggests that these lavas were at3.1. Volcanic rocks of the northern sectorleast partially produced by central volcanic struc-tures. Widespread hornblende- or pyroxene-bear-The northern sector includes the areas of

Huajuapan, Zapotitlan, Monte Verde, Chilapa and ing hypabyssal intrusions (dikes and small stocks)of intermediate composition that are emplaced atYanhuitlan (Fig. 4). The volcanic sequences in this

area are mostly Oligocene in age (Table 1) and lie different levels of the Tertiary sequences have beenrecognized throughout western Oaxaca and theon lower Tertiary age conglomerates or directly

on Mesozoic continental and marine sequences or adjacent parts of Puebla (e.g. Ferrusquıa-Villafranca, 1970; Ruiz-Castellanos, 1970). In thePaleozoic metamorphic rocks of the Acatlan

Complex. An andesitic hypabyssal intrusion, study area, hornblende concentrates of a stockand dike yielded K–Ar ages of 33.6±1.4 andemplaced in the reddish mudstones, sandstones

and tuffaceous beds of the Yanhuitlan Formation 34.2±1.4 Ma, respectively (samples CON-8A andCON-91, Table 1).(Fig. 5), yielded a hornblende K–Ar age of

40.5±1.7 Ma (sample CON-7, Table 1), which is Several K–Ar age determinations for whole rocksamples of lavas and hypabyssal intrusions in thisolder than the other ages obtained for volcanic

rocks of this area. This age and a few additional region have also been reported elsewhere. Sevenwhole-rock ages for the Zapotitlan-Huajuapanisolated Eocene ages that have been reported for

this area and the adjacent parts of the state of area range from 32±1 to 29±1 Ma (Galina-Hidalgo, 1996). The small variation between thesePuebla (Grajales-Nishimura, pers. commun.) seem

to represent the earliest manifestations of Tertiary whole rock ages and the ages obtained in thepresent study for mineral separates is probablymagmatic activity in this region. There is no evi-

dence that Eocene magmatism was widespread or due to the different material dated. Farther east,in the Tamazulapan–Yanhuitlan area, Ferrusquıa-volumetrically important in this region since, up

to now, all the other volcanic units in western Villafranca et al. (1974) and Ferrusquıa-Villafranca and McDowell (1991) report a K–ArOaxaca have been dated as Oligocene (Table 1)


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Table 1Age determinations of Tertiary magmatic rocks in western Oaxaca1

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

Northern sectorCON-7 Yanhuitlan 97°23∞36◊ 17°34∞05◊ Hornblende Andesitic laccolith 0.360 0.001023 K–Ar 40.5±1.7 aCON-8A Huajuapan 97°47∞16◊ 17°49∞43◊ Hornblende Andesitic stock 0.458 0.001076 K–Ar 33.6±1.4 aCON-75 N of Huajuapan 97°41∞48◊ 18°04∞51◊ Biotite Silicic tuff 5.662 0.012440 K–Ar 31.4±0.8 aCON-91 N of Huajuapan 97°40∞52◊ 18°02∞36◊ Hornblende Andesitic dike 0.496 0.001187 K–Ar 34.2±1.4 aFV69-180 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 sectorCON-59A L. de Guadalupe 97°51∞20◊ 17°11∞17◊ Hornblende Silicic tuff 0.484 0.001180 K–Ar 34.8±1.4 aCON-101 Tlaxiaco 97°36∞45◊ 17°21∞37◊ Biotite Silicic tuff 7.732 0.017810 K–Ar 32.9±0.9 a

Coastal plutonsCON-53 S. Ma. Zacatepec 97°58∞36◊ 16°53∞27◊ Biotite Granite 7.475 0.013330 K–Ar 25.5±0.7 aG-17 Jamiltepec 97°57∞01◊ 16°10∞21◊ Biotite Granite 7.809 0.0151 K–Ar 27.7±0.7 cMS-28 Progreso 97°45∞55◊ 16°09∞49◊ Biotite Granite 7.793 0.0133 K–Ar 24.4±0.6 cMS-34 Rıo Grande 97°26∞44◊ 16°00∞40◊ Biotite Granite 7.64 0.0125 K–Ar 23.5±0.6 cMS-35 Jamiltepec 97°49∞23◊ 16°16∞38◊ Hornblende Tonalite 0.874 0.0018 K–Ar 29.9±1.1 cMS-42 Progreso 97°47∞24◊ 16°15∞40◊ Hornblende Granodiorite 1.029 0.0019 K–Ar 27.7±1.0 cMu20 N of S.P. Amuzgos 98°03∞21◊ 16°40∞53◊ Zircon Granodiorite U–Pb 301 dMx12 NW of Progreso 97°45∞07◊ 16°09∞48◊ Zircon Tonalite U–Pb 281 dMu9 NW of Pochutla 96°38∞07◊ 15°51∞00◊ Zircon Granodiorite U–Pb 271 d

a Ref. (= reference): a: this work; b: Ferrusquıa-Villafranca et al. (1974) and Ferrusquıa-Villafranca and McDowell (1991); c:Hernandez-Bernal and Moran-Zenteno (1996); d: Herrmann et al. (1994).40Ar1=radiogenic 40Ar; l

b-(40K)=4.962×10−10/yr; (le+l∞e)=0.581×10−10; 40K/K=1.193×10−4 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-ing the different material dated, possible reheatingthe Yucudaac Andesite and a biotite K–Ar age

of 26.2±0.5 Ma for the Llano de Lobos of the tuff during the emplacement of the overlyinglavas or a lack of horizontal continuity within theFormation, a volcaniclastic sequence composed of

rhyolitic to andesitic tuffs, welded ignimbrites and volcanic sequences and variations in the volcanicstratigraphy so that the tuff dated is actuallyepiclastic deposits.

In the inland volcanic area, an eastward decreas- younger than the lava dated (Ferrusquıa-Villafranca et al., 1974). We consider that, in ordering age trend is seen between westernmost Oaxaca

and east-central Oaxaca where Miocene ages are to define the age trend between Huajuapan andYanhuitlan, it would be necessary to carry outreported (Ferrusquıa-Villafranca et al., 1974;

Ferrusquıa-Villafranca and McDowell, 1991). The additional age determinations on mineral concen-trates in this latter area.younger age obtained for the Llano de Lobos

Formation might also suggest a decreasing agetendency from Huajuapan to Yanhuitlan, but this 3.2. Volcanic rocks of the southern sectortrend is not clear. There is an apparent discrepancybetween the two ages from the Tamazulapan- The southern sector includes the areas of

Tlaxiaco, northwest of Chalcatongo, and smallerYanhuitlan area since the Llano de LobosFormation is shown to underlie the Yucudaac outcrops in the areas of Cuquila and Laguna de

Guadalupe of approximately 5 and 35 km2, respec-Andesite in the stratigraphic column presented byFerrusquıa-Villafranca (1976). There are several tively (Fig. 4). The Tertiary volcanic sequences in


this sector unconformably overlie Mesozoic sedi- along the southern limit, in the area of San PedroAmuzgos, this batholith is in contact with meta-mentary sequences and Tertiary conglomeratesmorphic rocks of the Xolapa Complex.(Fig. 5). The conglomerates in the Tlaxiaco area

The plutons are medium-grained granodioritescontain lithic fragments of volcanic and calcareousand granites containing biotite and/or hornblenderocks and siltstones and are generally <10 m thick.and are more highly differentiated than the inlandThe sequence in the southern sector is dominatedvolcanic rocks, particularly the predominant upperby intermediate to silicic volcaniclastic deposits ofunit lavas of the northern sector. K-feldspar isepiclastic and pyroclastic origin that include ash-generally microcline and commonly displays per-fall tuffs and reaches a thickness of up to 300 m.thitic intergrowths. Accessory minerals includeA biotite-bearing silicic ignimbrite caps thissphene, which sometimes occurs in large euhedralsequence in Cuquila. The samples dated (K–Ar)to subhedral crystals, apatite, iron-oxides andfrom the southern sector are Oligocene in age:zircon. Abundant swarms of aplitic dikes of NW–34.8±1.4 Ma was obtained for hornblende in aSE and NE–SW orientation intrude the plutonicvolcaniclastic rock and 32.9±0.9 Ma for biotite inrocks, especially near the southern border of thea silicic tuff (samples CON-59b and CON-101,La Muralla pluton, south of Santa MarıaTable 1). Although extensive lavas were not recog-Zacatepec, as well as in the coastal region.nized in the southern sector, abundant hypabyssal

Isotopic ages of the plutons are only slightlyintrusions, similar to those in the northern sector,younger than those of the inland volcanicare emplaced in the volcaniclastic sequence. Thesesequences. In the present study, a biotite concen-hypabyssal rocks display a porphyritic texture withtration yielded an K–Ar age of 25.5±0.7 Mapyroxene or hornblende phenocrysts in a microlitic(sample CON-53, Table 1) in a granite north ofplagioclase groundmass. The intrusions are daciticSanta Marıa Zacatepec, and Hernandez-Bernalto andesitic stocks and dikes of varying dimen-and Moran-Zenteno (1996) report five K–Ar cool-

sions. North and northeast of Tlaxiaco, severaling ages in biotite and hornblende (Table 1) of the

lava flows extending over a distance of approxi- Jamiltepec and Progreso areas that range frommately 5 km and displaying a general NE–SW 29.9 to 23.5 Ma. U–Pb crystallization agestrend, were recognized. obtained by Herrmann et al. (1994) for unde-

formed Tertiary age plutonic rocks from PinotepaNacional to Huatulco range from 30 to 27 Ma.

3.3. Granitoids of the coastal plutonic belt

In this paper, the intrusive rocks that areexposed in the La Muralla–San Pedro Amuzgos 4. Sample selection and analytical methodsregion in western Oaxaca (Fig. 4) are referred toas the La Muralla pluton. Similar plutonic rocks Tertiary age volcanic and plutonic rocks as wellare 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.Rıo Grande (Fig. 4) where they have been named The different stratigraphic units of the volcanicthe Rıo Verde batholith by Hernandez-Bernal and sequences as well as hypabyssal intrusions wereMoran-Zenteno (1996). The La Muralla pluton sampled in both the northern and southern areas.appears to be an extension of the Rıo Verde From the coastal plutonic belt, samples werebatholith, 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 morein southern Mexico. The La Muralla pluton is than 150 sampled rocks were studied to classifyemplaced between two distinct terranes. At the the rocks and select fresh samples for bulk chemicalnorthern limit, these rocks intrude Paleozoic meta- analyses, K–Ar determinations and other geochem-

ical studies.morphic rocks of the Acaltan Complex, whereas


Table 2Major 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 53.36Al2O3 17.94 17.02 16.76 17.15 16.84 16.70 16.97 18.24 17.71 16.80Fe2O3 4.52 8.38 8.33 7.39 6.35 8.34 6.80 6.11 8.85 8.21MnO 0.02 0.10 0.08 0.07 0.08 0.11 0.07 0.08 0.12 0.11MgO 1.41 5.00 5.18 4.13 3.30 4.66 3.84 4.03 5.57 5.98CaO 5.37 7.28 8.43 6.78 5.94 7.32 6.85 7.28 7.87 7.87Na2O 4.23 4.01 3.92 3.92 3.47 3.90 3.71 3.19 4.02 3.74K2O 1.31 1.01 1.17 1.25 1.66 1.11 1.55 1.20 0.81 1.00TiO2 0.80 1.29 1.36 1.24 0.89 1.37 0.87 0.90 1.34 1.24P2O5 0.29 0.33 0.48 0.34 0.25 0.34 0.24 0.33 0.31 0.32L.O.I. 2.88 0.90 2.62 1.63 2.05 1.57 1.75 3.78 1.74 1.34Total 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 459Rb 23 19 23 23 46 26 31 47 14 21Ba 412 274 460 381 511 312 436 335 219 309Th 3 1 4 2 4 2 3 2 1 2Nb 6 6 13 7 5 7 5 5 5 6Zr 158 150 165 148 161 146 137 128 130 139Hf 4 3 3Y 17 16 19 15 14 16 13 16 16 17Sc 9 14 17 13 12 14 14 13 15 16Cr 20 139 188 111 48 114 66 29 214 208Ni 10 68 78 48 23 50 28 14 101 74Co 19 31 40 32 38 42 50 23 37 45

La 28 15 30 19 20 17 17 18 12 16Ce 45 35 61 44 44 40 37 38 30 37Pr 7 5 8 6 5 5 5 5 4 5Nd 32 22 31 26 21 23 19 22 19 21Sm 6 5 7 6 5 5 4 4 5 5Eu 1.74 1.65 1.97 1.59 1.40 1.61 1.24 1.38 1.47 1.58Gd 5 4 5 4 3 4 3 4 4 4Tb 0.64 0.61 0.72 0.61 0.51 0.61 0.47 0.53 0.57 0.61Dy 3 3 4 3 3 3 2 3 3 3Ho 0.65 0.57 0.72 0.64 0.52 0.62 0.53 0.63 0.65 0.68Er 1.49 1.35 1.75 1.41 1.22 1.46 1.21 1.44 1.43 1.52Tm 0.22 0.20 0.27 0.19 0.19 0.21 0.18 0.21 0.22 0.23Yb 1.42 1.18 1.59 1.24 1.21 1.32 1.13 1.40 1.35 1.41Lu 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- Petrographiques et Geochimiques, CentreNational du Recherches Scientifiques, in Nancy,mined by inductively coupled plasma emission,

and all other trace elements by inductively coupled France. For conventional mineral K–Ar measure-ments, rock was crushed and sieved to retain theplasma mass spectrometry (ICP-MS) in the analyt-

ical laboratories of the Centre de Recherches 0.125–0.18 mm size fraction. Biotite was separated


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 58.91Al2O3 16.89 14.70 16.85 17.50 17.43 16.75 16.27 16.33 18.76 16.94Fe2O3 6.09 1.75 8.33 8.98 9.15 7.37 4.55 4.06 5.54 6.29MnO 0.09 0.02 0.11 0.07 0.10 0.09 0.07 0.05 0.07 0.06MgO 3.21 1.09 6.29 4.91 5.57 3.86 2.08 1.84 1.69 2.05CaO 5.76 2.79 7.50 7.73 7.67 6.45 4.91 4.34 5.69 5.67Na2O 3.49 3.78 3.79 3.64 3.90 3.35 3.60 3.79 2.96 3.79K2O 1.93 2.20 0.93 0.83 0.75 1.76 2.26 1.82 1.59 2.28TiO2 0.91 0.32 1.26 1.32 1.49 1.12 0.68 0.63 0.70 0.91P2O5 0.26 0.08 0.32 0.31 0.31 0.22 0.14 0.13 0.19 0.26L.O.I. 1.97 6.14 1.42 3.18 0.82 1.83 2.27 3.19 6.88 2.53Total 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 464Rb 40 131 15 15 13 51 53Ba 575 631 315 213 211 376 511Th 3 5 1 1 1 4 3Nb 6 6 5 5 5 5 6Zr 173 144 132 115 130 113 151Hf 4 4 3 3 3Y 14 11 15 16 17 19 16Sc 11 5 15 15 15 10 11Cr 51 14 202 224 181 28 36Ni 19 4 83 85 71 20 19Co 31 24 34 25 33 21 29

La 23 16 16 13 13 17 20Ce 48 31 36 28 30 29 39Pr 6 3 5 4 4 4 6Nd 24 12 21 19 20 17 24Sm 5 2 5 4 5 4 5Eu 1.35 0.89 1.51 1.51 1.69 1.33 1.38Gd 4 2 4 4 4 3 4Tb 0.53 0.31 0.56 0.57 0.62 0.51 0.59Dy 3 2 3 3 3 3 3Ho 0.59 0.42 0.59 0.66 0.65 0.63 0.62Er 1.37 1.09 1.39 1.55 1.54 1.41 1.35Tm 0.19 0.16 0.20 0.22 0.23 0.21 0.21Yb 1.24 1.17 1.29 1.43 1.45 1.31 1.18Lu 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-blende. Mineral concentrates of >99.5% purityelectronic mortar to separate the mica sheets and

free possible chlorite. Hornblende was separated were prepared and were analyzed by GeochronLaboratory Division of Krueger Enterprises, Inc.magnetically and with heavy liquids. Three of the

hornblende separates were acid-leached at room 87Sr/86Sr and 143Nd/144Nd ratios were measuredon a Finnigan MAT 262 mass spectrometer attemperature in an ultrasonic cleaner in 10% HF


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 65.38Al2O3 17.42 12.24 16.38 16.52 14.70 16.25 15.07 15.11 16.41 16.27Fe2O3 6.81 1.81 6.01 4.91 2.55 3.60 2.94 2.99 4.42 4.35MnO 0.10 Traza 0.06 0.05 0.02 0.05 0.05 0.05 0.05 0.05MgO 3.35 0.75 3.64 1.60 0.82 1.29 0.78 0.81 1.77 1.67CaO 7.00 1.93 5.85 4.64 1.32 3.37 2.97 2.93 4.12 3.99Na2O 3.62 0.97 3.60 2.90 1.75 4.04 3.85 3.88 3.97 4.03K2O 1.58 4.88 1.80 2.48 5.82 2.92 3.29 3.29 2.66 2.65TiO2 0.89 0.20 0.86 0.62 0.21 0.48 0.36 0.36 0.63 0.59P2O5 0.24 0.02 0.23 0.20 0.05 0.17 0.13 0.14 0.18 0.18L.O.I. 1.98 9.09 2.02 4.53 3.90 0.82 0.93 0.74 0.71 0.72Total 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 357Rb 37 171 49 66 124 83 86 85 80 75Ba 411 814 525 484 434 842 671 745 611 646Th 3 9 4 6 10 6 8 7 5 6Nb 5 6 5 5 6 7 7 7 5 5Zr 124 111 153 150 100 143 136 138 146 158Hf 4 3 4 4 4Y 16 26 14 12 9 12 11 10 11 11Sc 13 3 12 9 4 6 5 5 7 7Cr 34 4 110 25 11 7 5 4 11 10Ni 18 3 40 12 2 3 5 4 7 7Co 37 10 23 16 21 34 76 74 42 39

La 16 58 22 23 21 23 17 22 18 18Ce 34 56 46 47 40 47 35 43 41 39Pr 4 13 6 6 4 6 4 5 5 5Nd 19 49 23 21 16 19 17 19 19 19Sm 4 8 5 4 3 4 3 3 4 4Eu 1.22 1.37 1.32 1.12 0.49 0.86 0.92 0.92 0.93 0.94Gd 3 6 3 3 2 3 3 3 3 3Tb 0.53 0.92 0.51 0.45 0.34 0.43 0.39 0.37 0.43 0.43Dy 3 5 3 2 2 2 2 2 2 2Ho 0.62 0.95 0.52 0.48 0.34 0.45 0.41 0.36 0.43 0.43Er 1.50 2.31 1.25 1.24 0.79 1.16 0.93 0.89 1.04 1.01Tm 0.20 0.31 0.18 0.18 0.12 0.17 0.14 0.13 0.13 0.14Yb 1.32 2.03 1.08 1.18 0.79 1.12 0.99 0.80 0.95 0.97Lu 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 IsotopeGeochemistry (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 Geoquımico samples were determined on a Finnigan MAT 262mass spectrometer at the United States GeologicalIsotopico), UNAM. Lead isotopic compositions

of HF-leached feldspar separates and whole rock Survey in Reston, Virginia. Procedural Pb blanks


during this study were less than 200 pg and weretherefore negligible compared to the values mea-sured in the samples.

5. Geochemical results

Major- and trace-element compositions weredetermined in the Tertiary magmatic rocks of thestudy area; major elements were obtained in 31samples and trace elements in 28 (Table 2). Allvolcanic and plutonic rocks analyzed were classi-fied on an anhydrous basis. Previous geochemicalstudies carried out by Hernandez-Bernal andMoran-Zenteno (1996) on the Rıo Verde batholithinclude major and trace elements as well as Sr andNd isotope determinations of the Jamiltepec,Progreso and Rıo Grande intrusions.

5.1. Major- and trace-element geochemistry

Fig. 7. K2O–SiO2 classification diagram after Peccerillo andThe lavas are the least differentiated rocks of Taylor (1976). I=arc tholeiitic series; II=calc-alkaline series;

III=high-K calc-alkaline series; IV=shoshonitic series.the study area and vary from 53 to 61% SiO2 Western Oaxaca samples (this study): crosses=lavas, open cir-(anhydrous basis). Hypabyssal rocks are generallycles=hypabyssal stocks and dikes, diamonds=tuffs. Openmore evolved and range from 54 to 67%. Usingsquares=volcanic rocks from northeastern Guerrero (data

the classification system of Le Maitre (1989), the from Moran-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); mostsamples from both groups have medium-Kcontents. The volcanic rocks from western Oaxacaare characterized by being subalkaline (Fig. 6)with a calc-alkaline affinity. The K2O contents ofthese magmatic rocks are generally typical of thenormal calc-alkaline series based on Peccerillo andTaylor (1976), as shown in Fig. 7. Pyroclasticrocks are intermediate to felsic in composition.Oligocene volcanic rocks from the NE Guerreroarea (Taxco, Huautla and Buenavista areas) arealso more silicic than the intermediate lavas thatFig. 6. Total alkali — SiO2 for Oligocene age volcanic rocks ofpredominate in the northern sector of westernwestern Oaxaca for the classification of nonpyroclastic rocks

after Le Maitre (1989). B=basalt, BA=basaltic andesite, A= Oaxaca. In this part of Guerrero, the volcanicandesite; D=dacite, R=rhyolite, TB=trachybasalt, TBA= sequences consist principally high-K rhyolitic tobasaltic trachyandesite, TA=trachyandesite, T=trachyte.

dacitic ignimbrites and lava flows with no signifi-Division between alkaline and subalkaline fields from Irvinecant intermediate components (Moran-Zentenoand Baragar (1971). Crosses=lavas, open circles=hypabyssal

rocks. et al., 1998) (Fig. 7).


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

The plutonic rocks analyzed in this study are with enrichment in large-ion lithophile elements(LILE) ( K, Rb, Ba, Th) relative to the HFSfrom the La Muralla–San Pedro Amuzgos region

and plot in the granodiorite and granite fields elements (Nb, Ti±Zr) that are characteristic ofsubduction-related magmatism (e.g. Gill, 1981;(SiO2=66–70%) using the classification of Cox

et al. (1979) modified by Wilson (1989) (Fig. 8, Pearce, 1982, 1983; McCulloch and Gamble, 1991;Saunders et al., 1991). Compared to the lessTable 2). Closer to the Pacific coast, the Rıo

Grande and Progreso intrusions of the Rıo Verde differentiated inland volcanic sequences, the plu-tons display more negative spikes for Ti andbatholith show a very similar composition, with

the exception of the Jamiltepec intrusion, which is P2O5, are more enriched in incompatible elements,and show a greater depletion in compatible ele-less differentiated. The data corresponding to the

La Muralla pluton and the Rıo Verde batholith ments (Cr, Ni). The patterns for the immobileelements (Nb, Zr, Hf, Ti Y and Yb) on variationstraddle the boundary between the peraluminous

and metaluminous rocks using Shand’s index diagrams (Fig. 9a and b) show more similarity tothat of intra-plate basalts than to MORB. This(Maniar and Piccoli, 1989). These samples have

an A/CKN coefficient (Al2O3/CaO+K2O+ and the enrichment in LILE suggest an enrichedmantle source in the subcontinental lithosphereNa2O) of <1.1 (molar ratio) and relatively high

sodium contents (Na2O>3.2%), corresponding to modified by subduction fluids, which have addedthe more mobile elements (Rb, Ba, K) (Pearce,I-type granites based on the classification of

Chappell and White (1974). 1983; Wilson, 1989). The inland volcanic sequenceshave Ba/La ratios that vary from 15 to 25 andVariation diagrams for trace elements are shown

for the western Oaxaca samples in Fig. 9a and b. La/Nb from 2 to 5 which is within the range thatis typical of calc-alkaline lavas from other con-Trace-element patterns of the coastal plutons and

the volcanic rocks of the inland region are similar vergent plate boundaries (Gill, 1981).


Fig. 9. Trace-element variation diagrams in Tertiary magmatic rocks of western Oaxaca, MORB normalized using the values ofPearce (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 rareearth elements (HREE; Tb–Lu) (Fig. 10a and b).the samples of the inland volcanic sequences and

the coastal plutonic belt also show similar tenden- Although some granitoids display a very modestnegative Eu anomaly (Fig. 10b), no significantcies. Chondrite-normalized REE patterns display

light rare earth element enrichment (LREE; La– anomalies are observed in the volcanic rocks


Fig. 10. Chondrite-normalized rare earth element data for Tertiary magmatic rocks of western Oaxaca normalized using the valuesof 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 40to 90 times chondrite and Lu, four to seven timeswas not significant. The plutonic rocks have

slightly lower HREE concentrations; (La/Lu)cn chondrite. The LREE correlate positively withSiO2 in the Tertiary magmatic rocks although thisratios range from 6.0 to 13.6 in the lavas and

hypabyssal rocks, and from 11 to 16.7 in the tendency is not displayed by the HREE.


5.2. Isotope geochemistry for the volcanic sequences, and from 0.7042 to0.7044 for the granitoids in the La Muralla–SanPedro Amuzgos area (Table 3). eNdi valuesThe results of a first group of isotopic analyses

are presented here and consist of 12 Sr determin- obtained for the Tertiary magmatic rocks of thisstudy area range from close to 0 to +2.6 (Table 3,ations, 10 for Nd and 9 for Pb (Tables 3 and 4).

Despite the contrast in the degree of differentia- Fig. 11). Considering the isotopic heterogeneity ofthe crust in western Oaxaca, the narrow rangestion, the silicic coastal plutons and the intermediate

volcanic units of the northern volcanic sector show and generally low 87Sr/86Sr ratios and eNdi valuesnear and mostly above that of bulk earth suggestsimilar isotopic features within narrow ranges.

Initial 87Sr/86Sr ratios of the Oligocene samples a relatively low degree of crustal contamination.The Eocene age laccolith located in the easternare relatively low and range from 0.7042 to 0.7046

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

Sample Rb Sr Sm Nd (87Sr/ 87Rb/ (87Sr/ (143Nd/ 147Sm/ (143Nd/ (eNd)0 (eNd)ino. (ppm) (ppm) (ppm) (ppm) 86Sr)m 86Sr 86Sr)i 144Nd)m 144Nd 144Nd)i

Lavas and hypabyssal intrusionsCON 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 0.703688CON 9 13.2 535 5.16 22.1 0.704371±60 0.071 0.704336 0.512749±33 0.1414 0.512718 2.17 2.41CON 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 0.704511CON 18 35.5 508 4.72 23.4 0.704724±41 0.202 0.704626 0.512619±43 0.1219 0.512592 -0.37 -0.05CON 35 38.4 548 4.96 25.5 0.704715±45 0.203 0.704617 0.512623±20 0.1178 0.512597 -0.29 0.05CON 70 48.6 490 4.64 23.8 0.704692±36 0.287 0.704553 0.512665±16 0.1179 0.512639 0.53 0.87CON 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 plutonCON 52 86a 358 3.45 17.8 0.704668±46 0.695 0.704372 0.512726±33 0.1170 0.512703 1.72 2.02CON 53 85.2a 338 3.69 19.1 0.704677±40 0.730 0.704366 0.512723±45 0.1166 0.512700 1.66 1.96CON 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

Jamiltepec503 50 660 4 33 0.704339±59 0.263 0.704287 0.512614±29 0.138 0.51258 -0.2504 47 681 0.704313±46 0.220 0.704270

Progreso505 75 480 2 24 0.704701±36 0.466 0.704616 0.512617±40 0.118 0.51264 0.9506 46 450 4 23 0.704735±248 0.355 0.704678 0.512651±33 0.113 0.51263 0.5507 53 677 0.704271±34 0.284 0.704227508 94 352 5 18 0.70553±33 0.933 0.705387 0.512513±30 0.112 0.51249 -2.1

Rıo Grande509 72 413 0.704997±41 0.601 0.704905510 62 437 4 22 0.705394±32 0.533 0.705314 0.51247±34 0.090 0.51245 -3.0511 54 470 2 24 0.705444±41 0.417 0.70538 0.51247±51 0.090 0.51245 -3.0512 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 lavasand hypabyssal intrusions, with the exception of CON-7, which was calculated at 40.5 Ma, and assuming a present-day143Nd/144Nd (CHUR)=0.512638.b Rıo Verde batholith data from Hernandez-Bernal and Moran-Zenteno (1996).


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

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

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

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

Oaxaca ComplexCON-215 WR S of Oaxaca City Metagabbro 17.248 15.486 36.578CON-215 ksp Metagabbro 17.221 15.501 36.602CON-336 WR Nochixtlan–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 Quaternarymagmatic provinces of Mexico. The shaded field represents the Tertiary age magmatic rocks of western Oaxaca: open circles: plutonicrocks 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 Rıo Verde batholith (data from Hernandez-Bernal and Moran-Zenteno, 1996); squares: plutonic rocksof the Jamiltepec and San Pedro Amuzgos areas (data from Herrmann, 1994). Other magmatic provinces: SMO: Sierra MadreOccidental (SMO field includes data for the Upper Volcanic sequence of the northern SMO, with dashed lines enclosing typicalvalues, 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 ofsouthern Mexico: M=Manzanillo, J=Jilotepec, Z=Zihuatanejo, and H=La Huacana (data from Schaaf, 1990); A=Acapulco (datafrom Schaaf, 1990; Moran-Zenteno, 1992; Herrmann, 1994).


Fig. 12. Plot of 207Pb/204Pb–206Pb/204Pb for feldspars and whole-rock samples of Tertiary magmatic rocks of western Oaxaca andnortheastern Guerrero, and Precambrian basement rocks; data for preliminary Paleozoic Acatlan Complex field are from Lopezand Cameron (unpublished data) and Martiny et al. (1997). X=field for undeformed Tertiary plutons of the Xolapa terrane betweenAcapulco 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. (submittedfor publication). Reference lines are the two-stage terrestial lead evolution curve (Stacey and Kramers, 1975), graduated at 250 Maintervals (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 byHerrmann (1994) have similar Sr and Nd values.of Yanhuitlan, has a lower 87Sr/86Sr ratio of 0.7037

and could reflect less crustal involvement; this Pb isotopic ratios of whole rocks and leachedfeldspars of the Tertiary magmatic rocks of westernsample has an eNdi value of +2.0 (Table 3).

Dacites and rhyolites from northeastern Oaxaca determined in this study display a relativelyrestricted range, suggesting that the source of theseGuerrero are of a similar age (30.5–38.2 Ma), and

in Taxco, for example, five samples analyzed have rocks is similar. On Pb isotope diagrams, the ratiosof the volcanic, hypabyssal and plutonic rockshigher initial 87Sr/86Sr ratios that range from

0.7052 to 0.7063 (Moran-Zenteno et al., 1998), overlap and plot below the average Pb crust evolu-tion curve of Stacey and Kramers (1975) (Fig. 12).which could be explained by more crustal contami-

nation or a more evolved crustal component. The The volcanic rocks of western Oaxaca displaypresent-day ratios of (206Pb/204Pb)=18.67–18.72,volcanic rocks analyzed from Taxco are near the

boundary between the Mixteca and Guerrero (207Pb/204Pb)=15.59–15.61, and (208Pb/204Pb)=38.44–38.53. The granitoids show similar ratiosterranes.

Sr and Nd ratios obtained by Hernandez-Bernal of (206Pb/204Pb)=18.70–18.75, (207Pb/204Pb)=15.59–15.62, and (208Pb/204Pb)=38.48–38.59and Moran-Zenteno (1996) for the Rıo Verde

batholith show more variation than the granitoids (Table 4). The lead isotope range of the Tertiaryigneous rocks of the study area resembles that ofanalyzed in the present study (Table 3, Fig. 11).

Tonalitic intrusions of the Jamiltepec area display the orogene reservoir in the plumbotectonics modelof Doe and Zartman (1979).values similar to those of the La Muralla pluton

located farther inland, whereas the Progreso and In Fig. 12, the Tertiary magmatic rocks of west-ern Oaxaca appear to define a steep mixing trendRıo Grande intrusions, located to the east of

Jamiltepec, present similar or higher 87Sr/86Sri between a mantle component and a 207Pb-richreservoir. Steep trends are typical of some subduc-ratios and similar or lower eNdi values (Table 3).


tion-related rocks, such as the Aleutian, Cascades, event commenced in Oligocene times (Table 1).Oligocene magmatism in western Oaxaca and east-Mariana and Lesser Antilles arcs (Kay et al., 1978;

Woodhead and Fraser, 1985; White and Dupre, ern Guerrero is coeval with the displacement of theChortis block along the Pacific margin of southern1986) and have been interpreted as due to the

incorporation of radiogenic Pb from subducted Mexico (Herrmann et al., 1994; Schaaf et al., 1995)and the consequent migration of the trench–trench–sediments (Hawkesworth et al., 1991). The more

silicic volcanic rocks from NE Guerrero are slightly transform triple junction that constituted the inter-section between the North American, Farrallon andmore radiogenic; in a 207Pb/204Pb vs. 206Pb/204Pb

diagram, they plot to the right of the Stacey and Caribbean plates (Pindell et al., 1988; Ross andScotese, 1988; Herrmann et al., 1994; Schaaf et al.,Kramers curve and fall in a more scattered field

(Fig. 12) (Martiny et al., 1997). Pb ratios are also 1995; Moran-Zenteno et al., 1996). The relationshipbetween the ages obtained for the mylonitic zonesreported in leached plagioclase feldspars of five

undeformed Tertiary granitoids of the Xolapa ter- parallel to the coast and the plutons support thisinterpretation.rane (Herrmann et al., 1994) from the area that

extends from Acapulco to Pochutla (Fig. 12). On a regional scale, along-the-coast magmatismin southern Mexico during the Tertiary displays aCompared to the magmatic rocks of the study

area, these plutons have similar 206Pb/204Pb ratios rough decreasing age trend from northwest tosoutheast (Schaaf et al., 1995). The gradual extinc-and similar or slightly lower 207Pb/204Pb and

208Pb/204Pb ratios. tion of magmatism along the coast, at least to theeast of the Zihuatanejo region, is directly relatedPb isotopic compositions have been obtained for

the Precambrian and Paleozoic basement rocks in to the passage of the triple junction (Herrmannet al., 1994; Schaaf et al., 1995). In the inlandthe present study and by other workers who are

addressing problems related to the basement rocks volcanic belt, certain differences are displayed inthis decreasing age trend, particularly by the(Solari et al., 1998; Lopez et al., in press; Cameron

et al., submitted for publication). Whole rock Miocene ages in the region between the Valley ofsamples and feldspar separates from the igneous Oaxaca and Nejapa areas (Ferrusquıa-Villafrancaunits of the Precambrian Oaxaca Complex (meta- and McDowell, 1991) that lie north of thegabbro, metasyenite, charnockite, metagranite and Huatulco area where there are still Oligocene ageanorthosite) 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–Pb16.95–17.55; (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 andLopez et al., in press; Cameron et al., submitted Rıo Grande are slightly younger than those of thefor publication). Preliminary Pb isotopic composi- inland volcanic rocks (34.8–31.4 Ma) (Table 1).tions of the Paleozoic Acatlan Complex units are However, the K–Ar ages reported for the plutonsvery scattered on a 207Pb/204Pb–206Pb/204Pb dia- correspond to mineral cooling ages (biotite andgram (preliminary field shown in Fig. 12) and lie hornblende) and are not directly comparable toabove 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 theHuajuapan–Tlaxiaco area indicates that the extru-

6. Discussion sive rocks are slightly older. None the less, thereports of younger whole rock and mineral ages

6.1. Space–time trends of magmatism present in the inland area (FerrusquıaVillafrancaand McDowell, 1991; Galina-Hidalgo, 1996) pre-vent us from confirming a southward migration ofStratigraphic and geochronologic evidence indi-

cates that in western Oaxaca, a major magmatic the magmatism. Instead, we consider that the


western Oaxaca magmatic rocks constituted a (Progreso and Rıo Grande areas) also show morevariable isotopic compositions and have lower eNdbroad arc parallel to the coast in Oligocene time

(~35 to ~25 Ma). In western Oaxaca, as well as values and higher 87Sr/86Sr ratios (Table 3). Thisslightly greater range of Sr and Nd ratios in thein northeastern Guerrero, magmatic activity ceased

in the late Oligocene and recommenced to the Tertiary plutons of the Xolapa terrane might bethe result of greater crustal assimilation duringnorth at about 16 Ma in the Trans-Mexican

Volcanic Belt. The magmatic gap at this longitude magma ascent or assimilation of crust with a moreheterogeneous isotopic composition.was probably caused by changes in the geometry

of the subducted slab after the passage of the triple An even greater variability is seen in the Ndisotopic composition of the Tertiary coastal plu-junction (Moran-Zenteno et al., 1996). In central

and eastern Oaxaca, magmatism continued until tons of the Guerrero terrane. These plutons, withthe exception of Puerto Vallarta (Schaaf et al.,Miocene time (Ferrusquıa-Villafranca and

McDowell, 1991). 1995), have higher eNd values (+1 to +6.37)(Schaaf, 1990; Bohnel et al., 1992) than the westernOaxaca plutons (−3.0 to +2.6) (Fig. 11, Table 3).6.2. Geochemical patterns and variationsThe reason for this difference is not clear. TheGuerrero terrane is part of a relatively youngThere are certain differences in the geochemicalcrustal segment that was integrated with the Northbehavior between the extensive inland volcanicAmerican plate during the Mesozoic (Centeno-sequences of the predominant upper unit in theGarcıa et al., 1993), whereas the Tertiary magmaticnorthern sector and the Oligocene magmatic rocksrocks in western Oaxaca have an older basement.of other adjacent regions. The most evident differ-This difference could be explained by a lithosphericence is the degree of differentiation. In westernmantle that is more enriched in a subductionOaxaca, the SiO2 contents of the magmatic rockscomponent in western Oaxaca than in Guerreroincrease towards the coast. Basaltic andesite toor by assimilation of crust with variable isotopicandesitic compositions characterize the upper unitsignatures.in the northern volcanic sector, andesites and

Although the western Oaxaca Tertiary mag-dacites were identified in the southern volcanicmatic rocks were emplaced in Precambrian–sector, and in the coastal plutonic belt, granitesPaleozoic basement, the 87Sr/86Sr ratios are low,and granodiorites are prevalent (Table 2). Anand eNd values range from −3.0 to +2.6exception is the Jamiltepec intrusion, the least(Table 3). These eNd values are similar to thosedifferentiated pluton within the Rio Verde batho-displayed by the mid-Tertiary ignimbrites and lavaslith, which is of tonalitic composition (Fig. 8).of the Upper Volcanic sequence in the northernThere is also a significant contrast between theSierra Madre Occidental, where they range fromdegree of differentiation of the inland volcanic−1.8 to +4.1, although 87Sr/86Sr ratios show arocks of western Oaxaca and those of northeasternlarger range (0.7038–0.710) (Lanphere et al., 1980;Guerrero; in this latter region, the volcanic rocksVerma, 1984; Cameron and Cameron, 1985;display rhyolitic to dacitic compositions, and inter-Cameron et al., 1986; Smith et al., 1996).mediate units are not important (Fig. 7).

Pb isotopic compositions of the magmatic rocksThe Sr and Nd isotopic compositions of thein western Oaxaca, as with Sr and Nd isotopicintermediate lavas of the northern inland volcanicratios, show a very narrow range (Table 4, Fig. 12)sequence and the La Muralla pluton in westernalthough these rocks vary from intermediate toOaxaca have a restricted range, with relatively lowacidic compositions. This suggests a similar source87Sr/86Sr ratios and eNd values from near 0 toand evolution for these rocks. The distribution of+2.6. There is a difference between the 87Sr/86Srdata from Tertiary rocks of the study area on aratios of these western Oaxaca rocks and the more207Pb/204Pb–206Pb/204Pb diagram appears to definedifferentiated rocks of northeastern Guerrero, witha steep mixing line with a narrow range. Theslightly higher and more heterogeneous 87Sr/86Srpossible mixing end members cannot have beenratios observed in this latter region (Moran-

Zenteno et al., 1999). The plutons along the coast conclusively identified with the data available up


to now, although the general trend of the data stood but does not appear to be related to thepresence of a thick crust since this region wassuggests a mantle source contaminated with a

207Pb-rich component. The 207Pb-rich component affected by transtension even before the Oligocenemagmatism. In the Huajuapan–Tlaxiaco region,could correspond to the influence in the mantle

wedge of fluids derived from the subduction zone thick sequences of lacustrine-fluvial volcaniclasticdeposits and volcanic rocks accumulated in NNW–or assimilation of the Acatlan Complex. It is not

possible to identify the isotopic composition of a SSE-trending basins at the time of volcanic activityand the presence of oblique, lateral and verticalsubduction component by establishing an analogy

with the present-day sediments in the Acapulco striae in fault planes reflect the extensional environ-ment for this region. We consider that the greatertrench. The continental source for the trench sedi-

ments was most likely different in the early differentiation of the coastal plutons is related toa lower extensional deformation at the time of theOligocene since the Chortis block must have been

involved, and extensive exposures of Oligocene magmatism with respect to the inland regions andthe greater volume of magma involved in this zone.batholiths were lacking.

The preliminary data available up to now for Structural observations in the Chacalapa shearzone and the relationship with the Oligocene intru-the Paleozoic Acatlan Complex indicate that the

Sr, Nd and Pb isotopic composition is highly sions of the coastal region suggest that, previousto the magmatism, extension was the main compo-variable (Yanez et al., 1991; Martiny et al., 1997;

unpublished data from Lopez and Cameron). It nent of deformation, whereas the strike-slip com-ponent dominated afterwards (Tolson-Jones,seems that any significant degree of assimilation

would have resulted in a greater dispersion of the 1998). The peak of magmatism in the coastalregion of Oaxaca seems to have occurred duringdata for the Tertiary magmatic rocks, although we

do not completely discard assimilation of the the transition between these two strain regimes.We have documented that the volcanic rocks inAcatlan Complex. The Pb isotopic composition of

igneous units of the Oaxaca Complex (Table 4, this region, especially the predominant upper vol-canic unit of the northern sector, are lessFig. 12) indicates that these units of the

Precambrian basement were not incorporated into differentiated than the coastal plutons (Table 2).The northern sector volcanic sequences are alsothe Tertiary magmatic rocks to any notable degree.

There are several indications that fractional less differentiated than the volcanic rocks of theTaxco region, where Oligocene volcanism is associ-crystallization is probably the most important pro-

cess of magma differentiation for the western ated with NNE-trending strike-slip faults and nosignificant extensional features have been recog-Oaxaca magmatic rocks analyzed in this study.

Assuming a similar source for these rocks and nized (Moran-Zenteno et al., 1998). In central andsoutheastern Oaxaca, the occurrence of abundantgiven the heterogeneity of the basement rocks in

this region, the narrow range of Sr, Nd and Pb silicic ignimbrites also suggests a lower rate ofextension. Silicic rocks reported in the Oaxacaisotope ratios, particularly for the northern vol-

canic sector and La Muralla pluton, indicates a fault zone seem to have occurred in an extensionallow degree of crustal contamination. Coherent region, but since the Oaxaca fault is an old featurelinear 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 isand trace elements vs. SiO2 could, thus, be uncertain.explained by fractional crystallization. None the In arc regions, deformation and stress fieldsless, 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 magmabody ascends, it will also have an effect on theThe cause of the greater differentiation of the

plutons along the coast is not completely under- degree and type of differentiation (crustal contami-


nation and crystal fractionation) (Burkart and more mafic magmatism. The volcanic sequences inboth sectors are intruded by hypabyssal bodiesSelf, 1985; Glazner and Ussler, 1989). For exam-

ple, in the central Aleutian arc, basaltic lavas are that vary in composition from dacite to basalticandesite. The coastal plutonic belt is even moreassociated with the degree of intra-arc extension

along the volcanic axis that modified the thermal differentiated and is composed principally of gra-nitic to granodioritic plutons.and density structure of the lithosphere (Singer

and Myers, 1990). In the currently active exten- (4) The trace-element concentration of the mag-matic rocks in western Oaxaca is characteristic ofsional Trans-Mexican Volcanic Belt, Alaniz-

Alvarez et al. (1998) found a correlation between arc-related magmas. The relatively low Sr ratiosand eNdi ratios near that of bulk earth as well asthe type of volcanism (monogenetic vs. polyge-

netic) and the strain rate. Monogenetic volcanism the general low variability of Sr, Nd and Pbisotope ratios, especially for the inland volcanictends to be more mafic and is associated with

faults having a higher strain rate. In western region and the La Muralla pluton, suggest a lowdegree of crustal contamination. The narrow rangeOaxaca, a correlation also seems to exist between

the extensional strain rate and the general degree of isotopic ratios, which are more radiogenic com-pared to depleted mantle, indicate the subcontinen-of differentiation.tal lithospheric mantle contaminated by asubduction component as a probable source.

(5) The degree of differentiation in the mag-7. Conclusionsmatic rocks in western Oaxaca seems to have beeninfluenced by the different strain domains in the(1) K–Ar age determinations of igneous rocksregion. The higher degree of differentiation of thein western Oaxaca indicate that volcanic and plu-plutons along the coastal area and their slightlytonic activity occurred during the Oligocene (~35greater crustal contamination, compared to theto ~25 Ma); the volcanic sequences crop out inintermediate volcanic sequences that are dominantthe inland region, whereas the plutonic rocks arein the northern volcanic sector, seem to be relatedfound along the coast. These rocks form part ofto the lower extensional deformation in thean extensive magmatic arc in southern Mexicocoastal area.that roughly displays a decreasing age trend from

Paleocene in Colima to Miocene in eastern Oaxaca.(2) Although age determinations reported in

Acknowledgementsthis work for the western Oaxaca region appearto indicate that the coastal plutonic rocks areslightly younger than the inland volcanic Financial support by the National Council of

Science and Technology (CONACyT) (projectsequences, other data reported previously give noclear indication of a southward migration for the 3361 T9309) in Mexico is gratefully acknowledged.

Pb isotopic determinations were made possible bymagmatism and, instead, suggest a broad mag-matic arc parallel to the coast during the student grants received by one of the authors

(Barbara Martiny) given by the Program ofOligocene.(3) In general, the SiO2 content of the Tertiary Financial Aid for Graduate Studies (PADEP) at

UNAM and the Geological Society of Americamagmas of western Oaxaca increases from theinland region toward the coast. In the northern (Howard T. Sterns Fellowship Award). The

authors wish to thank G. Silva-Romo, S.A. Alaniz-sector of western Oaxaca, magmatism began withvolcanic activity of acidic to intermediate composi- Alvarez, A. F. Nieto-Samaniego and R. Lopez for

discussion and helpful comments; P. Schaaf andtion that produced a lower unit of epiclastic depos-its, ash fall tuffs and ignimbrites overlain by a J.J. Morales-Contreras for assistance with the ana-

lytical aspects of the isotopic determinations; R.predominant upper unit of basaltic andesite toandesitic lavas and autobreccias. In the southern Lozano-Santacruz for the XRF determinations;

M. Reyes-Salas for SEM analyses in the evaluationvolcanic sector dacitic to andesitic compositionsare predominant with insignificant amounts of of some of the samples for isotopic determinations;


Ortega-Gutierrez, F., 1993. Guerrero terrane of Mexico: ItsL. Alba-Aldave and T. Hernandez-Trevino forrole in the Southern Cordillera from new geochemical data.assistance in the field; A. Victoria-Morales forGeology 21, 419–422.providing the charnockite of the Oaxaca Complex;

Chappell, B.W., White, A.J.R., 1974. Two contrasting graniteand 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-Chavez, P., 1997. Deformazione, metamorfismo e mec-

canismi di segregazione migmatitica nel complesso pluton-McDowell for their helpful reviews. Additionalico-metamorfico del Terreno Xolapa, Messico. Ph.D. thesis,unpublished data for the preliminary AcatlanUniversita Degli Studi di Milano, 78 pp.Complex Pb isotope field are from R. Lopez, K.

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