Journal of South American Earth Sciences 12 (1999) 513±535

Tertiary arc-magmatism of the Sierra Madre del Sur, Mexico, and its transition to the volcanic activity of the Trans-Mexican Volcanic Belt

D.J. Mora n-Zenteno a,*, G. Tolson a, R.G. Martõ nez-Serrano b, B. Martiny a, P. Schaaf b, G. Silva-Romo c, C. Macõ as-Romo a, L. Alba-Aldave a, M.S. Herna ndez-Bernal a, G.N. Solõ s-Pichardo a

aInstituto de GeologõÂa, Universidad Nacional AutoÂnoma de MeÂxico, Cd. Universitaria, Mexico, D.F., 04510 bInstituto de GeofõÂsica, Universidad Nacional AutoÂnoma de MeÂxico, Cd. Universitaria, Mexico, D.F., 04510 cFacultad de IngenierõÂa, DivisioÂn de Ciencias de la Tierra, Universidad Nacional AutoÂnoma de MeÂxico, Cd. Universitaria, Mexico, D.F., 04510

Abstract

The Tertiary magmatic rocks of the Sierra Madre del Sur (SMS) are broadly distributed south of the Trans-Mexican Volcanic Belt (TMVB) and extend to the southern continental margin of Mexico. They represent magmatic activity that originated at a time characterized by signi®cant changes in the plate interactions in this region as a result of the formation of the Caribbean plate and the southeastward displacement of the Chortis block along the continental margin of southwestern Mexico. The change from SMS magmatism to an E±W trending TMVB volcanism in Miocene time re¯ects the tectonic evolution of southwestern Mexico during these episodes of plate tectonic rearrangement. The distribution and petrographic characteristics of the magmatic rocks of the SMS de®ne two belts of NW orientation. The ®rst is represented by the nearly continuous coastal plutonic belt (CPB), which consists of batholiths and stocks of predominantly felsic composition. The second belt is inland of the ®rst and consists of discontinuously distributed volcanic ®elds with piles of andesitic to rhyolitic ¯ows, as well as epiclastic and pyroclastic materials. These two belts were emplaced along a continental crust segment constituted by a mosaic of basements with recognizable petrologic and isotopic di€erences. These basements originated during di€erent tectono-thermal events developed from the Proterozoic to the Mesozoic. Major and trace element data of the SMS magmatic rocks de®ne a clear sub-alkaline tendency. Variations in the general geochemical behavior and in the Sr and Nd isotopic ratios indicate di€erent degrees of magmatic di€erentiation and/or crustal contamination. These variations, specially in the inland Oligocene volcanic regions of Guerrero and Oaxaca states, seem to have been controlled by the particular tectonic setting at the time of magmatism. In northwestern Oaxaca greater extension related to transtensional tectonics produced less di€erentiated volcanic rocks with an apparently lower degree of crustal contamination than those of northeastern Guerrero. The geochronologic data produced by us up to now, in addition to those previously reported, indicate that the Tertiary magmatic rocks of the SMS range in age from Paleocene to Miocene. The general geochronologic patterns indicate a southeastward decrease in the age of igneous activity, rather than a gradual northeastward migration of the locus of magmatism toward the present-day TMVB. SMS magmatic rocks exposed to the west of the 1008W meridian are dominantly Late Cretaceous to Eocene, while those to the east range from Oligocene to Miocene, also following a southeastward age-decreasing trend. Paleocene and Eocene magmatic rocks of the western region of the SMS seem to keep a general NNW trend similar to that of the Tertiary magmatic rocks of the Sierra Madre Occidental (SMO). In the eastern region of the SMS the Oligocene magmatic rocks show a trend that roughly de®nes an ESE orientation. The change in the trend of arc magmatism may be the e€ect of the landward migration of the trench, for a given longitude, as a result of the displacement of the Chortis block. The

* Corresponding author. Tel: +5-616-0557; fax: +5-550-6644. E-mail address: [email protected] (D.J. Mora n-Zenteno).

0895-9811/99/$ - see front matter # 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S0895-9811(99)00036-X 514 D.J. MoraÂn-Zenteno et al. / Journal of South American Earth Sciences 12 (1999) 513±535 transtensional tectonic regime developed in Oligocene time in NW Oaxaca probably accentuated this trend by facilitating magma generation and ascent in these northerly regions. The geochronologic data of the SMS, in conjunction with those of the TMVB, suggest that there is a spatial and temporal magmatic gap in south central Mexico between 97 and 1008W longitude during late Oligocene and middle Miocene time (24±16 Ma). This magmatic gap is interpreted in terms of a combination of the relatively rapid change in the subducted slab geometry after the passage of the Chortis block from a moderate to a shallow angle and the time needed for the mantle wedge to mature suciently to produce magmas. # 1999 Published by Elsevier Science Ltd. All rights reserved.

Resumen

Las rocas magma ticas terciarias de la Sierra Madre del Sur (SMS) se encuentran ampliamente distribuidas al sur de la Faja Volca nica Transmexicana (FVTM) y se extienden hasta la margen continental del sur de Me xico. La actividad magma tica que dio origen a estas rocas se desarrollo en un tiempo caracterizado por cambios signi®cativos en las interacciones de las placas tecto nicas en esta regio n, lo que resulto de la formacio n de la placa del Caribe y el desplazamiento al sureste del bloque de Chortis a lo largo de la margen suroccidental de Me xico. El cambio del magmatismo de la SMS al volcanismo con orientacio n E±W de la FVTM, en el Mioceno, re¯eja la evolucio n tecto nica del suroeste de Me xico durante estos episodios de transformacio n en la con®guracio n de las placas tecto nicas. La distribucio n y las caracterõ sticas petrogra ®cas de las rocas magma ticas de la SMS de®nen dos cinturones con orientacio nal NW. El primero de ellos esta representado por un cinturo n casi continuo de plutones ubicados a lo largo de la costa (CPB), el cual consiste principalmente de batolitos y troncos de composicio n dominantemente fe lsica. El segundo cinturo n se encuentra hacia el interior del continente y consiste de campos volca nicos discontinuamente distribuidos que incluyen secuencias con derrames de andesita a riolita, asõ como materiales epicla sticos y pirocla sticos. Estos dos cinturones fueron emplazados en un segmento de corteza continental constituido por un mosaico de basamentos con diferencias petrolo gicas e isoto picas reconocibles. Estos basamentos se originaron en diferentes eventos tectono-te rmicos desarrollados entre el Proterozoico y el Mesozoico. Los datos de elementos mayores y traza de las rocas magma ticas terciarias de la SMS de®nen una tendencia claramente subalcalina. Las variaciones en el comportamiento geoquõ mico general, asõ como en las razones isoto picas de Sr y Nd, indican diferentes grados de diferenciacio n magma tica y/o contaminacio n cortical. Estas variaciones, especialmente en las regiones volca nicas del Oligoceno del interior de los estados de Guerrero y Oaxaca, parecen haber sido controladas por el ambiente tecto nico particular en el tiempo del magmatismo. En el noroeste de Oaxaca, la mayor extensio n asociada a la tecto nica transtensional produjo rocas volca nicas menos diferenciadas con un aparente menor grado de contaminacio n cortical que aquellas del noreste de Guerrero. Los datos geocronolo gicos producidos por nosotros hasta ahora, asõ como a aquellos previamente reportados por otros autores, indican que las rocas magma ticas de la SMS varõ an en edad del Paleoceno al Mioceno. Los patrones geocronolo gicos generales indican una edad decreciente del magmatismo hacia el sureste, ma s que una migracio n gradual del eje del magmatismo hacia el norte, hasta la posicio n de la FVTM. Las rocas magma ticas de la SMS expuestas al oeste del meridiano 1008W, tienen dominantemente edades entre el Creta cico Tardõ o y el Eoceno, mientras que aque llas ubicadas al este varõ an entre el Oligoceno y el Eoceno, mostrando tambie n un patro n decreciente hacia el sureste en sus edades. Las rocas magma ticas del Paleoceno y el Eoceno de la regio n occidental de la SMS parecen guardar la tendencia general hacia el NNW que tienen las rocas magma ticas de la Sierra Madre Occidental (SMO). En la regio n oriental de la SMS las rocas magma ticas del Oligoceno muestran una tendencia en su distribucio n con una orientacio n cercana al ESE. El cambio en la orientacio n del magmatismo puede ser el efecto de la migracio n de la trinchera hacia el interior del continente, para una longitud dada, como resultado del desplazamiento del bloque de Chortis. El desarrollo de la tecto nica extensional en el Oligoceno en el NW de Oaxaca probablemente acentuo esta orientacio n, al facilitar la generacio n y acenso de magma en estas regiones septentrionales. Los datos geocronolo gicos de la SMS, en combinacio n con aquellos de la FVTM, sugieren que existio un `gap' temporal en el magmatismo para un intervalo entre el Oligoceno tardõ o y el Mioceno medio (24±16 Ma), entre los 978 y 1008W. Este `gap' magma tico es interpretado en te rminos de una combinacio n entre el cambio relativamente ra pido en la geometrõ a de la placa subducida de un a ngulo moderado a un a ngulo bajo, despue s del paso del bloque de Chortis, y el tiempo necesario para que la cunÄ a del manto madurara su®cientemente para producir magma. # 1999 Published by Elsevier Science Ltd. All rights reserved.

1. Introduction regional variations in the kinematic interactions among the North America, Cocos and Rivera plates The oblique orientation of the Trans-Mexican (Fig. 1, inset) (Urrutia-Fucugauchi and Del Castillo- Volcanic Belt (TMVB) with respect to the Acapulco Garcõ a, 1977; Sua rez et al., 1990; Pardo and Sua rez, (Middle America) Trench is a relatively unusual fea- 1995). Geochronological evidence indicates that ture of the circum-Paci®c that has been related to the TMVB activity started at about 16 Ma before the pre- D.J. MoraÂn-Zenteno et al. / Journal of South American Earth Sciences 12 (1999) 513±535 515

Fig. 1. Geologic map of southern Mexico showing the distribution of Cenozoic magmatic rocks with selected dates or age ranges, as well as the location of transects shown in Fig. 6. The methods and uncertainties associated with these dates are presented in Table 2. The inset shows the distribution of Tertiary magmatic provinces in Mexico and the present-day plate tectonic features (SMO=Sierra Madre Occidental, IVF=Inland volcanic ®elds of the SMS, CPB=Coastal plutonic belt of the SMS, TMVB=Trans-Mexican Volcanic Belt) (States: J=Jalisco, M=Michoaca n, G=Guerrero, O=Oaxaca, C=Chiapas; MC=Mexico City; IT=Isthmus of Tehuantepec). sent and evolved to E±W trending ma®c to intermedi- time with respect to the present-day arc-trench con®gur- ate volcanism that is active to this day (Ferrari et al., ation. The Tertiary magmatic rocks of the SMS consti- 1994). tute a broad province of intrusive and extrusive rocks Northwest of the TMVB, the volcanic arc products that extends from the State of Michoaca n to the Isthmus related to the convergent limit between the Farallon of Tehuantepec (Fig. 1). The southernmost outcrops of plate and the North America plate form the NNW the younger volcanic rocks of the TMVB de®ne the trending Sierra Madre Occidental (SMO) province northern limit of the SMS. To the south, the igneous constituted by andesitic to rhyolitic sequences ranging products of the SMS extend to the southwestern conti- in age from Paleocene to middle Miocene. The tran- nental margin of Mexico (Bellon et al., 1982). The distri- sition of volcanic activity from the SMO to the TMVB bution and petrologic characteristics of the Tertiary in western Mexico occurred without interruption, magmatic rocks of the SMS de®ne two di€erent NW whereas in some areas of the northern part of the cen- trending belts (Fig. 1). The ®rst, the CPB, is represented tral and eastern sectors of the TMVB, a volcanic hia- by a conspicuous chain of batholiths along with smaller tus has been recognized between 26 and 16 Ma intrusives that outcrop along the southwestern continen- (Ferrari et al., 1994). tal margin of Mexico and indicate relatively rapid uplift The aerial distribution, as well as the structural and since the Oligocene (Mora n-Zenteno et al., 1996). The geochronologic characteristics of the Tertiary arc-mag- other is represented by a series of relatively isolated vol- matic rocks of the Sierra Madre del Sur (SMS), suggest canic ®elds that are distributed between the CPB and the a di€erent tectonic scenario for pre-middle Miocene TMVB. This paired con®guration of the Tertiary mag- 516 D.J. MoraÂn-Zenteno et al. / Journal of South American Earth Sciences 12 (1999) 513±535

Table 1 Summary of the main petrologic characteristics and typical isotopic ranges of the tectono-stratigraphic terrane basements of southern Mexico. Isotopic ranges are based on recalculated data (30 Ma) from Ruiz et al. (1988a,b), YanÄ ez et al. (1991), Mora n-Zenteno (1992), Centeno-Garcõ a et al. (1993b), Talavera-Mendoza et al. (1995)

Guerrero Terrane Mixteca Terrane Oaxaca Terrane Xolapa Terrane

Basement or Mesozoic volcano-sedimentary Acatla n Complex Oaxaca Complex Xolapa Complex characteristic sequences sequences Age Jurassic to Lower Cretaceous Lower Paleozoic Precambrian (Grenville) Mesozoic metamorphism and metamorphism metamorphism anatexis of older metamorphic and plutonic units Petrologic Volcano-sedimentary arc Greenschist facies metapelites Granulite facies para- and Igneous and sedimentary features sequences unconformably and greenstones. Eclogite orthogneisses, meta- migmatites including underlain by oceanic-like facies metapelites, ultrama®c anorthosites, metagabbros metagraywackes, quartz- sequences. Post tectonic rocks and granitoids and charnockites amphibolites, quartz- plutons feldspathic gneisses and marble lenses 87Sr/86Sr 0.7035±0.7060 0.705±0.750 0.704±0.716 0.704±0.715 eNd from +1 to +7 from +3 to 12 from 9to13 from +3 to 12 matic rocks of the SMS portrays not only the exposure as well as tectonic anities (Ortega-Gutie rrez, 1981; of di€erent crustal levels but also some di€erences in Campa and Coney, 1983). The main characteristics of petrological characteristics and tectonic relationships. the basement units and their isotopic compositions are The SMS magmatic rocks play a crucial role toward summarized in Table 1. The inset of Fig. 2 shows the the understanding of the regional geologic framework tectonostratigraphic division of Campa and Coney of south central Mexico. They originated at a time (1983) for southern Mexico. According to this division, characterized by signi®cant changes in the plate tec- the SMS is distributed along the Guerrero, Mixteca, tonic interactions around the Paci®c margin of Oaxaca, Xolapa and Jua rez terranes. southern Mexico (Ross and Scotese, 1988; The Guerrero terrane is represented by island-arc Mammerickx and Klitgord, 1982). The stratigraphic sequences, mostly Early Cretaceous in age, although details, geochemical characteristics and particular tec- some Late Jurassic units have also been reported tonic relationships of the SMS Tertiary volcanic (Centeno-Garcõ a et al., 1993a, and references therein). sequences are essential to an understanding of the The stratigraphic relationships of these arc sequences transition to the present day trench-arc con®guration, as well as the nature and age of their basement have because they are the result of the changes which been the matter of debate (Campa, 1978; De Cserna et brought about this transition. In spite of this, studies al., 1978; Campa and Ramõ rez-Espinosa, 1979; Campa on regional geochronological or geochemical patterns and Coney, 1983; Lang et al., 1996). There is evidence are relatively scarce (Guerrero-Garcõ a, 1975; Schaaf; indicating that the Guerrero terrane sequences north- 1990; Mora n-Zenteno, 1992; Ferrusquõ a-Villafranca west of Acapulco unconformably overlie a pre- and McDowell, 1991; Herrmann et al., 1994; Schaaf et Cretaceous assemblage of oceanic anity (Centeno- al., 1995; Martõ nez-Serrano et al., 1997; Martiny et al., Garcõ a et al., 1993b), while the easternmost sector 1997; Mora n-Zenteno et al., in press). thrusts over continental crust probably related to the In this paper we discuss our geochronological and Mixteca terrane basement (Campa, 1978; Campa and geochemical database, as well as data previously Coney, 1983; Elõ as-Herrera and Sa nchez-Zavala, 1990). reported by other authors in order to evaluate their The Acatla n Complex, which constitutes the base- signi®cance in the interpretation of the regional ment of the Mixteca terrane, is represented by a tecto- changes in the magmatic patterns and their possible re- nically heterogeneous assemblage of mostly oceanic lationships with the plate tectonic interactions and the deformation of the continental crust. basin anity, including metamorphic units ranging from greenschist to eclogite facies (Ortega-Gutie rrez, 1978, 1993). The metamorphism and intense defor- mation of the Acatla n Complex have been interpreted 2. The basement of the SMS Tertiary magmatic rocks as the result of its accretion to the continental nucleus represented by the Oaxaca Complex (Ortega-Gutie rrez, The Tertiary plutonic and volcanic rocks of the 1993). The Acatla n Complex is covered by igneous SMS are found in di€erent basement terranes, each of and sedimentary sequences, including Late Paleozoic which display contrasting stratigraphic characteristics and Middle Jurassic to Cretaceous rocks (Ferrusquõ a- D.J. MoraÂn-Zenteno et al. / Journal of South American Earth Sciences 12 (1999) 513±535 517

Fig. 2. Map of southwestern Mexico showing di€erent deformation domains and indicating their age. Where the age is not constrained it is indi- cated by a question mark. Active features are indicated with an age of 0 Ma. For references to the timing of deformation see text. The inset shows the distribution of tectonostratigraphic terranes after Campa and Coney (1983) as well as the Sierra Madre Oriental, the Sierra Madre Occidental and the Trans-Mexican Volcanic Belt. Abbreviations used in the inset are, G=Guerrero terrane, J=Jua rez terrane, Ma=Maya ter- rane, Mi=Mixteca terrane, O=Oaxaca terrane, SM=Sierra Madre terrane, SMO=Sierra Madre Occidental, TMVB=Trans-Mexican Volcanic Belt, X=Xolapa terrane.

Villafranca, 1976; Mora n-Zenteno et al., 1993 and genic strata intensively deformed and thrusted with a references therein). general vergence to the east (Carfantan, 1986; The metamorphic basement of the Oaxaca terrane is Barbosa, 1994). The western margin of the Jua rez ter- represented by a Grenvillian age (900±1110 Ma) gran- rane north of the city of Oaxaca is de®ned by a west- ulite facies complex with ma®c and felsic ortho- dipping, NNW trending polygenic mylonitic shear gneisses, paragneisses as well as meta-anorthosites and zone which was reactivated in Middle Jurassic and charnockites (Ortega-Gutie rrez, 1981, 1993). The post- Tertiary times (Alaniz-A lvarez et al., 1996). metamorphic cover includes sedimentary sequences of Information about the deep structure and lateral the Tremadocian, Mississippian±Permian and variations of the lower crust in southern Mexico is lim- Jurassic±Cretaceous time intervals (Pantoja-Alor, ited, but the general arrangement and tectonic history 1970; Schlaepfer, 1970). portrayed by the exposed basement units suggest that The Xolapa terrane is composed of a lithologically gradually younger fragments of continental crust were heterogeneous mid-crustal amphibolite-facies assem- aggregated around the Oaxaca Complex during blage consisting of metagraywackes, quartz-amphibo- Paleozoic and Mesozoic times (Ortega-Gutie rrez, lites, quartz-feldspathic gneisses and marble lenses 1993). (Ortega-Gutie rrez, 1981; Corona-Cha vez, 1997; From an isotopic point of view, the basement rocks Tolson, 1998). A characteristic feature of this assem- of the Guerrero terrane are quite primitive compared blage is the occurrence of migmatites indicating di€er- to those of the Acatla n, Oaxaca and Xolapa complexes ent degrees of anatexis and subsolidus di€erentiation. (Table 1). Typical 87Sr/86Sr and eNd values for the The timing of metamorphism of the Xolapa terrane is igneous rocks of the Guerrero terrane sequences (recal- not yet clear, since ages between Lower Cretaceous culated to 30 Ma, i.e. the time of magmatism) range and Eocene have been reported (Mora n-Zenteno, from 0.7035 to 0.7060 and +1.0 to +7.0, respectively 1992; Herrmann, 1994). (Centeno-Garcõ a et al., 1993b; Talavera-Mendoza et The Jua rez terrane consists of Mesozoic sequences al., 1995). The other terranes exhibit di€erent ten- of marine and continental sedimentary and volcano- dencies toward more evolved 87Sr/86Sr isotopic ratios 518 D.J. MoraÂn-Zenteno et al. / Journal of South American Earth Sciences 12 (1999) 513±535 and negative eNd values (YanÄ ez et al., 1991; Ruõ zet west Mexico in combination with the presence of al., 1988a,b), probably as a result of the in¯uence of mylonitic shear zones with a component of left-lateral the Oaxacan Grenville basement nucleus. motion along the coast (Fig. 2) have been attributed by some workers to the eastward motion of the Chortis block toward its present position in Central 3. Cenozoic structural and tectonic setting of the SMS America during Neogene time (Ratschbacher et al., 1991; Herrmann et al., 1994; Schaaf et al., 1995). The plate tectonic setting and evolution of southern Comparative stratigraphy and the absence of similar Mexico during the Tertiary is complex. From the early mylonite zones to the northwest of Zihuatanejo Tertiary and until 29 Ma the Farallon plate was con- suggest that the initial position of the Chortis block sumed beneath southern Mexico. Subduction of the was no further west than Zihuatanejo (Schaaf et al., Guadalupe plate, formed by fragmentation of the 1995). The amount of displacement predicted by this Farallon plate, occurred during the period between 29 paleoposition (600 km) is less than that calculated by and 12.5 Ma. Since 12.5 Ma the Cocos and Rivera Rosencrantz and Sclater (1986) using ocean ¯oor plates have been overridden by North America along bathymetry and magnetic anomaly data (1000 km). the Acapulco Trench (Mammerickx and Klitgord, This discrepancy can be explained by E±W extension 1982). It is uncertain whether or not these major of the Chortis block, as suggested by prominent N±S changes in plate architecture and kinematics played a trending grabens (Schaaf et al., 1995) in Guatemala role in the determination of the patterns of magmatism and Belize. The displacement of Chortis along the associated with subduction in southern Mexico and in coast of Mexico brought rocks previously fairly distant the development of the complex geometry of the defor- from the continental margin to lie directly on the mation domains. In Oligocene to Miocene time the re- newly formed trench. Mora n-Zenteno et al. (1996) and arrangement of the oceanic plates did not produce Tolson (1998) have discussed in detail some of the geo- signi®cant variations in the age of oceanic crust being metric consequences of the eastward passage of subducted, but rather slight variations in the conver- Chortis, among which are the progressive exhumation gence vectors. of mid-crustal rocks and subduction erosion of upper At present, the Rivera and Cocos plates are being mantle lithosphere along the newly formed continental subducted along the Paci®c margin of Mexico and the margin. This o€shore passage of the Chortis block southern limit of the North America plate is the left- during the Oligocene is thus also a distinctly character- lateral Polochic-Motagua transform fault zone in istic tectonic event that a€ected the magmatic and de- northern Guatemala and Belize which extends toward formation patterns of southern Mexico as well as their the east into the Caribbean Sea (Fig. 1 inset). The timing. Cocos±Caribbean±North America triple junction is a The timing and kinematics of Cenozoic regional de- broad zone of subsidence o€ the coast of Salina Cruz formation in southern Mexico are still far from clear. de®ned by shallow normal fault seismicity (Guzma n- We have summarized the available data in Fig. 2, Speziale et al., 1989). including the ages of the structures where these are The conspicuous obliquity of the present-day volca- constrained. There is general agreement that at least nic arc to the Acapulco Trench has been explained by the central TMVB is undergoing present day N±S di- a lateral decrease in the dip angle of the subducted rected extension along E±W trending faults as a pos- Cocos plate in such a way that the 100 km depth con- sible result of gravitational collapse (Suter et al., 1992; tour line is roughly parallel to the southern margin of Ferrari et al., 1994; Suter et al., 1995a,b). The avail- the TMVB (Pardo and Sua rez, 1995). The angle of able data suggest that this extensional regime has been subduction of the Rivera plate at the western end of active since the late Miocene (Suter et al., 1995a). the TMVB and of the Cocos plate to the east of the There is also a general consensus regarding the de®- Isthmus of Tehuantepec is about 458 (Pardo and nition of the fault-bounded Jalisco block at the west- Sua rez, 1995; Ponce et al., 1992). The notably shallow ern end of the TMVB (Fig. 2) during the Miocene but Cocos plate trajectory beneath southern Mexico is details of its past and present kinematics and its inter- thus responsible for the obliquity of the TMVB with action with the Rivera plate are still debated (Barrier respect to the trench. Since the angle of subduction et al., 1990; Bandy et al., 1995; Quintero-Legorreta, a€ects the spatial distribution of magmatism with 1995; Ferrari, 1995; Rosas-Elguera et al., 1996). respect to the trench (e.g. Urrutia-Fucugauchi and Studies focusing on Cenozoic deformation inland Morton-Bermea, 1997), it seems logical to try to use from the Paci®c and south of the TMVB are still the timing and distribution of magmatism in this scarce but several are underway. Jansma and Lang region to trace the evolution of the subduction geome- (1997) have reported the presence of a NNW±SSE try of the Cocos plate with time. trending graben structure in the Arcelia region of The truncated Paci®c continental margin of south- Michoaca n and Guerrero states (Fig. 2) which they as- D.J. MoraÂn-Zenteno et al. / Journal of South American Earth Sciences 12 (1999) 513±535 519 sociate with Basin and Range extension, citing evi- di€erence in the ages of the two shear-zones, but also dence for its activity prior to 32 Ma. Studies by a di€erence in the kinematic details. As shown in Fig. Mora n-Zenteno et al. (in press) in the Taxco region 2, the Tierra Colorada shear-zone is a normal-left-lat- have recently indicated the presence of a NNE trend- eral oblique shear zone, with a moderate dip to the ing system of sub-vertical right-lateral faults which cut northeast (Mora n-Zenteno, 1992), while the Chacalapa ignimbrites of 38±36 Ma but whose upper age limit is shear-zone is a sub-vertical anastomosing shear-zone still uncertain. Further to the east, in the southern part with strictly left-lateral kinematic indicators (Tolson, of the State of Puebla and the northwest part of the 1998). In addition, in the region between Puerto A ngel State of Oaxaca, we have identi®ed a region in which and Salina Cruz, Delgado-Argote and Carballido- N±S trending en echelon left-lateral transfer faults Sa nchez (1990) and Tolson (1998) have documented bound down-thrown blocks with lacustrine sediment the presence of active left-lateral faults with E±W and ®lls interbedded with volcanic rocks of Oligocene age. NE±SW trends. The NW-trending Oaxaca fault (Fig. 2) exhibits a Meschede et al. (1997) published fault-slip analysis complex history of displacement beginning in the data from southern Mexico to which they applied Paleozoic followed by left-lateral motion in the inversion techniques in order to determine the stress Jurassic (Nieto-Samaniego et al., 1995). Its most recent ®eld. They used crosscutting and overprinting relation- activity has been described as normal fault motion ships to determine relative ages for their data and with the down-thrown block to the west and the strati- further subdivided them using kinematic compatibility graphic and geochronologic studies carried out by arguments. They compared the stress tensor orien- Ferrusquõ a-Villafranca et al. (1988) have left implicit tations resulting from the stress inversion to the avail- that the latest activity occurred during Miocene time. able data for Cenozoic stage poles for the Farallon However, research centered directly on the fault, its plate (and its derivatives) with respect to North kinematics, and timing has failed to provide conclusive America. On the basis of this comparison, Meschede evidence regarding the age of its most recent normal et al. (1997) concluded that there was an e€ective movement (Centeno-Garcõ a, 1988; Alaniz-A lvarez et stress transmission across the North American plate al., 1996). To the south of the Oaxaca fault, we have boundary and that the stress ®eld in southern Mexico recognized E±W oriented graben structures for which prior to 25 Ma had s2 in a sub-vertical orientation an early to middle Miocene age can be inferred on the and s1 with a northeasterly trend roughly parallel to basis of K±Ar dates of pyroclastic material from the the motion vector of the oceanic plate (with respect to basin ®ll (Ferrusquõ a-Villafranca, 1992). North America). Their data indicate that, since 25 Ma, Several studies have recognized the presence of the stress ®eld has a vertical maximum stress orien- mylonitic shear-zones along the Paci®c coast of south- tation and that s3 is perpendicular to the regional western Mexico (Fig. 2) (Ratschbacher et al., 1991; trend of topography throughout their study area in Mora n-Zenteno, 1992; Meschede et al., 1997; Corona- southern part of the State of Oaxaca. Cha vez, 1997; Tolson, 1998). Geochronological data The distribution and characteristics of the major tec- have constrained in detail the timing of crystal-plastic tonic features described above are not necessarily fully deformation of two of these shear-zones, suggesting a compatible with the stress-®eld evolution described by younging trend toward the southeast along the coast. Meschede et al. (1997), but the discrepancies may be The mylonite zones in the Zihuatanejo and Juchatengo explained by shear-zone reactivation and mechanical regions have been studied from a structural point of heterogeneity of the continental crust in southern view by Meschede et al. (1997), but details of the tim- Mexico. ing of deformation are lacking. The Tierra Colorada intrusive, which clearly truncates the Tierra Colorada mylonite zone (Riller et al., 1992), has yielded concor- 4. Geochronological, petrographic and geochemical dant U±Pb zircon ages between 35.2 and 34.3 Ma features of Tertiary igneous rocks of the SMS (Herrmann et al., 1994), setting a minimum age for the shear-zone in this region, at least for crystal-plastic de- Although the geochronology and geochemistry of formation. In the vicinity of Huatulco, Oaxaca, Tolson the plutonic rocks of the SMS have been studied with et al. (1993) and Tolson (1998) have shown that the 29 some detail in certain parts of southwestern Mexico Ma (Rb±Sr whole-rock and U±Pb zircon ages of (Guerrero-Garcõ a, 1975; Pantoja-Alor, 1983, 1992; Herrmann et al., 1994) Huatulco granodiorite is Schaaf, 1990; Delgado-Argote et al., 1992; Mora n- a€ected by crystal-plastic mylonitic processes along the Zenteno, 1992; Herrmann et al., 1994; Schaaf et al., Chacalapa shear-zone and that these mylonites are 1995; Herna ndez-Bernal and Mora n-Zenteno, 1996; truncated by granodioritic dikes which yielded a Calva-Guerra, 1996; Correa-Mora, 1997), the data 23.721.2 Ma K±Ar age using hornblende separates. available for the volcanic rocks are more limited. It is important to emphasize that not only is there a However, recent work on the Tertiary volcanic 520 Table 2a Geochronologic data for Tertiary igneous rocks of the SMS, mainly of the inland region, complementary to those presented by Schaaf et al. (1995) for the coastal plutonic belt. (a) Table includ- ing dates obtained by us and reported in published papers or in theses for which complete information is available

Locality Longitude (W) Latitude (N) Sample no. Rock type or formation Age Ma Material dated Method Referencef

MichoacaÂn Tzitzio 100856' 19836' Mx-88-17 Ignimbrite 33.421.7 Sanidine K±Ar 7 MeÂxico a a a

MataRedonda 100809'240 19808'120 AM-11 Rhyolitic tu€ 33.620.9 K-feldspar K±Ar 13 Mora D.J. Cerro El PenÄ on 100807'070a 19803'380a AT-2 Rhyolite 31.620.8 K-feldspara K±Ar 13 Arroyo El Castillo 100806'000a 19801'100a M-01 Lamprophyre 46.621.2 Biotitea K±Ar 13 Guerrero  Tetela del Rõ o 100805'a 17858'a CFE-2 Basalt 42.321.4 WRe K±Ar 2 513±535 (1999) 12 Sciences Earth American South of Journal / al. et n-Zenteno Xaltianguis 99850.43' 17809.95' FC19a,b,c Granodiorite to diorite 2821.5 3 pt. WR isochron Rb±Sr 12 Taxco 99838'550 18837'500 TX-21 Ignimbrite 32.420.8 Biotite K±Ar 15 Taxco 99838'150 18834'300 TX-10 Rhyolite 31.621.2 Plagioclase K±Ar 15 Taxco 99837.4' 18833.4' M48-51 Rhyolitic tu€ 4923WRK±Ar5 Taxco 99837'170 18833'500 TX-25 Vitrophyre 31.920.8 WR K±Ar 15 Taxco 99836'b 18834'b n.r.d Rhyolite 36.9, 35.521.3 K-feldspar,WR K±Ar 3 Taxco 99836'150 18835'500 TX-16 Vitrophyre 32.420.9 WR K±Ar 15 Taxco 99832'450 18834'150 TX-4 Vitrophyre 38.221.0 WR K±Ar 15 San Juan Tetelcingo area 99831'a 17856'a n.r. Tu€ 6622.3 Biotite K±Ar 4 San Juan Tetelcingo area 99831'a 17856'a CFE-1 Basalt 52.421.8 WR K±Ar 2 Sierra de Alquitra n99829'230a 17826'250a ALV96 Ignimbrite, Alquitra n Fm. 24.020.8 Volcanic glass K±Ar 11 Sierra de Alquitra n99828'400a 17819'540a AOV96 Ignimbrite, Alquitra n Fm. 22.520.6 Volcanic glass K±Ar 11 Coxcacla n99827'300a 18829'450a BV-21 Granodiorite 32.220.7 Biotite-WR Rb±Sr 14 San Marcos 99824'410a 16846'440a DM941b Granodiorite 33.520.7 Biotite-WR Rb±Sr 14 Buenavista 99824'050 18817'000 BV-17 Dacititc lava 30.521.1 Plagioclase K±Ar 15 Buenavista 99823'350 18817'000 BV-12 Dacitic lava 24.821.3 Hornblende K±Ar 15 Quetzalapa 99811'150 18821'000 SOL-5 Dacitic ignimbrite 31.920.8 Biotite K±Ar 15 Cruz Grande 99807'300 16843'130 DM94b Granodiorite 35.720.7 Biotite-WR Rb±Sr 14 DSDP Leg 66 site 493 98855.53' 16822.87' 493-59-1 Diorite 36.4, 34.522WR K±Ar6 DSDP Leg 66 site 493 98855.53' 16822.87' 493-59-2 Diorite 3522WRK±Ar6 Oaxaca Zacatepec 97859.06'b 16846.16'b JG73-16A Granodiorite 2521 Biotite-WR Rb±Sr 8 Sta. Ma. Zacatepec 97858'250 16853'270 CON-53 Granite 25.520.7 Biotite K±Ar 16 Chila 97852'080b 17857'330b SICHILA-1 Andesitic laccolith 3021WRK±Ar10 Laguna de Guadalupe 97851'230 17811'190 CON-59B Silicic tu€ 34.821.4 Hornblende K±Ar 16 Zapotitla n Palmas 97848'290b 17853'230b SVZA-3 Basalt 3221WRK±Ar10 Zapotitla n Palmas 97848'290b 17853'230b SVZA-1 Basalt 3121WRK±Ar10 Zapotitla n Palmas 97848'290b 17853'230b SVZA-4 Basaltic andesite 3021WRK±Ar10 Zapotitla n Palmas 97848'290b 17853'230b SVZA-2 Basaltic andesite 2921WRK±Ar10 Huajuapan 97847'170 17849'470 CON-8A Andesitic stock 33.621.4 Hornblende K±Ar 16 Huajuapan 97847'150b 17849'340b SILC-1 Basaltic andesite intrusion 3121WRK±Ar10 North of Huajuapan 97841'460 18804'510 CON-75 Rhyodacitic tu€ 31.420.8 Biotite K±Ar 16 North of Huajuapan 97840'520 18802'360 CON-91 Andesitic dike 34.221.4 Hornblende K±Ar 16 Tlaxiaco 97836'440 17821'400 CON-101 Rhyolitic tu€ 32.920.9 Biotite K±Ar 16 Tamazulapan 97834.08'b 17842.82'b FV69-180 Tu€, Llano de Lobos Fm. 26.220.5 Biotite K±Ar 1 D.J. MoraÂn-Zenteno et al. / Journal of South American Earth Sciences 12 (1999) 513±535 521

sequences in northwestern Oaxaca and northeastern Guerrero by some of the authors of this paper and by FerrusquõÂ a-Villafranca (1992) has provided infor- mation on the nature of these sequences and regional petrological and geochemical variations have been noted. The methods used and the materials dated in geo- chronologic studies of the magmatic rocks of the SMS are diverse. Our analysis was based on our own geo- chronologic data as well as on dates published in the literature or presented in theses that we considered re- liable. In the evaluation of dates published by other

o et al. (1996); 12. Correa-Mora (1997);13. authors, we took into consideration their general com- Ä s-Pichardo, PhD thesis (in prep.).  patibility with stratigraphic observations and the suit- Biotite-WRBiotite-WRBiotite-WRBiotite-WR Rb±Sr Rb±Sr Rb±Sr Rb±Sr 9 9 9 9 ability of the method applied with respect the material dated. The dating methods used for plutonic rocks are ndez-Trevin  indicative of di€erent stages in the crystallization-cool- 0.3 Biotite K±Ar 1 c c c c 2 ing process. For intrusive units dated with di€erent 0.4 Biotite-WR Rb±Sr 17 0.3 Biotite-WR Rb±Sr 17 0.3 Biotite-WR Rb±Sr 17 0.6 WR K±Ar 1 1.7 Hornblende K±Ar 16 0.5 0.5 0.5 0.4 0.3 Biottie-WR Rb±Sr 17 methods we gave preference in our regional analysis to 2 2 2 2 2 2 2 2 2 2 those data that better approximate the crystallization age. Nonetheless, when only one method was applied, we assumed in most cases that cooling occurred in a time interval short enough that it did not signi®cantly distort the regional geochronological patterns. This rrez (1980); 5. Linares and Urrutia-Fucugauchi (1981); 6. Bellon et al. (1982); 7.  assumption was based on the fact that most Tertiary plutons of coastal Oaxaca and Guerrero on which di€erent geochronologic methods were applied dis- played U±Pb, K±Ar and Rb±Sr mineral ages that indi- cate a relatively high cooling rate related to a rapid uplift of the continental margin (Mora n-Zenteno et al., 1996). A ®rst compilation of isotopic ages of the n-Zenteno et al. (in press); 16. Martiny et al. (in press); 17. Solõ  southwestern continental margin of Mexico has already been presented by Schaaf et al. (1995). In this paper we present a complementary list of dates com- H239 Granite 20.9 H234 Tonalite 13.6 H235 Quartz diorite 14.9 H230H113H247H250 Granite Granite Granite Tonalite 25.2 25.1 26.7 20.4 H029 Granite 16.6 CON-7 Dacitic laccolith 40.5

FV60-182FV69-185 Yucudaac Andesite Tu€, Suchilquitongo Fm. 28.9 17.4, 16.5 piled for the volcanic rocks and related intrusive units of the inland areas (Tables 2a and b). Table 2a lists those dates for which complete information has been given, including the locality, analytical uncertainties, as b ' 0 b 05 a a a a well as material and rock unit dated. Table 2b includes ' ' ' ' ' ' ' ' ' 53 25 20 34 02 34.78 15.49 50 47 46 56 those dates for which complete information has not 8 8 8 8 8 8 8 8 8 8 8 15 16 16 17 16 17 17 15 15 15 15 been published. Major and trace element compositions as well as isotopic data discussed in this paper are pre- sented in more detail elsewhere (Schaaf, 1990; a and Herrero-Bervera (1993); 9. Schaaf et al. (1995); 10. Galina-Hidalgo (1996); 11. Herna  Herna ndez-Bernal and Mora n-Zenteno, 1996; Mora n- b ' 0 Zenteno et al., in press; Martiny et al., in press). The 36 b a a a a ' ' ' ' ' ' ' ' ' ' 09 07 02 52 27 26 23 24 51.72 28 25 analytical details and related uncertainties of the data 8 8 8 8 8 8 8 8 8 8 8 are discussed in the cited papers. A description of the petrology, geochronology and geochemistry of the coastal plutonic belt as well as the volcanic rocks in four selected areas is summarized below. a-Villafranca (1976); 2. De Cserna (1981); 3. De Cserna and Fries (1981); 4. Ortega-Gutie  4.1. Coastal plutonic belt et al. (1991); 8. Guerrero-Garcõ n97  Á vez-Aguirre and Mendoza-Flores (1998); 14. Alba-Aldave et al. (1998); 15. Mora Â

Personal communication by author(s). Coordinates taken from mapAge or reported from previously information in inn.r.=not Schaaf original reported. et reference. al. (1995)WR=whole and rock. corrected in the1. present study. Ferrusquõ The composition of the plutons along the coastal rez 95 rez 95 Â Â f c e a b d HuatulcoHuatulcoXadaniAyuta 96 96 96 95 Jua Jua Bomba 95 Yanhuitla Huitzo-Etla 96 Huatulco 96 Cha Pasquare E. of Tamazulapan 97 margin generally varies from dioritic to granitic 522 D.J. MoraÂn-Zenteno et al. / Journal of South American Earth Sciences 12 (1999) 513±535

Table 2b (b) List of dates for which complete information is not available

Locality Rock type Age Ma Material Method Referenced or formation dated

Jalisco Techalutla Granodioritea 6922 Hornblendea K±Ar 7 Tamazula Batholith 5321 K-feldspara K±Ar 7 MichoacaÂn Punta San Telmo Granodioritea 5526 Hornblendea K±Ar 7 Ziquira n Porphyritic diorite 34.328 Hornblendea K±Ar 3 Guerrero Highway between Cd. Altamirano and Zihuatanejo, Andesite 61.221.3 Plagioclasea K±Ar 3 km 30 Between Cd. Altamirano and Zihuatanejo Dike 46.525% n.r.b K±Ar 5 Highway between Cd. Altamirano and Zihuatanejo, Porphyritic diorite 46.221.1 Plagioclasea K±Ar 3 km 70 Between Cd. Altamirano-Zihuatanejo Andesites 43-4622 n.r. K±Ar 6 Highway between Cd. Altamirano and Zihuatanejo, Andesitic dike 42.921 Plagioclasea K±Ar 3 km 67 10 km W of Cd. Altamirano Diorite 36.628WRa,c K±Ar 3 Between Cd. Altamirano and Zihuatanejo Dikes 33-4522 n.r. K±Ar 6 San Juan Tetelcingo area Basalt, Tetelcingo Formation 68.822.4 WR K±Ar 2 7 km E of Balsas railroad station Granodiorite 66.121.5 Hornblendea K±Ar 3 Vallecitos de Zaragoza granitoid Diorite to quartz monzonite 3625 n.r. Rb±Sr 6 Poliutla Rhyolitic ignimbrite 42.321 Plagioclasea K±Ar 3 El Ocotito, sample JG74-01 Quartz monzodiorite 3822 Biotite-WR Rb±Sr 1 Acapulco Granite 5020.5 Biotite-WR Rb±Sr 8 Km 91.9 Highway Chilpancingo to Acapulco, Quartz monzonite 32.723 Biotite-WR Rb±Sr 1 sample JG73-24 Oaxaca Suchilquitongo-Etla Tu€, Suchilquitongo Formation 20.6, 19.320.3 n.r. K±Ar 4 Tlacolutla-Mitla-Matatla n area Tu€ 16.015.3220.8 n.r. K±Ar 4 Nejapa area Tu€ 17.415.020.8 n.r. K±Ar 4

a Personal communication by author(s). b n.r.=not reported. c WR=whole rock. d 1. Guerrero-Garcõ a (1975); 2. Ortega-Gutie rrez (1980); 3. Pantoja-Alor (1986); 4. Ferrusquõ a-Villafranca and McDowell (1991); 5. Kratzeisen et al. (1991); 6. Frank et al. (1992); 7. Pantoja-Alor (1992); 8. Guerrero-Garcõ a and Herrero-Bervera (1993). although more ma®c units have been observed. Their the Isthmus of Tehuantepec area (Table 2a). To the

SiO2 content typically varies from 56 to 74 wt% and northwest, the older Puerto Vallarta and Manzanillo in a TAS diagram (Na2O+K2O vs SiO2), these plu- batholiths were emplaced in the Cretaceous. tons show a sub-alkalic trend (Fig. 3a). They are light- Crystallization ages for the Puerto Vallarta region are

REE enriched (LaN/LuN between 4 and 20) and in given by U±Pb zircon dating in granodiorites general display a ¯at pattern for heavy-REE (Fig. 4b). (10326.5 and 10123.4 Ma) and two Rb±Sr whole- These geochemical features are typical of fractional rock isochrons in granitoids (9924and9123 Ma). crystallization processes in mantle derived magmas. For granitoids in the Manzanillo area, a Rb±Sr whole- The plutonic rocks from the Acapulco and Xaltianguis rock isochron yielded 6923 Ma (Schaaf et al., 1995). intrusives show a negative Eu anomaly (Fig. 4a) that The ages of the Michoac n and Guerrero coastal plu- may be explained by plagioclase fractionation tons are similar to those obtained for inland intrusives (Negendank et al., 1987). located in south central Michoaca n. The Santa Elena The isotopic age data of the SMS (Table 2 of this and La Huacana plutons yielded ages of 66.121.5 Ma paper and Table 1 of Schaaf et al., 1995) show that (K±Ar) and 4224 Ma (Rb±Sr isochron), respectively the Tertiary intrusive rocks of the CPB in the region (Pantoja-Alor, 1986; Schaaf et al., 1995). Other plu- between the states of Michoaca n and Oaxaca show a tons (La Verde, Inguara n, San Isidro) give biotite, large variation in age, ranging from early Paleocene hornblende and sericite K±Ar age determinations of (6321 Ma hornblende K±Ar; Pantoja-Alor, 1983) in 35.620.8 to 31.820.7 Ma for granodiorites and re- Aquila, Michoaca n, near Punta San Telmo, to middle lated mineralization (Damon et al., 1983). These ages Miocene (13.620.3 Ma whole-rock-biotite Rb±Sr) in are somewhat anomalous in the regional context, D.J. MoraÂn-Zenteno et al. / Journal of South American Earth Sciences 12 (1999) 513±535 523

Fig. 3. Chemical classi®cation of Tertiary magmatic rocks of the Sierra Madre del Sur, Mexico. (a) Samples of the coastal plutonic belt (TAS diagram after Wilson, 1989); data of the Xaltianguis pluton after Correa-Mora (1997). (b) Samples of the inland Tertiary volcanic sequences (TAS diagram after Le Maitre et al., 1989). Note that in the northwestern Oaxaca region basaltic andesite and andesitic compositions are domi- nant. The three samples of rhyolitic composition correspond to minor pyroclastic units. Volcanic rocks of Guerrero and Morelos regions range from andesites to rhyolites but most of them fall in the dacite and rhyolite ®elds. The data plotted are in Martiny et al. (in press) and Mora n- Zenteno et al. (in press). which may suggest a local, post-intrusive thermal area, southeast of Zihuatanejo, yielded hornblende and event. biotite K±Ar ages ranging from 40.720.9 to Tertiary coastal plutons of the Guerrero terrane 37.421.5 Ma (Delgado-Argote et al., 1992; Schaaf et between Zihuatanejo and Acapulco were emplaced in al., 1995; Stein et al., 1994). In this same general area, late Eocene to Oligocene time (Fig. 1). A Rb±Sr the ultrama®c rocks of El Tamarindo and Loma Baya whole-rock isochron yielded an age of 36.525 Ma for are related to Cretaceous magmatism and have been the granitoids of the Zihuatanejo area (Schaaf et al., studied in detail by Delgado-Argote et al. (1992). 1995). The granitic batholith exposed in the Petatla n Farther to the southeast, a U±Pb zircon crystalliza- 524 D.J. MoraÂn-Zenteno et al. / Journal of South American Earth Sciences 12 (1999) 513±535

Fig. 4. Chondrite-normalized REE patterns of Tertiary magmatic rocks of the Sierra Madre del Sur (chondrite values of Nakamura, 1974). (a) Coastal plutonic belt; note the Eu anomaly in the intrusives of Xaltianguis and Acapulco. (b) Inland Tertiary volcanic sequences of northeastern Guerrero and western Oaxaca. tion age of 35 Ma was obtained for a granodiorite SE (Guerrero-GarcõÂ a and Herrero-Bervera, 1993). of Atoyac (Herrmann et al., 1994). The Tierra Furthermore, the Acapulco intrusive is remarkable in Colorada, Xaltianguis, San Marcos and Cruz Grande that it includes a granite with pseudorapakivi texture granodioritic plutons give biotite Rb±Sr ages ranging and a two pyroxene-bearing syenite facies. Fayalite from 35 to 26 Ma (Schaaf et al., 1995; Alba-Aldave et and ¯uorite have also been reported as accessories and al., 1998). In addition to these dates, U±Pb zircon ages some samples have trace-element distributions that are reported for the intrusives of Tierra Colorada (34.3 plot in the ``within-plate-granites'' ®eld of Pearce et al. Ma), San Marcos (31 Ma) and Cruz Grande (32 Ma) (1984). (Herrmann et al., 1994). Farther to the east, between Acapulco and The Acapulco granite displays notably older cooling Huatulco, there are younger intrusive rocks. The U± ages than other plutonic rocks in the region (43.420.9 Pb ages obtained by Herrmann et al. (1994) from Rb±Sr biotite-whole-rock; Schaaf, 1990; 5020.5 Ma Pinotepa Nacional to Huatulco range from 30 to 27 D.J. MoraÂn-Zenteno et al. / Journal of South American Earth Sciences 12 (1999) 513±535 525

Ma. The K±Ar dates from hornblende concentrates from 0.7051 to 0.7063 (Mora n-Zenteno et al., in reported by Herna ndez-Bernal and Mora n-Zenteno press). This inference is corroborated by the lead isoto- (1996) for the Rio Verde batholith, east of Pinotepa pic data reported by Martiny et al. (1997). Pb isotope Nacional, are 29.9 and 27.7 Ma. K±Ar biotite determi- ratios of the Tertiary magmatic rocks from western nations of rocks exposed to the north and east of Oaxaca show a narrow range [(206Pb/204Pb)=18.669± Pinotepa Nacional yielded 27.7 to 23.5 Ma 18.749; (207Pb/204Pb)=15.587±15.623; (208Pb/204Pb)= (Herna ndez-Bernal and Mora n-Zenteno, 1996; 38.442±38.588] whereas the more acidic volcanic rocks Martiny et al., in press). Between Puerto A ngel and from northeastern Guerrero are slightly more radio- Salina Cruz, K±Ar and Rb±Sr age determinations genic [(206Pb/204Pb)=18.745±19.000; (207Pb/204Pb)= from biotite concentrates obtained by Solõ s-Pichardo 15.599±15.651; (208Pb/204Pb)=38.557±38.818]. range from 2720.5 to 13.620.3 Ma (Schaaf et al., 1995; Table 2a, this paper). O€shore of Pinotepa 4.2.1. Western Guerrero Nacional the dioritic rocks cut by Leg 66 of the DSDP In the southwestern part of the State of Guerrero, yielded whole-rock ages of 34.522 to 36.421.8 Ma the Tertiary volcanic rocks of the Sierra Madre del (Bellon et al., 1982). Sur have been dated in the Zihuatanejo-Vallecitos area The plutons between Punta San Telmo and the area (Pantoja-Alor, 1986; Frank et al., 1992; Kratzeisen et west of Acapulco, with a few exceptions, typically al., 1991). Between Zihuatanejo and Cd. Altamirano show initial 87Sr/86Sr ratios between 0.7035 and 0.7049 87 86 the most widespread unit is a 1500 m thick series of and eNd values from +5.7 to +3.1. Initial Sr/ Sr high-K, and pyroxene-bearing porphyritic basaltic  ratios of the La Verde, Inguaran and San Isidro inland andesite to dacitic lava ¯ows, and intermediate to felsic plutons range from 0.7039 and 0.7055 (Damon et al., tu€s and ignimbrites (Kratzeisen et al., 1991). This 1983). In the Acapulco region, the Oligocene intrusives sequence is cut by numerous andesitic to dacitic dikes have slightly higher initial 87Sr/86Sr ratios of 0.7041± of varying dimensions trending NE±SW and NW±SE. 0.7050, and lower initial eNd of +2.9 and +1.6 The time range for these igneous sequences is given by (Schaaf, 1990; Correa-Mora, 1997). Between Acapulco K±Ar age determinations and varies from 46 to 43 Ma and Huatulco the initial 87Sr/86Sr ratios of the plutonic for the andesites and from 45 to 33 Ma for the dikes rocks range from 0.7040 to 0.7054; eNd values are gen- (Kratzeisen et al., 1991). Given the scarcity of data erally lower than to the west of Acapulco, and range from this region, and their broad range, it is not poss- from +3.5 to 3.0; (Mora n-Zenteno, 1992; ible to con®rm if these constitute a single event of 12 Herrmann, 1994; Herna ndez-Bernal and Mora n- million year duration or two separate events. In the Zenteno, 1996; Martiny et al., 1997). Cd. Altamirano area, dikes and tu€s were dated at 46.2±42.3 using K±Ar (Pantoja-Alor, 1986). The only 4.2. Inland volcanic ®elds other Oligocene volcanic rocks in the general region are exposed in the Mil Cumbres, Michoaca n, area The volcanic sequences of the Sierra Madre del Sur where a sanidine K±Ar age of 33.421.7 Ma has been are not as well studied as those of the CPB and the reported (PasquareÁ et al., 1991). data presented below were obtained from two transects perpendicular to the coast. The volcanic units consist of intermediate- to high-K, sub-alkalic rhyolites to 4.2.2. Northeastern Guerrero The Oligocene volcanic rocks in northeastern basaltic andesites, with SiO2 content between 53 and 76 wt% (Fig. 3b). Chondrite normalized REE patterns Guerrero are distributed in three main areas: Taxco, display typical magmatic arc behavior, with LREE Buenavista-Quetzalapa and Huautla. Other Tertiary volcanic rocks of Oligocene age have been reported in enrichment relative to HREE (LaN/LuN between 5 and 14; Fig. 4b). Eu anomalies in the volcanic rocks of the the Tejupilco area, State of Mexico, and include exten- Taxco area (Eu/EuÃ0 0.4) are probably related to crys- sive sequences of ignimbrites forming the Sierra de la tal fractionation of plagioclase. In the area of Goleta, Sierra de Nanchititla and the Mesa del Buenavista, Huautla and northwestern Oaxaca, the Naranjo. Similar rocks have also been reported in the volcanic rocks do not display a signi®cant Eu Tenancingo-Ixtapan de la Sal, State of Mexico, area anomaly. (De Cserna, 1982; Garcõ a-Palomo, 1997). The volcanic Initial 87Sr/86Sr ratios of andesites and basaltic ande- section in the Taxco area consists of rhyolitic ignim- sites from northwestern Oaxaca range from 0.7042 to brites, vitrophyres, ¯ow and ash fall tu€s, forming a 0.7046, and eNd values, from 0 to +2.6 (Martiny et sequence up to 800 m thick. K±Ar dates in volcanic al., in press). Dacites and rhyolites of similar age from glass, plagioclase and biotite show that the age of the the Taxco area show evidence of either a greater lower part of the sequence ranges from 38.2 to 35.5 degree of crustal contamination or more evolved Ma (De Cserna and Fries, 1981; Mora n-Zenteno et al., source rocks since their initial 87Sr/86Sr ratios range in press), whereas that of the upper part, from 32.4 to 526 D.J. MoraÂn-Zenteno et al. / Journal of South American Earth Sciences 12 (1999) 513±535

31.6 Ma (Mora n-Zenteno et al., in press). These results rhyolitic to andesitic lava ¯ows interbedded with lacus- suggest that the Taxco sequence originated from two trine deposits and intruded by rhyolitic to latitic hypa- distinct volcanic events. byssal rocks (Ferrusquõ a-Villafranca and McDowell, The Buenavista±Quetzalapa range, 30 km southeast 1991). Pyroclastic deposits are more abundant than of Taxco, is formed by a series of dacitic ignimbrites lava ¯ows in this region. The K±Ar age determinations and lava ¯ows with a maximum total thickness of of central and southeastern Oaxaca range from 20.6 about 900 m. A 31.921.1 Ma biotite K±Ar age was near the city of Oaxaca to 15.0 Ma in Nejapa obtained for the lower part of the sequence, rep- (Ferrusquõ a-Villafranca and McDowell, 1991). resented by dacitic ignimbrites of the Tilzapotla Formation, whereas the upper part, represented by dacites of the Buenavista Formation, yielded a plagio- 5. Geochronologic patterns clase K±Ar age of 30.521.1 Ma (Mora n-Zenteno et al., in press) (Table 2a). These rocks are related to a The geochronological database of the SMS mag- caldera about 20 km in diameter, which originated matic rocks (Table 2 and Schaaf et al., 1995) indicates during the early stage of volcanic activity and under- that arc-magmatism was active, with di€erent spatial went later resurgent activity. distributions, from Paleocene to middle Miocene times. Fig. 5 shows the distribution of arc magmatic 4.2.3. Northwestern Oaxaca rocks for four di€erent times. There is a recognizable In the Huajuapan±Tamazulapan±Yanhuitla n region, tendency of the older magmatic rocks to dominate in the volcanic sequences are dominated by andesite and the westernmost region of the SMS. This behavior was basaltic andesite lavas that overlie a sequence of epi- identi®ed by Schaaf et al. (1995) for the plutonic rocks clastic beds and some silicic ash fall and ash ¯ow tu€s along the continental margin between Puerto Vallata (Fig. 3b). Farther south in the Tlaxiaco region, the and Acapulco. Fig. 6a shows a plot of ages versus dis- Tertiary volcanic rocks range from felsic to intermedi- tance from Puerto Vallarta. The ages of intrusive ate pyroclastic and epiclastic deposits. The maximum rocks along the coastal segment between Puerto thickness of the Tertiary volcanic sequences in this Vallarta and Zihuatanejo range from 100 to nearly 35 region is about 500 m. Throughout northwestern Ma. From Zihuatanejo to the Huatulco region there is Oaxaca, numerous dacitic and andesitic hypabyssal also a decreasing age trend in the extinction of mag- intrusives (stocks and dikes) are emplaced at di€erent matism but with a higher rate of migration. Schaaf et levels of the volcanic sequence (Martõ nez-Serrano et al. (1995) calculated a rate of 1.2 cm/a for the ®rst seg- al., 1996). In some areas the volcanic rocks are inter- ment and 7.7 cm/a for the second. Based only on U± layered with lacustrine deposits, which have been inter- Pb zircon ages of di€erent plutons from the Acapulco preted as accumulated in fault-bounded tectonic basins region to Huatulco, Herrmann et al. (1994) calculated (Martiny et al., 1995). The time range for the volcan- a decreasing age rate of 5.59 cm/a. ism in northwestern Oaxaca is 34.8±31.4 Ma (Table The Late Cretaceous±Paleocene magmatism is rep- 2a) based on biotite and hornblende K±Ar ages of the resented by the Puerto Vallarta, Manzanillo and San tu€s and hypabyssal rocks (Martiny et al., 1997). Telmo plutons (Fig. 1) (Schaaf, 1990; KoÈ hler et al., These dates are relatively consistent with the whole- 1988; Schaaf et al., 1995). Paleocene to Eocene mag- rock K±Ar ages (32±29 Ma) for hypabyssal rocks and matic rocks, including the inland volcanic areas, lie lavas in the Huajuapan area (Galina-Hidalgo, 1996). mainly to the west of 1008W meridian, distributed In the Tamazulapan±Yanhuitla n area, Ferrusquõ a- from the coastal margin to a distance up to 300 km Villafranca and McDowell (1991) obtained a younger from the present-day trench (Fig. 5a). Eocene magma- whole rock K±Ar age of 28.9 Ma for an andesitic lava tism dominates in the Zihuatanejo region and inland and a biotite age of 26.2 Ma for a tu€. Up to now, in from it, including the extensive outcrops of andesites this region only one Eocene K±Ar hornblende age of of the Balsas River basin in the Presa del In®ernillo 40.521.7 Ma (Table 2a) has been obtained for a lac- region (Kratzeisen et al., 1991; Delgado-Argote et al., colith associated with the Yanhuitla n Formation that 1992; Pantoja-Alor, 1983). In the inland region of seems to represent the earliest manifestations of Michoaca n and western Guerrero, there are also sev- Tertiary magmatism in this region. eral calc-alkaline plutonic rocks for which Eocene ages have been reported (Pantoja-Alor, 1983; Alba-Aldave 4.2.4. Central and southeastern Oaxaca et al., 1996). The relatively scarce basaltic ¯ows inter- This region covers a broad area including NW of calated in the continental sequences of the Balsas the city of Oaxaca, Tlacolula-Mitla and Nejapa with Formation are indicative of isolated Eocene volcanism variations in the age and composition of volcanic pro- that extends toward the east to the eastern part of the ducts. In general, the volcanic sequences are consti- State of Guerrero (De Cserna, 1981). In addition to tuted by siliceous tu€s, volcaniclastic deposits, the basaltic intercalations of the Balsas Formation, ..Mora D.J.  -etn ta./Junlo ot mrcnErhSine 2(99 513±535 (1999) 12 Sciences Earth American South of Journal / al. et n-Zenteno

Fig. 5. Distribution of the Tertiary magmatic arc rocks of the Sierra Madre del Sur at di€erent times (a) Paleocene±Eocene; (b) Oligocene and (c) middle Miocene±Plio-Quaternary. 527 528 D.J. MoraÂn-Zenteno et al. / Journal of South American Earth Sciences 12 (1999) 513±535

Fig. 6. (a) Graph of pluton ages vs distance from Puerto Vallarta along the coast. Only data from the coastal plutonic belt are plotted. (b) Graph of age vs distance inland from the Acapulco Trench along section A±A'. (c) Graph of age vs distance inland from the Acapulco Trench along section B±B'. Section lines for (b) and (c) are shown on Fig. 1. For A±A' and B±B' sections age uncertainties are indicated. east of 1008W longitude, few localities of magmatic Zihuatanejo and Taxco±Tejupilco regions (Alba- rocks have been reported with Eocene ages (MartõÂ nez- Aldave et al., 1996) to west central Oaxaca, including Serrano et al., 1997). southern Puebla and the Huajuapan±Tlaxiaco region The early Oligocene magmatic rocks (36±30 Ma) are (Fig. 5b) (Martiny et al., 1996). In the Acapulco± distributed mostly in the eastern half of the SMS and Taxco sector Oligocene magmatism was in general coe- de®ne a broad zone that extends eastward from the val, indicating the occurrence of an arc of at least D.J. MoraÂn-Zenteno et al. / Journal of South American Earth Sciences 12 (1999) 513±535 529

200 km wide (Fig. 6b). In northwestern Oaxaca the The early Oligocene volcanic rocks of the Taxco± Oligocene rocks tend to be slightly older in the inland Tilzapotla area in northeastern Guerrero have in gen- volcanic zones (Huajuapan±Tamazulapan±Yanhuitla n) eral a higher SiO2 concentration than coeval volcanic with respect the plutonic rocks of the coastal zone rocks of northwestern Oaxaca (Fig. 3b). This higher

(Fig. 6c), however the K±Ar ages from 29.6 to 25.9 SiO2 content, particularly for the rhyolitic rocks of the (Table 2a) reported by Ferrusquõ a-Villafranca (1976) Taxco range, could be caused by a greater degree of for the inland sequences give indications of coeval crystal fractionation (plagioclase), as inferred from the magmatism in both regions at some time after 30 Ma. Eu anomaly displayed by the REE chondrite-normal- Late Oligocene arc-magmatic rocks (30±23 Ma) are ized diagrams. In addition to this, the Sr, Nd and Pb mainly distributed along the coastal region from isotopic data (Martiny et al., 1997) indicate that, for Acapulco to Huatulco and correspond mostly to felsic the Taxco±Buenavista volcanic rocks, there is a greater batholiths (Herrmann et al., 1994; Herna ndez-Bernal in¯uence of old crustal components or more evolved and Mora n-Zenteno, 1996). Early and middle Miocene source rocks than for the northwestern Oaxaca volca- rocks are distributed in the region between the city of nic zone. The tectonic and stratigraphic features in Oaxaca and the Isthmus of Tehuantepec (Fig. 5c) both areas indicate that western Oaxaca underwent left mainly within elongated, E±W trending tectonic basins lateral transtension that produced pull-apart basins of  located inland of the Puerto Angel-Salina Cruz sector. N±S orientation at the time of the volcanism. For the In this region it is evident that volcanism in the inland Taxco±Tilzapotla area there are no such indications of regions was active after the cessation of Oligocene plu- major extensional features contemporary with the tonism in the Huatulco region. silicic magmatism. The high rate of extension in north- western Oaxaca during early Oligocene time seems to have favored not only a greater volume of volcanism 6. Discussion and conclusions but also lower degrees of di€erentiation and crustal contamination, particularly for the upper andesitic The petrologic and geochemical variations of the sequences (Martiny et al., in press). Tertiary magmatic rocks along the SMS indicate di€er- In the CPB of Oaxaca and eastern Guerrero the in- ent degrees of magmatic di€erentiation and crustal ¯uence of the tectonic setting in the geochemical vari- contamination. The factors that controlled these vari- ations of the plutonic rocks is less evident. They were ations are not completely understood. There is no emplaced during a period of transtensional tectonics straightforward correlation between the degree of con- related to the Chortis block displacement. The beha- tamination by older crust, as inferred from the Sr and vior of major and trace elements of the plutonic rocks Nd isotopic data, and the age of the oldest crust in the as well as their isotopic compositions indicate vari- di€erent terranes of the tectonic mosaic in southern ations in the degree of magmatic di€erentiation and Mexico. In spite of this, there is a tendency for the Tertiary and Late Cretaceous magmatic units, crustal contamination. The causes of these variations emplaced in the Guerrero terrane (i.e. Manzanillo, for rocks emplaced in the same general tectonic setting Petatla n-Papanoa, Punta San Telmo), to display the remain elusive. lowest 87Sr/86Sr ratios and the highest eNd values The geochronological patterns described re¯ect in a (Schaaf et al., 1995). Other intrusives in the Guerrero general way the transition from a zone in¯uenced by terrane have 87Sr/86Sr and eNd values similar to those the subduction of the Farallon plate in western of the eastern SMS where older crustal components Mexico to a more complex plate tectonic scenario in seem to have played a greater role in the magmatic southern Mexico. The magmatic rocks of the late evolution. Paleocene±Eocene time interval, mainly distributed from Manzanillo to Zihuatanejo and inland up to the The SiO2 and alkali content of the SMS magmatic rocks, in conjunction with the rare earth element abun- 01008W meridian, seem to de®ne the southern exten- dances and isotopic data, show some variations that sion of coeval magmatic rocks of the Lower Volcanic may be the result of di€erent degrees of crystal frac- Complex of the SMO (Clark et al., 1982; Montigny et tionation. If we assume that the degree of fractionation al., 1987; Aguirre-Dõ az and McDowell, 1991). This is related to the rate of ascent of magmas to the upper fact suggests that the continuation of the magmatic arc crust (Burkart and Self, 1985; Glazner and Ussler, to the south of Michoaca n and western Guerrero was 1989) and that the rate of ascent will be controlled by probably removed laterally with the Chortis block. the stress regime and the particular deformation con- The scarcity of geochronological data in the Chortis ditions at the time of magma emplacement (Apperson, block and the extensive cover of Quaternary volcanic 1991; Takada, 1994), we can use the geochemical char- sequences prevent us from con®rming this interpret- acter of the igneous rocks to make a ®rst order guess ation at present. The volcanic members of the Balsas at the state of stress of the crust during magmatism. Formation in eastern Guerrero and other isolated 530 D.J. MoraÂn-Zenteno et al. / Journal of South American Earth Sciences 12 (1999) 513±535

Fig. 7. Cartoon showing the evolution of the continental margin geometry since the early Oligocene and associated igneous products. Abbreviations are: Ac=Acapulco, MC=Mexico City, Mz=Manzanillo, PA=Puerto AÂ ngel, Zt=Zihuatanejo.

Eocene localities in western Oaxaca were distant mani- displacement when the Caribbean plate was formed. festations of magmatic arc activity. The lack of correlative stratigraphic features along this The southeastern migration of the trench±trench± continental margin and the northern limit of the transform triple junction that accompanied the displa- Chortis block do not support a paleo-position of the cement of the Chortis block produced a gradual Chortis block o€ the coast of Jalisco, Colima and extinction of the magmatism since Eocene time as can northwestern Michoaca n (Schaaf et al., 1995). In ad- be observed in the age decreasing trend along the con- dition, no evidence has been found indicating Tertiary tinental margin southeast of Zihuatanejo (Herrmann et left lateral shear zones that could suggest a continu- al., 1994; Schaaf et al., 1995). The decreasing age trend ation of the kinematic conditions similar to those of in the segment northwest of Zihuatanejo does not the Zihuatanejo±Huatulco sector. Schaaf et al. (1995) seem to be directly related to the displacement of the interpreted this decreasing trend as the product of a Chortis block, at least not during the latest history of prior eastward migration of magmatism (Late D.J. MoraÂn-Zenteno et al. / Journal of South American Earth Sciences 12 (1999) 513±535 531

Cretaceous±Eocene) followed by episodes of tectonic tism farther north in the TMVB as a consequence of erosion of the continental margin. Lateral removal of the gradual development of the Acapulco Trench at continental slices, by oblique subduction, prior to the the truncated margin of southwestern Mexico (e.g. displacement of the Chortis block and possible subduc- Demant, 1978; Ratschbacher et al., 1991; Schaaf et al., tion erosion processes could have played a role in the 1995). Since the earliest manifestations of magmatic tectonic truncation of this margin. activity of the eastern and central TMVB are as old as The distribution of the Oligocene magmatic rocks middle Miocene (016 Ma; Ferrari et al., 1994 and indicates not only an eastward migration of the mag- references therein), a 010 Ma magmatic gap following matism, but also a change toward an ESE trend (Fig. the extinction of the late Oligocene magmatism can be 5b). There are no indications that the ESE orientation recognized for a broad region between 100 and 978W in the distribution of the early and late Oligocene (Fig. 7c). rocks from Guerrero to Oaxaca (Fig. 5b) could be the In addition to the emplacement of the arc magma- response of a change in the subducted slab geometry. tism in the present day oblique position of the TMVB, According to the reconstruction of the convergence the present-day geometry of the subducted slab vectors of the Farallon plate with respect to North beneath the eastern sector of the SMS (Pardo and America (Schaaf et al., 1995), the orientation of the Sua rez, 1995) is suggestive of a shallowing of the sub- arc at this time corresponds neither with a period of duction angle between the late Oligocene and the signi®cant increase of the convergence rate, nor with a middle Miocene (Fig. 7c). The relative rapid change of major change in the convergence direction. The age plate inclination and the time required for the mantle constraints of the shear zones in the continental mar- wedge beneath the TMVB to achieve the metasomatic gin and the paleogeographic reconstructions based on conditions necessary to produce arc-volcanism are the the ocean ¯oor magnetic data of the Cayman Trough most probable causes of the magmatic gap. The gra- (Rosencrantz and Sclater, 1986; Ross and Scotese, dual transition of SMO magmatism to the volcanic ac- 1988; Pindell and Barrett, 1990) indicate that a signi®- tivity of the western TMVB occurred in a di€erent cant segment of the Chortis block was still o€shore of scenario. The subducted slab of the present-day Rivera southern Oaxaca. Given the geometry of the northwest- plate seems to have remained at its present angle since ern part of the Chortis block, its displacement toward its formation by the fragmentation of the Guadalupe the ESE (Fig. 7a) produced a gradual northeastward plate at 12.5 Ma. migration of the trench. This migration probably caused the arc magmatism to reach the inland regions as far as northeastern Guerrero and northwestern Acknowledgements Oaxaca. In northwestern Oaxaca (Huajuapan±Tlaxiaco region), the occurrence of very active N±S transten- Di€erent phases of this research were funded by the sional tectonics could have favored the development of Consejo Nacional de Ciencia y Tecnologõ a the intermediate volcanism that gave the arc its rela- (CONACyT) (project 3361 T9309) and PAPPIT- tively broad con®guration. The largest volume of mag- DGAPA (grant IN102794) at the Universidad matism in late Oligocene time (<30 Ma) was Nacional Auto noma de Me xico (UNAM). We thank concentrated along the present-day continental margin Juan Julio Morales Contreras and Faustino Jua rez for of eastern Guerrero and Oaxaca (Fig. 7b). This distri- laboratory support at UNAM, Teodoro Herna ndez- bution seems to have been determined by the NW±SE TrevinÄ o for assistance in the ®eld and Margarita- shear zones associated with the transtensional tectonics Reyes-Salas for petrographic analyses. at the boundary between southern Mexico and the Chortis block (Herrmann et al., 1994; Tolson, 1998). Herrmann et al. (1994) suggested that the heating and References fracturing produced by extension facilitated the magma ascent during the eastward displacement of the Aguirre-Dõ az, G.J., McDowell, F.W., 1991. The volcanic section at Chortis block. Magmatism in the inland region Nazas, Durango, Mexico, and the possibility of widespread between the city of Oaxaca and Nejapa continued Eocene volcanism within the Sierra Madre Occidental. Journal of until early and middle Miocene times, whereas late Geophysical Research 96, 13,373±13,388. Alaniz-A lvarez, S.A., van der Heyden, P., Nieto-Samaniego, A.F., Oligocene plutonic rocks are predominant along the Ortega-Gutie rrez, F., 1996. Radiometric and kinematic evidence coastal area. This suggests a decrease in the displace- for Middle Jurassic strike-slip faulting in southern Mexico related ment rate of the Chortis block at this time that lead to to the opening of the Gulf of Mexico. Geology 24, 443±446. a more gradual northward migration of the magma- Alba-Aldave, L.A., Reyes-Salas, M., Mora n-Zenteno, D., Angeles-   tism. Garcõa, S., Corona-Esquivel, R., 1996. Geoquõmica de las rocas volca nicas terciarias de la regio n de Taxco-Huautla. Memoria del The removal of the Chortis block has been largely VII Congreso Nacional de Geoquõ mica, San Luis Postosõ . Actas invoked as the cause of the initiation of arc-magma- INAGEQ 2, 39±44. 532 D.J. 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