Print version ISSN 1026-8774
Rev. mex. cienc. geol vol.25 no.1 México 2008
Geochemical and SrNd isotopic characterization of the Miocene volcanic events in the Sierra Madre del Sur, central and southeastern Oaxaca, Mexico
Caracterización geoquímica e isotópica de eventos volcánicos del Mioceno en la Sierra Madre del Sur, centro y sureste de Oaxaca, México
Raymundo G. MartínezSerrano1,*, Gabriela SolísPichardo2, E. Leticia FloresMárquez1, Consuelo MacíasRomo2, and Jaime DelgadoDurán1
1 Instituto de Geofísica, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 México, Distrito Federal, México.* email@example.com
2 Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 México, Distrito Federal, México.
Manuscript received: November 9, 2006
Corrected manuscript received: September 20, 2007
Manuscript accepted: September 27, 2007
The Etla, MitlaTlacolula and Nejapa volcanic regions in central and southeastern Oaxaca comprise the southeastern part of a wide Cenozoic magmatic arc in the Sierra Madre del Sur. Most volcanic events in these regions ocurred between 22 to 15 Ma, almost contemporaneously with the initial volcanic events of the TransMexican Volcanic Belt. Petrographic, geochemical, and isotopic characteristics were determined for representative volcanic samples from the three regions, where ignimbrites, volcaniclastic and epiclastic deposits, lava flows and minor lacustrine deposits are found. In a SiO2 vs. alkalis diagram, chemical classification of volcanic products for the study area indicate variations from basaltic andesites to rhyolites, following a subalkaline trend, but with a bimodal pattern. Components with SiO2 concentrations between 58 to 67 wt. % are absent. The traceelement patterns for andesites and rhyolites are similar, with enrichment in the largeion lithophile elements relative to the highfieldstrength elements. Chondritenormalized REE patterns display light rare earth element enrichment (LaSm) with respect to the heavy rare earth elements (EuLu), which show flat patterns. These chemical characteristics are typical of volcanic arc rocks. Initial Sr and Nd isotopic data show certain differences between samples from the Etla and MitlaTlacolula regions 87Sr/86Sr: 0.7047 to 0.7066 and εNd: 1.15 to 1.75) and the Nejapa region 87Sr/86Sr: 0.7035 to 0.7048 and εNd: +0.52 to +1.42). These data and those reported for the basement rocks suggest greater involvement of continental crust for the magmas of the two first regions in comparison to magmas of the Nejapa region. The isotopic compositions are similar to those observed in other volcanic regions of the SierraMadre del Sur. Early to middle Miocene volcanic events in central and southeastern Oaxaca, together with the contemporaneous initial magmatic events in the TransMexican Volcanic Belt, could conform a magmatic arc with an anomalous orientation before attaining its present position.
Keywords: geochemistry, SrNd isotopic ratios, volcanic rocks, Miocene, Sierra Madre del Sur, Mexico.
Las regiones volcánicas de Etla, MitlaTlacolula y Nejapa, en la parte central y sureste de Oaxaca, conforman la porción oriental de un amplio arco magmático cenozoico dentro de la Sierra Madre del Sur. La mayoría de los eventos volcánicos ocurrieron entre los 22 y 15 Ma, casi de manera contemporánea con los primeros eventos volcánicos de la Faja Volcánica Transmexicana. Se determinaron las características petrográficas, geoquímicas e isotópicas de muestras volcánicas obtenidas de las tres regiones, donde existen ignimbritas, depósitos volcaniclásticos y epiclásticos, flujos de lava, y algunos depósitos lacustres. La clasificación química de las rocas en un diagrama SiO2 vs. álcalis varía entre andesitasy riolitas, siguiendo un patrón subalcalino bimodal. Al parecer no existen rocas con contenidos de SiO2 de entre 58 y 67 % en peso. Los patrones de comportamiento de elementos traza de las andesitas y riolitas son muy similares, con un enriquecimiento en los elementos litófilos respecto a los elementos de alto potencial iónico, mientras que los patrones de los elementos de tierras raras, normalizados con respecto a condrita, muestran un enriquecimiento de las tierras raras ligeras (LaSm) respecto a las tierras raras pesadas, con un comportamiento casi plano para estas últimas (EuLu). Este tipo de patrones se ha asociado con rocas de arco volcánico. Las relaciones isotópicas iniciales de Sry Nd muestran ciertas diferencias entre las regiones de Etla, MitlaTlacolula 87Sr/86Sr: 0.7047 a 0.7066 y εNd: 1.15 a 1.75) y Nejapa 87Sr/86Sr: 0.7035 a 0.7048 y εNd: +0.52 a +1.42). Con base en estos datos isotópicos, más los existentes para rocas del basamento de estas regiones, se sugiere que los magmas de las dos primeras regiones sufrieron una mayor interacción con la corteza continental en comparación con los magmas de la región de Nejapa. Las composiciones isotópicas de las rocas del área de estudio son similares a las observadas en otras regiones volcánicas cenozoicas de la Sierra Madre del Sur. Los eventos volcánicos del Mioceno temprano a medio de la parte central y sureste de Oaxaca, junto con los primeros eventos magmáticos de la Faja Volcánica Transmexicana podrían conformar un arco magmático continuo con una orientación anómala, antes de alcanzar su posición actual.
Palabras claves: geoquímica, isotopía de Sr y Nd, rocas volcánicas, Mioceno, Sierra Madre del Sur, México.
The PaleoceneMiocene magmatic rocks of the Sierra Madre del Sur (SMS), southern Mexico (Figure 1) have been identified as two broad belts approximately parallel to the Pacific coast. These rocks are part of a widespread magmatic arc that include an almost continuous WNWtrending plutonic belt made of granitic to tonalitic batholiths emplaced along the presentday coast of southern Mexico and a second volcanic belt discontinuously distributed inland, up to the northern part of the SMS. The volcanic belt shows compositional variations from basaltic andesites to rhyolites (MoránZenteno et al, 1999; Martiny et al, 2000a). The plutonic and volcanic rocks of this arc extend for about 600 km, from the southern part of Jalisco to the Isthmus of Tehuantepec, displaying a decreasing age trend from Paleocene in Colima to early Miocene in southeastern Oaxaca (Figure 1) (MoránZenteno et al, 1999). These arcrelated magmas were originated during subduction episodes along the Pacific margin previously or contemporaneously to the truncation of the continental margin by the displacement of the Chorus block (Ross and Scotese, 1988; Pindell et al, 1988; Ratschbacher et al, 1991; Herrmann et al, 1994; Schaaf et al., 1995; MoránZenteno et al, 1999) orby subduction erosion processes (Keppie and MoránZenteno, 2005;Keppieeía/.,2007).
Several stratigraphic, geochemical and isotopic studies have been carried out in some cenozoic volcanic fields of the SMS in the last fifteen years (e.g., FerrusquíaVillafranca, 1992; 2001; MartinezSerrano et al, 1997; 1999; MoránZenteno et al, 1998; 1999; 2004; 2007; Martiny et al, 2000a; AlanizÁlvarez et al, 2002). These studies showed that volcanic rocks display a decreasing formation age, from 46 Ma in the NW part of the state of Guerrero to 15 Ma in the SE part of the state of Oaxaca. Geochemical and isotopic results of these rocks, from several regions, suggest a mantle source in the subcontinetal lithosphere that has been enriched by subduction components. Crystal fractionation magma processes associated with a moderate degree of old crustal involvement probably produced these magmatic rocks (Martiny et al. 2000a; MoránZenteno et al. 1998, 1999).
Several volcanic successions located in the southeastern SMS, in the regions of Etla, MitlaTlacolula and Nejapa (central and southeastern parts of the state of Oaxaca; Figure 1), display KAr ages that range from 22 to 15 Ma (FerrusquíaVillafranca and McDowell, 1991; Iriondo et al, 2004). However, their geochemical and isotopic characteristics, and their tectonic context are not well known. These volcanic sequences may represent the final magmatic events in the Sierra Madre del Sur before reappearing in the TransMexican Volcanic Belt.
In this contribution, new petrographic, geochemical and isotopic data are provided for volcanic rocks of the Etla, MitlaTlacolula, and Nejapa regions, state of Oaxaca, in order to gain insight into the significance of these rocks in the tectonic evolution of southern Mexico during Miocene time.
Basement rocks and tectonic setting
The Cenozoic volcanic sequences in the central and southeastern parts of the state of Oaxaca cover two major tectonostratigraphic terranes (Figure 2): the Oaxaca or Zapoteco terrane and the Cuicateco or Juárez terrane (Campa and Coney, 1983; Sedlock et al, 1993). The oldest unit of the Oaxaca terrane is characterized by a granulitefacies metamorphic basement of Grenvillian age (9001100 to 1300 Ma) (OrtegaGutiérrez, 1981; Solan et al., 2003) overlain by Paleozoic and Mesozoic sedimentary sequences, and Cenozoic sedimentary and volcanic rocks (PantojaAlor, 1970; Schlaepfer, 1970; LópezTicha, 1985; FerrusquíaVillafranca, 1992; MoránZenteno et al, 1999; UrrutiaFucugauchi and FerrusquíaVillafranca, 2001). The presentday 87Sr/86Sr and eNd isotopic values of the metamorphic Oaxaca complex generally range from close to those of bulk earth to 0.717 (although one paragneiss has a reported 87Sr/86Sr ratio of 0.750) and from 9 to 12, respectively (Patchett and Ruiz, 1987; Ruiz et al, 1988a, 1988b).
The Juárez terrane (OrtegaGutiérrez et al, 1990; Sedlock et al., 1993) is a westdipping faultbounded prism of strongly deformed Jurassic and Cretaceous oceanic and arc volcanic rocks that structurally overlies the Maya terrane and underlies the Oaxaca terrane. Many aspects of the geology and geochronology of this terrane are unresolved. It is composed of Mesozoic marine and continental sedimentary sequences, and intensively deformed volcanogenic strata thrusted to the east (Carfantan, 1986; Barboza, 1994, NietoSamaniego et al., 2006). In the western margin of the Juárez terrane, the Etla area (north of the City of Oaxaca) is defined by a westdipping NNW trending polygenic mylonitic shear zone, which was reactivated in Middle Jurassic and Cenozoic times (AlanizÁlvarez et al, 1996). No geochemical and isotopic data are available for rocks from this terrane. The Miocene volcanic rocks of the Etla and MitlaTlacolula regions cover part of the stratigraphic sequences from the Oaxaca and Juárez terranes whereas the volcanic products of the Nejapa region were probably emplaced over Grenvillian rocks of the Oaxaca terrane (Figure 2). The influence of theses old continental rocks in magma composition might be observed in neogenic volcanic rocks.
According to NietoSamaniego et al. (2006) the major cenozoic structures in centralsouthern Mexico (Figure 2) were produced in three main successive tectonic events: the first event, in the late Cretaceous middle Eocene, corresponds to the Laramide orogeny during which deformation migrated from west to east. The Oaxaca fault system, which bounds the Oaxaca and Juárez terranes, displayed significant activity during this orogeny. The second event produced strikeslip faulting during NESW horizontal shortening from Eocene to Oligocene time. The third event produced normal and strikeslip faults, indicating NESW horizontal extension during OligoceneMiocene time. The major structures are roughly distributed along the borders of the main tectonostratigraphic terranes, but others are distributed within the terranes (Figure 2). In the Etla region, the NWtrending Oaxaca fault (Figure 2) exhibits a complex history of displacement beginning in the Paleozoic and followed by lateral motion in the Jurassic, although the sense of shear remains unknown (AlanizÁlvarez et al, 1996). Its most recent activity has been described as normal fault motion with the downthrown block to the west. The Oaxaca fault shows normal displacement as recently as the late Cenozoic (AlanizÁlvarez et al., 1996). To the southeast of the Oaxaca fault, several EW oriented graben structures were recognized by MoránZenteno et al. (1999), for which an early to middle Miocene age can be inferred on the basis of KAr dates of pyroclastic materials from the basin fill (FerrusquíaVillafranca, 1992). The MitlaTlacolula region is found in one of these EW grabens. Although structural and tectonic studies are lacking in the Nejapa region, FerrusquíaVillafranca (2001) described the presence of NWSE trending horst and graben structures.
The Cenozoic plate tectonic setting of southern Mexico is characterized by a predominantly convergent plate boundary, with several periods of oblique subduction and continental transform boundaries. Several authors proposed that the PaleoceneMiocene magmatic arc of the SMS was originated during subduction episodes along the Pacific margin previous to, and in part contemporary with, margin truncation attributed to the displacement of the Chorus block (Mailfait and Dinkelman, 1972; Ross and Scotese, 1988; Herrmann et al., 1994; Schaaf et al., 1995, Meschede et al., 1997). However, Keppie and MoránZenteno (2005) and Keppie et al. (2007) recently proposed a new model for the tectonic arrangement between the Chords block and the Pacific margin of southern Mexico, in which Chords was located southwest of its present position rather than off southwestern Mexico. These authors also suggested that the collision of the Chumbia Seamont Ridge with the Acapulco trench led to flattening of the subducting slab, inducing subduction erosion and exhumation of the southern Mexican margin.
The Cenozoic volcanic rocks in the study area are distributed in isolated outcrops and in this work these rocks have been grouped in three regions: Etla, MitlaTlacolula and Nejapa. Figures 3 and 4 show a schematic geological map and composite stratigraphic columns for each region.
The Etla region
The Neogene volcanic sequences in the Etla region are exposed in discontinuous outcrops of lava flows, epiclasticvolcaniclastic deposits, lacustrine, andpyroclastic deposits emplaced in an elongated valley located to northwest of the city of Oaxaca (area A in Figure 3). This valley displays a NWSE orientation and is bordered to the S W by metamorphic rocks of the Oaxaca complex (Grenvillian age) and to the NEby folded Mesozoic sedimentary rocks of the Juárez terrane. The limit of these two terranes is represented by the Oaxaca fault system that is over 150 km in length, and whose most spectacular expression is represented by a mylonitic belt in the Sierra de Juárez. Figure 4a summarizes the stratigraphic features of this region, where metamorphic rocks of the Oaxaca complex and sedimentary sequences of the Juárez terrane form the basement.
The Cenozoic sequences that have been dated are Miocene in age and lie unconformably on basement rocks, having a mean thickness of 700 m (FerrusquíaVillafranca, 1992 and FloresMárquez et al., 2001). Most Cenozoic sequences in the region are observed as tilted blocks in various directions (12° to 15°). It is possible that a main NW SE 45° fault system produced this structural pattern (FloresMárquez et al., 2001). A first volcanic event in the region is composed of several isolated outcrops of pyroxene andesites that are highly altered to chlorite and clay minerals. The age and thickness of this unit are unknown but FerrusquíaVillafranca (1990) proposed a Paleogene age based on its stratigraphic position. The Suchilquitongo Formation (Wilson and Clabaugh, 1970), composed of epiclastic tuffs, silicified lacustrine limestones and thick ignimbritic deposits, lies over the previously mentioned pyroxene andesites or directly over rocks from the metamorphic and sedimentary basement. This formation, with an estimated thickness of 300 m, is exposed as NW SE elongated hills within the valley (FerrusquíaVillafranca, 1990). It consists of two members. The first member is composed of intercalated thin layers (520 cm) of epiclastic tuffs, pyroclastic products and lacustrine deposits with angular to subangular detritic fragments (1.5 mm in size) of quartz, feldspar, pumice, volcanic fragments, and iron oxides in a cryptocrystalline matrix. Some layers of silicic lacustrine limestones are present in the sequences. Ages of middle to late Miocene have been assigned on the basis of abundant vertebrate fossils found in the sedimentary sequence (FerrusquíaVillafranca et al., 1974). An important wellindurated ignimbritic succession, designated as the Etla Ignimbrite by Wilson and Clabaugh (1970), overlies the lacustrine and volcaniclastic deposits. This second member is composed of silicic pumice, volcanic fragments, quartz, feldspar and abundant biotite included in an altered glassy matrix. The thickness of the ignimbrite was estimated at 30 m and in some outcrops it is covered by ashfall deposits. The source of the volcanic products is not known, but judging from the small thickness of deposits and their textures, it might be located outside of the Etla region. FerrusquíaVillafranca et al. (1974) determined two KAr dates for the Etla Ignimbrite (16.5±0.5 and 17.4±0.3 Ma, although the material dated is not indicated). Subsequently, UrrutiaFucugauchi and FerrusquíaVillafranca (2001) reported three KArbiotite and plagioclase ages (from 19.2±0.5 to 20.5±0.3 Ma). Apolymictic conglomerate about 70 m thick unconformably overlies the Suchilquitongo Formation. Even though its age is unknown, UrrutiaFucugauchi and FerrusquíaVillafranca (2001) proposed a Pliocene age. Important Quaternary alluvial deposits and soil (some tens of meters of thick) fill the Etla Valley covering the majority of the older rocks.
The MitlaTlacolula region
The Cenozoic volcanic rocks in the MitlaTlacolula region show a wider distribution in a valley with an almost EW orientation (area B in Figure 3). This valley is bordered to the south by rocks of the Oaxaca complex and to the north by Mesozoic sequences. Figure 4b summarizes the stratigraphic features. Mesozoic and Cenozoic sedimentary and volcanic sequences cover most of the basement rocks in the region. However, some outcrops of gneiss and other metamorphic rocks, probably associated with the Oaxaca complex, have been observed near the town of Tlacolula (Sample CON316, Figure 4b). Athick sedimentary sequence (400 m) composed of fossilbearing micritic marine limestones and intercalated shales overlies disconformably the metamorphic basement rocks. FerrusquíaVillafranca (1990) proposed an AptianCenomanian(?) age for this sequence. These rocks display different patterns of fractures and folds, and some silicic hypabyssal bodies are emplaced in the limestones, producing contact metamorphism with garnet, quartz, calcite, and sulfide mineralization.
Some isolated andesiticdacitic lava flows and dikes lie on Mesozoic sedimentary rocks in the northern part of the MitlaTlacolula valley, but a larger volume of andesitic lava flows are observed to the south covering the Oaxaca terrane basement. In this area, the andesitic rocks are strongly altered to chlorite and clay minerals, and in some regions hydrothermal Cu, Fe and Pb sulfide mineralization is observed. Although the age of these volcanic products is not known because of their advanced alteration, aPaleogene age is proposed on the basis of their stratigraphic position.
Several ignimbritic and tuffaceous deposits were emplaced in discordance over Mesozoic sedimentary rocks and Paleogene volcanic rocks. These ignimbrites and tuffs were designated as the Mitla Tuff Formation (FerrusquíaVillafranca, 1990), composed of two members. However, in the present study it is considered that the formation consists of several rhyolitic to rhyodacitic vitrocrystalline indurated tuffs, ignimbrites and also ashfall deposits. The thickness of this volcanic sequence was estimated at 500 m. The ignimbrites display several horizontal plateaus in the MitlaTlacolula valley. FerrusquíaVillafranca (1990) reported three KAr ages for this sequence that range from 14.4 ± 0.4 to 16 ± 0.4 Ma, although in his work it is not indicated the type of material dated nor its location. Iriondo et al. (2004) reported an 40Ar/39Ar age of 15.48 ± 0.2 for biotite in a rhyolitic tuff located in San Pedro Quiatoni. The Miocene volcanic sequence is affected by normal fault systems producing several tilted blocks. The source of volcanic rocks in this region is not known but it is considered to be near the valley given their great thickness. Some hypabyssal dikes or eroded volcanic necks 0.5 to 2 km in diameter were observed and sampled in the area.
The Matatlán Formation unconformably covers the Mitla Tuff Formation and other older sequences. It is composed of epiclastic deposits and some ashfall deposits emplaced in the MitlaTlacolula Valley and can attain a thickness of 200 m. Mammal fossils were identified in this formation by FerrusquíaVillafranca et al. (1974) suggesting early to middle Miocene ages. These paleontológica! ages are in disagreement with the KAr ages previously indicated. A Pliocene conglomerate covers some parts of the valley, and alluvial deposits and soil fill the region.
The Nejapa region
The Cenozoic volcanic and continental sedimentary sequences are broadly distributed in this region (Figure 3c) and in some areas reach a total thickness of almost 1,500 m. The stratigraphic characteristics of this region are summarized in Figure 4c. Some rocks from the basement were identified in discontinuous and rare outcrops southwest of the town of El Camarón (Figure 3c). Granulites containing pyroxene, plagioclase and biotite are the main rocks of these outcrops and could be associated with metamorphic sequences of the Oaxaca complex. FerrusquíaVillafranca (2001) described the El Limón Conglomerate Formation, with coarsegrained metamorphic and limestone fragments included in a finegrained matrix of silt and sand cemented by calcium carbonate. The estimated thickness of this formation is about 900 m and it overlies unconformably the metamorphic basement of the region. This formation has been observed as elongated hills in the central part of the region. These hills actually represent tilted blocks of conglomerates affected by NWSE fault systems. The age of this formation is not known but FerrusquíaVillafranca (2001) considered it as Paleogene because of the presence of Late Cretaceous lithic limestones in the conglomerate. Pyroxenehornblende andesitic lava flows with moderate hydrothermal alteration to clay minerals, chlorites and some sulfides were identified in some outcrops from the SW part of the region (Santa Maria Zoquitlán, Figure 3c). These altered volcanic rocks lie directly on the metamorphic basement in some areas, but in others they cover the El Limón Conglomerate Formation. Their ages are unknown but the stratigraphic position suggests a Paleogene age. A basalticandesite to andesite sequence, composed of some lava flows, cinder cone breccias, and hypabyssal bodies, has been observed in some outcrops overlying the hydrothermal altered volcanic rocks. Plagioclase, pyroxene and olivine phenocrysts included in a glassy matrix are the main components of these rocks. The total thickness of the sequence is not established but in some areas near the town of Yautepec, it can attain 15 m. Its age is probably early Miocene because it underlies in concordance the Yautepec Tuff Formation. An40 Ar/39 Ar age of 22.31 ± 0.03 (volcanic matrix) was determined by Mondo et al. (2004) for an andesite next to the Totolapan area (east of the region).
The most voluminous and widespread stratigraphic unit observed in the region is made up of silicic ignimbrites, several pyroclastic deposits and rhyoliticrhyodacitic domes. This sequence was denominated by FerrusquíaVillafranca (2001) as the Yautepec Tuff Formation and it can attain more than 1,000 m in thickness. These volcanic rocks and pyroclastic deposits conform most mountains and plateaus in the region, overlying in discordance the oldest formations (Figure 4c). The Yautepec Tuff is composed of at least three tabular bodies (FerrusquíaVillafranca, 2001) that display different morphological expressions. The first and oldest body is represented by limited exposures of moderately welded ignimbritic deposits with pumice and lithic fragments included in volcanic ash. The intermediate body has the greatest thickness in the area (600 m) and is composed of ignimbrites and other pyroclastic deposits displaying cross stratification. The main components are abundant pumice and lithic fragments, and broken quartz, feldspar andbiotite crystals in volcanic ash. These deposits form most plateaus and mountains in the area. Finally, the third body is lesser known and is composed of thin pyroclastic deposits overlying the ignimbrites. The composition of these bodies is predominantly rhyolitic with abundant subhedral to anhedral crystals of plagioclase (oligoclaseandesine), quartz, sanidine, biotite, and some pyroxenes, which are associated with pumice and lithic fragments in a silicic glassy matrix. The volcanic sequence has inclinations in several directions ranging from 15 to 40° as a result of at least two main fault systems with orientations of NWSE 65° and SWNE 25°. These structures have produced blocks tilted in different directions forming several horst and grabens. The sources of the volcanic deposits have not been identified yet because the block tilting has destroyed original volcanic structures. However, the textures and structures of the deposits suggest the idea that these sources were relatively nearby. The emplacement ages of the pyroclastic rocks were assigned to the middle Miocene by means of mammal fossils found in the lower body of the formation (FerrusquíaVillafranca 1990). KAr determinations indicate ages of 15.82 ± 0.70 to 16.47 ± 0.7 Ma for the basal units and 14.96 ± 0.85 Maforplagioclase andbiotite in an upper silicic tuff (FerrusquíaVillafranca, 2001). More recently, 40Ar/39Ar age determinations of 17.09 ± 0.06 (plagioclase) and 17.51 ± 0.05 (volcanic matrix) were obtained by Iriondo et al. (2004) for a porphyric andesite, next to the Totolapan area. In this work, a KAr age of 19.6 ± 0.5 Ma was obtained for a rhyolitic vitrophyre (whole rock) at the base of the volcanic unit (sample CON255A: 96°19'10.8"W, 16°38'13"N, K = 3.673%, 40Ar = 0.005015). These values confirm the early to middle Miocene ages of emplacement for the silicic and some andesitic rocks, but more isotopic ages are required to define the age range of the volcanic events in the region. In several outcrops, dikes and hypabyssal bodies of andesitic to dacitic composition intrude the Yautepec Tuff Formation. These intrusive bodies were also sampled and they display porphyritic to aphanitic textures with plagioclase and pyroxene phenocrysts included in a glassy matrix. Even though the age of emplacement of these magmatic bodies is unknown, their position with respect to the Yautepec Tuff and the upper units (the El Camarón Formation) might suggest a middle Miocene age.
The El Camarón Formation (FerrusquíaVillafranca, 2001), composed of epiclastic, fluvial and lacustrine deposits, covers in discordance the Yautepec Tuff in basins and valleys in the central portion of Nejapa region. The main deposits attain 200 m in thickness and are composed of volcaniclastic sandstones, although the grain size of the fragments varies from boulders to sand and silt, with predominance of quartz, feldspar, biotite and iron oxides, and minor volcanic lithics. The variable dips displayed by these deposits give evidence of the existence of regional fault systems. The abundant presence of terrestrial mammal fossils in the lower levels of this formation suggests a middle Miocene emplacement age (FerrusquíaVillafranca, 2001). Finally, Quaternary alluvial and soil deposits cover the older rocks in the valleys.
SAMPLE SELECTION AND ANALYTICAL METHODS
The Neogene volcanic sequence and basement rocks were sampled during several fieldwork campaigns in the Etla, MitlaTlacolula and Nejapa regions. In order to determine the petrographic and geochemical variations of the stratigraphic sequences, lava flows, ignimbrites, pyroclastic deposits and hypabyssal intrusions were sampled in each region. Thin sections of more than 45 sampled rocks were studied to determine the mineralogy and petrography, and fresh samples were selected for bulk chemical analyses and Sr and Nd isotopic determinations. Majorelements and Sc abundances of 12 representative samples were determined by inductively couple plasma emission, and all other trace elements, including the rare earth elements, by inductively coupled plasma mass spectrometry (ICPMS) in the analytical laboratories of the Centre de Recherches Pétrographiques et Géochimiques (CRPG), Centre National de Recherches Scientifiques (CNRS), in Nancy, France (SARM, 2004). The following errors are reported: <3% for major elements and <10% for trace elements. 87Sr/86Sr and 143Nd/144Nd isotopic ratios of 12 whole rock samples were measured using a Finnigan MAT262 thermal ion mass spectrometer at LUGIS (Laboratorio Universitario de Geoquímica Isotópica), UNAM. The spectrometer is equipped with a variable multicollector system (eight faraday cups) and all measurements were made in static mode. Sr and Nd samples were loaded as chlorides on double rhenium filaments and measured as metallic ions. Sixty isotopic ratios were determined for Sr and Nd on each sample. Elements were separated using standard ionexchange methods. Total procedure blanks during analysis of these samples were less than 10 ng Sr and 20 ng Nd.
A volcanic rhyolitic glass sample (CON255a) was prepared for KAr measurements and analyzed by Geochron Laboratory Division of Krueger Enterprises, Inc. Data results were indicated in a previous section (Nejapa region).
GEOCHEMICAL RESULTS Petrographic characteristics
Table 1 shows the modal mineralogical analyses and petrography of the majority of the samples. The first Cenozoic volcanic sequence in the study area shows a predominantly andesitic composition, although basaltic andesites have also been observed. Aphanitic to porphyritic textures are present, with phenocrysts (1mm) of plagioclase, orthopyroxene and clinopyroxene, hornblende with oxide reaction rims, and some olivine. The groundmass is composed of microlitic plagioclase, clinopyroxene, olivine in trace amounts, FeTi oxides and andesitic glass. Most andesitic rocks are strongly altered (20 to 55 vol. %) to clay minerals, chlorite and in some cases to sulfides.
The early to middle Miocene pyroclastic sequences in the three regions display a predominantly rhyoliticrhyodacitic composition with broken crystals (12 mm) of plagioclase (oligoclaseandesine), Kfeldspar, quartz and some biotite included in a glassy matrix (Table 1). However, ignimbritic deposits in the Nejapa region are more voluminous than in the MitlaTlacolula region, where crystalvitric tuffs with at least 50 to 60 vol. % of crystals predominate. The groundmass is composed of silicic glass (>27 vol. %) with a low to moderate degree of alteration to clay minerals, chlorite and quartz (from 1 to 10 vol. %). Quartz and feldspar are intergrown in spherulitic structures in these silicic rocks. Hypabyssal intrusions emplaced in the pyroclastic sequences display andesite to basaltic andesite composition, with porphyritic to microlitic textural variations. The degree of alteration to clay minerals of these intrusive rocks is relatively low (<10 vol. %).
Major and trace element geochemistry
Table 2 shows major and trace element concentrations determined in Neogene volcanic rocks for the three regions: Etla, MitlaTlacolula and Nejapa. Because no previous chemical data exist for these rocks, some comparisons have been made with chemical results from volcanic rocks of nearby areas in the Sierra Madre del Sur such as western Oaxaca (Martiny et al., 2000a) and NE Guerrero (MoránZenteno et al., 1998). Chemical classification of the rocks is given in the SiO2 vs. alkalis diagram in Figure 5a (Le Maitre et al., 1989). All volcanic rocks analyzed were classified on an anhydrous basis. The volcanic products of the three regions vary from basaltic andesites to rhyolites, but a bimodal pattern is observed for these rocks. A first group of rocks ranges from 53.17 to 57.82 wt. % SiO2 (basalticandesite to low SiO2 andesite), whereas the second group varies from 67.04 to 76.35 wt. % (silicic dacite to rhyolite). No SiO2 concentrations between 58 and 67 wt. % were observed in the samples from the study area. Although the small number of chemical analyses could bias this bimodal pattern, the petrographic descriptions of more than 45 samples indicate a predominance of basalticandesites, andesites and rhyolites with a remarkable absence of dacites. All volcanic rocks are subalkaline (Figure 5a) as defined by Irvine and Baragar (1971), lack iron enrichment, and display a typical calcalkaline trend in the AFM diagram of Figure 5b. This is equivalent to the low to mediumFe suite proposed by Arculus (2003). Chemical data of volcanic rocks from Martiny et al. (2000a) and MoránZenteno et al. (1998) display patterns similar to those of our data (Figure 5a) although without the bimodal distribution in SiO2. Oligocene volcanic rocks from the NE Guerrero area have rhyolitic, andesitic and dacitic compositions [from 57 to 76 wt. % SiO2 with a wide dispersion in alkalis (Na2O+K2O)] while no basalticandesites were observed. The volcanic rocks of western Oaxaca (Oligocene age) range in composition from basaltic andesites to dacites (53 to 67 wt. % SiO2), and rhyolitic tuffs were described at the base of the succession.
Negative correlations have been observed between SiO2 (wt. %) and TiO2, CaO and MgO (wt. %) in rock samples from the study area (Figure 6), although a certain amount of dispersion is present. Samples from NE Guerrero and western Oaxaca are also reported in various diagrams for comparison. Some samples from the Nejapa region (CON325, 328) and others from the western Oaxaca display relatively high (1.19,1.39 wt. %) TiO2 concentrations, which is not common in calcalkaline magmas. The K2O contents of the magmatic rocks in the study area are typical of calcalkaline series, with a positive correlation with respect to SiO2 (Figure 6), and with Nb contents smaller than 12 ppm.
A multielement diagram for trace elements is shown in Figure 7. Traceelement patterns for andesites and rhyolites are similar, with enrichment in largeion lithophile elements (LILE) relative to highfieldstrength elements (HFSE). This enrichment is typical of calcalkaline volcanic arcs. All rocks display negative Nb, Ta, P and Ti anomalies that are also characteristic of subductionrelated magmas (e.g.,Gill, 1981; Walker et al., 2001). In Figure 7, trace element patterns are variable for each sample because each analyzed sample represents a different volcanic unit in the study area, and thus a different magmatic process. The patterns of the immobile elements (Nb, Hf, Zr, Ti, Y and Yb) in the variation diagram and the enrichment in LILE suggest a depleted mantle source in the subcontinental lithosphere modified by subduction fluids, which have added the more mobile elements (Rb, Ba, K, and Pb) (Pearce, 1983; Wilson, 1989). The volcanic sequences in the study area have Ba/Nb ratios that range from 17 to 28 and La/Nb from 2.3 to 5. These ratios are similar to those found in volcanic rocks from western Oaxaca and are typical of calcalkaline lavas from other convergent plate boundaries (Gill, 1981).
The rare earth element (REE) abundances of the andesites and rhyolites from the study area show similar trends. Chondritenormalized REE patterns display light rare earth element enrichment (LREE: LaSm) and relatively flat patterns for the heavy rare earth elements (HREE: TbLu) (Figure 8). All rhyolitic samples and one andesite (CON335b) exhibit moderate to slight negative Eu anomalies indicating some degree of plagioclase fractionation, where the plagioclase probably crystallized first and was substantially removed from magmas before their ascent to the surface. This is consistent with Na2O and A12O3 concentrations that are almost constant or display a slight decrease with SiO2 contents (diagrams SiO2 vs. Na2O and A12O3 not shown). Andesites show slightly lower REE concentrations in comparison to rhyolites. This is commonly observed in continental volcanic arcs, in which abundances of incompatible trace elements (HFSE and REE) typically increase with SiO2. However, assimilation and crystallization processes (AFC; DePaolo, 1981) can not be discarded. Similar trace element patterns are observed for volcanic rocks from western Oaxaca (Martiny et al, 2000a) and NE Guerrero (MoránZenteno et al, 1998).
Isotopic analyses of Sr and Nd are given in Table 3 for several volcanic samples from the study area. Initial 87Sr/86Sr ratios of the Etla and MitlaTlacolula samples are relatively high (0.70472 0.70659) compared to those of the Nejapa region (0.70352 0.70482). Initial eNd values obtained for the volcanic rocks of the Etla and MitlaTlacolula regions range from 1.84 to +1.75 and in the Nejapa region from + 0.52 to +1.42 (Table 3). Figure 9 shows the initial SrNd isotopic ratios for rock samples from the study area and other volcanic regions of the Sierra Madre del Sur (SMS): western Oaxaca and northeastern Guerrero (Martiny et al., 2000a and MoránZenteno et al, 1998, respectively). Considering the isotopic heterogeneity of the crust in central and southeastern Oaxaca, the narrow ranges (without considering sample CON 326) and generally low 87Sr/86Sr, ratios and εNdi values near and mostly above that of bulk earth, suggest a relatively low degree of crustal contamination for volcanic rocks of the Nejapa region. In contrast, the initial Sr and Nd isotopic values for the Etla and Tlacolula sites are more variable and lower than those of bulk earth, possibly indicating more crustal contamination or a more evolved crustal component (see isotopic composition of Oaxaca complex in Figure 9). An example of extreme involvement of continental crust in magma genesis is the andesitic intrusion of Puente Negro, where the presence of abundant xenoliths from the Acatlan complex has strongly modified the original isotopic compositions (Figure 9) (Martiny et al., 2004). The majority of andesites and dacites from western Oaxaca display isotopic ratios similar to those of the Nejapa region (Figure 9). On the other hand, andesites and rhyolites from the Etla and MitlaTlacolula regions have higher initial 87Sr/86Sr and negative εNdi values, as was observed in volcanic samples from NE Guerrero. ForTaxco, Guerrero, MoránZenteno et al. (1998) indicated that initial 87Sr/86Sr ratios, in conjunction with chemical and mineralogical composition of volcanic rocks could be explained by moderate crustal contamination of magmas.
DISCUSSION AND CONCLUDING REMARKS
Regional stratigraphy and geochemical patterns
Cenozoic volcanic rock types found in the study area are, in order of decreasing abundance, rhyolites, basaltic andesites, dacites (SiO2 > 66 wt. %) and andesites (< 59 wt. % SiO2). Andesitedacite with SiO2 concentrations between 59 66 wt. % were not identified (Figure 5a). The first volcanic event in all these regions is represented by basaltic andesites and some andesitic lava flows that most of the time are highly altered and eroded. The age of emplacement of these rocks is notknown, butFerrusquíaVillafranca (1990) considered it as Paleogene. Overlying these sequences, thick rhyoliticrhyodacitic rocks observed in all regions are composed of ignimbrites, lava flows, epiclastic and pyroclastic deposits emplaced in different intermontane basins, associated with some andesitic lava flows. KAr dates for these sequences indicate that the main magmatic event occurred in the early middle Miocene (22 to 15 Ma). However, a migration pattern has not been fully determined, since rocks with similar ages were found in the three regions. Andesitic to basaltic andesite dikes cut the silicic sequences in MitlaTlacolula and Nejapa regions. Volcanic structures or vents were not observed to date, probably because of the presence of fault and fracture systems that dramatically cut these rocks. The emplacement of these andesitic to basaltic andesite dikes could represent the final magmatic events in the regions. In this work, they are considered of middle Miocene age on the basis of their stratigraphic position and lower degree of alteration.
The stratigraphic and certain petrographic features of the volcanic sequences in the study area show some differences in comparison to nearby regions in the Sierra Madre del Sur: western Oaxaca and northeastern Guerrero. In western Oaxaca (Huajuapan area), Martiny et al. (2000a) reported a thick volcanic pile composed principally of basaltic andesites to andesiticdacitic lava flows overlying minor silicic to intermediate volcaniclastic rocks. Some intermediate to silicic pyroclastic and epiclastic deposits were reported to the south in the Tlaxiaco area. Numerous andesiticdacitic hypabyssal intrusions are emplaced at different levels in the sequence. All volcanic rocks in westernmost Oaxaca rest on Paleozoic metamorphic rocks (Mixteca terrane; OrtegaGutiérrez, 1981), Mesozoic sedimentary units orPaleogene conglomerates. KAr ages for these sequences range from 29 to 34 Ma and some volcanic vents or central structures were identified at NE of Huajuapan (Martiny et al, 2000b). In northeastern Guerrero, the Cenozoic volcanic sequences are distributed in two main areas, Taxco and Tilzapotla. In both areas predominate thick sequences of rhyolites and minor andesites and dacites, which are represented by pyroclastic deposits, lava flows and hypabyssal dikes. The most important structure in this area is represented by a major resurgent caldera in the Tilzapotla area (MoránZenteno et al, 2004). All volcanic sequences display ages from ca. 38 to 32 Ma and are mainly deposited over Cretaceous marine rocks.
The Y and Yb concentrations for some rocks of the study area are relatively low (Y < 23 ppm and Yb <1.8 ppm whereas the SR concentrations are mederately high (328 to 591 ppm (Table 2 and Figure 10), with HREE depletions, suggestion that a possible slabmelt component has participated in the subduction process. Relatively high Sr/Y ratios and low Y contents have also been observed in some Oligocene volcanic rocks from western Oaxaca (Figure 10) ( Mariny, personal communication), and central southeastern Oaxaca indicating the existence of a geochemical adakitic signature. Positive anomalies of Ba and Pb, together with the high values of some element ratios (e.g., Ba/Zr and K/Nb) shown by certain volcanic rocks from the study area confirms that fluids and/or melts from the subducted slab contributed to magma genesis. This feature has not only been observed in volcanic arcs where young and hot slabs are being subducted (Defant and Drummond, 1990; Martin, 1999), but also when a flat subduction phenomenon produces the temperature and pressure conditions necessary for the fusion of a cold oceanic crust (Gutscher et al., 2000). Although more integrated investigations are required about geochemistry and tectonic patterns in the southeastern SMS, on the basis of the geochemical data it is proposed that melts of the subducted oceanic crust could had contributed to the magmas erupted in the Etla, TlacolulaMitla and Nejapa regions. This is not a rare feature in the Miocene to Quaternary volcanism linked to the subduction processes in southern Mexico, as has been recently reported for several volcanic sites of the TransMexican Volcanic Belt that show geochemical adakitic signatures (MartinezSerrano et al., 2004, GómezTuena et al., 2005, RincónHerrera et al., 2007), associated to flatslab subduction of the oceanic plate under the continental crust.
The initial 87Sr/86Sr and epsilonNd values for volcanic rocks in western Oaxaca (0.7042 to 0.7046 and 0 to +2.6) suggest a relatively low degree of old crust involvement (Martiny et al., 2000a). Whereas in NE Guerrero, initial isotopic compositions were explained by a greater crustal contribution to rhyolitic rocks from the Taxco region (0.7063 to 0.7075) (MoránZenteno et al, 1998), in the Tilzapotla caldera, the initial isotopic variations (87Sr/86Sri: 0.7034 to 0.7066; epsilonNdi: 1.07 to +5.41) are related to input of more primitive magmas into the magma chamber and the contamination of magmas with Cretaceous carbonates (MoránZenteno et al, 2004). In the study area, although most samples represent volcanic products derived from different events and structures, in general, the major and trace elements display coherent chemical patterns (low dispersion) suggesting crystal fractionation as the main magmatic process. However, certain differences in the isotopic and geochemical behavior are observed between the Etla, MitlaTlacolula and Nejapa regions. In Figure 11, initial epsilonNd isotopic compositions seem to change with increasing SiO2 contents for the Etla, MitlaTlacolula regions, whereas the isotopic compositions of samples from the Nejapa region are almost constant with respect to SiO2. The isotopic variations in the first regions could be due to a major interaction of magmas with an old continental crust, in addition to crystal fractionation processes. For the Nejapa region, the existence of more evolved rocks is probably associated with crystal fractionation processes with minor interaction between magmas and continental crust. In central and southeastern Oaxaca, most Cenozoic volcanic rocks are emplaced in the Oaxaca terrane (Grenvillian age), where presentday isotopic compositions show large variations (87Sr/86Sri = 0.705 0.717 and εNdi = 12 9; Patchett and Ruiz, 1987; Ruiz et al, 1988a, 1988b). In spite of these radiogenic isotopic compositions, the volcanic products in the study area do not evidence a large interaction of parental magmas with crustal rocks, especially for volcanic rocks from the Nejapa region. It is not the case for the Miocene andesitic dike from Puente Negro, Puebla, where the assimilation of an old continental crust produced dramatic changes in the isotopic compositions (Martiny et al, 2004). Although at present we do not have sufficient structural evidence, the minor interaction of magmas with older continental crust could be explained by the presence of large NW SE fault systems (FerrusquíaVillafranca, 2001), through which the magmas could ascend more easily without a significant degree of interaction with the crust.
Spacetime magmatic evolution in the SMS
Taking into account the geochronological database for the SMS magmatic rocks and the initial volcanic events of the TransMexican Volcanic Belt (data from Schaaf et al., 1995; MoránZenteno et al, 1999; 2005; Martiny et al, 2004; GómezTuena et al, 2005, and references therein, RincónHerrera, 2007), it is observed that arcmagmatism was active fromPaleocene to early middle Miocene. Figure 12 schematically displays the distribution of the magmatic arc rocks at different times, conforming NWSE belts with an orientation similar to that of the volcanic sequences of the Sierra Madre Occidental. The location of the radiometric ages indicates a migration of arcmagmatism from west to east (Eocene in Michoacán to earlymiddle Miocene in the southeastern Oaxaca study area). This NW SE migration could explain the age variations observed in the two main magmatic belts present in the SMS. One interesting point observed in the distribution of age data is the presence of early to middle Miocene (23.7 to 15.48 Ma) volcanic events in the central and southeastern parts of the state of Oaxaca (study area), and in the central and eastern parts of the TransMexican Volcanic Belt (TMVB) (Figure 12d). In the TMVB, these ages represent the first volcanic events in this province (ages reported by GómezTuena et al, 2005 and reference therein). The TMVB is considered to be a continental magmatic arc that transects central Mexico with an almost E W orientation, from the Pacific Ocean to the Gulf of Mexico (Figure 1). Although no continuous volcanic outcrops are registered between these two magmatic provinces (SMS and TMVB), in this work is proposed that a continuous early Miocene magmatic arc, with a relatively anomalous orientation, could have existed before the presentday position on the TMVB was attained. Today, several stratigraphic and geochemical studies have provided important evidence of the existence of this magmatic arc; for example, the andesitic subvolcanic body of Puente Negro, Puebla, with an apparent age of 22 Ma (Martiny et al., 2004); the Chalcatzingo rhyolitic domes with an age of 20.7 Ma and adakitic signature (RincónHerrera et al., 2007), and a silicic tuff in the valley of Tehuacán, Puebla where a KAr determination yielded an age of 16.4 ± 0.5 Ma in biotite (DávalosÁlvarez et al, 2007). The change from SMS magmatism to the E W trending volcanism of the TMVB in Miocene time probably reflects the tectonic evolution of southern Mexico during several episodes of plate tectonic rearrangement. At present, different tectonic models try to explain the Cenozoic magmatic migration in the Sierra Madre del Sur and its transition to the TransMexican Volcanic Belt. These models include the existence of the Chorus block and its displacement along the Pacific margin (Ross and Scotese, 1988; Pindell et al, 1988; Ratschbacher et al, 1991; Herman et al, 1994; Schaaf et al, 1995; MoránZenteno et al, 1999) or the existence of subduction erosion (MoránZenteno et al., 2007) and a Miocene collision of the Tehuantepec ridge with the Acapulco trench (Keppie and MoránZenteno, 2005). If an adakitic geochemical signature is demonstrated for some earlymiddle Miocene igneous rocks in southern Puebla (Chalcatzingo domes, RincónHerrera et al., 2007) and centralsoutheastern Oaxaca (this work), then a flatslab subduction process could have produced these geochemical characteristics. Some recent geodynamic models of subduction systems in southern Mexico (Manea and Manea, 2007) proposed the following evolution of the subducting slab: between 25 and 17 Ma the volcanic arc formed an approximately continuous belt in central and southeast Mexico, then the volcanic arc migrated to the north, suggesting that the subducting slab became subhorizontal at this time. These authors also proposed, in their geodynamic model, that the Chorus block migration along the MiddleAmericantrench might have created the conditions for cooling the mantle wedge and ceasing the existence of the Miocene volcanic arc in southeastern Mexico. However, future tectonic and geochemical studies in southern Mexico can contribute to the understanding of the magmatic evolution during the Miocene.
Financial support by the Consejo Nacional de Ciencia y Tecnologia (CONACYT) (project 3361 T 9303) and an internal project from the Instituto de Geofísica, UNAM are gratefully acknowledged. The authors wish to thank to Juan J. Morales Conteras and Maria del Sol Hernández Bernal for assistance with the analytical aspects of the isotopic determinations; Teodoro Hernández Treviño for assistance in the field and mechanical preparation of samples, and Barbara Martiny for revision of the English. Editorial handling by Angel Nieto and reviews by Gilberto Silva and one anonymous reviewer were very helpful and greatly appreciated.
AlanizÁlvarez, S.A., Van der Heyden, P., NietoSamaniego, A.F., OrtegaGutiérrez, F., 1996, Radiometric and kinematic evidence for Middle Jurassic strikeslip faulting in southern Mexico related to the opening of the Gulf of Mexico: Geology, 24, 443446. [ Links ]
AlanizÁlvarez, S.A., NietoSamaniego, A.F., MoránZenteno, D.J., AlbaAldave, L., 2002, Rhyolitic volcanism in extension zone associated with strikeslip tectonics in the Taxco region, southern Mexico: Journal of Volcanology and Geothermal Research, 118, 114. [ Links ]
Arculus, R. J., 2003, Use and abuse of the terms calcalkaline and calcalkalic: Journal of Petrology, 44, 929935. [ Links ]
Barboza, R., 1994, Regionalgeologische Erkundungen entlang der GEOLIMEXTraverse in Südmexiko, unter besonderer Berücksichtigung der Sierra de Juárez, Oaxaca: Universitat Clausthal, Germany, Unpublished Ph. D. Thesis, 105 p. [ Links ]
Bellon, H., Maury, R.C., Stephan, J.F., 1982, Dioritic basement, site 493: Petrology, geochemistry, and geodynamics, in Watkins, J.S., Moore, J.C et al, Initial Reptorts of the Deep Sea Drilling Project, v. 66: Washington, United States Government Printing Office, 723730. [ Links ]
Campa, M.F., Coney, P.J., 1983, Tectonostratigraphic terranes and mineral resource distributions in Mexico: Canadian Journal of Earth Sciences, 20, 10401051. [ Links ]
Carfantan, J.C, 1986, Du systeme cordillerain Nordaméricain au domaine Caraibe: Université de Savoie, France, Unpublished Ph. D. Thesis, 558 p. [ Links ]
DávalosÁlvarez, O.G., NietoSamaniego, A.F., AlanizÁlvarez, S.A., MartínezHernández, E., RamírezArriaga, E., 2007, Estratigrafía cenozoica de la región de Tehuacán y su relación con el sector norte de la falla de Oaxaca: Revista Mexicana de Ciencias Geológicas, 24, 2, 197215. [ Links ]
Defant, M.J., Drummond, M.S., 1990, Derivation of some modern arc magmas by melting of young subducted lithosphere: Nature, 347, 662665. [ Links ]
DePaolo, D.J., 1981, Trace element and isotopic effects of combined wallrock assimilation and fractional crystallization: Earth and Planetary Science Letters, 53, 189202. [ Links ]
Drummond, M.S., Defant, M.J., 1990, A model for trondhjemitetonalitedacite genesis and crustal growth via slab melting: Archean to modern comparisons: Journal of Geophysical Research, 95, 21, 50321, 521. [ Links ]
FerrusquíaVillafranca, I., 1990, Contribución a la diferenciación del Terciario continental de México, Parte 3, Geología Cenozoica del área Suchilquitongo, Estado de Oaxaca, México: Universidad Nacional Autónoma de México, Instituto de Geología, Unpublished report, 91135. [ Links ]
FerrusquíaVillafranca, I., 1992, Contribución al conocimiento del Cenozoico en el sureste de México y de su relevancia en el entendimiento de la evolución regional, in Actas de las sesiones científicas del III Congreso Geológico de España y VIII Congreso Latinoamericano de Geología: Salamanca, España, Universidad de Salamanca, Facultad de Ciencias, 4, 4044. [ Links ]
FerrusquíaVillafranca, I., 2001, Contribución al conocimiento geológico del Estado de Oaxaca, México, El área de Nejapa de Madero: Universidad Nacional Autónoma de México, Boletín del Instituto de Geología, 111, 100 p. [ Links ]
FerrusquíaVillafranca, I., McDowell, F.W., 1991, The Cenozoic sequence of selected areas in southeastern Mexicoits bearing in understanding regional basin development there, in Memorias de la Convención sobre la Evolución Geológica de México: Universidad Nacional Autónoma de México, Instituto de Geología, 4550. [ Links ]
FerrusquíaVillafranca, I., Wilson, J.A., Denison, R.E., McDowell, F.W., SolorioMunguía, J., 1974, Tres edades radiométricas oligocénicas y miocénicas de rocas volcánicas de las regiones de la Mixteca Alta y Valle de Oaxaca, Estado de Oaxaca: Boletín de la Asociación Mexicana de Geólogos Petroleros, 26, 249262. [ Links ]
FloresMárquez, E.L., Chávez, R.E., MartínezSerrano, R.G., HerreraBarrientos, J., TejeroAndrade, A., Belmonte, S., 2001, Geophysical characterization of the Etla Valley Aquifer, Oaxaca, Mexico: Geofísica Internacional, 40, 245257. [ Links ]
Gill, J.B., 1981, Orogenic Andesites and Plate Tectonics: Berlin, Springer, 390 p. [ Links ]
GómezTuena, A., OrozcoEsquivel, M.T., Ferrari, L., 2005, Petrogénesis ígnea de la Faja Volcánica Transmexicana: Boletín de la Sociedad Geológica Mexicana, Volumen Conmemorativo del Centenario, Temas Selectos de la Geología Mexicana, 57(3), 227285. [ Links ]
Gutscher, M. A., Maury, R, Eissen, IP, Bourdon, E., 2000, Can slab melting be caused by flat subduction?: Geology. 28, 535538. [ Links ]
Herrmann, U.R., Nelson, B.K., Ratschbacher, L., 1994, The origin of a terrane: U/Pb zircon geochronology and tectonic evolution of the Xolapa complex (southern Mexico): Tectonics, 13, 455474. [ Links ]
Iriondo, A., Kunk, M. J., Winick, J. A., CRM, 2004, 40Ar/39Ar dating studies of minerals and rocks in various areas in Mexico: USGS/CRM Scientific Collaboration (Part II): United States Geological Survey, OpenFile Report 041444, 46 p. [ Links ]
Irvine, T.N., Baragar, W.R.A., 1971, A guide to the chemical classification of the common volcanic rocks: Canadian Journal of Earth Sciences, 8, 523548. [ Links ]
Keppie, J.D., MoránZenteno, D.J., 2005, Tectonic implications of alternative Cenozoic reconstructions for southern Mexico and the Chortis block: International Geology Review, 47, 473491. [ Links ]
Keppie, D.D., MoránZenteno, D.J., Martiny, B.M., GonzálezTorres, E., 2007, Estimates of Cenozoic forearc subduction off southwestern Mexico: Constraints on EoceneMiocene reconstructions (abstract), in American Geophysical Union Joint Assembly, Acapulco, Mexico, Abstract U53A02. [ Links ]
Le Maitre, R.W., Bateman, P., Dudek, A., Keller, J., Lameyre, J., Le Bas, M.J., Sabine, PA., Schmid, R., Sorensen, H., Streckeisen, A., Woolley, A.R., Zanettin, B., 1989, A Classification of Igneous Rocks and Glossary of Terms: Oxford, Blackwell, 193 p. [ Links ]
LópezTicha, D., 1985, Revisión de la estratigrafía y potencial petrolero de la cuenca de Tlaxiaco: Boletín de la Asociación Mexicana de Geólogos Petroleros, 37, 4992. [ Links ]
Mailfait. B.T., Dinkelman, M.G., 1972, CircumCaribbean tectonic igneous activity and the evolution of the Caribbean plate: Geological Society of American Bulletin, 83, 252272. [ Links ]
Manea, V.C., Manea, M., 2007, Geodynamic modeling of subduction system in southern Mexico (abstract), in La conexión ChortisSur de México en el tiempo y el espacio, Symposium abstracts: Juriquilla, Querétaro, México, Universidad Nacional Autónoma de México, Abstract GC200710P. [ Links ]
Martin, H., 1999, Adakitic magmas: modern analogues of Archaean granitoids: Lithos. 46, 411429. [ Links ]
MartínezSerrano, R.G., MoránZenteno, D.J., Martiny, B., Macías, R.C., 1997, Geochemistry and geochronology of the Tertiary volcanic province of southern Mexico (abstract), in 9th Biennial Meeting of the European Union of Geosciences, EUG9, Strasbourg, France: Terra Abstracts, Abstract supplement, 1, Terra Nova, 9, p. 475. [ Links ]
MartínezSerrano, R.G., JiménezM. J.L., Macías R.C., Martiny, B., FloresM., E.L., 1999, Magmatic activity in central and southeastern Oaxaca, Mexico and its relation with the Tertiary plate tectonic rearrangement (abstract), in 10th Meeting of the European Union of Geosciences, EUG10, Strasbourg, France: Journal of Conference Abstracts, 4(1), 433. [ Links ]
MartínezSerrano, R. G., Schaaf, R, SolísPichardo, G., HernadezBernal. M., HernándezTreviño, T., MoralesContreras J.J., Macías J.L., 2004, Sr, Nd and Pb isotope and geochemical data from the Quaternary volcamsm of Nevado de Toluca, a source of recent adakitic magmatism, and the Tenango Volcanic Field, Mexico: Journal of Volcanology and Geothermal Research, 138, 77110. [ Links ]
Martiny, B., MartínezSerrano, R.G., MoránZenteno, D.J., MaciasRomo, C, Ayuso, R.A., 2000a, Stratigraphy, geochemistry and tectonic significance of the Oligocene magmatic rocks of Western Oaxaca, Southern Mexico: Tectonophysics, 318, 7198. [ Links ]
Martiny, B., SilvaRomo, G., MoránZenteno, D.J., 2000b, Estructuras oligocénicas del área de Huajuapan de León, Oaxaca (abstract), in 2a Reunión Nacional de Ciencias de la Tierra, Resúmenes y Programa: GEOS, 20(3), p. 327. [ Links ]
Martiny, B., OrtegaGutiérrez, F., MoránZenteno, D.J., Solé Viñas, J., 2004, Sr, Nd and Pb composition of the xenoliths in the Puente Negro intrusion, Puebla, Mexico, and relevance to the petrogenesis of the Tertiary volcanic rocks in western Oaxaca (abstract), in IV Reunión Nacional de Ciencias de la Tierra, Libro de resúmenes, Juriquilla, Querétaro: Sociedad Geológica Mexicana, p. 52. [ Links ]
Meschede, M., Frisch, W., Herrmann, U., Ratschbacher, L., 1997, Stress transmission across an active plate boundary: An example of southern Mexico: Tectonophysics, 266, 81100. [ Links ]
MoránZenteno, D.J., AlbaAldave, L.A., MartínezSerrano, R.G., ReyesSalas, M.A., CoronaEsquivel, R., AngelesGarcía S., 1998, Stratigraphy, geochemistry and tectonic significance of the Tertiary volcanic sequences of the TaxcoQuetzalapa region, southern Mexico: Revista Mexicana de Ciencias Geológicas, 15(2), 167180. [ Links ]
MoránZenteno, D.J., Toisón, G., MartínezSerrano, R.G., Martiny, B., Schaaf, P, SilvaRomo, G., MacíasRomo, C, AlbaAldave, L., HernándezBernal, M.S., SolísPichardo, G.N., 1999, Tertiary arcmagmatism of the Sierra Madre del Sur, Mexico, and its transition to the volcanic activity of the TransMexican Volcanic Belt: Journal of South American Earth Sciences, 12, 513535. [ Links ]
MoránZenteno, D.J., AlbaAldave, L.A., Sole, J., Iriondo, A., 2004, A major resurgent caldera in southern Mexico: the source of the late Eocene Tilzapotla ignimbrite: Journal of Volcanology and Geothermal Research, 136, 97119. [ Links ]
MoránZenteno, D.J., Cerca, M., Keppie, J.D., 2005, La evolución tectónica y magmática cenozoica del Suroeste de México: avances y problemas de interpretación: Boletín de la Sociedad Geológica Mexicana, Volumen Conmemorativo del Centenario, Temas Selectos de la Geología Mexicana, 57(3), 319341. [ Links ]
MoránZenteno, D. J., AlbaAldave, L., GonzalezTorres, E. A., Martiny, B., BravoDiaz, B.A., Sohn, E., 2007, Ignimbrite flareup in the northcentral Sierra Madre del Sur, Southern Mexico: a continuation of the Sierra Madre Occidental ignimbrite Province? (abstract), in American Geophysical Union Joint Assembly, Acapulco, Mexico, Abstract U53A03. [ Links ]
Nakamura, N., 1974, Determinations of REE, Ba, Fe, Mg, Na and K in carbonaceous and ordinary chondrites: Geochimica et Cosmochimica Acta, 38, 757775. [ Links ]
NietoSamaniego., A.F., AlanízÁlvarez, S.A., SilvaRomo, G., EguizaCastro, M.H., MendozaRosales, C.C., 2006, Latest Cretaceous to Miocene deformation events in the eastern Sierra Madre del Sur, Mexico, inferred from the geometry and age of major structures: Geological Society of American Bulletin, 118(1/2), 238252. [ Links ]
OrtegaGutiérrez, F, 1981, Metamorphic belts of southern Mexico and their tectonic significance: Geofísica Internacional, 20, 177202. [ Links ]
OrtegaGutiérrez, F., MitreSalazar, L.M., RoldánQuintana, J., SánchezRubio, G., De la Fuente, M., 1990, H3 Acapulco Trench to the Gulf of Mexico across Southern Mexico, scale 1:500,000: Boulder, CO, Geological Society of America, Centennial continent/ocean transect #13, 1 map with text. [ Links ]
PantojaAlor, J., 1970, Rocas sedimentarias paleozoicas de la región centroseptentrional de Oaxaca, in Segura, L.R., RodríguezTorres, R. (eds.), LibroGuía de la Excursión MéxicoOaxaca: México, D.F., Sociedad Geológica Mexicana, 6784. [ Links ]
Patchett, P. J., Ruiz, J., 1987, Nd isotopic ages of crust formation and metamorphism in the Precambriam of eastern and southern Mexico: Contribution to Mineralogy and Petrology, 96, 523528. [ Links ]
Pearce, J.A., 1983, Role of the subcontinental lithosphere in magma genesis at active continental margins, in Hawkesworth, C.J., Norry, M.J. (eds.), Continental Basalts and Mantle Xenoliths: Nantwich, Shiva, 230249. [ Links ]
Pindell III, J.L., Cande, S.C., Pitman, W.C., Rowley, D.B., Dewey, J.F., Labrecque, J., Haxby, W., 1988, Aplate kinematic framework for models of Caribbean evolution: Tectonophysics, 155, 121138. [ Links ]
Ratschbacher, L., Riller, U., Meschede, M., Herrman, U., Frisch, W., 1991, Second look at suspect terranes in southern Mexico: Geology, 19, 12331236. [ Links ]
RincónHerrera, N., GómezTuena, A., Mori, L., OrtegaGutiérrez, F., 2007, Rhyolite genesis by slab melting and mantle interaction: an example from the TransMexican Volcanic Belt (abstract), in American Geophysical Union Joint Assembly, Acapulco, Mexico, Abstract V53B03. [ Links ]
Ross, M.I., Scotese, C.R., 1988, Ahierarchical tectonic model of the Gulf of Mexico and Caribbean region: Tectonophysics, 155, 139168. [ Links ]
Ruiz, J., Patchett, P.J., OrtegaGutiérrez, F., 1988a, Proterozoic and Phanerozoic basement terranes of Mexico from Nd isotopic studies: Geological Society of American Bulletin, 100, 274281. [ Links ]
Ruiz, J., Patchett, P.J., Arculus, R.J., 1988b, NdSr isotope composition of lower crustal xenoliths Evidence for the origin of midTertiary felsic volcanic in Mexico: Contributions to Mineralogy and Petrology, 99, 3643. [ Links ]
Service d'Analyse des Roches et des Minéraux (SARM), 2004, Service d'analyse de roches et minéraux du CNRS: VandoeuvrelèsNancy Cedex, France, Centre National de la Recherche Scientifique <http://www.crpg.cnrsnancy.fr/SARM/index.html>. [ Links ]
Schaaf, P., MoránZenteno, D., HernándezBernal, M.S., SolísPichardo, G., Toisón, G., Kohler, H., 1995, Paleogene continental margin truncation in southwestern Mexico: Geochronological evidence: Tectonics, 14, 13391350. [ Links ]
Sedlock, R.L., OrtegaGutiérrez, F, Speed, R.C., 1993, Tectonostratigraphic terranes and tectonic evolution of Mexico: Geological Society of America, Special paper, 278, 153p. [ Links ]
Schlaepfer, C. J., 1970, Geología terciaria del área de YanhuitlánNochixtlán, Oaxaca, en in Segura, L.R., RodríguezTorres, R. (eds.), LibroGuía de la Excursión MéxicoOaxaca: México, D.F, Sociedad Geológica Mexicana, 8596. [ Links ]
Solari, L.A., Keppie, J.D., OrtegaGutiérrez, F., Cameron, K.L., Lopez, R., Hames, W.E., 2003, 900 1100 Ma Grenvillian tectonothermal events in the northern Oaxaca complex, southern Mexico: Roots of an orogen: Tectonophysics, 365, 257282. [ Links ]
Sun, S., McDonough, W., 1989, Chemical and isotopic systematics of oceanic basalts: Implications for mantle compositions and processes, in Saunders, A., Norry, M. (eds.), Magmatism in the Ocean Basins: Geological Society of London, Special Publication, 313345. [ Links ]
UrrutiaFucugauchi, J., FerrusquíaVillafranca, I., 2001, Paleomagnetic results for the Middle Miocene continental Suchilquitongo Formation, valley of Oaxaca, Southeastern Mexico: Geofísica Internacional, 40(3), 191205. [ Links ]
Walker, J.A., Patino, L.C., Carr, M.J., Feigenson, M.D., 2001, Slab control over HFSE depletions in central Nicaragua: Earth and Planetary Science Letters, 192, 533543. [ Links ]
Wilson, M., 1989, Igneous Petrogenesis: a Global Tectonic Approach: London, Unwin Hyman, 446p. [ Links ]
Wilson, J.A., Clabaugh, S.E., 1970, A new Miocene formation and descriptions of volcanic rocks in northern valley of Oaxaca, State of Oaxaca. México, in Segura, L.R., RodríguezTorres, R. (eds.), LibroGuía de la excursión MéxicoOaxaca: México, D.F., Sociedad Geológica Mexicana, 120128. [ Links ]