Geoquími artícula Moran

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Thermomechanical maturation of the continental crustand its effects on the late Eoceneearly Oligocenevolcanic record of the Sierra Madre del Sur Province,southern MexicoLaura Mori a , Dante J. Morn-Zenteno a , Barbara M. Martiny a , Enrique A. Gonzlez-Torresa , Mara Chapela-Lara a , Beatriz A. Daz-Bravo a & Julie Roberge aa Instituto de Geologa, Universidad Nacional Autnoma de Mxico, 04510, Mexico City,MexicoVersion of record first published: 22 Dec 2011.

To cite this article: Laura Mori , Dante J. Morn-Zenteno , Barbara M. Martiny , Enrique A. Gonzlez-Torres , Mara Chapela-Lara , Beatriz A. Daz-Bravo & Julie Roberge (2012): Thermomechanical maturation of the continental crust and its effects onthe late Eoceneearly Oligocene volcanic record of the Sierra Madre del Sur Province, southern Mexico, International GeologyReview, 54:13, 1475-1496

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International Geology ReviewVol. 54, No. 13, 10 October 2012, 14751496

Thermomechanical maturation of the continental crust and its effects on the late EoceneearlyOligocene volcanic record of the Sierra Madre del Sur Province, southern Mexico

Laura Mori*, Dante J. Morn-Zenteno, Barbara M. Martiny, Enrique A. Gonzlez-Torres, Mara Chapela-Lara,Beatriz A. Daz-Bravo and Julie Roberge

Instituto de Geologa, Universidad Nacional Autnoma de Mxico, 04510, Mexico City, Mexico

(Accepted 21 November 2011)

We interpret the voluminous late Eoceneearly Oligocene volcanic successions of the north-central Sierra Madre del Sur asthe eruptive manifestation of a progressive thermomechanical maturation of the crust, driven by sustained igneous activitythat affected the region since the early Eocene. Widespread Eocene magmatism and injection of mantle-derived melts intothe crust beneath the Michoacn-Puebla area promoted the development of a hot zone extending to upper crustal levels, andthe formation of a mature intracrustal magmatic system. Within this context, the intermediate siliceous compositions of theTilzapotla, Mueca, and Goleta explosive centres were generated through fractional crystallization, crustal contamination,and anatexis. In particular, decreasing bulk-rock Sr and Eu concentrations and Nd isotopes with increasing silica in theTilzapotla and Mueca suites document an evolution through low-pressure fractional crystallization of plagioclase-domi-nated assemblages, simultaneous with the assimilation of middleupper crustal materials. In contrast, marked Eu, Sr, andBa depletions coupled with high and variable Rb/Nd at constant 143Nd/144Nd in the Goleta rhyolites suggest their derivationfrom partial melting of biotite-bearing quartz-feldspathic lithologies. Ascent of the thermal anomaly induced by magmaemplacement and accumulation at shallow depths shifted the brittleductile crustal transition close to the surface, and pro-duced an ignimbrite are-up through caldera-forming eruptions. A different petrogeneticvolcanologic scenario developed innorth-western Oaxaca, where less profuse earlymiddle Eocene igneous activity and an ancient lower crustal basement madeup of refractory granulitic lithologies inhibited the expansion of the hot zone to shallow levels, and constrained magmaticevolution at depth. Here, composite and monogenetic volcanoes with intermediate compositions were produced throughhigh-pressure fractional crystallization and crustal contamination. Specically, increasing La/Yb and Sm/Yb with increas-ing silica in the Oaxaca suite, and negative correlations of Nd isotopes with SiO2 at low Rb/Nd, suggest garnet fractionationfrom parental basalts, coupled with the assimilation of Rb-depleted lower crustal materials.

Keywords: southern Mexico; Sierra Madre del Sur; mantle; thermomechanical maturation of the continental crust; silicicare-up

Introduction

Identifying the origin of the intermediate-silicic productsthat dominate the plutonic and volcanic record of con-tinental arcs worldwide is essential to comprehend themechanisms of magma generation and evolution at con-vergent margins, and has important implications for under-standing how andesitic-dacitic continents grow (Brown andRushmer 2006).

A major proportion of evolved igneous rocks emplacedat continental margins display bulk-rock geochemical char-acteristics that are consistent with a derivation fromintracrustal processing of basaltic precursors (Annenet al. 2006a). Indeed, during ascent, mantle-derivedmagmas can undergo fractional crystallization at vari-able crustal depths, contamination with different base-ment lithologies, and might also induce crustal melting,

*Corresponding author. Email: [email protected]

promoting the formation of a variety of residual and anate-ctic liquids (Annen et al. 2006a and references therein).

Recent works consider that intracrustal processing ofmac magmas in order to form evolved compositionswithin subduction settings mainly takes place in deep,hot crustal zones, which develop in response to the con-stant injection of mantle-derived basalts into the lowercrust (Annen et al. 2006a, 2006b). Starting from thisidea, other studies propose that the propagation of a hotthermal anomaly from the deep crust to upper crustal lev-els might be responsible for the generation of massiveevents of silicic volcanism and ignimbrite are-ups (deSilva et al. 2006; Bachmann et al. 2007). This modelenvisages the development of an extensive mantle-source-driven crustal magmatic system that advects heat throughthe entire crustal section. Abundant intrusion of mac

ISSN 0020-6814 print/ISSN 1938-2839 online 2012 Taylor & Francishttp://dx.doi.org/10.1080/00206814.2011.644745http://www.tandfonline.com

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arc magmas into the lower crust would develop a deep,hot zone, in which intermediate compositions are pro-duced through incomplete crystallization of mantle-derivedbasalts, crustal contamination, and anatexis (Annen et al.2006a). As the crustal magmatic system evolves, fuelledby continuous heat and mass input from the mantle, prop-agation of the thermal anomaly to shallower levels wouldresult in progressive thermal softening of crustal rocks,thus inhibiting the ascent of arc magmas, and insteadfavouring the accumulation of progressively larger magmabatches in the middleupper crust (de Silva et al. 2006;de Silva and Gosnold 2007). At these levels, magmaswould interact with more felsic and radiogenic lithologies,and undergo further crystallizationassimilation processes,forming more evolved compositions. Magma emplace-ment into the upper crust would elevate the brittleductile transition to very shallow levels and trigger fault-controlled caldera-forming eruptions, thus producing mas-sive ignimbrite outbursts as the climatic expressions ofthe progressive thermomechanical maturation of the con-tinental crust (de Silva et al. 2006; Bachmann et al.2007).

In this contribution, we present a geochemical studyof the Late EoceneEarly Oligocene volcanic sequencesemplaced in the north-central sector of the Sierra Madredel Sur (SMS) of southern Mexico (Figure 1). Magmaticactivity in this region produced two volcanic districtswith contrasting geochemical and volcanological char-acteristics: a siliceous are-up province representedin this study by the Mueca, Goleta, and Tilzapotla

explosive centres that emitted voluminous ignimbriteunits and intermediate-silicic lava ows; and the Oaxacavolcanic eld, made of composite and monogeneticvolcanoes with dominant mac-intermediate composi-tions. Our geochemical data and geological-volcanologicalobservations reveal two contrasting petrogenetic scenariosfor these coeval magmatic episodes, which appear to berelated to different stages of thermomechanical matura-tion of the continental crust beneath the two areas. Thesevolcanic successions therefore offer an excellent opportu-nity to examine how variable heat and mass inputs fromthe mantle to the upper plate, as well as differences inbasement lithology, affect the intracrustal processes thatproduce evolved compositions in a region of long-livedcontinental magmatism.

Geologic framework

Subduction-related Cenozoic magmatic activity alongthe Mexican convergent margin produced two extensiveprovinces of silicic volcanism, represented by the SierraMadre Occidental and the SMS (Figure 1A; Ferrari et al.2007; Morn-Zenteno et al. 2007). These continental arcsare characterized by similar eruptive styles and magmaticcompositions, but show some differences in terms ofspacetime distribution of magmatism (Ferrari et al. 2007;Morn-Zenteno et al. 2007) and tectonic regime active atthe time of magmatic activity (e.g. Nieto-Samaniego et al.1999; Alaniz-lvarez et al. 2002).

Figure 1. (A) Sketch map of the Mexican Pacic margin, showing the distribution of the subduction-related volcanic provinces of theSierra Madre Occidental (SMO), Trans-Mexican Volcanic Belt (TMVB), and Sierra Madre del Sur (SMS). (B) Schematic map of southernMexico, showing the distribution of the TMVB, and the volcanic and plutonic belts of the SMS magmatic province (modied from Morn-Zenteno et al. 1999). The studied volcanic centres are located in the north-central sector of the SMS, and are bordered in black. Importantcities, state names (in italics), and their limits (dotted lines) are included as reference.

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International Geology Review 1477

The SMS igneous province

The SMS is a broad magmatic province that extends for1100 km from the Puerto Vallarta area to the Isthmusof Tehuantepec, being delimited by the Miocene to recentTrans-Mexican Volcanic Belt (TMVB) to the north, and bythe Pacic coast to the south (Figure 1). It is composedof two WNW-trending belts with contrasting petrologiccharacteristics (Figure 1B): an inland series of volcaniccentres that produced mac to rhyolitic lava sequencesand hypabyssal intrusions, as well as voluminous silicicignimbrite deposits; and a chain of granitic-granodioritic-tonalitic batholiths and smaller intrusive bodies distributedalong the truncated and exhumed continental margin(Morn-Zenteno et al. 2007). The truncation and uplift ofthe Mexican Pacic margin have traditionally been relatedto the detachment and south-eastward displacement of theChortis block (Ross and Scotese 1988; Schaaf et al. 1995);more recently, it has been proposed that a process ofsubduction erosion may have caused rapid tectonic removalof a wide fore-arc, and induced the landward migration ofthe trench up to its modern location (Keppie et al. 2009).

The magmatic activity of the SMS took place over along period of time spanning from the Late Cretaceousto the late Oligoceneearly Miocene (Figure 2; Morn-Zenteno et al. 2007). Based on early geochronologicalresults, some authors considered that the SMS was aNNW-oriented magmatic arc that migrated from westto east between the Late Cretaceous and the Oligocene(e.g. Schaaf et al. 1995). Nevertheless, a growing bodyof isotopic ages documents that, during the Paleocene,magmatic activity was distributed in both the Jalisco-Colima region and central Guerrero (Figure 2A; Duceaet al. 2004; Cerca et al. 2007; Valencia et al. 2009).Moreover, the more comprehensive database now availablefor the SMS (Nelson et al. 2009 and references therein)indicates that, in Eocene and early Oligocene times, con-tinental magmatism formed a >200 km wide belt ori-ented parallel to the present-day Pacic coast, distributedbetween longitudes 102 and 97 (Figure 2B and 2C;Ducea et al. 2004; Morn-Zenteno et al. 2007; Keppie et al.2009). During the Late Oligocene, the igneous activity ofthe SMS was restricted to modest magmatic manifestations

Figure 2. Spacetime distribution of the SMS magmatic activity (modied from Morn-Zenteno et al. 2007), obtained using availableU-Pb ages for the plutonic rocks, and K-Ar, 40Ar/39Ar, and U-Pb ages for the volcanic sequences (data from Nelson et al. 2009). Stars rep-resent isotopic ages from the literature; shaded elds represent the inferred areal extent of magmatism at different times. Important cities,state names (in italics), and their limits (dotted lines) are included as reference. (A) Late Cretaceous igneous activity was concentrated inthe state of Jalisco, whereas it affected both Jalisco-Colima and central Guerrero in Palaeocene times; (B) during the earlymiddle Eocene,magmatism had a broad distribution and formed a belt oriented almost parallel to the modern Pacic margin; (C) in late EoceneearlyOligocene times, magmatism reached a broader distribution, and produced a belt of explosive silicic volcanic centres in the northcentralsector of the SMS (see inset); (D) late Oligocene activity of the SMS was limited to modest magmatic manifestations and ceased in earlyMiocene times.

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1478 L. Mori et al.

in Morelos, Guerrero, and southern Oaxaca (Figure 2D),and subsequently ceased in the early Miocene (Martnez-Serrano et al. 2008 and references therein), marking theinception of the TMVB (Morn-Zenteno et al. 1999;Gmez-Tuena et al. 2007).

Late Eoceneearly Oligocene volcanism in the SMS

During the late Eocene and early Oligocene, intensemagmatic activity developed in Michoacn, State ofMxico, Morelos, Guerrero, and Puebla (Figure 2C), pro-ducing one of the major pulses of explosive siliceousvolcanism in southern Mexico. Between 38 Ma and30 Ma, caldera-forming and ssure eruptions generateda WNW-trending belt of collapse structures, pyroclasticdike complexes, rhyolitic domes, small hypabyssal bod-ies, and intermediate-silicic lavas, which make up theNanchititla, San Vicente, Valle de Bravo, Mueca, Paredes,Goleta, Taxco, Tilzapotla, and Huautla volcanic centres(Figure 2C; Alaniz-lvarez et al. 2002; Morn-Zentenoet al. 2004; Gonzlez-Cervantes 2007; Gonzlez et al.2009; Martini et al. 2009; Daz-Bravo and Morn-Zenteno2011). This magmatic outburst produced ignimbrite suc-cessions with a total preserved volume of 2500 km3(Gonzlez-Torres et al. 2011); yet, considering that theregion has experienced deep erosion, these pyroclasticdeposits likely represent the vestiges of a much more exten-sive ignimbrite cover, which might have originally beentwice as large (Morn-Zenteno et al. 2007; Daz-Bravoand Morn-Zenteno 2011). In the state of Michoacn,Serrano-Durn (2005) and Gonzlez-Cervantes (2007) alsodocument the emplacement of mac dike swarms priorto and coeval with the ignimbrite are-up. According toAlaniz-lvarez et al. (2002), Morn-Zenteno et al. (2004),and Martini et al. (2009), the emplacement of the siliceoussuccessions and mac hypabyssal intrusions was favouredand controlled by a NW-trending left-lateral fault systemthat affected the region at that time.

Early Oligocene magmatism also developed in thenorth-western sector of the state of Oaxaca (3329 Ma;Martiny et al. 2000; Cerca et al. 2007; Figures 1B and 2C),but with very different volcanological and geochemicalfeatures. Here, igneous activity mainly generated com-posite volcanoes and monogenetic centres with dominantmac-intermediate compositions (Martiny et al. 2000).Volcanism in north-western Oaxaca was also associatedwith episodes of left-lateral strike-slip tectonics, whichcontinued after the cessation of magmatism (Martiny et al.2012).

Basement geology and volcanic stratigraphy of thestudy area

The late Eoceneearly Oligocene magmatic sequences ofthe north-central SMS are emplaced on two distinct crustal

blocks with contrasting ages, lithologies, and tectonic his-tories, whose boundaries are dened by major fault systems(Campa and Coney 1983; Sedlock et al. 1993).

The Mueca and Goleta volcanic elds rest overthe Mesozoic Guerrero terrane, recently recognized asa parautochthonous arc built on the thinned continentalmargin of North America, which was drifted in the palaeo-Pacic domain during episodes of back-arc extension, andsubsequently accreted back to the Mexican craton (Cabral-Cano et al. 2000; Elas-Herrera et al. 2000; Martini et al.2009, 2011). The Guerrero terrane is locally representedby the Early Cretaceous Tejupilco Schist and the LateCretaceous Arcelia-Palmar Chico Group (Elas-Herreraet al. 2000, 2009): the former is a polydeformed sequenceof phyllites, micaschists, and volcanic rocks intruded bysiliceous plutonic bodies; the latter is a succession of lime-stones, sandstones, and mac lavas that overthrust theTejupilco Schist. Ortho- and paragneiss xenoliths foundwithin a pyroclastic dike of the Goleta volcanic eld alsodocument the existence of granulite-facies lithologies ofEarly Jurassic age at deep levels of the middle crust (Elas-Herrera and Ortega-Gutirrez 1997; Elas-Herrera et al.2009).

The Guerrero terrane is not exposed in proximity ofthe Tilzapotla caldera (Morn-Zenteno et al. 2004); yet, theoccurrence of high-grade metapelite and quartz-feldspathicgneiss xenoliths hosted within the rhyolitic domes ofChalcatzingo in the TMVB, 50 km ENE of Tilzapotla,document the existence of deep granulitic lithologies petro-logically similar to those recognized beneath the Goletavolcanic eld (Ortega-Gutirrez et al. 2008), and thussupport that the Tilzapotla, Goleta, and Mueca volcaniccentres are built on the same basement.

In contrast, the Oaxaca volcanic eld rests over thePalaeozoic Acatln complex, which represents the exposedcrystalline basement of the Mixteco terrane, as well asone of the largest blocks of pre-Mesozoic metamorphicrocks in southern Mexico. The Acatln complex is avolcanosedimentary succession of oceanic and conti-nental afnity affected by greenschist-, amphibolite-,and eclogite-facies metamorphism, later intruded bygranites and affected by migmatization (Ortega-Gutirrez1981). According to Ortega-Gutirrez et al. (2008), theAcatln complex extends to a structural depth of 20 km,corresponding to the base of the middle crust; and isunderlain by granulite-facies lower crustal rocks, similarto those of the Grenvillian Oaxacan complex that cropsout in south-eastern Oaxaca (Ortega-Gutirrez et al. 1995;Keppie et al. 2003).

The Mueca volcanic centre

The Mueca volcanic centre (Figure 3A) has a preservedvolume of 25 km3. Its western boundary is marked by anarray of subvertical pyroclastic dikes made of pumiceous

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Figure 3. Geologic maps of the studied volcanic centres. (A) Mueca; (B) Goleta (modied from Daz-Bravo and Morn-Zenteno 2011);(C) Tilzapotla caldera (modied from Morn-Zenteno et al. 2004); (D) Oaxaca volcanic eld (modied from Martiny et al. 2012). Mainvillages are shown as reference.

ignimbrites with quartz, sanidine, minor plagioclase, andbiotite. Peripheral fracturing of the roof of a shallowmagma chamber probably triggered the explosive eventthat led to the emplacement of these dikes, and to the erup-tion of El Suz Tuff, which represents the basal unit of theMueca volcanic succession. The 420 m thick El SuzTuff (U-Pb age on zircon of 34.0 0.6 Ma; Chapela-Lara 2008) is composed of pumice-rich ignimbrites thatwere fed by the pyroclastic dike complex, as suggested bytheir textural and compositional similarities. El Suz Tuff isoverlain by El Potrero Andesite, a 500 m thick sequenceof andesitic-dacitic lava ows with phenocrysts of plagio-clase and pyroxene; and by the 400 m thick MuecaAutobreccia. Lava blocks from the Mueca Autobrecciahave intermediate compositions and display porphyritictextures with plagioclase, amphibole, and orthopyroxenephenocrysts. This unit is capped by the 250 m thickPea Larga Rhyolite, a succession of rhyolitic lava owswith plagioclase, amphibole, and pyroxene phenocrysts,intercalated with thin vitrophyric layers. In the southernsector of the volcanic centre, El Suz Tuff is intruded by theTonatilco porphyry, a rhyolitic body with large phenocrystsof plagioclase, sanidine, and scarce biotite, with an exposedarea of 3 km2. Smaller andesitic to rhyolitic intrusions arealso exposed (not shown in Figure 3A). These subvolcanicbodies likely represent the conduits through which theeffusive units of the Mueca volcanic eld were emitted.

The Goleta volcanic eld

The Goleta volcanic eld (Figure 3B) encompasses an areaof 400 km2, and has a preserved volume of 200 km3

(Daz-Bravo and Morn-Zenteno 2011). Its most spec-tacular feature is the presence of huge pyroclastic dikecomplexes that extend almost continuously along its west-ern and southern anks, and which acted as feeder conduitsfor the ignimbrite units deposited in the area. The mainunit is the basal Goleta Ignimbrite (U-Pb age on zircon of36.5 0.6 million years; Daz-Bravo and Morn-Zenteno2011), with a thickness of 200 m in the northern sectorand 600 m in the southern part of the volcanic centre.It is a massive succession of crystal-rich ignimbrites withsanidine, quartz, minor plagioclase, and biotite, which wasfed by the central and southern dike complexes (Daz-Bravo and Morn-Zenteno 2011). To the south, the GoletaIgnimbrite is intruded by the Tecomates porphyry (U-Pbage on zircon of 36.9 0.6 million years; Daz-Bravoand Morn-Zenteno 2011), a rhyolitic body with large phe-nocrysts of sanidine and quartz, with an exposed area of9 km2. The overlying ignimbrite units (the uppermostwith a 40Ar/39Ar age on sanidine of 34.4 0.5 millionyears; Daz-Bravo and Morn-Zenteno 2011) are pumice-rich, with a phenocryst assemblage of sanidine, quartz, pla-gioclase, and minor biotite. They are mainly distributed inthe northern sector of the volcanic eld, and were eruptedthrough the pyroclastic conduits exposed around the vil-lage of Sultepec (Daz-Bravo and Morn-Zenteno 2011).Here, the pyroclastic dikes do not delimit any collapse orsubsidence structure. On the other hand, the semicurvi-linear pattern of the pyroclastic dikes along the southernank of the volcanic centre, coupled with the greater thick-ness of the Goleta Ignimbrite, indicate the developmentof a partial collapse caldera in this area (Daz-Bravo andMorn-Zenteno 2011).

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The Tilzapotla caldera

The Tilzapotla caldera (Figure 3C) is a huge semiellipti-cal collapse structure that encircles a thick and continuousvolcanic succession. With a minimum volume of 600 km3

and a thickness of up to 600 m, the Tilzapotla Ignimbrite(40Ar/39Ar age on sanidine of 34.3 0.1 million years;Morn-Zenteno et al. 2004) represents the magmatic man-ifestation of the climatic event of caldera collapse: it isa massive sequence of dacitic tuffs with quartz, plagio-clase, biotite, and minor sanidine phenocrysts. This unitis covered by a post-collapse sequence that comprises theRodarte Ignimbrite and Gallego Formation. The former is avitroclastic sequence of ow units with pumice fragmentsand biotite phenocrysts; the latter is a thick sequence ofrheomorphic ignimbrites, vitrophyric ows, and daciticlavas with abundant plagioclase, sanidine, biotite, andquartz. Within the caldera, a NW-oriented uplifted blockis the main indicator of a resurgent stage (Morn-Zentenoet al. 2004). Lava ows and dikes overlying and intrudingthe Tilzapotla Ignimbrite and the post-collapse units wereerupted during this phase. The dikes are mainly distributedalong the structural borders of the uplifted block, andexhibit a compositional variation from andesite to rhyolite.The lava ows (40Ar/39Ar ages on plagioclase of 33.4 0.1 to 32.8 0.1 million years; Morn-Zenteno et al.2004) range in composition from two pyroxene andesitesto hornblende-bearing dacites.

The Oaxaca volcanic eld

The magmatic sequences of the Oaxaca volcanic eld(Figure 3D) have been grouped by Martiny et al. (2000)into two general units. The lower unit consists of siliceouspyroclastic and epiclastic deposits. In contrast, the pre-dominant and more voluminous upper unit is repre-sented by lava ows and autobreccias with intermedi-ate compositions. Specically, it consists of a thick pile(>400 m in some areas) of porphyritic basaltic andesiteand andesitic lavas with phenocrysts of ortho- and clinopy-roxene, olivine, plagioclase, and minor hornblende. Erodedvestiges of volcanic vents in the form of necks suggestthat these ows were at least partially emitted by cen-tral volcanic structures (Martiny et al. 2000). The volcanicsuccessions of the Oaxaca volcanic eld are intruded bynumerous hypabyssal bodies, represented by hornblende-or pyroxene-bearing stocks and dikes of intermediate com-positions (Martiny et al. 2000).

Analytical procedures

Major elements (Table 1) were determined by X-ray uo-rescence spectrometry using a Siemens SRS-3000 instru-ment at the Laboratorio Universitario de GeoqumicaIsotpica (LUGIS) of the Universidad Nacional Autnoma

de Mxico, following procedures by Lozano-Santa Cruzand Bernal (2005).

Trace element data (Table 1) were obtained by induc-tively coupled plasma mass spectrometry. A group ofsamples was analysed at LUGIS with an Agilent 7500ceinstrument, following procedures by Eggins et al. (1997);additional trace element compositions were obtained at theLaboratorio de Estudios Isotpicos (LEI) of UniversidadNacional Autnoma de Mxico using a Thermo Series XIIspectrometer, following procedures by Mori et al. (2007).Reproducibility of trace element data is based on multipledigestions of international rock standards; reproducibilityis better than 4% for the elements measured at LUGIS,and better than 3% for the element concentrations obtainedat LEI.

Sr and Nd isotope ratios (Table 2) were measured bythermal ionization mass spectrometry at LUGIS, using aFinnigan MAT 262 system equipped with eight Faradaycups. Sample preparation and measurement proceduresfor isotopic analyses are described in Schaaf et al. (2005).Results were corrected for mass fractionation by nor-malizing to 86Sr/88Sr = 0.1194 and 146Nd/144Nd =0.7219. At LUGIS, the long-term reproducibility of theNBS-987 standard is 87Sr/86Sr = 0.710236 0.000042(2 , n = 302); and the long-term reproducibility of theLa Jolla standard is 143Nd/144Nd = 0.511875 0.000042(2 , n = 148).

Geochemical results

Rocks from the study region display a wide compo-sitional variation spanning from basaltic andesites torhyolites (Figure 4). The volcanic products of the Oaxacasuite are represented by medium-K calc-alkaline basalticandesites and andesites, whereas rocks from the Tilzapotlaand Mueca volcanic elds range in composition frommedium-K andesites to high-K rhyolites; in contrast, vol-canic activity at Goleta mainly produced high-K calc-alkaline rhyolitic ignimbrites (Figures 4A and 4B). Rocksfrom the different volcanic centres display similar trends inmajor element variation diagrams, such as positive correla-tions between K2O and SiO2 (Figure 4B); negative correla-tions of TiO2, MgO, CaO (Figure 4C), Fe2O3tot and P2O5with silica; and almost constant Na2O abundances. Al2O3contents of the Oaxaca rocks show a parallel trend withSiO2, whereas they decrease with increasing differentiationin the other suites (Figure 4D).

The trace element patterns of all the rock sequencesdisplay enrichments in the large-ion lithophile elementsand Pb with respect to the high eld strength elements;and fractionated rare earth element (REE) patterns show-ing higher light REE (LREE) contents relative to the heavyREE (HREE) (Figure 5). Samples from the Oaxaca vol-canic eld are distinguished by their positive Sr spikesand depleted HREE contents (Figure 5A); they also show

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Tabl

e1.

Major

and

trac

eelem

enta

naly

sesof

thestud

ied

rock

suites

.

Sui

teM

uec

aM

uec

aM

uec

aM

uec

aM

uec

aM

uec

aM

uec

aM

uec

aGol

etaa

Gol

etaa

Gol

etaa

Gol

etaa

Sam

ple

VA-0

6VA-0

8VA-2

4VA-4

4VA-6

4VA-7

0VA-7

2VA-7

3GO-1

07-0

6GO-3

01-0

7VNP

GO-1

7-06

Lon

gitu

deW

100

10.3

00

100

10.2

00

100

10.2

87

100

11.2

27

100

10.2

59

100

11.0

46

100

11.0

38

100

10.5

59

100

01.9

16

100

00.4

35

100

06.8

9999

59.

578

Latitud

eN

18 5

3.21

018

53.

211

18 5

1.95

518

53.

073

18 5

2.02

018

51.

849

18 5

1.76

118

51.

993

18 3

5.71

418

47.

572

18 3

8.02

318

46.

121

Maj

orel

emen

ts(w

t.%)

SiO

258

.96

62.3

167

.23

61.5

955

.87

65.5

573

.80

55.7

271

.15

73.6

975

.49

73.2

4TiO

20.

830.

730.

470.

750.

940.

630.

431.

100.

120.

120.

080.

14Al 2O

316

.48

15.5

014

.42

15.9

515

.60

14.8

312

.63

16.9

712

.17

12.5

911

.91

13.4

4Fe

2O

3to

t7.

015.

683.

515.

488.

074.

533.

328.

281.

471.

721.

482.

81M

nO0.

100.

120.

050.

100.

130.

080.

030.

130.

030.

030.

030.

06M

gO4.

172.

320.

321.

265.

341.

410.

114.

040.

360.

280.

150.

24CaO

6.75

4.73

3.46

5.15

6.99

3.34

1.81

7.70

3.40

0.97

0.87

1.17

Na 2

O2.

943.

112.

653.

432.

433.

302.

613.

243.

343.

093.

393.

15K

2O

1.99

2.80

4.21

2.92

1.10

3.42

4.35

1.22

4.12

4.73

4.45

5.13

P2O

50.

190.

200.

160.

230.

260.

190.

170.

300.

030.

030.

030.

06LOI

0.80

0.95

3.53

2.97

3.26

2.81

0.57

1.13

3.82

2.94

2.60

0.56

Total

100.

2298

.45

100.

0199

.83

99.9

910

0.09

99.8

399

.83

100.

0210

0.19

100.

4810

0.00

Trac

eel

emen

ts(p

pm)

Lab

orator

yLUGIS

cLUGIS

cLUGIS

cLUGIS

cLUGIS

cLUGIS

cLUGIS

cLUGIS

cLUGIS

cLUGIS

cLUGIS

cLUGIS

c

Sc

21.1

15.4

9.5

14.6

17.6

9.6

7.0

19.9

12.7

8.3

9.0

9.3

V13

092

.536

.679

.914

969

.632

.416

517

.411

.37.

312

.8Cr

18.9

11.0

4.9

9.3

154

11.3

8.0

69.6

Co

21.4

11.2

4.5

9.2

25.4

7.8

2.7

19.2

2.2

1.9

1.3

2.3

Ni

29.4

10.7

8.4

21.9

46.9

3.8

2.6

11.8

7.0

4.6

4.4

10.1

Cu

2016

6278

319

1319

97

716

Zn

7382

6278

9258

3877

5648

5346

Li

17.4

45.1

25.3

22.1

48.5

16.2

45.4

13.0

42.6

36.5

20.0

49.9

Be

1.4

1.9

2.5

2.2

1.3

2.4

2.6

1.2

4.7

3.6

4.6

3.1

Rb

6897

181

107

3338

119

869

230

210

261

182

Sr

328

269

121

248

404

191

139

374

7616

041

67Y

25.9

27.9

29.5

30.4

21.0

31.7

37.1

24.2

48.5

35.9

36.9

22.1

Zr

157

151

143

163

174

166

134

148

7576

7293

Nb

7.06

10.4

9.83

10.8

8.71

10.3

11.0

7.50

13.7

10.7

11.8

8.50

Cs

2.36

3.03

8.48

4.65

8.83

31.6

5.57

36.5

38.4

59.1

29.7

7.00

Ba

644

511

543

528

375

581

618

482

7222

478

431

La

18.3

22.6

27.5

28.9

19.2

27.8

29.8

16.2

17.2

21.4

16.5

28.9

Ce

36.7

47.2

54.2

55.9

42.5

57.6

62.3

37.3

37.5

37.0

38.1

56.8

Pr

4.70

5.81

6.61

6.95

5.36

6.88

7.98

4.77

5.20

6.46

4.89

6.40

Nd

19.7

24.8

26.1

28.3

22.2

27.2

32.8

20.1

22.6

27.4

20.7

24.4

Sm

4.51

5.48

5.64

6.04

4.77

5.92

7.61

4.59

6.77

6.60

5.47

4.89

Eu

1.10

1.20

1.00

1.27

1.23

0.97

00.

930

1.24

0.20

00.

360

0.17

00.

500

Gd

4.50

5.00

5.30

5.74

4.37

5.54

6.94

4.40

7.11

6.12

5.66

4.26

Tb

0.80

00.

900

0.90

00.

960

0.73

00.

940

1.20

0.74

01.

361.

101.

080.

740

Dy

4.30

4.90

4.90

5.17

3.85

4.97

6.15

3.96

7.87

6.13

6.35

4.05

Ho

0.90

1.00

1.00

1.10

0.74

0.97

1.18

0.78

1.76

1.33

1.41

0.87

Er

2.50

3.00

2.80

2.92

2.20

2.85

3.36

2.31

4.73

3.60

3.84

2.41

Yb

2.43

2.72

2.69

2.73

2.06

2.70

2.99

2.20

4.46

3.36

3.65

2.34

Lu

0.37

00.

400

0.40

00.

420

0.30

00.

400

0.43

00.

330

0.67

00.

500

0.55

00.

350

Hf

4.23

4.50

4.44

4.71

4.39

4.81

4.20

3.81

3.50

3.30

3.20

3.40

Ta0.

721.

061.

221.

140.

580.

991.

140.

522.

101.

701.

901.

20Pb

8.7

1215

169.

615

176.

623

2221

26Th

6.33

8.79

12.5

9.06

3.96

13.2

15.4

3.84

13.3

10.9

10.7

10.5

U1.

912.

955.

483.

161.

284.

946.

241.

346.

005.

905.

704.

40

(Con

tinu

ed)

Dow

nloa

ded

by [U

NA

M C

iuda

d U

nive

rsita

ria] a

t 12:

45 0

7 Ja

nuar

y 20

13

1482 L. Mori et al.

Tabl

e1.

(Con

tinu

ed).

Sui

teGol

etaa

Gol

etaa

Tilza

potla

Tilza

potla

Tilza

potla

Tilza

potla

Tilza

potla

Tilza

potla

Tilza

potla

Tilza

potla

Tilza

potla

Tilza

potla

Sam

ple

GO-3

24-0

7GO-3

27-0

7TZ-0

1-09

TZ-0

2-09

TZ-0

4-09

TZ-0

5-09

TZ-0

6-09

TZ-0

7-09

TZ-1

7-09

TZ-1

8-09

TZ-1

9-09

TZ-2

0-09

Lon

gitu

deW

100

04.7

00

100

04.8

52

99 2

5.11

399

16.

844

99 2

5.26

199

16.

519

99 1

6.50

999

16.

442

99 1

7.04

999

17.

111

99 1

7.10

899

17.

134

Latitud

eN

18 3

8.90

418

39.

919

18 2

5.21

818

18.

121

18 2

5.83

818

27.

790

18 2

8.03

818

28.

033

18 2

6.55

818

26.

570

18 2

6.64

818

26.

729

Maj

orel

emen

ts(w

t.%)

SiO

275

.22

76.8

961

.15

65.2

372

.48

64.3

158

.98

59.5

961

.32

63.4

263

.94

60.2

5TiO

20.

180.

140.

580.

550.

370.

620.

790.

790.

660.

580.

570.

69Al 2O

312

.85

12.2

415

.40

13.8

313

.50

16.3

217

.82

17.6

116

.86

16.6

015

.91

16.9

7Fe

2O

3to

t1.

741.

655.

104.

312.

834.

916.

556.

515.

484.

984.

655.

99M

nO0.

030.

010.

070.

040.

010.

060.

110.

110.

080.

090.

060.

08M

gO0.

250.

283.

401.

610.

651.

921.

321.

362.

622.

241.

801.

99CaO

1.09

1.06

5.42

2.34

1.45

4.51

5.42

5.31

5.17

4.65

3.57

5.39

Na 2

O3.

203.

173.

282.

112.

793.

973.

743.

793.

703.

753.

723.

98K

2O

4.44

4.63

2.43

5.42

4.33

2.31

2.10

2.10

1.87

2.03

3.73

2.09

P2O

50.

050.

040.

130.

110.

080.

160.

260.

260.

150.

160.

150.

22LOI

0.68

0.50

2.46

4.15

1.80

1.59

2.75

1.80

1.48

1.92

2.31

2.23

Total

99.7

310

0.61

99.4

199

.70

100.

3010

0.68

99.8

499

.21

99.3

910

0.42

100.

4099

.87

Trac

eel

emen

ts(p

pm)

Lab

orator

yLUGIS

cLUGIS

cLEId

LEId

LEId

LEId

LEId

LEId

LEId

Sc

7.2

7.2

15.3

6.5

10.3

13.0

12.6

11.1

11.4

V13

.513

.912

246

.292

.810

068

.610

996

.5Cr

51.7

14.2

15.5

13.1

11.8

22.8

15.4

Co

2.0

1.7

13.9

4.9

12.6

17.9

15.8

15.0

13.4

Ni

3.5

9.5

2.2

13.0

10.3

10.7

13.5

11.0

Cu

56

176

4329

5836

25Zn

3529

6080

6175

7269

65Li

63.4

57.7

24.0

117

23.6

16.8

15.1

9.8

12.2

Be

3.8

3.8

1.8

1.8

1.6

1.7

1.5

1.4

1.6

Rb

175

185

8416

253

4539

5774

Sr

7552

352

149

422

876

676

495

479

Y24

.929

.422

.320

.017

.837

.629

.917

.218

.0Zr

8186

9828

115

7311

311

012

0Nb

8.80

9.70

6.95

6.81

5.49

5.29

5.18

4.76

5.43

Cs

12.1

9.80

4.27

5.02

2.02

2.30

0.61

2.12

2.78

Ba

370

432

424

415

480

521

507

570

612

La

18.0

31.7

15.8

17.2

15.3

21.0

18.0

16.8

17.3

Ce

36.9

60.1

33.5

35.4

26.8

36.2

34.3

33.1

30.5

Pr

4.72

7.32

4.32

4.36

4.31

6.29

5.58

4.80

4.88

Nd

19.0

28.2

17.1

16.0

18.0

26.9

23.6

19.4

19.3

Sm

4.58

5.88

4.04

3.46

4.13

6.40

5.59

4.41

4.31

Eu

0.44

00.

490

0.87

50.

538

1.01

1.66

1.46

1.16

1.07

Gd

4.39

5.26

3.91

3.13

3.84

6.42

5.32

3.93

3.89

Tb

0.81

00.

920

0.62

60.

522

0.57

10.

950

0.80

40.

592

0.56

3Dy

4.48

4.99

3.71

3.22

3.12

5.86

4.76

3.19

3.27

Ho

0.96

1.07

0.76

0.69

0.63

1.16

0.98

0.62

0.63

Er

2.64

2.92

2.01

1.89

1.57

3.19

2.60

1.56

1.63

Yb

2.53

2.83

2.08

2.02

1.57

2.85

2.52

1.55

1.61

Lu

0.38

00.

420

0.31

20.

309

0.23

50.

439

0.39

20.

228

0.23

8Hf

2.90

3.40

2.82

1.09

3.01

2.19

2.97

2.90

3.03

Ta1.

601.

600.

780.

780.

550.

480.

470.

430.

51Pb

1919

9.4

137.

56.

36.

67.

08.

4Th

10.2

10.7

6.36

9.19

3.41

3.47

3.82

2.95

3.29

U4.

504.

302.

932.

631.

951.

291.

111.

441.

76

(Con

tinu

ed)

Dow

nloa

ded

by [U

NA

M C

iuda

d U

nive

rsita

ria] a

t 12:

45 0

7 Ja

nuar

y 20

13


Tabl

e1.

(Con

tinu

ed).

Sui

teTilza

potla

Tilza

potla

Tilza

potla

Tilza

potla

Tilza

potla

Tilza

potla

Tilza

potla

Tilza

potla

Tilza

potla

Oax

acab

Oax

acab

Oax

acab

Sam

ple

TZ-2

1-09

TZ-6

2-09

TZ-1

36-0

9BV-1

2-09

BV-1

3-09

BV-1

7-09

BV-3

0-09

GF-1

7-09

vGF-1

7-09

dCON-8

8CON-9

0CON-7

7Lon

gitu

deW

99 1

7.34

399

17.

174

99 2

4.98

699

23.

140

99 2

3.51

599

23.

627

99 2

3.26

699

13.

847

99 1

3.84

797

40.

800

97 4

0.76

797

39.

400

Latitud

eN

18 2

6.69

418

27.

188

18 2

5.26

918

27.

596

18 2

7.48

718

27.

370

18 2

7.65

418

26.

945

18 2

6.94

518

02.

253

18 0

2.48

318

00.

034

Maj

orel

emen

ts(w

t.%)

SiO

261

.12

59.7

072

.00

63.6

764

.83

66.2

764

.48

72.5

677

.00

51.4

152

.68

53.0

3TiO

20.

730.

660.

280.

470.

440.

430.

480.

160.

151.

321.

491.

26Al 2O

316

.81

16.7

814

.37

16.1

415

.35

15.2

916

.21

12.9

411

.78

17.5

017

.43

16.8

5Fe

2O

3to

t5.

726.

002.

543.

484.

724.

435.

111.

471.

408.

989.

158.

33M

nO0.

060.

080.

060.

070.

080.

070.

040.

040.

020.

070.

100.

11M

gO2.

162.

550.

472.

951.

780.

671.

220.

340.

234.

915.

576.

29CaO

4.02

5.49

2.14

3.80

3.73

2.92

3.26

1.32

0.78

7.73

7.67

7.50

Na 2

O4.

153.

723.

593.

553.

613.

653.

783.

492.

403.

643.

903.

79K

2O

2.50

2.09

3.89

3.14

2.81

2.91

2.78

3.83

5.25

0.83

0.75

0.93

P2O

50.

190.

180.

090.

140.

130.

120.

130.

060.

050.

310.

310.

32LOI

2.64

2.64

1.05

3.12

2.34

2.77

1.94

3.94

0.99

3.18

0.82

1.42

Total

100.

1099

.89

100.

4710

0.53

99.8

099

.53

99.4

210

0.14

100.

0599

.88

99.8

799

.83

Trac

eel

emen

ts(p

pm)

Lab

orator

yLEId

LEId

LEId

LEId

LEId

LEId

LEId

Sc

11.4

12.8

6.6

9.0

8.8

3.7

2.5

15.1

14.8

15.0

V10

213

432

.925

668

.98.

07.

219

920

316

6Cr

18.3

25.8

12.1

3.3

3.0

2.6

2.6

224

181

202

Co

14.8

17.2

4.5

11.6

10.7

1.3

1.1

24.5

32.9

34.2

Ni

14.6

17.6

2.5

4.0

3.4

0.5

0.5

85.4

71.1

83.0

Cu

7147

721

203

633

2935

Zn

7369

4064

6334

2388

104

91Li

27.1

19.8

41.2

50.2

52.1

20.3

63.0

Be

1.6

1.4

3.0

1.8

2.0

3.0

3.2

Rb

5641

162

9791

185

203

1513

15Sr

377

641

193

404

516

139

9444

147

447

4Y

18.5

18.0

29.4

19.6

23.0

23.6

23.0

16.3

16.9

15.1

Zr

129

100

6166

6765

6011

513

013

2Nb

5.95

4.27

10.2

6.33

5.84

9.41

8.66

5.91

4.91

7.33

Cs

1.42

1.32

9.13

2.32

4.35

16.4

9.79

0.85

0.54

0.34

Ba

656

478

561

553

609

520

510

213

211

315

La

17.8

15.3

24.6

16.4

18.1

20.8

20.5

12.7

12.7

15.7

Ce

31.9

30.7

45.4

34.3

34.4

41.5

39.9

28.4

30.2

36.0

Pr

4.71

4.39

6.08

4.30

4.43

4.90

4.76

4.02

4.46

4.67

Nd

18.9

18.4

22.1

16.6

17.1

17.3

17.1

19.0

20.0

21.0

Sm

4.17

4.14

5.05

3.64

3.74

3.90

3.76

4.39

5.18

4.78

Eu

1.07

1.13

0.71

70.

790

0.80

60.

506

0.44

91.

511.

691.

51Gd

3.93

3.69

4.78

3.38

3.66

3.60

3.42

3.81

3.94

3.76

Tb

0.55

90.

544

0.76

20.

528

0.55

80.

595

0.59

40.

565

0.62

20.

561

Dy

3.24

3.05

4.53

3.25

3.42

3.45

3.48

3.30

3.37

2.86

Ho

0.62

0.63

0.92

0.65

0.70

0.74

0.74

0.66

0.65

0.59

Er

1.62

1.61

2.39

1.79

1.91

1.92

1.99

1.55

1.54

1.39

Yb

1.54

1.60

2.46

1.78

1.90

2.31

2.25

1.43

1.45

1.29

Lu

0.23

10.

247

0.36

60.

265

0.28

50.

351

0.34

00.

209

0.20

90.

182

Hf

3.23

2.71

2.09

2.06

2.07

2.18

2.13

3.10

3.32

3.24

Ta0.

500.

381.

410.

770.

711.

531.

460.

280.

270.

49Pb

7.5

7.5

189.

69.

917

163.

43.

65.

3Th

3.20

3.56

11.4

6.10

5.88

11.6

11.5

1.10

1.28

1.34

U1.

451.

384.

503.

972.

466.

424.

770.

349

0.47

60.

444

(Con

tinu

ed)

Dow

nloa

ded

by [U

NA

M C

iuda

d U

nive

rsita

ria] a

t 12:

45 0

7 Ja

nuar

y 20

13

1484 L. Mori et al.

Tabl

e1.

(Con

tinu

ed).

Sui

teOax

acab

Oax

acab

Oax

acab

Oax

acab

Oax

acab

Oax

acab

Oax

acab

Oax

acab

Oax

acab

Oax

acab

Oax

acab

Oax

acab

Oax

acab

Sam

ple

CON-1

4CON-1

8CON-2

9aCON-3

5CON-2

0CON-9

CON-3

2CON-2

7CON-7

0CON-6

0aCON-6

1aCON-3

3CON-2

8Lon

gitu

deW

97 4

1.85

97

41.

133

97 4

0.71

797

38.

800

97 4

0.81

797

45.

617

97 4

3.28

397

37.

633

97 3

9.28

397

50.

834

97 5

0.51

797

45.

000

97 3

7.88

3Latitud

eN

17 5

9.30

017

58.

733

17 5

8.38

417

57.

950

17 5

6.80

017

49.

367

17 4

4.90

017

41.

867

17 1

9.01

717

10.

467

17 1

0.10

017

44.

900

17 4

2.11

7

Maj

orel

emen

ts(w

t.%)

SiO

256

.03

58.7

551

.54

59.1

353

.96

54.4

053

.36

56.9

059

.26

58.9

156

.72

52.6

954

.82

TiO

21.

240.

891.

340.

911.

371.

291.

240.

870.

860.

910.

891.

130.

90Al 2O

317

.15

16.8

417

.71

16.8

916

.70

17.0

216

.80

16.9

716

.38

16.9

417

.42

17.0

618

.24

Fe2O

3to

t7.

396.

358.

856.

098.

348.

388.

216.

806.

016.

296.

818.

076.

11M

nO0.

070.

080.

120.

090.

110.

100.

110.

070.

060.

060.

100.

130.

08M

gO4.

133.

305.

573.

214.

665.

005.

983.

843.

642.

053.

355.

974.

03CaO

6.78

5.94

7.87

5.76

7.32

7.28

7.87

6.85

5.85

5.67

7.00

8.22

7.28

Na 2

O3.

923.

474.

023.

493.

904.

013.

743.

713.

603.

793.

623.

373.

19K

2O

1.25

1.66

0.81

1.93

1.11

1.01

1.00

1.55

1.80

2.28

1.58

1.34

1.20

P2O

50.

340.

250.

310.

260.

340.

330.

320.

240.

230.

260.

240.

350.

33LOI

1.63

2.05

1.74

1.97

1.57

0.90

1.34

1.75

2.02

2.53

1.98

1.50

3.78

Total

99.9

399

.58

99.8

899

.73

99.3

899

.72

99.9

799

.55

99.7

199

.69

99.7

199

.83

99.9

6

Trac

eel

emen

ts(p

pm)

Lab

orator

ySc

12.6

11.9

15.0

11.3

14.3

13.8

16.4

13.9

12.1

11.1

13.1

16.8

12.5

V16

113

320

012

117

817

919

315

713

514

517

820

113

6Cr

111

47.6

214

51.1

114

139

208

65.9

110

36.4

33.7

122

29.0

Co

31.7

37.7

36.8

31.2

41.6

31.2

44.5

49.6

22.9

28.8

37.2

31.0

23.0

Ni

47.5

23.3

101

18.5

50.4

67.7

73.9

27.9

39.7

18.8

18.0

34.0

14.0

Cu

2835

4219

4344

3917

2642

4223

13Zn

105

9295

9011

311

595

8078

9185

7479

Li

Be

Rb

2346

1440

2619

2131

4953

3723

47Sr

593

455

484

467

506

494

459

463

429

464

593

794

817

Y15

.013

.915

.913

.716

.015

.817

.013

.013

.715

.515

.820

.715

.7Zr

148

161

130

173

146

150

139

137

153

151

124

138

128

Nb

5.97

5.56

4.73

5.28

5.97

7.16

5.57

4.46

8.07

5.20

4.48

6.54

4.28

Cs

0.67

0.93

0.42

0.87

1.69

1.02

0.86

0.96

1.28

2.13

0.95

0.52

4.10

Ba

381

511

219

575

312

274

309

436

525

511

411

363

335

La

19.5

20.5

12.5

23.1

17.4

14.8

16.1

17.2

22.3

19.7

15.5

22.0

17.5

Ce

43.9

44.1

30.1

47.7

39.7

35.4

36.6

37.3

46.0

38.6

33.7

47.6

38.3

Pr

5.61

5.41

4.11

5.8

5.14

4.90

4.79

4.53

5.66

5.68

4.38

5.95

5.04

Nd

26.0

21.5

19.2

24.2

23.3

22.2

20.9

19.3

22.9

24.0

19.1

25.9

22.0

Sm

5.69

4.75

4.70

4.87

5.30

5.06

4.55

3.87

4.64

5.13

4.15

5.55

4.43

Eu

1.59

1.40

1.47

1.35

1.61

1.65

1.58

1.24

1.32

1.38

1.22

1.61

1.38

Gd

3.99

3.44

3.63

3.76

4.07

4.11

3.95

3.32

3.40

4.11

3.23

4.34

3.57

Tb

0.61

00.

510

0.56

80.

529

0.61

00.

610

0.61

00.

470

0.51

00.

590

0.53

00.

690

0.53

0Dy

3.01

2.78

3.19

2.80

3.16

3.13

3.17

2.44

2.78

3.14

3.10

4.00

3.02

Ho

0.64

0.52

0.65

0.59

0.62

0.57

0.68

0.53

0.52

0.62

0.62

0.86

0.63

Er

1.41

1.22

1.43

1.37

1.46

1.35

1.52

1.21

1.25

1.35

1.50

2.00

1.44

Yb

1.24

1.21

1.35

1.24

1.32

1.18

1.41

1.13

1.08

1.18

1.32

1.97

1.40

Lu

0.19

60.

177

0.21

40.

203

0.19

20.

179

0.23

00.

176

0.16

90.

174

0.19

30.

320

0.21

0Hf

3.57

3.79

3.16

4.10

3.53

3.59

3.50

3.36

3.98

3.91

3.22

3.39

3.22

Ta0.

400.

400.

300.

490.

120.

500.

400.

630.

550.

380.

390.

300.

30Pb

5.9

8.4

3.8

7.5

7.0

5.8

5.7

7.1

9.2

9.6

8.1

5.0

5.4

Th

2.26

3.72

1.25

3.41

2.11

1.48

2.03

2.83

4.44

3.46

2.72

3.42

2.44

U0.

640

0.78

10.

409

0.66

00.

805

0.51

70.

632

0.80

31.

211.

610.

853

0.96

00.

750

Not

es:a M

ajor

elem

entda

taof

Gol

eta

rock

sfrom

Daz-

Bra

voan

dM

orn

-Zen

teno

(201

1);bM

ajor

and

trac

eelem

entda

taof

the

Oax

aca

volcan

icsu

ite

from

Mar

tiny

etal

.(2

000)

and

Mar

tiny

(200

8);c A

tLUGIS

,ana

lytica

lpre

cision

was

gene

rally

better

than

4%re

lativ

estan

dard

devi

atio

n;dAtL

EI,

analyt

ical

prec

isio

nwas

gene

rally

better

than

3%re

lativ

estan

dard

devi

atio

n.

Dow

nloa

ded

by [U

NA

M C

iuda

d U

nive

rsita

ria] a

t 12:

45 0

7 Ja

nuar

y 20

13


Tabl

e2.

Sran

dNd

isot

opic

com

position

sof

selected

sam

ples

from

thestud

ied

rock

suites

.

Sam

ple

MZ20

04Rb

(ppm

)Sr(p

pm)

Sm

(ppm

)Nd

(ppm

)(8

7Sr/

86Sr)

m1

abs

87Rb/

86Sr

(87Sr/

86Sr)

ia(1

43Nd/

144Nd)

m1

abs

147Sm

/14

4Nd

(143Nd/

144Nd)

ia

Mu

ecasu

ite

VA-0

667

373

5.0

22.5

0.70

4927

330.

520

0.70

4658

0.51

2639

150.

136

0.51

2607

VA-0

810

228

35.

324

.60.

7054

3237

1.04

10.

7048

920.

5126

0416

0.13

10.

5125

73VA-2

417

712

95.

827

.50.

7072

3938

3.97

20.

7051

800.

5125

7018

0.12

80.

5125

40VA-4

410

826

56.

229

.30.

7057

7237

1.18

00.

7051

600.

5125

8119

0.12

80.

5125

50VA-7

27.

332

.60.

5125

6422

0.13

50.

5125

37VA-7

372

414

4.7

21.1

0.70

4664

330.

506

0.70

4448

0.51

2696

190.

134

0.51

2670

Gol

etasu

ite

VNP

287

476.

223

.00.

7156

8137

17.7

630.

7064

720.

5125

4719

0.16

20.

5125

08GO-3

24-0

78.

135

.90.

5125

6419

0.13

70.

5125

31GO-3

27-0

719

768

7.9

38.2

0.71

1001

358.

331

0.70

6682

0.51

2534

200.

126

0.51

2504

Tilza

potlasu

iteb

TZ-0

4-09

Tz-

4-98

223

141

8.7

39.0

0.70

7560

394.

561

0.70

5350

0.51

2631

170.

135

0.51

2601

TZ-1

7-09

Tz-

17-9

853

531

4.1

18.5

0.70

3773

480.

291

0.70

3634

0.51

2858

170.

133

0.51

2828

TZ-1

8-09

Tz-

18-9

867

457

5.7

28.1

0.70

3890

380.

422

0.70

3685

0.51

2820

220.

122

0.51

2793

TZ-2

0-09

Tz-

20-9

837

616

4.2

19.7

0.70

3534

380.

174

0.70

3450

0.51

2900

240.

130

0.51

2871

TZ-1

36-0

9Tz-

136-

0114

320

83.

419

.30.

7055

1841

1.98

20.

7045

580.

5125

8017

0.10

60.

5125

56BV-1

2-09

Bv1

298

374

2.4

10.5

0.70

4097

620.

758

0.70

3731

0.51

2778

190.

133

0.51

2748

BV-1

7-09

Bv1

789

471

11.0

51.6

0.70

4044

550.

547

0.70

3780

0.51

2769

480.

128

0.51

2740

Oax

acasu

itec

CON-7

715

556

4.9

22.4

0.70

4236

360.

079

0.70

4202

0.51

2755

190.

1316

0.51

2729

CON-1

424

646

5.7

27.8

0.70

4587

470.

105

0.70

4542

0.51

2712

400.

1248

0.51

2688

CON-1

836

508

4.7

23.4

0.70

4724

410.

202

0.70

4638

0.51

2619

430.

1219

0.51

2595

CON-3

538

548

5.0

25.5

0.70

4715

450.

203

0.70

4629

0.51

2623

200.

1178

0.51

2600

CON-2

018

604

5.6

25.0

0.70

4431

420.

086

0.70

4394

0.51

2726

170.

1351

0.51

2699

CON-9

1353

55.

222

.10.

7043

7160

0.07

10.

7043

410.

5127

4933

0.14

140.

5127

21CON-3

219

526

4.8

21.5

0.70

4334

530.

102

0.70

4291

0.51

2764

190.

1356

0.51

2737

CON-7

049

490

4.6

23.8

0.70

4692

360.

287

0.70

4570

0.51

2665

160.

1179

0.51

2642

Not

es:Rb,

Sr,

Sm

,and

Nd

conc

entratio

nswer

ede

term

ined

byisot

opic

dilu

tion

.Relativ

ere

prod

ucib

ilitiesfo

rRb,

Sr,

Sm

,and

Nd

abun

danc

eswer

e4.

5%,1

.8%

,3.2

%,a

nd2.

7%,r

espe

ctiv

ely

(1).

Sr,

Nd,

and

Sm

isot

opera

tios

wer

em

easu

red

with

aFi

nnig

anM

at26

2sp

ectrom

eter

equi

pped

with

eigh

tFar

aday

cups

,whe

reas

Rb

analys

eswer

epe

rfor

med

with

asing

leco

llec

torNBS

mas

ssp

ectrom

eter.I

soto

pic

mea

sure

men

tswer

em

ade

ina

static

collec

tion

mod

e,with

each

analys

isco

nsisting

of60

isot

opic

ratios

forRb,

Sr,

and

Nd,

and

20isot

opic

ratios

forSm

.The

1ab

ser

rors

forin

divi

dual

Sran

dNd

mea

sure

men

tsar

em

ultipl

ied

by10

6.R

elativ

eun

certaint

iesfo

r87

Rb/

86Sran

d14

7Sm

/14

4Nd

wer

e2%

and

1.5%

,res

pectiv

ely

(1).

a Ini

tial

Sran

dNd

isot

opera

tios

areca

lcul

ated

cons

ider

ing

anag

eof

34M

afo

rth

eM

uec

ase

quen

ce;3

6.5

Mafo

rth

eGol

etasu

ite;

34M

afo

rth

eTilza

potlaro

cks;

and

30M

afo

rth

eOax

acagr

oup.

bIsot

opeda

taof

theTilza

potlasu

itear

etake

nfrom

Mor

n-Z

enteno

etal

.(20

04).

Sam

ples

wer

eco

llec

ted

onth

esa

meou

tcro

psan

dar

eco

nsid

ered

tobe

equi

valent

toth

oselabe

lled

unde

rM

Z20

04.

c Iso

tope

data

ofth

eOax

acavo

lcan

icsu

itefrom

Mar

tiny

etal

.(20

00)an

dM

artiny

(200

8).

Dow

nloa

ded

by [U

NA

M C

iuda

d U

nive

rsita

ria] a

t 12:

45 0

7 Ja

nuar

y 20

13

1486 L. Mori et al.

Figure 4. Major element features of the studied rock suites (major element data of the Oaxaca and Goleta groups are taken from Martinyet al. 2000 and Daz-Bravo and Morn-Zenteno 2011, respectively). (A) Total alkali versus SiO2 diagram (Le Bas et al. 1986); (B) K2Oversus SiO2 discrimination diagram (Le Maitre et al. 1989); (C) CaO and (D) Al2O3 versus SiO2 variation diagrams. Abundances ofoxides are normalized to 100% volatile-free.

the strongest REE and HREE fractionations [(La/Yb)N =10.725.1; (Sm/Yb)N = 3.35.2]. The Tilzapotla andMueca suites present similar characteristics in multiele-ment diagrams, even though the former generally dis-plays slightly lower concentrations of incompatible traceelements (Figures 5B and 5C). Both rock groups havehigher abundances of large-ion lithophile elements than theOaxaca sequence at similar high eld strength element con-tents, as well as less-depleted HREE patterns. Andesiticsamples from the two suites exhibit positive Sr spikes sim-ilar to those observed in the Oaxaca rocks, whereas themore silicic products are depleted in Sr. REE ratios of theTilzapotla and Mueca volcanic products are lower thanthose of Oaxaca rocks, and vary within a more restrictedrange [(La/Yb)N = 8.714.1 and 9.012.9; (Sm/Yb)N =1.93.3 and 2.23.0, respectively]. Goleta ignimbrites arecharacterized by distinctive negative anomalies of Ba, Sr,and Ti in multielement diagrams (Figure 5D); REE pat-terns display seagull shapes (see, e.g. Glazner et al.2008), with fractionated LREE, at but enriched HREE,and pronounced negative Eu anomalies (Eu/Eu).

Relationships between Sr and Nd isotopes of thestudied rock suites and potential sources (age-corrected;see Table 2) are shown in Figure 6. The isotopic composi-tions of the Tilzapotla group display a negative hyperbolic

trend bracketed between a mantle-like end-member withhigh 143Nd/144Nd and low 87Sr/86Sr ratios, and a radio-genic crustal component. Despite their more mac char-acter, volcanic rocks from the Oaxaca area display lowerNd isotopes and slightly higher 87Sr/86Sr ratios than thoseof Tilzapotla andesites. Rhyolites from the Goleta volcaniccentre exhibit the highest Sr and lowest Nd isotopic compo-sitions, whereas the isotopic compositions of the Muecarocks are intermediate between those of the Oaxaca andGoleta groups.

Discussion

Petrogenesis

In this section, we identify the geologic components andprocesses that governed the geochemical features of theOaxaca, Tilzapotla, Mueca, and Goleta volcanic succes-sions, and constrain the origin of the different suites.

High-pressure fractional crystallization and crustalcontamination in the Oaxaca suite

Major element variations within the Oaxaca volcanic suc-cession are consistent with an origin of these magmas by

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Figure 5. Normalmid-ocean ridge basalt (N-MORB)-normalized trace element patterns of the studied rocks (normalization values afterSun and McDonough 1989). REE patterns shown in the insets are chondrite-normalized (McDonough and Sun 1995). (A) Oaxaca volcanicsequence (data from Martiny et al. 2000 and Martiny 2008); (B) Tilzapotla group; (C) Mueca rocks; (D) Goleta suite.

Figure 6. Nd versus Sr isotope variation diagram for thestudied rock suites and potential end-members (age-corrected;see Table 2). Also shown are the data eld of erupted macsequences of the Guerrero terrane (Gt; Centeno-Garca et al.1993); Grenvillian Oaxacan complex (Ruiz et al. 1988a, 1988b),granulitic xenoliths hosted in the Goleta pyroclastic dikes (Elas-Herrera 2004), xenoliths of the Tejupilco Schist (Martnez-Serrano et al. 2004), and representative granitic plutons of theSMS (Schaaf 1990).

fractional crystallization of a parental basalt composition(Figure 4). The low-pressure mineral assemblage observedin rock samples (i.e. olivine, pyroxene, plagioclase, andoxides) might in principle explain the coherent decreasein MgO, CaO, and Fe2O3tot contents with ongoing dif-ferentiation in this suite. Nevertheless, increasing La/Yb

and Sm/Yb ratios with increasing silica contents in theOaxaca group (Figures 7A and 7B) are inconsistent withthis hypothesis, because those phases have very low parti-tion coefcients for the REE, and thus exert a negligibleeffect on the REE ratios of residual liquids (Hart andDunn 1993; Dunn and Senn 1994). High LREE/HREEratios in the derivative magmas could be produced byhornblende fractionation (Castillo et al. 1999); but this pro-cess would also cause a decrease in Sm/Yb ratios, sinceSm is more compatible than Yb in amphibole (Bottazziet al. 1999). Instead, high La/Yb and Sm/Yb ratios cou-pled with marked HREE depletions are reliable indicatorsof garnet, which may be present in the residual parage-nesis of the source, or in the fractionating assemblage(Macpherson et al. 2006; Mori et al. 2009). In partic-ular, garnet-controlled REE ratios in subduction-relatedmagmas may reect partial melting of the eclogite-faciessubducted basalt (Defant and Drummond 1990); anatexisof mac underplates at the base of a thick arc crust(Atherton and Petford 1993); or partial melting of a deepgarnet-bearing mantle (Ulmer 1989). Alternatively, it hasbeen proposed that arc magmas may acquire the garnetsignature during high-pressure fractional crystallization ofmantle-derived basalts (Macpherson et al. 2006).

The mac-intermediate character of the Oaxaca vol-canic suite precludes derivation from partial meltingof basaltic lithologies, because this process essentially

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Figure 7. Fractional crystallization, contamination, and anatexis in the studied rock sequences (chemical data of the Oaxaca suite fromMartiny et al. 2000 and Martiny 2008; major element data of Goleta rocks from Daz-Bravo and Morn-Zenteno 2011). (A) La/Ybversus SiO2; (B) Sm/Yb versus La/Yb; (C) Eu anomalies [Eu/Eu=EuN/(SmN1/2GdN1/2)] versus SiO2; (D) Nd isotopes versus SiO2;(E) Nd isotopes versus Rb/Nb ratios. SiO2 abundances are normalized to 100% volatile-free; isotope ratios are age-corrected. The positivecorrelation between La/Yb (and Sm/Yb) with silica is indicative of high-pressure fractional crystallization (high-P FC) of garnet (Gt)within the Oaxaca suite; on the other hand, constant REE ratios with differentiation in the Mueca and Tilzapotla groups, coupled withprogressively decreasing Eu/Eu, reect low-pressure fractional crystallization (low-P FC) of mineral assemblages including plagioclase(Pl). Negative correlations between 143Nd/144Nd and SiO2 show that fractional crystallization in the Oaxaca, Tilzapotla, and Muecasuites occurred simultaneously with crustal contamination. The Oaxaca suite assimilated Rb-depleted lower crustal materials, whereasthe Mueca group assimilated Tejupilco Schist lithologies. Differentiation in the Tilzapotla sequence was accompanied by contaminationwith Rb-enriched materials. Marked negative Eu/Eu in the Goleta group, coupled with high and variable Rb/Nd ratios at constant Ndisotopes, indicate preferential melting of feldspar- and biotite-bearing rocks. Also shown are the data elds of mac amphibolites andmetapelites from the Acatln complex (Ortega-Obregn et al. 2009), erupted mac sequences of the Guerrero terrane (Gt; Centeno-Garca et al. 1993; Mendoza and Suastegui 2000); Grenvillian Oaxacan complex (Ruiz et al. 1988a, 1988b), representative granites of theSMS (Schaaf 1990), granulitic xenoliths hosted in the Goleta pyroclastic dikes (Elas-Herrera 2004), and xenoliths of the Tejupilco Schist(Martnez-Serrano et al. 2004).

produces magmas with dacitic to rhyolitic compositions(Sen and Dunn 1994; Rapp and Watson 1995). At thesame time, even though the geochemical features of theOaxaca rocks are consistent with a mantle origin, theirfractionated REE patterns could not be produced by dif-ferent degrees of fusion of a garnet peridotite, becausethis process would generate partial melts with variableLa/Yb and Gd/Yb ratios at almost constant silica con-tents (or variable within the range of basaltic composi-tions; Johnson 1994). In contrast, the REE ratios of theOaxaca group show a prominent increase during differen-tiation from mac to intermediate compositions, demon-strating that the garnet signature was governed by a pro-cess of fractional crystallization (Mntener et al. 2001;Alonso-Perez et al. 2009).

Garnet fractionation from mantle-derived melts hasbeen recently recognized as an important process in the

differentiation of subduction-related magmas (see, e.g.Macpherson et al. 2006; Mori et al. 2009). Experimentalstudies support this idea, indicating that garnet is a com-mon igneous phase in typical arc-like calc-alkaline deriva-tive liquids at 0.81.2 GPa, corresponding to deep crustalconditions (depth range of 2540 km; Mntener et al.2001; Alonso-Perez et al. 2009). Based on these con-siderations, we conclude that the magmatic evolution ofthe Oaxaca volcanic suite was controlled by high-pressurefractionation of garnet from a mantle-derived basaltic pre-cursor.

A negative correlation between silica contents (as wellas La/Yb and Sm/Yb ratios) and Nd isotope ratios indi-cates that magmatic differentiation in the Oaxaca volcanicsuite was accompanied by the assimilation of isotopicallyenriched materials (Figure 7D). A diagram of 143Nd/144Ndversus Rb/Nd ratios is used to constrain the nature of

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the crustal component involved in the petrogenesis of theOaxaca group (Figure 7E). Possible contaminants includethe Palaeozoic sequences of the Acatln complex andthe granulite-facies lithologies of the Grenvillian Oaxacancomplex, which represent the local basement beneath theOaxaca volcanic eld. Assimilation of mac amphibo-lites from the Acatln complex, which display similarNd isotope compositions but higher Rb/Nd ratios thanthose of Oaxaca rocks (Ortega-Obregn et al. 2009), canbe ruled out because it would produce a nearly paral-lel trend extending to higher Rb/Nd at almost constant143Nd/144Nd. In contrast, the Oaxaca samples exhibit anegative correlation, and trend towards a crustal compo-nent with low Rb/Nd and Nd isotope ratios (Figure 7E).Available geochemical data for basement rocks point togranulite-facies lithologies akin to the Oaxacan complex(Ruiz et al. 1988a, 1988b) or the high-grade metasedimen-tary sequences of the Acatln complex (Ortega-Obregnet al. 2009) as possible contaminants. Discriminatingbetween these two sources on pure geochemical grounds isa difcult task, because they would exert a similar effecton the composition of the studied magmatic sequences.Nevertheless, considering that crustal assimilation in theOaxaca suite should have occurred at lower crustal levelsin order to allow simultaneous fractionation of garnet fromthe hydrous parental basalt (2540 km; Alonso-Perezet al. 2009), the granulite-facies lithologies of the Oaxacancomplex appear to be a more appropriate contaminant rela-tive to the shallower metapelitic successions of the Acatlncomplex.

In summary, we consider that the Oaxaca volcanic suiteoriginated from mantle-derived parental basalts, whichintruded the base of the continental crust and underwentdeep fractional crystallization, together with assimilationof Rb-depleted lower crustal lithologies. The fact thatthe most mac products of the Oaxaca volcanic eld arebasaltic andesites with enriched isotope compositions sug-gests that all the volume of primitive parental magmasunderwent differentiation and contamination at depth. Yet,it is also possible that the low 143Nd/144Nd ratios in themost mac samples reect derivation from an isotopicallyenriched lithospheric mantle source.

Low-pressure fractional crystallization and crustalcontamination in the Tilzapotla and Mueca volcaniccentres

As previously described, andesites from the Tilzapotla vol-canic eld display the highest Nd isotope ratios among thestudied sequences (Figure 6). In this sense, their originmay be related to partial melting of a depleted peridotiteand subsequent differentiation of mantle-derived parentalbasalts, with negligible crustal contributions.

The coherent variation from andesitic to rhyoliticcompositions in the Tilzapotla volcanic suite suggeststhat these magmas evolved through a process of frac-tional crystallization. Almost constant La/Yb and Sm/Ybratios in this group (Figures 7A and 7B) indicate thatmagmatic differentiation did not involve the crystallizationof pressure-sensitive minerals with high partition coef-cients for the middle REE (MREE) and HREE, as observedin the Oaxaca suite. Instead, the coherent decrease of MgO,CaO, Al2O3, and Sr concentrations with increasing sil-ica contents (Figures 4C, 4D, and 5B), as well as thenegative correlation between Eu/Eu and the differentia-tion index (Figure 7C), support that the evolution of theTilzapotla suite was dominated by low-pressure fractionalcrystallization of pyroxene and plagioclase assemblages.

Decreasing Nd isotope ratios with increasing SiO2 con-tents in the Tilzapotla group (Figure 7D) indicate thatthese magmas also experienced crustal contamination dur-ing differentiation from andesites to rhyolites. Specically,the negative correlation between 143Nd/144Nd and Rb/Ndratios (Figure 7E) suggests assimilation of Rb-enrichedcrustal lithologies with unradiogenic Nd isotope composi-tions. The nature of the contaminant is difcult to dene,but could be represented by the middle crustal granulite-facies rocks found as xenoliths within the magmatic prod-ucts of the neighbouring Goleta centre (Elas-Herrera2004); or by felsic plutons intruded at middleupper crustallevels that may either belong to the SMS igneous province(e.g. Schaaf 1990), or have formed in previous times inrelation with the magmatic activity of the Guerrero terrane(Elas-Herrera et al. 2000).

In summary, we consider that the andesitic samples ofthe Tilzapotla volcanic suite were derived from simple frac-tional crystallization of mantle-derived basalts. These mag-mas evolved through low-pressure fractional crystallizationof plagioclase-dominated assemblages coupled with theassimilation of Rb-enriched middleupper crustal rocks,producing the dacitic and rhyolitic compositions.

Major and trace element variations within the Muecavolcanic succession are very similar to those observedand described in the Tilzapotla suite (Figures 4 and 5),and are consistent with a derivation of these magmas bylow-pressure fractional crystallization of gabbroic mineralassociations. Nevertheless, the isotopic compositions of theMueca samples are different from those of the Tilzapotlagroup when compared at similar silica contents, and alsoexhibit different trends, which reveal a distinct petrogenetichistory (Figure 7D).

First of all, the Nd isotope ratios of Mueca andesitesare lower than those of Tilzapotla rocks, and more similarto those of the Oaxaca suite. If the volcanic products ofthe Mueca and Tilzapotla eruptive centres were derivedfrom the same isotopically depleted mantle source, thenthis geochemical feature would indicate that the parental

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magmas of the Mueca succession suffered contamina-tion since the rst stages of differentiation (Figure 7D).Alternatively, as it has been previously proposed for theOaxaca suite, the mac precursor of the Mueca volcanicsuccession may have derived from an isotopically enrichedlithospheric mantle.

As recognized in the Tilzapotla sequence, crustal con-tamination in the Mueca group is documented by anisotopic shift to lower 143Nd/144Nd ratios that accom-panies the evolution from andesitic to dacitic magmas(Figure 7D). Possible contaminants include the TejupilcoSchist and the granulite-facies rocks recovered as xeno-liths within the adjacent volcanic centres, which representthe local basement beneath the Mueca volcanic eld. Thegeochemical characteristics of the Tejupilco Schist, in par-ticular its depleted Nd isotope compositions and relativelylow Rb/Nd ratios (Martnez-Serrano et al. 2004), suggestthat this lithology likely represents the assimilated material(Figure 7E).

On the other hand, the dacitic and rhyolitic samplesdisplay constant Nd isotope compositions (Figure 7D),which suggest that the advanced stages of magmatic evo-lution in the Mueca volcanic centre were dominated bysimple fractional crystallization, without further contami-nation. This may be explained by assuming that the lastphase of differentiation took place in a shallower magmaticreservoir, and considering that assimilation at uppermostcrustal levels becomes thermally ineffective (Schnurr et al.2007).

In conclusion, we consider that the Mueca andesiteswere derived from fractional crystallization and crustalcontamination of mantle-derived parental basalts. Low-pressure fractional crystallization of pyroxene and plagio-clase, coupled with the assimilation of the Tejupilco Schistlithologies at middleupper crustal levels, was responsiblefor the generation of andesitic and dacitic magmas withprogressively lower 143Nd/144Nd ratios. A last stage of dif-ferentiation in a shallower magma chamber likely producedthe most siliceous compositions through simple fractionalcrystallization.

Rhyolitic volcanism by crustal anatexis in the Goletavolcanic eld

The geochemical features of the Goleta volcanicsequence may in principle suggest that these rocksformed by fractional crystallization of mac-intermediatecompositions analogous to those of the neighbouringvolcanic elds (Figure 4). Nevertheless, the Goleta ign-imbrites display more pronounced Ba depletions andnegative Eu/Eu than Mueca and Tilzapotla rocks atsimilar silica contents, and therefore do not plot along theliquid line of descent dened by these suites (Figure 7C).Furthermore, and as pointed out by many studies (e.g.

Sisson et al. 2005), extreme degrees of fractional crys-tallization would be required to yield high-silica rhyoliticliquids from a basaltic precursor, and in any case the inter-mediate compositions should be volumetrically relevant.In contrast, the whole magmatic column at the Goleta vol-canic eld is constituted by high-silica rhyolites. Based onthese observations, we consider that the Goleta successionsare most likely derived by a process of crustal anatexis, andthat mantle-derived magmas contributed with heat ratherthan mass transfer to their generation. In this sense, theisotopic and elemental compositions of these siliceoussequences should provide insights into the chemical andmineralogical characteristics of their source rock.

A favourable scenario for the generation of silicicmagmas at subduction settings envisages partial melt-ing of a mac lower crust composed of underplatedarc basalts and cumulates (Petford and Gallagher 2001;Sisson et al. 2005). Indeed, juvenile hydrous materials s

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