petrology of crustal xenoliths, puente negro intrusion · disgregadas en la andesita. asociaciones...

Revista Mexicana de Ciencias Geológicas

ISSN: 1026-8774

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Universidad Nacional Autónoma de México

México

Ortega-Gutiérrez, Fernando; Martiny, Barbara M.; Morán-Zenteno, Dante J.; Reyes-Salas, A.

Margarita; Solé-Viñas, Jesús

Petrology of very high temperature crustal xenoliths in the Puente Negro intrusion: a sapphire-spinel-

bearing Oligocene andesite, Mixteco terrane, southern Mexico

Revista Mexicana de Ciencias Geológicas, vol. 28, núm. 3, diciembre, 2011, pp. 593-629

Universidad Nacional Autónoma de México

Querétaro, México

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Petrology of crustal xenoliths, Puente Negro intrusion 593

PetrologyofveryhightemperaturecrustalxenolithsinthePuenteNegrointrusion:asapphire-spinel-bearingOligoceneandesite,

Mixtecoterrane,southernMexico

Fernando Ortega-Gutiérrez*, Barbara M. Martiny, Dante J. Morán-Zenteno, A. Margarita Reyes-Salas, and Jesús Solé-Viñas

Instituto de Geología, Universidad Nacional Autónoma de MéxicoCiudad Universitaria, C.P. 04510, Mexico City, Mexico.

* [email protected]

ABSTRACT

This study presents petrologic, chemical, geochronological and isotopic data, as well as petrogenetic interpretations about a unique subvolcanic locality in southern Mexico that contains deep-seated xenoliths and xenocrysts (igneous and metamorphic), albeit affected by extreme pyrometamorphism during rapid ascent in a composite andesitic dike. The intrusion has a K-Ar age of 29.2 ± 0.3 Ma on volcanic matrix and 30.5 ± 0.6 Ma on hornblende xenocrysts,and it is part of an arc-related regional magmatic event in southern Mexico. This magma at Puente Negro intruded quartzo-feldspathic gneisses and micaceous schists of the Paleozoic Acatlán Complex. Xenoliths consist of high-grade garnet-bearing gneisses, aluminous metapelites, impure quartzites, and abundant hornblende-rich gabbroic rocks. Garnet, corundum (including purple-blue sapphire), spinel, and aluminous orthopyroxene constitute the main types of deep-seated xenocrysts derived from disaggregation of metamorphic rocks in the andesite. Low pressure assemblages with tridymite, spinel, Al-silicates (mullite and sillimanite), two pyroxenes, Fe-Ti oxides, and high-silica anatectic glasses indicate peak temperatures of pyrometamorphism above 1100 ºC. Decompression coronas of spinel-plagioclase-orthopyroxene ± corundum ± glass about polyphase garnet porphyroblasts in plagioclase-orthopyroxene-spinel restitic gneisses, Al-rich orthopyroxene coring Al-poor orthopyroxene xenocrysts, spinel-plagioclase-corundum xenocrystic pseudomorphs probably after garnet, and local preservation of orthopyroxene-sillimanite and garnet-hypersthene-spinel-quartz assemblages strongly support interaction of original basaltic magmas with the lower crust. Aluminum in orthopyroxene (up to 11.6 Al2O3 wt. %) coexisting with spinel, ilmenite-magnetite pairs, and Fe/Mg partitioning between orthopyroxene and spinel in garnet coronas yield decompression metamorphic temperatures around 990 ºC, whereas coexisting hornblende-plagioclase and two pyroxenes in gabbroic xenoliths yield magmatic temperatures of 800 to 950 ºC. The first basaltic hydrous magma represented by the gabbroic xenoliths differentiated in a magmatic chamber in the middle crust at 4–6 kbar based on Al-in-hornblende barometry. The subsequent injection of this partially to totally crystallized magma chamber by a new basaltic batch apparently caused disaggregation of the hornblende-rich rocks and transported the xenolith-xenocryst load to the surface. Based on major and trace elements and Sr, Pb, and Nd isotope data, we conclude that the Puente Negro “andesite” was the end product of a mantle-derived, relatively long-lived plumbing system of original basaltic composition that in the early Oligocene (29–30

Revista Mexicana de Ciencias Geológicas, v. 28, núm. 3, 2011, p. 593-629

Ortega-Gutiérrez,F.,Martiny,B.M.,Morán-Zenteno,D.J.,Reyes-Salas,A.M.,Solé-Viñas,J.,2011,PetrologyofveryhightemperaturecrustalxenolithsinthePuenteNegrointrusion:asapphire-spinel-bearingOligoceneandesite,Mixtecoterrane,southernMexico:RevistaMexicanadeCienciasGeológicas,v.28,núm.3,p.593-629.

Ortega-Gutiérrez et al.594

Ma) interacted with the continental crust at lower, middle and shallow levels, which are represented, respectively, by xenoliths of granulite facies quartzites and metapelites, mafic and ultramafic gabbroic rocks, and sanidinite facies quartzo-feldspathic buchites.

Key words: crustal xenoliths, pyrometamorphism, sapphire xenocrysts, arc basalt, Mixteco terrane, southern Mexico.

RESUMEN

Este estudio presenta datos petrológicos, químicos, geocronológicos e isotópicos, así como interpretaciones petrogenéticas sobre una localidad volcánica única en el sur de México que contiene xenolitos y xenocristales ígneos y metamórficos de origen profundo, afectados por pirometamorfismo extremo durante su ascenso rápido en un dique andesítico compuesto. La intrusión tiene una edad de de 29.2± 0.3 Ma (matriz volcánica) y de 30.5±0.6 Ma (xenocristales de hornblenda), y es parte de un evento magmático regional en el sur de México que, localmente en Puente Negro, intrusionó gneisses cuarzo-feldespáticos y esquistos micáceos del Complejo Acatlán paleozoico. Los xenolitos consisten en gneisses granatíferos, metapelitas aluminosas, cuarcitas impuras y abundantes rocas gabroicas ricas en hornblenda. Granate, corindón (incluyendo safiro azul-púrpura), espinela y ortopiroxena aluminosa constituyen los tipos principales de xenocristales profundos procedentes de rocas metamórficas disgregadas en la andesita. Asociaciones de presión baja con tridimita, espinela, mullita, silimanita, clinopiroxena y ortopiroxena, óxidos de Fe y Ti y vidrios anatexíticos altos en sílice indican temperaturas pico en el campo del pirometamorfismo superiores a 1100 ºC. Las coronas de descompresión alrededor de granates polimetamórficos formadas por espinela-plagioclasa-ortopiroxena ± corindón ± vidrio en gneisses restíticos de plagioclasa-ortopiroxena-espinela, ortopiroxena aluminosa en los centros de xenocristales de ortopiroxena pobre en aluminio, pseudomorfos de espinela-plagioclasa-corindón probablemente reemplazando granate, y la preservación local de asociaciones de ortopiroxena-silimanita y granate-hiperstena-espinela-cuarzo, claramente apoyan la interacción de un magma basáltico original con la corteza inferior. El contenido de aluminio en ortopiroxena (hasta 11.6 % Al2O3), que coexiste con espinela, pares de ilmenita-magnetita y la distribución Fe/Mg entre ortopiroxena y espinela en las coronas alrededor de granate, indican temperaturas metamórficas durante la descompresión cercanas a los 990 ºC, mientras que pares coexistentes de hornblenda-plagioclasa o de dos piroxenas en los xenolitos gabroicos, proporcionan temperaturas de cristalización de 800 a 950 ºC. Este primer magma basáltico hidratado, representado por los xenolitos gabroicos, se diferenció en la corteza media a presiones de 4–6 kbar, calculadas por barometría del contenido de aluminio en las anfíbolas. La inyección subsecuente a través de esta cámara magmática parcial o totalmente cristalizada de nuevos magmas basálticos, aparentemente causó la desintegración de sus rocas ricas en hornblenda, emplazándose muy cerca de la superficie con su carga de xenolitos y fundiendo parcialmente los gneisses encajonantes. Con estas bases y datos isotópicos (Sr, Pb y Nd) y geoquímicos adicionales, concluimos que la “andesita” Puente Negro fue el producto final de un sistema magmático de larga duración relativa y composición basáltica derivado del manto, el cual, durante el Oligoceno temprano (30–29 Ma), interactuó con niveles inferiores, intermedios y someros de la corteza continental representados respectivamente por xenolitos de metapelitas y cuarcitas en facies de granulita, rocas gabróicas máficas y ultramáficas y buchitas cuarzofeldespáticas en facies de sanidinita.

Palabras clave: xenolitos corticales, pirometamorfismo, xenocristales de safiro, basalto de arco, terreno Mixteco, sur de México.

INTRODUCTION

Thestudyofdeep-seatedxenolithsinvolcanicrocksprovidesdirectinformationaboutthenatureandprocessesatdepth that remain otherwise difficult or impossible to sample andunderstand(RudnickandFountain,1995).Inparticular,xenolithsconstitutesamplesfromtheburiedcrustanduppermantle,whichinmanyimportantwaysconstrainthetectonichistoryandcrustalgenesisatagivenarea(Hallidayet al.,1993;Lee,et al.,2001;Ghentet al.,2008;Rayet al.,

2008),andalsoilluminatefundamentalaspectsofmagmaticprocessessuchasascentratesandthermalinteractionsofthemagmawiththetraversedcrust(e.g.,Beardet al.,2005).Moreover,whilstdeep-seatedxenolithsinintraplate-typevolcanicsofcentralandnorthernMexicohavebeenusedsuccessfullytotracethenatureofthelocallowercrustanduppermantle(Ruizet al.,1988;Hayobet al.,1989;Luhret al.,1989;RudnickandCameron,1991;Cameronet al.,1992;Schaafet al.,1994;LuhrandAranda-Gómez,1997),withfewexceptions(e.g.,BlatterandCarmichael,1998;


IzNt

Trans-Mexican BeltVolcanicGuadalajara

Guerreroterrane

Mexico City

Oaxaca

Mixtecoterrane

Chatino terrane

Cuicatecoterrane

200 km

Pacific Ocean

105°W 100°W

PnCha

Ma

Gulf

ofM

exico

Veracruz

Zap

oteco

terraneAcapulco

20°N

95°W

15°N

Sm

Maya terrane

Eastern sectorWestern sector

Colima

Trench

Vs Am

PeEp

Central sector

MP

Al

North America Plate

Po

Acapulco

a)

18° 15’

98° 20’ 98° 00’

Mesozoic cover Eclogitic rocks

Granitoids Quartz phyllites and greenstones

18° 00’

AcatlánPiaxtla

10 km

Puebla

Oaxaca

Tecomatlán

PuenteNegro

Las PeñasEl Papayo

b)

P

A H

TECTONOSTRATIGRAPHIC SETTING

PuenteNegroisasubvolcanicintrusionemplacedinrocksofthePaleozoicAcatlánComplex,basementoftheMixtecoterrane,stateofPueblainsouthernMexico(Figure1a),whichisoneof themostextensivepre-MesozoicmetamorphiccomplexesexposedintheforearcregionoftheTrans-MexicanVolcanicBelt(CampaandConey1983;Sedlocket al.,1993).Theoldestbasementofsouth-ernMexicoincludestheGrenvillianOaxacanComplex(Zapotecoterrane)juxtaposedagainsttheAcatlánComplexalongtheCaltepecPermianfaultexposed75kmtotheeastofPuenteNegro(Elías-HerreraandOrtega-Gutiérrez,2002).ThewesternlimitoftheAcatlánComplexismarkedbytheLaramidicPapalutlathrustagainst theMorelosCretaceouscarbonateplatform(Guerreroterrane), thebasementofwhichisnotexposed.ThehostrocksofthePuenteNegrointrusion(Figure1b,2a)consistofretrogradequartzo-feldspathicgneisses(EsperanzaGranitoids)withpolyphasemetamorphicfabricsofPaleozoicagecomposedofwhitemica,chlorite,quartz,andepidote,withamphibole,epidote,garnet,andalbiteascommonporphyroclasts.The

Aguirre-Diazet al.,2002)thishasnotbeenpossibleinsouthernandcentralMexicoessentiallybecauseofthelackofvolcanicrocksbearingdeepcrustalxenoliths.Thus,theratheruniquenatureofthePuenteNegroshallowintrusionanditsxenolithslocatedinsouthernMexicoconstitutesaninvaluableopportunityforassessingthelithotectonicstructureofthecrustunderlyingthepolyphasemetamorphicAcatlánComplexintheforearcregionoftheTrans-MexicanVolcanicBelt,andalsoprovidesextraordinaryinformationaboutthethermalandpetrologicevolutionofbasaltic-andesiticorogenicmagmasastheytraversetheentirecrust.Thus, the main goals of this work are a) to present the first detailedpetrographic,mineralogical,andpetrogeneticstudyofboththeandesitichostrockanditsdiverseloadofxenolithsandxenocrysts,andb)todiscussimportantphysicalaspectsofthemagma-xenolithsystemevolution.Inparticular,thepresenceofcorundum/sapphire,greenspinel,andaluminousorthopyroxenexenocrystsintheandesiticrock, togetherwiththepolyphase,veryhightemperaturesdocumentedinthemetamorphicxenoliths,provideintriguinginsightsintotheP-ThistoryofPuenteNegromagmas.

Figure1.(a)Tectonostratigraphiclocation.LocationofthePuenteNegro(Pn) xenolith locality (large filled rhomb) in relation to the Transmexican Volcanic Belt, and localities where granulite or mantle xenoliths (small filled rhombs) have been reported in the volcanic arc (see Ortega-Gutierrez et al.,2008).ValledeSantiago(Vs),Amealco(Am),ElPeñon(Ep),Pepechuca(Pe),NevadodeToluca(Nt),Chalcatzingo(Cha),Iztaccihuatl(Iz),Malinche(Ma),PicodeOrizaba(Po),AltoLucero(Al),SanMartinTuxtla(Sm),LosHumeroscaldera(H),LosAzufrescaldera(A),LaPrimaveracaldera(P).Majorstratovolcanoesareshownbywhiteasterisks,andlargestcalderasbydoublecircles.(b)GeneralgeologicsettingofthePuenteNegrointrusionarea.


100 m

Xayacatlán Formation

Esperanza Granitoidsand screen

Cosoltepec FormationPuente Negroandesite

Alluvial deposits

Intermittent stream

Village

b)

El Papayo

Puente Negrobridge

Na)

18°13'45"

18°14'00"

18°13'45"98°11'00"

0 200 m

unexposedrocksintrudedbytheandesiticdike(Figure2b)probably consist of retrograde eclogitic mafic and ultra-mafic rocks of the Xayacatlán Formation, and low-grade quartzosephyllitesoftheCosoltepecFormation,whichformelsewherethemainpartoftheexposedAcatlánComplex(Ortega-Gutiérrez,1978).

Puente Negro intrusion

The igneous body is exposed at 18˚13.74’ north lati-tude, and 98˚ 10.98’ west longitude, between the towns of ElPapayoandLasPeñas(Figures1band2a).Theintrusion(Figures2band3a)apparentlyrepresentsasplayedandtaperedshallowdikecomplexterminationwithanovaloutcropexpression,andamaximumapparenttotalwidthofabout160meters.Itiscomposedofatleastthreeseparatebodies(Figure2b),twoofwhichcontainabundantxeno-lithsandxenocrystsshowingthroughoutasteepigneousfoliationtrendingNWtoWNW.Adiscontinuousscreenabout3mwideofglassyrocksinterpretedaspartiallymeltedcountrygneiss(buchite)islocatedbetweenthetwomaindikes.

Althoughhigh-grademetamorphicxenoliths(Figure3bto3f)andquartz,garnet,andamphibolexenocrystsarecommon,igneousgabbroicandquartziticgneissespredominateinthexenolithpopulation.Thegabbrosarehighlyporphyritic,richinamphibole,andattainamaximumsizeofabout10cm,whereasthelargestquartzosexenolithfoundisaroundedblock40×30cmacross.Manyofthequartzosexenolithsshowmantlesornarrowveinsofglassand rare ultramafic xenoliths (hornblende orthopyroxenites) attainsizesuptoafewcentimeters.Amphibolemegacrystsinthegabbroicxenolithsmaybeseveralcentimeterslong,whereasthelargestgarnetxenocrystfoundwasaboutonecentimeterindiameter.

Themaximumdepthfortheintrusivebodyisesti-matedtobeafewhundredmeters(afewtensofbars),asdeducedfromtheaphanitictextureoftheigneousmatrixandperfectpreservationofglassinbothandesiteandxenoliths.Similardikes,sillsandplugswithxenolithsandgarnetxenocrystsintrudingtheAcatlánComplexarecommonelsewhere(e.g.,LasMinaspluglocatedabout40kmNNWofPuenteNegro,andReyesMetzontla,65kmtotheeastofPuenteNegro),butnoneisasdiverseorwithxenolithsaslargeasthoseatPuenteNegro.

Figure2.a)Localgeology,andb)schematiccrosssectionofthePuenteNegrodikecomplexintrudedintotheAcatlánComplex


Figure3.FieldaspectsofPuenteNegrointrusionanditsxenoliths.a)PuenteNegromainoutcrop.b)Rheomorphicxenolithentrainedintheandesiticmagma;coinsinb)ande)are3cmacross.c)Quartzo-feldspathicbandedgneiss;peninc)andd)is13cmlong.d)Calcsilicatexenolith.e)Quartzitexenolith.f)Closeupoftheequigranular(granulitic)textureofametapeliticxenolith;theimageisabout2cmacross.


PETROGRAPHY OF INTRUSION AND XENOLITHS

BecausethePuenteNegroxenolithlocalityisde-scribed for the first time and both, host andesite and its xeno-lithsshowaverycomplexmineralogyandunusualtexturalfeatures,thefollowingdetailedpetrographicdescriptionsandpetrologicdiscussionoftherocksandmineralsareconsideredessentialtothisandfuturepetrogeneticstudiesofthelocality.Apetrographicsummarybasedonthestudyofover70thinsectionsofthePuenteNegroandesiteandxe-nolithsisshowninTable1.Sixdomainsweredistinguishedintheandesite:aphaniticvolcanicmatrix,phenocrysts,“glomerocrysts”,xenoliths(gabbroicandmetamorphic),xenocrysts,andxenocrysticpolycrystallinepseudomorphs,allofwhichprovideimportantpetrogeneticinformationontheconditionsthatcontrolledtheevolutionofthemagma-xenolithsystemen routetothesurface.

Thedikesinoutcropshowporphyriticandaphanitictextures,butinthinsectionthedominantmicrotextureisdefinedbypilotaxiticplagioclase(2–200µm)setinafine-grainedmatrixrichinFe-Tioxides(1–20µm),orthopyroxene,andminorclinopyroxene;often,abundantbrownglassisalsopresent(Figure4a).Thematrixofarepresentativeglass-richsample(PN21)isformedbyPl

(63%),glass(20%),Opx(15%),TiMag(titanomagnetite)(2%),andCpx(<1%).

Abbreviations forminerals in thisworkareaf-terWhitneyandEvans(2010),exceptwhenindicated.Note thatmodalpercentageswerevisuallyestimated.Microphenocrysts(250–400µm)andphenocrysts(>400µm)areplagioclase,orthopyroxene,andsomeclinopy-roxene. Amphibole is the main mafic phase in the andesite (Figure4b),butoccursinlargetosmallcrystalsthatatfirst sight may be considered phenocrysts and therefore cogeneticwiththeandesite.However,theintensealtera-tiontoanhydrousgabbroicassemblages(often100%),andstrongresemblancetoamphibolesinthegabbroicxenolithssuggestaxenocrysticorigin;itischaracterizedbysimpletwinning,magmaticembayment,andmultipleoscillatoryzoningmarkedbyalternatingbandsofbrowntocolorlessorgreenishbrowncolors.Orthopyroxeneoccursintheaphaniticmatrixwithopaqueoxides,plagioclasemicrolitesandglass,andasfaintlypleochroic(paleyellowtocolorlessphenocrystsupto2.2mmlong);afewcrys-talscontainabundantinclusionsofhercynite.Plagioclase“phenocrysts”(probablyxenocrysts)areupto1.3mm,sharplyzoned,andcommonlyshowsievetexture,aswellasinclusionsoforthopyroxene,opaquephases,greenspinel(Figure4c),andcorundum.Primaryclinopyroxenepresent

Sample Qz Trd Sa Pl Glass Spl Sil Mul Crn Opx Cpx Grt Hbl Bt Cal Rt Fe/Ti oxide

Ap Zrn Crd Rock type

PN21* X X X X AndesitePN0 X X X X X X X X QuartzitePN1 X X X X X X ? ? X X X X X X X ? QuartzitePN3 X X X X X HblgabbroPN4 X X X X X X X WebsteritePN6 X X X X X X X GrtgneissPN9 X X X X X ? X X X X X X GrtgneissPN10 X X X X X HblgabbroPN12 X X X X X HblgabbroPN14 X X X X X X X X X X QuartzitePN17 X X X X X X GrtgneissPN18 X X X X X X X X GrtgneissPN19 X X X X X QuartzitePN23 X X X X X X X X X X QuartzitePN24 X X X X X X X X ? BuchitePN27 X X X X X X X QuartzitePN29 X X X X X X X X X ? GrtgneissPN41 X X X X HblgabbroPN42 X X ? X X X X X X X CalcsilicateFO29698 X X X X X X X X X X X X ? QuartziteFO29998 X X X X X X X X X HblgabbroFO6605 X X X X X X X X X X BuchiteFO7505 X X X X X X QzgneissFO11679 X X X X X X X BuchiteFO21098 X X X X X X X Qzgneiss

Table1.PetrographicsummaryofthePuenteNegroandesite-xenolithsystem.


line euhedral grains rimming glass patches. Xenocrysts of orthopyroxenemayshowopaciticreactionrimsandductiledeformationrevealedbywavyextinctionandmechanicaltwinning.“Glomerocrysts”arecommoninmostthinsec-tionsstudied;theyconsistofallpossiblecombinations

in“glomerocrysts”mayshowilmeniteinclusionscoredbyrutile(PN22),whereassecondaryclinopyroxenerimmingorcompletelyreplacingquartzxenocrystsformsclustersupto1.2cminsize(Figure4d).Clinopyroxenealsoisfoundinthegabbroiccoronasofamphibole,andaspolycrystal-

Figure4.Photomicrographsof:a)abundantglass(Gls)intheandesitematrix,probablyderivedfrommeltingoftheadjacentgneissxenolith(planepolarizedlight,PN23a).b)Agroupofamphibolecrystals(Hbl)withsharpopacite(Op)rimssetinaphaniticvolcanicgroundmass(planepolarizedlight,FO375).c)Sharplyzonedplagioclasexenocrystwithabundantglassandspinel(Spl)inclusions(crossednicols,FO375).d)Totalreplacementofquartzxenocrystsbyclinopyroxene(planepolarizedlight,PN23a).e)Purplish-bluesapphirexenocrystorphenocryst(?)insidethematrixoftheandesite(planepolarized light, PN32). f) Sharply zoned orthopyroxene xenocryst displaying intense pleochroism (Al-rich) in the core (green along Z and brown along X), andcolorlessintheouterrims(Al-poor)(planepolarizedlight,PN6).g)ColorlesscorundumenclosedbycalcicplagioclaseinsidetheandesiticmatrixofPN21(planepolarizedlight).h)CorundumrimmedbyspinelintheandesiticmatrixofPN24(planepolarizedlight).


ofplagioclase,pyroxene,hornblende,andrarebiotite,thusranginginpetrographiccompositionfromgabbroic,anorthositicandpyroxenitictohornblenditic;thetexturalarrangementandeuhedralaspectofthecomposingmineralsindicateacumuliticorigin.Decompressionaffectedtheseinclusionsbecausemostshowreactioncoronasagainstthevolcanicmatrix.Rareclustersofplagioclase-apatite,apatite-Fe/Tioxide-zircon,andplagioclase-spinelwerealsoobserved.Whenpresent,glassisabundant(upto60%ofthethinsection),brownincolor,andusuallyassociatedwithquartzosexenolithsindicatinglocalanatecticderivation.

Wall-rock Paleozoic gneiss

ThePaleozoicgneissnexttotheintrusionremainedessentiallyunaffected,suggestingquickcoolingofthein-trusion,althoughmeter-sizedmyloniticraftsofthegneissinsidetheintrusion(FO11679,PN9,FO6605)weresub-stantially melted (≥ 30%) and pervasively reconstituted to theassemblageglass-Qz-Trd-Opx-Pl-TiMag-Hem-Rt-Zrn.Intheserocks,whichareinterpretedasbuchitesaccordingtothe definition of Grapes (2006, p. 2): “…glassy, pyrometa-morphicrockwithpartialtototalmelting”,feldsparacquiredamottledappearanceanditiscomposedofsymplectiticintergrowthsofalteredbrownglassandternaryfeldspar.Thesepseudomorphsandisolatedplagioclasegrainsarealwaysseparatedfromthequartz/tridymitegroundmassbyawiderimofcolorlessglass,indicatingdrymeltingoffeldsparinthepresenceofquartzortridymite.Amphibolewaspseudomorphosedbycompositeaggregatesofortho-pyroxene,clinopyroxene,Fe/Tioxides,andpossiblycalcicplagioclase(sausuritized);epidoteappearsreplacedbyfine aggregates of plagioclase-titanomagnetite, whereas mostquartzandrarelytridymiteincontactwithglassweremantledbyprismatictoacicularcrystals(10–20µm)oforthopyroxeneorclinopyroxene.Otherpseudomorphsformedduringcontactmetamorphisminthegneissconsistoftinytitanomagnetitegrainsassociatedwithbroadzonesofglassandacicularclinopyroxene.Thesexenolithicraftsoccasionallypreservedunalteredvestigesoftheoriginalgneissmineralssuchaschlorite,epidote,zircon,titanite,andgarnet.

Gabbroic (PN2, PN3, PN20, PN26, PN41) and ultramafic xenoliths (PN4, PN5)

MostxenolithsinthePuentoNegrointrusionareofgabbroiccomposition.Texturallyandmineralogically,theymay be classified as plutonic and hypabyssal rocks, with the latter forming fine to coarse-grained amphibole-pla-gioclase±clinopyroxene±glassporphyriesandamarkedidiomorphictextureconsistingofamphibolecrystalsuptoseveral centimeters long set in a feldspathic, fine-grained hypocrystallinegroundmass.Inthinsection,amphibole

showsexsolutionofilmeniteinbandsandirregularzones;it defines short prismatic to quenched skeletal or arborescent shapesdispersedinamatrixofsmallergrainscomposedofthreetypesofplagioclase:(a)sieve-textured,(b)hollow,and(c)microlitic,allinsidelargepatchesofalteredglass,chalcedonyandcalcite.Isolatedprismsofclinopyroxene,rareorthopyroxene,darkredrutile,andneedlesofapatiteprobablywerederivedfromthetotaldehydrationofamphi-bole,whichmorecommonlyremainedmantledbyCpx-Pl-Fe/Tioxides,exceptwhenisolatedinsideplagioclase.TheanhydrousassemblageOpx+Kfs+Fe/Tioxidesreplacedbiotitedevelopedfromthebreakdownofamphibole.Inextremecases(e.g.,FO7405)theoriginallyhydrated,horn-blende-richxenolithswerealmostcompletelydehydratedtothepseudomorphicassemblagePl-Cpx-Opx-TiMag±Bt ± glass, and in one case to a fine aggregate very rich in spinel,butretaininginsomecases(hypabyssalsamples)theintersertal,formerlyhornblende-richmatrixtextureoftheoriginalrock.DeutericCO2andH2O-rich fluids often causedastrongalterationofthequenchedmatrixtocarbon-atesandclays.

Although rare, small ultramafic xenoliths are present inthePuenteNegrointrusion.TwosamplesexaminedinthinsectionhavetheprimarymodalassemblageOpx(60-80%)-Cpx(7%)-Hbl(10-30%)-glass(<5%)-Pl(<5%)-Hc(<1%),inwhichmostamphibolewaspartlydehydratedtoPl-glass,orrecrystallizedtoCpx-Pl-Bt-opaqueaggregates.Orthopyroxeneshowsraggedgrainboundariesandcom-monlyisweaklyalteredtochlorite;itshowsfaintpleoch-roismincontrasttothestronglypleochroicorthopyroxeneassociatedwithglassinmetamorphicxenolithsandxeno-crystswithintheandesite.SomegrainscontaininclusionsofclinopyroxeneandroundedprismsorblebsofFe-Tioxides.Amphiboleisstronglypleochroicwithbrown,yellowandreddishshades.Itmayshowinclusionsofilmenite/rutile,clinopyroxene,andorthopyroxene.Clinopyroxeneoccursinminorquantitiesassmall,subhedraltoanhedralinclusionsin orthopyroxene, and as twinned crystals in the ultramafic assemblage. Clinopyroxene crystallized first because it is foundasinclusionswithinorthopyroxeneandamphibole.Rarecrystalsofolivegreenspineloccurincontactwithclinopyroxeneandmaybeanearlymagmaticphase.Mostglassy matrix was devitrified to spherulitic smectite, en-closingacicularplagioclase.Acumulateoriginforthesexenoliths is inferred from the euhedral arrangement of mafic phasestouchingeachotherinaquenchedplagioclase-glass(altered)minorgroundmass,whichdemonstratesveryrapidcoolingafterentrainmentbecausesomeoftheglassremainedfreshinsidesieveplagioclase.

Quartzose gneiss and quartzite xenoliths

Xenoliths of this type (e.g.,PN0,PN1,PN9,PN14,PN19,PN27,FO29698,FO8305,PN605)aredominatedbyquartzandtridymite(oftenepitaxiallyinvertedtoquartz


paramorphs),accompaniedbyvariableamountsofgarnet,plagioclase,orthopyroxene,clinopyroxene,greenspinel,Al-silicatess(aluminumsilicatessolidsolutions,cf.Cameron,1977),corundum,zircon,rutile,hematite,opaquephases,rarelycristobalite,andglass.Quartz,usuallymantledbytridymite(Figure5a),occursinfinetocoarse-grained(0.1–7.5mm)granoblasticmosaicswithabundantcurvedfractures,commonlyenclosingpocketsofbrownglasswithnumerouscrystalsofclinopyroxeneorplagioclase.Rare,globularinclusionsinquartzformedbyAl-orthopyroxene,greenspinelandneedlesofrutileimmersedinglassmayrepresentformerinclusionsofincongruentlymeltedbio-tite.OthersymplectiticpseudomorphsinthesexenolithsconsistingofIlm-Rt-Spl±Zrn,Rt-Ilm,Opx-Spl-glass,Pl-Opx-TiMag-Crn-glass,andOpx-Spl-Pl,mayrepresent,respectively,pseudomorphsafterarmalcolite,ferropseudo-brookite,biotite,amphibole,andgarnet,butnovestigesofthesephases,withtheexceptionofgarnet,werepreserved.Zirconprobablywasexsolvedfromhightemperaturerutilebecauseitisveryoftenacompanionphaseofrutile-ilmeniteaggregatesinthequartzosematrix.

Almostpurequartzitexenolithsdisplayagranoblastictexture with quartz grains defined by curved broad contacts occupiedbycolorlessglass,plagioclase,clinopyroxeneandtridymite,whichcommonlymergeintotridymititeorpocketsofquenchedmagma(brownglass+plagioclaselaths).Inadditiontoquartzandtridymite,quartzitesin-cludesmallamountsofstronglypleochroicorthopyrox-ene(browntopaleorangewithoccasionalinclusionsofgreenspinelandcorundum)coexistingwithplagioclase,Al-silicatess, titanomagnetite, rutile,andbrownglass.Thisorthopyroxenesometimesreactedwithintergranularanatecticmelts(glass)acquiringnarrowmantlesofclino-pyroxene. Elsewhere, orthopyroxene defines slender to stubbypleochroiceuhedralcrystalsupto250µmlongthatareembeddedinglass(Figure5b),anditisalsopresentaspolycrystallinepseudomorphsandcoronasreplacinggarnet.Ithasamarkedpleochroisminshadesofgreen,yellow-brownandbrown,andiscommonlyaccompaniedbyroundedgrainsorrhombsofgreenspinel.Multiplemi-crolitesofplagioclaseandtridymite(quartzparamorphs)nucleatedatrightangletomostquartzgrainmarginsorinquenchedpockets,althoughplagioclasealsooccursinsymplectiticaggregateswithspinel,Al-silicatess,andor-thopyroxene.Microprobeanalyses(seebelow)andopticalpropertiesofplagioclaseindicateabytownitecomposition.Glass,occasionallyassociatedwithclinopyroxene,formsanintergranularmatrixenclosingmostquartzgrains.Garnetisfoundasfractured,colorlessporphyroblastsupto8mmacrossmantledbypolycrystallinecoronaswithabundantsymplectitesofspinelandplagioclaseinamatrixofor-thopyroxene,plagioclaseandglass.Absenceofcordieriteinthesecoronasindicatesanoriginalcalcium-richgarnetsource.Someofthelargestgarnetcrystalswererotatedinthefoliatedmatrixoftheoriginalmetamorphicrock,whereassmallcrystalsarecompletelyalteredtoaspinel,

orthopyroxene,plagioclaseandglassaggregate.Zirconoccursinsideglassasroundedtosubhedralstubbygrainsupto170µm,commonlywitheuhedralovergrowthsandgranularaggregatespossiblyrelatedtothermalrecrystal-lizationandneocrystallization.

Othertypesofquartzosexenoliths(PN9)showlens-shapedfoliateddomainsofQz/Trd-Zrn-Rt-opaque-glassalternatingwiththeassemblagesOpx-glass,Pl-glass,andOpx-Pl-glassthatprobablyrepresentincongruentlymeltedbiotiteorgarnet.Thus,manyofthe“quartzite”xenolithsmayberestitesafterstrongmeltingoforiginalquartz-biotite-plagioclase-garnetgneisses.OtherzonesinthequartzitesshowtheassemblagePl-Spl-Alsilicatess-Opx-Trd/Qz-glass-opaque.Tridymite,mostlyinvertedtoquartz,formsthinrindsaboutalloriginalquartzgrains,whereasdarkgreen,darkbrownishgreen,oropaquespineloccursinsquaredandrhombicshapesalwaysassociatedwithorthopyroxene,glassandAl-silicatess.Orthopyroxenealsooccursasstubby,distinctlypleochroicprismaticcrystalswithinpatchesofdarkbrownglasscontainingafewlongneedlesofAl-silicatess.

Metapelitic gneiss xenoliths

TheextremelyaluminousassemblagesPl-Sil-Spl-Crn-St,andSpl-Crn-Mul/Sil(Al-silicatess)werenoticedinonlyonexenolith(FO8305),withstauroliteandsillimanitepartlyreplacedbyspinelorcorundum.Commonly,corundumandsillimanitearemantledbygreenspinel,suggestingthehigh-temperaturereducingreactionsMag+3Crn=3Hc+½O2,andMag+3Sil=3Hc+3Trd+½O2,whereascoronasof Spl-Qz-Opx around relict garnets and fine aggregates of Al-silicatess,Opx,Qz,andSplindicatedecompressionofthexenolithsatultrahightemperaturebythereactionsGrt=2Opx+Spl+Qz,andSpl+2Qz=Al-silicatess+Opx.

Someofthesexenolithsaregarnet-richandshowgranoblastic textures typicalofgranulites(Figure3f)with finely banded structure. Their high-temperature as-semblagesincludeOpx-Al-silicatess-Pl-Spl-Rt-Zrn-Fe/Tioxides(FO29678),andGrt-Pl-Spl-Qz-Opx-Zrn-glass(PN5,PN17),whererelictgarnetsformporphyroblastsupto7mminsize.Garnetdisplaysclean,unfracturedcoressur-roundedbyathickfracturedzonethatinturnismantledbycoronasofOpx-Spl-Pl,orPl-Crn-Mag(seeFigures5c-5d).Theclosespatialassociationofspinel,glass,andorthopyroxeneinbothmatrixandcoronasaroundgarnetmaybeascribedtogarnetbreakdownduringxenolithascentinthemagmabythereaction4Grt+Qz+½O2=5Opx+4An+Mag,implyinginthiscaseanincreaseintheactiv-ityofoxygen.Calcium-richplagioclaseenclosingeuhedralorthopyroxeneandspinelformsdiscontinuousanorthositicbands;orthopyroxeneisstronglypleochroicinshadesofgreen,yelloworbrown.MostofthesexenolithsalsoshowcoronasofAl-OpxaboutspinelincontactwithplagioclaseprobablyoriginatedbythereactionSpl+Pl=Opx+melt


(Figure5e).ThereactionMgTsOpx(tschermackiticOpx)=Spl+Trdwasalsotexturallyobserved(PN23b),indicat-ingdecompressionandveryhightemperatures.Additionalpseudomorphicaggregatesincludesymplectitesoftitano-magnetiteandglass(PN17)probablyafterbiotite,andof

Al-silicatess-Crd?-Spl,interpretedasformersapphirine.Euhedral,primaryanhydritecrystalsandstrontianbaritewerealsofoundtogetherwithzircon,whereasopaqueminerals include titanomagnetiteandilmenite,whichwereusedforgeothermometry(seebelow).Otherpoorly

Figure5.Photomicrographsof:a)Fracturedquartzrimmedbyformerhightridymite(Trd)laths(crossednicols,FO7405).b)High-Alorthopyroxene(Opx)(darkgray,planepolarizedlight)withinclusionsofspinel(black)inamatrixofglass(gray,Gls)andquartz(white)(PN1).c)MultipledecompressioncoronasofSpl-Pl-Opx-Crn-glassaroundagarnetporphyroblastsetinaplagioclase-richfoliatedmatrix(planepolarizedlight,PN29).d)Detailofthedecompressionalcoronashowninc)(planepolarizedlight).e)Al-Opxrimsaroundspinelincontactwithplagioclaseingarnetiferousgneissxenolith(planepolarizedlight,PN29a).f)Pseudomorphafterbiotite(?)meltedincongruentlytotheassemblageOpx-Spl-glass(planepolarizedlight).g)Pseudomorphofspinel(black),plagioclase(white)andcorundum(gray)probablyaftersapphirineorstaurolite(crossednicols,PN32).h)Spinel-mullite-glassassemblageprobablyformedbytheincongruentmeltingofanaluminoussilicate(planepolarizedlight,FO29988).


defined bands composed of garnet-plagioclase-orthopy-roxene,plagioclase-hercynite-biotite-corundum(mantledbyhercynite),andorthopyroxene-plagioclase-glasszonesarepresentinsomeofthesexenoliths(PN18a).Asmallxenolithimmersedintheandesite(PN23)showsallthestagesofgarnetbreakdowntotheultimatepseudomorphicassemblagesSpl-Pl,Spl-Pl-Crn,andSpl-Pl-Opxthatpre-servedtheformerexternalshapeofgarnetporphyroblastsandremnantsoftheoriginalmineral.Thedifferencesinmin-eralogymaybeascribedtodifferentialreactionofgarnetswiththemetamorphicmatrixenclosingthegarnetconsistingofPl-Spl-Opx-Crn-TiMag-Cpx-glass.Orthopyroxeneandcorundum,althoughnotindirectcontact,coexistwithinadistanceoftensofmicrons,andareprobablyrelatedtothedecompressionreactionSpl+Qzmelt=Opx+Crn.

Quartzo-feldspathic gneiss xenoliths

Graniticgneissxenolithsupto10cmacrossarecoarse-grainedandhaveawelldevelopedbandedstructurewith alternating mafic and felsic domains and abundant glass(Figure3c).Oneofthesegneissesexaminedinthinsection(PN24)showedafoliatedtextureofintermediategrainsizeformedby theassemblageTrd/Qz-Opx-Pl-glass-opaque-Zrn.Quartz,withoccasionalinclusionsofrutileandzircon,formspolycrystallineribbonstypicalofquartzo-feldspathicgranuliticgneisses.Allcrystalsofquartzincontactwithglassshowareactioncoronaformedbytinylathsofplagioclaseandtridymite.Occasionally,orthopyroxenerimmedbytridymiteoccursasinclusionsinsidequartz.SeveralpseudomorphicaggregatescomposedofOpx-Spl-glass,Pl-glass,orOpx-Crd?-Spl-opaquesug-gestthetotalbreakdownorpartialmeltingofphasessuchasbiotite,amphibole,plagioclase,andprobablygarnetintheoriginalgneiss.Complexdendritic,micrographicandsymplectiticintergrowthsofsanidineandquartz/tridymitewereobserved,indicatingpartialmeltingwithrapidcrys-tallization.Mostimportantly,sampleFO7505containspossiblerelictsoftheultrahigh-temperature,deep-seatedassociationOpx-Sil,albeitthesephaseswerenotobservedindirectcontact.

Glassofthesexenolithsoccursinamatrixtraversedbyacicularplagioclaseandrareorthopyroxene,andasdistinctpatcheswithlarge,euhedralprismsoforthopyroxeneform-ing an intersertal texture within the glass. The first type of glassisdarkbrown,whereasthesecondtypeisofalightercolor.Aconsiderableproportionofbothglasseswasalteredtoradialclayaggregates.

Calcsilicate xenoliths

Xenolith PN42 (Figure3d)showsinthinsectionaroughlybandedstructurewiththecompositehigh-Tas-semblageCpx-Pl-Trd/Qz-Mw(?)-Ttn-Ilm-Zrn-Ap-glass.

Almostpurequartzitebandsareincontactwiththeandesiticmagma,followedinsidethexenolithbyanotherbandofCpx-glass,anditscoreiscomposedofcalcicplagioclaseandclinopyroxeneaccompaniedbyaphaseopticallyre-semblingmerwinite.Mostquartzisrimmedbytridymite,andilmenitebytitanite,whichmayalsoformsymplectiticaggregateswithclinopyroxenecores.Primarycarbonateiscoarse-grainedandhasstraightcontactsagainstplagioclase,glassandclinopyroxene,butitmaybesecondary,becauseitisalsopresentaspatchesintheandesitehostingthexenolith.Clinopyroxeneiscommonlycolor-zoned,withbrowncoresandgreenrims;itdisplaysapoikiloblastictextureenclosingnumerousgrainsofplagioclaseandglass,suggestingthebreakdownofhornblendeinthepresenceofquartzandcalcite.Titaniteformsmantlesaroundilmenite,whereas“merwinite”isabundantinpatchesaspolycrystal-line,roundedtoeuhedralgrainsintexturalequilibriumwithtridymite,bytownite,andglass.Somefeldspar(plagioclaseorsanidine)showscomplexmicrographicintergrowthswithglass.Pyrrhotiteisquiteabundant,indicatingreduc-ingconditionsduringhightemperaturecrystallization.Thisxenolithisconsideredpositiveevidenceforanon-localsourceofmanyofthePuenteNegroxenolithsbecausecalcsilicate-quartzitebandedrocksarenotpresentintheunderlyingorexposedAcatlánComplex.

Xenocrysts and xenolithic polycrystalline pseudomorphs

Inoutcrop,themainxenocrystsfoundinthePuenteNegrodikeareamphibole,quartz,andredgarnet. Inthinsection,additionalsmallxenocrystsofcolorlessandpurplish-bluecorundum(sapphire)(Figure4e),greenspinel,rutile,zonedAl-orthopyroxene(Figure4f),ilmenite,andzircon,aswellaspolycrystallinepseudomorphsprobablyafterbiotite(Figure5f),andhighlyaluminousaggregatesofspinel,corundumandAl-silicatesswerediscovered.Quartzcommonlyformsovalxenocrystswithcompleteorincompletecoronasofclinopyroxene,althoughmanycrystalsandquartzitefragmentsdidnotreactwiththeandesiticmatrix,whileotherswerecompletelyreplacedbyclinopyroxene.Orthopyroxenexenocrystsaredistinguishedby their intense pleochroism (Z = brown, X = green) in grainsupto250µmmantledbycolorlesshyperstheneovergrowths.Allgarnetsshowkelyphiticrims50to500µmwideconsistingofspinel,plagioclase,orthopyroxene,andminorcorundumorglass.GarnetsyieldlowMgandhighMncontents,withacalcium-richfracturedzonemantlingCa-poor,unfracturedcores.Mostimportantly,andesitesamplesPN43,PN32,PN24,andPN16containovalpseudomorphicxenocrystsupto1cminsizeconsistingofSpl-Pl-Crn±Opx±Rt±Bt(Figure5g)withmodalabundancesofspinel(20–70%),plagioclase(20–65%)andcorundum(10–30%).Thesemineralsandtheirrelativeproportionsareremarkablysimilartometasomatizedorthogneissic


and paragneissic xenoliths in the Voisey’s Bay troctolitic intrusionofLabrador,Canada(Marigaet al.,2006a)andinprinciplecouldhaveacomparableoriginatdepthbeforeentrainmentintheandesite,asdiscussedlater.Euhedralcalciclabradorite(An65)mantlesthepseudomorphsincontactwiththeenclosingvolcanicmatrix.Abundanceofspinelandcorundumsuggeststhatsomepseudomorphsprobablyformedaftercalcium-richgarnet;otheraluminousphases such as sapphirine, staurolite, pargasite, ormetasomatizedanorthiteareconsideredlesslikelyprecursorminerals.Additionalhighlyaluminouspseudomorphsfound(FO29698)showsymplectitesofAl-silicatess-Crd?-Spl,andAl-silicatess-Spl-Crn-Pl.Staurolitewasfoundcoringoneofthecorundum-bearingpseudomorphs(FO7505)formedbyabout50–65%spinel,20–40%plagioclase,0–10%corundum,and5–10%staurolite.Indeed,probablythemostimportantpetrogeneticfeaturesfoundinthePuenteNegroandesitearethecommonpresenceofsubhedralcrystalsofcorundummantledbyplagioclase(Figure4g)orspinel(Figure4h),andaxenocrystofpurplish-bluesapphireabout300µmlongisolatedinthevolcanicgroundmassofsamplePN32(seeFigure4e);thistypeofsapphireinvolcanicrocksmaybeuniquebecauseofitsverysmallsizeandassociationwithcalc-alkalinemagmatism,asopposedto themegacrystic,gem-qualitysapphiresdescribedfromseveralalkali(extensional)basalticorsyenitic fields of the world (Guo et al.,1996;Sutherlandet al.,1998;Garnieret al.,2005).Thesapphirexenocrystwas identified by its specific optical properties (uniaxial negative,negativeelongation,straightextinction,extremerelief,andcharacteristicpleochroism(palegreenalongεanddeeppurplish-bluealongω), and also confirmed by microprobeanalysis(seebelow).

AGE OF PUENTE NEGRO INTRUSION

TheisolatedcharacterofthePuenteNegrodike,intrudedonlyintoPaleozoicmetamorphicrockslackingayoungercover,promptedtheneedtobetterconstrainitsagebyisotopicmethods.Becausetheintrusionanditsigneousxenolithsarerichinfreshhornblende,thismineral,togetherwiththevolcanicmatrixwerethematerialschosentodatethemagmaticevent.

Analytical methods and results

SamplesPN10,PN22andPN44werecrushedwithasteeljawandsievedat180–250µm.Aftercleaninginultra-sonicbath,bothamphiboleandmatrixwereseparatedusingaFrantzmagneticseparator.Theconcentrateswerecleaned,driedandsplitintwoparts,oneforKdeterminationandtheotherforArmeasurement.KforthematrixsamplewasobtainedfollowingthemethodofSoléandEnrique(2001).Briefly, 100 mg of sample were fused with 50% lithium

metaborate+50%lithiumtetraborate.Thefusedpearlwasmeasured with a Siemens 3000 XRF spectrometer calibrated againstseveralinternationalstandardspreparedinthesameway.Resultswereaccurateto1%(1σ)orbetter.PotassiumfromamphiboleshasbeenmeasuredbyID-TIMS,becausemeasurements by XRF were not reproducible. Results from ID-TIMSareaccurateto1%(1σ)orbetter.

Argonwasmeasuredbyisotopedilution(38Artracer)withaMM1200Bnoblegasmassspectrometeroperatedinstaticmode.Between8and22mgofsamplewerefusedwitha50WinfraredCO2laserinaUHVchamber.Afterfusion, evolving gases were purified with a cold finger andtwoSAESgetters,oneoperatedat400ºCandtheotheratroomtemperature.Eightseriesofmeasurementsofeachmassweremadesequentiallyandextrapolatedtogasinlettime.Thesignalwasacquiredwithasecondaryelectron multiplier. Variation coefficients for 40Arand38Araregenerallybelow0.1%andbelow0.5%for36Ar.AllanalysesweremadeattheInstitutodeGeología,UNAM.TheconstantsrecommendedbySteigerandJäger(1977)wereusedthroughout.TheobtainedagesareshowninTable2.Theweightedmeanageofamphiboleis30.5±0.6Ma,whichisabout1Maolderthantheageofvolcanicmatrix,datedat29.2±0.3Ma.Thus, it ispossibletoassumethattheamphiboleandhencethegabbroicsourcemaybesomewhatolderthantheandesite.Inconclusion,weassignanageof29.2±0.3Ma(Oligocene,Rupelian)tothePuenteNegrointrusiveevent,whichispartoftheextensivemagmatismthatformedtheSierraMadredelSurvolcanicprovinceintheMixtecaregionofsouthernMexico(Morán-Zentenoet al.,2000),butisolderthantheearliestmagmatismassociatedwiththeTrans-MexicanVolcanicBeltconsideredtobeataround19Ma(Gómez-Tuenaet al.,2005).ThecommonpresenceofandesiticdikesthroughouttheMixtecoterranewherethePaleogenevolcanicrockshavebeenerodedsuggeststhatthesedikesarefeedersoftheOligocenevolcanicprovince.

Analysisnumber

Sample K%

40Ar*moles/g

40Ar*%

AgeMa (± 1σ)

Amphiboles712 PN-10 0.480 2.540·10-11 70.5 30.3±0.8889 PN-22 0.507 2.778·10-11 58.8 31.3±1.8892 ” ” 2.716·10-11 64.2 30.6±1.51380 PN-44 0.465 2.486·10-11 40.5 30.6±1.8

Mean 30.5 ± 0.6Matrix1321 PN-44 1.32 6.732·10-11 78.4 29.2±0.81375 ” ” 6.753·10-11 76.8 29.3±0.9

Mean 29.2 ± 0.3

Table2.AnalyticaldataforK-Armeasurements.Meandataforamphibolesandmatrixhavebeencalculatedusingaweightedmeanandaweightedstandarddeviation.Aminimumof1%uncertaintyhasbeenappliedtothestandarddeviation.


CHEMISTRY AND PETROLOGY OF THE PUENTE NEGRO INTRUSION AND ITS METAMORPHIC AND IGNEOUS XENOLITHS

Whole rock chemistry

Majorwholerockelementchemicalanalysesofeightrepresentativesamplesperformedontheandesiticintrusion(threeanalyses),fourxenolithsandtheintrudedgneiss(PN39W)areshowninTable3.Althoughthesilicacontentandbulkcompositionofthedikesaretypicalofandesite(Gill,1981;LeBaset al.,1986),theaveragenormativelabradorite(An52),andrelativelyhighTiO2contentcorrespondmorewithanoriginalbasalticcompositionconsideringthatsmallbutcommonquartzxenocrysts(estimatedatca.2%modalfor<500µmgrains),onlyevidentinthinsection,couldnotbealleliminatedfromtheanalyzedrocks,thusrisingtheirbulksilica.Metamorphicxenolithsstudiedaremainlyquartzosegneisses(e.g.,PN1,PN19),ahigh-Almetapelite(PN17),andabuchite(FO6605).Notetheclosechemicalcorrespondencebetweenthebuchiteandthecountrygneiss(PN39W),indicatingalocalsourcefor thisxenolith.Thequartziticxenolith(PN1)yields83.91wt.%normativequartzand1wt.%corundum.Theexcess alumina is probably fixed in modal Al-orthopyroxene andspinel,whereassodiumandcalciumarefixed inplagioclase(Ab67An33). PN17 is a fine-grained garnetiferous gneisswith4wt.%normativecorundum,verylowquartz(1wt.%),veryhighfeldspar(70wt.%),andhighzirconium(356 ppm), possibly reflecting a sedimentary, somewhat desilicated protolith. PN4, the only ultramafic xenolith analyzed,shows(CIPWnorm)55wt.%ofhypersthene,17wt.%quartz,andabout24wt.%plagioclase(Ab32An68).Itapparentlyrepresentscumulateassemblagesinamagmachamberdifferentiatedathighwaterpressure.Thehighsilicacontentoftheultramaficxenolithmaybeduetolate-stage hydrothermal silicification (chalcedony). Major elementsofthreexenolithswereanalyzedbyICP-OESandtraceelementsbyICP-MSinthelaboratoriesattheCentredeRecherchesPétrographiquesetGéochimiquesoftheCentre National de la Recherche Scientifique in Nancy, France.Majorandtraceelementsofonexenolith(FO6605)were carried out by X-ray fluorescence at the Instituto de Geología,UNAM,usingproceduresdescribedinLozanoandBernal(2005).MajorandtraceelementsoftheintrusiverocksandintrudedgneisswerecarriedoutusingdifferentmethodsattheInstitutodeGeología,UNAM(seeTable3).IsotopedeterminationsweremeasuredbythermalionizationmassspectrometryatLUGIS(LaboratorioUniversitariodeGeoquìmicaIsotópica)attheNationalUniversityofMexicousingaFinniganMAT262system,followingtheproceduresoutlinedinSchaafet al.(2005).

Mineral chemistry

Chemicalmicroanalysesofmajorelementsinigneous

andmetamorphicmineralsandglassesweremadebyWDSandcomplementaryEDSelectronprobeatthepetrologiclaboratoriesoftheInstitutodeGeologíaandInstitutodeGeofísica,bothat theNationalUniversityofMexico,using a Jeol JXA 8900R superprobe, and a Jeol C35 SEM, respectively.ForWDS,abeamcurrentof10–20nAandacceleratingvoltageof15–20kVwereused,withcountingtimesof20to40secondsandafocusedbeam(1µm),exceptfortheglasses,inwhichcasethecountingtimewasloweredto10secondsandthebeamdefocusedto5µm.Naturalminerals(albite,diopside,kaersutite,jadeite,sanidine,obsidian,apatite,quartz,pyrope,almandine,plagioclase,andCr-diopside),aswellassyntheticproducts(rutile,chromiumoxide,andilmenite)wereusedasstandards.CorrectionsweremadeusingtheZAFsoftwareprovidedbyJeol.

Puente Negro intrusion and related gabbroic and ultramafic xenoliths

Microprobechemicalanalysesofselectedphasescomposing the Puente Negro andesite and an ultramafic ig-neousxenolithareshowninTables4and5,respectively.

Pyroxenes. Various kinds and compositions ofigneous orthopyroxene are present in the Puente Negroandesitebutwerenotsystematicallyanalyzed:(a)Colorlessto distinctly pleochroic phenocrysts up to 1.4 mm, (b)colorlessmicrophenocrystsandmicrolitesintheandesiticgroundmass, and (c) in “glomerocrysts”. Orthopyroxeneformsthemainphaseinthehornblendepyroxenitexenolith.Allorthopyroxenesfallneartheenstatite-ferrosilitebaseofthepyroxenequadrilateral,withMg#numbers[100*Mg/(Mg+Fe)] up to 77 in the ultramafic xenolith and 78 in the matrix,butnotethattheAl2O3contentoftheorthopyroxeneintheandesitematrix(3.89wt.%)ishigherthanthatoftheultramafic xenolith (average of 2.22 wt. %), but both contain muchlessaluminathanthoseinthemetamorphicxenoliths.Zoningtowardsamoreferrosiliticmarginisshownbythecore-to-rim traverse across the ultramafic orthopyroxene (seeTable 5). Clinopyroxene of the ultramafic xenolith falls in the ferroaugite field (av. En42.7Wo42.7Fs10.4),but israreintheandesitegroundmassandabsentasphenocrysts;it iscommonin“glomerocrysts” togetherwithabundantamphibole, plagioclase, and orthopyroxene and also aspartialtocompletereplacementofquartzxenocrysts.Notealso that clinopyroxene in the ultramafic xenolith shows a relativelyhighcontentofaluminaandchromiumandthesametendencyasorthopyroxenetoslightironenrichmentfromcoretorim.

Amphibole.Amphibole xenocrysts are abundantin the andesite (2–20% of thin section area) and it is acharacteristic phase in gabbroic and ultramafic (pyroxenites) igneous xenoliths, but it is completely absent in themetamorphicxenoliths.Table4showsselectedamphiboleanalyses from the andesite, and Table 5 from the ultramafic xenolith.All amphiboles, gabbroic and xenocrystic, arepargasiteswithMg#varyingfrom59to77andmoderate


Sample 119-79* PN15b§ PN44§ Av. And PN1b ◊ PN17 ◊ PN4 ◊ FO6605 ‡ PN39W ‡Rock type Andesite Dacite Andesite Andesite Xenolith Xenolith Xenolith Xenolith Gneiss

Major elements (wt. %)SiO2 56.19 63.00 56.03 57.60 90.43 53.65 58.87 64.48 59.85TiO2 1.11 1.00 1.03 0.77 0.13 1.17 0.57 0.53 0.64Al2O3 15.50 15.51 17.26 17.30 3.70 21.39 7.16 15.53 15.44FeO 4.50 3.10Fe2O3 3.46 5.13 7.43 4.30 1.22 7.75 8.89 5.26 6.78MnO 0.11 0.10 0.13 0.15 <d.l. 0.36 0.10 0.13 0.13MgO 5.61 3.03 4.82 3.60 0.56 3.34 15.14 2.61 4.42CaO 6.93 6.66 7.47 7.20 0.72 4.78 3.64 3.90 5.02Na2O 3.05 2.66 3.08 3.20 0.78 4.79 0.87 3.07 2.46K2O 1.50 1.27 1.46 1.50 0.18 1.05 0.13 2.11 0.91P2O5 0.22 0.24 0.25 0.21 0.05 0.11 0.13 0.11 0.13H2O+ 1.02 1.90 0.96 1.00 2.02 1.52 4.40 2.30 3.52H2O- 0.57 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.Total 99.77 100.50 99.92 99.93 99.79 99.91 99.90 100.03 99.30

Trace elements (ppm)Sc 23.5 26.5V 205 229 17 135 212 143 121Cr 127 111 16 107 442 102 76Co 24 47 1 14 54 20 24Ni 74 21 <d.l.. 20 64 28 29Cu 34 29 8 24 68 11 23Zn 128 242 14 158 171 81 104Ga 3 23 11Be 2.3 2.2 <d.l. 3.2 <d.l.Rb 24 40 25 19 6 109 66Sr 700 784 143 424 422 529 242Y 21.2 27.3 10.0 46.4 11.2 32 28Zr 147 165 277 356 65 244 173Nb 7.2 8.6 2.5 19.2 3.0 14 14Sn 1.7 1.8 <d.l. 0.53 0.60

Table3.Majorandtraceelementcompositionsoftheintrusivebody,xenolithsandlocalcountryrocksatPuenteNegro.

continues

contentsofTiO2thatareslightlyhigherinthexenocrysts(up to 2.39 wt. %).Aluminum is quite variable in thexenocrysts(10.49–14.33Al2O3 wt. %), probably reflecting the polybaric differentiation and contamination of theandesite,andisquiteuniform(11.56–12.47Al2O3wt.%)in the ultramafic xenolith (PN4). Sodium is moderate in thegabbroicamphiboles(av.2.21wt.%),butmuchhigherinxenocrystsoftheandesite(av.4.23wt.%).TheMgO/FeO ratio is about the same in xenocrysts and ultramafic xenoliths.Amphibole in the latter bears inclusions oftitanomagnetitewithTiO2upto9.4wt.%.

Plagioclase. Plagioclase associated with gabbroicxenoliths (Table 5) is calcic to intermediate bytownite(An81–An74),whereasmicroliticplagioclaseintheandesite(Table4)yieldsanaveragecalcicandesine(An45)withminoramountsofiron.Measuredplagioclaseinthedehydrationrims of amphibole xenocrysts of the ultramafic xenolith is closetopureanorthite(An97).EDSanalysesperformedonplagioclase in the quenched matrix of the ultramafic xenolith

showacalcicbytownitecomposition,butpotassium,whenanalyzed,wasalwaysverylow.

Glass. Magmatic residual glasses from severalandesiticdikesamplesandglass fromapartiallymeltedgabbroicxenolithwereanalyzedbyelectronmicroprobe,withresultsshowninTable4fortheandesiticglassesandinTable5forthegabbroicxenoliths.Glassesintheandesiteyieldarhyoliticcomposition,withsilicacontentsfrom75.5to77.6SiO2wt.%andnormativecorundumupto5.78wt.%.Glasses in thecoarsegabbroicxenolithsare stronglyaltered toaclayphaseandwerenotanalyzed,but thosein the fine-grained quenched types (PN20) show a similar compositiontoandesiteglasses(Table5)exceptfortheirmorehydratedstate.

High-Al xenocrysts and xenolithic pseudomorphsXenocrysts and xenolithic pseudomorphs with high

contentsofaluminumarecharacteristicofthePuenteNegroandesiteinclusions.Spinelandcorundummaybefound


Sample 119-79* PN15b§ PN44§ Av. And PN1b ◊ PN17 ◊ PN4 ◊ FO6605 ‡ PN39W ‡Rock type Andesite Dacite Andesite Andesite Xenolith Xenolith Xenolith Xenolith Gneiss

Trace elements (ppm)Sb 0.25 0.52 <d.l. 0.46 0.76Cs 2.06 0.74 7.61 1.57 3.62Ba 393 384 68 423 132 630 654La 19.89 20.22 8.82 65.10 6.80Ce 46.01 44.00 17.75 131.52 15.59Pr 5.55 5.80 2.21 15.65 2.27Nd 23.89 25.73 7.77 59.38 9.84Sm 5.22 5.72 1.49 10.53 2.25Eu 1.38 1.60 0.43 2.46 0.56Gd 4.80 5.33 1.36 8.51 2.17Tb 0.75 0.85 0.24 1.28 0.33Dy 3.86 4.60 1.50 8.30 2.11Ho 0.76 0.94 0.33 1.72 0.38Er 2.14 2.62 1.03 5.01 1.12Tm 0.276 0.378 0.15 0.81 0.16Yb 2.00 2.57 0.98 5.87 1.15Lu 0.30 0.40 0.15 0.92 0.17Hf 3.91 4.17 6.54 9.28 1.82Ta 0.21 0.49 0.22 1.34 0.24W 0.303 2.032 0.772Pb 11.1 21.9 6.7 30.5 21.6 16 27Th 5.21 4.68 2.18 17.50 1.91 8 12U 1.371 0.950 0.660 3.983 0.569CIPW NormQ 10.46 22.28 5.94 12.48 83.91 0.94 17.43 23.03 20.66Or 9.04 7.62 8.75 8.98 1.06 6.32 0.83 12.76 5.61Ab 26.32 22.85 26.32 27.33 6.77 41.21 7.70 26.57 21.75An 24.61 26.99 29.26 28.73 3.35 23.39 15.97 19.08 25.08C - - - - 1.04 4.00 0.00 1.40 1.67Di 7.17 4.08 5.44 4.86 - - 1.63 - -Hy 14.65 13.69 21.75 11.11 3.50 21.64 55.00 15.88 23.64Mag 5.10 - - 4.54 - n.d. - - -Il 2.15 1.92 1.98 1.48 0.25 2.26 1.14 1.03 1.27Ap 0.51 0.56 0.58 0.49 0.12 0.26 0.32 0.26 0.32

isolatedasxenocrystsandwithinplagioclase,whereaspeculiarpseudomorphs(Table6)probablyaftergarnetorotheroriginalphasesofunknownnature(sapphirine?)arecomposedofsymplectiticaggregatesofspinel,plagio-clase,corundum,withsomeorthopyroxene,Fe/Tioxides,andminorzircon,biotite,andrareternaryfeldspargrains(Ab64.8Or26.8An8.6).

ThreeformsofxenocrysticcorundumarefoundinthePuenteNegroandesite:(a)Purplish-bluesapphirexe-nocrysts,(b)pale-bluetocolorlesscorunduminaluminouspseudomorphs,and(c)colorlesscorunduminclusionsincal-cic plagioclase. Table 6 shows analyses of the first two types. Notethatsapphireshowsahigherironcontentthancolorless

corunduminthepseudomorphs,consistentwithitspossiblyigneousanddeepercrystallization(e.g.,Garnieret al.,2005).Colorlesscorundum(notanalyzed)inCa-richplagioclase,ontheotherhand,maybetheproductofmagmaticpre-cipitationathighpressure,oroftheincongruentmeltingofplagioclaseduringmagmaticrechargethroughthereactionNaCa-richPl=Crn+CaNa-richPl+Na-Si-richmelt(e.g.,Marigaet al.,2006b).Spinelinthepseudomorphs(Table6)ishercynitewithanaverageMg#of27,andtheassociatedsymplectiticplagioclaseisbytownite(An69Ab28Or3).Spinelalsoexistsinotherforms(notanalyzed)withintheandesiteas:(a)somewhatcorrodedeuhedralgreencrystals,(b)darkgreensubhedralxenocrystsisolatedinthevolcanicmatrix,

Majorelementsareinwt.%andtraceelementsinppm.PN1b:quartziticgneiss;PN17:garnetiferousgneiss;PN4:amphiboleorthopyroxenite;FO6605:buchiticxenolith;PN39W:adjacentcountryrock.*Wetchemicalanalysis;§majorelements,Cr,Co,Ni by XRF, and remaining trace elements by ICP-MS; ◊ major elements and Sc by ICP emission, trace elements by ICP-MS; ‡ XRF. d.l.: detection limit; n.d.: not determined. Average andesite after Gill (1981).

Table3(continued).Majorandtraceelementcompositionsoftheintrusivebody,xenolithsandlocalcountryrocksatPuenteNegro.


Plag

iocl

ase

mic

rolit

esO

pxA

mph

ibol

e xe

nocr

ysts

Deh

ydra

ted

rim

s on

am

phib

ole

xeno

crys

tsG

lass

esC

px

Opx

PlIl

m

Oxi

de w

t. %

Aver

age

SiO

256

.54

56.3

364

.63

64.8

452

.80

43.4

243

.40

41.1

040

.54

41.9

941

.02

40.3

41.2

040

.842

.88

51.6

040

.89

0.00

75.5

077

.10

77.6

076

.72

TiO

20.

020.

040.

050.

051.

402.

242.

012.

172.

262.

142.

162.

322.

232.

391.

500.

600.

9948

.30.

300.

280.

480.

35A

l 2O3

26.0

025

.91

22.3

622

.33

3.89

10.4

910

.68

11.9

111

.71

10.9

111

.34

13.4

14.3

313

.51.

562.

9228

.04

0.07

11.6

12.0

11.1

11.5

8Fe

OTo

tal

0.37

0.43

0.33

0.35

13.3

612

.51

8.85

13.5

513

.59.

8914

.06

12.1

12.2

12.8

8.03

19.3

70.

2649

.31.

541.

591.

501.

54M

nOn.

d.n.

d.n.

d.n.

d.0.

540.

120.

100.

090.

130.

080.

030.

450.

000.

54n.

d.n.

d.n.

d.n.

d.0.

000.

010.

020.

01M

gO0.

010.

030.

040.

0224

.84

12.8

314

.92

11.9

312

.114

.20

11.5

013

.513

.713

.914

.12

22.8

90.

001.

630.

160.

180.

130.

16C

aO8.

939.

968.

698.

641.

1311

.31

11.8

311

.57

11.6

11.9

511

.19

10.9

11.3

11.7

27.5

01.

3827

.10

0.00

0.55

0.55

0.62

0.57

Na 2

O6.

956.

675.

355.

10n.

d.3.

604.

215.

926.

005.

975.

742.

142.

222.

250.

000.

000.

410.

001.

751.

801.

641.

73K

2On.

d.n.

d.n.

d.n.

d.n.

d.0.

730.

680.

660.

680.

690.

730.

860.

880.

740.

00n.

d.0.

140.

172.

592.

342.

442.

46To

tal

98.8

299

.37

101.

4510

1.33

97.9

897

.25

96.6

898

.90

98.4

597

.82

97.7

795

.96

98.1

498

.69

95.5

998

.76

97.8

399

.41

94.0

395

.84

95.5

095

.12

Cat

ions

8O

xyge

ns23

Oxy

gens

CIP

W n

orm

Si2.

574

2.56

2.81

52.

823

1.93

6.46

6.41

6.13

6.09

6.23

6.20

6.08

6.05

6.01

1.66

1.92

2.01

0.00

Qz

56.3

57.4

59.0

57.5

7Ti

0.00

10.

001

0.00

20.

002

0.04

0.25

0.22

0.24

0.26

0.24

0.25

0.26

0.25

0.26

0.04

0.02

0.04

1.87

C5.

295.

784.

815.

29A

l1.

395

1.38

71.

148

1.14

60.

171.

841.

862.

092.

071.

912.

022.

372.

482.

350.

070.

131.

630.

00A

b15

.715

.914

.615

.40

Fe2+

0.01

40.

016

0.01

20.

013

0.41

1.56

1.09

1.69

1.69

1.23

1.78

1.53

1.50

1.57

0.26

0.60

0.01

2.13

An

2.88

2.83

3.22

2.98

Mn

n.d.

n.d.

n.d.

n.d.

0.02

0.02

0.01

0.01

0.02

0.01

0.00

0.06

0.00

0.07

0.00

0.00

0.00

0.00

Or

16.3

14.4

15.1

15.2

5M

g0.

001

0.00

20.

003

0.00

11.

352.

853.

292.

652.

703.

142.

593.

033.

003.

060.

821.

270.

000.

13H

y2.

913.

062.

442.

80C

a0.

436

0.48

50.

405

0.40

30.

041.

81.

871.

851.

861.

901.

811.

761.

781.

851.

140.

061.

43n.

d.Ilm

0.61

0.55

0.95

0.70

Na

0.61

30.

588

0.45

20.

43n.

d.1.

041.

211.

711.

751.

721.

680.

630.

630.

640.

00n.

d.0.

04n.

d.K

0.00

0n.

d.n.

d.n.

d.n.

d.0.

140.

130.

130.

130.

130.

140.

170.

160.

140.

00n.

d.0.

01n.

d.To

tal

5.03

45.

039

4.83

64.

818

3.95

15.9

616

.10

16.5

016

.56

16.5

116

.47

15.8

715

.86

15.9

54.

004.

005.

164.

13

An

41.5

49.2

47.3

42.3

97.0

Mg#

7864

7563

6172

5966

6766

7668

6

Tabl

e4.

Ele

ctro

nm

icro

prob

ech

emic

ala

naly

seso

fmaj

orp

hase

scom

posi

ngth

ePu

ente

Neg

roa

ndes

ite.


Plag

iocl

ase

Am

phib

ole

Clin

opyr

oxen

eO

rtho

pyro

xene

Gla

ss

Oxi

de w

t. %

Cor

eR

imC

ore

Rim

SiO

249

.92

49.0

050

.61

48.9

742

.96

42.0

943

.81

43.2

143

.03

41.4

441

.22

41.2

852

.03

51.4

152

.95

52.1

252

.20

51.9

854

.07

54.0

053

.91

54.7

552

.75

73.0

0Ti

O2

0.01

0.02

0.01

0.02

1.98

2.07

1.91

2.25

1.95

2.07

2.03

1.98

0.53

0.54

0.30

0.55

0.45

0.49

0.18

0.14

0.15

0.11

0.28

0.27

Al 2O

330

.65

31.3

030

.20

31.2

912

.43

12.2

711

.63

12.3

011

.56

12.4

612

.47

12.4

63.

253.

452.

223.

243.

283.

002.

612.

302.

272.

061.

8410

.32

Cr 2O

30.

000.

000.

000.

000.

000.

000.

040.

000.

000.

000.

000.

000.

960.

970.

600.

820.

700.

870.

080.

010.

000.

020.

04n.

d.Fe

OTo

t0.

360.

120.

350.

1212

.99

13.5

39.

968.

0610

.10

13.6

612

.91

13.1

15.

755.

996.

076.

666.

396.

8314

.81

14.8

014

.42

14.1

722

.01

1.91

MnO

n.d.

n.d.

0.00

0.03

0.14

0.13

0.12

0.09

0.14

0.25

0.16

0.27

0.08

0.14

0.14

0.12

0.11

0.16

0.26

0.20

0.90

0.22

0.37

0.02

MgO

0.03

0.02

0.03

0.02

13.1

712

.42

15.3

314

.45

14.5

312

.09

12.5

712

.84

14.6

414

.53

14.9

614

.51

14.5

714

.83

25.5

527

.14

26.9

627

.16

22.4

30.

16C

aO14

.27

15.3

214

.34

15.7

811

.23

11.2

111

.41

11.3

411

.47

11.1

511

.12

11.2

220

.87

20.5

720

.95

20.2

719

.74

20.0

51.

080.

910.

930.

901.

090.

83N

a 2O

2.73

1.93

2.73

1.99

2.22

2.13

2.20

2.50

2.19

2.14

2.10

2.18

1.07

0.78

1.11

0.99

1.38

1.29

0.39

0.12

0.19

0.08

0.03

0.27

K2O

0.21

0.16

0.21

0.16

0.60

0.67

0.61

1.07

0.60

0.67

0.67

0.60

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

2.28

Tota

l98

.18

97.8

798

.48

98.3

897

.72

96.5

297

.02

95.2

795

.57

95.9

395

.25

95.9

499

.18

99.3

899

.30

99.2

898

.82

99.5

099

.03

99.6

299

.73

99.4

710

0.84

89.0

6

Cat

ions

16O

xyge

ns23

Oxy

gens

6O

xyge

ns6

Oxy

gens

CIP

W n

orm

Si2.

316

2.28

12.

340

2.27

36.

332

6.31

06.

413

6.40

56.

410

6.27

06.

260

6.23

01.

921.

921.

961.

931.

941.

931.

971.

951.

951.

971.

95Q

tz66

.59

Ti0.

000

0.00

10.

000

0.00

10.

219

0.23

30.

210

0.25

10.

220

0.24

00.

230

0.23

00.

010.

020.

010.

020.

010.

010.

010.

000.

000.

000.

01O

r15

.13

Al

1.67

61.

717

1.64

51.

711

2.15

92.

168

2.00

62.

149

2.03

02.

220

2.23

02.

220

0.14

0.15

0.10

0.15

0.14

0.13

0.11

0.10

0.10

0.09

0.08

Ab

2.54

Cr

0.00

00.

000

0.00

00.

000

0.00

00.

000

0.00

50.

000

n.d.

n.d.

n.d.

n.d.

0.03

0.03

0.02

0.02

0.02

0.03

0.00

0.00

0.00

0.00

0.00

An

4.61

Fe2+

0.01

40.

005

0.01

40.

005

1.60

11.

696

1.21

90.

999

1.26

01.

730

1.64

01.

650

0.18

0.19

0.19

0.21

0.20

0.21

0.45

0.45

0.44

0.43

0.68

Hy

3.92

Mn

0.00

00.

000

0.00

00.

001

0.01

70.

017

0.01

50.

011

0.02

00.

030

0.02

00.

040

0.03

0.00

0.00

0.00

0.00

0.01

0.01

0.01

0.03

0.01

0.01

C6.

64M

g0.

002

0.00

10.

002

0.00

12.

894

2.77

63.

345

3.19

33.

230

2.73

02.

840

2.89

00.

810.

810.

830.

800.

800.

821.

391.

461.

451.

461.

23Ilm

0.57

Ca

0.70

90.

764

0.71

00.

785

1.77

31.

800

1.78

91.

801

1.83

01.

810

1.81

01.

810

0.83

0.82

0.83

0.81

0.79

0.80

0.04

0.04

0.04

0.04

0.04

Na

0.24

60.

174

0.24

50.

179

0.63

40.

619

0.62

40.

718

0.63

00.

630

0.62

00.

640

0.08

0.06

0.08

0.07

0.10

0.09

0.03

0.01

0.01

0.01

0.00

K0.

012

0.00

90.

012

0.00

90.

113

0.12

80.

114

0.20

20.

110

0.13

00.

130

0.12

0n.

d.n.

d.n.

d.n.

d.n.

d.n.

d.n.

d.n.

d.n.

d.n.

dn.

d.To

tal

4.97

54.

952

4.96

74.

965

15.7

4315

.747

15.7

4115

.729

15.7

4015

.790

15.7

8015

.830

4.03

4.00

4.02

4.01

4.00

4.03

4.00

4.01

4.02

3.99

4.00

An

7581

7481

7271

7168

7068

7577

7777

65M

g#64

6273

7772

6164

64En

42.6

943

.03

42.8

342

.47

42.6

242

.60

72.3

674

.65

73.9

575

.70

62.6

7Fs

9.34

10.1

89.

9411

.12

10.6

311

.19

24.0

023

.12

23.5

422

.50

35.0

3W

o43

.73

43.7

843

.10

42.6

441

.541

.39

2.20

1.80

1.83

1.80

2.19

Ac

4.

063.

004.

133.

775.

254.

82

1.44

0.43

0.68

n.d.

0.11

Tabl

e 5.

Ele

ctro

n m

icro

prob

e ch

emic

al a

naly

ses o

f mai

n ph

ases

in th

e ul

tram

afic

xeno

lith

PN4.


(c)palegreeninclusionsincalcicplagioclasephenocrysts,and(d)mantlingorreplacingcorundumxenocrystsinthevolcanicmatrix.

Orthopyroxene(En65.2Fs34.5Wo0.30)isveryaluminous(upto5.91wt.%Al2O3),withahighMg#of65,andrathercalcium-poor.Accompanyingbiotite(Table6)showshighTiO2content(4.74wt.%TiO2)andMg#(72)thattogetheryieldatemperatureof800ºCusingtheHenryet al.(2005)Ti-in-biotitegeothermometerforbiotitecrystallizationinthepseudomorph.Coexistingtitanomagnetiteandilmenitealsowereusedtoobtainanindependentestimateofthecrystallizationtemperatureforthepseudomorph,asdis-cussedbelow.

Non-stoichiometricgarnetxenocrystswithhighpyropecontent(Al54Pyr44Sp1.36Gr0.32)immersedinasani-dine-richmatrix,andtitanitexenocrystsalsofoundintheintrusionmayindicateassimilationofdeepcrustalmaterialsbytheandesiticmagma.

Evolution of the Puente Negro plumbing system

Althoughhornblendexenocrysts,“glomerocrysts”,andmicrophenocrystsintheandesiticmagmareproducethe assemblages in the gabbroic xenoliths (mafic and ul-tramafic), the order of crystallization cannot be established forthevolcanicrockbecauseofthestrongmineralandtextural modifications caused by interaction with the rocks thatprovidedthegabbroicxenoliths.Thus,thecrystal-lizationsequenceshowninFigure6isdeducedonlyfromthe petrography of ultramafic cumulitic xenoliths, which representanimportantplutoniccomponentofthePuenteNegroplumbingsystem.“Glomerocrysts”intheandesiteandcumuliticgabbroicxenoliths,bothrichinpargasiticamphibole,indicatefractionalcrystallizationathighwaterpressuresinanevolvingmagmachamberemplacedinthemiddlecrust,followedbytheirdisaggregation,possiblyduringmagmaticrechargeandrapidascentofasubsequentmagmabatchwithadifferentisotopiccomposition(seebe-

low).Sapphire,colorlesscorundum,high-Alorthopyroxene,andgreenspinelrepresentphasesintheandesitethatmayberestiticxenocrystsafterpartialmeltingandassimilationofpeliticandquartzo-feldspathicgneissesatvariousdepthsinthe lower crust or, alternatively, the first fractionated phases athighpressuresinthemantle.Theeuhedralaspectofspinelandcorundumxenocrysts(Figure4g-4h)andtheirpresenceasinclusionsinplagioclaseoftheandesite(Figure4c)sug-gesttheirveryearlycrystallizationintheplumbingsystemratherthanpluckedcrystalsfromahigh-grademetamorphicrock.Infact,inthebasalticsystemCMAS(cf. MilhollandandPresnall,1998)spinelandcorundumdevelopsmallwindowsfortheearlycrystallizationofthesephasesathightemperaturesandpressuresinthemantle.

Themagmabatchparentaltothegabbroicxenolithsandhornblendexenocrystsintheandesiteunderwentsequentialcrystallization(Figure6) inthemiddleorlowercrust,apparentlystartingwithminorclinopyroxene,hercyniteandilmenite,closelyfollowedbyabundantorthopyroxeneand amphibole, and finally by abundant plagioclase and amphibole.Absenceofolivineandearlypresenceofclinopyroxeneandsomespinelintheliquidusindicate,in

PHASE

Crn ?Spl ?CpxOpxAmpPl

TiMagIlm/RtGlass

MAGMATIC STAGEDEEP INTERMEDIATE SHALLOW

Table6.Electronmicroprobechemicalanalysesofmainphasescomposingthe“sapphirine”pseudomorphandasapphirexenocrystinPN32.CoexistingTiMagandIlmwereusedforgeothermometryofthepseudomorphformation.

Oxide wt% Spl TiMag Ilm Opx Pl Crn Sapphire Bt

SiO2 0.31 0.48 0.07 0.02 0.05 0.04 0.21 0.00 48.80 49.64 55.34 0.03 0.01 38.93TiO2 0.44 0.59 0.77 10.29 9.76 46.28 43.09 48.53 0.24 0.33 0.00 0.34 0.33 4.74Al2O3 58.82 58.82 50.36 4.74 3.88 0.09 0.25 0.08 5.06 5.91 27.47 100.6 97.92 12.62Cr2O3 0.00 0.00 0.09 0.04 0.07 0.03 0.07 0.06 0.04 0.00 0.00 0.06 0.06 0.00FeOTotal 28.53 28.17 37.43 78.79 80.46 51.39 52.06 50.12 20.98 20.58 0.07 0.36 1.38 11.17MnO 0.13 0.14 0.98 0.42 0.40 0.32 0.57 0.38 3.40 1.84 0.00 0.00 0.01 0.05MgO 11.72 12.17 10.28 4.89 4.64 2.20 3.18 1.36 20.85 21.77 0.01 0.04 0.03 16.41CaO 0.08 0.10 0.03 0.15 0.08 0.01 0.11 0.02 0.22 0.13 11.24 0.01 0.02 0.05Na2O n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 2.55 n.d. n.d. 0.27K2O n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 0.38 n.d. n.d. 11.06Total 100.03 100.47 100 99.34 99.34 100.36 99.54 100.55 99.59 100.20 97.06 101.4 99.76 95.30

Figure6.InferredpolybaricsequentialcrystallizationinthePuenteNegro“glomerocrysts”,gabbroicxenoliths,andhigh-Alxenocrystsdomains.


Mantle Melting

Basaltic Magma 1

Gabbroic rocks

Deep xenoliths

Basaltic magma 2

PN dike + xenoliths

~30 Ma ~29 Ma

15-20 km

~29 Ma

principle,fractionalcrystallizationathighpressure(e.g., ElthonandScarpe,1984),whereasthelateappearanceofplagioclasemaybeduetohighwaterpressureduringcrystallizationofthebasicmagmaatdepth.Virtuallyallamphibolesshowsimpletomultipledecompressionrimsofopaciteoranhydrouspyroxene-plagioclase-titanomagnetiteassemblages,andtheiroscillatorycolorzoningmaybeduetoisobaricT-fH2Ochangescausedbyrepeatedinjectionofbasalticmagmaintoadifferentiatinghydrousmaficmagmachamber(e.g., BachmannandDungan,2002).Somemagmaticpulseswithextremelyrapidascentratesareindicatedbyverynarrowtoabsentopaciterimsforsomeamphiboles,asopposedtoothercrystalswithbroaderopaciterimsorcompleterecrystallizationtotheassemblagesPl-Cpx-Mag-Ilm±Bt±Rt±Fa,andPl-Opx-TiMagthatindicateslowercoolingrates(Buckleyet al.,2006).Infact,giventhepargasiticcompositionofallamphiboles,itspresenceinthesilica-saturatedandesite-dacitedikessuggeststhatthisphaseprobablywasderivedfromthepostulatedgabbroicbody.Magmaticrecharge,possiblybyfreshbasalticbatches,mayhavetriggeredrapidascenttoshallowlevelsofthedifferentiatedmagma,asdeducedfromthequenchedplagioclase-glasspatchesandthepartialtototaldehydrationofhornblendeingabbroicxenoliths(cf.RutherfordandDevine,2003).QUILF(quartz-ulvospinel-ilmenite-fayalite)(Andersenet al.,1993)calculationsoncoexistingaugiteandorthopyroxeneinthehornblendepyroxenitecumuliticxenolith(seeTable5)yield,at4–6kbar,temperaturesof944–949±57ºCforthecrystalcores,and900–906±9ºCfor theirrims, in thiscaseprobablyindicatingminorcationreadjustmentduringcooling.Temperaturesof909and920ºCareobtainedforHbl-Plpairs(HollandandBlundy,1994)forpressuresof4and6kbar,respectively.Heatingbycrystallizationanddegassing,asproposedbyBlundyet al.(2006)formanyandesitesinmagmaticarcs,alsoremainsastrongpossibilitytoexplainmanyfeaturesofPuenteNegro,suchastheabundanceofmicrolitesofplagioclaseandorthopyroxeneintheandesiticmatrix,dehydrationofhornblende,andthehigh temperatures of final emplacement above the quartz-tridymitephasetransition.Furthercontaminationbypartialmelting,assimilationandphysicaldisruptionofgraniticgneissesnearthesurfaceresultedinthepresentcomplexmineralogyandbulkandesitic-daciticcompositionofthePuenteNegrodikes.

Figure 7 provides a simplified model for the possible plumbingsystemofPuenteNegro,andTable7showsphasedistributionaccordingtothepetrographicdomainsdistinguishedinthePuenteNegroandesite.

High-grade metamorphic xenolithsGiventhegreatdiversityofmineralandmeltassem-

blagesfoundinthemetamorphicxenoliths,thefollowingdata,obtainedbymicroprobeanalysesofalimitednumberofsamplesrepresentonlyafew,albeitessentialpetro-logicaspectsofthecomplexevolutionofthePuenteNegro

magma-xenolithsystem.Thecompositionofimportanthigh-gradephasesis

giveninTable8(silicatesandoxides)andTable9(garnetanditsdecompressioncorona),butseveralotherphases,suchasmullite,corundum,staurolite,sillimaniteandclinopyroxene,werenotanalyzedduetotheirverysmallsize.Figure8givesourinterpretationontheoriginoftheanalyzedmetamorphicphasesintermsoftwomajorevents:deep-seatedgranulitefaciessuperimposedbyveryshallowsanidinitefacies.

Silica phases. SiO2-rich phases in most xenolithsconsistofquartz,tridymite,invertedtridymite(paramorphiclaths in optical continuity with quartz), opal, and twofibrous phases, possibly chalcedony and metastablecristobalite. However, whereas optical properties oftridymitearediagnostic(biaxialpositive,triangularfacets,pie-shaped twinning, and very low birefringence andrelief), identification of pristine cristobalite (very low relief, isotropic)istentative.MostgrainsoftridymiteareclosetopureSiO2,butshowincursorymicroprobeanalyses(Table8)smallcontents(<1wt.%)ofCaO,MgO,andFeO,withAl2O3rangingfrom0.48to1.02wt.%,indicatingincipientsolidsolutionwithmullite.Thebrittlebehavior(polygonalcracking)ofallquartzatthesehightemperaturesatteststotheextremelydryenvironmentinwhichitrecrystallizedandmayindicatecontractionfromhightolowquartz.

Al-silicatess. Long prismatic to acicular silicatephasesresemblingsillimanitebutoccurringinbundlesandintimatelyassociatedwithotherhightemperaturealuminousphases suchas spinel andAl-richhyperstheneandglass

Figure 7. Simplified plumbing system model for the Puente Negro intrusion, includingtheuptakeofxenolithsatdifferentlevelsinthecrust.


areinterpretedasmullitesolid-solutionphases(Figure5h)basedonEDSanalyses(molAl2O3/SiO2>1).Unfortunately,quantitativeWDSanalyseswerehamperedbytheminutewidths(<3µm)ofthesecrystals.Ontheotherhand,relictsillimaniteapparentlycoexistingwithorthopyroxeneandquartz in a microxenolith was identified optically in sample FO6605,andalsoinsideacomplexpoikiloblastcomposedofbytownite-spinel-corundum-garnet-sillimanite-stauroliteinFO8305.

Spinel.Spinelispresentinallhigh-gradexenolithsand it is compositionally inhomogeneous. It is almost apure hercynite-spinel solid solution (Hc57–75) varying incolorfromopaquetopalegreen,olivegreenorbrown,andis usually associated with titanomagnetite, glass, calcicplagioclase, aluminous orthopyroxene, quartz/tridymite,mullite,corundum,ilmenite,andrutile,dependingontheoriginalmineralassemblagereplacedduringdecompressionand pyrometamorphism.Analyses shown in Table 8correspondtocrystalsintheglass-richquartzosegneisses.Notethelargerhercynitecontentsofspinelsdevelopedinthedecompressionmantlesofgarnets(Table9).Thestableassemblage Opx-Spl-Trd/Qz, in the absence of garnet,maybe explainedby the coupled reactionsGrt+Crd=2Opx+3Spl+6Qz/Trd,andGrt+2Sil=3Spl+5Qz/Trdduringheatinganddecompression.Commonly,spinelandcorundumappeartogradeintoeachother,butspinelalsoappears replaced by the assemblage Crn-Mag in somedecompression coronas of garnet porphyroblasts (PN29)throughthereaction3Hc=3Crn+Mag.Elongatedcrystalsofspineloccurinonequartzosegneiss(FO8305)ascorestoanAl-silicatess.

Garnet.Textural and chemical features of garnetporphyroblastsrevealdifferentP-Thistories,compositions,degreeoffracturing,andzoningpatterns.Analyzedgrains(Tables 9 and 10) show high manganese and moderatecalcium, low magnesium, and a remarkable chemicalhomogeneityoftheirinteriorcores.Resultsofamicroprobetraverseperformedononeofthesecoresanditsfracturedenvelope are shown in Table 10 and Figure 9, where a flat profile stands out for all cations across the area. The internal zonearoundthecoreisstronglyfracturedandmuchricherincalcium,probablyindicatingcontrastingP-Tinheritedconditionsofrecrystallizationpriortoincorporationinthemagma.TheouterzoneofmostgarnetsconsistsofkelyphiticcoronaswiththeassemblagePl-Opx-Spl/Mag±glass±Crn±Rtwherethepresenceofcorundumandmagnetitewasprobably controlled by the reaction 3Hc + O2= Mag +3Crn, and rutile from possible biotite inclusions in thegarnet. Minor clinopyroxene (Jd1.9CaTsch16.1Hd15.4Di66.6)wasalso foundduring routinemicroprobestudiesof thecorona. These features are important petrogeneticallybecause they demonstrate that garnets are polyphaseand remained at high temperature and relatively highpressureforalongtimeuntilentrainmentandfastupliftinthemagma.

Orthopyroxene. It isubiquitous inallmetamorphicxenolithswithgranulitefaciesandsanidinitefaciesthermalimprintsas:(a)stronglycolored,stubbytoacicularprismsimmersedinglassofpartiallymeltedgneissesandquartzites(Table8),(b)subhedralcrystalsinthecoroniticalterationof garnet (Table 9), (c) mantling quartz/tridymite ovalgrainsinsideglasspatchesofthestronglymeltedshallow

Domains and phases

Metsedxenoliths

Garnet corona

Gabbroicxenoliths

Quenchedgabbroic matrix

Volcanic matrix

Pheno-crystal

Xeno-crystal

Glomero-crystal

Hbl rims Pseudo-morph

Pl X X X X X X X X X XCpx X X X X X X X XOpx X X X X X X X X XHbl X X XSpl X X X X XCrn X X X XTiMag X X X XIlm X X XGrt X X XSil X XMul X XQz X XTrd XCrs XSt X XBt X XSa X XRt X X X XZrn X XGlass X X X X X

Table 7. Interpreted origin of most relevant petrographic objects (phases and domains) identified in the Puente Negro andesite and its metasedimentary andgabbroicinclusions.


Ort

hopy

roxe

neSp

lPl

agio

clas

eIl

men

iteTr

idym

ite

Oxi

de w

t. %

SiO

247

.32

46.9

347

.35

48.8

049

.64

0.09

0.08

n.d.

n.d.

57.2

357

.69

61.0

952

.96

53.8

955

.82

0.04

0.02

97.3

296

.06

TiO

20.

420.

40.

380.

240.

330.

350.

360.

460.

410.

040.

060.

040.

000.

010.

0050

.62

50.3

00.

000.

00A

l 2O3

11.6

11.2

311

.33

5.06

5.91

60.7

960

.91

59.6

759

.61

27.6

027

.16

24.7

330

.25

29.8

928

.78

0.22

0.26

0.48

1.02

FeO

Tota

l21

.923

.14

23.0

820

.98

20.5

827

.97

28.0

628

.21

28.7

30.

210.

360.

240.

380.

280.

2143

.76

440.

310.

65M

nO0.

150.

110.

123.

401.

840.

110.

120.

100.

10n.

d.n.

d.n.

d.0.

000.

000.

000.

220.

230.

000.

02M

gO19

.83

18.8

618

.46

20.8

521

.77

11.9

511

.77

11.6

511

.75

0.03

0.04

0.05

0.10

0.09

0.08

4.03

4.18

0.16

0.39

CaO

0.16

0.14

0.13

0.22

0.13

0.09

0.09

0.06

0.05

9.58

9.37

6.82

12.9

612

.33

11.1

50.

010.

000.

110.

52N

a 2O

n.d.

n.d

n.d

n.d

n.d

n.d

n.d

n.d.

n.d.

6.81

6.40

8.34

4.25

4.62

5.22

n.d.

n.d.

0.07

0.06

K2O

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

0.07

0.07

0.11

0.03

0.04

0.06

n.d.

n.d.

n.d.

n.d.

Tota

l10

1.38

100.

8110

0.85

99.5

510

0.20

101.

3510

1.39

100.

1510

0.65

101.

5310

1.15

101.

4210

0.93

101.

1510

1.32

98.9

098

.99

98.4

598

.72

Cat

ions

6O

xyge

ns32

Oxy

gens

16O

xyge

ns

Si1.

732

1.73

81.

750

1.84

61.

843

n.d.

n.d.

n.d.

n.d.

2.54

02.

560

2.69

02.

382

2.41

22.

483

Ti0.

011

0.01

10.

010

0.00

70.

009

0.05

60.

058

0.07

50.

067

n.d.

n.d.

n.d.

0.00

00.

000

0.00

0A

l0.

500

0.49

00.

493

0.22

60.

259

15.3

4715

.373

15.2

7915

.223

1.44

01.

420

1.28

01.

603

1.57

71.

509

Fe2+

0.67

00.

716

0.71

30.

663

0.63

95.

015.

025

5.12

55.

206

0.01

00.

010

0.01

00.

014

0.01

00.

008

Mn

0.00

50.

003

0.00

40.

109

0.05

80.

020.

022

0.01

80.

018

n.d.

n.d.

n.d.

0.00

00.

000

0.00

0M

g1.

081

1.04

01.

016

1.17

61.

205

3.81

63.

758

3.77

43.

796

n.d.

n.d.

n.d.

0.00

70.

006

0.00

5C

a0.

006

0.00

50.

005

0.00

90.

005

0.02

10.

021

0.01

40.

012

0.46

00.

550

0.71

00.

624

0.59

10.

531

Na

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

0.59

00.

550

0.31

00.

371

0.40

10.

450

Kn.

d.n.

d.n.

d.n.

d.n.

d.n.

d.n.

d.n.

d.n.

d.0.

000

0.00

00.

010

0.00

20.

002

0.00

3To

tal

4.00

54.

003

3.99

14.

036

4.01

824

.270

24.2

5724

.285

24.3

225.

040

5.09

05.

010

5.00

35.

001

4.99

0

An

4445

3163

5954

Mg

#62

5959

6465

4343

4242

Wo

0.4

0.3

0.3

0.5

0.3

En61

.559

.158

.660

.063

.2Fs

38.1

40.6

41.1

39.5

36.5

Tabl

e8.

Ele

ctro

nm

icro

prob

ech

emic

ala

naly

seso

fmai

nph

ases

com

posi

ngh

igh-

grad

em

etas

edim

enta

ryx

enol

iths(

PN1,

PN

17,P

N19

).Fo

rgar

net,

see

Tabl

es9

and

10.


(buchite)xenolithsand(d)inpseudomorphicassociationswithhigh-Alphasesandglass.Deformedorsharplycolor-zoned orthopyroxene crystals found occasionally in theandesite may be xenocrystic. Orthopyroxene coexistingwithgreenspinelandimmersedinglassinsideanimpurequartzite (PN1, Table 8) is extremely rich in alumina(averageofthreegrainsis11.4±0.19Al2O3wt.%)andpoorin calcium, whereas the coronitic orthopyroxene aroundgarnetonlycontainsupto1.72wt.%Al2O3.Intheshallowxenolith(Table11),orthopyroxeneshowsupto0.92wt.%Al2O3,andconsiderablemanganeseandcalcium.Intensegreentobrownorpaleorangepleochroismdistinguishesthe most aluminous types, including grains that containspinelandcorunduminclusions(FO8305).Thesetexturesand compositions suggest variable P-T conditions oforthopyroxene crystallization recorded by the xenoliths.TheMg#inthecoroniticpyroxenearoundgarnet(Table9)rangesfrom65to69,whereasaluminousorthopyroxeneinthehigh-grademetasedimentaryxenolithsshowsslightlylowervaluesof59to62(Table8).

Feldspar. Plagioclase in metamorphic xenoliths iscommonandof several types (Tables8,9,11)one type

crystallizedasneedleswithinglass,anothertypeformedslender prisms nucleated on most quartz grain surfaces(PN19), and other grains form part of the symplectiticcoronas around decompressed garnet (PN17), or occurclosely associated with orthopyroxene. Plagioclaseassociatedwithgarnetbreakdownisoftwotypes,onebeingnearlypureanorthiteandtheotherlabradorite.Anorthitecontentsinplagioclasevariesaccordingtotheirpositionintheinternalorexternalpartsofthecorona(Table9).Sodicsanidine(Ab64Or32An4)wasrarelyfoundinthegroundmassofasmallxenolithinPN32,perhapsbecausemostalkaliswerepartitionedtotheglassphaseonmeltingoftheoriginalbiotite-plagioclasegneissicxenoliths.

Fe-Ti oxides.Fe-TioxidesarepresentinallxenolithsandigneousrocksofPuenteNegro.Representativeanalysesof ilmenite-magnetite-ulvospinel solid solutions fromvariousdomainsappearinTables4,6,8and9.Ilmenitein the quartzo-feldspathic xenoliths is notable for itshighcontentofMgO(geikielite)reachingupto4.2MgOwt.%withsmallercontentsofMnO(pyrophanite)(Table8).TitanomagnetitealsoyieldssubstantialcontentsofMgO,andupto39%ulvospinelcomponent.Onetitanomagnetite

Components of garnet coronaSpl Spl Opx Opx Pl Pl Pl Ilm TiMag

Oxide wt. %SiO2 0.54 0.07 52.71 53.44 45.20 44.79 46.66 0.04 0.70TiO2 0.80 0.77 0.12 0.19 0.50 0.34 0.06 46.28 17.52Al2O3 45.84 50.36 0.77 1.72 34.02 35.54 35.24 0.09 3.46Cr2O3 0.09 0.09 n.d. n.d. n.d. n.d. n.d. 0.03 0.47FeOTotal 43.39 37.43 23.16 20.28 0.90 0.79 0.66 51.39 69.18MnO 1.32 0.98 0.25 0.19 n.d. n.d. n.d. 0.32 0.90MgO 8.57 10.28 23.72 24.86 0.10 0.04 0.09 2.20 3.90CaO 0.04 0.03 0.73 0.55 17.35 17.97 17.39 0.01 0.40Na2O n.d. n.d. n.d. n.d. 1.00 1.21 1.31 n.d. n.d.K2O n.d. n.d. n.d. n.d. 0.08 0.06 0.08 n.d. n.d.ZnO 0.22 0.35 n.d. n.d. n.d. n.d. n.d. n.d. n.d.Total 100.8 100.4 101.46 101.2 99.15 100.7 101.5 100.4 96.53Mg# 26 33 65 69 n.d. n.d. n.d. n.d. n.d.

CationsSi n.d. n.d. 1.944 1.944 2.120 2.060 2.120 n.d. n.d.Ti 0.145 0.134 0.003 0.005 n.d. n.d. n.d. n.d. n.d.Al 13.02 13.75 0.033 0.074 1.880 1.930 1.890 n.d. n.d.Cr 0.003 0.003 0.000 0.000 n.d. n.d. n.d. n.d. n.d.Fe2+ 8.745 7.253 0.714 0.620 0.040 0.030 0.030 n.d. n.d.Mn 0.269 0.192 0.008 0.010 n.d. n.d. n.d. n.d. n.d.Mg 3.079 3.551 1.304 1.350 n.d. n.d. n.d. n.d. n.d.Ca 0.010 0.007 0.029 0.020 0.870 0.890 0.850 n.d. n.d.Na n.d. n.d. 0.000 0.000 0.090 0.110 0.120 n.d. n.d.K n.d. n.d. 0.000 0.000 0.000 0.000 0.000 n.d. n.d.Total 25.27 24.89 4.035 4.023 5.000 5.020 5.010 n.d. n.d.

Table9.Electronmicroprobechemicalanalysesofmineralscomposingthegarnetcoronainthegarnetiferousgneiss(PN17).

continues


Garnet internal zoneRim Core Rim

Oxide wt. %SiO2 37.65 37.68 37.56 37.37 37.37 37.38 37.58 37.58 37.52 37.34 37.32 37.39TiO2 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.Al2O3 21.19 21.11 21.09 20.95 21.11 21.14 21.05 21.11 21.16 21.13 20.95 21.11Cr2O3 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.FeOTotal 25.69 25.62 25.65 25.51 25.67 25.48 25.38 24.98 25.59 25.74 25.92 25.98MnO 12.98 12.83 13.18 13.04 12.98 13.24 12.96 13.17 12.97 13.14 13.17 13.04MgO 1.38 1.39 1.42 1.42 1.37 1.36 1.39 1.37 1.38 1.38 1.36 1.36CaO 1.54 1.60 1.63 1.73 1.69 1.70 1.64 1.65 1.69 1.65 1.54 1.56Na2O n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.K2O n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.ZnO n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.Totals 100.4 100.22 100.53 100.01 100.19 100.30 99.99 99.86 100.31 100.37 100.25 100.45Mg# 9 9 9 9 9 9 9 9 9 9 8 8

Cations 12OxygensSi 3.030 3.040 3.030 3.030 3.020 3.020 3.040 3.040 3.030 3.020 3.020 3.020Ti n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.Al 2.010 2.010 2.000 2.000 2.010 2.010 2.010 2.010 2.010 2.010 2.000 2.010Cr n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.Fe2+ 1.730 1.730 1.730 1.730 1.740 1.720 1.720 1.690 1.730 1.740 1.760 1.750Mn 0.890 0.880 0.900 0.890 0.890 0.910 0.890 0.900 0.890 0.900 0.900 0.890Mg 0.170 0.170 0.170 0.170 0.170 0.160 0.170 0.170 0.170 0.170 0.160 0.160Ca 0.130 0.140 0.140 0.150 0.150 0.150 0.140 0.140 0.150 0.140 0.130 0.130Na n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.K n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.Total 7.960 7.970 7.970 7.970 7.980 7.970 7.970 7.950 7.980 7.980 7.970 7.960

Table9(continued).Electronmicroprobechemicalanalysesofmineralscomposingthegarnetcoronainthegarnetiferousgneiss(PN17).

analyzedforvanadium(EDSnotshown)yielded1.42wt.%V2O3.Rutileiscommoninquartzosegneissesasindividualcrystals,andcoringorrimmingilmenite,butoftenformscomposite, coronitic and symplectitic aggregates withilmenite, spinel, and rarely zircon. These aggregatessometimes suggest the breakdown of rutile-thialite andarmalcolite or ferropseudobrookite solid solutions.Hexagonalplatesofareddishbrownphasepresentinglass-richportionsofthequartzosexenolithsandassociatedwiththe high-Al xenocrystic pseudomorphs were tentativelyidentified as pyrophanite based on its distinctive optical properties.

Clinopyroxene. Clinopyroxene in the metamorphicxenolithsoccursasstubby,euhedralprismsisolatedwithinstrongly brown glass (FO6605) and abundantly in thecalcsilicatexenolith,whereitshowscolorsfrombrowninthecentertogreeninthemargin,suggestingarathersodiccomposition.Onlytwoanalysesweremadeonthismineralcorresponding to a secondary augite formed within thebuchitexenolithFO6605(Table11).

Glasses. Glasses are abundant and diverse in allmetamorphic xenoliths. Representative microprobe

analyses (Table 12) are from restitic quartzose gneisses(PN1,PN19,PN23),garnetiferousgneisses(PN17,PN18),and a buchite shallow xenolith (FO6605, Table 11).Compositionaldifferences are evident,withmuch lowersilica and higher alumina, alkalis, and calcium contentswithin thegarnetiferousxenoliths (deep-seated) than thesilica-richglassesof the shallowquartzosegneisses thatcontainnormativecorundum(upto9.16mol%),andlargeamounts of normative quartz (up to 70%).All glassescontainsubstantialamountsofiron.

Brownandcolorlessglassesintheshallowxenolithsdifferinironandsilicacontents,andwatercontentmaybe5%lowerinthebrownglass(seeTable11).However,bothtypescommonlymergeintoeachother,andthecolorlessglassalsoformsmicroscopicveinsthatcutacrossallotherphases,includingthebrownglass.Thesilicicglassesformedinpartiallyfusedxenolithsandwerelocallymixedwiththemagmabearingthewallrockxenolith,assuggestedbythegradualmergingofthebrownandclearglasses.Brownglassoftenremainedin situforming,withcalcicplagioclase,symplectiticpseudomorphsprobablyafteramphiboleandsodicplagioclase.

continues


Geothermobarometry of the xenoliths

Quantification of the peak temperatures attained dur-ingdeep-seatedmetamorphismcanbederivedfromtheFMASphasediagrambasedonexperimentalandtheoreticaldataforAl-orthopyroxenecoexistingwithsapphirineandquartz(LiermannandGanguly,2003;HollisandHarley,2003).Accordingtothelatterauthors,atpressuresbetween12and16kbars,andtemperaturesof1250and1350ºC,aluminumsolubilityinorthopyroxeneshowsastrongdepen-denceontemperature,butveryslightlyonpressure.TheAlcontentinOpxmeasuredinthoseexperimentsvariedfrom0.374to0.547a.p.f.u.(atomsperformulaunit),whichifcomparedtovaluesof0.49to0.50a.p.f.u.measuredforthemostaluminousorthopyroxeneofPuenteNegroquartzosexenoliths,itwouldindicateveryhightemperaturesandhighpressures,albeitnotpreciselydeterminableatPuenteNegrobecauseoftheactualabsenceofsapphirineduetoinappropriatechemicalcomposition(relativelyhighFe/Mgbulkcompositions)ratherthandifferentP-Tconditions.Theupto44%a.p.f.u.contentoftheMg-Fe-Tschermakiteendmemberinthemostaluminousorthopyroxeneofquartzose

xenolithsofPuenteNegrocoexistingwithspinelisconsis-tentwithtemperaturesinexcessof1100ºC(Kelseyet al.,2003;Kosyakovaet al.,2005).Moreover,themaximumAlcontentoftheenstatitefoundbyGrochauandJohannes(1997)duringaexperimentalstudyofphlogopitestabilityonlyyielded7.6Al2O3wt.%,whereasPuenteNegroor-thopyroxenescontainupto11.60wt.%alumina,probablyreflecting higher temperatures for the metamorphism that affectedthexenoliths.Finally,zonedAl-orthopyroxeneintheNapierComplex(Antarctica),withAl2O3contentssimilartothoseofPuenteNegro,formedattemperaturesabove1120ºCandpressuresaround10–12kbar(HarleyandMotoyoshi,2000).Unfortunately,pressuresforthecrystal-lizationofPuenteNegrohigh-Alorthopyroxenecannotbeaccuratelyobtainedsolelyfromthissystem.

SeveralmineralogicalandgeothermometriccriteriamaybeappliedtoPuenteNegroxenolithstoestimatetheveryhightemperaturesofmetamorphism,mostofwhichconverge tominimumvaluesabove1000ºC.Variousmagnetite-ilmenite thermometerscalculatedwith theILMATExcelworksheetofLepage(2003)yieldvariable,albeitconsistenttemperaturesbetween966and1072ºC,with fO2between(log10)-7.60and-10.71for ilmenite/titanomagnetitepairsinthedecompressioncoronasofgarnetofthegarnetiferousgneisses(PN17),andbetween961and1035ºCwithfO2(log10)-9.07to-11.52forpairsinthedecompressionalxenocrystwithintheandesitePN32(seeTable6).TheQUILFprogramofAndersenet al.(1993)wasalsousedforthegarnetcoroniticoxideassemblage

Table9(continued).Electronmicroprobechemicalanalysesofmineralscomposingthegarnetcoronainthegarnetiferousgneiss(PN17). Phase

QuartzTridymitePlagioclaseSanidineGarnetOrthopyroxeneClinopyroxeneSpinelCorundumSillimaniteMulliteRutileIlmeniteTitanomagnetiteBiotiteGlass

Granulite Facies Sanidinite Facies

Figure8.PhasestabilitybardiagraminferredforthetwomainmetamorphiceventsregisteredinthePuenteNegroxenoliths.

Garnet external zoneRim Core Rim

Oxide wt. %SiO2 37.87 36.78 37.98 38.51 37.62 37.39TiO2 n.d. n.d. n.d. n.d. n.d. n.d.Al2O3 21.19 21.88 21.45 21.32 20.83 21.02Cr2O3 n.d. n.d. n.d. n.d. n.d. n.d.FeOTotal 22.42 24.05 22.70 22.67 23.24 24.42MnO 10.87 10.42 10.11 9.83 9.30 8.47MgO 0.94 0.88 0.93 0.92 0.92 1.06CaO 7.34 7.35 7.32 7.67 7.70 6.99Na2O n.d. n.d. n.d. n.d. n.d. n.d.K2O n.d. n.d. n.d. n.d. n.d. n.d.ZnO n.d. n.d. n.d. n.d. n.d. n.d.Totals 100.63 101.36 100.49 100.91 99.62 99.34Mg# 7 6 7 7 7 7

Cations 12OxygensSi 3.020 2.940 3.030 3.050 2.990 3.020Ti n.d. n.d. n.d. n.d. n.d. n.d.Al 1.990 2.060 2.010 1.990 2.050 2.000Cr n.d. n.d. n.d. n.d. n.d. n.d.Fe2+ 1.500 1.600 1.510 1.500 1.550 1.650Mn 0.730 0.700 0.680 0.660 0.630 0.580Mg 0.110 0.100 0.110 0.110 0.110 0.130Ca 0.630 0.630 0.620 0.650 0.660 0.600Na n.d. n.d. n.d. n.d. n.d. n.d.K n.d. n.d. n.d. n.d. n.d. n.d.Total 7.980 8.030 7.960 7.960 7.990 7.980


yieldingsimilarvaluesat1020±81to965±72ºC,withoxygenfugacityof(log10)-10and-10.5,respectively(oxidesinTable9).Garnet-orthopyroxenegeothermometry(e.g.,LeeandGanguly,1988)appliedtothecoronasofdecompressedgarnetsyieldunrealisticlowtemperatures,probablybecausegarnetisnolongerinequilibriumwiththecoroniticorthopyroxene.TheTi-in-biotitethermometerofHenryet al.(2005),basedonthepresenceofsomebiotiteinthealuminouspseudomorphs(seeTable6),yieldsatemperatureof800±12ºCforformationoftheSpl-Pl-Crn±Opx±Bt±Rtpseudomorph,whichisprobablyaminimumvaluebecausequartzandgraphitearenotpresentintheassemblage.

Unfortunately,precisegeobarometrycouldnotbeperformedwiththeassemblagesofPuenteNegrobecausegarnetisnolongerstablewiththerestoftheminerals,andcordierite was not positively identified. However, minimum depthconditionsforthexenolithP-TpatharesuggestedbytexturalevidenceofreactionssuchasAl-Opx=Spl+Qzcoexistingwithmelt,andthetotalabsenceofcordieriteintheplutonicassemblages(garnetcoronas),whichrequirespressuresabove3.5kbaraccordingtothermodynamicphase-meltequilibriaatveryhightemperature(e.g., VielzeufandHolloway,1988).Higherminimumpressuresarederivedfrom the crystallization of the cumulate ultramafic xenolith PN4(Table5),whoseaveragepressurewascalculatedonthebasisoftotalmolarAlinhornblende(cf.Johnsonand

Rutherford,1989)atabout5.75±0.8kbarforthegabbroicamphiboles,and4.85±0.8kbarfortheandesiticamphiboles.Thesepressures,however,maybeoverestimatedbyabout1.2kbar,astheassemblagesaccompanyingamphibolearequartz-free,implyingasilicaactivitylowerthanunity.Othersimilar,butstillminimumpressuresaffectingsomeofthexenolithsduringentrainmentinthehot,hornblende-

Table10.ElectronmicroprobechemicalanalysesacrossthecorezoneofagarnetporphyroblastofPN17andtwogarnetxenocrystsintheandesite.

mol

%

80

70

60

50

40

30

20

10

01 2 43 5 6 7 8 9 10 11 12 13

Point number

ALMPRPSPSGRS

Rim Core Rim

Figure9.Microprobetransectacrosstheunfracturedcoreofagarnetxenocryst.Notelackofzoningforallelements.

1*Averageofsixpointsfromcoretorim.2**Averageoffourrandompointsinthegarnetxenocryst.

Left Rim Central Zone Right Rim 1* 2**

Oxide wt. %SiO2 37.16 37.45 37.59 37.13 37.19 37.39 37.14 37.64 37.34 37.26 37.15 37.10 37.74 36.69 37.81TiO2 0.05 0.05 0.05 0.07 0.05 0.06 0.08 0.05 0.05 0.06 0.05 0.09 0.11 0.05 0.08Al2O3 19.60 19.67 19.92 19.95 20.45 19.89 19.57 19.69 19.66 19.09 19.57 19.60 19.87 20.71 20.87FeOTotal 31.50 31.14 30.76 30.64 29.84 30.31 30.16 30.21 30.65 32.02 30.42 30.09 30.53 29.94 27.46MnO 6.94 7.46 7.62 8.06 7.85 8.48 8.46 8.31 8.20 7.74 8.37 8.17 7.46 7.70 6.44MgO 2.17 2.09 1.99 2.02 2.10 1.94 2.01 1.91 2.00 1.91 2.04 1.94 2.10 2.59 1.95CaO 2.60 2.83 2.67 2.86 2.68 2.91 3.01 2.81 2.74 2.82 2.81 3.00 2.94 2.54 3.35Total 100.02 100.69 100.60 100.73 100.16 100.98 100.43 100.62 100.64 100.90 100.41 99.99 100.75 100.21 97.96

Cations 12OxygensSi 3.03 3.03 3.04 3.01 3.01 3.02 3.02 3.05 3.03 3.03 2.99 3.03 3.04 2.97 3.08Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00 0.00AlTotal 1.88 1.88 1.90 1.90 1.95 1.89 1.88 1.88 1.88 1.83 1.85 1.88 1.89 1.98 2.00Fe2+ 2.15 2.15 2.08 2.08 2.02 2.05 2.05 2.04 2.08 2.18 2.18 2.05 2.06 2.03 1.87Mn 0.48 0.51 0.52 0.55 0.54 0.58 0.58 0.57 0.56 0.53 0.57 0.56 0.51 0.53 0.44Mg 0.26 0.25 0.24 0.24 0.25 0.23 0.24 0.23 0.24 0.23 0.24 0.24 0.25 0.31 0.24Ca 0.23 0.25 0.23 0.25 0.23 0.25 0.26 0.24 0.24 0.25 0.24 0.26 0.25 0.22 0.29Total 8.03 8.07 8.01 8.03 8.00 8.02 8.03 8.01 8.03 8.05 8.07 8.03 8.01 8.04 7.92

Alm 68.9 67.6 67.7 66.5 66.4 65.8 65.3 66.2 66.6 68.3 67.4 65.9 67.0 65.6 65.8Prp 8.5 8.1 7.8 7.8 8.3 7.5 7.8 7.5 7.8 7.3 7.6 7.6 8.2 10.1 8.3Spes 15.4 16.4 17.0 17.7 17.7 18.6 18.6 18.4 18.0 16.7 17.6 18.1 16.6 17.1 15.6Gros 7.3 7.9 7.5 8.0 7.6 8.1 8.4 7.9 7.6 7.7 7.5 8.4 8.3 7.1 10.3


Plag

iocl

ase

Spin

elO

rtho

pyro

xene

Clin

opyr

oxen

eG

lass

esc

cc

b

Oxi

de w

t. %

SiO

252

.96

53.8

955

.82

51.8

560

.03

n.d.

n.d.

n.d.

n.d.

53.1

553

.97

54.2

451

.55

50.1

550

.16

53.7

175

.22

78.7

077

.61

80.1

9Ti

O2

n.d.

n.d.

n.d.

0.20

0.01

0.57

0.47

0.48

0.46

0.12

0.14

0.14

0.23

0.97

0.40

0.29

0.33

0.21

0.48

0.18

Al 2O

330

.25

29.8

928

.78

28.3

923

.17

59.6

560

.08

59.7

860

.16

0.47

0.92

0.43

0.58

0.84

9.30

1.29

12.4

710

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11.0

610

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380.

280.

212.

140.

6528

.43

28.2

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.20

28.9

622

.29

19.9

121

.53

23.2

427

.58

10.5

914

.50

1.10

1.21

1.50

2.06

MnO

n.d.

n.d.

n.d.

n.d.

0.04

0.13

0.10

0.14

0.12

0.73

0.69

0.66

1.24

0.97

0.29

0.46

0.02

0.00

0.02

0.07

MgO

n.d.

n.d.

n.d.

0.06

0.14

12.0

212

.13

12.0

511

.66

23.3

923

.71

24.4

821

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17.0

96.

446.

790.

100.

120.

130.

18C

aO12

.96

12.3

311

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13.8

17.

220.

050.

060.

060.

070.

460.

910.

281.

541.

9420

.66

21.4

81.

190.

280.

620.

85N

a 2O

4.25

4.62

5.22

3.49

6.51

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

0.03

n.d.

0.47

0.98

0.21

1.12

1.73

1.64

1.62

K2O

0.03

0.04

0.06

0.34

1.40

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

0.00

n.d.

0.00

0.07

0.03

1.99

2.47

2.44

2.60

Tota

l10

0.83

101.

0510

1.24

100.

2899

.17

100.

8510

1.04

100.

7110

1.43

100.

6110

0.25

101.

7999

.98

100.

0198

.89

98.7

693

.61

94.7

895

.50

98.5

9

Cat

ions

16O

xyge

ns32

Oxy

gens

6O

xyge

ns6

Oxy

gens

CIP

W n

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Si2.

382.

410

2.48

02.

380

2.71

7n.

dn.

dn.

dn.

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969

1.98

01.

975

1.94

81.

940

1.89

01.

980

Qz

61.8

160

.66

57.4

658

.95

Tin.

d.n.

d.n.

d.n.

d.0.

000

n.d

n.d

n.d

n.d

0.00

30.

004

0.00

40.

007

0.02

80.

010

0.00

0C

6.92

4.23

5.78

4.81

Al

1.60

1.58

01.

510

1.54

01.

236

n.d

n.d

n.d

n.d

0.02

10.

040

0.01

80.

026

0.03

80.

410

0.05

0A

b10

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15.4

815

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14.5

5Fe

2+0.

010.

010

0.01

00.

080

0.02

55.

145.

015.

035.

140.

691

0.61

20.

655

0.73

40.

892

0.33

00.

400

An

5.79

1.49

2.83

3.23

Mn

n.d.

n.d.

n.d.

n.d.

0.00

2n.

d.n.

d.n.

d.n.

d.0.

023

0.02

10.

020

0.04

00.

032

0.01

00.

020

Or

12.5

915

.42

14.4

215

.07

Mg

n.d.

n.d.

n.d.

n.d.

0.00

93.

873.

843.

833.

691.

292

1.29

81.

329

1.21

70.

985

0.36

00.

560

Hy

1.90

2.31

3.04

2.44

Ca

0.62

0.59

00.

530

0.68

00.

350

n.d.

n.d.

n.d.

n.d.

0.01

80.

036

0.01

00.

062

0.08

00.

830

0.97

0Ilm

0.66

0.41

0.55

0.95

Na

0.37

0.40

00.

450

0.31

00.

571

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

0.03

50.

070

0.03

0K

0.00

0.00

00.

000

0.02

00.

081

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

n.d.

0.00

00.

000

Tota

l4.

980

4.99

04.

980

5.01

04.

991

n.d.

n.d.

n.d.

n.d.

4.01

73.

991

4.01

14.

034

4.03

03.

910

4.01

0

An

37.2

40.3

45.7

30.8

34.9

Ab

62.6

59.5

53.9

67.3

57.0

Or

0.20

0.20

0.30

2.00

8.10

Mg

#43

4343

4265

6867

6253

5158

Wo

0.9

1.8

0.5

3.0

4.04

54.2

49.9

En63

.866

.066

.059

.349

.53

23.5

28.9

Fs35

.332

.233

.537

.746

.43

22.3

21.2

Tabl

e11

.Ele

ctro

nm

icro

prob

ech

emic

ala

naly

seso

fmai

nsi

licat

esa

ndg

lass

esc

ompo

sing

the

buch

itera

ftxe

nolit

h(c

:cle

arg

lass

,b:b

row

ngl

ass)

.


richmagma,yieldabout5kbarusingtheempiricalsemiquantitativethermobarometerofErnstandLiu(1998)attemperaturesaround900ºCcalculatedwithQUILFoncoexistingCpx-Opxinthecumulitichornblendepyroxenitexenolith.Inaddition,thepaucityofplagioclasephenocrystsandcommonOpx-Hbl-Cpx“glomerocrysts”,foranandesiticcomposition,wouldindicate(BlatterandCarmichael,1998)minimumcrystallizationpressuresaround4.3kbarorabout15kmofdepth.

Contiguoushornblendeandplagioclaseintheultra-mafic xenolith, using the method of Holland and Blundy (1994),yieldedcrystallizationtemperaturesbetween886and930ºC,whereasatemperatureof950ºCwasobtainedusingthemethodofSaxenaet al.(1986)basedonthedistributionofFeandMgbetweenorthopyroxeneandclinopyroxeneinigneousandmetamorphicrocks.

Onthesebases, itmaybeconcluded thatmaxi-mumtemperaturesattainedbythePuenteNegroxenolithsinsidethemagmaorinthesolidcrustvariedbetween961ºC(pargasitichornblendestable)tomorethan1100ºC(mullite-tridymite-highlysiliceousmelts),whereaspeakpressuresareonlyconstrainedtoaminimumofabout5kbarbutprobablyreached8kbarormore.Quemographicrepre-sentationsofstablemineralassemblagesofPuenteNegrometamorphichigh-Alxenolithsandthegarnetcoronasare

illustratedin Figure10,whilstagraphicsummaryofthepeakthermalconditionsasderivedfromtheircriticalandinferredmineralassemblagesisgiveninFigure11.

Geochemistry of the Puente Negro intrusion

ThewholerockgeochemistryofthePuenteNegroandesite(PN44,Table3)iscomparabletothatofandesitictobasalticandesitelavasofsimilarageexposedtotheeast,inthewesternsectorofthestateofOaxaca(Martinyet al.,2000).InHarkerdiagramsofmajorandtraceelements(notshown)aswellasinchondrite-normalizedrareearthelement(REE)diagramsandprimitivemantlenormalizedmulti-elementvariationdiagrams(Figures12aand13a),theleastcontaminatedsamplecollectedatPuenteNegro(PN44)displayspatternssimilartotheOaxacalavas,althoughitisslightlymoreenrichedinPbandREE,particularlytheheavyREE(HREE)andheaviermiddleREE(MREE).EnrichmentinPbcanbeexplainedbytheparticipationofacrustalcomponentinthegenesisofthemagmas.Amphibole,whichisabundantinthePuenteNegroandesite,controlsenrichmentintheheavierMREErelativetoLREEandHREE (partition coefficients are particularly high from Dy toEr)(e.g.,Arth,1976;GreenandPearson,1985;Schnetzler

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Oxide wt. %SiO2 61.77 62.95 63.10 61.48 65.73 78.72 80.41 75.22 78.70 77.61 79.33 79.72 80.19 80.41TiO2 0.11 1.85 0.07 1.85 0.03 0.00 0.18 0.33 0.21 0.48 0.18 0.29 0.18 0.18Al2O3 19.74 17.33 19.14 17.35 14.35 12.92 10.47 12.47 10.06 11.06 9.95 12.61 10.85 10.47FeOTotal 0.30 0.63 0.25 0.64 3.84 0.10 1.90 1.10 1.21 1.50 1.27 1.44 2.06 1.90MnO 0.00 0.06 0.01 0.06 0.01 0.01 0.07 0.02 0.00 0.02 0.03 n.d. 0.07 0.07MgO 0.00 0.11 0.00 0.11 2.81 0.00 0.11 0.10 0.12 0.13 0.13 0.22 0.18 0.11CaO 2.06 1.75 2.10 1.75 3.36 3.73 0.84 1.19 0.28 0.62 0.26 0.44 0.85 0.84Na2O 6.81 6.01 6.80 6.01 4.32 4.30 0.99 1.12 1.73 1.64 1.29 1.46 1.62 0.99K2O 5.15 4.79 4.73 4.79 0.00 0.00 2.32 1.99 2.47 2.44 2.42 0.44 2.6 2.32F 0.03 0.00 0.00 0.00 n.d. n.d 0.00 0.07 0.00 0.00 0.05 n.d. 0.00 0.00Total 95.97 95.48 96.20 94.04 94.46 99.81 97.29 93.61 94.78 95.50 94.91 96.62 98.59 97.29

CIPW normQz - 6.60 1.39 5.13 27.55 60.08 64.05 61.81 60.66 57.46 64.17 69.49 58.95 64.05C - - - - 1.20 - 4.94 6.92 4.23 5.78 5.00 9.16 4.81 4.94Ab 56.29 53.22 59.82 54.07 38.67 36.47 8.63 10.15 15.48 15.91 11.51 12.78 14.55 8.63An 8.4 6.46 8.03 6.63 17.66 15.99 4.27 5.79 1.49 2.83 1.34 2.48 3.23 4.27Or 31.74 29.67 29.08 30.08 0.00 - 14.06 12.59 15.42 14.42 15.07 2.72 15.07 14.06Ne 2.05 - - - - - - - - - - - - -Hy - - - 0.3 14.87 - 3.67 1.9 2.31 3.04 2.55 2.81 2.44 3.67Di 0.73 - - - - 0.38 - - - - - - - -Wo 0.52 - 0.83 - - 0.89 - - - - - - - -Ilm 0.21 1.52 0.13 1.57 0.06 - 0.36 0.66 0.41 0.55 0.36 0.57 0.95 0.36Rt - 0.39 - 0.4 - - - - - - - - - -Ttn - 1.84 - 1.83 - - - - - - - - - -

Table12.ElectronmicroprobechemicalanalysesofglassesinPuenteNegroxenoliths


1 bar 2 kbar

>10% wt Al 02 3

2 kbar 10 kbar

1.5 kbar

P = 5 kbar

RANGE OF THERMAL STABILITY (°C)

800 900 1000 1100 1200 1300

ASSEMBLAGE

Trd-QzBoyd and England, 1960

Opx-Spl-QzHensen and Green, 1973

Rt-Spl-Opx-QzPowell and Sandiford,

1988

MulliteHolm, 2001

Spl+Qz=MeltHollis and Harley, 2003

Al-Opx isoplethsLane and Ganguly, 1980

Dry granite liquidusHoltz ., 2001et al

High cristobaliteBerman, 1988

andPhilpotts,1970)andcanexplainMREEenrichment,whereastheprobablepresenceofminutegarnetxenocrystsintheandesitecouldexplainHREEenrichment.

PreliminarydataonSr,NdandPbisotopicdatapre-sentedinthisworkforPuenteNegrointrusivebody(Table13) confirm substantial assimilation of ancient crust by the originalmagmas,sincethemoreradiogenicsample(PN15b)displaysaninitial(87Sr/86Sr)ivalueof0.706252,andinitial(εNd)iof-2.4(Figure14).Thedepletedmantlemodelage(TDM)is0.98Ga.Present-day206Pb/204Pb,207Pb/204Pband208Pb/204Pbratiosofthissampleare18.846,15.637,and38.759,respectively.AnothersampleofthePuenteNegrointrusivebody(PN44)displaysasmallerdegreeofassimilationandshowsa(87Sr/86Sr)iratioof0.704635,

an(εNd)iof-1.6,andasimilarTDMageof1.01Ga.PN44lies on the radiogenic side of the field corresponding to thewesternOaxacalavas(Figure14),whichareslightlylesscontaminatedbyoldcrustalmaterial.Present-day206Pb/204Pb,207Pb/204Pband208Pb/204PbratiosofPN44are18.768, 15.620, and 38.634, falling within the field of early OligocenelavasfromwesternOaxaca,whichdisplayalineartrendwithapositiveslopeinPbisotopediagrams(Figures15aand15b).ItisinterestingtonotethatthelineararrayoftheselavaspointstowardthegneissicxenolithsfromPuenteNegro(Figures15aand15b).Thelineartrendofthelavassuggestsamixinglinebetweenamantlesourceandacrustalcomponentrichin206Pb,207Pb,and208Pb,suchastheuppercrustorsediments,similartothecompositionofthemetapeliticgneiss(PN17).

Geochemistry and isotopic composition of the Puente Negro xenoliths

Inchondrite-normalizeddiagrams,theREEpatternsofthexenolithsanalyzedareroughlysub-parallel,particu-larlythetwogneisses(PN17andPN1b).Thedeep-seatedgarnetiferousgneississtronglyREEenriched,withelevatedHREEconcentrationsof27timesthechondriticvalues(Figure12b)thatcanbeaccountedforbythepresenceofgarnet.PN17isalsocharacterizedbyLREEenrichmentwithchondrite-normalized(La/Yb)N=7.3,andaveryslightnegativeEuanomaly(Eu/Eu*=0.80).Thequartzosegneiss(PN1b)hasmuchlowerREEconcentrations,withHREE

TrdSiO2

Mul

CrnTiMagSpl

FeO/MgO/TiO2

Al O2 3

Opx

a)

b)

TiMagFeO/MgO/TiO2

Al-rich xenoliths

Opx

Spl

SiO2

Al O2 3

PlGrt

Garnet corona

Crn

Figure10.a)QuemographicphaserelationsinthesystemSiO2-Al2O3-FeO/MgO/TiO2(PN1,PN17,PN19,PN29,29698).RutilemaybestableinsteadofTiMag.Notethealuminousnatureoforthopyroxene.FromtexturalobservationsOpxwithcorundumormullitemaynotbeinequilibrium,suggestingthedecompressionreactionsOpxss+2Crn=Spl+Al-Silss,andAl-Silss+Opx=Spl+2Trd.b)QuemographicphaserelationsinthesystemCaO-Al2O3-FeO/MgO/TiO2(partiallymeltedxenolithsFO6605and11679).Notetheabsenceofaluminousphases(includingAl-Opx)inthesexenolithsentrainedataveryshallowlevel.Zoisite,althoughstablewithglassandtridymite/quartz,maybeincompatiblewithorthopyroxeneconsideringtheoxidizing-dehydrationreactionOpx+2Zo+Qz+½O2

=3An+Di+H2O,suggestedbycrossingtielinesinthetriangleandthehightemperaturenatureoftheevent.

Figure11.ThermalstabilityrangeforphasesandassemblagesthatcharacterizePuenteNegro.


1

10

100

Ce Eu YbLa Pr Nd Sm Gd Tb DyHo Er Tm Lu

1000

1

10

100

Ce Eu YbLa Pr Nd Sm Gd Tb DyHo Er Tm Lu

1000 b)

a)Sample/chondrite

Sample/chondrite

Andesite PN 44

PN17 (garnetiferous gneiss)PN1b (quartzose gneiss)PN4 (ultramafic)

concentrationsof4–5timesthechondriticvalues,andshowsacomparableLREEenrichment[(La/Yb)N=6].Theamphibolepyroxenite(PN4)displaysaslightlyfractionatedREEpattern[(La/Yb)N=4],hasaveryslightnegativeEuanomaly(Eu/Eu*=0.78),andHREEconcentrationsof5timesthechondriticvalues.ThepresenceofamphiboleinthisxenolithcanexplaintheslightenrichmentintheheavierMREE.

Inprimitivemantlenormalizedmulti-elementvaria-tiondiagrams,traceelementcontentsofthexenolithsdis-playstrongenrichmentinCs,Pb,Th,andU.Therelativedepletion in fluid immobile incompatible elements, such asNb,TaandTi(Figure13b)suggestsasubductioncom-ponentintheprotolith.EnrichmentinZr,Hf,andU,andThin thegneissxenoliths is likelycontrolledbyzir-con,whichhashighdistributioncoefficientsfortheseelements.

TheSr,NdandPbisotopiccompositionsofthePuente

NegroxenolithsprovideapreliminaryinsightonthenatureofthebasementundertheAcatlánComplex(Table13).Forreference,SrandNdisotoperatioswerealsodeterminedforthebasementrocksoutcroppingintheuppermostcrustin-trudedbythePuenteNegroandesite(EsperanzaGranitoidsoftheAcatlánComplex,Figure2b).Thegneissicxenoliths(PN1,PN1b,PN17),whichoriginatedinthelowercrust,displaylittlevariationininitial87Sr/86Srratios(calculatedfor30Ma),withratiosbetween0.7144and0.7148,and(εNd)ivaluesvaryingfrom-10.9to-8.2.SrandNdisotoperatiosofthecountryrock(EsperanzaGranitoids)ofthePuenteNegroandesiteismuchmoreradiogenic(Figure14,Table13),indicatingthatthereisprobablynogeneticrelation-ship to the xenoliths. Surprisingly, the ultramafic cumulate xenolith(PN4)ismoreradiogenicthanthehostingandesite[(87Sr/86Sr)i=0.711,(εNd)i=-3.7](Table13),suggestingparticipationofatleasttwomagmabatches,onesubstantial-lymorecontaminatedatdepthandparentaltothegabbroicxenolithsandhornblendexenocrysts,andanewbasalticmagmathatentrainedallthexenolithicmaterial;thesemagmasfurtherassimilateduppercrustalrocksyieldingthepresentandesitic-daciticcompositionofthePuenteNegrointrusion.Pbisotoperatiosoftheamphibolepyroxeniteareslightlymoreradiogenic(19.011,15.683,39.001)thanthoseofthegneissicxenoliths(206Pb/204Pb=18.808–18.878,207Pb/204Pb=15.650–15.677,and208Pb/204Pb=38.790–38.937)(Figure15,Table13).

Thegneissicxenolithsanalyzedhavepre-Grenvillianmodelages(TDM=1.26,1.40and1.52Ga),whichhintatthepresenceofoldcomponentsinthelocallowercrust.IthasbeensuggestedpreviouslythattheAcatlánComplexoverliesanunknownPrecambriancontinentalcrust,tenta-tivelyconsideredGrenvillianinage(Ortega-Gutiérrezet al.,1990).ModelagesoftheGrenvillianOaxacanComplex(1.49–1.60),situatedtotheeast,arerelativelyclosetothoseofthestudiedgneissicxenolithsandseemtosupportthishypothesis.

DISCUSSION

Polymetamorphic evolution of the gneissic xenoliths

Petrologicallydistinguishingbetweenoriginalmeta-morphicassemblagesandsourceofxenolithsfromthesu-perimposed low pressure, purely thermal event was difficult toestablish.Thermalresettingandabsenceofappropriatemineralgeobarometers(i.e.,absenceofquartz,olivine,orcordieriteinthedecompressioncoronasofgarnet)didnotpermitpressuredeterminations,butthiscentralissueeventuallymaybeaddressedthroughthestudyofabundantfluid-solid and melt inclusions present in most xenoliths of PuenteNegro(e.g.,SachsandHansteen,2000).

PyrometamorphismPhasestabilityinthesanidinitefaciesandpyrometa-

Figure12.Chondrite-normalizedrareearthelement(REE)compositionof(a)theOligocenePuenteNegroandesite(PN44)and,forcomparison,andesitesandbasalticandesitesofsimilaragefromnearbywesternOaxaca(shadedarea).SlightenrichmentoftheheavyREE(Dy-Lu)inthePuenteNegrodikeswithrespecttowesternOaxacalavasmaybeduetothepresenceofcommongarnetxenocrysts.(b)PuenteNegroxenoliths.DataforwesternOaxacalavasfromMartinyet al.,(2000).ChondritevaluesfromNakamura(1978)andHaskinetal.(1968).


morphism(e.g.,Grapes,2006andreferencestherein)iswellestablishedatPuenteNegrobytheabundanceofglassandtridymite,bothfresh(hightridymite)andinvertedtoquartz,aswellasbycoexistingsanidine,orthopyrox-ene,clinopyroxene,spinel,mullite,andcalcicplagio-clase.Theformerpresenceofhigh-tridymiterimsaboutquartzinmostquartzosexenolithswouldindicatemini-mumtemperaturesof867ºCat1barpressure,andabove1100ºCatpressuresaround1kbar(BoydandEngland,1960).Moreover,texturalandmineralogicaldataindicateevenmoreextremecontactmetamorphisminvolvingseveraldrypartialmeltingreactionssuchasSpl+Qz/Trd=Mul+melt,Pl=Mul+melt,andAb+Qz=melt,orOr+Qz=melt,allofwhichwouldoccurattemperaturesabove1000ºCatlowpressures.PseudomorphicassemblagesinxenolithsofOpx-glass,Spl-Opx-glass±Al-silicatess,Spl-Opx-Pl-Crn±glassaftergarnetorbiotite,andSpl-Pl-Cpx-Ilmafteramphibole,constituteevidenceofincongruentmelting of those mafic phases at very high temperature and variablepressures.SillimaniteinfactshouldbeconvertedtomulliticAl-silicatessabove1000ºCatlowpressure(cf.PuginandKuskov,1968;Schreyer,1976;Cameron,1976;Grapes,2006).Basedontheinterpretationoftextures

andsecondarymineralassemblagesinsymplectites,itisassumedthatbiotitewaspresentinsomeofthemetasedi-mentaryxenoliths,andthat itmeltedincongruentlytosymplectiticpseudomorphsofOpx-Spl-glass±Al-silicatess.Temperaturesinexcessof1100ºCatlowpressuresalsomayexplainthevirtualabsenceofcordieriteinthePuenteNegroxenoliths,asitprobablywasreplacedinthereactionCrd=2Spl+5Trd(cf.Harris,1981).Atnearsurfaceconditions,theanhydrousassemblagealbite+quartzmeltsat~1015ºC(WenandNekvasil,1994),areactionsuggestedatPuenteNegrobycolorlessglassmantlingalbitegrainsinformercontactwithquartz.Finally,orthorhombicmullite,wouldrequiretemperaturesabove1150ºC(Johnsonet al.,2001;Grapes,2006),whilstthepresenceofSpl-Opx-Qz/Trd-Rt-IlminmanyofthePuenteNegroxenolithsalsoimpliestemperaturesinexcessof1100ºCandlowa(O2)(PowellandSandiford,1988).

Thebreakdownof igneouspargasitebeforeandduring the final ascent of the gabbroic xenoliths from the middlecrustsuggestsmagmaticrechargeandheatingabove1000ºC,whereastheoccasionalpresenceoftridymiteingneissicxenolithsstopedfromtheadjacentcountryrockswouldimplyminimumemplacementtemperaturesnearthequartz-tridymitetransitionat867ºC.Atthemaximumtemperatures(>1000ºC)andverylowpressureenvisagedforthePuenteNegrointrusion,highcristobalitewouldformmetastablyfromhighquartzinanarrowwindowbetween1.5and2kbar(Berman,1988).Hightridymiteatthesepres-sures would form in its own stability field at temperatures near1200ºC,remainingstabletotemperaturesofabout900 ºC during the final emplacement of the andesite. The stabilityoftridymiteisconsistentwithtemperaturescalcu-latedfromilmenite-magnetitepairspresentintheshallowbuchiticxenolith(FO6605),thevaluesofwhichvaryfrom861to909ºCapplyingtheILMATprogramofLepage(2003),andgeothermometrybyPowellandPowell(1977),SpencerandLindsley(1981),andAndersenandLindsley(1988).Finally,titanitecoexistingwithorthopyroxeneandspinelsuggests(cf. Xirouchakis et al.,2001)highfO2andtemperatures below 650 ºC at some point in the final cooling historyoftheintrusion.

Deep-seated metamorphism?Theevidencesuggestingthatsomeofthexenoliths

andhigh-Alxenocrystsrepresentveryhigh-grademeta-morphicrocksthatsufferedconsiderabledecompressionbeforecontactmetamorphismconsistsof:(a)xenolithswithabundantrelictgarnetshowingunfractured,inclu-sion-freecalcium-poorunzonedcoresinsharpcontactwithcalcium-richfracturedrimsmantledbykelyphiticcoronaswiththeassemblagePl-Spl-Opx±Crn±glass,(b)xenocrystsofAl-richorthopyroxenerimmedbyanAl-poortypeassociatedwiththedecompressioncoronasofsomegarnets,(c)xenocrystsofsapphire,corundum,andspinelintheandesiticintrusion,and(d)therareoccurrenceofOpx-Sil(PN42),implyingpressuresclosetoorabove8

Sample/prim

itive

mantle

1000

100

10

1

Rb BaThU KNb Ta LaCePb Sr P ZrNd SmHf Ti Y Yb LuPrCs

1000

Sample/prim

itive

mantle

100

10

1

Rb BaThU KNb Ta LaCePb Sr P ZrNd SmHf Ti Y Yb LuPrCs

a)

b)

Andesite PN 44

PN17 (garnetiferous gneiss)PN1b (quartzose gneiss)PN4 (ultramafic)

Figure13.Primitivemantlenormalizedmulti-elementvariationdiagramsfor(a)PuenteNegroandesiteandnearbywesternOaxacalavas(shadedarea)ofsimilarage,and(b)PuenteNegroxenoliths.DataforwesternOaxacalavasfromMartinyet al.,(2000).PrimitivemantlevaluesfromSunandMcDonough(1989).


(a) S

ampl

eR

ock

type

Rb

SrSm

Nd

(87Sr

/86Sr

) m1s

abs

87R

b/86

Sr(87

Sr/86

Sr) i

(143 N

d/14

4 Nd)

m1s

abs

147 S

m/14

4 Nd

(143 N

d/14

4 Nd)

i(ε

Nd)

0(ε

Nd)

iT

DM

Puen

te N

egro

PN15

bPu

ente

Neg

roin

trusi

on(d

acite

)22

.164

54.

3620

.70.

7062

95±

360.

0991

0.70

6253

0.51

2503

±18

0.12

720.

5124

78-2

.6-2

.40.

98PN

44Pu

ente

Neg

roin

trusi

on(a

ndes

ite)

28.9

662

5.27

23.5

0.70

4689

±37

0.12

640.

7046

350.

5125

44±

160.

1356

0.51

2517

-1.8

-1.6

1.01

PN17

Gar

netif

erou

sgne

issx

enol

ith19

.246

810

.77

58.2

0.71

4481

±37

0.11

910.

7144

300.

5121

99±

190.

1118

0.51

2177

-8.6

-8.2

1.26

PN1

Qua

rtzos

egn

eiss

xen

olith

20.6

127

1.31

6.8

0.71

4855

±40

0.47

030.

7146

550.

5120

64±

260.

1170

0.51

2041

-11.

2-1

0.9

1.52

PN1b

Qua

rtzos

egn

eiss

xen

olith

23.7

143

1.42

7.6

0.71

4999

±39

0.48

030.

7147

940.

5121

14±

160.

1130

0.51

2092

-10.

2-9

.91.

40PN

4A

mph

ibol

epy

roxe

nite

xen

olith

5.4

421

2.31

9.5

0.71

0988

±36

0.03

750.

7109

720.

5124

39±

160.

1470

0.51

2410

-3.9

-3.7

1.37

Acat

lán

Com

plex

Cha

z-1

Schi

st(C

hazu

mba

For

mat

ion)

81.6

254

6.03

32.1

0.71

4651

±51

0.93

160.

7142

540.

5121

20±

130.

1134

0.51

2098

-10.

1-9

.81.

39C

os-1

Schi

st(C

osol

tepe

cFo

rmat

ion)

226.

585

10.6

155

.40.

7512

38±

447.

7070

0.74

7954

0.51

1973

±16

0.11

570.

5119

50-1

3.0

-12.

71.

63C

ON

-300

Aug

eng

neis

s(Es

pera

nza

Gra

nito

ids)

124.

911

810

.29

54.6

0.73

2326

±38

3.06

880.

7310

180.

5122

36±

220.

1139

0.51

2214

-7.8

-7.5

1.24

MD

202

Aug

eng

neis

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Pb

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a)

b)

38

39

206 204Pb/ Pb

206 204Pb/ Pb

208

204

Pb/

Pb

18.0 18.5 19.0 19.5

Acatl nComplex

á

Acatl nComplex

á

W Oaxacalavas

W Oaxaca lavas

PN15bPN44

PN17 (garnetiferous gneiss)P 1b (quartzose gneiss)N

PN4 (ultramafic)PN1 (quartzose gneiss)

Xenoliths Puente Negro andesite

SK

0.25SK

0 Ga

0 Ga

0.25

kbar(e.g.,CarringtonandHarley,1995;Harley,2004).ThestablepresenceofcorundumwithorthopyroxeneinsomeofthepseudomorphsandgarnetcoronasmayindicateveryrestrictedbulkchemicalcompositionsandP-Tconditionsproperofthelowercrust(KellyandHarley,2004).IngranulitesofSriLanka,coronasofOpx-Crnaroundgarnetformedondecompressionfrom9to7.5kbar(KriegsmanandSchumacher,1999).Thefactthatrelativelyhigh-pres-surephasessuchasgarnetandaluminoushornblendesuffereddecompressionmelting,asdiscussedabove,alsosuggestsrelativedeepcrustallevelsfortheentrainmentofsomeofthexenoliths.Hayobet al.(1989)describedrimsoforthopyroxeneandgreenspinelwithinterstitialglassorplagioclasearoundgarnetsindeep-seatedgranulitesfromnorthernMexico,andsuggestedanoriginbyincongruentmeltingofgarnet,asxenolithsascendedinthebasalticmagmafromadeepsource.SimilarconditionsmayhaveprevailedduringthekelyphiticbreakdownofgarnetsatPuenteNegro.

Analternativeexplanationforthepresenceofhigh-AlxenolithsinPuenteNegroistoconsiderthepresenceatcrustaldepthofahigh-temperaturethermalaureolearounda gabbroic intrusion similar in many aspects to the Voisey’s BayIntrusionofLabrador,Canada(Marigaet al.,2006a,2006b).There,smallxenoliths(2–10cm)wereconvertedthroughreactionofcountrygneisseswiththegabbroicmagmaintoaggregatesofspinel-plagioclase-corunduminproportionsverysimilartothosefoundinthehigh-AlpseudomorphsatPuenteNegro.Here,furtherinjectionofsubcrustalmagmasmayhavetraversedthishypotheticalau-reolecarryingthepseudomorphicxenolithsneartosurfaceconditions.However,gabbroicxenolithsotherthanthehy-drous porphyries and ultramafic cumulates are lacking, and thehightemperaturesrequiredtoproducetheassemblagesdescribedabovecouldnotbeprovidedbyahydrousmagmafromwhichthehornblende-richrockscrystallized.

Ascent history and source of xenoliths

Insightsintothemagmaascentratesforthefinalpulsemaybegainedfromthenatureofopaciteandgab-broiccoronasdevelopedvirtuallyonallamphiboles,bothintheandesiteandinthegabbroicxenoliths.Theserimsusuallyformbydehydrationduringascentanddewater-ingofhydrousmagmas(BlatterandCarmichael,2001).Becauseopaciticrimsmayrangefromalmostabsenttothecompletereplacementofamphibole,injectionofdifferentbatchesofmagmaintoachamberorvariablepositioninthe flow channel of the Puente Negro intrusion probably controlledtheascentratestothesurface.Theanhydrousgabbroicopaciticreplacementofpargasitewouldrequireinteractionwithahotmagmaandascenttimesmuchlonger

87 86Sr/ Sri

0.704 0.7120.708

Nd i

0.716

+10

+5

0

-5

-10

-15

MORB

W Oaxacavolcanic rocks

Esperanza Granitoids(0.725 to 0.731, -7.5 to -11.5)

Gneissxenoliths

PN15bPN44

PN17 (garnetiferous gneiss)PN1b (quartzose gneiss)

PN4 (ultramafic)PN1 (quartzose gneiss)

Xenoliths

Puente Negro andesite

Figure14.InitialSrandNdisotopevaluesforthePuenteNegrointrusionandxenoliths,EsperanzaGranitoids,andearlyOligocenelavasfromwesternOaxaca.DataforwesternOaxacalavasfromMartinyet al.,(2000).

Figure15.PbisotoperatiosforthePuenteNegrointrusionandxenoliths,andearlyOligoceneandesiticandbasalticandesitelavasfromwesternOaxaca. The field for representative samples from Acatlán Complex is indicated.ShownforreferencearetheNHRL(NorthernHemisphereReferenceLine,definedbyHart,1984),alongwhichmostnorthernhemisphereMORBandOIBslie,andSK,thePbisotopicevolutioncurve,as defined by Stacey and Kramers (1975).


thanafewdays,whereaspristineamphiboleinsomepulsesofthePuenteNegromagmawouldrequireascentrates>2cm/s(RutherfordandDevine,2003).Assumingameandepthofabout15kmfor thegabbroicdifferentiatingbody,thisdistancewouldbetraversedinlessthanaweek,asenvisagedfortheSantaElenaeruptiononthebasisoftheabsenceofopaciterimsinamphibole(RutherfordandHill,1993).

Moreover, thecommonpreservationofglassandabundanceofmicrolitesintheandesite,aswellasglasswith quenched plagioclase in the gabbroic and ultramafic xenoliths,suggest(e.g., CashmanandBlundy,2000)explo-siveascentrates(ca.1m/s)forsomepulsesofthePuenteNegrointrusion.

Figure16showsselectedveryhightemperaturephaseequilibriainthesystemFMASandatentativeP-TtrajectoryforthePuenteNegrogranuliticxenoliths,whereasFigure17illustrates a preferred albeit simplified mantle-crust scenario wherethemagma-xenolithplumbingsystemmighthaveevolved. An original basaltic batch envisaged for the first intrusiveeventatPuenteNegro(hydrousgabbroicmagma)requiresaprimarysourceintheuppermantleunderlyingahotCenozoiccrustmorethan40kmthick(ca.11kbar)(cf.Valdéset al.,1986);anditstransientstagnationinthemiddlecrust(15–20km)attemperaturesnear1000ºCandpressuresof5–6kbar.There,thebasalticmagmacooledanddifferentiatedbyfractionalcrystallizationinachamberathighwaterpressures,withthesuccessivebutoverlappingcrystallizationofclinopyroxene,orthopyroxene,amphi-bole, and plagioclase, forming a mafic-ultramafic cumulate complexrepresentedby“glomerocrysts”andhornblende-pyroxenerichxenoliths.Themoresiliceousresidualmelteventuallymixedwithhightemperaturebasalticbatchesthatrechargedtheevolvingmagmachamberandtriggeredtherapidextrusionoftheresidualmagmaandbothgabbroicandmetamorphicxenoliths.Interactionsbetweenmultiplebasalticsillsemplacedattemperaturesabove1200ºCandahotlowercrustcouldintheorymeltandassimilatemuchofthatcrust,leavingresiduesofrefractoryphases,suchasanorthite,corundum,spinel,Al-orthopyroxene,sillimanite,andgarnet,whicharepresentintheactualxenocrystsandxenolithicphasesofthePuenteNegrointrusion.Refractoryxenolithsandxenocrystswouldhavebeentakenfromthelocallowercrustbythenewmagma,decompressedandcooleden route to its final emplacement near the surface, togetherwithfragmentsofthepartiallycrystallizedhorn-blende-richgabbroicbatch.

Althoughanotherpossiblesourceforthehigh-gradexenolithscouldbetheexposedunitsofthelocalAcatlánComplex(augengneisses),whichduringpyrometamor-phismmayinprinciplealsoyieldtheanhydrous,high-Alassemblagesthatcharacterizethegneissicxenoliths,theisotopiccompositionsofthoserocksareindeeddifferent(Table13). Also,theabundanceofgarnetinPuenteNegroxenolithsandxenocrystsexcludesthelow-gradephyllitesandquartzitesoftheunderlyingCosoltepecFormationasthe

sourceforthehigh-gradexenoliths.Likewise,thedifferentcompositionofgarnetsfromtheAcatlánComplexhigh-pressurerocks(Reyes-Salas,2003)alsoseemstoexcludethegneissicrocksexposedinthesurfaceasthesourceforthePuenteNegrogarnetiferousxenoliths.Infact,thelocalAcatlánComplexlacksstaurolite,anditspossiblepresenceinthecorundum-spinel-plagioclasepseudomorphs,orincombinationwithspinel,garnet,sillimanite,andcorundum,thusstronglysuggestsentrainmentofmetapeliticxenolithsfromcrustallevelsundertheAcatlánComplex.Moreover,eclogiticgarnetsofthatcomplexusuallycontainabundantinclusionsofepidote,rutile,andquartz,contrarytotheclean,unfracturednatureofallgarnetxenocrystsinthevolcanicrock,andporphyroblastsfoundinthePuenteNegroxenoliths.Ontheotherhand,polyphase,veryhigh-gradedeep-seatedmetamorphisminthexenolithsissuggestedbyassemblageswithspinel,plagioclase,orthopyroxene,garnet, and titanomagnetite defining foliated fabrics, where spinelormagnetitearemantledbyAl-orthopyroxene,andgarnetporphyroblastsdisplaycompositionsunlikethoseofgarnetsinthelocal,highpressurequartzo-feldspathicgneissesoftheAcatlánComplex(Reyes-Salas,2003),which

Figure16.AmodelproposedfortheoriginandevolutionofthePuenteNegroxenolithsandtransportingmagma.Positionandtemperaturesofmagmaticbodiesareinferredfromgeothermobarometricdataasdeterminedforthedeep,intermediateandshallowemplacementlevelsrepresentedbythestudiedpetrologicsystems.


areextremelyrichincalcium(10–20CaOwt.%).Rarecoronitictexturesofspinel-orthopyroxenearoundgarnetthatexcludeplagioclasesuggeststrongdecompressionofalmandine-pyropegarnetatUHTM(ultrahightemperaturemetamorphism)conditionsbythereactionGrt=2Opx+Spl+Qzmelt(73%molarvolumeincrease).

ThiscomplexpetrologiccontentofPuenteNegrogneissicxenolithsisconsideredtobetheresultofhighorul-trahightemperaturepre-MesozoicgranuliticmetamorphismfollowedbyOligocenesanidinitefaciespyrometamorphism.ThepresenceofrelictorthopyroxeneandsillimaniteintwoofthestudiedxenolithsstandsasdirectevidenceofultrahightemperaturegranulitefaciesmetamorphicrocksunderlyingtheAcatlánComplex.Finally,thecommonpresenceofspinelandcorunduminclusionsinplagioclasephenocrysts,andsapphire,spinel,andhigh-Alorthopyroxenexenocrystsintheandesiticvolcanicmatrix,wouldalsosupportthesubcrustalorlowercrustalprovenanceandthereforeoriginalbasalticcharacterofthePuenteNegroandesite.

CONCLUDING REMARKS

TheabundanceanddiversityofxenolithsinthePuenteNegroshallowintrusionandparagenetictexturalrelation-shipsofthemagmatic-metamorphicsystemposedseveralquestions,someofwhichremainpoorlyresolved:(a)Didthehostmagma(nowandesitic)originallyhaveabasalticcomposition?.Theanswerinthiscasemaybepositivebecausethecoexistenceofbasalticmagmastagnatedinthe middle crust and represented by xenoliths of mafic-ul-tramafic cumulates (hydrous, relative low temperature, low silica)andthefollowing“andesitic”magma(highsilica,relativehightemperature)inprincipleindicatethepresenceofatypicalbasalticmagmachamberperiodicallyreplen-ishedbyhotbasalticmagma(e.g.,Murphyet al.,2000).

Theandesiticdikeshostingthexenolithswouldrepresentthelatest,originallybasalticbatchthatinteractedwiththedeep crust (high-grade metapelites), sampled the mafic cumuliticproductsofpreviousbasalticmagmadifferenti-atedinthemiddlecrust(gabbroicxenoliths),andstronglyinteractedwiththeupperexposedcrust(buchiticxenoliths).The final result of these processes was the present andesite-dacitesubvolcaniccomplexanditsloadofxenolithstakenatthreedifferentlevelsinthecrust.Textural,chemical,geochronological,andisotopicdata,asdiscussedabove,supportthisconclusion.

(b)DosomeofthexenolithsrepresentPaleogeneresiduesofpartialmeltingatdepthbeneaththeAcatlánComplex?Theanswertothisquestionalsomaybepositivebecausetheminimumemplacementpressurescalculatedfortheunderplatedgabbroicmagmaequivalentto15–20kmrequirethatsomexenolithswereentrainedatthisorgreaterdepth,whererocksoftheAcatlánComplexmostprobablyarenolongerpresent.Moreover,theextremelyaluminouscompositionofmanyxenolithsandtheactualpresenceofhigh-gradecalcsilicatexenolithsindicateprotolithsthatarerareorabsentintheAcatlánComplex.Finally,complexdecompressionalcoronasandcompositezoningpatternsofgarnetxenocrystsandingarnetiferousxenolithsalsoindicateadeepanddistinctcrustalsourceotherthantheAcatlánComplex.

(c)Arethehigh-aluminaphasesandpseudomorphsintheandesiticmagmaformerphenocryststhatcrystal-lizedatmantlepressures,orjustxenocrystsremainingasaproductofmetapeliticxenolithdisaggregation?Thisques-tionremainsopenbecauseevidenceofmagmaticcrystal-lizationofcorundumorspinelistexturallyinconclusive,whilemanyofthemetamorphicxenolithsarecorundumandspinel-bearing,thusprovidinganotherplausiblesourceforthesecrystals.

(d)Whatwerethemagmaascentrates?Magma-crustinteractionsoccurredatdifferentlevelsinPuenteNegro,onebeingveryshallow(afewhundredsmetersatmost),butother,muchdeeperlevelsareevident.Complexdecompres-sionreactiontexturesandpreservedmineralassemblagesstableatmiddleorlowercrustallevels(hypersthene-sil-limanite,staurolite-spinel-sillimanite,hypersthene-corun-dum-spinel),andquenched,glass-richresidualmagmaingabbroicxenoliths,requirefastmovementofsomemagmastowardsthesurface.Thepresenceofrarepristineamphibole(withoutdehydrationrims)ingabbroicxenolithsandhostmagmaisconsideredstrongevidenceforexplosiveascentratesofmagmasfromitsmiddlecrustchamber.

(e)Whichwerethemainprocessesofmagmaticdifferentiation?Fractionalcrystallizationissupportedbyabundanceofcumulatemafic-ultramaficxenolithsand“glomerocrysts”ofgabbroiccompositionintheextrusiveandesite,whereasassimilationisphysicallyvisibleinthemeltingandmetasomaticreactionsthatoccurredbe-tweenxenolithsandmagmaatdifferentlevelsinthecrust.Moreover,abundantquartzxenocrysts,extensivemelting

100

Crn-Spl (?)Basaltic bubble

Hbl-Cpx-Opx-PlMul-Trd-Spl

Puente Negro andesite

Uppercrust

Lower

Moho

MANTLEWEDGE

OCEANIC MANTLE

1100-870 °C950 °C

900 °C

~280 km to the paleotrench (~30 Ma)

km

20

40

60

80

0

crustGrt-Opx-Sil-Spl

Oceaniccrust

Aqueous fluids

Figure17.Schematicgeologicalmodelforthedepth-temperaturehistorypathof thePuenteNegromagma-xenolith-xenocrystsystem,frominceptioninthemantlewedge(basalt)toemplacementnearthesurface(andesite).


ofxenoliths,thepresenceofotherexotichigh-Alphasesthatcouldbeconsideredxenocrysts(corundum,spinel,Al-orthopyroxene),andhighlyradiogenicNd,Sr,andPbisotopesofthemagmasfurthersupportcrustalassimilationasanimportantadditionalprocessintheevolutionofthePuenteNegromagmas.

ACKNOWLEDGMENTS

ThisresearchwaspartlyfundedbyPAPIITgrantIN104706 from the DGAPA at UNAM, and benefited by the kindness of many people at UNAM: Rufino Lozano, and Patricia Girón made some of the XRF analyses, and Diego Apariciopreparedandpolishedthinsectionsonwhichthestudywasbased;wespeciallythankHugoDelgadoandCarlosLinaresforfacilitatingtheuseoftheelectronmicroprobeattheLaboratorioUniversitariodePetrologíaatUNAM.WethankGillesLevresse,oftheCentrodeGeociencias,UNAM,forICP-MSanalysesofthexeno-liths.WearealsogratefultothefollowingpersonsattheInstitutodeGeologíaandtheInstitutodeGeofísica,UNAM,whohelpedindifferentwayswiththeanalyticalwork:AttheLaboratorioUniversitariodeGeoquímicaIsotópicaGabrielaSolisassistedinthecleanlab,andJuanJulioMoralesmeasuredtheisotopes,MaríadelSolHernándezmeasuredtheKcontentoftheplagioclaseusedfordatingtheandesite,andElenaLounejevacarriedoutsomeoftheICP-MSanalyses.ThisworkwassubstantiallyimprovedbytheextremelycarefulandconstructivereviewofJ.JorgeAranda-Gómez.

REFERENCES

Aguirre-Díaz,G.,Dubois,M.,Laureyns,J.,Schaaf,P.,2002,NatureandP-TconditionsofthecrustbeneaththecentralMexicanVolcanicBeltbasedonaPrecambriancrustalxenolith:InternationalGeologyReview,44,222-242.

Andersen,D.J.,Lindsley,D.H.,1988,InternallyconsistentsolutionmodelsforFe–Mg–Mn–Tioxides:Fe–Tioxides:AmericanMineralogist,73,714-726.

Andersen,D.J.,Lindsley,D.H.,Davidson,P.M.,1993,QUILF:aPascalprogramtoassessequilibriaamongFe-Mg-Mn-Tioxides,pyroxenes,olivine,andquartz:ComputersandGeosciences,19,1333-1350.

Arth,J.G.,1976,Behavioroftraceelementsduringmagmaticprocesses–Asummaryoftheoreticalmodelsandtheirapplications:JournalofResearch,U.S.GeologicalSurvey,4(1),41-47.

Bachmann,O.,Dungan,M.A.,2002,Temperature-inducedAl-zoninginhornblendesoftheFishCanyonmagma,Colorado:AmericanMineralogist,87,1062-1076.

Beard, J.S.,Ragland,P.C.,Crawford,M.L., 2005,Reactivebulkassimilation:Amodelforcrust-mantlemixinginsilicicmagmas:Geology,33,681-684.

Berman,R.G.,1988,Internally-consistentthermodynamicdataformineralsinthesystemNa2O-K2O-CaO-MgO-FeO-Fe2O3-Al2O3-SiO2-TiO2-H2O-CO2:JournalofPetrology,29,445-522.

Blatter,D.L.,Carmichael,I.S.E.,1998,HornblendeperidotitexenolithsfromcentralMexicorevealthehighlyoxidizednatureofsubarcuppermantle:Geology,26,1035-1038.

Blatter,D.L.,Carmichael,I.S.E.,2001,Hydrousphaseequilibriaofa

Mexicanhigh-silicaandesite:Acandidateforamantleorigin?:GeochimicaetCosmochimicaActa,65,4043-4065.

Blundy,J.,Cashman,K.,Humphreys,M.,2006,Magmaheatingbydecompression-drivencrystallizationbeneathandesitevolcanoes:Nature,443,76-80.

Boyd,F.R.,England,J.L.,1960,Thequartz-coesitetransition:JournalofGeophysicalResearch,65,749-756.

Buckley,V.J.E.,Sparks,R.S.J.,Wood,B.J.,2006,HornblendedehydrationreactionsduringmagmaascentatSoufrièreHillsvolcano,Montserrat:ContributionstoMineralogyandPetrology,151,121-140.

Cameron,K.L.,Robinson,J.V.,Niemeyer,S.,Nimz,G.J.,Kuentz,D.C.,Harmon,R.S.,Bohlen,S.R.,Collerson,K.D.,1992,Contrastingstylesofpre-Cenozoicandmid-TertiarycrustalevolutioninnorthernMéxico:evidencefromdeepcrustalxenolithsfromLaOlivina:JournalofGeophysicalResearch,97,17353-17376.

Cameron,W.E.,1976,Coexistingsillimaniteandmullite:GeologicalMagazine,113,497-592.

Cameron,W.E., 1977, Nonstoichiometry in sillimanite: Mullitecompositionswithsillimanite-typesuperstructures:PhysicsandChemistryofMinerals,v.1,p.265-272.

Campa,M.F.,Coney,P.J.,1983,Tectono-stratigraphicterranesandmineralresourcedistributionsinMexico:CanadianJournalofEarthSciences,20,1040-1051.

Carrington,D.P.,Harley,S.L.,1995,Partialmeltingandphaserelationsinhigh-grademetapelites:anexperimentalpetrogeneticgridintheKFMASHsystem:ContributionstoMineralogyandPetrology,120,270-291.

Cashman,K.,Blundy,J.,2000,Degassingandcrystallizationofascendingandesiteanddacite:PhilosophicalTransactionsRoyalSocietyLondon,358,1487-1513.

Elías-Herrera,M.,Ortega-Gutiérrez,F.,2002,Caltepecfaultzone:anEarlyPermiandextraltranspressionalboundarybetweentheProterozoicOaxacanandthePaleozoicAcatláncomplexes,southernMexico,andregionaltectonicimplications:Tectonics,21,doi:10.1029/2000TC001278.

Elthon,D.,Scarpe,C.M.,1984,High-pressurephaseequilibriaofahigh-magnesiabasaltandthegenesisofprimaryoceanicbasalts,AmericanMineralogist,69,1-15.

Ernst,W.G.,Liu,J.,1998,Experimentalphase-equilibriumstudyofAl-andTi-contentsofcalcicamphiboleinMORB–Asemiquantitativethermobarometer:AmericanMineralogist,83,952-969.

Garnier,V.,Ohnenstetter,D.,Giuliani,G.,Fallick,A.E.,PhanTrong,T.,HoaNgQuang,V.,PhamVan,L.,Schwarz,D.,2005,Basaltpetrology,zirconagesandsapphiregenesisfromDakNong,southernVietnam:MineralogicalMagazine,69,21-38.

Ghent,E.D.,Edwards,B.R.,Russell,J.K.,Mortensen,J.,2008,GranulitefaciesxenolithsfromPrindlevolcano,Alaska:ImplicationsforthenorthernCordillerancrustallithosphere:Lithos,101,344-358.

Gill,J.,1981,Orogenicandesitesandplatetectonics:NewYork,Springer-Verlag,390pp.

Gómez-Tuena,A.,Orozco-Esquivel,T.,Ferrari,L.,2005,PetrogénesisígneadelaFajaVolcánicaTransmexicana:BoletíndelaSociedadGeológicaMexicana,57,227-283.

Grapes,R.,2006,Pyrometamorphism:Berlin,Springer-Verlag,275pp.Green,T.H.,Pearson,N.J.,1985,ExperimentaldeterminationofREE

partition coefficients between amphibole and basaltic to andesitic liquidsathighpressure:GeochimicaetCosmochimicaActa,49,1465-1468.

Grochau,B.,Johannes,W.,1997,Stabilityofphlogopiteingraniticmelts,anexperimentalinvestigation:ContributionstoMineralogyandPetrology,126,315-330.

Guo, J., O’Reilly, S.Y., Griffin, W.L., 1996, Corundum from basaltic terrains:amineralinclusionapproachtotheenigma:ContributionstoMineralogyandPetrology,122,368-386.

Halliday,A.N.,Dickin,A.P.,Hunter,R.N.,Davies,G.R.,Dempster,T.J.,Hamilton,J.P.,Upton,B.G.J.,1993,Formationandcompositionofthelowercontinentalcrust:EvidencefromScottishxenolithsuites:JournalofGeophysicalResearch98,B1,581-607.

Harley,S.L.,2004,Extendingourunderstandingofultrahightemperature


crustalmetamorphism:JournalofMineralogicalandPetrologicalSciences,99,140-158.

Harley,S.L.,Motoyoshi,Y.,2000,Alzoninginorthopyroxeneinasapphirinequartzite:evidencefor>1120ºCUHTmetamorphismintheNapierComplex,Antarctica,andimplicationsfortheentropyofsapphirine:ContributionstoMineralogyandPetrology,138,293-307.

Harris,N.,1981,Theapplicationofspinel-bearingmetapelitestoP/Tdeterminations:AnexamplefromsouthIndia:ContributionstoMineralogyandPetrology,76,229-233.

Hart,S.R.,1984,Alarge-scaleisotopeanomalyintheSouthernHemispheremantle:Nature,309,753-757.

Haskin,L.A.,Haskin,M.A,Frey,F.A.,Wildeman,T.R.,1968,Relativeandabsoluteterrestrialabundancesoftherareearths,inAhrens,L.H.(ed.),OriginandDistributionoftheElements:Oxford,Pergamon,889-912.

Hayob,J.L.,Essene,E.J.,RuizJ.,Ortega-Gutiérrez,F.,Aranda-Gómez,J.J.,1989,Younghigh-temperaturegranulitesfromthebaseofthecrustincentralMexico:Nature,342,265-268.

Henry,D.J.,Guidotti,C.V.,Thomson,J.A.,2005,TheTi-saturationsurfaceforlow-to-mediumpressuremetapeliticbiotites:ImplicationsforgeothermometryandTi-substitutionmechanisms:AmericanMineralogist,90,316-328.

Hensen,B.J.,Green,D.H.,1973,Experimentalstudyofthestabilityofcordieriteandgarnetinpeliticcompositionsathighpressuresandtemperatures.IIIsynthesisofexperimentaldataandgeologicalapplications:ContributionstoMineralogyandPetrology,38,151-166.

Holland,T.J.B,Blundy,J.D.,1994,Non-ideal interactionsincalcicamphiboles and their bearing on amphibole-plagioclasethermometry:ContributionstoMineralogyandPetrology,116,433-447.

Hollis,J.A.,Harley,S.L.,2003,Aluminasolubilityinorthopyroxenecoexistingwithsapphirineandquartz:ContributionstoMineralogyandPetrology,144,473-483.

Holm,J.L.,2001,Kaolinites-mullitetransformationindifferentAl2O3-SiO2systems:Thermo-analyticalstudies:PhysicalChemistryandChemicalPhysics,3,1362-1365.

Holtz,F.,Becker,A.,Freise,M., Johannes,W.,2001,Thewater-undersaturatedanddryQz-Ab-Orsystemrevisited.Experimentalresultsatverylowwateractivitiesandgeologicalimplications:ContributionstoMineralogyandPetrology,141,347-357.

Johnson,B.R.,Kriven,W.M.,Schneider,J.,2001,Crystalstructuredevelopment during devitrification of quenched mullite: Journal ofEuropeanCeramicSociety,21,2541-2562.

Johnson,M.C.,Rutherford,M.J.,1989,Experimentalcalibrationofthealuminum-in-hornblendegeobarometerwithapplicationtoLongValleyCaldera(California)volcanicrocks:Geology,17,837-841.

Kelly,N.M.,Harley,S.L.,2004,Orthopyroxene–corunduminMg–Al-richgranulitesfromtheOygardenIslands,EastAntarctica:JournalofPetrology,45,1481-1512.

Kelsey,D.E.,White,RW.,Powell,R.,2003,Orthopyroxene-sillimanite-quartz assemblages: distribution, petrology, quantitative P-T-X constraintsandP-Tpaths:JournalofMetamorphicGeology,21,439-453.

Kosyakova,N.A.,Aranovich,L.Y.,Podlesskii,K.K.,2005,EquilibriaofaluminousspinelwithorthopyroxeneinthesystemFeO-MgO-Al2O3-SiO2:newexperimentaldataandthermodynamicassessment:DokladyEarthSciences,400,57-61.

Kriegsman,L.M.,Schumacher,J.C.,1999,Petrologyofsapphirine-bearingandassociatedgranulitesfromcentralSriLanka:JournalofPetrology,40,1211-1239.

Lane,D.L.,Ganguly,J.,1980,Al2O3solubilityinorthopyroxeneinthesystemMgO-Al2O3-SiO2;areevaluation,andmantlegeotherm:JournalofGeophysicalResearch,85,B12,6963-6972.

LeBas,M.J.,LeMaitre,R.W.,Streckeisen,A.,Zanettin,B.,1986,Achemical classification of volcanic rocks based on the total alkali-silicadiagram:JournalofPetrology,27,745-750.

Lee,C.-T.,Rudnick,R.L.,Brimhall,G.H.,Jr.,2001,Deeplithospheric

dynamicsbeneaththeSierraNevadaduringtheMesozoicandCenozoicasinferredfromxenolithpetrology:GeochemistryGeophysicsGeosystems,2(12),doi:10.1029/2001GC000152.

Lee,H.Y.,Ganguly,J.,1988,Equilibriumcompositionsofcoexistinggarnetandorthopyroxene:ExperimentaldeterminationsinthesystemFeO-MgO-Al2O3-SiO2,andapplications:JournalofPetrology,29,93-113.

Lepage,L.D.,2003,ILMAT:anExcelworksheetforilmenite-magnetitegeothermometryandgeobarometry:Computers&Geosciences,29,673-678.

Liermann,H.P.,Ganguly,J.,2003,Fe2+-Mgfractionationbetweenorthopyroxeneandspinel:experimentalcalibrationinthesystemFeO-MgO-Al2O3-Cr2O3-SiO2,andapplications:ContributionstoMineralogyandPetrology,145,217-227.

Lozano,R.,Bernal,J.P.,2005,Characterizationofanewsetofeightgeochemical reference materials for XRF major and trace elementanalysis:RevistaMexcianadeCiencasGeológicas,22,329-344.

Luhr,J.F.,Aranda-Gómez,J.J.,1997,Mexicanperidotitexenolithsandtectonicterranes:correlationsamongventlocation,texture,temperature,pressure,andoxygenfugacity:JournalofPetrology,38,1075-1112.

Luhr,J.F.,Aranda-Gómez,J.J.,Pier,J.G.,1989,Spinel-lherzolite-bearingQuaternaryvolcaniccenters inSanLuisPotosí,Mexico:I.Geology,mineralogyandpetrology:JournalofGeophysicalResearch,94,B6,7916-7940.

Mariga,J.,RipleyE.M.,Li,C.,2006a,Petrologicevolutionofgneissicxenoliths in theVoisey´sBayintrusion,Labrador,Canada:Mineralogy,reactions,partialmelting,andmechanismsofmasstransfer:GeochemistryGeophysicsGeosystems,7(5),doi:10.1029/2005GC001184.

Mariga,J.,Ripley,E.M.,Li,C.,McKeegan,K.D.,Schmidt,A.,Groove,M.,2006b,Oxygenisotopicdisequilibriuminplagioclase-corundum-hercynite xenoliths from the Voisey’s Bay Intrusion, Labrador, Canada:EarthandPlanetaryScienceLetters,248,263-275.

Martiny,B.,Martínez-Serrano,R.G.,Morán-Zenteno,D.J.,Macías-Romo,C.,Ayuso,R.A.,2000,Stratigraphy,geochemistryandtectonicsignificance of the Oligocene magmatic rocks of western Oaxaca, southernMexico:Tectonophysics,318,71-98.

Milholland,C.S.,Presnall,D.C.,1998,LiquidusphaserelationsintheCaO-MgO-Al2O3-SiO2systemat3·0GPa:thealuminouspyroxenethermaldivideandhigh-pressurefractionationofpicriticandkomatiiticmagmas:JournalofPetrology,38,3-27.

Morán-Zenteno,D.J.,Martiny,B.,Tolson,G.,Solís-Pichardo,G.,Alba-Aldave,L.,Hernández-Bernal,M.S.,Macías-Romo,C.,Martínez-Serrano,R.G.,Schaaf,P.,SilvaRomo,G.,2000,GeocronologíaycaracterísticasgeoquímicasdelasrocasmagmáticasterciariasdelaSierraMadredelSur:BoletíndelaSociedadGeológicaMexicana,53,27-58.

Murphy,M.D.,Sparks,R.S.J.,Barclay,J.,Carrol,M.R.,Brewer,T.S.,2000, Remobilization of andesite magma by intrusion of mafic magmaattheSoufriereHillsvolcano,Montserrat,WestIndies:JournalofPetrology,41,21-42.

Nakamura,N.,1974,DeterminationofREE,Ba,Fe,Mg,NaandKin carbonaceous andordinary chondrites:Geochimica etCosmochimicaActa,38,757-775.

Ortega-Gutiérrez,F.,1978,EstratigrafíadelComplejoAcatlánenlaMixtecaBaja,EstadosdePueblayOaxaca:Revista,2(2),112-131.

Ortega-Gutiérrez,F.,MitreSalazar,L.M,RoldánQuintana,J.,SánchezRubio,G.,delaFuente,M.,1990,NorthAmericanOcean-ContinentTransectsProgram,TransectH-3fromAcapulcoTrenchtotheGulfofMéxicoacrosssouthernMéxico:Boulder,CO,GeologicalSocietyofAmerica,DecadeofNorthAmericanGeology(DNAG),Transect13,mapwithsectionsandtext,9pp.

Ortega-Gutiérrez,F.,Elías-Herrera,M.,Dávalos-Elizondo,M.G.,2008,OnthenatureandroleofthelowercrustinthevolcanicfrontoftheTrans-MexicanVolcanicBeltanditsfore-arcregion,southernandcentralMexico:RevistaMexicanadeCienciasGeológicas,


25,346-364.Powell,M,Powell,R.,1977,Plagioclase-alkalifeldspargeothermometry

revisited:MineralogicalMagazine,41,253-256.Powell,R.,Sandiford,M.,1988,Sapphirineandspinelphaserelationships

inthesystemFeO-MgO-Al2O3-SiO2-TiO2-O2inthepresenceofquartzandhypersthenes:ContributionstoMineralogyandPetrology,98,64-71.

Pugin,V.A.,Kuskov,O.L.,1968,P-TequilibriainthesystemAl2O3-SiO2accordingtothermodynamicdata:GeochemistryInternational,5,804-808.

Ray, R., Shukla, A.D., Sheth, H.C., Ray, J.S., Duraiswami,R.A.,Vanderkluysen,L.,Rautela,C.S.,Mallik,J.,2008,HighlyheterogeneousPrecambrianbasementunderthecentralDeccanTraps,India:Directevidencefromxenolithsindykes:GondwanaResearch,13,375-385.

Reyes-Salas,A.M.,2003,MineralogíaypetrologíadelosGranitoidesEsperanzadelComplejoAcatlán,surdeMéxico:Morelos,México,UniversidadAutónomadelEstadodeMorelos,FacultaddeCienciasQuímicaseIngeniería,Ph.D.thesis,165pp.

Rudnick,R.L.,Cameron,K.L.,1991,AgediversityofthedeepcrustinnorthernMexico:Geology,19,1197-1200.

Rudnick,R.L.,Fountain,D.M.,1995,Natureandcompositionofthecontinentalcrust:a lowercrustalperspective:Reviews inGeophysics,33,267-309.

Ruiz,J.,Patchett,P.J.,Ortega-Gutiérrez,F.,1988,ProterozoicandPhanerozoicbasementterranesofMexicofromNdisotopicstudies:GeologicalSocietyofAmericaBulletin,100,274-281.

Rutherford,M.J.,Hill,M.,1993,Magmaascentratesfromhornblendebreakdown:anexperimentalstudyappliedtothe1980-1986MountSt.Helenseruptions:JournalofGeophysicalResearch,98,B11,19667-19685.

Rutherford,M.J.,Devine,J.D.,2003,Magmaticconditionsandmagmaascentasindicatedbyhornblendephaseequilibriaandreactionsinthe1995–2002SoufrièreHillsmagma:JournalofPetrology,44,1433-1454.

Sachs,P.M.,Hansteen,T.H., 2000,Pleistoceneunderplating andmetasomatismofthelowercontinentalcrust:axenolithstudy:JournalofPetrology,41(3),331-356.

Saxena,S.K.,Sykes,J.,Eriksson,G.,1986,Phaseequilibriainthepyroxenequadrilateral:JournalofPetrology,27,843-852.

Schaaf,P.,Heinrich,W.,Besh,T.,1994,CompositionandSm-NdisotopicdataofthelowercrustbeneathSanLuisPotosi,centralMexico:Evidencefromagranulitefaciesxenolithsuite:ChemicalGeology,118,63-84.

Schaaf,P.,Stimac,J.,Siebe,C.,Macías,J.L.,2005,GeochemicalevidenceformantleoriginandcrustalprocessesinvolcanicrocksfromPopocatépetlandsurroundingmonogeneticvolcanoes,centralMexico:JournalofPetrology,46,1243-1282.

Schnetzler, C.C., Philpotts, J.A., 1970, Partition coefficients of rare-earth elementsbetweenigneousmatrixmaterialandrock-formingmineralphenocrysts–II:GeochimicaetCosmochimicaActa,34,331-340.

Schreyer,W.,1976,Experimentalmetamorphicpetrologyatlowpressuresandhightemperatures,inBaileyD.K.MacDonald,R.(eds.),TheEvolutionoftheCrystallineRocks:London,AcademicPress,261-331.

Sedlock,R.L.,Ortega-Gutiérrez,F.,Speed,R.C.,1993,TectonostratigraphicterranesandtectonicevolutionofMexico:GeologicalSocietyofAmericaSpecialPaper278,153.

Solé, J., Enrique, P., 2001, X-ray fluorescence analysis for the determination ofpotassiuminsmallquantitiesofsilicatemineralsforK-Ardating:AnalyticaChimicaActa,440,199-205.

Spencer,K.J.,Lindsley,D.H.,1981,Asolutionmodelforcoexistingiron-titaniumoxides:AmericanMineralogist,66,1189-1201.

Stacey,J.S.,Kramers,J.D.,1975,Approximationofterrestrialleadisotopeevolutionbyatwo-stagemodel:EarthandPlanetaryScienceLetters,26,207-221.

Steiger,R.H.,Jäger,E.,1977,Subcommissionongeochronology:Conventionontheuseofdecayconstantsingeo-andcosmochronology:EarthandPlanetaryScienceLetters,36,359-362.

Sun,S.-s.,McDonough,W.F.,1989,Chemicalandisotopicsystematicsofoceanicbasalts:implicationsformantlecompositionandprocesses,inSaunders,A.D.,Norry,M.J.(eds.),Magmatismintheoceanbasins,GeologicalSocietySpecialPublication42,313-345.

Sutherland,F.L.,Hoskin,P.W.O.,Fanning,C.M.,Coenraads,R.R.,1998,Modelsofcorundumoriginfromalkalibasalticterrains:areappraisal:ContributionstoMineralogyandPetrology,133,356-372.

Valdés,C.M.,Mooney,W.D.,Singh,S.K.,Meyer,R.P.,Lomnitz,C.,Luetgert,J.H.,Helsley,C.E.,Lewis,B.T.R.,Mena,M.,1986,CrustalstructureofOaxaca,fromseismicrefractionmeasurements:BulletinoftheSeismologicalSocietyofAmerica,76,547-563.

Vielzeuf,D.,Holloway,J.R.,1988,Experimentaldeterminationofthefluid-absent melting relations in the pelitic system. Consequences forcrustaldifferentiation:ContributionstoMineralogyandPetrology,98,257-276.

Wen,S.,Nekvasil,H.,1994,Idealassociatedsolutions:Applicationtothesystemalbite-quartz-H2O:AmericanMineralogist,79,316-331.

Whitney,D.L.,Evans,B.W.,2010,Abbreviationsfornamesofrock-formingminerals:AmericanMineralogist,95,185-187.

Xirouchakis, D., Lindsley, D.H., Andersen, D.J., 2001, Assemblages with titanite(CaTiOSiO4),Ca-Mg-Feolivineandpyroxenes,Fe-Mg-Tioxides,andquartz:Part1.Theory:AmericanMineralogist,86,247-253.

Yañez,P.,RuizJ.,Patchett,P.J,Ortega-Gutiérrez,F.,GehrelsG.E.,1991,Isotopicstudiesof theAcatlanComplex,southernMexico:ImplicationsforPaleozoicNorthAmericantectonics:GeologicalSocietyofAmericaBulletin,103,817-828.

Manuscriptreceived:June9,2010Correctedmanuscriptreceived:July22,2011Manuscriptaccepted:July27,2011

petrology of crustal xenoliths, puente negro intrusion · disgregadas en la andesita. asociaciones...

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