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Mesozoic evolution of the Xolapa Complex 201
ABSTRACT
The Xolapa Complex in the Acapulco-Tierra Colorada area, southern Mexico, is made up of orthogneisses and paragneisses, both affected by a variable degrees of migmatization. These rocks are intruded by several episodes of Jurassic-Oligocene, calc-alkaline granitic magmatism. Three phases of ductile deformation affected the gneisses and migmatites. D1 produced an amphibolite-facies metamorphic banding (M1) and a penetrative S1 foliation, axial planar to isoclinal, recumbent F1 folds with axes parallel to a NE to NW, gently plunging, L1 stretching lineation. D2 consists of a S2 foliation defined by hornblende, biotite and garnet, synchronous with M2 migmatization that generated leucosomes, which are generally parallel to S1. D3 is made up of asymmetric, chevron F3 folds that deform the composite S1/S2 foliation during greenschist to lower amphibolite metamorphism (M3). U-Pb SHRIMP (Sensitive High-Resolution Ion Micropobe) geochronology carried out on zircon separated from two orthogneisses yielded an Early Jurassic magmatic event (178.7 ± 1.1 Ma) and the age of migmatization (133.6 ± 0.9 Ma). Two episodes of Pb loss were also recognized, the first at 129.2 ± 0.4 Ma, and the second during the earliest Paleocene (61.4 ± 1.5 Ma); they are probably associated with two episodes of magmatism. The Early Jurassic magmatic arc may be correlated with a magmatic arc in the eastern Guerrero terrane. The Early Cretaceous migmatization is inferred to have resulted from shortening, possibly due to the accretion of an exotic block, such as the Chortís block along whose northern margin contemporaneous, high-pressure metamorphism has been recorded.
Key words: migmatization, U-Pb, Mesozoic, Xolapa Complex, southern México.
RESUMEN
El Complejo Xolapa es el terreno metamórfico más extenso en el sur de México. En el segmento que va de Acapulco a Tierra Colorada está constituido por ortogneises y paragneises, afectados por un grado variable de migmatización. El basamento del Complejo Xolapa está también afectado por diversos episodios de magmatismo granítico de afinidad geoquímica calcialcalina y edades que van del Jurásico al Oligoceno. Tres fases de deformación dúctil afectan los gneises y migmatitas. Un bandeamiento metamórfico se desarrolla durante D1, y está asociado con una foliación penetrativa S1, en facies de anfibolita, que actúa como plano axial de pliegues de recumbentes a isoclinales F1. Una lineación de estiramiento L1 está presente en toda el área de estudio, con buzamiento tanto al NE como NW. La
MesozoicgeologicevolutionoftheXolapamigmatiticcomplexnorthofAcapulco,southernMexico:
implicationsforpaleogeographicreconstructions
Rosalva Pérez-Gutiérrez1, Luigi A. Solari2,*, Arturo Gómez-Tuena2, and Uwe Martens 3
1 Instituto de Geología, Universidad Nacional Autónoma de México, Cd. Universitaria,Del. Coyoacán, 04510 México D.F., Mexico.
2 Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla,Blvd. Juriquilla 3001, 76230 Querétaro, Qro., Mexico.
3 Department of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Bldg 320, Stanford, CA 94304, United States.
Revista Mexicana de Ciencias Geológicas, v. 26, núm. 1, 2009, p. 201-221
Pérez-Gutiérrez et al.202
INTRODUCTION
High-grade metamorphic terrains form a largeportionofthecontinentalcrustexposedinPhanerozoicandPrecambrianorogens.Inasmuchasthesekindsofmetamorphic terrains representexhumedportionsofthemiddleandlowercontinentalcrust,understandingtheirgeologicevolutionshedslightonthedeepcrustalprocesses.
ThegeologyofsouthernMexicoiscomposedofamo-saicoftectonostratigraphicterranes,(Figure1)(CampaandConey,1983;Sedlocket al.,1993;Keppie,2004).Alongthe Pacific coast, the Xolapa (or Chatino) terrane, ~600 km long and ~50–80 km wide, is a composite plutonic and metamorphicterrane.TheXolapaterranetruncatestheter-ranesexposedtothenorth,whichconsist(fromWtoE)of:(a)theMesozoicGuerreroterrane(Talavera-Mendozaet al.,2007;Centeno-Garcíaet al.,2008),(b)thepolymetamor-phic,Paleozoic-bearingMixtecaTerrane(Ortega-Gutiérrezet al.,1999),and(c)theMesoproterozoic,granulite-bearingZapotecoterrane(Keppieet al.,2001;Keppieet al.,2003;Solariet al.,2003;Solariet al.,2004).TheMixtecaandZapotecoterraneswereamalgamatedduringthePermian,whereastheGuerreroterranewasaccretedintheMesozoic(Centeno-Garcíaet al.,1993;Elias-HerreraandOrtega-Gutiérrez,2002;Keppie,2004;Centeno-Garcíaet al.,2008).TheallochthonousorautochthonousnatureoftheXolapaComplexisstilldebated(e.g.,Keppie,2004;Duceaet al.,2004;Corona-Chávezet al., 2006). One view sug-geststhattheXolapaComplexisautochthonous,originatedin situbythinningandreheatingofthecontinentalcrustduringTertiary(e.g.,Herrmannet al.,1994;Schaafet al.,1995; Morán-Zenteno et al., 1996; Ducea et al.,2004).AcontrastingideasuggeststhattheXolapaisallochthonous,andwasaccretedtothecontinentalmarginofsouthern
Mexico during Mesozoic–Tertiary (e.g.,Corona-Chávezet al., 2006).
Inanattempttoresolvethisdebate,weundertookacombination of fieldwork, petrologic and geochronologic analysestobetterdocumentthesequenceoftectonother-maleventsintheXolapaComplex.Akeyquestiontobeaddressedisthetimingofmigmatizationandhigh-grademetamorphismoftheXolapaComplex.Varioustimerangeshavebeenproposed:LateJurassictoearliestCretaceous(e.g.,Duceaet al.,2004;Solariet al.,2007),LateCretaceousto earliest Paleocene (~60–110 Ma, Corona-Chávez et al.,2006), and Early Tertiary (46–66 Ma; Herrmann et al.,1994).Inthispaper,wepresentdetailedstructuraldescrip-tionoftheXolapaComplexnorthofAcapulco(Figures1and2),thegeochemicalcharacteristicofsomeofthehigh-grademetamorphicrocks,andU-Pbzircongeochronology.ThesedataarethenappliedtoTertiaryreconstructions(e.g.,Solariet al.,2007).
REGIONAL GEOLOGY
TheXolapaComplexismadeupbyasequenceofhigh-grademetasedimentaryandmetaigneousrocksthatarefrequentlyintrudedbybothdeformedandundeformedplutonicrocks.Previousworkwasgenerallylimitedtoeitherfield descriptions, (i.e., De Cserna, 1965), combined geo-chemicalandgeochronologicalanalyses(Morán-Zenteno,1992;Herrmannet al.,1994;Duceaet al.,2004),orcom-bined field and petrological work without geochronology (Corona-Chávezet al., 2006). As there is some confusion about what constitutes the Xolapa Complex, we define theXolapaComplexasthoserocksthatareaffectedbymigmatizationandductiledeformation,whichpredatetheintrusion and further solid-state shearing of the ~130 Ma
orientación de las estructuras D1 controla el emplazamiento de los lentes de leucosoma D2, generados durante el proceso de migmatización. Afuera de los dominios leucosomáticos, la foliación S2 está caracterizada por la asociación hornblenda + biotita + granate. Pliegues asimétricos de tipo chevron relacionados a la fase F3 deforman la foliación compuesta S1/S2, en condiciones de facies de esquistos verdes hasta anfibolita. La geocronología de U-Pb por SHRIMP (microsonda iónica de alta resolución) realizada en zircones separados de dos muestras de ortogneises permitió reconocer un evento magmático jurásico temprano (178.7 ± 1.1 Ma), así como la edad de migmatización en esta porción del complejo de Xolapa calculada en 133.6 ± 0.9 Ma. Dos episodios de pérdida de Pb se pueden reconocer en algunos de los zircones de las muestras fechadas. El primero es posterior al pico de migmatización (129.2 ± 0.4 Ma), y el segundo ocurrió durante el Paleoceno temprano (61.4 ± 1.5 Ma), y probablemente ambos son un efecto térmico asociado a dos de los episodios magmáticos posteriores. Estos datos sugieren que un arco magmático del Jurásico temprano habría constituido el basamento del Complejo Xolapa; se sugiere que este mismo arco corresponda a la porción este del terreno Guerrero, en donde algunas edades magmáticas similares han sido reportadas. La migmatización ocurrió durante el Cretácico temprano como consecuencia del acortamiento producido por la accreción de un bloque exótico. El bloque de Chortís es un candidato posible para tal colisión, ya que registra un evento metamórfico de alta presión contemporáneo con el evento de migmatización en el Complejo Xolapa.
Palabras clave: migmatización, U-Pb, Mesozoico, Complejo Xolapa, sur de México.
Mesozoic evolution of the Xolapa Complex 203
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Sierra de Juarez Mylonitic Complex
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Precambrian Oaxacan Complex
Precambrian Guichicovi Complex
Major shear zones
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Studied area(Figure 2)
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Acapulco,Solariet al.(2007)presentedU-Pbconcordantages of ~129 Ma on undeformed to slightly deformed gran-ites, and ~55 Ma ages for other granites locally affected by ductile shearing. The ~129 Ma ages are in the range of the deformedgranitesofMorán-Zenteno(1992),whereasthe~55 Ma ages are similar to those previously determined byDuceaet al.(2004)ontheAcapulcosyeno-granite.GeochronologyofdeformedsamplesfromtheXolapaComplexindicatesthepresenceofprotolithssimilartotheneighboringmetamorphiccomplexes.Forinstance,northofPuertoÁngel(Figure1)rockshavebothigneousandmeta-morphicGrenvillianages,similartothoseintheOaxacanComplex(e.g.,Keppieet al.,2001;Duceaet al.,2004).NorthofPuertoEscondido,Duceaet al.(2004)reportedPermianagesinabiotite-bearinggneissthataresimilartoagesreportedingraniteandgranodioritebelongingtotheAcatlánComplex(Yañezet al.,1991;Keppieet al.,2004,2008),theJuchatengocomplex(Grajales-Nishimura,1988;Grajales-Nishimuraet al.,1999),andthestitchinggranitesalongtheCaltepecfaultzone(Elias-HerreraandOrtega-Gutiérrez,2002).IntheAcapulcotransect,agesreportedsofararenoolderthanMiddleJurassic,andaresimilarto
deformedgranites(e.g.,Solariet al.,2007,andseebelow).Previous definitions of the Xolapa Complex included all the metamorphicandigneouspre-Oligocenerocksexposedinthearea(e.g., De Cserna, 1965; Alaniz-Álvarez and Ortega-Gutiérrez,1997.
ThenorthernboundaryoftheXolapaterrane(Figure1)ismarkedbynormalandleft-lateralductileshearzones,whichwereactivefromtheEoceneonthewest(TierraColoradashearzone,Rilleret al.,1992;Solariet al.,2007),totheOligoceneintheeast(Chacalapashearzone,Tolson,2005). While protolith ages of the metasediments have yettobedetermined,theoldestmagmaticarcintrudingtheXolapaComplexisdocumentedasJurassictoEarlyCretaceous(e.g.,Guerrero-Garcíaet al.,1978;Morán-Zenteno,1992;Herrmannet al.,1994;Duceaet al.,2004).TheyoungestundeformedplutonsintrudingtheXolapaComplex range from ~34 Ma in the Acapulco – Tierra Coloradaarea(Hermannet al.1994;Duceaet al.,2004),and become younger toward SE (~25–29 Ma in Puerto Escondido – Huatulco; Hermann et al.1994;Duceaet al.,2004).Atleasttwootherintrusiveeventsoccurredbetweentheoldestandyoungestmagmaticevents.Thus,northof
Figure 1. Simplified terrane subdivision of southern Mexico. Main basement complexes are also indicated. Modified from Solari et al.(2007).TerranesubdivisionaccordingtoCampaandConey(1983),Sedlocket al.(1993),andKeppie(2004).
Pérez-Gutiérrez et al.204
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ParotillasLa ConcepciónSan Isidro Gallinero
San JoséCacahuatepec
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Tertiary volcanicsUndeformed granite (~30 Ma)El Pozuelo g 129 Ma, Solari et al., 2007)ranite (
Migmatitic orthogneiss
Migmatitic paragneissOrthogneiss (165 Ma, Guerrero-García et al., 1978)Paragneiss
LEGENDAlluvium
OrthomigmatiteParamigmatite
Marble
USED SYMBOLS
Geologic section traceA A´
Foliation and lineation1247
Thrust fault
Left-lateral fault
Foliation, with dip47
Fold axis9 O
Lineation, with plunge18
Inferred fault
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Mesozoic evolution of the Xolapa Complex 205
thoseintheGuerreroTerrane(Talavera-Mendozaet al.,2007;Centeno-Garcíaet al.,2008).ThesecorrelationscanbeinterpretedastheprimeevidencefortheautochthonousnatureoftheXolapaterrane.
Corona-Chávezet al. (2006) recognized a single high-grade metamorphic event (830–900 °C and 6.3–9.5 kbar) withaclockwisepressure-temperature(P-T)pathintheXolapa Complex of the Puerto Escondido – Puerto Ángel area(Figure1).Fluidsreleasedbythebreakdownofbiotiteandamphiboleproducedintensemigmatization.ThevastmajorityofplutonicrocksoftheXolapaComplexarecalc-alkalinegranitesandgranodiorites(Herrmann,1994).TheTertiary(PaleocenetoOligocene)granitesandgranodioriteshavesimilarcompositions(Morán-Zenteno,1992;Schaafet al., 1995; Morán-Zenteno et al., 1996; Morán-Zenteno et al.,1999;Morán-Zentenoet al., 2005).
ROCK TYPES
Thestudyarea,locatednorthofAcapulco,coversabout700km2.Previousstudiesinthisareawerefocusedon general field descriptions of the northern part of the XolapaComplexintheBarrancadeXolapa(Figure2),where the Xolapa Complex was first defined (De Cserna, 1965). Subsequently, Alaniz-Álvarez and Ortega-Gutiérrez (1997)describedmediumtohigh-grademetasedimentsatthislocality(quartz-K-feldspar-biotite-muscovite-sil-limanite-garnet,andrarecordieriteandcorundum)thatareintrudedbydifferentgenerationsofvariablydeformedpegmatite, mafic dikes and granites. Other studies dealt with regionalgeologicalmapping(e.g.,Sabanero-Sosa,1990),orcombinedgeologicmappingwithisotopicdetermina-tions(Morán-Zenteno,1992).ForinstanceMorán-Zenteno(1992)producedaregionalmap,andgeochemical,Rb-SrandSm-Nddataonthedeformedgranitoids,whichyieldedan age of ~130 Ma, interpreted as the age of intrusion. Other studiesnorthofAcapulcoprovidedgeochronologicaldataon gneisses (~134–140 Ma, Ducea et al.,2004)andunde-formed,Oligoceneplutons(e.g.,Herrmannet al.,1994;Schaafet al., 1995; Ducea et al.,2004),butthelackofdetailed mapping makes it difficult to establish a complete geologicalrecord.Northeastofthestudyarea,Torres-deLeón (2005) and Solari et al.(2007)documentedtheal-lochthonousnatureoftheTierraColoradaarea(Figure1).Torres-de León (2005) and Solari et al.(2007)establishedthestructuralandmagmaticevolutionoftheTierraColoradaarea as a series of alternating magmatic (~130, ~55, and ~34 Ma) and deformation pulses (>130, ~45, and between 45 and 34 Ma).
Ourgeologicmapping(Figure2)revealedtheex-
istenceofseverallithologiesexposednorthofAcapulcothatbelongtotheXolapaComplex.Therecognizedpre-migmatiticrocksweregroupedintopara-andortho-gneiss-es,whichareaffectedbyvariabledegreesofmigmatization.Their petrographic characteristics and field relationships are describedinthefollowingsection.
Metasediments
Metasediments,includingpsammitic-peliticparag-neissesaswellasmarbles,aremainlydistributedtothesouth and southeast of the study area and constitute ~30% of the area (Figure 2). They generally consists of 5–30 cm thick bandsofgreytoochre-browncoloredquartzofeldsphaticmicaschists,whosemainconstituentsaregranoblasticquartz,plagioclase,biotite,muscovite,cordierite,garnet,±sillimanite,±staurolite.Stauroliteandmuscovitearenotpresentinthemigmatiticrocks.Theassociationbiotite+sillimanite+cordierite+corundum+hercyniteisquitecommoninthemetasedimentaryrestites.Zirconandapatitearecommonaccessoryminerals.Tourmalineisanabundantaccessorymineralinsomesamples.Biotite,muscoviteand sillimanite (often present as fibrolite) are generally intergrown,andalignedalongfoliationplanes.Themin-eralogysuggestsagreywacke/minorpelitesprotolith.Upto1kmwidemarblebodieswithinthesequencearemadeup of ≤85% calcite, minor dolomite, and accessory quartz, plagioclase,diopsideanddetritalzircons.
Orthogneisses
Orthogneisses,themostabundantlithologyinthestudyarea,aremainlyleucocratictomesocratic,pervasivelyfoliatedgranitesandgranodiorites(notdistinguishedonFigure2).Theorthogneissesarebandedandoftencharac-terizedbyupto20cmthickdarkbandsmainlycomposedofbiotite,hornblende,plagioclaseandopaqueminerals.Leucocraticbandsarecomposedofquartz,plagioclase,potassicfeldspar,biotite,±hornblende.Commonaccessorymineralsarezirconandapatite.Chloritedevelopsasasec-ondarymineralphaseattheexpenseofbiotite.Orthoclaseporphyroclastsarepresent,sometimesdevelopingcoreandmantletextures,withsodium-richmantlesaroundpotas-sium-rich cores. The foliation is defined by biotite and feld-sparporphyroclastsintheleucocraticbands,andhornblendeandbiotiteinthemelanosomes(e.g.,Figure3a).Thesharpcontactsbetweenparagneissesandorthogneisses,aswellassomepreservedcross-cuttingrelationships,indicateoforiginalintrusiverelationships.
Figure 2. Geologic map of the Xolapa Complex between Acapulco and Tierra Colorada, Guerrero, southern Mexico. Modified from Pérez-Gutiérrez (2005).
Pérez-Gutiérrez et al.206
Figure3.Fieldphotographsillustratingthemainaspectsofthemigmatitescroppingoutinthestudiedarea.a:Medium-grainedorthogneiss,showingS2 foliation made up of aligned biotite and hornblende: the black marker is ~12 cm long. b: Close to isoclinally folded stromatic migmatite: the black marker on the top center is ~12 cm long. c: Pinch and swell, to agmatic migmatite, with injecting leucosomatic vein, lacking a preferred orientation of intrusion; the notebook is ~16×20 cm in size. d: Raft (Schöllen) migmatite, where dm-sized block of melanosome float into a heterogeneous leucosome; the notebook is ~12×17 cm in size. e: Diktyonitic migmatite, with a vein of leucosome that is intruding the granitic mesosome, showing diffuse contacts; vertical pencil, in the mid of the picture, is ~15 cm long. f: Orthogneiss with granitic leucosome pods, with diffuse contacts, is characterized by centimetric crystals of biotite and hornblende: the pencil is ~15 cm long. g: Leucosome veins, or lens, are intruding the paleosome parallel to S1surfaces,buttheyarenot deformed themselves; the notebook is ~12×17 cm in size. h: ca.1m-thickmuscoviteandgarnet-bearing,F5foldedpegmatite.FoldaxisisroughlysubhorizontalandEWoriented.AxialplaneisNNE-steeplyplunging,ESE-WNWtrending.
a)
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Mesozoic evolution of the Xolapa Complex 207
(2007)arguedthatthecrystallizationofElPozuelogranitepostdatesmigmatizationintheXolapaComplex.Duceaet al. (2004, 2006) also reported ~134 to ~140 Ma U-Pb zirconageson“syntectonic”hornblende-biotitegneissesinthestudyarea.
Undeformed, post-Laramide plutonsAsecondgroupofpost-migmatiticgranitestypically
have ~55 Ma ages. They slightly postdate Laramide defor-mationinsouthernMexico(Cercaet al.,2007;Solariet al.,2007).Suchbodieslackofsolid-statedeformation,althoughtheyarelocallyaffectedbylow-gradeductileshearingorbrittledeformation.Forinstance,theAcapulcogranite,intrudingtheXolapamigmatitesandgneissesjustSWofthemappedarea,wasdatedbyDuceaet al. (2004) at 54.9 ±2Ma.TheSalitreperaluminousandLasPiñasgraniteswere dated at 55.3 ± 3.3 and 54.2 ± 5.8 Ma, respectively (Solariet al.,2007),andcropoutnortheastofthestudyarea,justwestoftheLaVentashearzone.Theplutonsareeitherfoliated(ElSalitre)orunfoliated(LasPiñas),andlo-cally,alongLaVentashearzone,theydisplayS-Cfabricsassociatedwiththeshearing.
Eocene-Oligocene undeformed granitoidsAthirdgroupofundeformed,Eocene-Oligocene
granitoidscropoutinthestudiedarea(Herrmannet al.,1994;Schaafet al., 1995; Ducea et al.,2004;Hernández-Pineda, 2006). These plutonic bodies lack ductile, solid-state deformation,buttheygenerallyshowmagmaticbandingaswellasbrittlefaultsandjoints.
Severalgenerationsofdioritic-quartzodioriticdikes,pegmatites,andminoraplites,intrudeboththeXolapabasement,andtheEocene-Oligocenegranitoids.SomepegmatitescroppingoutintheBarrancadeXolapaweredated by Morán-Zenteno (1992) at ~59 Ma, and by Solé (2004) at 60–62 Ma. Although no isotopic ages are available forthedioriticdikes,theyoungestgenerationpost-datesthe34MaTierraColoradagranite.
STRUCTURAL EVOLUTION
Threephasesofductiledeformationwererecognizedinthegneissesandmigmatitesofthestudyareathatareabsentinthepost-migmatizationgranitoids.Post-migmati-zationdeformationwaspreviouslydescribedbySolariet al.(2007) as ~45 Ma greenschist facies extension, 45–34 Ma brittle-semibrittleSW-vergentthrustingandopenfolding.
D1: metamorphic banding and initial stages of migmatization
S1metamorphicbandingconsistsofalternatingleu-cocratic and melanocratic horizons, which range from 0.5 to10cmintheorthogneissesbutareasthinasfewmil-limetersinparagneisses.Penetrativeductiledeformationisassociatedwithdevelopmentofagentlytomoderately,
Ortho- and para-migmatites
Migmatizationissuperimposedonbothortho-andpara-gneisses(Figure2).Themainrecognizabletypesare(following Mehnert, 1968, and modifications in Milord and Sawyer,2003,andBrown,2004):(1)stromatic,i.e.bandedmigmatites,whereupto10cmthickmelanosomaticbands,madeupofhornblende,biotite,plagioclase,andopaqueminerals,alternatewithleucosomesmadeupofquartz,plagioclase,±feldspar,±garnet,±muscovite(Figure3b);(2)Agmatictodiktyoniticmigmatites,inwhichabreccia-likepaleosomeisinjectedbyanetworkofneosomeveins(Figure3c)oftrondhjeimitictotonaliticcompositions(thelatteroftenshowsgreenpoikiloblastsofhornblende,±garnet±biotite);(3)Raftmigmatites(Figure3d),inwhichsmall pieces of paleosomatic gneisses float in a heteroge-neousleucosome;(4)nebuliticmigmatites(Figure3e),inwhichtheadvanceddegreeofmigmatizationalmostcom-pletelyerasestheoriginalstructuresoftheformergneisses.Podsofgraniticleucosomearecommon,andtheygenerallyshowcentimetricporphyroblastsofbiotiteandhornblende(Figures3eand3f).Basedonthedegreeofmigmatization,fourdifferentunitsweremapped(Figure2):(1)migmatiticparagneisses,i.e.rockswithwell-preservedgneissosity,<30% of partial melting, and sedimentary (peraluminous) protolith;(2)migmatiticorthogneiss,sameas(1)butwithigneousprotolith;(3)para-migmatite,wheretheleucosomesconstitute >30% and the rock lack of clear, traceable gneiss bands;theabundantAl-richphases,suchassillimaniteandgarnet,areindicativeofasedimentaryorigin;and(4)ortho-migmatite,sameas(3),butwithaclearigneouspro-tolithsuggestedbytheabsenceofAl-richphases,togetherwiththepresenceofbiotiteandhornblendeasprincipalmafic phases.
Post-migmatization units
Threegroupsofpost-migmatizationrocktypeswererecognizedinthestudyarea(Figure2).
Deformed plutonsThedeformedplutonshavecompositionsranging
fromgranitetogranodioriteandlocallytonalite.PlutonicrockscroppingouttowardstheSEofthestudyareausuallydisplayamagmaticfoliationthatwaslateroverprintedbysolid-statedeformation(e.g.,Morán-Zenteno,1992).Basedon field relations and geochronologic data, Morán-Zenteno (1992)arguedthattheemplacementofdeformedgraniteswassyntectonicwiththemaintectonothermaleventthataffectedtheprotolithsoftheXolapaComplex.However,intheN-NNW-vergent,LaVentashearzone,the129MaElPozuelogranite(Figure2)wasdeformedundergreen-schist facies conditions at ~45 Ma (Torres-de León, 2005; Solariet al.,2007).AstheElPozuelograniteisnotshearedoutside the ~100 m wide La Venta shear zone, Solari et al.
Pérez-Gutiérrez et al.208
N
N = 140
N
N = 42
N
N = 93
N
N = 58
S average axial plane3
F fold axis3
S poles3
N
Pole to S /S foliation 1 2Pole to S /S foliation 1 2
S /S a1 2 verage foliation plane S /S a1 2 verage foliation plane
N
Pole to foliation S4 L stretching lineation4
L mineral lineation3
(northern sector)
a) b)
c)
e)
d)
f)
L mineral lineation1/L2L mineral lineation1/L2
L (355º/34º avg) in2
supracrustal rocks,La Venta (from Solari et al., 2007)
Density curves: S poles in2
supracrustal rocks, La Venta(from Solari et al., 2007)
N = 187
S a4 verage foliation plane
N = 183
L mineral lineation3
(southern sector)
3
Figure4.Lowerhemispherestereoplotsofstructuralmeasuredfabrics.a:polestoS1/S2foliationandL1/L2minerallineationinthewesternpartofthestudyarea.b:polestoS1/S2foliationandL1/L2minerallineationintheeasternpartofthestudyarea.c:F3foldaxesandtheaverageS3axialplane.d:DispersedL3minerallineation.e:PoletoS4foliationsmeasuredintheXolapaComplex.DensitycurvesareanaverageplotofpolestofoliationforthesamephaseofdeformationinthesupracrustalrocksoftheLaVentaarea(Solariet al.,2007).f:L4stretchingminerallineations.Kinematics,asobservedinsupracrustalrocks,isnormalwithatop-to-theNNWcomponent(cf. Solariet al.,2007).SquaresindicatetheaveragestretchinglineationofthesamephaseofdeformationinthesupracrustalrocksoftheLaVentaarea(Solariet al.,2007).
NEtoENEdippingS1foliationinthewestandsouthwestofthestudyarea,whereasitismoderatelyWtoWNWdip-pingintheeast(stereonetsofFigures4aand4b).Pre-D1graniticdikesintrudingtheparagneisses(e.g.,Figure3c)weredeformedbyrecumbenttoisoclinal,rarelyrootless,F1folds,withaxialplanessubparalleltotheS1foliation.S1 is defined by polygonal quartz, micas and sillimanite in theparagneisses,andbybiotite,hornblendeandquartzinorthogneisses.Thegrainsizeofthequartzvariesfrombandtoband.Stretchedquartzandelongatedfeldsparporphyro-clasts define the gently NE- to NW-plunging L1lineation(Figures4aand4b).
D2 /M2: ductile deformation, migmatization and leucosome emplacement
Inthepara-gneissesoutsidethemigmatiticzone,afoliation,S2, ismadeupofalignedsillimanite+bi-otite±muscovite±staurolite.Garnetisoftenpresentinthefoliationplanes.Intheorthogneisses,alignedbrownhornblende+biotite+titanite±garnetconstituteS2(e.g.,Figure3a).S1andS2appeartoformacompositefoliation,S1/2,formedatthehighestmetamorphicgradeaspartofonecontinuoustectonothermalevent.Undeformed,1to7cmthickleucosomaticlensesinjectedintothepaleosomeparalleltoS1(Figure3g)haverandomlyorientedminer-
Mesozoic evolution of the Xolapa Complex 209
Bt1 mm
S3
S3
NNW SSE
a)
b)
c)
alssuggestingthatthemigmatizationpostdatestheD1deformation.However,itispossiblethattheheatassoci-atedwithD1frictionwasresponsibleforpartialmelting,andactedasthedrivingmechanismforneosomegen-erationduringthehigh-grademetamorphism(cf.Brown,2005). The absence of a S2foliationinthemigmatitesandmigmatiticgneissesisinferredtobeduetothehighductilityofmoltenleucosomeduringD2,whichprecludedtherecrystallizingmineralsfrombecomingoriented(e.g.,Solariet al.,2003).
D3: ductile deformation of migmatitic structures TheS1/2foliationandbandingarelocallydeformed
bychevron,gentlytomoderatelyNWtoSEplunging,subverticaltosteeplyNNEdipping,slightlyasymmetricF3 folds (Figures 4c and 5a). These folds are common in boththeparagneissesandmigmatiticgneisses,butarelesscommoninorthogneisses.Inparagneisses, theS3axialplanesarecharacterizedbyalignmentofbiotiteandmuscovite;intheorthogneissesS3 is defined by aligned hornblende and biotite (Figure 5b). This suggests that the
Figure 5. Field and microstructural character of post-migmatitic structures in the Xolapa Complex. a: Mesoscale, chevron F3folds;theruleris30cmlong.b:microscopicdetailofF3chevronfoldscutperpendiculartothefoldhinges:biotitecrystalsgrowparalleltotheS3 axial planes. c: Deflection of theuppermostlimbofaS3foldbytop-to-theNNW-vergentS4 shear planes; 5 cm scale.
Pérez-Gutiérrez et al.210
metamorphicgradeassociatedwithD3wasintheuppergreenschisttoamphibolitefacies.AL3minerallineationvariesinorientationfromnorthtosouthinthestudyarea(Figure4d).ThisvariationisprobablyduetothelaterD4shearingandD5refolding.
D4: greenschist facies extension in the post-migmatization units
Top-to-theNNWD4shearzones,developedundergreenschistfaciesconditions,affectedbothXolapaComplexandpost-XolapaComplexunitsinthenortheasternpartofthestudyarea(Figure2).Forexample,F3foldlimbsarede-flected in the shear zones (e.g., Figure 5c). Whereas S4shearzonesaremappableinsupracrustalrocksoftheChapolapaFormation, and in both post-migmatization ~130 and ~55 Magranites(cf.Solariet al.,2007),intheXolapagneisses,S4isgenerallyparalleltoS2,constitutingacompositeS2/S4foliation,associatedwithacompositeL2/4stretchinglinea-tion(Figures4eand4f,respectively).
D5: folding of ~55 Ma pegmatitesGarnet and muscovite-bearing pegmatites, up to 60
cmthick,werecloselyfoldedduringD5.TheS5axialpla-narcleavageisnotpresentinfoldedpegmatites,butitisvisibleinthehostparagneisses.F5foldaxes(e.g.,Figure3h)areroughlysubhorizontal,andEWtrending,whereasaxialplanearesubverticaltoNNWsteeplyplunging,andESE-WNWtrending.
SUMMARY OF STRUCTURAL EVENTS IN THE XOLAPA COMPLEX AND ITS COVER
Fivephasesofductiledeformationhavebeenrecog-nizedintheTierraColoradaareaoftheXolapaComplex:
(a)D1andD2areassociatedwithhigh-grademeta-morphismandmigmatization;
(b)D3ischaracterizedbychevronfolds;(c)D4ispresentinthenorthwesternpartofthestudy
area, where it is congruent with the ~45 Ma, normal to left-lateralductileshearingintheLaVentashearzone(D2ofSolariet al.,2007);
(d)D5producedfoldsinsomepegmatitescroppingoutinthenorthwesternportion,anditcanbecorrelatedwiththeadjacentSSW-vergentthrustingoftheMorelosFm.overtheChapolapaFm.(D3ofSolariet al.,2007).
ANALYTICAL METHODS
GeochemicalanalyseswereperformedatActlabsLabortories(Texas,EE.UU).Majorelementsweredeter-minedbyInstrumentalNeutronActivation(INA),whereastraceelementsweremeasuredbyInductivelyCoupledPlasma–Mass Spectrometry (ICP-MS) employing lithium metaborate/tetraboratefusions.CommonPbanalyseswere
performedatLaboratorioUniversitariodeGeoquímicaIsotópica,UNAMonfeldsparsseparatedfromthecrushedrocksusingmethodsdescribedinSolariet al.(2004).
Zirconswereseparatedbymeansofstandardcrush-ing and concentration techniques, such as Wilfley shaking table,Frantzisodynamicmagneticseparator,andheavyliquids(e.g.,Solariet al.,2007).About10kgofsamplewereprocessedtoensurethateverymorphologicaltypeofzircon was adequately represented in the final concentrate. Representativezirconswerethenhandpickedinethanol,mountedinepoxyresintogetherwithchipsofstandardzirconR33(Blacket al.,2004).Themountwasthengroundtoexposezirconstoapproximatelyhalftheirthickness,andpolishedusingdiamondcompound.Polishedmountswere cleaned in 1N HCl, to avoid surficial common Pb contamination,andgoldcoated.Cathodoluminescenceimaging(CL)wasperformedonthepolishedzirconstogainknowledgeontheirinternalmorphology,guidetheanalysisandhelplaterwiththeageinterpretations.CLim-aging was performed in a JEOL 5600 LV SEM, equipped withaHamamatsuCLDetector,inStanford-USGSfacility.A total of 36 analyses, 20 on zircons belonging to sample Xo0301, and 16 on zircons belonging to sample Xo0303 wereperformedforU-Th-Pbgeochronology,followingthemethodologyreportedinNourseet al. (2005) and Weber et al., (2006), and utilizing the Stanford-USGS SHRIMP-RG (Sensitive High Resolution Ion Microprobe – Reverse Geometry)facilities.Oxygenwasusedasprimaryionbeam, and operated at about 2–4 nA, excavating an area of about 25–30 µm in diameter to a depth of about 1 µm. Sensitivity ranged from 5 to 30 cps per ppm Pb. Data for each spot were collected in sets of five scans through the massrange.IsotoperatioswerecorrectedforcommonPbusingthemeasured204Pb.Thereduced206Pb/238UratioswerenormalizedtothezirconstandardR33whichhasaconcordant TIMS age of 418.9 ± 0.4 Ma (2σ) (Black et al.,2004).FortheclosestcontrolofPb/Uratios,onestandardanalysiswasperformedaftereveryfourunknownsamples.Uraniumconcentrationsweremonitoredbyanalyzingastandard (CZ3) with ~550 ppm U. U and Pb concentrations are accurate to about 10–20%. SHRIMP isotopic data were reducedusingSQUID(Ludwig,2001)andplottedusingIsoplotEx(Ludwig,2004).
GEOCHEMISTRY
Afterextensivepetrographicscreeningtoeliminatealteredsamples,sixorthogneissandtwoparagneisswerechosenforgeochemicalanalysis(Table1).Thesesamplesinclude unmigmatized samples (RB71 and RB76), gneisses (Xo0303,Xo0230,Xo0214),andmigmatites(Xo0301,RB105, RB109). As most of the analyzed samples are high-grade metamorphic rocks, classification and interpretation usingdiagramsdesignedforunmetamorphosedrocksshouldbeviewedwithcaution.Nevertheless,comparisonsbetween
Mesozoic evolution of the Xolapa Complex 211
Sample# RB 71 RB 76 RB 105 RB 109 XO 0301 XO 0303 XO 0230 XO 0214Rockdescription
Paragneiss Orthogneiss MigmatiteOrthogneiss
MigmatiteParagneiss
MigmatiteOrthogneiss
MigmatiticOrthogneiss
MigmatiticOrthogneiss
MigmatiticOrthogneiss
Mainminerals Qz-Pl-Bt-GntSil-Op
Pl-Hbl-Qz-Ttn
Qz-Pl-Bt-Gnt Pl-Bt-Hbl-Gnt-Qz
Qz-Pl-Bt-Hbl-Gnt-Op
Qz-Pl-Bt-Kfs-Gnt-Op
Qz-Pl-Bt-Gnt-Op
Qz-Pl-Bt-Gnt-Op
Lat N16˚52´23” N16˚54´07” N16˚52´37” N16˚46´32” N16˚57´29” N17˚00´56” N 16°53’36” N 16°59’22.1”Long W99˚46´31” W99˚43´25” W99˚52´35” W99˚36’19” W99˚44’51” W99˚39’31” W 99°44’54” W 99°47’45.5”
SiO2 70.59 58.78 74.73 68.50 69.32 67.79 69.80 66.25Al2O3 15.21 15.69 13.80 15.68 15.23 15.45 14.69 15.41Fe2O3 2.49 8.00 1.60 3.74 4.22 3.97 4.18 3.63MnO 0.033 0.116 0.034 0.074 0.073 0.047 0.075 0.050MgO 0.61 3.60 0.24 0.93 0.80 1.35 0.96 0.77CaO 2.87 8.91 1.82 4.22 4.41 2.90 3.64 2.01Na2O 3.74 2.24 3.85 3.58 3.61 3.58 3.89 3.49K2O 2.99 0.91 3.25 2.15 1.28 3.00 1.83 5.22TiO2 0.294 0.787 0.141 0.356 0.391 0.640 0.437 0.558P2O5 0.07 0.12 0.04 0.08 0.09 0.14 0.09 0.14LOI 0.81 0.64 0.51 0.51 0.51 1.13 0.29 1.52TOTAL 99.69 99.78 100.01 99.81 99.93 99.99 99.87 99.05Sc 7 29 4 10 12 9 12 8Zr 125 180 86 134 268 180 168 295Be 2 1 2 2 2 3 2 1V 34 190 9 55 47 75 49 26Cr -20 38 -20 -20 -20 30 -20 -20Co 2 16 -1 3 5 4 5 4Cu -10 -10 -10 -10 20 11 -10 11Zn 31 50 -30 38 33 43 77 41Ga 16 15 15 15 17 17 17 16Ge 0.8 1.8 0.9 0.8 1.4 0.7 1.2 0.9Rb 83 12 89 52 52 85 88 108Sr 181 312 98 290 215 201 171 198Y 16.0 34.2 30.2 13.1 36.7 27.1 30.5 21.6Zr 116 176 94 119 264 185 177 287Nb 4.6 5.9 5.3 4.1 6.3 10.3 7.1 8.6Sn 3 2 1 -1 2 2 2 1Cs 2.9 0.2 2.4 1.9 1.4 3.5 2.6 0.9Ba 667 147 884 1,120 448 1,240 506 2127La 17.1 16.7 23.1 26.0 27.7 29.5 27.2 50.5Ce 32.3 35.8 45.6 42.7 57.5 59.0 53.6 100Pr 3.75 4.48 5.17 4.39 7.19 6.79 5.44 10.3Nd 13.9 18.6 19.9 15.4 29.1 26.4 21.1 40.2Sm 2.75 4.56 4.53 2.63 6.39 5.49 4.64 6.77Eu 0.889 1.03 0.654 0.982 1.46 1.46 0.952 1.31Gd 2.33 4.95 4.54 2.28 6.29 4.99 4.80 5.97Tb 0.41 0.93 0.81 0.36 1.10 0.84 0.83 0.78Dy 2.41 5.33 4.69 2.02 6.80 4.51 4.96 3.99Ho 0.49 1.07 0.95 0.41 1.38 0.87 0.99 0.76Er 1.61 3.18 2.87 1.23 4.04 2.61 3.05 2.28Tm 0.263 0.512 0.438 0.202 0.569 0.382 0.465 0.339Yb 1.75 3.31 2.85 1.40 3.50 2.45 2.99 2.27Lu 0.279 0.490 0.428 0.233 0.515 0.380 0.445 0.341Hf 3.5 5.1 3.3 3.3 7.3 5.0 4.4 6.8Ta 0.40 0.45 0.39 0.30 0.40 0.68 0.39 0.61W -0.5 1.8 -0.5 -0.5 -0.5 0.6 -0.5 -0.5Tl 0.85 0.14 0.76 0.55 0.29 0.93 0.65 0.81Pb 10 8 15 8 7 23 9 17Bi 2.1 1.6 0.5 0.2 1.0 1.2 0.2 0.2Th 6.67 6.92 9.77 8.10 6.67 9.75 9.10 12.0U 1.89 2.57 1.87 1.63 1.74 1.81 1.96 1.38206Pb/204Pb† N.D. N.D. 19.00897 19.29560 19.1791 18.91633 19.0978 18.7343207Pb/204Pb† N.D. N.D. 15.67733 15.68175 15.6874 15.66799 15.6826 15.6554208Pb/204Pb† N.D. N.D. 38.81786 38.85647 38.9359 38.83747 38.8639 38.7383
Table1.Geochemicalanalyses,XolapaComplex,AcapulcoTierraColoradasector,southernMexico.
Major element concentration in wt. %; trace element concentrations in parts per million, determined by ICP-MS at ACTLABS laboratories. Negative numbersarebelowthedetectionlimit.†:Feldsparmeasuredinitialratios,followingSolariet al. (2004). 2-sigma errors on isotopic ratios are <0.06% (206Pb/204Pb ratios), <0.08% (207Pb/204Pb ratios) and <0.1% (208Pb/204Pbratios),respectively.
Pérez-Gutiérrez et al.212
0.1
1
10
100
1000
Cs
Rb
Ba Th U Nb Ta
K2O La Ce
Pb Pr
Sr
Nd Zr Hf
Sm Eu
TiO2
Gd Tb Dy
Ho Er
Yb Y Lu
Orthogneisses
Paragneisses
1
10
100
1000
La Ce PrNdSmEuGdTbDyHoErYbLu
Rock/PrimitiveMantle
Rock/Chondrite
calc-alkaline
tholeiitic
0.0
0.4
0.8
1.2
1.6
0.0 0.5 1.0Rb/Sr
F
A M
Nb/La
OrtGnParaGnPopoMORBMarianas
intracrustaldifferentiation
+fluids
+sediments or crust
0.0
0.2
0.4
0.6
0 20 40 60 80Ce/Th
U/Th
OrtGnParaGnPopoMORBHuiznop
+UC
+LC
15.4
15.5
15.6
15.7
16 17 18 19 2036.0
36.5
37.0
37.5
38.0
38.5
39.0
39.5
16 17 18 19 20
+UC
+LC
1
10
100
1000
1 10 100 1000
Yb+Nb
VAG
WPG
ORG
syn-COLG
Rb
OrtGnParaGnPopoMarianas
a) b) c)
d) e)
f) g) h)
206 204Pb/ Pb206 204Pb/ Pb
208
204
Pb/Pb
207
204
Pb/Pb
thecompositionsoftheanalyzedsamplesandpristineigne-ousrockscanprovidesomeusefulinsightsintothesourcesandprocessesthatleadtotheirformation.
The rocks can be classified as metaluminous I-type dioritestogranites,displayingavariablerangeinSiO2(58.8 to 74.7 wt%) that correlates negatively with TiO2,
MgOandFe2O3contents.OntheAFMdiagram,therocksplot in the calc-alkaline field (Figure 6a). MgO contents
(3.6–0.2 wt%) and Mg# (51–26) of the studied rocks indi-cate that they are significantly more differentiated than any magmainequilibriumwiththeuppermantle(LangmuirandHanson,1980),andthereforecannotbeconsideredasprimarymantlemelts.
Chondrite-normalized rare earth patterns (Figure 6e) areenrichedinlight-rareearthelements(LREE;La/SmN2.3–6.2), and unfractionated in the heavy rare earths (HREE;
Figure 6. Geochemical data for migmatites and gneisses of the Xolapa Complex plotted in diagrams. a: AFM diagram; b: Rb/Sr vs.La/Nbdiagram;c:Ce/Thvs.U/Thdiagram;d:Traceelementgeochemicalcompositionofselectedsamples,normalizedtotheprimitivemantlevaluesofSunandMcDonough(1989); e: REE elements normalized to the chondrite values of McDonough and Sun (1995); f: Y+Nb vs.Rbtectonicdiscriminationdiagram,afterPearceet al.(1984).Syn-COLG:syn-collisionalgranites;VAG:volcanic-arcgranites;WPG:within-plategranites;ORG:Ocean-ridgegranites.g:Uranogenic206Pb/204Pbvs.207Pb/204Pbdiagram;h:thorogenic206Pb/204Pbvs.208Pb/204Pbdiagram.ReferencedataofMORBanalysesreportedinFiguresb,c,fandgarefromLehnertet al.(2000).GeochemicaldataoftheMarianaarcinFiguresbandcarefromElliottet al.(1997).Popocatépetlvolcanogeochemicaldata(PopoinFiguresb,c,f,andg)arefromSchaafet al. (2005). Huiznopala gneiss isotopic data (Huizno in Figures f and g) from Lawlor et al.(1999).
Mesozoic evolution of the Xolapa Complex 213
Dy/Ybn 1.0–1.4). The Xolapa orthogneisses also display strong negative Eu anomalies (Eu/Eu*=0.44–1) that are coupled with high Rb/Sr ratios (Figure 6b). These geochemi-calfeaturesindicatethatmagmaticdifferentiationoccurredin the stability field of plagioclase.
Incompatibletraceelementpatternsshowenrichmentsoflargeionlithophileelements(LILE)overhigh-fieldstrengthelements(HFSE)typicalofarcmagmas(Figure6d). The rocks also display low Nb/La ratios (Figure 6b) andincompatibleelementrelationshipsthatinvariablyplot within the field of volcanic-arc granites in the tectonic discriminationdiagramsofPearceet al. (1984) (Figure 6f). Nonetheless,theXolaparocksalsotendtohavemuchlowerU/Th and Ce/Th ratios (Figure 6c) than is typical for oceanic islandarcs,whichtendtopreservethechemicalsignatureof fluids released from the downgoing slab. Thus, the rela-tive enrichment in the fluid-immobile element Th likely indicatesstrongcontinentalcontributionstotheoriginalmagmas,eitherthroughsubductedsedimentsorbymeansofcrustalcontamination.
CrustalcontributionsarealsosupportedbytheoverallradiogenicPbisotopiccompositionsoffeldsparsfromtheorthogneisses,whichformarestrictedpositivecorrelationthatisbracketedbetweenanevolvedupper-crustal-likecomponentsimilartotheisotopiccompositionoftheana-lyzedparagneiss,andanunradiogeniclower-crustal-likecomponentwithhigher207Pb/204Pband208Pb/204Pbratiosthatistypicallyobservedinmagmasderivedfromthepartialfusion of the upper mantle (Figures 6g and 6h). In summary, thechemicalcompositionsoftheXolaparocksuiteslikelyreflect the differentiation stages of a magmatic arc that ei-thergrewincloseproximitytoacontinent,orwasdirectlyconstructedoveramaturecontinentalcrust.
GEOCHRONOLOGY
Twosamplesofmigmatiticorthogneisswerecho-senforSHRIMP-RGU-Pbanalysis.SampleXo0301isamigmatiteleucosomemadeupofquartz,plagioclase(Ab30–50%), poikilitic garnet, brown biotite, and rare horn-blendeporphyroclasts.Largeallanitecrystals,abundantzirconandapatitearecommonaccessoryminerals.SampleXo0303isabandedmigmatiticgneiss,inwhichleucocraticportionsaremadeupofquartz,plagioclaseandsubordi-nateK-feldsparandzircon,whereasmesocraticbandsarealmostentirelycomposedofbiotite,magnetite,tinyrelictgarnet,apatiteandsubordinatezirconsasinclusionswithinbiotite.SampleslocationsareinFigure2,whereassamplescoordinatescanbefoundinTable1.
Sample Xo0301: migmatitic leucosome
ZirconsfromsampleXo0301arepaleyellowtohoney-colored, range from 60 µm to >250 µm, and show
oscillatoryzoningincathodoluminescence(CL)images(Figures7ato7l),whichisgenerallyinterpretedasigne-ouszoning(Connelly,2001;Corfuet al.,2003).Someofthezirconsshowthepresenceofxenocrysticcores,whosestructuresarediscordantwithigneousoscillatoryzoning,indicatingthatigneouszircongrewaroundpreexisting,un-dissolved,olderzircons.SHRIMPanalyseswerefocusedontheigneouszoningandlate-stagerecrystallizationratherthantheinheritedcores.Analysesyieldedtwoagegroups(Figure8a):(i)sevenanalyses(discontinuousellipsesinFigure8b)areconcordanttonearlyconcordant,andhave206Pb/238U ages between 135.5 ± 1.0 and 131.3 ±0.7 Ma (1 sigma); and (ii) five analyses (thick line ellipses in Figure 8b)areconcordantwithintheanalyticalerror,andhave206Pb/238U ages between 129.8 ± 1.0 and 126.2 ± 1.3 Ma (1sigma).ComparingtheanalyzedspotswiththeCLim-ages(e.g.,Figures7ato7l),itisevidentthatthetwoagegroupsarerelatedtodifferentevents.Whereas,someoftheanalyzedzirconsshowcontinuousoscillatoryzonationwithsimilaragesinbothcoreandrim(e.g.,Figures7b,7c,and7e),othersshowacleardiscontinuitybetweencoreandrim. In one case (Figure 7d), a 129.5 Ma zircon rim grew overanoscillatory-zonedcorewitha206Pb/238U age of 135.5 Ma. The Early Cretaceous age group, clustering at 133.6 ± 0.9Ma(Figure8b)isinterpretedastheageofmigmatiza-tion.Thesecondagegroupclusteringat129.2±0.4Ma,isinterpretedaseitheranepisodeofPblossand/orpartialrecrystallization.
Anothergroupofanalyzedspotsyieldedmoredis-cordantisotoperatios,generallywithlargererrors(Figure8a).However,theapparent206Pb/238Uagesofsuchzirconsdefine a mean of 61.4 ± 1.5 Ma (Figure 8c), which is in-terpretedasEarlyPaleocenePblossorzircongrowth.CLimagingofsuchdomains(e.g.,Figures7a,7h,7i,7l)revealthat they cannot be texturally distinguished from the first episode of Pb loss or recrystallization at ~129 Ma.
Sample Xo0303: banded migmatitic orthogneiss
ZirconsseparatedfromsampleXo0303areyellowtoreddish colored, <280 µm in size, and range from prismatic tostubbywithorwithoutbypyramidalterminations.ImagedunderCL,mostofthemshowwell-developedoscillatoryzoning.Inheritedcoresarevisibleinsomecrystals,buttheydo not show any significant age difference (e.g.,Figure6m) when compared to the oscillatory overgrowths (e.g.,Figures7nto7r).Theisotopicanalysesareconcordantwithinanalyticalerror(Figure9aandTable2).Together,they define a cluster yielding an average concordia age of 178.7±1.1Mainterpretedasthecrystallizationageoftheorthogneiss(Figure9b).Somezirconcrystalshavebrighterrimsaroundoscillatoryzones(e.g.,Figures7sto7z),andanalysesoftheserimsyieldeddiscordantages(Figure9a)with206Pb/238U ages ranging between 168.7 ± 1.2 Ma and 116.2 ± 1.0 Ma, suggesting variable degrees of Pb loss. Two
Pérez-Gutiérrez et al.214
1C
1R
129.8 ±1.0
60 ±3.5
4C
4R
129.5 ± 0.9
135.5± 1.0
2M133.3± 0.6
5M
5C133.7± 0.8
133.8± 1.13M
134.6± 0.6
Xo0301
100 m
a) b) c) d) e)
5R139.9 ± 2.8
6R1
6R2
178 ±1.4
179.1 ± 0.9
9R161.3 ±1.3
10R
10M
174.9 ±1.2
116.2 ±1.0
11M179.8 ± 0.9
12M176.3 ±1.2
178.9 ±1.1
13M
172.1 ±1.2
15M
4R
168.7 ± 1.2
1R178.8 ± 0.8
100 m
Xo0303
f) g) h) i) l)
m) n) o) p) q)
2R
133 ±.2.6
r)
9M
13R61.2 ± 5.0
11R
15R
58.0 ± 4.9
60.1 ± 1.3
7R
128.4 ± 0.8
129.6 ± 1.3
s) t) u) v) z)
oftheanalyzedcrystals,althoughdiscordant(Figures7vand7z,andTable2),have206Pb/238Uagesthataresimilarto the migmatization age of 133.6 ± 0.9 Ma determined on sampleXo0301.Theyoungest,discordantanalysis(Figure9a)isprobablytheresultofayoungerPblossastheyoung-estzirconsanalyzedinsampleXo0301.
DISCUSSION AND CONCLUSIONS
Early Jurassic magmatism
The178±1.1MaigneousagecalculatedfromthemigmatiticorthogneissXo0303is thefirstreportofa
LowerJurassicmagmaticeventintheXolapaComplex(timescaleofGradsteinet al.,2004).Becauseofitscalc-alkalinegeochemicalsignature,wearguethattheigneousactivityisrelatedtoamagmaticarc.Thisissimilarto:(a)a Middle Jurassic age of 165 Ma reported by Guerrero-Garcíaet al. (1978) from the Acapulco – Tierra Colorada transect of the Xolapa Complex; (b) a ~158 Ma age reported byDuceaet al.(2004)fromnorthofPuertoEscondido;and(c)anEarlyJurassicmagmaticagereportedbyElías-Herreraet al. (2000) in the Tizapa metagranite (186.5 ± 7.4Ma,lowerinterceptofdiscordantdata)intheeasternGuerreroTerrane,Teloloapansubterrane,200kmnorthofthestudiedarea(Figure2).TheLowerJurassicagehasseveralpaleogeographicimplications.First,itindicatesthe
Figure7.Cathodoluminescenceimagesofselectedzirconsofthedatedsamples.atol:zirconsfromsampleXo0301.mtoz:zirconsfromsampleXo0303.Whitebarsrepresent100micrometersscale.Seetextforfurtherexplanations.
Mesozoic evolution of the Xolapa Complex 215
200 160 120 80
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
30 50 70 90 110 130238 206
U/ Pb
data-point error ellipses are 2s
Xo0301all analyses
140 136 132 128 124
0.040
0.044
0.048
0.052
0.056
45 47 49 51 53
207
206
Pb/
Pb
60
Migmatization133.6 ± 0.9 Ma
Pb loss at129.2 ± 0.4 Ma
50
54
58
62
66
70
206 238Mean of Pb/ U ages
61.4 ±1.5 (95% conf.)
MSWD = 0.25, probability = 0.86
207
206
Pb/
Pb
238 206U/ Pb
Age
,M
a
206 238Mean of Pb/ U ages61.4 ±1.5 (95% conf.)
MSWD = 0.25, probability = 0.86(error bars are 2s )
a)
b) c)
presenceofapre-Toarciansedimentarysequence,whichwasintrudedbytheEarlyJurassicigneousprotolithandthenmetamorphosedtoformtheXolapaparagneiss.Ortega-GutiérrezandElías-Herrera(2003)arguedthatammonite-bearingsedimentsofthemid-JurassicTecocoyuncaGroup(Burckhardt,1927;Corona-Esquivel,1981),whichareextensivelyexposednorthoftheTierraColorada-Chacalapashearzone,gradeintothemetasedimentaryprotolithoftheXolapaComplex.However,thissuggestionisnotsupported
bythedatapresentedinthispaperbecausetheTecocoyuncaGroupisyoungerthanpre-178MaparagneissesoftheXolapaComplex.Anotherpossibilityis that therocksinthestudyareaarecorrelativesoftheeasternmostpartoftheGuerreroTerrane,i.e.,theTeloloapansubterrane(Elías-Herreraet al.,2000;Centeno-Garcíaet al.,2008).TheTeloloapanterranewasaccretedtothebackboneofMexico(MixtecaandZapotecoterranes)eitherduringtheMiddle-LateJurassic(Elías-Herreraet al.,2000),duringthe
Figure8.U-PbconcordiaplotsforsampleXo0301.a:U-PbTera-WasserburgconcordiaofalltheanalysesperformedonzirconsfromsampleXo0301;b:Detailoftheconcordiaplot(a)withtheinterpretedaveragerepresentingthemigmatizationandPb-lossevents;c:Meanof206Pb/238UagesfortheEarlyPaleoceneovergrowthsrecognizedinsomeoftheanalyzedzircons.Seetextforfurtherexplanations.
Pérez-Gutiérrez et al.216
182 178 174
0.044
0.046
0.048
0.050
0.052
0.054
0.056
34.4 34.8 35.2 35.6 36.0 36.4 36.8
data-point error ellipses are 2s
238 206U/ Pb
207
206
Pb/
Pb
Concordia Age = 178.71 ±1.1 Ma(95% confidence, decay-const. errs included)
MSWD (of concordance) = 4.2
Xo0303
200 180 160 140 120
0.040
0.044
0.048
0.052
0.056
0.060
0.064
0.068
3034 38 42 46 50 54 58
Xo0303All analyses
data-point error ellipses are 2s
238 206U/ Pb
207
206
Pb/
Pb
a)
b)
EarlyCretaceous(DickinsonandLawton,2001),orduringtheLateCretaceous(Talavera-Mendozaet al.,2007).Thus,the178Mamagmatisminthehigh-gradeXolapaComplexcouldhavebeenpartofanEarlyJurassicmagmaticarcfringingtheMixtecaTerraneofsouthernMexico,whichwassubsequentlyaccretedanddeformed.
Early Cretaceous Migmatization
Ourzirconagesindicatethathigh-grademetamor-phismandmigmatizationoftheXolapaComplexinthe
study area occurred ~133 Ma ago. This interpretation is consistent with field observations reported by Solari et al.(2007),whoconcludedthatmigmatizationoccurredpriortothe ~129 Ma crystallization age of undeformed granites intrudingmigmatites.OurstructuraldataandpetrographicstudiesofXolapahigh-graderocksarealsoconsistentwiththoseofCorona-Chávezet al. (2006) in the Puerto Escondidoarea(Figure1),whereonlyonehigh-grademeta-morphiceventinthelowergranulitefaciesisdiscernibleinmigmatitesandgneisses.Similarlithologies,metamorphicgradeandintrusiverelationships(cf.Robinsonet al.,1989;Herrmannet al.,1994;Corona-Chávezet al., 2006) through-
Figure9.U-PbconcordiaplotsforsampleXo0303.a:U-PbTera-WasserburgconcordiaofalltheanalysesperformedonzirconsfromsampleXo0303;b:Detailoftheconcordiaplot(a)withtheinterpretedaveragerepresentingthemagmaticageofsampleXo0303.Seetextforfurtherexplanations.
Mesozoic evolution of the Xolapa Complex 217
Gra
in
Spot
206 P
b* pp
mU
pp
mT
h pp
mT
h/U
Obs
erve
d ra
tios
App
aren
t age
s20
4 Pb/
206 P
b20
6 Pb c
(%
)20
7 Pb* /20
6 Pb*
± 1σ
%20
8 Pb* /20
6 Pb*
± 1σ
%20
6 Pb* /23
8 U±
1σ %
206 P
b/23
8 U(M
a)
± 1σ
208 P
b/23
2 Th
(Ma)
±
1σ
(1)
(2)
(3)
(3)
(3)
(4)
(4)
Xo0
301
0301
-16R
0.5
541
0.02
0.03
820
9.81
0.12
490
8.4
0.19
386
11.0
0.00
558
2.7
19.2
12.5
---
---
0301
-1R
0.1
130
0.01
(-)
-0.9
50.
0396
824
.80.
0851
331
.30.
0167
58.
460
.03.
5--
---
-03
01-1
5R2.
226
52
0.01
0.00
197
1.16
0.05
640
5.3
0.03
809
9.3
0.01
680
4.4
60.1
1.3
---
---
0301
-11R
0.1
130
0.02
0.00
509
2.04
0.06
344
20.2
0.10
006
21.9
0.01
706
5.0
58.0
4.9
---
---
0301
-13R
0.4
461
0.01
0.00
801
1.95
0.06
288
12.0
0.07
112
16.2
0.00
986
4.4
61.2
5.0
---
---
0301
-14R
1.4
140
10.
010.
0049
21.
020.
0556
08.
90.
0496
913
.90.
0079
13.
268
.54.
1--
---
-03
01-6
R1.
612
111
0.10
0.00
193
1.55
0.06
020
6.1
0.07
986
7.2
0.00
965
1.8
93.2
2.3
---
---
0301
-19M
14.0
969
569
0.61
0.00
004
-0.3
80.
0451
81.
90.
2038
63.
00.
0388
914
.810
7.2
0.6
113
403
01-1
2M8.
348
614
10.
300.
0001
10.
130.
0496
03.
60.
0942
52.
60.
0349
43.
812
6.2
1.3
120
503
01-7
R9.
856
116
40.
300.
0002
90.
080.
0492
72.
50.
1010
12.
30.
0374
84.
712
8.4
0.8
122
803
01-9
M3.
118
058
0.33
(-)
0.01
0.04
869
4.1
0.10
758
3.9
0.03
551
3.8
129.
61.
313
25
0301
-4R
8.4
479
630.
140.
0000
8-0
.17
0.04
729
4.8
0.04
741
3.7
0.03
307
4.7
129.
50.
913
48
0301
-1C
8.3
474
240
0.52
0.00
017
-0.0
10.
0485
92.
70.
1672
62.
10.
0314
50.
612
9.8
1.0
126
403
01-8
M14
.179
426
90.
350.
0000
4-0
.06
0.04
819
2.0
0.11
538
1.8
0.03
643
3.6
131.
30.
713
53
0301
-10M
11.2
628
265
0.44
(-)
-0.1
50.
0474
73.
00.
1389
91.
90.
0355
83.
413
2.4
0.7
133
303
01-2
M16
.591
639
90.
450.
0000
70.
040.
0490
31.
90.
1453
11.
60.
0363
24.
213
3.3
0.6
133
203
01-5
C15
.385
149
50.
600.
0000
4-0
.06
0.04
824
2.0
0.19
158
1.5
0.03
584
4.3
133.
70.
813
42
0301
-5M
7.1
391
106
0.28
0.00
039
0.41
0.05
200
2.9
0.09
159
3.1
0.03
159
1.9
133.
81.
111
711
0301
-3M
20.6
1135
818
0.74
0.00
004
-0.0
10.
0486
51.
70.
2349
61.
10.
0362
13.
913
4.6
0.6
133
203
01-4
C11
.060
134
50.
59(-
)-0
.12
0.04
778
2.6
0.18
632
1.8
0.03
649
2.4
135.
51.
013
43
Xo0
303
0303
-10R
6.8
429
150.
040.
0004
6 0.
740.
0542
23.
00.
0261
85.
90.
0266
90.
911
6.2
1.0
9176
0303
-2R
1.0
5622
0.42
0.00
056
0.27
0.05
087
7.9
0.19
519
5.2
0.04
347
1.7
133.
02.
617
717
0303
-5R
2.2
111
20.
020.
0018
81.
120.
0578
05.
20.
0186
917
.30.
0200
15.
813
9.9
2.8
---
---
0303
-9R
6.5
298
330.
110.
0001
6 0.
290.
0515
93.
00.
0415
64.
40.
0444
03.
216
1.3
1.3
161
2003
03-4
R7.
934
751
0.15
0.00
012
-0.0
60.
0489
92.
70.
0507
13.
70.
0463
42.
416
8.7
1.2
162
1003
03-1
5M7.
833
514
00.
43(-
)0.
280.
0517
22.
60.
1392
52.
20.
0473
43.
417
2.1
1.2
176
403
03-1
4M5.
824
893
0.39
0.00
010
0.04
0.04
988
3.1
0.12
658
2.8
0.04
652
3.5
174.
31.
417
66
0303
-10M
8.4
357
141
0.41
(-)
0.16
0.05
087
2.5
0.13
355
3.6
0.04
814
3.6
174.
91.
218
17
0303
-7M
10.0
423
870.
21(-
)0.
130.
0506
42.
30.
0667
22.
80.
0471
02.
717
5.9
1.1
175
503
03-1
2M8.
133
810
80.
330.
0000
70.
210.
0513
12.
60.
1074
82.
50.
0464
43.
017
6.4
1.2
178
603
03-6
R2
6.9
285
470.
170.
0000
90.
330.
0522
42.
90.
0568
13.
80.
0471
42.
717
8.3
1.4
176
1003
03-1
R29
.812
3224
0.02
0.00
009
0.08
0.05
027
1.3
0.00
707
4.9
0.04
940
3.7
178.
50.
710
542
0303
-13M
11.0
455
232
0.53
0.00
011
-0.1
10.
0487
92.
20.
1661
81.
70.
0509
72.
917
8.3
1.1
174
503
03-6
R1
14.6
602
130.
020.
0001
70.
400.
0528
71.
90.
0126
95.
30.
0507
73.
717
9.3
1.0
150
8303
03-8
M11
.045
427
40.
620.
0000
7-0
.04
0.04
932
2.3
0.19
124
1.7
0.04
432
5.8
179.
41.
217
24
0303
-11M
17.7
728
145
0.21
0.00
010
-0.0
70.
0491
11.
80.
0671
62.
20.
0487
53.
217
9.4
0.9
176
6
Tabl
e1.
U-P
bda
tata
ble,
Xol
apa
Com
plex
,Aca
pulc
oTi
erra
Col
orad
ase
ctor
,sou
ther
nM
exic
o.
(1):
Con
tribu
tion
ofc
omm
on20
6 Pb
toto
tal20
6 Pb
in %
; (2)
: Con
cent
ratio
n of
radi
ogen
ic (*
) 206 P
b; (3
): M
easu
red
isot
ope
ratio
s. Er
rors
are
pro
vide
d as
1-s
igm
a in
%; (
4): A
ppea
rent
age
s, an
d 20
4 Pb
corr
ec-
tions
are
cal
cula
ted
usin
gSQ
UID
softw
are
(Lud
wig
,200
1).F
orP
hane
rozo
icz
ircon
sin
gene
ral,
206 P
b/23
8 Ua
ges,
are
cons
ider
eda
sthe
mos
trel
iabl
e.-20
4 Pb
notd
etec
ted;
---20
8 Pb/
232 T
hag
ean
der
rors
wer
eno
tcal
cula
ted,
bec
ause
Th
isv
irtua
llya
bsen
tin
such
zirc
ons,
and
unce
rtain
tyis
thus
too
big.
Pérez-Gutiérrez et al.218
outtheXolapaComplexsuggestthattheEarlyCretaceousageofmigmatizationcanbeextrapolatedacrossthewholecomplex.Supportforthisisprovidedbythe131.8±2.2MaagecalculatedbyHerrmannet al.(1994)onamigmatitecroppingoutnorthofPuertoEscondido.
Late Mesozoic to Early Cenozoic evolution
High-gradegneissesandmigmatitesinthestudyareaunderwentlimitedre-heatingduringtheLateCretaceous-PaleoceneasindicatedbyPblossinzirconsofsampleXo0301.Fourtectonothermaleventswerepreviouslyre-portedfortheearlyCenozoicintheXolapaComplex:(1)magmatism at ~55 Ma (Acapulco, El Salitre and Las Piñas granites,Morán-Zenteno,1992;Duceaet al.,2004;Solariet al.,2007);(2)NNW-vergent,normalandleft-lateralductileshearing at ~45 Ma (Riller et al.,1992;Solariet al.,2007);(3) SW-vergent brittle thrusting between ~45 and ~34 Ma, and (4) magmatism at ~30–34 Ma (Tierra Colorada and Xaltianguisundeformedgranites,Herrmannet al.,1994;Schaafet al., 1995; Ducea et al.,2004;Hernández-Pineda,2006). Such events are probably responsible for the reo-rientationofpreviousstructures(e.g.,D4,seeabove),Pblossinzircons,recrystallizationorresettingofisotopicchronometersinmicasduetointrusionorhydrothermalfluid circulation, specially along the boundaries of major intrusions(e.g.,Solé,2004).
Tectonic implications
PreviousstudiesofthegeologicalrecordoftheXolapaComplexhavebeeninterpretedintermsofextensionandexhumation during the Late Cretaceous – Early Tertiary causingupliftoftheisothermsandpartialmelting(e.g.,Robinsonet al.,1989;Herrmannet al.,1994;Meschedeet al., 1997). Such models were partly based on a 66–46 Ma ageforthemigmatization(Herrmannet al.,1994).TheseauthorssuggestedthatexhumationoftheXolapaComplexwasrelatedtothesoutheastwarddisplacementoftheChortísblockfromalocationoffPuertoVallartaalongthesouthernMexican coast at ~100 Ma, towards a location off Huatulco at ~29 Ma.
OurpresentdatasuggesttwoalternativemodelsfortheLowerCretaceousmigmatization,bothinvolvingaccretionofoutboardterranes:(1)accretionoftheGuerreroterranetothebackboneofsouthernMexicocouldpossiblyhaveoccurredduringtheEarlyCretaceous(butseediscussionabove);or(2)accretionoftheChortísblocktosouthernMexico.Theabsenceofanexposedsutureeastof theGuerreroterranearguesagainsthypothesis#1,whichsug-geststhattheGuerreroterranemayhavebeenacontinentalmarginarc(cf.Elías-HerreraandOrtega-Gutiérrez,1998).Moreover,theaccretionoftheGuerreroterranecannotexplain the synchronous migmatization as far as 500 km
SEofthestudyarea.SeverallinesofevidenceindicateapossiblecorrelationbetweentheXolapaComplexandthenorthernChortísblock:(i)asimilarmetamorphicgradeoccursintheexposedbasementofLasOvejasComplexinsouthernGuatemalaandnorthwesternHonduras(Horneet al., 1976; Schwartz, 1977; Ortega-Gutiérrez et al.,2007;Martenset al., 2007), (ii) a ~170 Ma magmatic arc is presentintheChortísblock(Martenset al.,2007),and(iii)thepresenceofeclogitesintheMotaguamélange(centralGuatemala),whichhaveSm-Ndmineralisochronagesspanning ~125 to ~136 Ma (Brueckner et al., 2005; Martens et al.,2007).TheselatteragesareinterpretedintermsofcollisionbetweentheChortísblockandsouthernMexico(cf.Harlowet al.,2004;Martenset al.,2007).Thesimilar~133 Ma migmatization age in the Xolapa Complex could representcollisionbetweentheXolapaComplexandthe(current)northernmarginoftheChortísblockmarkedbytheHPmetamorphismintheMotaguamelange.Exhumationofsucheclogitesinblueschistmetamorphicconditionsat ~70–75 Ma (e.g.,Harlowet al.,2004;Martenset al.,2007)couldmarktheseparationoftheChortísblockfromsouthernMexico.ThisisconsistentwiththeconclusionsofCorona-Chávezet al. (2006), who suggested that the clock-wiseP-TpathfollowedbytheXolapaComplexrepresentacompressionalregimeintherootsofacontinentalmagmaticarc.Otherprocesses,suchasshorteningduetoaccretionofnappesand/orterranes(e.g.,Whitneyet al.,1999),ormagma-loadingprocesses(e.g., Brown, 1996; Warren and Ellis, 1996) are normally invoked in such a tectonic sce-nario.RemovaloftheChortísblockduringthemid-LateCretaceouswouldhaveallowedreorganizationofplates,andonsetofsubductionalongtheMiddleAmericatrenchto generate the ~55 and ~30 Ma arc plutons that intrude the XolapaComplex(Duceaet al.,2004;Solariet al.,2007)andtheGuerreroterrane(e.g.,Levresseet al.,2004).
ACKNOWLEDGEMENTS
Severalpeoplearethankedfortheirsupportdur-ingfieldwork(e.g.,RafaelTorresdeLeón,GuillermoHernándezPineda,TeodoroHernándezTreviño,BernardoVillacura),andforfruitfuldiscussionsonthegeologyoftheXolapaComplexinvariousstagesofthiswork,suchasPedroCorona-Chávez,FernandoOrtega-Gutiérrez,MarianoElías-Herrera,J.DuncanKeppie.Theauthorsacknowledgea CONACyT scholarship to RPG, CONACyT grant 54559 toLAS,andPAPIIT-DGAPAgrantIN101407toLAS,which funded field and analytical work. Diego Aparicio madealotofthinsectionswestudiedduringdifferentstagesofthiswork,andConsueloMacíashelpedwithmineralseparations.WealsothankJoeWooden,USGSatStanfordUniversity,forassistanceduringSHRIMPanalyses.ThoroughreviewsbyBodoWeber,PedroCorona-ChávezandDanteMorán-Zentenoimprovedthereadabilityoftheconceptsexpressedinthiswork.
Mesozoic evolution of the Xolapa Complex 219
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Manuscriptreceived:January22,2008Correctedmanuscriptaccepted:August13,2008Manuscriptaccepted:September9,2008
