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ABSTRACT

Anarak Paleozoic ophiolite is located in western part of the Central - East Iranian Microcontinent. This metaophiolite is covered by Paleozoic schist and marble. Blueschists of the Anarak ophiolite are exposed along the northern Anarak east-west main faults and are considered as remnants of the Paleo-Tethys suture zone in Central Iran. Anarak blueschists are formed by metamorphism of primitive basic lavas. In some cases, they preserve the primary pillow structure. Petrography and microprobe analyses show that they are composed of riebeckite, actinolite, plagioclase (albite), sphene, magnetite, white mica and apatite. Secondary minerals are chlorite (pycnochlorite), epidote, pyrite and calcite. Mineralogical assemblages are consistent with blueschist facies metamorphism, which is followed by a retrograde metamorphism in greenschist facies. Estimation of the metamorphic conditions suggests 300-450 oC and 4-9 kbar.

Whole rock geochemical analyses show that these rocks can be classified as alkaline basalts. Chondrite-normalized rare-earth element (REE) patterns of the studied rocks display 10-150 times enrichment, high light REE and relatively low heavy REE contents. These geochemical characteristics are representative of mantle-derived magmas. Primitive mantle normalized spidergram of the Anarak samples exhibit negative anomalies of Ba, U, K and Sr, and positive anomalies of Cs, Rb, Th, Nb, Ta and Zr. Similar geochemical features of all analyzed rocks indicate that they were all derived by more than 12% partial melting of an enriched/carbonated garnet lherzolite and underwent similar degree of partial melting. Geochemically, the studied blueschists resemble intraplate alkali-basalts. The presence of Paleozoic ophiolitic rocks along the main faults of central and northern Iran are indicative of a multi-suture closure of the Paleo-Tethys ocean.

Key words: Late Permian, ophiolite, blueschist, Paleo-Tethys, Anarak, central Iran.

RESUMEN

La ofiolita Anarak del Paleozoico se encuentra en la porción occidental del Microcontinente Iraní Centro-Oriental. Esta metaofiolita está cubierta por esquisto y mármol del Paleozoico. Esquistos azules de la ofiolita afloran a lo largo de las fallas principales este - oeste del norte de Anarak, y se consideran relictos de la zona de sutura del Paleo-Tetis en Irán central. Los esquistos azules de Anarak se formaron por metamorfismo de lavas máficas primarias. En algunos casos, estos esquistos preservan la estructura almohadillada primaria. Análisis petrográficos y de microsonda muestran que los esquistos están compuestos por riebeckita, actinolita, plagioclasa (albita), esfena, magnetita, mica blanca y apatita. Los

LatePermianblueschistfromAnarakophiolite(CentralIran,Isfahanprovince),amarkofmulti-suture

closureofthePaleo-Tethysocean

Ghodrat Torabi

Department of Geology, University of Isfahan, Azadi Square, 81746-73441 Isfahan, Iran. Torabighodrat@yahoo.com

Revista Mexicana de Ciencias Geológicas, v. 28, núm. 3, 2011, p. 544-554

Torabi,G.,2011,LatePermianblueschistfromAnarakophiolite(CentralIran,Isfahanprovince),amarkofmulti-sutureclosureofthePaleo-Tethysocean:RevistaMexicanadeCienciasGeológicas,v.28,núm.3,p.544-554.

Late Permian blueschist from Anarak ophiolite, Central Iran 545

minerales secundarios son clorita (picnoclorita), epidota, pirita and calcita. Los ensambles minerales son consistentes con metamorfismo en facies de esquistos azules, seguido por metamorfismo retrógrado en facies de esquistos verdes. Las condiciones de metamorfismo se estiman en 300-450 oC y 4-9 kbar.

Análisis geoquímicos de roca total muestran que estas rocas pueden ser clasificadas como basaltos alcalinos. Patrones de elementos de las Tierras Raras (REE) normalizados a condrita de las rocas estudiadas muestran enriquecimiento entre 10-150 veces, valores altos de REE ligeras y relativamente bajo REE pesadas. Estas características geoquímicas son representativas de magmas derivados del manto. El diagrama multielemental normalizado a manto primitivo de las muestras de Anarak exhibe anomalías negativas de Ba, U, K y Sr, y anomalías positivas de Cs, Rb, Th, Nb, Ta y Zr. Carcterísticas geoquímicas similares de todas las muestras analizadas indican que todas derivaron de más del 12% del fusión parcial de una lherzolita de granate enriquecida/carbonatada, y fueron sometidas a grados de fusión similares. Geoquímicamente, los esquistos azules estudiados se asemejan a basaltos alcalinos de intraplaca. La presencia de rocas ofiolíticas del Paleozóico a lo largo de las fallas principales del centro y norte de Irán es indicativa de un cierre multi-sutura del Paleo-Tetis.

Palabras clave: Pérmico Tardío, ofiolita, esquisto azul, Paleo-Tetis, Anarak, Irán central.

INTRODUCTION

Ophiolitesandassociatedmetabasitesprovidecrit-icalevidenceforpaleo-oceansandsubductionzones.Glaucophaneandriebeckitearethekeyindexmineralofblueschistfaciesmetamorphicrocks,andarewidelyusedasindicatorsofpaleo-subductionzones(e.g.,ErnstandLiou,2008).Linearexposureofophioliticblueschistsalongregionalstructuresandmainfaultscanbeconsideredaspaleo-sutures.

TheAnarakophioliteissituatedinwesternpartoftheCentral-EastIranianMicrocontinent(CEIM)(Figure1),andiscoveredbyPaleozoicmetamorphicrocks(Figure2).Inthisophiolite,theblueschistsareexposedalongthemainfaultsofnorthernAnarakfollowinganeast-westtrend.FieldstudiesshowsthatthesePaleozoicblueschistsoriginallywerebasicpillowlavasmetamorphosedinblueschistfacies.Insomecases,theprimarypillowstructuresarepreserved(Figure3d).Inthisarticle,petrologicalandgeochemicalas-pectsofformation,P-Tconditionofmetamorphism,aswellasgeotectonicsettingofthesemetavolcanicsarepresented.This research is the first study carried out on the petrology oflate-PaleozoicblueschistsofCentralIran.Accordingtogeographicalandstratigraphicalpositionoftheserocks,itisexpectedthatthisresearchwillbeusefultoidentifythe Paleo-TethyspositionandclosureinIran.

REGIONAL GEOLOGY

Zagros,ZagrosThrustzone(ZTZ),Sanandaj-Sirjan,Urumieh-Dokhtarmagmaticarc(UDMA),CEIM,Alborz,KopehDagh,EasternIran,andMakranarethemainstruc-turalunitsofIran(Figure1).TheCEIMislocatedbetweentheconvergentArabianandEurasianplates.Itisaffectedbyacomplexsystemofactiveintra-continentalstrike-slipfaultscausinganintensivenorth-southdextralshearingof

thewholearea(Figure1)(Zanchiet al.,2009).TheCEIMcomprisesthreemajorcrustaldomainsfromeasttothewest:theLutblock,theKerman(Tabas)block,andtheYazd(Naein)block.ThestudyareaislocatedinthewesternpartoftheYazdblockandsouthofGreatKavirfault(GKF).Theleft-lateralGreatKavir(Doruneh)fault,whichisoneofthelongestandmostprominentfaultsofIran,isveryclosetothestudyarea(Figures1and2).ItplaysanimportantroleintheregionaltectonicsofCentralIran(Torabi,2009a,2010).ThedrasticdirectionchangeofthisfaultintheAnarakareamarksthewesternborderoftheCEIM.

OphiolitesofIrancanbedividedintofourgroups(Torabi,2009c,2011;Torabiet al.,2011):1)PaleozoicophiolitesofnorthernIranwhicharelocatedalongtheAlborzmountainrangeandincludetheRashtophiolites.TheseophiolitesareconsideredasremnantsofPaleo-Tethys(Bagheri,2007;Torabi,2004;Torabiet al.,2011);2)MesozoictoCenozoicophiolitesoftheZagrossuturezoneincludingtheNeyrizandKermanshahophioliteswhichareapparentlyanextensionoftheOmanophiolites(Torabi,2009b);3)MesozoictoCenozoicophiolitesandophioliticmélangesthatmarktheboundariesoftheCEIM;and4)PaleozoicmetamorphosedophiolitesoftheYazdandPosht-e-Badamblocks,locatedinthewesternpartoftheCEIM.TheselastophiolitesincludetheAnarak,Jandaq,Bayazehandposht-e-Badammetaophiolites(Figure1).Presenceofseveralophioliticbeltsalongthelargeactiveintracontinen-talfaultsofIranindicatestheopeningandclosureofseveraloceanicbasinsduringitscomplexgeologicalhistory.

TheAnarakmetamorphicrocks(schistandmarble)includemountainswithnearlyeast-westtrend,whichhavecoveredtheAnarakophiolite.Thisophioliteiscomposedofmantleperidotites,cumulates,gabbros,basicandultrabasicdykes,pyroxenite,riebeckitebearingmetabasite(blue-schist),rodingite,andlitswaenite(Sharkovskiet al.,1984;Torabi,2004).Chromititehasnotbefoundyet(Torabi,2009c).PetrographyofmantleperidotitesoftheAnarak

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TriassicaccretionarywedgethatwasactiveintheAnarakregiontothesouthofNakhlak(Bagheri,2007).DetailedpetrographicalstudyoftheNakhlakTriassicsuccession(Alam,BaqoroqandAshinformations)showsthatitconsistsofpelagicsedimentaryrocksincludingRossoammoniticofacies,volcanicarenites,alluvialfanconglomeratesandsandstones,nodularlimestoneandmarl,andturbiditesdepositedinavolcanicarcsettingalonganactivemargin(Baliniet al.,2009).Thissuccessionwasdepositedinarapidlysubsidingsedimentarybasin(Zanchiet al.,2009).Previousauthors(e.g.,Soffelet al.,1996;Alaviet al.,1997)consideredthattheAnarakophioliteisaremnantofPaleo-Tethys,transferredtoitspresentpositionfromNEofIran,by135oanti-clockwiserotationofCEIM.Whereas,newpaleontologicalandpaleomagneticstudies(e.g.,Baliniet al.,2009;Muttoniet al.,2009)donotsupportlargeanti-clockwiserotations.

ANALYTICAL TECHNIQUES

ChemicalanalysesofmineralswerecarriedoutwithaCamecaSX-100electronprobemicro-analyzerattheInstituteofMineralogy,LeibnizUniversity,Hannover,

ophioliteshowsthat thisophioliteisofthelherzoliticophiolitetype(LOT)(Torabi,2004,2009c).AllrockunitsoftheAnarakophiolitehavebeenmetamorphosed,andunderwentahighdegreeofserpentinization(Torabi,2004).

40Ar-39ArisotopicanalysesontheAnarakmetamorphicrocksindicateagesof318.99±1.63and333.87±1.91Ma(Bagheri,2007).Furthermore,analysisofsodicamphiboleinAnarakblueschistbythe40Ar-39Armethodyieldedanageof285.42±1.65Ma(Bagheri,2007),whichpointstoLatePermianmetamorphismoftheprimitivepillowlavas.

Undeformedtrondhjemiticintrusions(stocksanddykes)crosscuttheAnarakophioliteanditscoveringmeta-morphicrocks(Figures2and3).U-PbanalysisofzirconfromAnaraktrondhjemitereports262.3±1.0Ma(Bagheri,2007).Thisindicatesthatthetrondhjemiticmagmatismoccurredabout23m.y.aftertheformationofblueschistsintheAnarakophiolite.FieldevidencesandwholerockschemistryoftrondhjemitesrevealthattheyhavecrystallizedfrommagmasderivedbymeltingofsubductedAnarakoce-aniccrust(Torabi,2004).GeochemicalcharacteristicsoftheAnarak trondhjemites all reflect melting of a mafic protolith atrelativelyhighpressures(>10kbar)toformagarnet-rich,plagioclase-freeeclogiticresidue(Torabi,2004).

TheAnarakareaisconsideredasremnantofaPermo-

Figure1.MainstructuralunitsofIran;GKF:GreatKavirfault;KDF:KopehDaghFault;NBF:Naibandfault;NF:Nehbandanfault;SSZ:Sanandaj-Sirjanzone;PTSZ,Paleo-Tethyssuturezone;UDMA:Urumieh-Dokhtarmagmaticarc.An,Jn,ByandPBrepresenttheAnarak,Jandaq,BayazehandPosht-e-Badammetaophiolites,respectively.

Late Permian blueschist from Anarak ophiolite, Central Iran 547

Nakhlak

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Ashin-Zavar mesozoic ophiolite

Cretaceous limestone

Eocene volcanic rocks

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Germany,andaJEOLJXA-8800Rat theCooperativeCentreofKanazawaUniversity,Japan.Themachineswereoperatedatavoltageof15kVandabeamcurrentof15 nA with 3-μm probe beam diameter. Natural minerals andsyntheticmaterialsofknowncompositionwereusedasstandards.TheZAFprogramwasusedfordatacor-rections.TheamountsofFe3+andFe2+inmineralswereestimatedbyassumingidealmineralstoichiometry.TheMg#calculatedasMg/(Mg+Fe2+)atomicratioofminer-als.RepresentativechemicalanalysesofthemineralsandcalculatedstructuralformulasarepresentedinTables1.MineralabbreviationsinphotomicrographsarefromKretz(1983).

Geochemicalanalyseswerecarriedoutonwhole-rocksamplesusingaBrukerS4PioneerXRFattheCentralLaboratoryoftheUniversityofIsfahan,andbyNeutronActivationAnalysis(NAA)attheActivationLaboratoryofIsfahan(MNSRDepartment).Thequalityassuranceofthe analytical results was evaluated by analyzing certified StandardReferenceMaterialspreparedbytheCanadaianCertified Reference Material Project (CCRMP), Republic ofSouthAfricaBureauofStandards(SACCRM),andChinaNationalAnalysisCenter.Analyticaluncertain-tiesofallmajorelementswerebetween1-4%.Precision

fortraceelementsanalyseswereestimatedtobebetterthan5%onthebasisofrepeatedanalysisofstandardmaterials.SixteensamplesofblueschistfromtheAnarakophiolitewereselectedforanalyses.Whole-rockchemicaldataarepresentedinTable2.

PETROGRAPHY AND MINERAL CHEMISTRY

Blueschists are medium to fine grained in hand specimen. Theoriginalpillowstructureandmarginalcavities,whicharefilled by the secondary minerals, are still visible. In some cases, theydisplayapervasivefoliationmarkedbythepreferredshapeorientationofamphiboleandchlorite.Maintexturesarenematoblasticandgranoblastic.Theserockscontainamphibole,plagioclase,whitemica,quartz,sphene(titanite),chlorite,epidote,rutile,apatite,hematite,calciteandpyrite(Figure4).Spheneandrutilearepartlytransformedtoleucoxenebysecondaryalteration.Metamorphicdifferentiationcausedseparationofdarkandlightmineralsasindividualbandsinsomesamples(Figure4d).

Electronmicroprobeanalysesofminerals(Table1),showthatplagioclaseisalbitic(Ab99.6%)(Figure5a),which indicates that sub-sea floor metamorphism of primary

Figure 2. Simplified geological map of the Anarak region (Isfahan province, Central Iran). Presence of both Paleozoic and Mesozoic ophiolites in the Anarakareaisevident.

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pillowlavashaschangedtheprimarycalcicplagioclasestosecondarysodicones.Amphibolesaremagnesio-riebeckite(Figure5b)andactinolite(Table1)incomposition.Allac-tinolitesarefreshbutMg-riebeckitesarepartlychangedtochlorite.Mg#ofamphibolesare0.602to0.735and0.755forMg-riebeckitesandactinolite,respectively.Chloritesarepycnochloriteincomposition.Mg#ofchloriteandwhitemicasis0.706and0.680,respectively.

Theabovementionedmineralogicalassociation,probablyindicatesatransitionfromblueschisttogreen-schistfaciesP-Tconditionsbyretrogrademetamorphism.TheequilibriumphaseassemblagefortheblueschistP-TconditionsincludesMg-riebeckite,whitemica,quartz,titanite,plagioclaseandhematite(Zanchiet al., 2009). ‏On theotherhand,actinolite,chlorite,epidoteandleucoxenearecharacteristicofthegreenschistfacies.

WHOLE ROCK GEOCHEMISTRY

Wholerockgeochemicaldata(Table2)showthatSiO2,TiO2,Al2O3,MgO,CaO,Na2OandK2Ovaluesofthestudiedblueschistsrange47.96–52.91,1.86–2.64,12.93–

16.76,4.39–9.95,2.81–7.62,4.18–6.69,and0.30–1.34(wt.%),respectively.Considerablevaluesoflostonignition(LOI:2.40–4.44wt.%)canbeattributedtothepresenceofhydrousminerals(e.g.,Mg-riebeckite,actinolite,mica,chlorite and epidote). According to the sub-sea floor meta-morphismandspilitizationoftheAnaraksamples,itcanbeconcludedthatmostofthemajorelementshaveprobablybeen modified by secondary alterations and that only im-mobileelements(e.g., high field strength elements, HFSE, andrareearthelements,REE)shouldbeconsideredforgeochemicalinterpretations.IntheNb/YversusZr/TiO2classification diagram (Winchester and Floyd, 1977), all studied rocks plot in the alkali-basalt field (Figure 6a).

Chondrite-normalizedREEpatterns(Figure6b),oftheAnaraksamplesexhibitparallelpatternsandenrichmentinlightREE(LREE)relativetoheavyREE(HREE).Theyare10to150timesenrichedinREErelativetochondrite.NoevidentEuanomalyisobserved.AbundancesofLREEareslightlymorevariablethanthatofHREE.

Inaprimitivemantle-normalizedmulti-elementsdiagram(Figure6c)allanalyzedsamplesdisplayparallelpatternsfromthelarge-ionlithophileelements(LILE)totheHFSEandREE.Inthisdiagram,thesamplesexhibitnega-

Figure3.FieldphotosoftheAnarakophiolite,blueschistsandassociatedrocks;a)AnarakophioliteexposedneartotheAnarak-Khurmainroad;b)BlueschistsarecoveredbyPaleozoicmetapelites;c)Anarakophiolitetrondhjemitecrosscuttheblueschists;d)Blueschistspreservedtheprimarypillowstructure.

Late Permian blueschist from Anarak ophiolite, Central Iran 549

tiveanomaliesofBa,U,KandSr,andpositiveanomaliesofCs,Rb,Th,Nb,TaandZr.Thestudiedblueschistsaresimilarinpetrographyandgeochemistry;thereforetheirparentalmeltsprobablyderivedfromthesameorfromverysimilarmantlesources.

Geochemicalcharacteristicsandpatternsoftheana-lyzedsamplesshowninFigures6ato6candtheircomparisonwithnon-metamorphosedPaleozoicalkali-basalts(Torabi,2009a),aswellasyoungandfreshonesfromCentralIran(Torabi, 2004 and 2010) confirm that originally they were basaltandbelongtothealkalinemagmaticseries.

DISCUSSION

Magma generation

AnalyzedsamplesoftheAnarakareaexhibithighTiO2(1.86to2.64wt.%)valuesandstrongLREEenrich-mentrelativetoHREE(Figure6b)(La/Luratio=29.7-124.8), presumably indicating alkaline mafic partial melts. RelativelylowHREEcontentsofthesemetabasaltsindicatethatgarnetwaspresentasaresidualphaseintheirmantle

sources.Althoughtheasthenosphereisdominantlygarnetperidotite,drygarnetperidotiteisnotasuitablesourcefortheAnarakophioliteprimaryalkalibasalts.InordertogeneratemeltswithsuchhighLREEandincompatibletraceelementcontents,themantlesourcemusthavebeenpreviouslymetasomaticallyenriched.Thechondritenormal-izedREEpatternsofallindividualsamplesareremarkablyparallel,therebyimplyingthattheywereallderivedfromasimilarmantlesourceregionandunderwentsimilarde-greeofpartialmelting.Additionaly,parallelREEpatternsindicatefractionalcrystallizationofaco-geneticsuite.PartialmeltingofamantlesourcewithgarnetwillbufferHREEatlowconcentrationsandvariableLREE.ThehighincompatibleelementenrichmentsoftheAnarakstudiedrocksarecoupledwithcomparativelylowabundancesofAl2O3,YbandSc.Thiseffectisalmostcertainlycausedbythepersistenceofgarnetinthemeltresidue,whichhashighpartition coefficients for these elements, and keeps them bufferedatrelativelylowabundances(Hofman,2005).Residualgarnetisstableinperidotitesatdepthsgreaterthan~80km(Hofman,2005).

ThehighHFSEcontentsofthestudiedalkalibasaltssuggest that theirsourcewassignificantlyenrichedin

Figure4.Photomicrographsofthestudiedblueschists.a,canddincrossedpolarizedlight(XPL);b:sameas(a)inparallelpolarizedlight.Blueriebeckiteneedlesandseparationofdarkandlightminerals(d)bymetamorphicdifferentiationareevident.

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Sanidine

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al., 2002). But significant variations of Zr/Hf (40–62), Zr/Ta (68–114),Nb/Ta(11–19)andGd/Yb(1.3–4.0)ratioscannotbeexplainedbysuchsources(Gómez-Tuenaet al.,2011;Zenget al.,2010).Becauseofthehighsolubilityofzirconinpyroxenite-eclogiteoriginatedalkalinemelts,itcannotsurviveinsuchmeltsandevenifzircondidcrystallize,itwould not significantly fractionate Zr from Hf (Linnen and Keppler,2002)andZr/Hfratiowillbelow.SuperchondriticZr/Hfratioinbasaltsandmantlerocksarerarelyobservedandappeartoberestrictedtocarbonatemetasomatism(Weyeret al.,2003).Universally,carbonatiteisoneofthemostincompatible-elementsenrichedrocks.Meltingofacarbonatedmantlewillproduceenrichedmelts(Dasguptaet al.,2007).AccordingtotheverysimilargeochemicalbehaviorofZrandHf,theelevatedvaluesandwiderangeofZr/Hfratios(40–62)arerelatedwithanenrichmentbycarbonate-rich metasomatic melts/fluids (e.g.,Rudnicket al.,1993;Pearsonet al.,2005).Itissuggestedthatsuchgeochemicalcharacteristicsofalkalibasaltsareinheritedfromanasthenosphericsourcethathadundergoneenrich-mentwithcarbonatiticliquids.Onthebasisoftheabovementionedevidences,itseemsthatthebestcandidateforsourcerockoftheAnarakalkalinebasaltsisacarbonatedperidotite.

Highlyincompatibleelementalratioscanbeusedtotracepetrogeneticprocesses.Forinstance,theZr/Yratiogenerallyremainsconstantduringfractionalcrystalliza-tion,butvariesduringpartialmeltinginbasalticsystems(NicholsonandLatin,1992).ZrismoreincompatiblethanYinthemantle,soZr/Yratiostendtobehigheratsmalldegreesofpartialmelting.DifferencesinFeOcontentsinprimarymagmacouldberelatedtothevariousdepthsand/orsourcecompositions(NicholsonandLatin,1992;Tanget al.,2006).TheAnaraksampleshavelowZr/Yratios(4.76–7.61)andmoderatetotalFeOcontents(9.42–14.70wt.%;average:11.22wt.%).ThesefeaturessuggestthattheAnarakbasaltsweregeneratedbyintermediatedegreesofpartialmelting.La/YbversusSm/Ybdiagram(Figure6d)(Zenget al., 2010), confirms this result and indicate more than12%partialmeltingofamantlegarnetperidotiteforthestudiedrocks.

Insummary,moderatedegreesofpartialmeltingofacarbonatedgarnet-bearinglherzoliticmantlesourcearerequiredtoexplainthetraceandREEpatternsobservedinthesebasalts.Thesystematicpresenceofgarnetasaresidualphaserequiresmeltingdepthsinexcessof70–80km,wheregarnetbecomesstable(Tanget al.,2006).

P-T conditions of metamorphism

Estimationofpressureandtemperatureofmeta-morphicepisodesforAnarakophioliteblueschistsisnotstraightforward(Zanchiet al.,2009).Theabsenceofomphaciticclinopyroxene,indicatesthattheupperpres-surelimitofpeakmetamorphismshouldbelowerthan12

theseelements.ComparedwithOIB(TiO2=2.87wt.%)(McDonoughandSun,1995),Anarakmeta-alkalibasaltshavelowerTiO2concentrations(1.86to2.64wt.%;aver-age:2.14wt.%).

Thepetrogenesisofintra-platealkalinebasaltsre-mainscontroversial,andfollowingsourceshavebeenproposedforsuchOIB-likemelts(Zenget al.,2010):(1)hornblenditeproducedbyhydrousmetasomatism,(2)silica-deficient eclogite and garnet pyroxenite, and (3) carbon-atedperidotite.TiO2contentofAnarakmeta-alkalibasalts(1.86to2.64wt.%;average:2.14wt.%)islowerthantheexpectedvaluestobeoriginatedfromanhornblenditesource.TheelevatedZr/HfratiosoftheAnarakmetabasalts(40–62),whicharehigherthanchondriticZr/Hfvalues(~36),canhaveresultedfrommeltingofaneclogite(orgarnetpyroxenite)withhighclinopyroxene/garnetmodes(≥70) in their source (Gómez-Tuena et al.,2011;Klemmeet

Figure5.a)PlagioclasesoftheAnarakblueschistsarealbiteincomposi-tion(Deeret al.,1992);b)Blueamphibolespresentmagnesio-riebeckitecomposition(Leakeet al.,1997).

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kbar.Inaddition,thelackofaragoniterevealsthatcalcitewasthestablecarbonatephaseduringpeakconditions,andconstrainstheupperpressureboundaryto8-9kbar.Anuppertemperaturelimitisalsoprovidedbytheabsenceofgarnet(AptedandLiou,1983;Guiraudet al.,1990),whichisstableabove400-450oCforpressureconditionsrangingbetween4and9kbar.Theabsenceoflawsonitecouldbetentatively used as a lower temperature boundary, confining thepeakphasemineralassemblagewithNa-amphibole,

albite,whitemicaandtitaniteatatemperatureabove300oCfora4–9kbarpressurerange.Thepressurepeakrecordedbythephaseassemblagewasfollowedbyadecreaseinpressureandanincreaseintemperature,suggestedbythecrystallizationofactinoliteandchlorite.ThissuggeststhattheblueschistsoftheAnarakophioliteweresubductedintoanaccretionarywedgeatadepthofatleast15–20km(Zanchiet al.,2009).

Mineralassemblagesandgeochemistry(Na2Ocon-

Figure6.GeochemicaldiagramsoftheAnarakblueschists.a)Nb/YversusZr/TiO2 classification diagram (Winchester and Floyd, 1977);b)ChondritenormalizedREEpatternsoftheanalyzedrocks.TheREEcontentsofchondritearetakenfromSunandMcDonough(1989);c)Primitivemantle-normalizedmulti-elementvariationdiagram.NormalizingvaluesarefromMcDonoughandSun(1995);d)La/Ybvs.Sm/YbdiagramforAnarakmetamorphosedalkalinebasalts(Zenget al.,2010);e)OIB-normalizedmulti-elementpatternsoftheAnarakblueschists.OIBdataarefromSunandMcDonough(1989).

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a)

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tent)ofthestudiedrocksindicatethattheypassedaprogrademetamorphisminblueschistfacies(M2)fromasub-seametamorphosedandspilitizedbasalt(M1)andchangedtoblueschistinresponsetosubduction.TheM2wasfollowedbyretrogrademetamorphimingreenschistfacies(M3).Ithasbeendemonstratedthattransformationofblueschist-fa-ciesmineralassemblagestogreenschistoramphibolite-fa-ciesassemblagesbyvolatizationreactionscanbetriggeredby channelized infiltration of external fluids during the uplift oftherocks(Schulzet al.,2001).

Tectonic setting

ThestudiedmetabasaltspresentLREEenrichment(La/Yb=4.60–16.12)andnoEuanomalies,whichistypicalofOIBandintraplatealkalibasalts(TurnerandHawkesworth,1995).OIB-normalizationofthestudied

rocksinamulti-elementdiagram(Figure6e)indicatesthattheAnarakblueschistspresentsomesimilaritiestoOIB.ThenegativeSranomalyoftheAnaraksamplesinboththeprimitivemantleandtheOIB-normalizedmulti-elementdia-gramscanbeattributedtothealterationofprimitivecalcicplagioclase. The variable Sr contents (49–130 ppm) confirm differentdegreesofalterationintheanalyzedrocks.

Severalgeotectonicdiscriminationdiagrams(Figures7a,7b,7cand7d)havebeenusedfortheserockstode-termine their tectonicsetting.Alldiagramssuggestawithin-platesetting.Geologicalsituationoftheserocksconfirm these diagrams. The Paleo-Tethys ocean spread-inginCentralIrancommencedinLateOrdovician-EarlyDevonianandterminatedintheLatePaleozoic-Triassic(Torabi,2009a,2009c;Torabiet al.,2011).SubductionofPaleo-TethysisthecauseofvolatileenrichmentofthemantleandOIB-likemagmatismintheUpperPaleozoic.Foranextensivesubduction-relatedmetasomatismofthe

Figure7.DiscriminationgeotectonicdiagramsoftheAnarakrocks.a)Th-Hf/3-Nb/16triangle(Wood,1980);b)Ta/YbversusTh/Yblog-logdiagram(GortonandSchandl,2000);WPB,WPVZandACMareabbreviationofwithinplatebasalt,withinplatevolcaniczoneandarcmagmatism,respectively;c)Zr–Ti*100-Y*3triangle(PearceandCann,1973);d)ZragainstZr/Yplot(PearceandNorry,1979).Theanalyzedsampleslieinthewithin-platealkaline basalt field.

Late Permian blueschist from Anarak ophiolite, Central Iran 553

mantle,aconsiderabletimespanisnecessary.Therefore,theformersubductionofPaleo-TethysfromlowerPaleozoictoLatePermianandEarlyTriassicisthecauseofthemantleenrichmentinvolatilesandintraplatealkalinemagmatismintheupperPaleozoicofAnarakarea.ItiswellknownthatcarbonatecanbeeffectivelytransportedintotheEarth’sinte-riorbysubductionprocesses(Shawet al.,2003).Therefore,aformerlysubductedslabcanhavebeenthesourceofthecarbon-richagentthatmetasomatizedthemantlesourcebeneaththeAnarakarea.

TheoccurrenceoftheAnarak,Jandaq,BayazehandPost-e-BadamPaleozoicophioliteswithincontinentalIranposesseveralquestionsregardingitsevolutionandespeciallyonthenumberofPaleo-Tethyssutures(singleratherthanmultiple)betweenEurasiaandIran(Zanchiet al.,2009).Previousauthors(e.g.,Soffelet al.,1996;Alaviet al.,1997)explainedtheanomalouspositionoftheNakhlak–Anarakregionbysparsepalaeomagneticdatathatapparentlyshowedapost-Triassic135Oanti-clockwiserotationofcentralIran.ThatrotationwassupposedtoberesponsiblefortransferringalargefragmentofthePaleo-Tethyssuturefromthepresent-dayAfghanistan-Iranborder(AghdarbandandMashhadareas)tocentralIran.However,newpalaeomagneticdataobtainedfromtheTriassicsucces-sionofNakhlak(Muttoniet al.,2009)challengesthatinter-pretationandsuggeststhatnolargerotationshaveoccurredintheareasincetheMiddleTriassic(Zanchiet al.,2009).FaunastudiesbyBaliniet al.(2009),regionalfaciesanalysisofUpperSilurian–LowerCarboniferoussuccessionsbyWendtet al.(2005),andLateTriassicpaleomagneticdataofBesseet al.(1998),alsoarenotconsistentwithalarge135OanticlockwiserotationofcentralIran.

Onthebasisoftheabovementionedgeologicaldata,itcanbeconcludedthatthepresenceofPaleozoicophioliticrocksalongthemainfaultsofcentralandnorthernIranrevealsmulti-sutureclosureofthePaleo-Tethysoceaninlate-PaleozoictoEarlyMesozoic.

CONCLUSIONS

FieldandpetrographicalstudiesoftheAnarakophio-litebluschistsrevealthattheywereoriginallybasaltswithpillowstructurethathavepassedthreemetamorphicepi-sodes. The order of metamorphic phases is sub-sea floor metamorphismandspilitization(M1),blueschistfaciesmetamorphism(M2),andmetamorphismingreenschistfacies(M3).Geochemically,thestudiedrocksaresodicalkalinebasaltswithtraceelementpatternscharacteristicofwithin-platealkalibasalts.Geochemicalfeaturesofthesemetabasiteindicateroleofametasomatized/carbonatedasthenosphericgarnetlherzoliteintheirpetrogenesis.ThemetasomatismofPaleozoicasthenosphericmantleisattrib-utedtosubdutionofPaleo-Tethysoceaniccrust.ThelongtermsubductionofanoceaniccrustfromLateOrdoviciantoLatePaleozoicisthecauseofthemantlemetasomatism

andOIB-likemagmatism.Theexposureofblueschistswithpillowlavastructure

alongthemainfaultsoftheAnarakareaareconsideredasremnantsofasubductionandsuturezone.ThepresenceofPaleo-TethysoceanremnantsalongthedeepintracontinentalfaultsoftheYazdandPosht-e-Badamblocks,andnorthernIran,aswellasnewpaleontologicalandpaleomagneticdataindicateaPaleo-Tethysmulti-sutureclosureinIran.

ACKNOWLEDGEMENT

TheauthorthankstheUniversityofIsfahan,andKanazawaandLeibnizUniversitiesforfinancialsup-portsandgeochemicalanalyses,respectively.Dr.HashemBagheri,UniversityofIsfahan,andananonymousrevieweraregratefullyacknowledgedfortheirthoughtfulreviewsofthemanuscript,andDr.FranciscoVegaforeditorialhandling.

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Manuscriptreceived:July23,2010Correctedmanuscriptaccepted:June20,2011Manuscriptaccepted:June26,2011