甲玛西文原文.docx

甲玛西文原文.docx

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甲玛西文原文

TheIndus–YarlungZangboophiolitesfromNangaParbattoNamcheBarwasyntaxes,southernTibet:

Firstsynthesisofpetrology,geochemistry,andgeochronologywithincidencesongeodynamicreconstructionsofNeo-Tethys

Abstract:

ThepurposeofthisfirstsynthesisistosummarizefindingsontheYarlungZangboSutureZone（YZSZ）ophiolitesanddiscussstillremainingproblems.TheYZSZstudiedforalmost30yearsandistheyoungestofthesuturesrecognizedonTibetPlateau.ItisnowacknowledgedthattheYZSZisacomplexassemblageofsedimentary,metamorphicandigneousrockscreatedduringandshortlyafterthecollisionbetweenIndiaandEurasia.TheagesofthevariouslithologiesspanatimeintervalfromtheJurassictotheMiddleMiocene,withsomePermianandDevonianexoticblocksfrommélangezone.TheYZSZischaracterizedbyophioliticcomplexesandophioliticmélange.Theophiolitesareoftwotypes:

nondismemberedanddismemberedsections.Thenondisturbedsections,althoughtectonicallyreworked,areobservedalongthesegmentfromDazhuqutoJidinginXigazeareaandSpontangophiolite.ThedismemberedsequencesarefoundinvariouslocationssuchasNidar,Kiogar,Jungbwa,Saga,Sangsang,Xigugabu,andLuobusa.Theincompletestratigraphiclogcouldbeconnectedtointraoceanicororogeneticorigins.Theophiolitesaredistributedintotwogroupsofages:

theLuobusa,ZedangandKiogarsequencesbeingJurassic-LowerCretaceouswhereasallothersequencesareofLowerCretaceousage.Compilationofgeochronologicaldatasuggestthatsomeophiolitesequencesmighthaveevolvedforovermorethan70Myfromtheirinitialgenesistoobductionwhichoccurredaround70–90Myago.Ophiolitesdifferintermsofpetrologicalandgeochemicalaspectshowever,theywereallgeneratedinsuprasubductionzoneandmorespecificallyinarc（fewfore-arc）andback-arcsettings.Synthesisofmorethan700geochemicalanalysesshowvariablemixingofcomponentsfromN-MORB-typetoIAT-CABandtoOIBend-members.TheJurassicophiolitesshowthemaximumofarccomponentwhiletheLowerCretaceousonesshowlittletostrongmixing.Inaddition,mostophioliteswerecreatedinshortlived（30My）basinsandgeneratedclosetotheEurasiaticcontinentalmargin.WeproposethatLadakh–TibetophiolitesweregeneratedinasuprasubductioncontextsimilartoMarianaarc,interarcandback-arcorTonga–Lausystem.Thevariablearcsignatureoftheseophiolitesisdirectlyrelatedtotheirinitialpositionwithinthesuprasubductionsystem.

Highlights:

►Synthesisonthegeodynamicsofophioliticsequencesandrelatedrocks.►ReconstructionofNeo-Tethysbasininvolvesanactivecontinentalmarginandcomplexmarginalbasins.►Veryshortlifetimeforthebasin,withophiolitesequencesrepresentingageodynamicpuzzle.

Keywords:

Ophiolite;YarlungZangboSutureZone;Suprasubduction;Geodynamics;Tibet

1.Introduction

TheYarlungZangboRiversutureareaisthoughttobethegeologicalfeatureofthemostimportantgeologicaleventtohavehappenedonEarthsincetheCretaceous-Tertiaryboundary:

thecollisionofIndiawithEurasia（e.g.,Aitchisonetal.,2011）.Thiscollisionwasresponsibleformajorseawatercompositionshift,accentuatedsedimentologicprocesses,andclimaticchanges.TheHimalayabeltandTibetanPlateauwereformedasaresultofthiscollision.Deformationfeatures,thermalmodificationofthecrustandthemantlecanbedetectedwellbeyondtheregion.ThejourneyofIndiaplatestarted85Maagoaccordingtoplatepaleokinematicreconstruction（Klootwijketal.,1992）.Thecollisionitself,stillamatterofdebate,isbelievedtohavetakenplacesome50Maago,afterconsumptionoftheNeo-TethysOceanwhichoccupiedthespacebetweenthenorthmovingGreaterIndiaPlateandEurasiaticcontinent（MolnarandTapponnier,1975,Gansser,1980,PatriatandAchache,1984andRowley,1996）.TheclosingoftheNeo-TethysspaceiscurrentlybelievedtobeaccommodatedbysubductionofalargeportionofoceanicandcontinentallithosphereunderEurasia（Allègreetal.,1984,Coulonetal.,1986,Deweyetal.,1989,YinandHarrison,2000,Guilmetteetal.,2008andGuilmetteetal.,2009）.Althoughthisorogenhasbeenstudiedformorethan80years（Argand,1924,Gansser,1964,Gansser,1974andGansser,1980）,severalmajorquestionsremainunanswered.Forinstance,theprocessesofclosingtheNeo-TethysOcean,thetimingofthecollisionbetweenIndiaandEurasia,thetiminginupliftofTibetanPlateau,andthegeodynamicsignificanceoftheophioliticbeltareamongtheinvestigatedproblems.Thispaperisanattempttosynthesizethestateofknowledgeconcerningthe2500kmlongophioliticbeltwhichgeologicallymarksthelimitbetweenHimalayanandTibetanterranes,theEurasiaticandIndiandomainsanddelineatingtheIndus–YarlungZangboSutureZone（heresimplifiedasYZSZforthepurposeofthispaper）.WeproposeaMariana-orTonga–Laubasin-typesuprasubductionsystemtorestorepossiblegeometryoftheevolutionoftheNeo-Tethysbasedongeochronologicalandgeochemicaldata.

2.Generalgeologicframework

TheHimalaya–TibetorogenicbeltissynthesizedinFig.1.FromSouthtoNorth,thesystemisorganizedintoseverallithotectonicdomains:

1.theHimalayanfold-thrustbeltandtheGreaterHimalayanCrystallines,2.theLhasaBlock,3.theQiangtangBlock,theSongpan–GanziHoh-XilBlock,4.theKunLun–QiadamBlock（Gansser,1980）.Thesedomainscanbesubdividedintomajorrockformationasfollows:

theLesserHimalayanmetasedimentaryseries,theHighGreaterHimalayametamorphicrocks,theTethyansedimentarysequence,theearlyTertiaryLinzizongvolcanicrocks,theCretaceousplutonicdomainoftheGangdesebatholithandKohistanarc,theTertiarysedimentaryrocks,thePaleozoicstrataoftheQiantangBlock,andthePaleozoicandMesozoicplutonicrocksoftheKunLunandQilianBlocks.ThereaderisreferredtothesynthesisofYinandHarrison（2000）fordetailsaboutthehistoryofthisgiganticsystem.

Fig.1.SimplifiedgeologicalandtectonicmapoftheHimalaya–TibetorogeniczoneandtheYarlungZangbosuturezoneareashowingthewesttoeastlocalizationandagesofthemajorophioliticmassifsoftheYarlungZangboSutureZone.SourcesforagesarelistedinTable1andclassifiedbymassifs.Ageswritteninitalicarebiostratigraphicages（sediments）.Acronyms:

SSZ,Shyoksuturezone;ZSZ,Zanskarsuturezone;BNSZ,BangongNujiangsuture;YZSZ,YarlungZangboSutureZone.TheHimalayanfold-thrustbeltissubdividedintostructuralblocksaccordingtothemajordiscontinuities:

GCT,GreatCounterthrust;STD,SouthTibetdetachment;HHT,HighHimalayathrust,MCTZ,MainCentralthrustzone;MBT,mainBoundarythrust,MFT,MainFrontalthrust.ThemapismodifiedafterYinandHarrison,2000andGoscombeetal.,2006.

TheTibetanPlateautothenorthofYZSZwhichisthehighestofitskindintheworld,resultsfromthecontinentalcollisionbetweenIndiaandEurasiaplates（Yinetal.,1988）.Itissubdivided,fromnorthtosouth,intoblocksorterranesaccordingtonearlyeast–westmajorsuturezones:

1.theAyimaqin–KunlunMutztaghsuture;2.theJinshasuture;3.theBangong–Nujiangsuture;4.theYarlungZangbosuture（Fig.1）.Thesesuturesallcontainfragmentsofoceanicbasinsthatexistedpriortothesuturingofthedifferentblocks（YuandZhen,1979,MercierandLi,1984andMurphyetal.,1997）.NumerousstudiesconcerningtheupliftingofTibet（Turneretal.,1993,Chungetal.,1998andColemanandHodges,1995）andtheactualdeepthermalstructureoftheplateau（Xiao,1988,LinandWatts,1988andWilliamsetal.,2001）aresummarizedinVanderVooetal.,1999andZhouandMurphy,2005.ItisbelievedthattheIndianPlateisbeingsubductedtodepthsasgreatas165to260kmandunderlaytheentireplateau.Velocityislowinthecrustandhighinthelithosphericmantleatdepthsbetween75and120km.AnasthenosphericwedgeisinterpretedbetweentheIndianplateandtheTibetanplateauatdepthsof120to165km.ZhouandMurphy（2005）presentedamodelinwhichtheamountofshorteningintheHimalayasisrelatedtotheamountofunderthrustedIndianmantlelithosphereandtheypredictthattheentireTibetanPlateauisunderlainbyayoungmantlegeochemicalsignature（Milleretal.,1999andHouetal.,2004）.BasedontomographicimagingofthesubductionoftheTethyanregion,Hafkenscheidetal.（2006）concludedthatextensiveback-arcbasinsmusthaveopenedduringtheconvergenceofIndiaandEurasiafortheJurassicandtheCretaceousperiods.Thesearcandbackarcbasinsgavebirthtotheancestraligneoussequencesofthefutureophiolites.TheYZSZistheyoungestofTibetansuturesandcontainsremnantsoftheNeo-TethysOceanandisthetopicalstructuralfeatureofthispaper.

2.1.GeologyofYZSZ

BecauseTibetisanexceptionalnaturallaboratory,ithasbeenstudiedbyalargenumberofearthscientistsfromaroundtheworldandfromdiversescientificbackgrounds.

Firstofall,itisimportanttodefinewhichgeologicaltermstheYZSZincludes.TheYZSZisborderedtothenorthbytheLhasaBlock,madeofMiddleProterozoictoLowerCambriancrystallinebasementandPaleozoictomid-Cretaceoussediments（ChangandCheng,1973,Burgetal.,1983andXuetal.,1985）.TheLhasaBlockincludesthefollowingunitsandtheirattributedages:

SangriGroup（UpperJurassictoLowerCretaceous）,Gangdesebatholiths（133±10to66–57Ma;Chiuetal.,2009andreferencestherein）andpossiblyasoldas188.1±1.4Ma（Chuetal.,2006）,Ladakhbatholiths（66.6±2.1to45±0.56Ma（Upadhyayetal.,2008）;46.3±0.6to29.8±0.2Ma（Bhutanietal.,2004））,Kohistanbatholiths（29–25Ma（Georgeetal.,