甲玛西文原文.docx
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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.,
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