ELSEVIER Earth and Planetary Science Letters 161 (1998) 215–

ELSEVIER Earth and Planetary Science Letters 161 (1998) 215–230 U=Pb zircon ages constrain the architecture of the ultrahigh-pressure Qinling–Dabie Orogen, China

ELSEVIER

U=Pbzirconagesconstrainthearchitectureoftheultrahigh-pressure

Qinling–DabieOrogen,China

BradleyR.Hackera, ,LotharRatschbacherb,LauraWebbc,TrevorIrelandd,DougWalkere,

DongShuwenf

b

DepartmentofGeology,UniversityofCalifornia,SantaBarbara,CA93106-9630,USA

Institutfu¨rGeologie,St.NorbertStr.6,Universita¨tWu¨rzburg,D-97299ZellamMain,Pleicherwall1,D-97070Wu¨rzburg,Germany

cGeologicalandEnvironmentalSciences,StanfordUniversity,Stanford,CA94305-2115,USAdGeologicalandEnvironmentalSciences,StanfordUniversity,Stanford,CA94305-2115,USAe120LindleyHall,GeologyDepartment,UniversityofKansas,Lawrence,KS66045,USAfChineseAcademyofGeologicalSciences,534EastZongshanRd,Nanjing210016,China

Received3March1998;revisedversionreceived26June1998;accepted3July1998

a

Abstract

NewSHRIMPandTIMSzirconages,40Ar=39Arages,andeclogitelocationscontributesigni cantlytoourunderstand-ingoftheultrahigh-pressureDabieShan.(1)ThegeographicextentoftheYangtzecratonthatwassubductedtoultrahighpressureextendstothenorthernedgeoftheDabieShan.(2)ThenorthernhalfoftheDabieShanisamagmaticcomplex,intrudedovera10-Myrintervalbetween137and126Ma,thataccommodated¾100%N–Sstretchingofthepre-existingcollisionalarchitecture.(3)Graniticorthogneissesandenclosingultrahigh-pressureparagneisseshaveindistinguishablezirconpopulations.ThepopulationofTriassiczirconagesrangesfrom¾219to¾245Ma,leadingustoquestiontheprevailingassumptionthat219Mazirconsformedatultrahighpressure,andtoproposeinsteadthattheyre ectlateretrogressionatcrustalpressuresfollowingthebulkofexhumation.©1998ElsevierScienceB.V.Allrightsreserved.Keywords:DabieMountains;zircon;absoluteage;Sino–Koreanplatform;highpressure;orogenicbelt

1.Introduction

TheDabie–Hong’an–Tongbai–Qinlingarea2000-kmlongultrahigh-pressure(UHP)orogenformedchie yintheTriassicbyattemptedN-directedsubductionoftheYangtzecratonoramicrocontinentbeneaththeSino–Koreancraton(Fig.1)[1,2].InvestigationofUHPtectonicshasfocusedontheDabieShanbecauseofthewide

Corresponding

author.Tel.:C18058937952;Fax:C1805

8932314;E-mail:hacker@geology.ucsb.edu

varietyofcontinentalcrustalrocksthatweremeta-morphosedunderacompleterangeoflowtoultra-highpressuresandtemperatures.FromStoN,themainrockunitsareafold-and-thrustbelt,blueschist,high-pressureamphibolite,quartzeclogite,coesiteeclogite,theNorthernOrthogneissunit(NOU),theLuzhenguangGroup,andtheFozilingGroup(Fig.2).AllareintrudedbyvoluminousCretaceousplutons,andunitsonthemarginsofthemoun-tainsareoverlainbyCretaceousandyoungerallu-vium(referencesinFig.1).Theblueschistthrougheclogiteunitsconstituteaprogrademetamorphicse-

0012-821X/98/$19.00©1998ElsevierScienceB.V.Allrightsreserved.PIIS0012-821X(98)00152-6

ELSEVIER Earth and Planetary Science Letters 161 (1998) 215–230 U=Pb zircon ages constrain the architecture of the ultrahigh-pressure Qinling–Dabie Orogen, China

216B.R.Hackeretal./EarthandPlanetaryScienceLetters161(1998)

215–230

Fig.1.CollisionalorogenincentralChina,comprisingtheWudang,Qinling,Tongbai,Hong’an,DabieShan,andSu-Luareas.

quence,withthecoesite-bearingeclogitesindicatingsubductionofcontinentalcrustto>120km[3,4].Hackeretal.[5]interpretedtheseunitsasasingletranspressionally(dextral)shortenedcrustalsegmentexhumedbyextrusionovertheSino–Koreancratoncombinedwitherosion.TheNOUisamagmatic-structuraldomeformedduringCretaceousNW–SEsubhorizontalextension[5]thatoverprintsanearliergranulite-faciesbasementsequence.TheLuzhen-guangandFozilingGroupsarelesswellknownunitsthatmayrepresentcontinentalbasementandcover,respectively.

IntheHong’anarea(Figs.1and2),blueschist-faciesrocksarebetterdevelopedanddistinctgreenschist,progradeamphibolite,andeclogite-ret-rogressed-to-amphiboliteunitshavebeenmapped(e.g.,[6]).Also,awidervarietyofmetamorphicrocksnotgenerallygroupedwiththeultrahigh-andhigh-pressureunitscropoutinE–Wtrendingfault-boundedunitsatthenorthernlimitofthemountainrange(i.e.,theSujiahethroughErlangpingGroupsintheNWcornerofFig.2).ManyoftheseunitsarewellknownintheQinlingarea(Fig.1;e.g.,[7]).Thispapersummarizesnewgeochronological=petrological=structuraldatathataddresssigni cantquestionsaboutthearchitectureofthisorogen:(1)WhereisthesuturebetweenthecollidedSino–KoreanandYangtzecratons?(2)Whatisthespa-

tialextentoftheUHPmetamorphism?Were,forinstance,theNorthernOrthogneiss,Luzhenguang,orFozilingunitssubjectedtoUHP?(3)WereallrocktypesintheUHPunitsmetamorphosedatUHPoraresomeofthemlowerpressurerocksthatwerejuxtaposedatalaterdate?(4)WhatisthetemporalrelationshipbetweentheUHPmetamorphismandthegranulite-faciesmetamorphismoftheNOU?

2.Qinling–DabieorogenicarchitectureandUHPmetamorphism

TheTriassicsuturebetweentheSino–KoreanandYangtzecratonshasbeenproposedtobelocatedeitherinthecentralDabieShan(Fig.2)betweentheNOUandthecoesiteeclogiteunit(e.g.,[8]),oralongtheXiaotian–MozitangFaultnearthenorth-ernedgeoftheDabieShan(e.g.,[9]).Neitheroftheseisaprimaryfeature;theformerisaCretaceousnormalfault,andthelatterisaCretaceoussinistraltranstensionalfault[5].ThelocationofthesutureconstrainsthesizeandcompositionofthematerialthatwassubductedtoUHPdepthsandiscentraltounderstandingthescaleandmechanismofex-humation.WereportanewlocationforthesuturebasedonU=Pbzirconages,40Ar=39Arages,andthedescriptionofaneweclogitelocality.

ELSEVIER Earth and Planetary Science Letters 161 (1998) 215–230 U=Pb zircon ages constrain the architecture of the ultrahigh-pressure Qinling–Dabie Orogen, China

B.R. Hack

er et al./ Earth and Planetary Science Letters 161 (1998) 215–230 217 Fig. 2. Map of the Dabie Shan and Hong’an areas drawn from our mapping in 1993–1997 and Regional Geological Sur …… 此处隐藏：43368字，全部文档内容请下载后查看。喜欢就下载吧 ……