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REE Geochemical Evolution and Its Significance of Early Precambrian Metamorphic Terrain,Wuyang,Henan

By Bernard Webb,2014-09-22 16:05
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REE Geochemical Evolution and Its Significance of Early Precambrian Metamorphic Terrain,Wuyang,Henanof,OF,REE,and,Its,Early,ree,AND,EARLY,its

    REE Geochemical Evolution and Its

    Significance of Early Precambrian

    Metamorphic Terrain,Wuyang,Henan

Vo1.IINo.2CHINESEJOURNALoFGEoCHEMISTRY1992

    ;REEGeochemicalEvolutionandItsSignificance

    ;ofEarlyPrecambrianMetamorphic

    ;Terrain,Wuyang,Henan

    ;CHENYANJING(陈衍景)

    ;(InstituteofGeochemistry,AcademiaSinica,Guiyang) ;FUSHIGU(富士谷),HuSHOUXI(胡受奚)

    ;(DepartmentofEarthSciences,NanjingUniversity) ;ANDZHANGYUANYOU(张源有)

    ;(GeologicalTeam4,HenanBranchofChinaNonferrousMetalIndustryCompany)

    ;Abstract

    ;ThesupracrustalrocksoftheWuyangmetamorphicterrainaredividedintotheZhat’anzhuang,

    ;TieshanmiaoandYangshuwanFormations.ThesethteeFormationsweredatedat3Ooo2550Ma,

    ;255023ooMaand230022ooMa,respectively.ZREEandLa/Yb0ftheZ

hat’anzhuangFor-

    ;marionvolcanicrocksareobviouslyhigherthanthoseoftheTieshanmiaoFormationequivalents,sug?

    ;gestingasedimentarygap(255OMaboundary)betweenthesetwoformations.TheZhat’anzhuang

    ;FormationisolderthantheTieshanmiaoFormation.ThesedimentsofthesetwoFormationsshowno

    ;obviousditferencesinREEandaregenerallycharacterizedbvlowZREEandpositiveEuanomalies.

    ;Onthecontrarv,thesedimentsoftheYangshuwanFormationarecharacterizedbyhigh?REEand

    ;negativeEuanomalies.DetaileddiscussionsdemonstratethattheYan2shuwanFormationwasdepos?

    ;itedinanoxidizingenvironmentwhereastheothertwoformationswereformedinareducingenviron-

    ;merit.AttheendoftheevolutionoftheTieshanmiaoFormationabout2300Maago,the

    ;sedimentaryenvironmentwastransformedfromreducingtooxidizing.Onthebasisofthe

    ;SHAB(soft/hardacidandbase)theory,anoxidation?reductionmodelforsedimentaryREEevolu-

    ;tionhasbeenestablished.Itisproposedthatthemantletendstobecomegradual

lydepletedin

    ;REE,especiallyinLREE,andtheindicesZREEandLa/Yb)nofmantle-derivedvolcanicrocksal-

    ;sotendtobecomelowerandlower.

    ;GeologicalSetting

    ;TheWuyangEarlyPrecambrianmetamorphicterrainislocatedinthe ;HuaxiongblockatthesouthernmarginoftheNorthChinacraton.Inthel950’s,

    ;theTieshanmiaotypeFedepositswerefoundinthisterrainbyChuXinchunet

    ;a1.Inl960alargerFedeposit—theZhat’anzhuangdepositwasfound

    ;therebyZhangYuanyoueta1.Sincethen,muchworkhasbeendoneonthege- ;ologyofthoseFedeposits.Themetamorphicstrataoftheterrainaresubdivided

    ;fromthebottomupwardsintotheZhat’anzhuangFormation,theTieshanmiao

    ;FormationandtheYangshuwanFormation.Thesethreeformationsweredatedat

    ;Archaean.Thecorrelationbetweenthestratigraphicdivisionandthestratigraphic

    ;sequencehasfurtherbeenconfirmed.Itfollowsthattheagesofthesethreeforma

    ;tionsare30002550Ma,25502300Maand23002200Marespectively,

and

    ;arepetrOgraphically ;greenstonebeltand ;1991:Hu,1988).

    ;distinguishedastheprimarygreenstonebelt,thesecondary

    ;thekhondaliteseries(Fig.1)(Cheneta1.,1988,1989,

    ;

    ;l34CHINESEJOURNALoFGEoCHEMISTRYVo1.Il

    ;2200Ma…

    ;TangshansouFr.

    ;Yangcunwan

    ;2550Ma

    ;Tleshanmiao

    ;2300Ma

    ;Zhaoanzhuang

    ;SampiesandAnalysis ;NO1

    ;NO5

    ;NO3

    ;NO12

    ;N0U

    ;NO2

;N07

    ;N06

    ;NO10

    ;NO8

    ;123

    ;5678

    ;Fig.1.Thestratigraphicsequenceofthe ;Wuyangmetamorphicterrairland ;samplelocalities.

    ;1.Bandedirollformation(BIF):2.volcanic ;rock;3.fineelasticrock;4.coarseclastic ;rock;5.graphite-bearingelasticrock; ;6.biocalcilutite:7.Alrichsediment:

    ;8.dolomititeanddolomiticlimestone. ;No.7,Wol5:

    ;metamorphicelastic

    ;No.8,W027

    ;ListedinTablelare

    ;andrare-earthelements

    ;thecontentsofmajor

    ;inthesupracrustal

    ;rocksofdifferenthorizonsintheWuyang

    ;metamorphicterrain.Themajorelementdata ;wereprovidedbyZhangYuanyou.TheREE ;contentswereanalyzedwiththeICPtechnique ;atthegeologicallaboratoryofHubei ;ProvincialGeologicalBureau.Mostofthe ;sampleswerecollectedfromdrillcores,with ;nosignofbeingweathered.Inthefollowing ;arethepetrographyandmineralogydatafor ;thesamples:

    ;No.1,W026:diopsidel0%:carbonate ;minerals90%:chemicalsediment.

    ;No.2,W030:diopside40%:quartz

    ;40%:magnetite20%:chemicalsediment. ;No.3,W003:diopside57%:quartz

    ;20%:magnetite20%:hematite3%:chemical ;sediment.

    ;No.4,W033:graphite6%:carbonate ;minerals10%;quartz20%;~ldspar20%; ;others(actinolite,epidote,chlorite,biotite) ;44%:biocalcilutite.

    ;No.5,W034:quartz40%:~ldspar

    ;20%:calcite5%:biotite35%:calcareous

;shale.

    ;No.6,Wol4:quartz90%:~ldspar

    ;l0%:elasticrock.

    ;quartz97%:~ldspar2%;muscovite1%;migmatized

    ;rock.

    ;quartz20%;andesine20%;garnet30%;amphibole30%

    ;pyroxenel0%:komatiite.

    ;No.9,Wo2l:magnetite45%:serpentine48%;calcite6%;apatite1%;al. ;teredultrabasicvolcanicrock.

    ;No.10,WO24:magnetite5%:serpentine50%;pyroxene35%;olivine ;10%;alteredultrabasicvolcanicrock.

    ;No.1l,Wo3l:magnetitel3%serpentine85%:phlogopite2%;altered ;ultrabasicvolcanicrock.

    ;No.12,W003:magnetite5%:pyroxene95%;ultrabasicvolcanicrock. ;EnvironmentsoftheSedimentsinSpecialReferencetoTheirREEGeochemistry

    ;FromTables12andFigs.12

    ;No.7whichwasmigmatized,sharethe

    ;TheindexEu/Euforthesediments

    ;itisshownthatallthesamples,except

    ;followingtwocharacteristicfeatures:(1)

    ;youngerthan2300Ma(Nos.45)is

;

    ;NO.2CHINESEJOURNALoFGEoCHEMISTRYl35

    ;<0.80,olderthan2300Ma(No.1,No.2,No.6)>1.00andatabout2300 ;Ma(No.3)0.801.00.ThistrendisconsistentwiththevariationofEu/Euin ;the31samplesfromtheEarlyPrecambriansedimentsatthesouthernmarginof ;theNorthChinacraton(Cheneta1.,1990).(2)The?REEofthesediments

    ;youngerthan2300Maisobviouslyhigherthanthatofthesedimentsolderthan ;2300Ma.

    ;2

    ;O

    ;2

    ;O

    ;4

    ;7

    ;5

    ;6

    ;LaCe.PrNdSmEuGdTbDyHOErTmYbLu

    ;Fig.2.Chondfite.normalizedREEdiagramofsedimentsintheWuyangmetamorphicterrain

    ;(a):Clasticrock;(b):chemicalsediment.

    ;AsproposedbyTayloretaI.(1985),thepost-Archaeansedimentsarede- ;pletedinEU,andsuchaEu-depletioniSattributedtothedevelopmentof

    ;K-granitesattheendoftheArchaeanratherthantootherfactorssuchas ;oxidation-reduction.Moreover,intheWuyangterrainanditsvicinity,no ;K-graniteolderthan2200Mahasyetbeenreported,andthesediment ;Eu-depletionoccurredatabout2300Ma.Hence,suchaEu?depletioncannotbe

    ;simplyattributedtotheemplacementofK-granites.WeconsiderthattheEu ;anomalyofthesedimentsshouldbedeterminedbythegeochemicalbehaviorofEu

    ;andsedimentaryenvironment.ItiSwellknownthatEucanexisteitheras ;Eu2orasEuwhiletheotherREEcanexistastrivalentionsonly,i.e., ;R3.OnthebasisoftheSHAB(soa/hardacidandbase)theory(Dai,1987), ;EuandR”arebothhardacids.thussharingmuchincommonwithrespectto

    ;theirproperties.EuanomaliesseemtohaveresultedfromtheseparationofEu ;=-coIIuJcrI\一厶E??一鼬orI

    ;

    ;136CHINESEJOURNALOFGEOCHEMISTRYV01.11

    ;Si02

    ;Ti02

    ;Al203

    ;Fe203

    ;FeO

    ;Mn0

    ;MgO ;CaO ;Na20 ;K20 ;P2O5 ;0thers

    ;La ;Ce ;Pr ;Nd ;Sm ;Eu ;Gd ;Tb ;Dv ;Ho ;Er ;Tm ;Yb ;Lu ;Y

    ;Eu/Eu’

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