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Chronology

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    Chronology

Chin.J.Geochem.(2008)27:276-284

    ;DOI:10.1007/sl16310080276Y

    ;ChronologyandcoolinghistoryoftheTianmen-

    ;shanplutoninSouthJiangxiProvinceandtheir

    ;geologicalsignificance

    ;ZENGQingtao,,MAOJianren,HUQing,ZHAOXilin,andYEHairnin

    ;NanjingInstituteofGeologyandMineralResource,Nanjing210016,China ;.EarthScienceCollegeofJilinUniversity,Changchun130061,China ;Correspondingauthor,Email:zengqingtao@gmail.corn

    ;ReceivedApril24,2007;acceptedJuly13,2007

    ;AbstractGeochronologicalstudiesonthecrust-derivedTianmenshanplutonwereundertakenbySHRlMPz~con

    ;UPbdatingandAr-Ardatingofbiotite,muscoviteandKfeldspar,givingapetrogeneti

    cageof167Ma.Owingto

    ;theclosuresystemsjndifferentminerals,thecoolinghistoryoftheplutoncanbedeterminedwithan

    ;agetemperaturediagram.Thelatehydrothermaleventhasbeenrecognized,whichisrelatedprobablywithminer

    ;alization.Intermsofthecomparativegeochronologicalandpetrologicrecords,itisconcludedthattherearesome

    ;constrainsontectonicevolutionandthattheformationoftheTianmenshanplutonproceededinatransitionperiod

    ;fromIndosinianpostorogenyextensiontostrongcompressivetectonics.Andthetimingofthehydrothermalevent

    ;matchesthecompressiveclimaxoftheYanshanianorogeny.Thetemporalg

    betweengraniteemplacementand

    ;wolframitemineralizationcould1ast1o_20Maowingtothe1owcoolingrateofthepluton. ;Keywordsgranite;coolinghistory;tungstendeposit;Tianmenshan;SouthJiangxiProvince

    ;1IntroductiOil

    ;MesozoicplutonsandbatholithsinSouthChina

    ;(SC,arecharacterizedbyvastoccurrenceandabun-

    ;dantassociatedtungsten.tinmineralization.Therela-

    ;tionshipsbetweenore.formationandassociatedmag-

    ;maticrockshavebeenconsideredasahotsubjectin

    ;geologicalresearchforalongtime.Recently,withthe

    ;furtherstudiesonSCgranites,researchershavebegun

    ;tocorrelatetheregionalmineralizationwiththe1itho

    ;sphericevolution(MaoJingweneta1.,2004;Hua

    ;Renmineta1.,2005).Aftercollatingthedataofgran- ;iteassociatedW.Sndeposits,HuaRenmin(2005)no- ;ticedthatthereisanevidenttemporalgapbetweenthe ;formationofcrust.derivedgranitesinSouthChinaand ;relevantmineralization,anddiscussedthecausations ;intermsofthebackgroundofsourceandthegeotec- ;tonicrelationsbetweengraniteemplacementandmet- ;allogenesis.

    ;InthisPaDer’SHRIMPzirconU—Pbdatingand

    ;4OAr/39Ardatingofbiotite,

    ;muscoviteandK.feldspar

    ;wereconductedfromtheTianmenshanplutonin ;SouthJiangxiProvince.Theauthorsstudiedthegeo- ;chronologyandcoolinghistoryoftheTianmenshan ;plutonanddiscussedthetectonicsettingofthepluton ;www.gyig.ac.cnwww.springerlink.com

    ;anditssignificanceinoredepositgeology.Incombi- ;nationwiththepreviousstudiesinthisarea,refined ;chronologicalandpetrologicconstrainsareprovided ;fortheore.formingtectonicenvironment.Onthebasis ;ofthecalculatedtempora1spanofore.formationand ;mineralization,anewexplanationispresentedofthe ;

    ;10Matempora1gapbetweengraniteemplacement ;andmetaUOgenesis.

    ;2BasiccharacteristicsoftheTianmenshan ;pluton

    ;TheTianmenshanplutonislocatedinChongyi ;County.SouthJiangxiProvince.China.Itisshaped ;likeanE.W-trendingspindle.Itistectonicallylocated ;ontheNWmarginofaCathaynianblock,asuper

    ;posedzoneonthewesternflankoftheWuyiUplift ;andinthemiddlesegmentoftheLuoxiaofold,be- ;longingtothenorthoftheXihuashan.Yangmeisire

    ;gionintheChongyi.Dayu.ShangyouW-polymetallic ;concentrationarea.Thisgranitecoversanexposure ;areaof15km.Themaintypesoftheplutonsare ;greyishwhite,lightpink,medium-granularporphy’

    ;riticbiotitegranites,varyingingrainsizefromme- ;dium.fineinthecoretofineintheouterparts.There ;aresomelate.stagegraniticporphyriesoccurringin ;Springer

    ;

    ;cGe.chem

;.

    ;(2008)27:2784

    ;

    ;278

    ;biotitegranitewithporphyriticstructure,measuredat ;

    ;1.2ITUTIingrainsizeandcharacterizedbyamineral ;assemblageofKfeldspar(30%),plagioclase(28%), ;quartz(35%),biotite(5%)andmuscovite(<2%). ;Biotitehasbeenchloritizrd;muscoviteisdividedinto ;twotypes:magmatic.ndhydrothermalthelatterof

    ;whichcanbeobservedoccasionallyinalteredbiotite; ;sericitizationofplagioclaseisdeveloped,andsome ;crystalpiecesoflatestagemuscovitearevisibleon

    ;thesurfaceofplagioclase,andthereisnoK.feldspar ;overgrowth.InordertoacquireSUfficientevidenceto ;constrainthecoolinghistoryoftheTianmenshanplu. ;ton.weperformedSHRIMPzirconU.Pbdatingand ;Ar.ArmonomineraldatingattheElectronMicro. ;probeLaboftheInstituteofGeology,ChineseAcad. ;emyofGeologicalSciences(CAGS)inBeijingand ;theInstituteofGeologyandGeophysics,Chinese ;Chin.J.Geochem.(2008)27:276--284

    ;AcademyofSciences,respectively.Weacquiredthe ;accurateandreliableisotopedataanddrewmeaning

    ;fulconclusions.

    ;3.1ZirconU?PbSHRIMPdating

    ;3.1.,Samplepreparation

    ;Arepresentativesample(SampleNo.04Gn.031 ;waschosenforisotopicdating.Representativezir. ;congrainsf>1000)werepickedoutunderabinocular ;microscopeafterseparationbyusingconventional ;heavyliquidandmagnetictechniquesattheRegional ;InvestigationLabinHebeiProvince.TheYwere ;handpickedandmountedontoanepoxyresindiscin ;thestandardofRESE(GeoscienceDepartment,AUS. ;tralianNationalUniversity),andthenpolished. ;Fig.2.CLimageandU-PbageanalyticspotsofthezirconsinsampleNo.04Gn-03fromtheS

    hangyoupluton.ThenumbersindicatespotsinTable1 ;Table1.SHRIMPU-PbdataforzirconsfromtheTianmenshangranite(04Gn-03)

    ;InSHRIMPzirconU.Pbanalysis,zirconswith ;magmaticcharacteristicswerechosenasdescribed ;bellow:thezirconsampleswerefreefromflawinthe ;surface,andtransparentwiththelength.widthratioof

    ;about3:1.Afterartificialhand.selectionandtransmis. ;sionlightandreflectedlightphotograph.takingwere ;conductedonthehand.pickedzircons,cathodelumi

    ;nescenceimageanalysiswasconductedtoshedlight ;ontheinternalstructureofthezircons(Fig.2,.The ;standardTEMORA17Ma..PbPU--0.06683) ;wasusedininterelementfractionation.Forthedetails ;ofthemeasurement,pleaserefertoSongBiaoeta1. ;

    ;Chin.J.Geochem.(2008)27:276-284

    ;(2002).

    ;3.1.2Analyticalresults

    ;AnalyticalresultsarelistedinT{lb1e1.Forteen ;grainsofzirconyielded14determinationsthatare ;mpstl~he.o

    ;plottedonconcordia

    ;,

    ;cur

    ;W

    ;ve

    ;D=

    ;to

    ;1.

    ;g

    ;)

    ;ivea

    ;pb/238Uageof167~5Ma(n=14MS3(Fig.

    ;3,.TheagesofGn03.4.1andGn03?11.1deviatefrom ;theconcordiacurvef185Maand184Ma),respec? ;tively.FromtheCLimagesofthezircons,wecansee ;thatNos.4andl1zirconsarerelativelydarkincolor ;duetothehighcontentsofinitialU.

    ;0.034

    ;0.032

    ;0.030

    ;0.028

    ;0.026

    ;0.024

    ;0.022

    ;0.020

    ;0.00.10.20.3

    ;.Pb/U

    ;Fig.3.ZirconUPbConcordiaoftheTianmenshangranite. ;3.2MonomineralAr-Ardating

;3.2.Samplepreparation

    ;4uAr,jArdatingwasconductedonthefreshand ;wel1.crystallizedbiotite,muscoviteandK?feldspar ;separatedfromSampleNo.04Gn.03.

    ;Biotite,muscoviteandK.feldsparfromSample ;No.04Gn.03wereanalyzedbytheincremental ;step.heatingmethod.Ar-Ardatingwasconductedat ;theIsotopeLab.oftheInstituteofGeology,Chinese ;AcademyofGeologicalSciences(CAGS1inBeijing. ;Neutronirradiationofthesamplesandstandardswas ;carriedoutinthe”SwimmingPoolReactor”atthe

    ;ChineseScientificInstituteofAtomicEnergy.The ;domesticstandardZBH.25wasusedwithanageof ;132.7MaandaKcontentof7.6%.Step.heatingwas ;doneinanelectronbombardmentstove.Thesamples ;weredegassedbyheatinginvacuumfor6-8hours, ;witheachheatingandpurifyingsteplasting30min. ;utes.Massspectrometricanalysiswascarriedouton ;anMM.1200Bmassspectrometer,with8groupsof ;dataforeachDeakvalue.Thedatawerecorrected ;everytimewhenthedataresetfromzero,including ;blank.massdiscrimination,memoryeffectandradio? ;activedecayofAt.Thecorrectioncoefficientforthe ;279

    ;interruptionisotopesproducedintheprocessofneu’

    ;tronirradiationcanbeobtainedbyanalyzingtheiIra? ;diatedK2SO4andCaF2.Theplateauageandpositive ;andnegativeisochronswerecalculatedbythe ;ISOPLOTProgram(Ludwig,1995).A11errorsin? ;volvedintheplateauagewerereportedatthe281eve1. ;Detailedlaboratoryprocedurescanbefoundinrelated ;literaturefChenW_eneta1.,2002).

    ;.

    ;2.2Analyticalresults

    ;Analyticalresultsforbiotite,muscoviteand ;K.feldsparfromtheTianmenshangranitearelistedin ;b1e2.

    ;Theplateauagesandisochronagesofbiotite, ;muscoviteandK.feldspararegiveninFig.4. ;Theplateauagesofbiotite,muscoviteandfeld? ;sparare165.7~1.3Ma.159.8?1.1Maand143.5?1.4 ;Ma.andtheiriSOChronagesare166.2~2.1,160.6~1.8 ;and143.7~2.2Ma(18),respectively.Theinitial ;4uAr,jArratiosareclosetothoseofatmosphereargon,

    ;whichindicatesthatthereisnoexcessofAr.AUOf ;thecontentsofArreleasedduringthemiddlesteps ;arehigherthan87.1%,withonecaseuptO99.22%,so ;theplateauandisochronagesareconsideredreliable. ;Furthermore,thecoincidentplateauandisochronages ;indicatethattheanalyticalresultsmaywellrepresent ;theageofmineralcrystallization.

    ;4Discussion

    ;4.1Coolinghistoryoftheplutonanditstectonic ;significance

    ;Sincetheclosuretemperaturetheorywaspro’

    ;posedbyDodson(1973).ithasfoundwideapplica? ;tionsinthermalchronologytoconstrainthermalhis? ;torybothathomeandabroad(WuZhenhaneta1., ;2000;Rogereta1.,2000;Maluskieta1.,2001;Kirby ;eta1.,2002;YlangJinhuieta1.,2004;ZhengDewenet ;a1.,2004).Theconceptionofclosuretemperaturewas ;developedfromthetheory.Theclosuretemperature ;referstothefinaltemperaturewhenradioactiveiso? ;topesstopdiffusionandmigrationinrocksandminer’

    ;alsandthistemperaturevariesfrommineraltomin’

    ;era1andforeachisotopicsystem(GangulyandTirone, ;1999).Previousstudiesshoweddifferencesinselect? ;ingtheclosuretemperatureforzirconisotopicsystems. ;ZhengYongfeieta1.(1997)tooktheclosuretempera? ;tureof700~50?fortheU.Pbisotopicsyrstem;Leeet ;a1.(1997)studiedthemechanismofUThdiffusionin

    ;zircon,indicatingthattheclosuretemperatureofzlr- ;conishardtochoseunder800?.IncOnsideratiOnOf

    ;theaboveinformation,thetemperatureof750~Cis ;takeninthispapen

    ;

    ;280Chin.

    ;J.Geochem.(2008)27:276-284

    ;Table2.4oA3Ardatingresultsforbiotite,muscoviteandK.reidsparfromtheTianmenshan

    ;Underidealcircumstances,theinitialtiming ;pointsofmono.mineralsaredelayedastheirclosure ;temperaturetendedtobecomelowerintheprocessof ;coolingofthepluton,whichyieldedyoungerages. ;However,muscoviteisabnormalinbehaviorinthis ;study.Itscrystallizationageis6Mayoungerthanthat ;ofbiotite,althoughitsclosuretemperatureissug. ;gestedtobehigherthanthatofthebiotite.Closure ;temperaturesmaybeaffectedbyvariationsinmineral

    ;composition(Harrisoneta1.,1985),grainsize ;(Markleyeta1.,2002),inclusions(Ramaeta1.,1965), ;retrogression(DiVincenzoeta1.,2003),anddeforma. ;tionorrecrystallization(Mulcheta1.,2002).Interms ;oftherationaldiflusioncoefficientandactivationen. ;ergyfHamesandBowring,1994),theclosuretern. ;peratureofmuscovitedifferinginsizestillCanvary

    to490?.whichindicatesthatmuscovite ;from270?

    ;couldfinishitsclosuretoargonatlowertemperature ;thanbiotite.ThatcouldexplainwhytheageofmUS. ;coviteisyoungerthanthatofbiotite.Inthisstudy, ;relativelystrongsericitizationonbiotiteandplagio

    ;clasecanbeobservedundermicroscope,someminer- ;alshavebeenalteredintoflakyFemuscovite.The ;analysistargetofmuscovitewasamixtureofmag

    ;maticmuscoviteandhydrothermalmuscovite.Li ;XianhuaandGuiXuntang(1990)proposedthatKAr

    ;fAr.Ar)datesalwaysrevealthelatesthydrothermal ;eventsinthecaseofinvolvementofhydrothermally ;alteredminerals.Theflakymagmaticmuscoviteas ;observedunderthemicroscopelendsasupportsug

    ;gestingthatthemixedageofhydrothermaleventsis ;advanced.Inotherwords,thecoolingageofmag. ;

    ;Chin.J.Geochem.(2008)27:276-284

    ;maticmuscoviteisyongerowingtotheinfluenceof ;hydrothermalactivities.Becauseitishardtoevaluate ;thesignificanceofhydrothermaleventsinthereten? ;tionofargon,itseemsimpossibletogodeintothe

    ;t.dataHowever,wecansaywithcertaintythat ;thelatehydrothermaleventshavebeenrecordedin ;hydrothermalmuscovite.andthestartOfAr.Ardating ;180

    ;160

    ;

    ;140

    ;

    ;120

    ;0

    ;100

    ;80

    ;60

    ;40

    ;c

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    ;0

    ;?

    ;

    ;

    ;?

    ;

    ;.

    ;(a)

    ;__

    ;U

    ;DPlateauage1657?13Ma(2s)

    ;“Arinvolvedinthecalculationofplateau

    ;age934%ofthetotal.Ar

    ;..”..’

    ;O20406080l00

    ;Cumulative9Arpercent

    ;fc)

    ;n.fif}

    ;Plateauage1598?I.1Mar26)

    ;Arinvolvedinthecalculationofplateau ;age=9922%ofthetotalAr

    ;04Gn-O3-mtlSt~OVlte

    ;0204060801OO

    ;Cumulative.Arpercent

    ;(e)

    ;Plateauage143.5?14Maf26)

    ;.ArinvolvedinthecalculationOf’plateau

    ;age871%ofthetotalr

    ;O20406080l00

    ;Cumulative.Arpercent

    ;281

    ;hasbeendelayed,probablycontributingtotheforma.

    ;tionofW-bearingquartzveins.Consequently,we ;haverecognizedthattheageofmuscoviteimpliesthe

    ;commenceoftungstenmineralizationiscloselyre?

    ;latedwiththelatehydrothermalevents.Inthispaper

    ;400?and350?aretakenastheclosuretempera. ;turesformuscoviteandbiotite,respectively. ;r/Ar

    ;Ar/Ar

    ;Ar/36Ar

    ;Fig.4.Plateauandisochronagesofbiotite(aandb),muscovite(candd)andKfeldspar(eandf)fromtheTianmenshangranite.

    ;ThecapacityofK.feldspartoretainArisnot ;strong,soitiseasyforargontodifluseandlose.In ;thisstudy,weobtainedasmoothageplateauwithlittle ;fluctuationandtheinitial”Ar/Arratioiscloseto

    ;atmosphericargonratio,indicatingthatthecorrection ;ofatmosphericargonusedisreasonableandthereis ;noexcessinitialargon.TheplateauageofK.feldspar ;isreliable.Furthermore,theplateauandisochronages ;arecoincidentwitheachother(about144Ma1.indi. ;catingthereliabilityoftheresultsofagedetermina. ;tion.Theclosuretemperatureof150?forK.feldspar

    ;wasadoptedinthisstudy(McDongallandHarrison. ;1988).

    ;Consequently,accordingtotheagesoftheTian. ;menshangraniteandthecorrespondingclosuretem. ;perature,acurveofcoolinghistoryisshowninFig.5. ;Forhydrothermaloredepositsassociatedwithgranites. ;however,thetemperatureoforedepositionisUSUally ;/J/J

    ;??如加m

    /J ;v

    ;OOOOO0,

    ;

    ;282

    ;muchlowerthanthetemperatureofmagmacrystalli

    ;zation.Themineralizationisthussignificantlylater ;thanthemagmacrystallization(Zhaoeta1.,2004). ;Thesolidlinerepresentsthenormalcoolingpathway ;ofmagmaticevolution;thedashedlinerepresentsthe ;temperaturecurveinfluencedbyhydrotherI-flalevents. ;60155150

    ;AgeGilt

    ;Fig.5.CoolingcurveoftheTianmenshanpluton. ;ofw0}nmlf

    ;Forthegranite,theaveragecoolingrateduring ;theperiodof167-165MawasestimatedtObe

    ;200?/Ma.andthatduringtheperiodof165-144Ma, ;9.5?/Ma.Itisevidentthatthecoolingratesofthe ;granitesaredistinctindifferentperiods.Acommon ;fe:atureisthatthecoolingrateswerehighinthefirst ;stagefromtheageyieldedbyzircontothatyieldedby ;biotiteandbecamelowinthesecondstagefromthe ;ageyieldedbybiotitetothatyieldedbyK.feldspar.It ;isspeculatedthatthetemperaturegradientbetween

    ;thegranitesandthesurroundingcountryrockswas ;large,resultingintherapidcoolingofgranitemagma ;duetosignificantthermaldiffusionandheatconvec

    ;tion.Withrapidcoolingofgranitemagmaduringthe ;firststage,thetemperaturegradientbetweenthejust ;crystallizedgraniteandthesedimentaryrocksbecame ;smallerandsmaller.Thatcouldexplainwhythereis ;anobviouslylowcoolingrateinthesecondstagesoon ;aftercrystallizationofbiotite.Accordingtothetec

    ;tonicsdata.160MaistheclimaxageoftheYansha

    ;nianorogenyinSouthChina.Itishighlypossiblethat ;thehydrothermaleventrepresentedbythemuscovite ;isamicroscopicreflectionofthisstrongcompressive ;orogenV.Letuscalculatethecoolingrateagaininthe ;periodfromtheageyieldedbymuscovitetothat ;yieldedbyKfeldspar(400150~C)/(16O-144

    ;Ma)=l5.6~C/Ma].Inthisway,wecanconcludethat

    ?/Mareflectsthetruether ;thecoolingrateof10

    ;modynamiccharacteristicsoftheTianmenshangranite. ;Inviewofthethermallyevolutionalstudy,thecon

    ;stantcoolingrateof1015?/Maandmagmaticac

    ;tivitiesduring165-144Mashowedthatthetectonic?

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