Chronology
Chin.J.Geochem.(2008)27:276-284
;DOI:10.1007/sl1631—008—0276一Y
;ChronologyandcoolinghistoryoftheTianmen-—
;shanplutoninSouthJiangxiProvinceandtheir
;geologicalsignificance
;ZENGQingtao,,MAOJianren,HUQing,ZHAOXilin,andYEHairnin
;NanjingInstituteofGeologyandMineralResource,Nanjing210016,China ;.EarthScienceCollegeofJilinUniversity,Changchun130061,China ;Correspondingauthor,E—mail:zengqingtao@gmail.corn
;ReceivedApril24,2007;acceptedJuly13,2007
;AbstractGeochronologicalstudiesonthecrust-derivedTianmenshanplutonwereundertakenbySHRlMPz~con
;U—PbdatingandAr-Ardatingofbiotite,muscoviteandK—feldspar,givingapetrogeneti
cageof167Ma.Owingto
;theclosuresystemsjndifferentminerals,thecoolinghistoryoftheplutoncanbedeterminedwithan
;age—temperaturediagram.Thelatehydrothermaleventhasbeenrecognized,whichisrelatedprobablywithminer—
;alization.Intermsofthecomparativegeochronologicalandpetrologicrecords,itisconcludedthattherearesome
;constrainsontectonicevolutionandthattheformationoftheTianmenshanplutonproceededinatransitionperiod
;fromIndosinianpost—orogenyextensiontostrongcompressivetectonics.Andthetimingofthehydrothermalevent
;matchesthecompressiveclimaxoftheYanshanianorogeny.Thetemporalg印
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 ;一
;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 ;greyish—white,lightpink,medium-granularporphy’
;riticbiotitegranites,varyingingrainsizefromme- ;dium.fineinthecoretofineintheouterparts.There ;aresomelate.stagegraniticporphyriesoccurringin ;垒Springer
;
;cGe.chem
;.
;(2008)27:2784
;
;278
;biotitegranitewithporphyriticstructure,measuredat ;！
;1.2ITUTIingrainsizeandcharacterizedbyamineral ;assemblageofK—feldspar(30%),plagioclase(28%), ;quartz(35%),biotite(5%)andmuscovite(<2%). ;Biotitehasbeenchloritizrd;muscoviteisdividedinto ;twotypes:magmatic.智ndhydrothermalthelatterof
;whichcanbeobservedoccasionallyinalteredbiotite; ;sericitizationofplagioclaseisdeveloped,andsome ;crystalpiecesoflate—stagemuscovitearevisibleon
;thesurfaceofplagioclase,andthereisnoK.feldspar ;overgrowth.InordertoacquireSUfficientevidenceto ;constrainthecoolinghistoryoftheTianmenshanplu. ;ton.weperformedSHRIMPzirconU.Pbdatingand ;Ar.Armono—mineraldatingattheElectronMicro. ;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.ZirconU—PbConcordiaoftheTianmenshangranite. ;3.2Mono—mineralAr-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)studiedthemechanismofU—Thdiffusionin
;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)proposedthatK—Ar
;fAr.Ar)datesalwaysrevealthelatesthydrothermal ;eventsinthecaseofinvolvementofhydrothermally ;alteredminerals.Theflakymagmaticmuscoviteas ;observedunderthemicroscopelendsasupportsug—
;gestingthatthemixedageofhydrothermaleventsis ;advanced.Inotherwords,thecoolingageofmag. ;
;Chin.J.Geochem.(2008)27:276-284
;maticmuscoviteisyongerowingtotheinfluenceof ;hydrothermalactivities.Becauseitishardtoevaluate ;thesignificanceofhydrothermaleventsinthereten? ;tionofargon,itseemsimpossibletogode印intothe
;t.dataHowever,wecansaywithcertaintythat ;thelatehydrothermaleventshavebeenrecordedin ;hydrothermalmuscovite.andthestartOfAr.Ardating ;180
;160
;—
;140
;订
;三120
;0
;100
;80
;60
;40
;c日
;一
;0
;?
;《
;邑
;?
;《
;.
;(a)
;__
;U
;DPlateauage1657?13Ma(2s)
;“Arinvolvedinthecalculationofplateau
;age934%ofthetotal.Ar
;】..”..’曲
;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)andK—feldspar(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
;加??如加m
/J ;v艚
;OOOOO0,加
;
;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 ;yieldedbyK—feldspar『(400—150~C)/(16O-144
;Ma)=l5.6~C/Ma].Inthisway,wecanconcludethat
?/Mareflectsthetruether— ;thecoolingrateof一10
;modynamiccharacteristicsoftheTianmenshangranite. ;Inviewofthethermallyevolutionalstudy,thecon—
;stantcoolingrateof10—15?/Maandmagmaticac—
;tivitiesduring165-144Mashowedthatthetectonic?