[TheJournalofGeology,2001,volume109,p.755 - PDF document

[TheJournalofGeology,2001,volume109,p.755–770]2001byTheUniversityofChicago.Allrightsreserved.0022-1376/2001/10906-0005$01.00GeodynamicSignicanceoftheKontumMassifinCentralVietnam:ArandU-PbAgesfromPaleozoictoTriassic ElizabethA.Nagy, Figure1.SimpliedgeologicmapoftheKontummassifinVietnamshowingsamplingsitesandregionsdiscussedintext.Mapiscompiledfromexistinggeologicmapsandpresenteldinvestigations.MapunitsareafterTranVanTri(1973)andNguyenXuanBaoetal.(1994).Heavydashedlinesareinternationalborders;lightdashedlinesarerivers.TheKannackcomplexconsistsofgranulitefaciesrocksandsyntectoniccharnockiteintrusions.ThePaleozoicamphibolitefaciesrocksincludeacidictobasicorthogneissesknownastheDak-ToˆUnit.Undatedamphibolitefaciesrocks,whichincludemigmatiticgneisses,orthogneisses,andmetasediments,aretheNgocLinhcomplexandtheKhamDucformation.MesozoicterrigenoussedimentsareTriassicandJurassic.,MapofVietnamshowingmajorfaultzones,cities,andtownsmentionedinthetextandthelocationofthemaingeologicmap.BienPhufaultzone;RedRiverfaultzone;SongChaymassif;SongCafaultzone;SongMafaultzone;Tamky–PhuocSonfaultzone. JournalofGeologyKONTUMMASSIF,VIETNAM7571,inset).EarlyProterozoicK-Aragesof1.7–2.0Ga(TranNgocNam1998),aswellasNdmodelagesof3.4–3.1Ga(Lanetal.1999),havebeendocu-mentedalongtheRedRiverfaultzone,althoughsingle-grainU-Pb-HfisotopeanalysesfromdetritalzirconandbaddeleyitecrystalsfromtheRedRiverarenotinfavorofcrustolderthan2.5Gaalongthismajordrainage(BodetandScha¨rer2000).In-heritedzirconcomponentscorroboratemeltingofPrecambrian(1.6–0.8Ga)crustinOligo-MiocenegneissesfromtheRedRiverandBuKhangareas¨reretal.1990,1994;Nagyetal.1999,2000),althoughdirectevidenceforPrecambrianrockshasnotbeenreportedfromtheBuKhangregion.AgesfromtheKontummassifincludeafewearlytomid-dleProterozoicK/ArandRb-Srages,aswellasaCambrianRb-SrageandaDevonianK/Arage,al-thoughanalyticaldetailsaremissingfrommanyoftheseearlystudies(FaureandFontaine1969;Snell-ing1969;HurleyandFairbairn1972;PhanTruongThi1985;TranQuocHai1986;Hutchison1989).RecentgeochronologicstudiesintheKontummas-sifndNdmodelagesof2.0–1.5Ga(Lanetal.2000)andSHRIMPU-Pbzirconagesfromonegranulitesampleof254Ma(TranNgocNametal.2001).WepresenttherstArandU-PbcombinedgeochronologicresultsforavarietyofrocksfromtheKontumbasement,includingtheKannackmetamorphiccomplex,andshowthattheserocksdonotrepresentPrecambrianlowercrustbutratherrecordPaleozoictoTriassicmagmatismandlow-temperaturethermalevents.Ourresultshaveim-portantimplicationsfortheextentofmagmatismrelatedtothelatePaleozoicclosingofthePaleo-TethysSea.GeologicSettingandSampleLocationsTheKontummassif(Saurin1944)insouth-centralVietnamisanupliftedblockofhigh-grademeta-morphicrocksintrudedbygraniticbodiesandlargelycoveredbyNeogene-Quaternarybasalts(Ranginetal.1995;HoangandFlower1998;Leeetal.1998).TheE-to-W-strikingTamky–PhuocSonfaultzone(g.1andinset)separatestheKontumblockfromtheTruongSonfoldbelttothenorth.TheTruongSonbelt,whichstrikesNWandex-tendsfromcentralVietnamthroughnorth-centralVietnamandintonortheasternLaos,consistspri-marilyofelongatedzonesofmetamorphicrocks.TheSongCaandSongMafaultzones(g.1,inset)are250–300-km-longcontinuousshearzoneswith-intheTruongSonbeltthatshowmetamorphicfab-ricsparalleltotheoveralltrendoftherange(e.g.,Lepvrieretal.1997).SouthoftheTamky–PhuocSonfaultzone,thesupposedlyPrecambrianKannackmetamorphiccorecomplex,locatedprimarilyintheeast-centralpartoftheKontumblockwithsmallerisolatedex-posuresinthesouthnearM’DracandinthewestnearDak-Toˆ(g.1),isintrudedbyPaleozoicandundatedamphibolitefaciesrocksandbyMesozoicnonfoliatedgranites.Mesozoicsedimentsarepre-sentaswell.Alloftheserockshavebeenexten-sivelyfaultedbystructuresgenerallystrikingNtoNNW,whereasNeogene-Quaternarybasaltsandsedimentsintheregionarenotdeformedbybrittledeformation.TheKannackmetamorphiccomplexincludesseveralformationspreservinggranulitefaciesrocktypes(DGMVN1997).Allrocksinthecomplexarerelatedtothe2-pyroxenegneissfaciesofregionalmetamorphism(Hutchison1989),correspondingtoconditionsofformationatdeepcrustallevelsaround800–850Cand7–8kbar(TranQuocHai1986).Thebaseofthecomplexisformedbyanaluminousmetasedimentaryseries(pelitesandAl-quartzites)intercalatedwithcarbonate(moreorlessdolomitic)sediments.Theseundatedsedimen-taryprotolithsarehighlyfoliated(sometimesmy-lonitic)andmetamorphosedtogranulitefacies,giv-ingrisetokhondaliticandkinzigiticgneisses,granuliticmetaquartzites,marbles,andskarns.Thefoliationisattomoderatelydipping.Intermediatepressuresaresupportedbyprismaticbiotiteandsil-limanite-garnet-K-feldsparmineralogy.Prismaticrutile,Mg-ilmenite,Ti-magnetite,sulfur,apatite,zircon,andgraphiteareabundant.Thepresenceofstablebiotitewithquartzinsomesamplesindi-catesthatanatecticconditions,probablyunderlowO,wereattained.Syntectonicbodiesofcharnockiteintrudethegranuliticmetasedimentaryformation.Evidenceforthesyntectonicnatureoftheintrusionsin-cludesmagmaticfoliationthatparallelsthefolia-tionofthegranulitesandfoliation-parallelintru-sionsoflargesills.Mostofthecharnockitesbelongtoeitheracalc-alkalinemagmaticseriesoracon-tinentaltholeiiticserieswithplagioclaseandor-thopyroxene.Themagmaticsuiteisdifferentiatedintoacidic,intermediate,andbasicmembers,giv-ingrisetocharno-enderbites,Q-enderbiteswithor-thopyroxeneandbiotite(PhanCuTienetal.1988),andtruenoriticgabbros.Themostacidicchar-nockiticrockscouldbeproductsofanatexisofthegranuliticmetasedimentsunderlowpHOcondi-tions.Thecharnockitesandgranulitesdisplayalate(secondary),retrogradegreenschistfaciesmeta-morphism(e.g.,chloritizationofbiotite,Felossinbiotites,growthofopaquesandsecondarywhite- 758E.A.NAGYETAL.micas).Cordieritemaybepinitizedandbrownhornblendecanbetransformedintoactinoliteandepidote.Thisretrogressiveepisodemaysimplybeduetocoolingofthesyntectonicmagmasoritmayhaveformedastheresultofindependent,late,low-grademetamorphism.OurstudyincludesaQ-enderbitecharnockite(VN357)analyzedwithU-Pb,Rb-Sr,andArmethods(14NorthandsouthoftheKannackcomplex,largeamountsofmigmatiticgneissesandstronglyfoli-atedgranitesareexposed.TheserocksdisplayaN-to-NE-strikingfoliationandageneralantiformalstructure(seecrosssectionsing.2).Shearcriteriaacrosstheareaindicatenormalmovementsrelatedtoexhumationofthemassif.Thedirectionofex-tensionisapproximatelyNWtoSE.Themineralassemblagecorrespondstoamphibolitefacieswithmuscoviteandsillimanite.IncontrasttotheKan-nackcomplex,migmatizationoccursinthepres-enceofwater.Wehaveanalyzedacharnockite(Q-enderbite)enclave(VN343)fromwithinthisunitfromthesouthernmostpartoftheKontummassif(g.1)withtheU-PbandRb-Srmethods(12InthewesternpartoftheKontumblock,aNNW-trendingzonealongtheKrongPokoRiverbetweenthevillagesofDak-ToˆandKontum(g.1)consistsofamphibolitesandgranitoidsmoreorlessgneis-sied(PhanCuTienetal.1988).Tothewest,theserocksareinfaultcontactoverasliceofKannackcomplexrocks;Neogenesedimentscovertheeast-erncontact.Therocksdonotshareanyfeatureswiththepreviouslydescribedcharnockites,andfromageochemicalpointofview,theybelongtoaFe-K-richseries.Wesampledthreedifferentam-phibolitefaciesmetabasitesrepresentingalkalinetocalc-alkalineafnities.Sampledoutcropscon-sistofaN110E-striking,subverticalfoliationandsubhorizontallineations.Abiotite-enrichedpeliticserieswithintheorthogneissex-pressesstrongfoliationandisoclinalfolding.Shearcriteriatendtoindicateadextralsenseofmotion.Oursamplesincludeaweaklydeformedgrano-diorite(VN386;14;107),anortho-gneiss(VN387;14;107),andabiotiteamphibolite(VN388;samelocationasVN387).WeanalyzedsampleVN386withU-PbandRb-SrmethodsandallthreesamplesforAr.MineralogyandGeochemistryCharnockites.SampleVN357isaQ-enderbitethatistypicalofintermediate-to-acidicunitsfoundintheKannackcomplex.Itcontainsquartz,andesineplagioclase(AncontainingantipertheticK-feldspar(AbmyrmekiticK-feldspar,hypersthene(Fe),Ti-biotite,Ti-hornblende,apatite,zircon,andTiopaques.(AllphenocrystcompositionsquotedhereweredeterminedwithaCamecaSX100-5electronmicroprobeattheUniversityofMontpellier2.)Thebiotiteoccursasasyn-tolate-crystallizationphase,formedaftertheinitialcrystallizationoforthopy-roxene,andbrownhornblendecrystallizedevenlater.SampleVN343isaQ-enderbiteoccurringwithinmigmatitesandconsistsofquartz,metastablecer-iticizedplagioclaseexhibitingpolysynthetictwin-ning,myrmekiticK-feldspar,orthopyroxene,chlor-itizedbiotite,muscovite,secondaryamphibole,apatite,andzircon.Thereisevidenceforbothpri-maryandsecondarymuscovite.Thepetrofabricin-dicatesbrittledeformation.AmphiboliteFaciesRocks.SampleVN386isaweaklyfoliatedgranodioritecontainingquartz,K-feldspar,oligoclaseplagioclase,Fe/Ti-biotite,andzircon.SampleVN387isanorthogneissconsistingofquartz,K-feldspar,oligoclaseplagioclase,Fe/Ti-biotite,Fe-hornblende,allanite,titanite,apatite,zircon,andopaques.Finally,VN388isabiotiteam-phiboliteofdioriticcomposition,consistingofquartz,andesineplagioclase(An),green-brownamphibole(Fe-pargasite/hastingsitewithFe0.9–1.13wt%TiO),Fe/Tibiotite(Fe;2.65wt%TiO),epidote(var.pistacite),titanite,allanite,ilmenite,apatite,andzircon.Geochemistry.ChemicalanalysesofpowderedwholerocksplitsfromthethreesamplesanalyzedforU-PbandRb-Sr(charnockitesVN357andVN343andgranodioriteVN386)wereperformedusingemission-ICPformajorelementdetermina-tionsandICP-MSfortraceandrareearthelement(REE)analyses(table1[tables1,3,and5areavail-ablefromTheJournalofGeologyDataDepositoryfreeofchargeuponrequest]).Rockclassicationforeachsampleisassignedusingasilica-alkalidi-agram(g.3).Majorandtraceelementdatashowthatthethreeintrusionsreectcalc-alkalinemag-maslyingintheeldsofdiorite(VN357),grano-diorite(VN386),andgranite(VN343).AllsamplesareLREE(lightrareearthelement)enrichedwithLa/(g.3).CharnockiteVN357istheleastfractionatedofthethreesamples,andcharnockiteVN343isthemostfractionatedandsignicantlydepletedinHREE(heavyrareearthelement)(La/),suggestinggarnetfractionationinthesourcemagma.Euanomaliesareslightlynegative(VN357)toabsent;plagioclasefractionationisthereforenotstronglysupported.Intraceelement Figure2.Geologiccrosssectionscorrespondingtoingure1andshowingMesozoicandolderrocks.Theamphibolitefaciesrocksareshownherewithoutthelithologicdistinctions(metasediments,migmatites,orthogneiss)showningure1.Elongatesymbolsshowgeneraldirectionoffoliationinthevariousrocktypes.Shortdashesaremicaschists;otheramphibolitefaciesunitsareshownwithlongdasheswithsinglecrossbar. 760E.A.NAGYETAL. Figure3.GeochemicalresultsfromsamplesVN343,VN357,andVN386illustratedby()atotalalkali-silicadiagram(boundarypositionsafterLeBasetal.1986)and)rareearthelementpatternsnormalizedtochondriticcomposition(normalizingvaluesfromAndersandEbi-hara1982).discriminationdiagramsappliedtograniticrocks(Pearceetal.1984),thegranite(VN343)andgran-odiorite(VN386)samplesgenerallyfallwithinthesyn-collisionalandvolcanic-arcgraniteelds,re-spectively.GeochronologicDataPreviousData.Previousstudieshavenotquan-titativelyidentiedArcheanagesforrocksfromanypartoftheKontummassif.Ofparticularsig-nicance,ArcheanagesinferredfortheKannackmetamorphiccomplexinthecentralandeasternregionsoftheKontummassifarebasedexclusivelyuponlithologiccorrelationsinferredbetweenthecharnockitesandpetrologicallysimilarPrecam-brianrocksinGondwana(PhanTruongThi1985;TranQuocHai1986;Hutchison1989,p.152).K-ArgeochronologyintheKontummassifprovidedEarlyProterozoicages(1650–1810Ma;TranQuocHai1986;Hutchison1989)thataresimilarto,yetslightlyolderthan,Rb-Sragesof1400–1600Ma(PhanTruongThi1985);a2300-MaPb-isochronagehasalsobeenmentionedintheliterature(Hutch-ison1989,p.300).AsignicantlyyoungerK-ArbiotiteageofMahasbeenreportedfromtheDak-ToˆregionnorthwestofKontum(g.1;FaureandFontaine1969;Snelling1969).Radio-metricageshaveonlyrecentlybeenreporteddi-rectlyfortheKannackcomplex(manyoftheagesreportedbyHutchison[1989]areestimates)andin-cludeK-Aragesof241–244Mafromsynkinematicbiotitesinacordierite-sillimanite-biotitegneiss(TranNgocNam1998),a240-MaArageforacharnockite(MaluskiandLepvrier1998;Maluskietal.1999),andU-PbSHRIMPagesaround250Maforagranulite(TranNgocNametal.2001).U-PbandRb-SrAnalyses.ZirconsfromsamplesVN357,VN343,andVN386weredatedwiththeU-Pbtechnique,andK-feldsparsfromthesampleswereanalyzedforRb-SrandPbisotopesystematics.Measurementswereperformedusingtheisotopedilutionmethodongrain-by-grainselectedfrac-tionsofzirconandK-feldspar,concentratedusingconventionalmagneticanddensityseparationtechniquesfollowingcrushingof3–5-kgrocksam-ples.TheU-PbandRb-Sranalyticalresultsarelistedintables2and3,respectively.Mostgrainswereabradedpriortodissolution(indicatedintable2).AllU-Pbdateswerecorrectedforinitialcom-monPbasdeterminedoncoexistingK-feldsparsfromeachsample(table3[tables1,3,and5areavailablefromTheJournalofGeologyDataDe-positoryfreeofchargeuponrequest]).Detailsoftheanalyticalproceduresandmass-spectrometricanal-ysiscanbefoundinthefootnotesoftables2and3andinearlierpublications(e.g.,Scha¨reretal.1994;ZhangandScha¨rer1999).Concordiadiagramsforthedioriteandgranitecharnockitesandtheamphibolitefaciesgranodior-iteareshowningure4,respectively.SevenzirconfractionsfromVN357(g.4)yieldvecon-cordantandtwoslightlydiscordantdates.Themeanagedenedbytheveidenticallyconcordantzirconfractionsandtheveryslightlydiscordantfractionsis(2)Ma.Aregressionline1.7includingallsevenfractionsgivesanupperinter-ceptageofabout1.1Ga,approximatingtheageofzirconsfromthemeltedcrustalmaterial.Sevenzir-confractionsfromVN343(g.4)yieldtwocon-cordantagesandoneveryslightlydiscordantdatethattogethergiveameanageofMa,1.5 Table2.U-PbAnalyticalResultsforZirconsfromtheKontumRegion Sampledescriptiontion(ppm) RadiogenicPbinatomic% Atomicratios Apparentagesin Urad. Dioriticcharnockite(VN357):45mediumtolargeequantgrains,transparent,abraded.162631313.065181.84.213.9.03944.2816.05177324925227530largeegg-shapedtoelongategrains,trans-parent,abraded.098434914.251484.34.311.4.03996.2810.05100925325124135largeelongategrains,transparent,nonabraded.179838315.6116582.64.213.1.03913.2770.0513392472482567largeelongategrains,transparent,nonabraded.072240215.7116685.54.410.1.03882.2726.0509382452452385mediumtolargeegg-shapedtoroundgrains,transparent,stronglyabraded.036926311.0150880.74.215.0.03919.2833.05242624825330414mediumtolargebrokenpiecesofgrains,trans-parent,abraded.07901606.656881.74.214.1.03887.2761.05151824624826415largeelongategrains,transparent,nonabraded.156537715.8322183.64.412.0.04077.2952.052509258263308Graniticcharnockite(VN343):13largeelongategrains,transparent,abraded.048449822.0457677.24.018.9.03958.2810.05147825025126211largeelongatetoroundgrains,transparent,abraded.025147021.1101276.33.919.7.03978.2828.05156225125326615mediumtolargeegg-shapedgrains,transparent,stronglyabraded.037751319.9306388.84.66.6.04009.2861.05175525325527510largeelongategrains,transparent,nonabraded.0673218372.711,34991.54.73.9.03543.25014.05120622422725022mediumtolarge,roundtoegg-shapedgrains,trans-parent,abraded.1190152163.611,49889.25.75.1.04341.38073.06361627432872917mediumequantgrains,transparent,abraded.0496122059.8911688.65.85.5.05056.4583.06574631838379824smalltomediumegg-shapedgrains,transparent,wellabraded.0360116447.9259788.04.97.1.04210.3260.056159266287459Granodiorite(VN386):13mediumwell-roundedgrains,transparent,wellabraded.043562043.8561388.75.06.3.07298.5667.05632145445646512mediumtolargeangulargrains,transparent,abraded.060858641.631787.84.97.3.07238.5539.05550245044843326smalltomediumequantgrains,transparent,abraded.032560342.1409689.25.05.9.07238.5554.05565345044943919smalltomediumegg-shapedgrains,transparent,abraded.042947132.031188.45.06.6.06993.5455.0565824364424758mediumtolargeelongategrains,transparent,non-abraded.0567121574.5780290.45.14.5.06451.4986.0560644034114555mediumtolargeroundgrains,transparent,abraded.062257433.1116989.84.95.3.06028.4578.055082377383416 Note.Individualanalyseswereperformedoneuhedral,unbroken,crack-freegrainsofhighesttransparencypossible.Smallgrains80mmlong,medium-sizegrainsare80–120mmlong,andlargegrainsare120mmlong.ZirconsweredissolvedinHFat220Cfor4dinTeonbombs;thechemicalprocedureisfromKrogh(1973),andabrasionwasperformedaccordingtoKrogh(1982).UdecayconstantsusedarethosedeterminedbyJaffeyetal.(1971)asrecommendedbySteigerandJa¨ger(1977).Forisotopicmeasurements,PbandUwereloadedtogetheronsingleRelamentswithSigelandHandwererunat1350–1450Cand–1550C,respectively,onaThomson206solid-sourcemassspectrometer.Massdiscriminationis%/amuforPbandU.Dioriticcharnockite(VN357)mean(1–6):(2)Ma;graniticcharnockite(VN343)mean(8–10):Ma;1.7252.5granodiorite(VN386)mean(15–17):Ma,mean(15–18):Ma.2.6448.1Ratiocorrectedformassdiscriminationandisotopictracercontribution.Ratiocorrectedformassdiscrimination,isotopictracercontribution,10pgofPbblank,1pgofUblank,andinitialcommonPb(table4)asdeterminedinleachedcoexistingfeldspar(Scha¨rer1991)fromthesamples.Togetherwithmass-spectrometricprecisionsanduncertaintiesfromspikecalibration,suchcorrectionforinitialPbyieldsanalyticaluncertaintiesof0.5%–0.7%for0.6%–1.0%forU,andabout0.2%–0.5%forPb,dependentonPbmeasured. 762E.A.NAGYETAL. Figure4.U-Pbconcordiadiagramsforsamples(VN357,()VN343,and()VN386.whereasthreeotherfractionsaresignicantlydis-cordant,indicatingthepresenceof1.8–2.7-Gain-heritedcomponentsextractedfromthemagmasourcerocks.Anonabradedfractionplotsatayoungeragerelativetoallothersamples,undoubt-edlyduetopostcrystallizationPblossatthecrystalsurfaces.SixzirconfractionsfromVN386(g.4yieldthreeconcordantdatesdeningameanageofMa,andthreeotherfractionsshow451.2variousdegreesofdiscordancethatcanbeascribedtopostcrystallizationPbloss.IncludingtheleastdiscordantfractionwiththeconcordantonesgivesameanageofMa.448.1Sourcecharacteristicsofthedioritic,granitic,andgranodioriticmagmaswereexaminedusingRb-SrandPbisotopesanalyzedinK-feldspar(tables3,4).TocalculateinitialSrratios(Sr),wecorrectedfortheinsitudecayofRbusingtheU-PbagesoftherocksandthemeasuredRb/Srratio.TheinitialPbisotoperatios(Pb)weremeasureddirectlyonK-feldsparmineralfractions,whicharegenerallydevoidofU.ThefeldsparswereleachedwithweakHF/HBrpriortodissolutiontoavoidanalyzingchemicallyalteredcrystalrimsthatpo-tentiallyincludeUandPbfromintergranularvaluesforthethreesamplesliebetween0.70939and0.71639(table4).The253-Magranitecharnockite(VN343)showsthelargestcrustalcon-tribution,whichisinagreementwith(1)itsgraniticcomposition,comparedtothelessdifferentiatedgranodioritic-dioriticcompositionsoftheothertworocks,and(2)thepresenceofsignicantinheritedcomponentsinitszircons(g.4).TheSrturesinthedioriteandgranodioriteindicatealesssignicantcrustalcomponentintheirsourcemag-mas,consistentwiththeirchemicalcharacter.valuesforallthreesamplesareslightlyhigherthanthemodelcurvesforPbevolutioninaverageoruppercontinentalcrust(diagram,g.5BecausePbisbufferedbycontinentalcomponents,whichcarrythedominantportionofPbcomparedtomantlemelts,acontributionofonlyasmallper-centageofuppercrustalmaterialtoabasalticmagmadominatesthePbisotopecompositionatthetimeofplutonemplacement(e.g.,Scha¨rer1991).Oursamples,includingthediorite,thereforeplotalongthemodelcurvesforcontinentalcrustbutinaslightlyhigherposition,implyingassimi-lationofoldcrustalcomponentswithvalueshigherthanbothaverageandupperconti-nentalcrustvalues(StaceyandKramers1975;Zart-manandDoe1981).ThisconclusionfromthePbsystematicscorroboratesthepresenceof1.1–2.7-Gainheritedcomponentsinthezircons(g.4).An JournalofGeologyKONTUMMASSIF,VIETNAM763 Figure5.diagrams.AlsoshownaremodelcurvesforPb-isotopeevolutionofupper(ZartmanandDoe1981)andaverage(StaceyandKramers1975)continentalcrust.Table4.SummaryofAgesandInitialSrandU-Th-PbSignatures Arage(Ma),U-Pbage(Ma),magmasourcePb()ofmagmasourceTh/U()ofmagmasource Dioriticcharnockite(VN357)2434248.81.7.710829.503.92Graniticcharnockite(VN343)…252.51.5.716399.643.90Granodiorite(VN386)3398451.22.6.709399.673.74Orthogneiss(VN387)3806…………Dioriticamphibolite(VN388)42410………… Note.(iscalculatedfromtheRb-Srdatalistedintable3andtheU-Pbagelistedabove.arecalculatedfromthePbdatalistedintable3andtheU-Pbagelistedabove,usingasingle-stageevolutionmodelofa4.56-Ga-oldEarthhavingPbinitialisotopicratiosasmeasuredintheCanyonDiabloFe-meteorite(Tatsumotoetal.1973).diagramofPb(g.5)revealsthatmagmasourcesforthecharnockites(VN343,VN357)areenrichedinThrelativetoU(Th/U[]:3.92and3.90;table4).Theseratiosarehigherthantypicalofmodelupperoraveragecrust,possiblyindicatingassimilationoflowercrustmaterial(i.e.,depletedinU).Incontrast,theoldergranodiorite(VN386)hasaratio(3.74)thatsuggestsmagmacontribu-tionfromtypicaluppercrust.ArBiotiteAnalyses.Foursampleswereir-radiatedwithuxmonitorsattheOsirisReactor(CEA)inSaclay,France.AnalyseswereperformedbystepwiseheatingofpuriedbiotiteseparatesandmeasuredusingamodiedThomsonraregasmassspectrometer.Analyticalprocedurescanbefoundinearlierpublications(Maluski1989;Maluskietal.1995;Lepvrieretal.1997).Dataarepresentedintable5(tables1,3,and5areavailablefromJournalofGeologyDataDepositoryfreeofchargeuponrequest)andillustratedingure6.AplateauageofMa(2)isgivenbythe243lastthreeheatingstepsofcharnockiteVN357(g.).Thisageaccountsforabout50%ofthedegassedbetween1050Candthetemperatureofcompletefusion.TheagecalculatedfortheentiredatasetisMa,whichoverlapswiththe239formervaluegivenuncertainties.Agesreachtherangeoftheplateauvaluebylowtemperatures(e.g.,Maforthe700Cstep).Thissuggeststhatpostcoolingthermaleffectswereinsignicant.Thethreeamphibolitefaciessamples(VN386,VN387,VN388)giveverysimilaragespectrapat-terns.Theirplateauagesarereachedearlyinthestep-heatingprocedure,generallyby650C,andtheplateausincludemorethan75%oftheextractedAr(g.6).PlateauagesareMa,Ma,andMa,respectively.6424Ourgeochronologicresults(table4)showthatmag-matismproducingcharnockitesandamphibolitefaciesintrusionsintheKontummassifismuch 764E.A.NAGYETAL. Figure6.Arplateaudiagramsforsamples()VN357,()VN386,()VN387,and()VN388youngerthanthePrecambrianagespreviouslyin-ferred(e.g.,PhanCuTienetal.1988;Hutchison1989,p.152).TheKontummassifappearstobeacompositeblockmadeupofremnantsofatleasttwomagmaticevents,oneinvolvingPaleozoicmiddlecrustalplutonismandtheothergeneratingPermo-Triassiclowercrustalcharnockites.Itfol-lowsthattheKontummassif,andinparticulartheKannackmetamorphiccorecomplex,didnotriftfromthePrecambriangranulitebeltofGondwana,whichispresentlydispersedinEastAntarctica,In-dia,SriLanka,andAustralia(Katz1993).Itshould,nevertheless,stillbeconsideredtheprinciplepartoftheIndochinablockinpaleogeographicrecon-structions.FollowingageochronologicsummaryofthemagmatichistoryoftheKontummassifasre-cordedinoursamplesandadiscussionofmagmasourcecharacteristics,weexaminetherelationshipbetweenthetectonichistoryoftheIndochinablockandmagmatismrecordedintheKontumregion.MagmaticHistoryintheKontumMassif.Theam-phibolitefaciesgranodiorite(VN386)givesaU-Pbemplacementageof450MaandanArage340Ma.Thedifferenceintheseagesmaybeduetoslowcooling.Alternatively,theweakfoli-ationdelineatedbythebiotitefabricmayhaveformedduringapost-450Ma,low-temperaturetec-tonometamorphiceventthatalsoresettheargonsystematics.Retrogressivegrowthofepidote(pis-tacite)andchloritesupportssuchanevent.BecausebiotitesinsampleVN386shownoevidenceofpar-tialresettingoftheargonsystem(i.e.,nosigni-cantlyyoungerAragesatlow-temperatureextractionsteps),weinferthatsuchareheatingeventprobablyoccurredat340Ma,completelyresettingtheargonisotopesinthebiotiteofthisparticularsample.Thisscenarioisfurthersup-portedbyPblossinthreeslightlydiscordantzirconfractionsinVN386(g.4)thatinterceptconcordiaatupperPaleozoictime.Onthebasisofeldrelationships,andintheabsenceofU-Pbdata,weinferthattheotheram-phibolitefaciesrocks(VN387andVN388)wereem- JournalofGeologyKONTUMMASSIF,VIETNAM765placedcontemporaneouslywithVN386atMa.AllthreesamplesappeartobepartofthesameseriesattheoutcroplevelintheKrongPokoRiverregion,andtherearenotectonicorintrusivecon-tactsbetweenthem.SamplesVN387andVN388Arplateauagesof380Maand424Ma,respectively.Again,theseagesmightreectslowcoolingofOrdovicianintrusions,therateofwhichmusthavevariedbetweenthethreeamphibolitesamplesgiventhethreedifferentArages.If,however,alow-temperaturethermaleventdidoc-curaround340Ma,assuggestedabove,theolderagesofVN387andVN388suggestonlypartialar-gonresetting.IncontrasttoVN386,theagespectraobtainedfromthebiotitesinVN387andVN388showevidenceofArdiffusionatlowdegassingtem-peratures,correspondingtolowagesrelativetotheplateauagesandsupportingpartialresettingbyapostemplacementtemperaturerise.Clearly,themostsurprisingresultfromtheam-phibolitefaciesmagmaticsuiteisthePaleozoic,ratherthanPrecambrian,U-PbandArages.OuralternativeinterpretationtosimplecoolingofOrdovicianintrusionsatvariousrates,whichisthatasubsequentthermalevent(340Maoryounger)resettheargonsystematicstovaryingde-grees,isspeculativebutcertainlyaviablepossi-bilitygiventheargonanddiscordantzirconresults.FaureandFontaine(1969)alsosuggestedalow-temperaturereheatingeventbasedonaPaleozoicK/Arage(398Ma)determinedforthesamegran-odioriteassampleVN386.Wewouldfavoranear-lierage(340Ma)fortheactualreheatingeventbasedontheobservationsoutlinedabove.ThecharnockitesamplefromtheKannackcom-plex(VN357),previouslyassignedanArcheanage(e.g.,PhanCuTienetal.1988),yieldssignicantlyyoungeragesof249Ma(U-Pb)and243Ma(Ar)(table4).TheconvergenceofagesbythetwodifferentradiometricsystemsimpliescoolingtoCwithin6Ma,indicatingrapidcrystalli-zationandcooling(40–60C/Ma)duringPermo-Triassictimes.AsimilarU-Pbage(253Ma)forthecharnockitesamplefromthesouthernKontummassif(VN343)isinagreementwithpreviouslyre-portedPermo-TriassicK/Ar(PhanCuTienetal.1988)andAr(MaluskiandLepvrier1998;Maluskietal.1999)agesfromthisregion.Resultsfromthetwogeochronometerssuggestrapidex-humationofthenewlygeneratedlowercrust,whichis,apriori,afundamentalrequirementtobringcharnockiticrocksthroughtheamphiboliteandgreenschiststabilityeldstothesurface.Itissignicantthattheamphibolitefaciessam-plesyieldingmiddlePaleozoicAragesintheKrongPokoRiverregionshownoeffectsfromthePermo-Triassicthermaleventthatisrecordedinthenearbycharnockitesamples.Itseemslikelythatthereisamajor,post-250-MastructureacrosswhichthePaleozoicandPermo-Triassicrocksarejuxtaposed.Giventhecomplexityoffaultsshownincurrentgeologicmaps(e.g.,g.1),itislikelythatoneormoresuchstructuresispresentintheMagmaSources.RelicinheritedcomponentsinzirconsfromcharnockitesamplesVN357andVN343showU-PbevidenceforProterozoic,andperhapsevenArchean,crustalsourcematerialpre-sentinthemagmasourceregionorassimilateddur-ingmagmaascentthroughthelowercrust.InitialSrandPbsignatures(tables3,4;g.5)alsoindicatetheinvolvementofoldercontinentalcrustinthegenesisofboththe250Maand450Mamagmas.Someoldercrustalmaterial,potentiallyintheformofsediments,musthavebeenpresentduringbothorogeniccycles;however,thisdoesnotimplythatPrecambriancrustisexposedatthesurfaceoftheKontumcomplex.Althoughthecalc-alkalinein-trusionsfromKontummostlikelyrepresentmix-ingofmantlewithcrustal-derivedmagmas,suchasindicatedbytheSrsignaturesandthedioriticcompositionofVN357,thenatureofmeltedcrustcannotberigorouslydetermined.Pbisotopicval-uesimplythatcrustalsourcematerialsdidnothavesignicantlyevolvedsignaturesatthetimeofmelt-ing,whichiscompatiblewithmagmaformationwithinandfromthelowercontinentalcrust,trig-geredbyinjectionofbasalticmagmasfromthemantle,agenerallyacceptedscenarioforthefor-mationofacalc-alkalineseries(e.g.,DePaoloetal.1991).Evidently,thishypothesisisbasedonalim-iteddatasetandtakesintoaccountthetectonicItisusefultodistinguishwhetherthecharnock-ites(VN357andVN343)aremetamorphic(e.g.,de-hydrationproductsofamphibolitesduringgranu-litefaciesmetamorphism)origneous(e.g.,meltingofafertilegranulitethatwasdehydratedduringanearliermetamorphism)becausetheformerisin-ferredtooccurat700–800C(e.g.,Hansenetal.1987),whereasthelattercanoccurattemperaturesashighas1000C(KilpatrickandEllis1992).ThelowSiOcontentofcharnockiteVN357(58%;table1)suggeststhatitmaybeaderivativemagmaofcumulatesassociatedwithmagmaticcharnockites,whoseparentalcompositionsaregenerally62%(KilpatrickandEllis1992),whereasthehigh-charnockite(VN343)mightrepresentacon-temporaneousfractionatedmagmaextractedfromthecharnockiticparent.Therelativelyhighabun- 766E.A.NAGYETAL.dancesofKOandLILE(largerionlithophileele-ments)andlowabundanceofCaOinVN343sup-portamagmatic(Ctype),ratherthanmetamorphic,charnockitesource(KilpatrickandEllis1992).Thecrystallizationofbiotitepriortohornblendeisalsocharacteristicofigneouscharnockites(KilpatrickandEllis1992).Perhapsmostsignicantly,gran-ulitemetamorphismwouldnotproducethenec-essarytemperaturesandpressuresrequiredtoresettheU-Pbsystemswithinthezirconstogenerateconcordant,ornearlyconcordant,grains(g.4Hightemperatures,suchasthoseinferredfortheemplacementofmagmaticcharnockites,canbeat-tainedbylargevolumesofbasaltinjectionintothelowercrustorbyprocessesoccurringwithinthebasallevelsofthickenedcrust,suchasproducedinmajorcontinentalcollisionzones(KilpatrickandEllis1992).Thislendssupporttoasyn-collisionalmagmaticoriginfortheserocksduringamajorPermo-Triassicorogenesis(discussednext).RelationshipbetweenMagmatismandTectonicHis-tory.ThePhanerozoictectonichistoryofIndo-chinaiscomplicatedandcontroversialbecausedatafromtheregionarerare.NumerousPaleozoicriftingeventstoreapartthenorthernmarginofGondwana,producingmicrocontinentsthateven-tuallycollidedintoAsiaanderadicatedmuchoftheearliergeologicrecord.Thelackofgeochron-ologicconstraintson,andscarcityof,obductedophiolitesmakesitdifculttoconstrainthetimingofthenumerousocean-formingeventsandmicro-plateseparations.Mostpaleogeographicrecon-structions,consequently,relyonthepresenceorabsenceoforalandfaunalassemblageslinkingvariouscontinentalblocks.AbriefoverviewofthetectonichistoryoftheIndochinablock,whichin-cludesthecentralandsouthernpartsofVietnam,isprovidedhereandshowshowourgeochronologicresultsmayconstrainthetimingofvariouscolli-sionalevents.Itisgenerallyagreedthat,bytheearlytomiddleCarboniferous,theIndochinaandSouthChinablocksamalgamatedalongtheSongMafaultzone(SMFZing.1,inset)innorthernVietnamandYunnan(e.g.,Hutchison1989;Metcalfe1996;Sen-¨randNatal’in1996).YinandNie(1996,p.473)suggestthatthesemicroplatescollidedinthelateTriassic;however,thereissignicantevidenceforaCarboniferoussuturebetweenthetwoblocks.Ac-cordingtoHutchison(1989),theAnnamiticplu-tonicarcformedabovearegionofsubductioninthelateDevonian,andsuturingwascompletewiththeformationoftheTruongSonfoldbelt(g.1,inset)andtheemplacementof330-Magranites.TheagesdiscussedbyHutchison(1989)arepre-dominantlyK-Ardeterminationsthattheauthoradmitsneedtobeveriedwithmorerigorousra-diometrictechniques.AdditionalevidenceforaLateDevonian/EarlyCarboniferousagefortheSongMasuturezoneincludesmiddleCarbonifer-ouscarbonatesthatblanketthezone,differentpre-middleCarboniferousfaunasoneithersideofthezoneoverlainbysimilarmiddleCarboniferousfau-nas,andlarge-scalefolding,thrusting,andnappeformationintheearlytomiddleCarboniferous(Metcalfe1998).WehavesuggestedthattheSiluriantoCarbon-Aragesforthethreeamphibolitefa-ciesrocksfromtheKrongPokoRiverregioninwesternKontum(VN386,VN387,VN388)mayre-ectalow-temperature(tectonometamorphic)re-heatingeventofOrdovicianintrusionsduringtheEarlyCarboniferous(340Ma).Itseemsreasonablethatthisthermaleventcorrespondstoatempera-tureincreaseinthecrustdirectlyrelatedtocrustalthickeningandmicroplatecollisionalongtheSongMasuture.Ifthisisthecase,itimpliesthattheKontumblock,whichwasformerlydescribed(e.g.,TranVanTrietal.1979)asanindependentunitfromtheTruongSonfoldbeltnorthoftheTamky–PhuocSonfaultzone(g.1andinset),wasinfactincontactwiththeTruongSonbeltpriortoPermo-Triassictimesandsubsequentlysufferedthesametemperatureanddeformationhistory.AmajorPermo-TriassiccollisionalandsuturingeventbetweenIndochinaandSibumasu(includingpeninsularMalaysia,Thailand,Burma,andYun-nan)closedthePaleo-TethysSea.Werefrainfromusingtheterms“Indosinian”or“Cimmeride”orogenyinordertoavoidsomecontroversysur-roundingtheirappropriateness(e.g.,Hutchison1989;Sengo¨randNatal’in1996),althoughitisclearthatwidespreadorogenicandmagmaticeventsoc-curredinIndochinaduringlatePaleozoic/earlyMesozoictimes.ThereisgeneralagreementthattheclosingofthePaleo-TethysSea,whichpro-ducedseveralmajorN-to-NW-trendingsuturezones,occurredintheTriassicwithorogenesiscon-tinuingintotheJurassic(e.g.,Hutchison1989;Met-calfe1998).AfairamountofArandU-Pbdatahascon-rmedthekeyroleofthisPermo-Triassictecton-ometamorphiceventintheNW-strikingTruongSonfoldbelt(g.1)betweentheSongMasutureandnorthernKontum(Maluskietal.1995;Lep-vrieretal.1997;NagyandScha¨rer1999;TranNgocNametal.2001),aswellasintheSongChaymassif(g.1,inset)northoftheRedRiverfaultzone(Ma-luskietal.1999,1999).AregionalstudybyLep- JournalofGeologyKONTUMMASSIF,VIETNAM767vrieretal.(1997)foundconsistentArplateaucoolingagesonmicasaround245Maforcalc-alkalinemagmaticrocksandhigh-grademeta-morphicrocksdistributedthroughoutNW-trend-ingshearzonesinVietnamnorthoftheKontumEmplacementagesdeterminedinthisstudyof249Ma(VN357)and253Ma(VN343)forchar-nockiticintrusionsintheKontummassifareingoodagreementwiththeagesfoundtothenorth.ThePermo-Triassicintrusions,thus,probablyrep-resentcollision-related,batholithic-typemagma-tismrelatedtotheclosingofthePaleo-TethysSea.Trace-elementgeochemistryofVN343(table1)issimilartothatfoundinothersyn-collisionalgran-itesthatexperiencedvolatile-inducedenrichmentinelementssuchasRbandTa(Pearceetal.1984).ThesimilaritybetweentheU-PbandAragesforthetwocharnockitessuggestsrapidexhuma-tionofnewlygeneratedcrust,whichmostlikelyoccurredduringoraftercollision.Rapidmigrationofthecharnockitefromthemagmasourceregionmayhavebeenfacilitatedbypreexistinginclinedshearzonesformedduringearliermicroplatecol-lisionsorevenbyactivecrustal-scaleshearzonesmovingcontemporaneouslyduringemplacement.Inasimilarmanner,activetranscurrentshearzoneswereperhapsresponsiblefortheemplacementandupwardmigrationofaPan-Africangranite-charnockiteplutoninNigeria(Ferre´etal.1997).OurresultsextendtothesouththeknownregioninIndochinaaffectedbytheextensivePermo-Triassicorogeny.ThetectonometamorphiceventisroughlycontemporaneouswithclosureofalargebasinthatformedtheE-to-W-trendingQinling-DabieshanorogenicbeltbetweentheSouthandNorthChinablocks.TheageofthiscollisioninChinaismostlikelyearlyTriassic,possiblyiniti-atinginthelatestPermian(e.g.,Eideetal.1994;YinandNie1996;Rowleyetal.1997;Hackeretal.1998);thusitappearsthatseveralmicroplatecollisionsoccurredsimultaneouslyinAsiaduringPermo-Triassictimes.ThisdiscussionimpliesaboveallthattheKon-tummassifdidnotriftfromthePrecambriangran-ulitebeltofGondwana.Rather,itrepresentsare-gionofPaleozoicandPermo-Triassicmagmatism.ThePaleozoicrocksfromtheareaaroundtheKrongPokoRiver(g.1)weresomehowprotectedfromthethermalanddeformationaleffectsoflaterPermo-Triassicmagma-generatingorogenesis,sug-gestingpost-250-MatectonicjuxtapositionofthesedifferentpartsoftheKontummassif.OurresultsfromtheKontummassifquestionthePrecambrianageassignedtoothermassifsinthearea,suchastheChonBurimassifinThailandandthePalinmassifinCambodia(Hutchison1989).WehavemuchtolearnandupdateregardingthegeologyofIndochina,whichhasbeenrelativelyinaccessiblefordecades.GeochronologicdatafromtheKannackcomplexintheKontummassif,Vietnam,implythattheAr-cheanagecommonlyinferredfortheregionisnolongersupported.Instead,newArandU-PbagesindicatethatcharnockitesandamphibolitefaciesrocksformedfromOrdoviciantoPermo-TriassictimesandthusdidnotriftfromthePre-cambriangranulitebeltofGondwana.Wenote,however,thatthepresenceofacrustalsourcebear-ingPrecambrianmaterial,potentiallyintheformofsediments,issubstantiatedbyinheritedcom-ponentsretainedinthezirconsanalyzedforU-Pb.Mostsignicantly,theseresultsconrmthesig-nicantroleofawidespreadmagma-generatingorogenythatbeganintheLatePermian/EarlyTri-assicandwastheresultofclosingofthePaleo-TethysSea.TheabsenceofthermaleffectsintheamphibolitefaciesrocksbythePermo-Triassicoro-genesisimpliespost-250-MatectonicjuxtapositionofthesedifferentpartsoftheKontummassif.Priortothisstudy,noevidenceexistedfora450-MamagmaticeventintheIndochinablock.TherelativelylowYandYbconcentrationsinVN386aresimilartoconcentrationsfoundingranitesformedinvolcanicarc(i.e.,subductionrelated)set-tingsratherthaninwithin-plategranitesandocean-ridgegranites(Pearceetal.1984).Ontheotherhand,arift-relatedoriginforVN386issup-portedbytraceelementgeochemicalpatternsthat,normalizedtoocean-ridgegranites,arequitesim-ilartowithin-plategranitesintrudedintostronglyattenuatedcrust(e.g.,relativelyhighconcentra-tionsofK,Rb,andThandvaluesofHf,Zr,andSmclosetothenormalizingvalues;Pearceetal.1984).TheearlyPaleozoichistoryoftheIndochinablockisuncertain.SomeauthorsfavorOrdovicianorSi-lurianriftingofIndochinafromGondwana(Hutch-ison1989,p.126),whereasotherssuggestthatrift-ingoccurredinLateDevoniantimesfollowingmajorintracontinentalextensionofthenorth-westernAustralianshelfintheEarlyOrdovician(Metcalfe1996,1998andreferencestherein).Ifthecalc-alkalineOrdovicianmagmatismidentiedherewasgeneratedinavolcanicarcsetting,thiswouldnotendorseearlyriftingscenarios.Instead,itmightsupportasouth-to-northprogressionofsubduction-relatedmagmatismfromtheIndochina 768E.A.NAGYETAL.blocktotheSouthChinablockwhere420–435-Mamagmatism(Lietal.1989;Li1994;Rogeretal.2000)hasbeenassociatedwithaphaseofgra-niticmagmatismthatoccurredduringamajormid-dleSilurian(“Caledonian”)collisioninsoutheastChinathatproducedwidespreadfolding,meta-morphism,andgraniticemplacement(e.g.,Hutch-ison1989).OurgeochronologicresultsdrawintoquestionProterozoicandearliestPhanerozoicK-ArandRb-SragespreviouslyreportedfortheKontummassif(e.g.,PhanTruongThi1985;TranQuocHai1986;Hutchison1989).Inseveralofthesepublications,theagesaregivenwithoutsupportingdatatablesorobtainablereferences,andinsomecases,wholerockratherthansinglecrystalagesweredeter-mined,whichcanbelessreliableindicatorsoftheageofcrystallization.HurleyandFairbairn(1972)reportedarelativelyyoungRb-Srwholerockage(530Ma)foragroupofrockssampledbetweenDaNangandtheKontumregion(g.1,inset).Theirresultsarequestionable,however,becauseofthelargegeographicdistributionofthesamples,theirdifferentoriginandnature,theirestimatedinitialratioof(considerablylowerthananyvaluesanalyticallydeterminedhere;seetable4),andthefactthatmanysamplesintheirstudydonotfallontheirreferenceisochron.Furthergeo-chronologyisneededtoshowthatArchean,orevenPrecambrian,crustexistsinIndochina.Presently,onlysmallslicesofheavilyoverprintedPrecam-briangneisseshavebeenidentiedinVietnamalongtheRedRiverfaultzone(g.1).ThegenerallyacceptedPrecambriancrystallizationageoftheBuKhangcomplexincentralVietnamhasrecentlybeenshowntobeCenozoic(Jolivetetal.1999;Nagyetal.1999,2000).Signicantly,arecentstudybyLanetal.(2000)ndsNdmodelagesfortheKannackcomplexof2.0–1.5Ga,arguingagainstArcheancrustalformation.WearesincerelygratefultoourVietnamesecol-leagues,includingTongDzuyThanh,NguyenVanVuong,TrinhVanLong,andNguyenXuanBao,fortheinvaluablehelptheyhaveofferedduringmanydifferenteldtrips.WethankF.Bodetforrewritingdata-processingprogramsandhelpfuldiscussions,L.delaCruzforhelpwithRb-Sranalyses,andF.RogerforprovidingapreprintofherresultsfromtheSongChaymassif.Wethankthreeanonymousreviewersforhelpfulsuggestions.ThisworkwassupportedbyCentreNationaldelaRechercheScientique-INSUthroughtheProgrammeInter-nationaldeCoope´rationScientique“Vietnam”andpartiallyfundedbyaNationalScienceFoun-dationInternationalResearchFellowshiptoE.A.Nagy. 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