Trace-element-characteristics-of-graywackes-and-te

ContribMineralPetrol(1986)92:181-193ContributionstoMineralogyandPetrology9Springer-Veriagt986TraceelementcharacteristicsofgraywackesandtectonicsettingdiscriminationofsedimentarybasinsMukulR.Bhatia*andKeithA.W.CrookDepartmentofGeology,AustralianNationalUniversity,CanberraA.C.T.2600,AustraliaAbstract.ThegraywackesofPaleozoicturbiditesequencesofeasternAustraliashowalargevariationintheirtraceelementcharacteristics,whichreflectdistinctprovenancetypesandtectonicsettingsforvarioussuites.Thetectonicsettingsrecognisedareoceanicislandarc,continentalislandarc,activecontinentalmargin,andpassivemargins.Immo-biletraceelements,e.g.La,Ce,Nd,Th,Zr,Nb,Y,ScandCoareveryusefulintectonicsettingdiscrimination.Ingeneral,thereisasystematicincreaseinlightrareearthelements(La,Ce,Nd),Th,NbandtheBa/Sr,Rb/Sr,La/YandNi/CoratiosandadecreaseinV,ScandtheBa/Rb,K/ThandK/Uratiosingraywackesfromoceanicislandarctocontinentalislandarctoactivecontinentalmargintopassivemarginsettings.Onthebasisofgraywackegeo-chemistry,theoptimumdiscriminationofthetectonicset-tingsofsedimentarybasinsisachievedbyLa-Th,La-Th-Sc,Ti/Zr-La/Sc,La/Y-Sc/Cr,Th-Sc-Zr/10andTh-Co-Zr/10plots.Theanalysedoceanicislandarcgraywackesarechar-acterisedbyextremelylowabundancesofLa,Th,U,Zr,Nb;lowTh/UandhighLa/Sc,La/Th,Ti/Zr,Zr/Thratios.ThestudiedgraywackesofthecontinentalislandarctypesettingarecharacterisedbyincreasedabundancesofLa,Th,U,ZrandNb,andcanbeidentifiedbytheLa-Th-ScandLa/ScversusTi/Zrplots.ActivecontinentalmarginandpassivemargingraywackesarediscriminatedbytheTh-Sc-Zr/l0andTh-Co-Zr/10plotsandassociatedparame-ters(e.g.Th/Zr,Th/Sc).Themostimportantcharacteristicoftheanalysedpassivemargintypegraywackesisthein-creasedabundanceofZr,highZr/ThandlowerBa,Rb,SrandTi/Zrratiocomparedtotheactivecontinentalmar-gingraywackes.IntroductionTraceelementgeochemistrycontributessignificantlyinde-cipheringthephysicalandchemicalprocesses(e.g.partialmelting,fractionation,mixingandcontamination)intheevolutionofmagmaticrocks.Understandingthechemicalvariabilityoftheserockshasalsoledtotheidentificationofchemicalsignatureswhicharecharacteristicofvariousplatetectonicregimes.Variousschemesoftectonicsettingdiscriminationofmagmaticrockshavebeenestablishedon*PresentAddress:ElfAquitainePetroleumAustraliaPty.Lim-ited,P.O.Box725,NorthSydneyNSW2060AustraliaOffprintrequeststo.M.R.Bhatiathebasisoftraceelementswhichareregardedasinsensitivetoalteration(e.g.PearceandCann1973;WinchesterandFloyd1977;Woodetal.1979;Bailey1981).Thecompositionofsedimentaryrocksmakesimportantcontributionsininterpretingpre-Mesozoicplatesettingsbe-causemanyprovenanceregionshavebeendestroyedandtheonlyrecordliesinthesedimentsderivedfromthem.Thus,therelationshipbetweensedimentcompositionandplatetectonicsprovidesapowerfultoolforrecognisingan-cienttectonicsettings.Manystudieshaveshownthatframeworkmodesofterrigenoussandstonesreflectprove-nancedifferencesthatdependuponplatetectonicsettings(Crook1974;Schwab1975;DickinsonandSuczek1979).Theframeworkgrainsofsandstonesareliabletobemodifiedwithburialandmetamorphismandassuchcannotalwaysbeusedtoinfertectonicsettingsandprovenancetype.Themajorelementgeochemistryofsandstoneshasbeenfoundtoreflectthetectonicsettingofthebasin(Bha-tia1983).Thetraceelements,particularlythosewhicharerelativelyimmobile,canalsobeveryuseful.ElementsLa,Ce,Nd,Y,Th,Zr,Hf,Nb,TiandScaremostsuitedforprovenanceandtectonicsettingdetermi-nationsbecauseoftheirrelativelylowmobilityduringsedi-mentaryprocessesandtheirlowresidencetimeinseawater(Holland1978).Theseelementsaretransportedquantita-tivelyintoelasticsedimentaryrocksduringweatheringandtransportationandthuswouldreflectthesignatureoftheparentmaterial(McLennanetal.1983b).However,suchstudiesrelatingtraceelementgeochem-istryofsedimentaryrockstotheprovenanceandtectonicsettingareveryfew(e.g.BhatiaandTaylor]981;Petermanetal.1981).Suchanapproachsufferssignificantlybecauseoflackofdataonsedimentsofmoderncontinentalmarginandoceanicbasins.ThepurposeofthepresentpaperistodocumentthevariationinthetraceelementgeochemistryofgraywackesfromwelldefinedsuitesofeasternAustraliaandtoproposecriteriaforthediscriminationofprovenanceandtectonicsettinginsedimentarysequencesofcontinentalmarginsandoceanicbasins.Thetermgraywackehasbeenusedinthepresentworktodenotesandstonesofwidemineralogicalrange,nowformingpartofmobilbelts,andoncedepositedinadeepseaenvironmentbyturbiditycurrentsorothermass-flowprocesses(Crook1974).Thesesedimentsarenormallychar-acterisedbysedimentarystructuresrepresentativeofBoumasequence(Bouma1962).Typicallythesandstonesaremoderatelytopoorlysortedwithappreciablesecondarymatrix(epimatrixorpseudomatrixofDickinson1970).182140Fig.1.LocationmapofthestudiedsedimentarybasinsandsuitesineasternAustralia.1:TamworthTrough;2:HillEndTrough(includesHillEndandCookmansuites);3:HodgkinsonBasin;4:BendigoTroughThepresentworkisbasedongeochemicalanalysesofthefollowingPaleozoicgraywackesuitesofeasternAustra-lia(Fig.1):Tamworth(Devonian),HillEnd(Silurian-De-vonian),Hodgkinson(Devonian),Bendigo(Ordovician)andCookman(Silurian).Assho