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ANALYTICALMETHODS

Agedeterminationsbylaser40Ar/39Aranalysiswereperformedonsinglecrystalsofplagioclase,amphibole,andbiotite,whichwerehandpickedfromcrushedandsievedsplitsofSamples161977A60X1,07cm,61X1,07cm,62X1,1319cm,and978A47R1,06cm.Mineralseparateswereetchedin15%hydrofluoricacid(plagioclaseandamphibole)andcleanedusinganultrasonicdisintegrator(allseparates).Sampleswereirradiatedfor72hoursataneutroncurrentof2.1x1012N/cm2atthe5MWreactoroftheGKSSResearchCenter(Geesthacht,FederalRepublicofGermany),withcrystalsinaluminumtraysandirradiationcanswrappedin0.7mmcadmiumfoil.

Analysesof40Ar/39ArwereconductedattheGeomarTephrochronologyLaboratory.Plagioclaseandamphibolecrystalswerefusedinasinglestep,whereasanalysesofbiotitewereperformedbylaserstepheating(40mW2000mW).PurifiedgassampleswereanalyzedusingaMAP216seriesnoblegasmassspectrometer.Rawmassspectrometerpeakswerecorrectedformassdiscrimination,background,andblankvalues(determinedeveryfifthanalysis).TheneutronfluxwasmonitoredusingTCRsanidine(TaylorCreekRhyolite,27.92MaDalrympleandDuffield,1988DuffieldandDalrymple,1990)andinternalstandardSAN6165(0.470MavandenBogaard,1995).VerticalvariationsinJvalueswerequantifiedbyacosinefunctionfit.LateralvariationsinJwerenotdetected.CorrectionsforinterferingneutronreactionsonCaandKarebasedonanalysesofopticalgradeCaF2andhighpurityK2SO4saltcrystalsthatwereirradiatedtogetherwiththesamples.Correctionfactorsweredeterminedas36Ar/39ArCa=0.4650.00137Ar/39ArCa=977238Ar/39ArK=0.02220.0001and40Ar/39ArK=0.0020.001.Analyticalerrorsarequotedas1 .

Formajorandtraceelementanalyses,thevolcanicpebbleswerecleaned,crushedto0.5cmsize,handpicked,andthenwashedwithdistilledwater.Wholerockchipsweregroundtorockpowderinanagatemill.BothH2OandCO2contentsweredeterminedusingaRosemountInfraredPhotometerCSA5003.Bulkrockanalysesofmajorandtraceelements(Ba,Rb,Sr,Zr,V,Cr,Co,Ni,Cu,Zn,andGa)wereperformedonfusedtablets.ThetabletswereanalyzedattheGEOMARResearchCenterusingaPhilipsXuniquePW1480Xrayfluorescencespectrometer.InternationalreferencestandardsRGM1(rhyolite),BM(basalt),SY2(syenite),andSDC1(micaschist)werealsoanalyzedandareusedtoevaluatetheprecisionandaccuracyofthemeasurements(Table2).Themaximumstandarddeviationofthestandardsis0.20wt%forSiO2and0.02wt%forallothermajorelementsexceptNa2O(0.10wt%).ForthetraceelementsmeasuredbyXRF,themaximumstandarddeviationis5ppm,withtheexceptionofBa,forwhichitis14ppm.

Additionaltraceelements(Pb,Th,U,Hf,Nb,Ta,Y,andallrareearthelements[REE])wereanalyzedusinganupgradedVGPlasmaquadPQ1ICPMSattheGeologicalInstitute,UniversityofKiel,afterthemethodofGarbeSchoenberg

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(1993).TwoblanksandinternationalstandardsBHVO1(basalt),RGM1(rhyolite),andSTM1(syenite)wereanalyzedwiththesamples.ResultsarepresentedinTable3.Precisionoftheresultswasestimatedfromduplicatedigestionsandduplicatemeasurementsandwasfoundtobe