GeochemicalCharacteristicsofVolcanicRocksfromODPSite794,YamatoBasin:ImplicationsforDeepMantleProcessesoftheJapanSea

DeepmantleprocessesandthedynamicmechanismofmagmatismintheJapanSeaBasinareimportantprocessesthathavenotbeenstudiedindetail.Inthispaper,systematicevaluationofbasaltsamplesfromtheoceandrillingprogramSite794intheJapanSeawasperformed,whichincludedpetrography,whole-rockmajor-andtrace-elementanalysis,Sr?Nd?Pbisotopiccomposition,andelectronmicroprobeanalysisofplagioclaseandclinopyroxene.ThesebasaltsbelongtothetholeiiticserieswithporphyritictextureandmassiveCa-richplagioclase,clinopyroxene,andminorolivinephenocrysts.Thebasaltsarecharacterizedasflatrareearthelementsandhigh-field-strengthelementswithremarkablylowratiosof(La/Yb)N(0.75？2.51),significantpositiveanomaliesofBa,Sr,andRbandnoEuanomaly(δEu=0.99？1.36).Thesamplesshowedrelativelyhigh87Sr/86Sr(0.70425？0.70522),207Pb/204Pb(15.511？15.610),and208Pb/204Pb(38.064？38.557)valuesandalow143Nd/144Ndratio(0.51271？0.51295).ThebasaltsfromSite794canbedividedintoupper,middle,andlowervolcanicrocks(UVR,MVR,andLVR)onthebasisoftheirstratigraphiclevel.TheMVRwasgeochemicallyderivedfromthedepletedmantle,whereastheUVRandLVRoriginatedfromanondepletedandrelativelyenrichedmantlesourcewithcontributionsfromsubductedPacificplatefluidandsediments.UseofgeothermobarometersindicatesthatthecrystallizationpressurefortheUVRandLVR(6.25？11.19kbar)wassignificantlyhigherthanthatoftheMVR(3.48？5.84kbar).TheUVRandLVRmayhavebeenderivedfromthelow-degree(5%？10%)partialmeltingofspinellherzolite,whiletheMVRoriginatedfromashallowermantlesourcewithahighdegree(10%？20%)ofpartialmelting.Inaddition,thegeochemicalcharacteristicsofthesamplesareconsistentwithayoungerage(13？17Ma)andthedepletedcompositionoftheMVRandanolderage(17？23Ma)andslightlyenrichedcompositionoftheUVRandLVR.Therefore,temporalchangesinthemantlesourcefromoldandenrichedtoyounganddepletedandsubsequentlytooldandnondepletedmayhavebeenassociatedwithprogressivelithosphericextensionandthinning,aswellasatleasttwoepisodesofdiverseasthenosphericupwellingandpull-aparttectonicmotionintheYamatoBasin.