Geochemical and Mineral Characteristics of Jurassic Volcanic Rocks fromODP Sites 304, 1149, and 801： Implications for Magmatic Evolution in the Northwest Pacific

TheregularvariationsinmagmaticactivitiesalongtheNorthwestPacificplatehavebeenlittlestudiedinspiteoftheirimportance.Inthiscontribution,systematicanalyseswereconductedontholeiiticbasaltsfromthreeOceanDrillingProgramsites（Sites304,1149,and801）,includingthepetrographicfeatures,majorandtraceelements,Ndisotopiccompositions,andmineralstructureandcompositionsofwholerocks.VolcanicrocksfromSites304,1149,and801belongtotholeiitesandexhibitdepletedlightrareearthelements（LREE）,largeionlithophileelementalcontents（LILE）,andrelativelydepletedNdisotopicratios（143Nd/144Nd=0.513139-0.513211）,similartothoseofnormalmid-oceanridgebasalts（N-MORB）.Comprehensivedataonmineralcompositions,whole-rockgeochemistry,andgeochronologydemonstratethataregularvariationtrendexistsinthenorth-southdirectionalongtheNorthwestPacificplate.The143Nd/144Ndvalues（0.513139-0.513211）andtrace-elementratiosforwholerocks（Sm/Th=15.35-30.00;Zr/Hf=28.53-35.76;Zr/Y=2.58-3.67;Th/La=0.04-0.06;Th/Y=0.33-0.70）,aswellasthetrace-elementratios（Zr/Hf,La/Yb,Ti/Zr）ofclinopyroxenesfromSites1149and801tholeiitesshowlargervariationscomparedtothosefromSite304tholeiites（143Nd/144Nd=0.513185-0.513195;Sm/Th=18.19-20.58;Zr/Hf=31.07-33.26;Zr/Y=2.62-3.03;Th/La=0.05-0.06;Th/Y=0.48-0.57）.MineralzoningtextureswereobviousintholeiitesfromSites1149and801butwererarelyobservedinSite304.Theseregularfeatureswerelikelyattributedtothedifferencesintheheterogeneityofthemagmasource,theprocessofmagmatieevolution,theplate-spreadingrate,andtheeffectiveandineffectivemixing.