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GeochemicaJlournal,Vol.31,pp. 135to154,1997 Agerelationsandpetrologyofalkalicigneousrocks fromOldDozen,SouthwestJapan PAULA.MORRIS,1,T6ETSUMARIUTAYA,2SHIGERUIIZUMI,1HIROOKAGAMI3, R.JOHNWATLING4andHISASHIMURAKAMI5 1DepartmenotfGeoscienceS,himaneUniversity,1060NishikawatsuM,atsue690,Japan 2ResearchInstituteofNaturalSciences,OkayamaUniversityofScience,1-1Ridaicho,Okayama700,Japan 3lnstituteforStudyoftheEarth'sInterior,OkayamaUniversity,Misasa,Tottori982-02,Japan 4ChemistryCentreofWesternAustralia,125HayStreet,EastPerth6004,WesternAustralia 5CosmoKensetsuConsultancyY,aoNishimachi,Saigo,OkiDogo685,Japan 6GeochemistrySection,GeologicalSurveyofWesternAustralia,100PlainStreet,EastPerth6004,WesternAustralia (ReceivedJuly 22,1996;AcceptedDecember2,1996) K/Aragedata,whole-rockmajor,traceandrareearthelementgeochemistry8,7Sr/86Srand143Nd/144Nd isotopechemistry,andcomputermodellinghavebeenusedtoclarifythetimingandpetrogenesisoflate MioceneigneousactivityonOkiDozenIslandintheJapanSea.Commencingatabout7 Ma,quartz syenitewasintrudedonthesoutheasterpnartofpresen-tdayNishinoshi.maAtthesametime,compositionally similarmagma(trachyte)waseruptedaslavaandpyroclasticflows.At approximatel6yMa,alkaliolivine basal-ttrachyandesitweaserupted,formingavolcanicshield(sommaphase.)Followingthis,thelocusof volcanicactivity,andthecompositionofthemagmachanged,witheruptionoftrachyteandrhyolitefrom satellitevents(parasiticphase).Dikesc,ompositionallysimilartoextrudedmagma,wereintrudetdhroughout thisvolcanicepisod.eK/Ardatingcannotresolvetheindividualphasesofvolcanicactivity,whichpartly overlapswithlateMiocenevolcanismon OkiDog.o Thecompositionofsommaandparasiticphase volcanicswerecontrolledbyfractionatioonfobservedphenocrystphases,butdecouplingoftraceelements andsmallisotopicshiftsindicatethetransitionfromsommatoparasiticvolcanismwasnotaclosed- systemproces.sSyeniteandcomagmaticcentralconetrachytearecompositionallsyimilartoanorogenic granitoidsa,ndmayresultfrommeltingofafelsiclowercrustbyupwellingbasalticmagma. Despite weakageoverlap,divergenttraceelementandisotopictrends,andsmalldifferencesinrareearthelement chemistryindicatethatalkalicvolcanicrocksonOkiDozenandOkiDogotappeddifferenmtantlesource.s
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