volcanologypetrologyrocksgeochronologygeochemistryseafloor phenomenaoceanic crustmineralsnortheast Honshu magmatic arcKuroko formationlate Cenozoic ageback-arc spreadingYamato back-arc basin basaltBABBNorthern Honshu rift systemNH riftisland-arc upliftingsea floor basalt lavadolerite sheetAosawa volcanic beltBabame volcanic beltAni volcanic beltKuroko volcanic beltbimodal volcanismcaldera formingTi tholeiitic basaltisland-arc tholeiite typeIATextensive felsic hyaloclastitefelsic/dacitic pyroclasticsrhyolite lava domeintermittent basalt eruptionmudstonepumice explosionpost-ore felsic activitypetrographyaphyric/plagioclase phyric aphanitic featurepost-ore dacitequartz phenocrystnormative compositionQz-Ab-Or-An diagrampre-ore rockandesiteNb-SiO/sub 2/ diagrampetrochemical signaturerelational transitional periodaphyric dacite21 to 18 Myr