347 The C atwdianM iru ralo gis t Vol.35, pp. 347-365 (1997) ASPECTS OF MAGMASOURCES AND PROCESSES IN THEHONSHU ARC DAVIDA. GUST1 School of Geology, Queewlanl University of TechrclogX, GPO BOX 2434, Brisbatrc, Quzenslwtd 4001, Australia RICHARD J. ARCTILUS Nationnl Key Centre for the Geochentical Evolation and Menllogeny of Contincnts, Departnent of Geology, Australian National University, Cotrbena" ACT A00, Australia ANNIEB. KERSTING Isotope Sciences DivisioA Lanvrence Livermore National Laboratory, I-231, P.O. Box 808, Livermore, Califomia 94551, U.S-A.. ABSTRACT The volcanoes of the northernlzu - south-cental Honshuand northeastem Honshuarcs,Japan, occur in a region where vmiables such asage, conposition, andnature ofthe sediment coveroftle subducting plate arerelatively uniform. In this study, v/e attempt to isolate some ofthe variables associated with suprasubduction zone (SSZ)magma genesis, in order to provideinsight into ttre natureof sources andpetrogenetic processes. The chemicalcompositions of selected parental and evolvedmagmas in fifteeu Plio-Pleistocene centers along the strikeofthe volcanicfront allow us to assess tle significance of over-riding lithospheric input for SSZ magmatism. These volcanoesare distributed acrossa number of major terrane boundaries, including the lithosphere-penetrating Tanakura Tectonic Line. Conpositionsrangefrom basaltto rhyolite, with a preponderance of basaltic andesite andandesite; an exception is basalt-dominated Fuji. Most basalts aresigniffcmfly fractionate4with low MgO (<7 wtTo), Mg# $0, andlow Ni (d0 ppm). Significant differences among volcanic suites are established on the basis of isotopic @b, Sr, and Nd), major- and trace-element data-The predominantsourceof thesema€mas is a relatively refractory (compared to mid-oceaa ridge basalt)mantle-wedge are4 with subchondritic MZr and ZrlIIf. A numberof volcanoes also havelow ZrN (< MORB), but several haveboth elevated hN (>3) andDy/Yb, indicative of gametinvolvement in their gercsis.The Nd and Sr isotope ratios also indicateregionalvariations in chemical composition of the source(s) (ong-term depleted to enriched). Pb isotopic data are distinctivein northern Honshu, with elevated 48Pb/2I/.Pb afi,NTPbPePb at specific 2n6Pb2oaPb (south versas north of the Tanakura Tectonic Line), arecollectivelyelevated above the NorthemHemisphere Reference Line (and lzu-Mariana arcs), appear similax to portions of the Philippines andTertiary basalts of eastern Asia and probably reflect an Asian lithospheric heritage.The combinedtrace-element and isotope geochemistry of parentalbasalts distinguishdifferent domainswithin the Izu-Bonin, centraland northeastern Honshuarcs. The ultimate cause of these is most likely multicomponent, and some crustal assimilation is higbly probable. However,tlese domains may in some paxtrepresent nagma interaction with a subcontinental lithosphere dominated by Indian-Ocean-type mantle,or alt€matively, derivation from an Indian-Ocean-type asthenosphere. The correlation of some magma compositional characteristics with nature of the basement terrane shows that the lithosphere plays a subtle but significantrole in determining parental island-arc basalt chaxacteristics. The longevity andcharacter ofthe continental lithospherein Asian margin arc systems, and its interaction with asthenosphere-derived melts, is a1 imFcntant process in island-arc basalt genesis. Keywords: ars m:gmatism,petrogenesis, litlosphere, source, tace-element geochemisty,isotopic data Honshu arc, Japaa, Sovnraarns Les volcans desarcsdu nord d'Izu - secteur sud-cenfal de Honshu et du nord-est de Honshu, au Japon, sont situ6s dans une r6gionotl'ige, lacompositionetlanaturedelacouche s6dimentairedelaplaqueen subduction semblentrelativementuniformes. Nous essayons ici d'isoler quelques unesdesvariables associ6es i la gendse desmagmas li6s i la zonede suMuction, afin de E-mail addre s s:. d.gust@ qutedu.au