OXYGEN ISOTOPIC COMPOSITIONS OF VOLCANIC-ROCKS FROM THE EAST JAPAN ISLAND ARCS AND THEIR BEARING ON PETROGENESIS

18O/16O ratios were measured for 118 volcanic rocks and coexisting minerals and groundmass from the East Japan island arcs. The whole-rock σ 18O values relative to SMOW range from +4.45 to +7.80‰ and show a tendency for 18O enrichment with increasing SiO2 content. Tholeiite from the Hachijo-jima volcano, which is located in an immature island arc close to the axis of down-going sea floor, has a σ 18O value of +5.66‰, which is identical to the values of mantle-derived basalts from other oceanic regions (mid-ocean ridges, marginal basins and oceanic islands). This suggests direct melting of the upper mantle in magma genesis in the early stage of island arc development. The oxygen isotopic compositions of andesites and dacites from the Hachijo-jima volcano are explained by 18O enrichment in residual magma by crystal fractionation of tholeiitic magma. Basalts and their differentiates from an alkalic rock suite on the continental side of the arc are isotopically indistinguishable from a tholeiitic rock suite on the oceanic side. Volcanic rocks from the main islands of Honshu and Hokkaido, both tholeiitic and calc-alkalic rock suites, have scattered σ 18O values against SiO2 content and 0.5-1.7‰ enrichment in 18O relative to the Hachijo-jima tholeiitic rock suite. This 18O enrichment could be attributed to larger isotopic fractionation in magmatic differentiation under hydrous conditions, or to contamination with 18O-rich crustal materials underlying the volcanic areas in the main islands. Melting of subducted oceanic crust could have produced the calc-alkalic andesites on the continental side of the arc. © 1979.