Petrological Characteristics of Volcanic Rocks from "the Boninitic Seamount" in Ogasawara Forearc: Implications for the Early Stage Island-Arc Volcanism

During KH84-1 cruise (R/V Hakuho Maru, Univ. Tokyo), more than 600 volcanic rocks including boninite, volcaniclastic rocks, and sedimentary rocks were dredged from a small seamount called "the Boninitic Seamount (BSM)" between the Ogasawara ridge and the Ogasawara trench (Ishii, 1984). Boninite is a kind of unique andesites characterized by very high MgO content and relatively high SiO₂ contents. The Chichijima Island and Mukojima Island in the Ogasawara ridge are mainly composed of Tertiary submarine volcanic rocks of boninite series rocks and calc-alkali series rocks. Based on the geochemical and mineralogical features, volcanic rocks dredged from the BSM are divided into three groups: A) boninite group, which are similar to those from the Chichijima Island; B) pyroxene andesite-hornblende dacite group, which are classified into calc-alkali series and have similar chemical composition to volcanic rocks from the Mikazukiyama formation of the Chichijima Island (Umino, 1986); C) alkali basaltic andesite group. Boninites (group A) from the BSM contain the most abundant bronzite crystals (Mg# = 0.87) among three groups. In group B, bronzite crystals (xenocryst), which have the same chemical compositions (Mg# = 0.87), also occur with hypersthene and augite phenocrysts. With increasing of SiO₂ content, the quantity of bronzite crystals decreases. Furthermore, bronzite crystals frequently exhibit disequilibrium compositional zoning indicating reaction with andesitic (or dacitic) magma. These observations may indicate that boninite magma was injected into pyroxene andesite-hornblende dacite magma. Volcanic rocks from the Mikazukiyama formation were produced by fractionation from Cpx-HMA (e.g. Kuroda et al., 2001). If it assumes that pyroxene andesite-hornblende dacite magma was produced by same mechanism, a layered magma chamber, consist of the hypersthene-augite andesite magma, augite-dacite magma and hornblende quartz dacite magma from bottom to top, are proposed. We can interpret that the lower part of the magma chamber was injected by a great number of boninite magma, in contrast, the upper part of the magma chamber might be injected by a small quantity of boninite magma.