Petrology and geochemistry of boninitc-series volcanic rocks, Chichi-Jima, Bonin Islands, Japan

An Eocene submarine boninite-series volcanic center is exposed on the island of Chichi-jima, Bonin Islands, Japan. Five rock types-boninite, bronzite andesite, dacite, quartz dacile, and rhyolite-were distinguished within the boninite volcanic sequence on the basis of petrography and geochemistry. Boninites contain high Mg, Ni, and Cr contents indicative of primitive melts, but have high Si contents relative to other mantle-derived magmas. All boninite-series rocks contain very low incompatible-element concentrations, and concentrations of high-field-strength elements in primitive boninites are less than half of those found in depleted mid-ocean ridge basalts. Abundances of large-ion lithophile elements are relatively high in boninite-series rocks, similar to the enrichments observed in many island-arc lavas. Trends for both major and trace-element data Suggest that the more evolved members of the boninite magma series were derived primarily through high-level fractional crystallization of boninite. Textural features, such as resorption and glomeroporphyrocrysts, and reverse chemical zonations suggest that magma mixing contributed to the development of the quartz dacites.