PETROLOGY OF MAGNETITE-BEARING ULTRAMAFIC AND MAFIC INCLUSIONS FROM IKI ISLAND, JAPAN

Abstract

Magnetite-bearing olivinc gabbro, gabbro, anorthosite and hypersthene gabbro inclusions, which are the most common series of cognate ultramafic and mafic inclusions in Iki Island, Japan, occur in several alkali basalt flows. New analyses are presentatcd for one host basalt, ten inclusions, one plagioclasc, five clinopyroxenes and one orthopyroxene.
It is reasonable to conclude from petrographic and chemical evidence that magnetite-bearing olivine gabbro, gabbro and anorthosite inclusions would be produced from hypersthenc-normative alkali basalt magma which has been moderately differentiated and reached the middle stage of differentiation under high oxygene partial pressures at a depth of less than 15km, while hypersthene gabbro inclusion would be formed from calc-alkaline basalt magma derived from alkali basalt magma with contamination of sialic rocks.