Clinopyroxene megacrysts from Itino-megata volcano, northeastern Japan

Abstract

Clinopyroxene and olivine megacrysts with welldeveloped crystal faces up to 2cm and 6mm in size, respectively occur sporadically in air-fall lapilli tuff and explosion breccia of olivine-bearing titanaugite alkali basalt from Itino-megata volcano. They are accompanied by various types of accidental ultramafic and mafic inclusions derived from upper mantle and lower crust. Two clinopyroxenes have been chemically analysed. The clinopyroxene megacrysts contain considerable amounts of Ca-Tschermak's component (7.6 and 8.9 per cent, respectively). The clinopyroxene and olivine megacrysts are thought to crystallized from enclosing alkali basalt magma as liquidus phases at a depth of about 20 to 30km in the lower part of the crust in this region, from their mineralogy and chemistry and the experimental work at high pressures and temperatures on natural rock systems.