Abstract
Chemical compositions of the rock-forming minerals in a high-alumina basalt, calc-alkaline basalts, calc-alkaline andesites from Chokai volcano were analyzed with the electron microanalyzer. The rocks of the Chokai volcano can be divided into olivine-bearing and olivine-free types on the basis of their mineralogical character. The cores of phenocryst (orthopyroxene, clinopyroxene, plagioclase and magnetite) in the olivine-free calc-alkaline andesite of stage I are unimodal in chemical compositions, and the rim of phenocrysts and microphenocrysts have similar composition to the core of the respective phenocryst. These phenocrysts tend to change the chemical composition with increasing differentiation of the host andesite. The olivine-free andesite of stage IIc is also characterized by the unimodal phenocryst compositions. The phenocrysts of olivine-bearing rocks (high-alumina basalt and olivine-bearing calc-alkaline andesite of stage I, and most of the rocks from stage II and stage III) have normally-zoned phenocrysts (olivine, calcic plagioclase and magnesian clinopyroxene) and reversely-zoned phenocrysts (sodic plagioclase, ferriferous clinopyroxene and hypersthene) in the same rock. The cores of the phenocrysts are almost constant in composition regardless of the bulk chemical compositions of the host rocks.
