Title: Fractional crystallization of the island arc tholeiitic magma in Hotaka volcano.
Abstract: A liquid line of descent is well demonstrated in the Hotaka volcano. Mineral assemblage and chemical composition of phenocrysts change systematically with the change of the chemical composition of the groundmass (magmatic liquid). Crystallization sequence with increasing SiO2 content of the groundmass is as follows. Olivine, hypersthene, augite, plagioclase and magnetite coexisting in the liquid whose SiO2 content ranges from 53.5 to 57 wt%. In liquids with more than 57 wt% SiO2 content, olivine disappears. Augite, hypersthene, magnetite and plagioclase continuously crystallize until the SiO2 content of liquid attains at least 70 wt%. The amount of the crystallized minerals during fractional crystallization is estimated to be 46 wt% (plagioclase, 22; hypersthene, 6; augite, 11; magnetite, 7), while the SiO2 content of liquid changes from 57 to 70 wt%. Plagioclase accumulation during fractional crystallization is inferred from the difference between the estimated ratio of plagioclase to other phenocrysts and the observed ratios in the modal compositions of phenocrysts. The assemblage of groundmass pyroxenes changes with increasing SiO2 content of the groundmass in the following order: augite+pigeonite→pigeonite→pigeonite+hypersthene→hypersthene. This change reflects the decrease of the normative diopside content of the liquid. The chemical and mineralogical features of fractional crystallization is close to that of other island arc tholeiites in Japan, though the normative diopside content of the rocks of the Hotaka volcano in slightly lower than that of the others and groundmass hypersthene occurs in the rocks with more than 65 wt% SiO2.