美和高山ピクライト玄武岩の産状と起源

抄録

Picrite basalts are distributed in the Mt. Miwa-Takayama area of Shizuoka city, Shizuoka Prefecture. This area spans the Setogawa tectonic belt sediments composed of middle Eocene sandstone and shale. We found picrite basalt with pillow lavas at Yuyama near the Mt. Miwa-Takayama. Pillow lavas are found with weathered sediments with pillow breccia flows. This is the first record of picrite pillow lavas in Setogawa tectonic belt.
Miwa-Takayama picrite basalts contained many large phenocrysts of olivine, in which have high mg# [= Mg/(Mg+Fe) atomic ratio] and many chromian spinel inclusions. Some olivines have kink banding deformation characteristics. Whole rock chemical compositions showed that Miwa-Takayama picrite basalts are classified to komatiite or meimechite. Miwa-Takayama picrite basalts belong to ocean island basalts (OIB) by the chemical composition of Al₂O₃ and TiO₂ of spinel inclusions trapped in the olivine. Equilibrium temperatures of the Miwa-Takayama picrite basalts are calculated to about 1140°C or over, relative to the olivine-spinel geothermometer. Miwa-Takayama picrite basalts are part of the accretionary prism in the Setogawa tectonic belt.