GEOLOGICAL AND PETROLOGICAL STUDY ON MAGNESIAN ANDESITES IN ISLAND ARC

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1. Study on Bonin Island boninites. Shiraki and co-worker discovered the most Mg-rich, clinoenstatite boninites in Takinoura, Ani-jima. This contains the most Fo- and NiO-rich olivine in the boninites, and the partition coefficient of Ni between olivine and liquid revealed a high value of 12. Maruyama et al. compiled the geology of Ani-jima. Kuroda and co-investigator published a petrological study on ferropigeonite-bearing dacites, which are the latest differentiates from boninite-forming magma.2. Study on sanukites and related rocks in the Setouchi volcanic belt. Shiraki et al. discovered a large number of magnesian andesites in southeastern Yamaguchi Prefecture and carried out detailed geological, petrological, minetalogical and geochemical studies on them. Shiraki and Nomura reported a magnesian andesite from the lower Miocene Hachiya formation in Minokamo-shi, Gifu Prefecture.3. Study on magnesian andesitet from other region. Shiraki et al. and Shijo and Kato described Magnesian andesites from Iriomote-jima and Kume-jima, Ryukyu Islands, respectively. Nanao and Sakaguchi found magnesian andesites in primitive magmas of the Hisatsu volcanic rocks. Takahashi discovered magnesian andesites in the base of the Nishina group in Izu Peninsula. Magnesian andesites in the Japan Sea side were described by Furuyama, Sato et al. and Okamura and Yosida, respectively.