Comparative petrologic study of ophiolitic rocks in the Japanese Islands and Russian Far East

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(1) Research in Japan. Western part of the Yakuno ophiolite exposed in the Kamigori area constitutes a crustal section of the late Paleozoic oceanic island arc. The coexistence of Ca-rich plagioclase and Fe-rich clinopyroxene in the metagabbro as well as highly depleted incompatible trace elements in amphibolite support this identification, which is also confirmed by the finding of shoshonitic rocks. We measured K-Ar age of hornblende from metacumulates of the Oeyama ophiolite, and confirmed their early Paleozoic age (400-440 Ma). We find eclogitic glaucophane schist from the Renge blueschist belt in the Omi area. This finding proves that the Renge metamorphism reached the eclogite facies. In the Jurassic accretionary complex of the North Kitakami belt in Aomori Prefecture, we find that the accreted volcanic rocks (greenstones) are mostly of ocean-island tholeiite origin. This clearly contrasts to the reported alkali basalt origin, which predominates greenstones in the southern Iwate Prefecture area, and proves magmatic diversity of the accreted seamounts.(2) Research in Russia. The Elistratova ophiolite in Taigonos Peninsula at the southern edge of the Koryak Mountains shows peculiar occurrence, where a stratified sequence of an island-arc ophiolite intrudes into the oceanic mantle peridotite. Extreme diversity in the degree of depletion of mantle peridotite along the eastern coast of Taigonos Peninsula is also revealed. In Primorye, we find that the Cambrian Khanka ophiolite shows distinct "passive continental margin" occurrence clearly different from the other "accretionary" early Paleozoic ophiolites in the Sikhote Alin accretionary system. Our petrologic and geochronologic study of the associated blueschist revealed close analogy between Oeyama ophiolite-Renge blueschist pair in Japan and the "accretionary" early Paleozoic ophiolite-late Paleozoic blueschist pair in Primorye.