Kobe University, Japan

A large number of serpentinite seamounts in the Izu-Ogasawara-Mariana forearc reflect serpentinite diapirism under a tensional stress field. The Franciscan Complex, California, and the Kamuikotan metamorphic belt, Japan, are accompanied by serpentinite melange including deep, high-grade metamorphic rocks, whereas the Sanbagawa metamorphic belt, Japan, contains lesser amounts of serpentinite and is not accompanied by serpentinite melange. The former were probably formed under a tensional stress regime, and the latter under a compressional one. The restricted occurrence of serpentinite to high-grade pelitic schists in the Sanbagawa belt suggests that the serpentinites were transported from the mantle wedge into pelagic sediments on the top of the subducting plate at the initiation of accretion. Tremolite rocks and phengite-chlorite schists near serpentinite in the Sanbagawa belt and from Chamorro Seamount, Mariana forearc, may suggest widespread metasomatism along the boundary between the mantle-wedge serpentinite and pelagic sediments.