抄録
This paper concerns the occurrence and chemical compositions of “in-situ” greenstones in the Nago and Kerama Formations in the Okinawa and Kerama Islands, which belong to the Shimanto Belt of Cretaceous-Paleogene age. The in-situ greenstones are composed of metabasaltic pillow lava, pillow breccia, hyaloclastite and sills. These are closely interbedded with, or grade upward into, pelitic rock (in Nago Fm.) and sandstone (in Kerama Fm.), indicative of a volcanism near trench. The greenstones have geochemical features of N-MORB in terms of high-field strength element (HFSE) and rare-earth element (REE) chemistry. The relict Cr-spinel composition also support this bulk chemical characteristics. They may have been produced at the spreading ridge between the Kula and Pacific plates, which is thought to have passed northward along the SW Japan arc through Cretaceous and Paleogene. Assuming a ridge migration rate of 4 cm/year, we infer that ages of the Nago and Kerama Formations are about 95-100 Ma.
