Characterizating the igneous basement and reconstructing the volcanotectonic history of the Philippine Sea are the major targets of surveys for delineating Japan's continental shelves operated by Ministry of Economy, Technology and Industry. Sampling using R/V Hakurei-maru No. 2 equipped with Deep-sea Boring Machine System (BMS) and arm dredges covers a wide area of the Philippine Sea. The results are summarized of 40Ar/39Ar dating and chemical and isotopic analyses on igneous rock samples from the Philippine Sea, particularly from the West Philippine Basin, and its surrounding area, and Kinan Seamount Chain in the Shikoku Basin. Both regions are characterized by Ocean Island Basalt (OIB)-like magmatism which formed oceanic plateaus (e.g., Urdaneta Plateau, Oki-Daito Rise) and seamounts (e.g., seamounts in Minami Daito Basin, Kinan Seamount Chain). However, the geochemical characteristics of OIB-like basalts and pattern of spatial and temporal variations of this type of magmatism indicate that sources and processes involved in magmatism in these two areas are clearly distinct.
