Sanukite is a type of black vitreous Mg-rich volcanic rocks first recognized in the northern part of Shikoku in Japan. It occurs mainly in the Miocene (11～14 Ma) Setouchi volcanic belt, and is typically high in Mg number (Mg/Mg+Fe2+>0.6), SiO2 (andesitic to dacitic), Cr and Ni contents and K/Na ratios (0.33～0.52). The Setouchi sanukite is located in fore_ or back-arc basins, and originated in subduction of the young and hot Philippine Sea plate and pull-apart of the Shikoku basin. Shirey and Hanson (1984) first used ‘sanukitoid' to name some Archaean intrusive and extrusive rocks with the above geochemical characteristics. Geochemically, sanukite resembles adakite except for somewhat higher Mg, Cr and Ni contents, indicating that it can be produced directly from a mantle source through partial melting, whereas adakite is merely produced by partial melting of a basaltic source. Sanukite is widespread in Archaean, indicating a higher geotherm of the mantle wedge or the subducting plate. The early sanukite has only been found in Late Archaean (<3.0 Ga), probably indicating that the modern style plate tectonics was initiated at that time and that plate subduction came to be an important geological process only after 3.0 Ga.