(K-Ar ages of muscovite from greenstone in the Ino formation and schist blocks associated with the Kurosegawa tectonic zone near Kochi City, central Shikoku.)

The Kurosegawa tectonic zone is a serpentinite melange zone in which various kinds of tectonic blocks occur. Muscovites coexisting with high-P minerals gave ages of 352-394 m.y. They indicate that so-called semischists were neither metamorphosed at the time of formation of the Kurosegawa tectonic zone, nor by the Sanbagawa metamorphism, but show a distinct metamorphic event older than Middle Carboniferous. This is the oldest radiometric age among glaucophane schists in Japan. Muscovites from the schist xenoliths of albite-epidote amphibolite facies grade were dated at 317 and 327 m.y. by the K/Ar method. Hornblende and barroisite are sometimes replaced by actinolite or riebeckite along their cleavages or irregular cracks. Muscovite is also deformed to show kink-band folding and is chloritized parallel to cleavage or along irregular cracks. Therefore, the values 317 and 327 m.y. may not show the time of albite-epidote amphibolite facies metamorphism, but are older than those of the Sanbagawa schists.-M.Ak.