MESOZOIC TECTONOTHERMAL DEVELOPMENT OF THE SAMBAGAWA, MIKABU AND CHICHIBU BELTS, SOUTH-WEST JAPAN - EVIDENCE FROM AR-40 AR-39 WHOLE-ROCK PHYLLITE AGES/

Five whole-rock Ar-40/Ar-39 plateau ages from low-grade sectors of the Sambagawa belt (Besshi nappe complex) range between 87 and 97 Ma. Two whole-rock phyllite samples from the Mikabu greenstone belt record we ll-defined Ar-40/Ar-39 plateau ages of 96 and 98 Ma. Together these ag es suggest that a high-pressure metamorphism occurred in both the Samb agawa and Mikabu belts at c. 90-100 Ma. The northern Chichibu sub-belt may consist of several distinct geochronological units because metamo rphic ages increase systematically from north (c. 110 Ma) to south (c. 215 Ma). The northern Chichibu sub-belt is correlated with the Kuma n appe complex (Sambagawa belt). Two whole-rock phyllite samples from th e Kurosegawa terrane display markedly older metamorphic ages than eith er the Sambagawa or the Chichibu belts. Accretion of Sambagawa-Chichib u protoliths began prior to the middle Jurrasic. Depositional ages dec rease from middle Jurassic (Kuma-Chichibu nappe complex) to c. 100 Ma (Oboke nappe complex) toward lower tectonostratigraphic units. The age s of metamorphic culmination also decrease from upper to lower tectono stratigraphic units. The Kurosegawa belt and the geological units to t he south belong to distinctly different terrances than the Sambagawa-C hichibu belts. These have been juxtaposed as a result of transcurrent faulting during the Cretaceous.