Geochronological study of pyrophyllite deposits in East Asia

Numerous pyrophyllite deposits occur in East Asia, including those in south west Japan, the Korean Peninsula, and in southeast China. Deposits distribu ted along the margines of the Japan Sea, the Yellow Sea and the East China Sea are aligned along a NE-SW trend when Japan is restored to its original position prior to the middle Miocene opening of the Japan Sea. Most of the deposits occur in Cretaceous volcanic rocks (rhyolites and andesites). Form ation of the clay deposits (pyrophyllite, illite and kaolinite) is due to h ydrothermal activity associated with the volcanism. This association of vol canism, hydrothermal alteration and clay formation suggests that the genesi s of the East Asia pyrophyllite deposits is related to tectonics. K/Ar dati ng of illite from the clay deposits shows that the southeast China deposits are older (91-99 Ma) than those in the Korean Peninsula (51-92 Ma) and sou thwest Japan (63-87 Ma). The age of the hydrothermal alteration which forme d the pyrophyllite deposits thus decreases to the northeast. The younging o f hydrothermal alteration from southeast to northwest may therefore reflect migration of igneous activity caused by late Cretaceous to early Paleocene subduction of the Kura-Pacific Ridge along the East Asian paleotrench, as proposed by Kinoshita and Ito (1988).