Origin of schist clasts of Upper Cretaceous Onogawa Group, southwest Japan

Abstract

The Upper Cretaceous Onogawa Group in east Kyushu is a thick turbiditic sequence that contains large clasts of schists of unknown origin. Newly determined K-Ar ages of white micas from these schist clasts are concentrated in 199-182 Ma (Early Jurassic) and they clearly deny the previous belief that the schist clasts were derived from the Sanbagawa Belt. The first surface exposure of the Sanbagawa metamorphic rocks was recorded not by the Late Cretaceous Onogawa Group, but by the Middle Eocene (52-40 Ma) Kuma Group in west Shikoku. K-Ar ages around 199-182 Ma suggest that the schist clasts of the Onogawa Group were derived most probably from the high P/T Sangun metamorphic rocks in the Inner Zone of Southwest Japan. -Authors