K-Ar ages of mica clay minerals in clay veins found in granitic and rhyolitic rocks of Hiroshima Prefecture, Japan

Two mica clay minerals in clay veins developed in Hiroshima granitic rock and Takada rhyolitic rock distributed in Hiroshima Prefecture were dated by K-Ar method. The age of both minerals are 68.4 m.y. and 79.5 m.y. respectively. The ages of mica clay minerals are similar to that of the Hiroshima granitic rocks (70 - 91 m.y.). This result may be suggested that the mica clay minerals found in both host rocks are formed by hydrothermal solution subsequent to the post granitic activity. (author)