Abstract
K-Ar ages were determined for the Miocene Setouchi-vocanic rocks in the western Setouchi Inland Sea region, southwestern Japan. The Setouchi-vocanic rocks in this area are olivine andesite, pyroxene hornblende andesite and biotite dacite. The obtained ages of nine andesitic rocks range from 16 Ma to 12 Ma, and most of them concentrate between 15.5 Ma and 14 Ma. Detailed petrographic observation of the specimen suggests that the effect of alteration is negligible in the obtained ages. These ages are about two million years older than the those reported previously. They will be important constraints on discussions regarding the genesis of the Setouchi-vocanic rocks and Miocene tectonics around this area.
