K-Ar ages for the Setouchi volcanic rocks in Shitara district, central Japan.

Abstract

K-Ar ages were determined for the Shitara volcanic rocks, which belong to the middle Miocene Setouchi volcanic rocks in southwestern Japan arc. The Shitara volcanic rocks are mainly composed of alkaline volcanic rocks, whereas high-Mg andesites (HMAs), which are the main constituents of the Setouchi volcanic rocks of other districts, were not observed. The samples for K-Ar age determination were selected from intrusive rocks that belong to the latest magmatic activity in this district. The obtained ages range from 13.6±0.4 Ma to 12.6±0.4 Ma, and these ages are similar to those of the other districts. These indicate that the magma activity in the Shitara district ceased at about 13 Ma, and the origin of the Shitara volcanic rocks is closely related to that of the HMAs, although the petrological characteristics of the Shitara volcanic rocks are different from those of the HMAs.