Middle Miocene andesite and dacite and Late Miocene basalt from the Kurahashi Jima and Hashira Jima districts in the western Setouchi region, Japan

Abstract

Miocene volcanic rocks were discovered in the Kurahashi Jima and Hashira Jima districts. The volcanic rocks compose of MgO-rich olivine-clinopyroxene andesite, biotite-hornblende dacite, and high-alkali tholeiitic olivine basalt, occurring as dikes or necks with minor pyroclastic rocks.
     Whole rock K-Ar age determinations were carried out on two magnesian andesites, a daciteand a basalt. The magnesian andesites and the dacite yielded Middle Miocene ages of 14.9±0.7, 13.3±0.7, and 12.2±0.6 Ma respectively. These results indicate that the andesites and the dacite belong to the Setouchi volcanic rocks, whereas the basalt gives a Late Miocene age of 8.4±0.4 Ma suggesting another younger volcanic activity.