瀬戸内海西部，倉橋島及び柱島地域の中期中新世安山岩―デイサイトと後期中新世玄武岩

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.