K-Ar ages for the Yahazudake volcanic rocks from southwest Kyushu, Japan

We determined K-Ar ages for four andesites of the Hisatsu volcanic rocks from Minamata and for rhyolite of the Gesujima welded tuff from Amakusa Shimoshima, southwest Kyushu Japan. The andesites include three Yahazudake volcanic rocks and one Ontake volcanic rocks. The obtained K-Ar ages for the Yahazudake type andesite range from 1.98 to 2.08 Ma. The ages of Ontake type andesite and Gesujima welded tuff are 2.15 Ma and 2.89 Ma respectively.     From our new K-Ar age data combined with previously reported geological, petrological and chronological data of the Hisatsu volcanic rocks, we discussed the space and time distribution of these volcanic rocks. Andesites from Hisatsu volcanic rocks are divided into two categories, “Ontake type andesite (K2O>1.7 wt. %)” and “Yahazudake type andesite (K2O<1.2 wt. %)”, based on potassium content in andesite at given SiO2 range (57-63 wt.%). In terms of this criterion, most of the Hisatsu volcanic rocks and andesites from Kirishima volcano are defined as the “Ontake type andesite”. Consequently, the Ontake type andesite distributes whole area of the Hisatsu volcanic province extending from Nagashima area to Kirishima volcano. Andesites of this type, as a whole, have a positive correlation between latest eruption age of each volcano and the distance from the present volcanic front. This spatial arrangement of the Ontake type andesite implies that the width of volcanic zone has narrowed from Nagashima to Kirishima volcano during last three million years. Although the distribution area of Yahazudake type andesites overlaps the Ontake type andesites, the volcanism of the Yahazudake type andesites is quite limited in age ranging from 1.6 to 2.0 Ma. The production of each magma types may belong to different tectonic events.