K-Ar ages determined for post-caldera volcanic products from Aso volcano, central Kyushu, Japan

摘要

We determined the K-Ar ages for post-caldera lavas from Aso volcano in central Kyushu, Japan using the unspiked sensitivity method. The following three peaks of highly-frequent volcanism are recognized in the post-caldera stage: 70-50 ka, 40-20 ka and later than 10 ka. The two peaks of 70-50 ka and 40-20 ka are characterized by activities of compositionally diverse magmas from basalt to rhyolite. The volumes of silicic magmas are significantly larger than those of basalt magmas in these peaks. On the other hand, the peak later than 10 ka is characterized by predominantly basaltic eruptions without silicic magma activities. The active center of the silicic magma production (=crustal assimilation of injected basalt) had presumably migrated northeastward from the southwestern part of the caldera between the peaks of 70-50 ka and 40-20 ka. The predominant basaltic activity in the peak later than 10 ka represents that the production rate of silicic magma decreased, and does not indicate the accumulation of voluminous silicic magma beneath the recent Aso caldera.