K-Ar geochronology of rhyolitic rocks in the Hohi volcanic zone, central Kyushu, Japan

We dated two rhyolitic lava plateaus, the Yamakogawa rhyolite and the Haneyama rhyolite, that are assumed to have erupted within a short time interval, in the middle of the Hohi Volcanic Zone, central Kyushu, Japan. The groundmass with phenocrysts removed was dated by the conventional K-Ar method using peak-height comparisons without an Ar-38 spike. Our results show that the Yamakogawa rhyolite erupted more than once over a short time interval, from 1.17 to 1.22 Ma. The Haneyama rhyolite is distinctly younger, ranging from 0.67 to 0.71 Ma. The Yamakogawa rhyolite is slightly older than, and therefore probably unrelated to the Yabakei pyroclastic-flow deposit and other known widespread tephra. (c) 2005 Elsevier Ltd. All rights reserved.