K-Ar age of a rhyolite lava (Kohideyama rhyolite) on the northwestern flank of Mt. Kohide, Nakatsugawa City, Gifu Prefecture

Abstract

K Ar age of a rhyolite lava (Kohideyama rhyolite) on the northwestern flank of Mt. Kohide, Nakatsugawa City, Gifu Prefecture (Yasuo ISHIZAKI) * (Ken ichi YASUE) ** Mt. Kohide (Nakatsugawa City, Gifu Prefecture) is located 7 km north of the Atera fault. A partial collapse of the northwestern flank of Mt. Kohide has exposed a 5 m wide dyke like conduit 15 30 m beneath the extrusive rhyolite lava that was fed by it. The rocks of the extrusive lava and the feeder are nearly aphyric (< 0.5 vol% phenocrysts), biotite rhyolite (73 wt% whole rock SiO 2). The extrusive lava yields a K Ar age of 23.01 0.36 Ma. Within the measurement errors, this age coincides with the previously determined K Ar ages of biotite rhyolite dykes intruded along the Atera fault (23 22 Ma) and the rhyolitic welded tuff of the Hachiya formation (40 km southwest of Mt. Kohide; 22 Ma) as well as the K Ar and fission track ages of the rift type alkalic moonstone rhyolites from the Hokuriku area (25 22 Ma). This suggests that rhyolitic volcanism occurred contemporaneously in central Japan between the Late Oligocene and Early Miocene.