岐阜県中津川市，小秀山北西斜面に分布する流紋岩質溶岩 (小秀山流紋岩) のK-Ar年代

抄録

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 SiO₂). 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.