福井県米ヶ脇層崎浦溶結凝灰岩の岩石学的特徴およびK-Ar年代

抄録

Major and trace element compositions and a K-Ar age are reported for the Sakiura welded tuff of the Miocene Komegawaki Formation in the Mikuni coastal area, Fukui prefecture, central Japan. The rock samples are densely welded, generally show eutaxitic texture, and contain 9-17 vol.% plagioclase, minor (<1 vol.%) hornblende, and 1-14 vol.% lithic fragments. Plagioclase in the Sakiura welded tuff yields a K-Ar age of 13 Ma, which is within the 12-14 Ma age range of Komegawaki volcanic rocks. Sakiura welded tuff samples are rhyolitic (SiO₂ = 73.3-74.5 wt.%) and are near the SiO₂-rich end of the compositional trend of Komegawaki volcanic rocks on major element versus SiO₂ cross-plots. The MORB-normalized trace element patterns of Sakiura welded tuff samples are similar to those of the Komegawaki volcanic rocks. These observations indicate that the origin of the rhyolite magma that produced the Sakiura welded tuff is associated with the Komegawaki volcanic rocks. Evidently, multiple rhyolitic eruptions occurred concurrently with the formation of the Miocene volcanic rocks after the opening of the Japan Sea.