北アルプス,鷲羽・雲ノ平火山の地質

抄録

Three small volcanoes exist in the central part of the Japan Alps; i. e., Kumonotaira, Washibaike and Warimodake. They are collectively called Washiba-Kumonotaira volcano. The earliest volcanism of the Kumonotaira formed a small stratovolcano (1.0-0.9 Ma). Thereafter, a quantity of gravels of basement rocks were deposited around it. The volcanic activity restarted at approximately 0.3 Ma, and the main body of the Kumonotaira (lavas and pyroclastics) was formed overlying the gravel bed. The Washibaike consists of lavas and pyroclastics which erupted on the slope of the mountain of basement granitic rocks (approximately 0.1 Ma). The Warimodake is a minor mass of andesite lava and thought to be the remnant of a lava flow lying at the crest of the ridge. Rocks of these volcanoes are augite-bearing olivine basalt (SiO₂-51-52%), andesite (55-61%) with complex phenocryst assemblage and glassy dacite (63-64%) rich in hornblende, biotite and quartz phenocrysts. Among them, the andesite is dominant. The basalt constitutes the older stratovolcano of the Kumonotaira, and the dacite composes a part of the Kumonotaira and the Washibaike. On the FeO^*/MgO vs. SiO₂ diagram, these rocks are grouped into three. That is, 1. basalt with low SiO₂ and high FeO^*/MgO ratio 2. andesite with moderate SiO₂ and moderate FeO^*/MgO ratio 3. andesite and dacite with high SiO₂ and low FeO^*/MgO ratio Their total chemical variation cannot be explained by a single mechanism such as fractional crystallization. Complicated mechanisms including magma mixing would have caused the chemical variety of the volcanoes.