島弧火山の深部構造とマグマ変遷の仕組み

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Distributions of basalt, andesite and rhyolite dominant volcanoes and their close relation with thickness and chemical composition of the basement crust clearly indicate that the mantle-derived basalt magmas are being modified to the silicic arc magmas within crustal magma feeding system. Systematic relations between liquidus temperature of hydrous arc magmas with SiO_2=50 to 75 wt% and water contents have been established, Based on the magmatic temperatures recorded in the volcanic rocks and the above systematics, it is concluded that the tholeiitic andesite and dacite magmas contain 3 to 5 wt% of H_2O which is much higher than previously though. On the other hand, calc-alkaline andesites in most subduction zones are hybrid products of low temperature silicic melts with 5-8 wt% of H_2O and high-temperature basaltic melts. The mechanism of generation of hehybrid magma was studied at the Usu volcano in Hokkaido. The Usu magma chamber was originally simple (stratification of evolved basalt and nearly aphyric rhyolite) but as a consequence of its historic eruptions, the magma chamber has been evolving into a complex magma plumbing system with heterogeneous lithology of hybrid volcanic rocks. Based on seismological view of the deep structure of Japancse Quaternary volcanoes, petrologic informations of the arc magmas, and the geologic cross section of the deep structure of young island arc exposed in the Hidaka metamorphic belt in Hokkaido, a model on the crustal magma-feeding system of subduction zone volcanoes is proposed.
特定非営利活動法人日本火山学会の論文
1997-04-25

島弧火山の深部構造とマグマ変遷の仕組み

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