東伊豆単成火山群における珪長質マグマの成因

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The activity of Higashi-Izu monogenetic volcano group produced basalt, andesite, dacite, and rhyolite, which can be classified into rock types by whole-rock composition and phenocryst assemblage. It has been widely accepted that felsic upper crust has an important role in forming compositionally diverse ejecta; andesites are formed through the assimilation of felsic crust by the basaltic magma and the felsic magmas through remelting of the felsic crust. To reinvestigate genesis of felsic magma, I examine the physicochemical conditions of both pre-eruption felsic magmas and the felsic crust. Beside observed rhyolite, felsic magmas can be recognized in dacite as endmember of magma mixing. The pairs of the magnetite and ilmenite yield the temperature of 820-790℃ for the rhyolite and 850-820℃ for the dacite. The plagioclase geohydrometer shows that the rhyolitic magmas had PH20 of 2 kbar indicating their residence within the felsic upper crust at the depth of 6-15 km before eruption. Some andesite could be formed through bulk assimilation, a simple mixing between felsic crust and basaltic magma. The compositions of the felsic crust are obtained using mixing ratios of the felsic crust and basalt magrna. The felsic crust which could range from rhyolitic to andesitic, differs from those of the rhyolitic magmas; the rhyolitic magrnas are lower in HREE (Gd, Er, Dy) and Sr, and higher in LILE (Rb, Ba, K_2O) than those of the felsic crust. In addition, rhyolitic magmas cannot be formed by the fractional crystallization from the magma formed by the remelting of the felsic crust; Rb/K_2O ratio of the rhyolitic magmas is higher than that of the felsic crust. However, partial melting of the felsic crust, leaving amphibole and plagioclase as the residue could produce the rhyolitic magmas.
2000-07-10

東伊豆単成火山群における珪長質マグマの成因

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