カルクアルカリ安山岩多元成因論 : 渡島大島等での実例
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概要
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Igneous petrologists realize the difficulty in giving a meaningful definition to the calcalkaline andesite, yet emphasize its importance in the development of orogenic belts. The difficulty may indicate that the andesite which we treat in the lump, has in reality more than two geneses. In this paper the author tries to find out andesite localities where certain hypothesis on the genesis is convincingly applicable, and also tries to understand mechanisms operating in nature to produce andesites. Fractionation of basaltic magma is controlled mainly by amphibole cryatallization: The Oshima-Oshima island, off the coast of Hokkaido, is a stratovolcano which is composed of 70% alkali olivine basalt and 30% calcalkaline andesite. The andesite bears ultramafic and mafic nodules (dunite, gabbro, clinopyroxenite, hornblendite). Hornblend is an essential phase in all these rocks. A brief petrographic study has revealed that they are of cognate origin to the Oshima-0shima basalt and andesite. The same phases as the nodule constituents can commonly be observed as phenocrysts in the lavas. The Rayleigh distillation law, applied to these basalt and andesite, yields partition coeffcients of elements between magma and unidentified mineral precipitate. Comparison of these coefficients with the compiled data for various mineral/magma pairs indicates amphibole as a major cause for the fractionation of the magmas. As expectedly, the ^<87>Sr/^<86>Sr ratio is constant at 0.703 for the basalt and andesite, and the ^<18>o/^<16>o ratio slightly increases as magmas fractionate. All the evidence shows that the alkali olivine basalt fractionated to produce the calcalkaline andesite, and that the nodules were the complementary precipitate.
- 1977-12-15
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