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 ⁸⁷Sr/⁸⁶Sr ratio is constant at 0.703 for the basalt and andesite, and the ¹⁸o/¹⁶o 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.

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Emergent studies on some water and ash samples collected immediately after the August 7,1977 eruption of Usu volcano have been made by a joint group of geochemists. This paper reports a part of the results obtained for the samples collected in August and September. The items measured were temperature, pH, major and minor dissolved chemical components including radon, hydrogen and oxygen isotope ratios for ground water, thermal water around the Usu volcano and fumarolic condensates of the Showashinzan, the adjacent newer volcano to the Usu, chemical and mineral composition of ash including its soluble components. Through a short period of observation, we could not find any significant change in chemical and isotopic compositions of a variety of waters after the eruption. However, a rough idea of hydrothermal and hydrological situation around the Usu volcano has been obtained. In this comection, we set standing stations for water sampling and the observation through these stations has been still continued. Ash fall contained montmorillonite and chlorite. This fact indicates that clay minerals formed beneath the crater during the dormant period was expelled together with pumice originated from the fresh magma pertinent to this eruption. When the chemical composition of the water-soluble components of the ash is also taken into consideration, temperature of the coexisting gas phase with ash is suggested to be fairly high.

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