2. Chemical Composition of the Rocks and Rock-forming Minerals(VI. Materials Erupted during the 1970-71 Activity of Akita-Komagatake)

Abstract

A large number of chemical analyses of the historic and prehistoric lavas are now available. New ejectas (Table 2) and lava flow (Table 3) are very homogeneous in chemical composition as shown by analyses from different laboratories. Compositions of mechanically separated groundmass (Table 4) and glass composing major portion of the groundmass (Table 5, by EPXMA) are also soown. Variation diagram (Fig. 1) indicates that the 1970 magma is more felsic than the prehistoric lavas of the volcano, including Medake from the top of which the 1970 magma was erupted. The variation trends (Figs. 1 and 2) indicate that they are in agreement with the petrography of the rocks that prehistoric and new lavas belong to the tholeiitic (pigeonitic) rock series of KUNO with the 1970 lavas falling very close to the boundary between the tholeiitic and calcalkali (hypersthenic) rock series. Very marked concentration of iron is apparent in the groundmass glass which is quite off the general trend of the Japanese volcanic rocks. Enrichment of Na with respect to K is also pronounced. Plagioclase ranges its size from 2mm down to less than several microns, making the distinction between phenocryst and groundmass rather arbitrary. Few phenocrysts have cores as calcic as An₉₀. Range in composition is generally from An₇₀ to An₅₀, with total Fe as FeO as high as 1.0%. Augite and hypersthene (Table 8 and Fig. 5) occur only as phenocrysts and microphenocrysts. They appear to have ceased crystallizing in the groundmass stage. Ca-poor clinopyroxenes (pigeonite and subcalcic augite) crystallized in the groundmass stage with Wo content from 7 to 23 mol%. There is no well-defined trend in the pyroxene quadrilateral and the Al content varies very mucy haphazardly. These may indicate that Ca-poor pyroxenes crystallized metastably from the magma cooling rather rapidly. Opaque minerals are titanomagnetite (Table 9) with 41-46% ulvospinel molecule. Groundmass magnetite is more Ti-rich than the phenocryst. Xenoliths composed mainly of tridymite with anhydrite and clinopyroxene are found in the 1970 lavas.