1. Petrography and Petrogenesis of the New Lava of Akita-Komagatake(VI. Materials Erupted during the 1970-71 Activity of Akita-Komagatake)
1971 Volume 16 Issue 2-3 Pages 174-183
Details
1971 Volume 16 Issue 2-3 Pages 174-183
The new lava erupted in the 1970 activity of Akita-Komagatake comprises volcanic bombs, volcanic blocks and lava flows of hypersthene-augite andesite (Vd→c type). The mode comprises phenocrysts of plagioclase, augite, hypersthene and magnetite in a groundmass of plagioclase, clinopyroxene, hypersthene, magnetite and brown glass. Rarely anhydrite-bearing xenoliths, consisting of tridymite, anhydrite, augite and glass, are found. These may have been formed by pyrometasomatism of basement rock rich in opaline silica. Petrochemical features of the new lava are discussed by means of various diagrams. It noted that K2O is very low, though total alkalis are nearly the same as in the older lavas. The new lava plots near the boundary line between the tholeiitic series and calc-alkalic rock series, whereas the older ones fall always in the central part of the tholeiitic series in the MgO-(FeO+Fe2O3)-(Na2O+K2O) diagram. When plotted in the (FeO+Fe2O3)/(FeO+Fe2O3+MgO)-SiO2 diagram, the new lava shows a trend different from that of the older ones. This change may have been brought about by increase of pO2 as the result of increasing water content in the magma reservoir. If this trend goes further, a pyroxene andesite of Vd type with about 60% SiO2 might be erupted by this volcano in the future.
