Studies on the Volcanoes in Japan The CaO/(Na2O+K2O):FeO/MgO in the volcanic rocks

Abstract

The writer examined the relations between the chemical and mineral compositions of Japanese volcanic rocks., The difference between "pyroxene andesites" (rocks carrying no hornblende as phenocrysts) and "hornblende andesites" (all the rocks carrying hornblende as phenocrysts) is shown most clearly by the diagram in which the ordinate gives the ratio CaO/Na2O+K2O) and the abscissa FeO/MgO., In this diagram the former rocks all into an area with higher CaO/(Na2O+K2O) and FeO/MgO ratios, and the latter ones into an area with lower ratios, the two areas being bounded by a curve corresponding to {CaO/(Na2O+K2O)}×(FeO/MgO)=K (const., ) approximately., Moreover basic members of the pyroxene andesites tend to lie away from the boundary curve while acidic ones near the curve., The two areas are designated as "hornblende field" and the "pyroxene field"., The position of the boundary curve is not fixed, being variable in different provinces or volcanoes (Ref., Fig., 1-Fig., 7)., For example, when CaO/(Na2O+K2O)=FeO/MgO, √(K)is 1., 7 for Iizuna and Kurohime volcanoes, 1., 3 for Kayagatake volcano, 1., 1 for Asama volcano, etc., ., This figure may be used as an index showing chemical characteristics of each volcanoes or petrographic provinces., The writer proposes to call √(K)"the CaO/(Na2O+K2O):FeO/MgO Index (in short CaO·Alkalies·FeO·MgO) Index".,