Calc-alkaline volcanic rock series derived from alkali-olivine basalt magma

Abstract

In the Quaternary Japanese alkaline rock province which occupies the coast of the Sea of Japan, calc-alkaline basalt and andesite occur in association with alkali basalts. Although basalt and andesite of the calc-alkaline series in this province has the same mineral assemblage as that of other Japanese provinces, such as the high-alumina basalt and tholeiite provinces, it has a higher concentration of alkalis and a lower concentration of CaO than the other two provinces. There is a general increase of Peacock's alkali-lime index from the Pacific coast to the Japan Sea with values of 66.8, 63.7, and 61.2 across the Japanese Islands. Geologic, petrographic, and petrochemical studies indicate that the basalt of the Japanese alkaline province is derived from an alkali-olivine basalt magma. If this alkali-olivine basalt magma assimilated about 5 to 10% of its weight of granitic material in early stages of fractionation, it would become a silica-saturated basaltic magma, from which andesite and dacite could have been produced in the middle and later stages of crystallization.