The so-called “quartz basalts” from Iki island

Abstract

The so-called “quartz basalt” are distributed in the northern part of Iki island in lava flows and dikes in Pleistocene time. These rocks belong to the calc-alkali rock series derived from the contaminated alkali olivine basalt magma as pointed out by K. Aoki. From their petrographic characters, the rocks are classified into following two types, pyroxene basalt-andesite and hornblende andesite. The latter is smaller in its area and more basic than the former. Fourteen of these rocks are newly analysed and it is found that they are generally poor in MgO and richer in FeO+Fe2O3 than the quartz basalts from the Abu district, Yamaguchi Prefecture. Alkali contents of the quartz basalts are richer than that of the calc-alkali rock series derived from tholeiite magma and poorer than the alkali rock series. It is characteristic of these rocks to increase rapidly K2O/Na2O ratio as SiO2 increases.