Petrological studies of the Cenozoic basaltic rocks from the western San-in district (II)

Basaltic rocks from the Abu district

Abstract

The Cenozoic volcanic rocks distributed in the Abu district, Yamaguchi Prefecture are divided into the following three groups from geologic relations and petrographic characters: 1) andesites and rhyolites of the calc-alkali rock series in Miocene age, 2) basalts of the tholeiitic series in Pliocene, 3) trachybasalts and trachyandesites of the alkali rock series, and andesites (so called quartz basalt) of the calc-alkali rock series in Quaternary. In this district, Quaternary trachybasalts, trachyandesites, and andesites are widely distributed contrasting to the other two groups. Of these, one of the second group and sixteen of the third are chemically analysed. Abundant quartz xenocrysts and granitic xenolithes are contained in some trachybasalts and trachyandesites, and in all andesites. On the other hand, it is notable that olivine nodules are included in some andesites. These facts are suggestive to consider that andesites are also to have derived from the alkali rock series by the contamination of granitic rocks as already pointed out by several petrologists.