The optical and chemical studies have been made on basaltic rocks of the Tara-dake volcano in the northwestern part of Kyushu, Japan. The volcanic rocks are classified into three groups, such as rhyolitic, andesitic and basaltic rocks. The basaltic rocks are subdivided into three rock series as follows;
(1) Alkali rock series
(2) Tholeiitic rock series
(3) Calc-alkali rock series.
The extrusion of the basal basaltic rocks took place during Pliopleistocene descending order of (1), (2) and (3). The extrusion of the Taradake volcanic rocks may occur in Pleistocene with some basaltic rocks following the activities of the basal volcanic rocks. It is noted that the chemical compositions of the Tara-dake basaltic rocks are slightly higher in CaO and MgO contents and are somewhat lower in total FeO and K
2O contents than those of basaltic rocks of the adjacent areas. Most of the basaltic rocks of this district belong to either alkali rock series or calc-alkali rock series. The basalt of alkali rock series may be formed by differentiation of parental alkali olivine basalt magma, however, the basalt and andesite of calc-alkali rock series may be derived from contaminated alkali olivine basalt magma. The calc-alkali rock series shows two differentiation trends, the one of which is a fractional trend of basaltic magma contaminated at early stage and the other is at somewhat later stage.
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