Abstract
We obtained a K-Ar age of 3.32 Ma (Pliocene epoch) for an alkali basalt sample from Kyuroku-shima Island in the northeast margin of Japan Sea. The sample is normally magnetized and probably correlates to the Gauss normal epoch. The N-type MORB-normalized incompatible element pattern and the K2O/TiO2 ratio. indicate that the alkali basalt has a character of typical island arc. The petrography and chemistry of the alkali basalt is similar to those of basalts from present Chokai Volcanic Zone, especially those from Oshima-Oshima volcano. A regional study of Pliocene volcanoes by Shuto (1989) and ours revealed an existence of the lateral variation back then at least within our study area.
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