1985 年 30 巻 4 号 p. 273-286
The 87Sr/86Sr ratios and the abundances of rubidium and strontium have been determined for 40 samples of Unzen, Tara and Kumamoto-Kimpo volcanoes, and Neogene basaltic rocks from the surrounding areas in Kyushu, Southwest Japan. The 87Sr/86Sr ratios in these Quaternary volcanic rocks range from 0.70418 to 0.70509. In each volcano, almost all of the magma seems to be subjected to the assimilation with different 87Sr/86Sr ratios during crystal fractionation. The 87Sr/86Sr ratios of the Quaternary volcanic rocks are clearly distinguished between the northern part and the southern part of Kyushu region marked by the Shiobara-Beppu Tectonic Graben in the central Kyushu. The 87Sr/86Sr ratios in the tholeiitic basalts, alkaline basalts and calc-alkaline basalts from the areas, range from 0.70362 to 0.70397, from 0.70404 to 0.70487 and from 0.70451 to 0.70470 (except for a calc-alkaline basalt from Kumamoto-Kimpo area, 0.70363), respectively. In the southwest Japan, the 87Sr/86Sr ratios of Cenozoic basalts from the Kyushu region are lower than those of basalts from the San-in region. This means that there exists a regional heterogeneity of Sr isotopic ratios under the Southwest Japan arcs.