Strontium Isotopic Ratios of the Volcanic Rocks in the Western Part of the San-in Region and the Surrounding Area, Southwest Japan

Abstract

The isotopic ratios of strontium in the volcanic rocks in the western part of the Sanin region and the surrounding area, Southwest Japan, have been determined. The distribution of the Cenozoic volcanic rocks can be divided into three districts. The ⁸⁷/Sr⁸⁶Sr ratios in the alkaline basalts and calc-alkaline andesites from the region, range from 0.7042 to 0.7059 and 0.7036 to 0.7043, respectively, with some exceptions. The formation of the calc-alkaline andesites by contamination of the upper mantle-derived alkaline basalt magma with crustal materials is not required by the isotopic evidence, but it may be formed by direct melting of the underlying lower crustal materials with isotopic inhomogeneity. The ⁸⁷Sr/⁸⁶Sr ratios in the volcanic rocks of the Aono volcano group, lie in the range from 0.70352 to 0.70389. Such isotope ratios are characteristic of most young island arc volcanic rocks. Similar ratios are found in the volcanic rocks of Fuji, Hakone and Izu areas in the Izu-Ogasawara arc as well, against in the range of 0.70485-0.70522 for Daisen and Sambe volcanics. If the basaltic rocks in the southwest Japan were formed by melting of the upper mantle, the ⁸⁷Sr/⁸⁶Sr ratios would be close to the range from 0.7043 to 0.7065 for the San-in region and 0.7038 to 0.7043 for the northwest Kyushu region, respectively. Besides, a view that East Japan and West Japan have different geological structures, has lately become prevalent. The ⁸⁷Sr/⁸⁶Sr ratios of Cenozoic volcanic rocks are clearly different between the East Japan arcs and the Southwest Japan arcs divided by a tectonic line-Fossa Magna in central Japan.