Strontium isotopic ratios of the volcanic rocks from Dogo of the Oki Islands, Japan

Abstract

The isotopic composition of strontium and the abundances of rubidium and strontium in volcanic rocks from Dogo of the Oki Islands and Takashima in the northwest Kyushu, West Japan, and Ulrungdo of Korea, have been determined. The rubidium and strontium contents for alkaline basalts range from 27.6 to 51.2 ppm and 444 to 723 ppm, and 148 to 208 ppm and 3.7 to 205 ppm for intermediate to felsic suites, respectively.
The alkaline basalts are divided into two groups with ⁸⁷Sr/⁸⁶Sr ratios of the restricted ranges of 0.70481-0.70496 and 0.70540-0.70575, respectively. However, the ⁸⁷Sr/⁸⁶Sr ratios of intermediate to felsic rocks of Dogo are relatively high and variable ranging from 0.70706 to 0.71019, which probably reflect the contamination and/or produced by body or partial melting of the basement rocks in this area without crustal assimilation of basaltic magma. In addition, the lead isotopic results indicate that the melting of Precambrian basement rocks possibly yields less radiogenic lead.
In the southwest Japan, the ⁸⁷Sr/⁸⁶Sr ratios of Cenozoic basaltic rocks are clearly different between the San-in and the northwest Kyushu regions, which includes Jeju island, The higher ⁸⁷Sr/⁸⁶Sr ratios of basalts from the San-in region than that of basalts from the northwest Kyushu region also reflect the different properties of the upper mantle, which means there is regional heterogeneity of Sr isotopic ratios under the southwest Japan arcs. Furthermore, the relatively high and variable ⁸⁷Sr/⁸⁶Sr ratios of volcanic rocks are particularly concentrated in the southwestern Japan arcs which has probably more continental properties than the northeastern Japan arcs.