Bimodal associations of mafic and silicic rocks are observed at Kikai volcano in the Ryūkyū arc, Japan. We determined 87Sr/86Sr ratios of 16 representative volcanic rocks of this volcano. The ratios scatter in the narrow range between 0.70477 and 0.70508, despite the varying SiO2 content of the rocks, except for one sample with a ratio of 0.70539. This suggests that both basaltic and rhyolitic rocks at Kikai volcano are generated from the same source material, in contrast to the separate origin required for the bimodal volcanism typically observed in the western United States. The origin of the bimodal volcanism in the island-arc setting is distinctly different from that in the continental setting, in view of source materials of basaltic and rhyolitic magmas.

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