| Summary: | 87Sr/86Sr ratios were examined for volcanic rocks from Quaternary volcanoes in the Kurile arc in Hokkaido, Japan. The results show that the ratios decrease immediately away from the volcanic front, then increase in the middle part of the arc, but decrease again toward the back-arc side. The complex lateral variations of the ratios can be ascribed to breakdown of different minerals in the mantic wedge, i.e. amphibole decomposes beneath the volcanic front, whereas phlogopite breaks down beneath the middle part of the arc. Geochemical characters of the rocks from Rishiri volcano, located furthest away from the volcanic front arc quite different from those commonly observed in the trench-arc systems. The characters are similar to those of lavas from Deception island, located in Bransfield Strait formed by bad-arc spreading. This suggests that the Rishiri volcanism is independent of the arc magmatism.
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