ABSTRACT

A number of volcanoes are located on the continental side of the volcanic front in central Japan, where the Northeast Japan and Izu-Ogasawara arcs join. These volcanoes are distributed not only along the volcanic front but also in an area extending up to 150 km away from the volcanic front. We determined ⁸⁷Sr/⁸⁶Sr ratios, and Rb and Sr contents of 40 volcanic rock samples from 11 volcanoes in this area and summarize, together with data already published, the spatial variation of the ratios. The range of ⁸⁷Sr/⁸⁶Sr ratios of volcanic rocks from volcanoes of the Hakusan group, situated far from the volcanic front, is 0.70648-0.70842 (above 0.7070 for most samples). While, the range of the ratios from volcanoes of the Norikura group, situated between the Hakusan group and the volcanic front, is 0.70515-0.70730 (0.7060-0.7070 for most samples). Since the ⁸⁷Sr/⁸⁶Sr ratios of volcanic rocks sampled along the volcanic front lie around 0.70400, the ratios increase with distance from the volcanic front. This trend is opposite to that observed in the Northeast Japan and Izu-Ogasawara arcs. The spatial variation in chemical composition of volcanic rocks from central Japan is neither systematic nor corresponding to Sr isotopic variation. These features suggest that the crustal processes control the magma compositions near the arc junction in central Japan, rather than the mantle-slab interaction.