Strontium isotopic characteristics of arc volcanic rocks at the initial stage of subduction in western, Japan

Abstract

In western Japan, the Philippine Sea plate subducts northwestward from the Ryukyu Trench and the Nankai Trough, to form two volcanic arcs, the Ryukyu and Southwest Japan arcs, respectively. These arcs are thought to be in the initial stage of subduction, because microearthquake data show that the subducting slab extends only a short distance from the trenches. In the Southwest Japan arc, volcanoes are aligned parallel to the Nankai Trough, although the leading edge of the subducted slab does not reach under the volcanic arc. To the south, the subducted slab reaches just under the Ryukyu volcanic arc. We determined 87Sr/ 86Sr ratios and Rb and Sr contents for 39 volcanic rock samples from 14 Quaternary volcanoes of both arcs and summarized the spatial distribution of the ratios. The ranges of 87Sr/ 86Sr ratios of volcanic rocks from the Ryukyu and Southwest Japan arcs are 0.7040–0.7069 and 0.7035–0.7066, respectively. In the Ryukyu arc, the regional variation in 87Sr/ 86Sr ratios of volcanic rocks is hidden under the wide scatter of ratios of different volcanic rocks from each one volcano. In the Southwest Japan arc, the ratios of volcanic rocks from volcanoes on the volcanic front vary in the along-arc direction; 0.7040–0.7048 for the western part, 0.7035–0.7039 for the central part and 0.7048–0.7066 for the eastern part. The across-arc variations in the ratios are different from place to place. These isotopic features suggest the absence of slab-derived components in the source mantle and reflect the heterogeneous nature of 87Sr/ 86Sr ratios in the upper mantle, modified by crustal processes. This coincides with the seismological observation that the subducting Philippine Sea slab does not reach under the volcanic arc at all.