Spatial distribution of ~(87)Sr/~(86)Sr ratios of stream sediments in Shikoku Island and the Kii Peninsula, Southwest Japan

摘要

Geochemical mapping of ~(87)Sr/~(86)Sr ratios in Shikoku Island and the Kii Peninsula of Southwest Japan was performed using 233 samples of stream sediments (<180 μm). The spatial distribution of ~(87)Sr/~(86)Sr ratios successively increases from northern to southern geological units, with a gap at the Median Tectonic Line (MTL). In the Inner Zone divided by the MTL, a zone with Cretaceous granitoid rocks of the Ryoke Belt and sedimentary rocks of the Izumi Group, the ~(87)Sr/~(86)Sr ratios are measured at ~0.710. In the Outer Zone, a zone with Cretaceous metamorphic rocks of the Sanbagawa Belt and Jurassic to Miocene accretionary complexes of the Chichibu Belt and Shimanto Belts, the ~(87)Sr/~(86)Sr ratios tend to fall in the range from 0.706 to 0.715, tend to increase from north to south, and are highest in the youngest accretionary complex of the Southern Shimanto Belt. The ~(87)Rb/~(86)Sr-~(87)Sr/~(86)Sr plots for stream sediments more clearly reveal the differences and similarities of their bed- rocks, such as the different trends among the stream sediments of the Inner and Outer Zones. Samples collected from the Inner Zone show almost the same ~(87)Rb/~(86)Sr-~(87)Sr/~(86)Sr field as their source rocks, while most of the samples derived from accretionary complexes in the Outer Zone are plotted around a mixing line of older continental detritus and igneous-rock- derived materials. In light of the other geochemical characteristics observed, particularly the REE chemical features of the stream sediments and zircon ages in the basemental sedimentary rocks, the accretionary complexes in the Outer Zone may be mainly formed from the same source rocks, namely, igneous-rock-derived materials and continental detrital materials with different mixing ratios and a larger ratio of continental materials in the Southern Shimanto Belt. This study indicates that ~(87)Sr/~(86)Sr ratios of stream sediments strongly reflect the isotopic composition of the source rocks and are very useful for distinguishing the various geological and geochemical settings of the source rocks.