Thermal Structure Around the Shibukawa Ultramafic Body of the Mikabu Greenstones in the Low-Grade Sanbagawa Schists, Northwest Shizuoka, Central Japan

Abstract

The Cretaceous high-P/T metamorphic complex such as Sanbagawa schists and Mikabu greenstones is located in the outer zone of the Median Tectonic Line in the Shibukawa area, northwestern Shizuoka Prefecture, central Japan. In this area, a 2 × 1 km Shibukawa ultramafic body, a member of the Mikabu greenstones, occurs within the chlorite zone of the low-grade Sanbagawa schists. The upper part of this ultramafic body is covered by pelitic schists (classified as the Upper unit). The peak temperatures estimated from the Raman spectra of carbonaceous material (CM) in the pelitic schists were 277°C–354°C. The temperatures of the Upper unit range from 277°C to 293°C, which are lower than those of the surrounding Sanbagawa schists, ranging from 295°C to 354°C. Considering the elevation and large-scale structures of the body, the Upper unit is inferred to be a shallow, lower-grade Sanbagawa schist unit preserved in the ultramafic body, as it has a geological structure similar to that of the surrounding schists. This suggests that although the occurrence of the Shibukawa ultramafic body has complicated the geological structure of the Sanbagawa schists, the metamorphic temperature increases toward apparently lower structural levels. In addition to constraining metamorphic temperatures, it shows that the Raman CM geothermometer is a powerful tool for revealing structural characteristics based on geological and thermal relationships within low-grade metamorphic rocks in shallow subduction zones.