Structural and thermal characteristics around an ultramafic body in the low-grade Sanbagawa pelitic schists, central Honshu Island, Japan

We investigated the microstructures in the pelitic schists of the high-P/low-T Sanbagawa metamorphic belt in the Shibukawa area, northwestern Shizuoka Prefecture, Japan, which are mostly classified as low-grade, non-spotted schists, and delineated the thermal structure and microstructure of the Sanbagawa pelitic schists in the vicinity of an ultramafic block using a Raman carbonaceous material (CM) geothermometer and SEM-EBSD crystallographic analysis. The pelitic schists consist of alternating mica-rich and quartzofeldspathic layers. The mica-rich layers consist mainly of muscovite, chlorite, and CM, whereas the quartzofeldspathic layers consist mainly of quartz and albite. The maximum experienced temperatures estimated from the carbonaceous material (CM) were 277–354°C, corresponding to the metamorphic temperature of the chlorite zone. In the quartzofeldspathic layers, both quartz and albite showed subgrain boundaries with undulatory extinctions, indicating plastic deformation. The crystallographic preferred orientations (CPOs) of the quartz grains within the pelitic schists show weak but distinct patterns somehow resembling a type-I cross girdle, whereas those of the albite locally show (100)[001] patterns. The average sizes of both quartz and albite grains were at around 10 µm for all samples and increased slightly with increasing modal composition, independent of the Raman CM temperature. This suggests that the duration time of the peak metamorphic temperature may not be long enough to mature metamorphic textures in the pelitic schists. As a consequence, the microstructures in the pelitic schists would result from the interaction between the grain growth due to heating and the grain size reduction due to deformation in the subduction zone.