Thermal Structure and EBSD Microstructural in Pelitic Schists of the Sanbagawa High-P/T Metamorphic Belt, Japan

The Sanbagawa Metamorphic Belt is a typical high-P/low-T metamorphic belt. We studied thermal structure and microstructures within pelitic schists in the Tenryu area, Shizuoka Prefecture, Japan. Previous studies of the Tenryu area based on the Degree of Graphitization (GD) of carbonaceous materials have reported a complex thermal structure^[1] and subdivided the area into chlorite, garnet, and, in some parts, biotite zones based on mineral assemblages. However, the relationship between GD values and peak metamorphic temperature remains unclear. In this study, Raman carbonaceous material geothermometry^[2][3] and SEM-EBSD analyses were applied to investigate the thermal structure and microstructure of pelitic schists collected from the Shirakura Unit, western Tenryu area. The main mineral assemblages of the pelitic schist included quartz, albite, muscovite, chlorite, and carbonaceous material, with garnet and calcite observed in some of the samples. The temperatures estimated using Raman carbonaceous material geothermometry ranged from 329 to 458 °C, with samples from the northeastern part of the study area exhibiting higher temperatures. The mean grain size of quartz ranges from 12 to 60 µm, whereas that of albite ranges from 15 to 75 µm. A positive correlation was partly observed between the Raman temperature estimates and the GD values. The mean grain sizes of quartz did not exhibit systematic variations with temperature, whereas those of albite we correlated with temperature. These findings clarify the tectono-metamorphic characteristics of the Tenryu area in the Sanbagawa Metamorphic Belt.

 

References

[1] Tagiri et al. (2000) Island Arc, 9, 188–203. [2] Aoya et al. (2010) Journal of Metamorphic Geology, 28, 895–914. [3] Kouketsu et al. (2014) Island Arc, 23, 33–50.