12:00 PM - 12:15 PM
[SMP28-12] Structural characteristics of the Sanbagawa schists in the Tenryu area, NW Shizuoka, Japan
Keywords:Sanbagawa Belt, Tenryu, Pelitic schist, Raman carbonaceous material geothermometer
The Sanbagawa Belt in the northwestern part of Shizuoka Prefecture is divided into the Tenryu area in the northeast and the Shibukawa area in the southwest, bound by the Atago-gawa fault. 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 reliability of GD as a temperature indicator requires re-evaluation because the process of powdering carbonaceous material may influence the measurement results. In this study, Raman carbonaceous material geothermometry[2][3] and SEM-EBSD analyses were applied to investigate the thermal structure and microstructure of primarily pelitic schists collected along the Okuyama-gawa and Izunma-gawa River routes in the 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 samples. The temperatures estimated using Raman carbonaceous material geothermometry ranged from 364 °C to 458 °C, with samples from the northeastern part of the study area exhibiting higher temperatures. Garnets were identified only in samples that exceeded 395 °C. The mean grain size of quartz ranges from 12 to 60 µm, whereas that of albite ranges from 15 to 75 µm. The c-axis fabric of quartz predominantly exhibits Type-Ⅰ cross-girdle patterns. A positive correlation was observed between the Raman temperature estimates and GD values, suggesting that GD may serve as an index of the thermal structure in the Tenryu area. Additionally, a reconstructed cross-section showing schistosity revealed a fold structure with a NE–SW trend axis, and the temperature distribution was inferred to be parallel to the schistosity. The mean grain size of quartz did not exhibit systematic variations with temperature; on the other hand, that of albite was correlated with temperature. Based on these findings, combined with the development of a crystallographic fabric in quartz, we discuss the structural development in the pelitic schists of the Tenryu Sanbagawa Belt.
[1] Tagiri et al. (2000) Island Arc, [2] Aoya et al. (2010) J. Metamor. Geol., [3] Kouketsu et al. (2014) Island Arc.
[1] Tagiri et al. (2000) Island Arc, [2] Aoya et al. (2010) J. Metamor. Geol., [3] Kouketsu et al. (2014) Island Arc.