5:15 PM - 5:30 PM
[R8-08] Pressure and temperature conditions for the occurrence of index minerals in pelitic schists in the Sanbagawa belt, Kanto Mountains, Japan, inferred from Raman carbonaceous material geothermometry
Keywords:Raman carbonaceous material geothermometry, Pseudosection modelling, Garnet, Biotite, Oligoclase
The Sanbagawa belt in the Kanto Mountains has long been studied as a type locality for the Sanbagawa metamorphic belt, but the occurrence of index minerals and the graphitization degree (GD) show a complicated distribution, and the boundaries of isograd vary among researchers. In this study, we applied Raman carbonaceous material (CM) geothermometry to pelitic and siliceous schists collected in the Ayukawa-Sanbagawa area to estimate their temperature structures, and conducted pseudosection modelling to analyze the stability field of index minerals, such as garnet, biotite, and oligoclase. The application of Raman CM geothermometer to the Ayukawa-Sanbagawa area resulted in temperatures ranging from 360°C to 520°C: Chlorite zone is about 400-440°C, Garnet zone is about 360-470°C, and Biotite zone is about 470-520°C, respectively. A pseudosection diagram of the pelitic schist of the Biotite zone (AM41P, Miyashita 1997), which shows the average bulk rock composition of the Kanto Sanbagawa belt, shows that garnet, biotite, and oligoclase occur with increasing temperature. The stability field of garnet strongly affected by the bulk rock composition of MnO, as shown in previous studies, which explains the overlap of the temperature ranges of the Chlorite and Garnet zones. Compared with the temperature data, the biotite zone in the Kanto Mountains was formed at about 0.8 GPa, and may have experienced lower pressure conditions than the Shikoku. Oligoclase (XAn > 0.1) appears at about +20-30°C after the appearance of biotite under low pressure conditions up to about 0.9 GPa, but the stability field tends to shift to the higher temperature side under high pressure conditions above 0.9 GPa. This result is consistent with the hypothesis that there is a pressure difference between the Kanto Mountains, where the oligoclase appears homogenously in the biotite zone, and the Shikoku, where the Albite-biotite zone and the Oligocase-biotite zone are clearly separated.