[S-MP24] Deformed rocks, Metamorphic rocks and Tectonics

[SMP24-P16] Thermal structure of the Sanbagawa belt in the Ayukawa-Sanbagawa area, Kanto Mountains, central Japan

*Yui Kouketsu¹, Ichiko Shimizu² (1.Department of Earth and Environmental Sciences, Graduate School of Environmental Studies, Nagoya University, 2. Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University)

Keywords:Sanbagawa belt (Sambagawa belt), Raman carbonaceous material geothermometry, Kanto Mountains, Pseudosection analysis

The thermal structures of the Ayukawa-Sanbagawa area of the Sanbagawa metamorphic belt in the Kanto Mountains were estimated using Raman carbonaceous material geothermometry. The temperatures of the pelitic schist in the chlorite zone, garnet zone, and biotite zone were estimated to be about 360–440°C, 400–460°C, and 470–520°C, respectively. The overlap between the chlorite and garnet zones is in harmony with previous reports that the occurrence of garnet is affected by the bulk rock chemical composition of Mn, and a similar trend is observed in the Sanbagawa belt in other regions such as the Kii Peninsula and Shikoku Island. On the other hand, the transition temperature from the garnet zone to the biotite zone in the Sanbagawa belt in the Kanto Mountains is around 460°C, while the transition temperature of the Asemigawa area in Shikoku is around 490°C. This indicates that the temperature at which biotite appears varies depending on the region. Two reasons for this difference were examined: (1) the effect of the bulk rock composition and (2) the effect of pressure. Pseudosection analysis suggests that the stability field of biotite varies with the amount of K₂O in the bulk rock composition. However, there is no significant difference in K₂O content among the bulk rock compositions reported for the Kanto Mountains and Shikoku. On the other hand, the stability field of biotite expanded toward lower temperatures with decreasing pressure. Therefore, it is suggested that the biotite zone of the Sanbagawa belt in the Kanto Mountains may have undergone metamorphism under lower pressure conditions than the Shikoku. The results of pseudosection analysis indicate that biotite appears at 470°C under pressure conditions of about 0.7 GPa, which is 0.1–0.25 GPa lower than the constrained pressure condition of 0.80–0.95 GPa in the albite-biotite zone of the Shikoku Sanbagawa belt. The cooling age of the Sanbagawa belt in the Kanto Mountains is reported to be about 20 Ma younger than that of the Asemigawa area in Shikoku Sanbagawa belt. The reason for the lower pressure condition may be due to the shallowing of the subducting slab caused by plate rejuvenation due to the approach of the ridge.