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[SMP24-01] Structural relationship between the Mikabu ultramafic body and Sanbagawa pelitic schists in the Shibukawa area, NW Shizuoka Prefecture
Keywords:Sanbagawa metamorphic belt, Mikabu belt, Raman carbonaceous material geothermometry, pelitic schist, ultramafic rock
The Sanbagawa Belt is a low-temperature, high-pressure metamorphic belt that generally locates between the Median Tectonic Line at the northern boundary and the Chichibu Belt at the southern boundary. The Sanbagawa Belt is an important geological unit for understanding the structural development and material cycles in the subduction zone. In the Shibukawa area of NW Shizuoka Prefecture, the Sanbagawa and the Mikabu Belts are alternately distributed on the outer side of the Median Tectonic Line. Although the Shibukawa ultramafic body, which is 3 × 2 km in size, occurs in the pelitic schists characterized by the low-grade chlorite zone of the Sanbagawa Belt, it appears to be an accumulation rock that belongs to the Mikabu Belt based on its lithological characteristics (Shioya et al. 2021). In this study, we conducted field surveys and sample analyses to reveal the structural relationship between the Shibukawa ultramafic body of the Mikabu Belt and the surrounding pelitic schists of the Sanbagawa Belt. The pelitic schists in the Shibukawa area commonly showed well-developed fold structures in outcrops. Outcrops of pelitic schists are also present in the Shibukawa ultramafic body. Polished thin sections of the sampled pelitic schists showed that the major minerals were quartz, albite, chlorite, and carbonaceous material, and that some samples contain apatite, rutile, calcite, lawsonite, pyrite, and titanite.
The analytical methods are metamorphic temperature estimation by applying Raman carbonaceous material geothermometry, which estimates the maximum experienced temperature from the crystallinity of carbonaceous material within the pelitic schists (Kouketsu et al. 2014, Kaneki & Kouketsu 2022), and grain size measurements of quartz and albite using scanning electron microscope. The results showed a temperature range of 277–479°C. The pelitic schists in the Shibukawa ultramafic body tended to have relatively lower temperatures than those outside the ultramafic body. Mean grain sizes of quartz showed a slightly positive correlation with increasing temperature, whereas the mean grain size of albite showed a stronger positive correlation than that of quartz. Based on the relationship between the structural attitudes of the pelitic schist and the estimated temperatures, it is suggested that the Shibukawa area has a geological structure where the metamorphic temperature increases toward the lower structural level. In addition, the pelitic schists in the Shibukawa ultramafic body were characterized by lower temperatures than the surrounding pelitic schists, even considering the difference in elevation and thermal structure. This suggests that the Shibukawa ultramafic body of the Mikabu Belt was structurally higher than its present form during the peak metamorphism occurred in the pelitic schists of the Sanbagawa Belt.
[References]
Kouketsu, Y., et al., 2014, Island Arc, 23, 33–50.
Kaneki, S., Koukestu, Y. 2022, Island Arc, 31, e12467.
Shioya, H., 2021, J. Geol. Soc. Japan, 127, 59–65.
The analytical methods are metamorphic temperature estimation by applying Raman carbonaceous material geothermometry, which estimates the maximum experienced temperature from the crystallinity of carbonaceous material within the pelitic schists (Kouketsu et al. 2014, Kaneki & Kouketsu 2022), and grain size measurements of quartz and albite using scanning electron microscope. The results showed a temperature range of 277–479°C. The pelitic schists in the Shibukawa ultramafic body tended to have relatively lower temperatures than those outside the ultramafic body. Mean grain sizes of quartz showed a slightly positive correlation with increasing temperature, whereas the mean grain size of albite showed a stronger positive correlation than that of quartz. Based on the relationship between the structural attitudes of the pelitic schist and the estimated temperatures, it is suggested that the Shibukawa area has a geological structure where the metamorphic temperature increases toward the lower structural level. In addition, the pelitic schists in the Shibukawa ultramafic body were characterized by lower temperatures than the surrounding pelitic schists, even considering the difference in elevation and thermal structure. This suggests that the Shibukawa ultramafic body of the Mikabu Belt was structurally higher than its present form during the peak metamorphism occurred in the pelitic schists of the Sanbagawa Belt.
[References]
Kouketsu, Y., et al., 2014, Island Arc, 23, 33–50.
Kaneki, S., Koukestu, Y. 2022, Island Arc, 31, e12467.
Shioya, H., 2021, J. Geol. Soc. Japan, 127, 59–65.