[S-MP28] Deformed rocks, Metamorphic rocks and Tectonics

[SMP28-11] Comparison between the peak temperature and microstructures of pelitic schist in the Sanbagawa Belt, Shinshiro area, Chubu District, Japan.

*Akane Matsuzaki¹, Yui Kouketsu¹, Katsuyoshi Michibayashi^1,2 (1.Department of Earth and Planetary Sciences, GSES, Nagoya University, 2.Volcanoes and Earth's Interior Research Center, IMG, JAMSTEC)

Keywords:Sanbagawa metamorphic belt, Raman carbonaceous material geothermometer, pelitic schist

The Sanbagawa belt is a low-temperature, high-pressure metamorphic belt extending from Kanto to Kyushu on the outer belt side adjacent to the Median Tectonic Line. The Sanbagawa crystalline schist and the intermittently distributed Mikabu greenstones are distributed in the southern part of Shinshiro City, Aichi Prefecture, and the metamorphic grade of the Sanbagawa crystalline schists in the southern part of Shinshiro City belongs to chlorite zone^[1]. The deformation mechanisms of the Sanbagawa Belt have been interpreted differently in different study areas, and the structural development of the Sanbagawa Belt is unknown as a whole, so studies in different areas are needed. This study focused on the pelitic schist of the Sanbagawa crystalline schist in this region, with the aim of clarifying microstructural characteristic associated with the thermal structure. Twenty-four schist samples were observed in polished thin sections using polarized light microscopy. The main constituent minerals of the pelitic schist were quartz, muscovite, albite, chlorite and carbonaceous material. Raman CM geothermometer^[2–5] was applied to 16 pelitic schist samples to estimate their peak temperatures by Raman spectra of the carbonaceous material. The grain size and crystallographic orientation of quartz and albite in the pelitic schist were measured by electron backscatter diffraction (SEM-EBSD) of a scanning electron microscope. As a result, the temperatures calculated by the Raman CM geothermometer were in the range of about 300–400°C. Pelitic schists distributed close to the Median Tectonic Line to the north of the green schists layers have relatively higher temperatures of around 350–400°C, whereas those to the south have lower temperatures of around 300–330°C. SEM-EBSD analysis revealed that quartz and albite coexist at all temperatures. Recrystallized and relict grains of quartz are separated following to Cross et al. (2017)^[6]. The results showed that the average size of the recrystallized quartz grains was 9–14 µm irrespective of temperature, whereas the average size of relict grains increased from 15 µm to 43 µm, as increasing temperature. The maximum grain size ranged from 30–100 µm and was positively correlated with the peak metamorphic temperature. The coexisting albite average grain size was from 10 µm to 25 µm and a maximum grain size was from 22 µm to 144 µm, as increasing temperature. We discuss microstructural development based on these results.