R8: Metamorphic rocks and tectonics

[R8-09] Microstructural and petrological characteristics of the Ryoke mylonite occurred along the Median Tectonic Line in the Shinshiro area, Aichi Prefecture

*Miharu Niwa^1,2, Katsuyoshi Michibayashi^1,3, Kenichiro Tani⁴, Takuma Nishimura⁵ (1. Nagoya Univ. Env., 2. TMNH, 3. JAMSTEC, 4. NMNS, 5. HMNH)

Keywords:Mylonite, Median Tectonic Line, Ryoke belt, Fault rock

The Median Tectonic Line (MTL) in Shinshiro City, Higashimikawa area, Aichi Prefecture, has a complex geology, with the Ryoke metamorphic rocks and plutonic rocks in the inner zone, the Sambagawa metamorphic rocks in the outer zone, and the Shidara Group that overlie the basement rocks. Although the fault rocks are exposed in this area along the MTL as like in Nagano, Shizuoka and Mie Prefecture, detail structural and petrological reports are rare compared to other areas. The fault rocks in this area are mostly cataclasite, and mylonite has been found near in the Horai-cho, Shinshiro City, we found continuous outcrops of mylonite in the Sakurabuchi Park, Shinshiro City, furter southwest from Hoai-cho. In this study, we observed microstructure with polarized light microscope, measured quartz crystal orientation with SEM-EBSD system, analyzed geochronological, in order to estimate deformation temperature, shear sense, and original rocks. Mylonite in this area is greenish-white in outcrop, and is characterized plagioclase porphyroclasts with white spots, as like Kashio mylonite. There are tonalitic rocks, composing of main constituently minerals, quartz, plagioclase, and biotite, with minor amount of K-feldspar, allanite, and zircon. The microstructure consists of a typical porphyroclastic texture composing of coarse-grained plagioclase and an asymmetric texture with plagioclase and fine quartz strain shadows. The quartz c-axis fabric shows a pattern of Y-maxima, which is formed at relatively intermediate temperature during deformation. In combination with the plagioclase-quartz porphyroclasts structure, which shows deformation during the progressive retrogression period, it is inferred that the quartz was formed by plastic shear deformation during the peak temperature from 350-450℃. Furthermore, the shear sense of the mylonite along the MTL, which is common in Nagano Prefecture, shows sinistral shear sense, whereas the shear sense of the mylonite in this area shows dextral shear sense.Based on these results, we discuss the regional structural characteristics of the Mikawa area in comparison with other areas.