2:45 PM - 3:00 PM
[SMP28-17] Development of mylonite along the Median Tectonic Line in the Shinshiro area, Aichi Prefecture
Keywords:mylonite, Median Tectonic Line, Ryoke metamorphic belt
The Median Tectonic Line (MTL) is one of the most prominent faults in Japan, and its tectonic boundaries separate the Ryoke metamorphic belt in the Inner Zone from the Sanbagawa metamorphic belt in the Outer Zone. Although many studies have been conducted on the MTL, there are still many uncertainties regarding its deeper structure. In the Shinshiro area, Higashi-Mikawa, Aichi Prefecture, outcrops around the MTL contain a complex geology, with Ryoke metamorphic rocks and granitoids, Sanbagawa schists, and Shitara Group.
In this study, microstructural observations using microscopy, quartz c-axis CPOs using the SEM-EBSD system, and zircon U-Pb geochronology of mylonite collected from outcrops along the MTL in the Shinshiro area, Aichi Prefecture, were performed to investigate the deformation temperature, shear sense, and protolith. The mylonite is tonalitic and has an asymmetric porphyroclastic texture, consisting mainly of coarse-grained plagioclase, indicating dextral shear in this area, in contrast to the other mylonite showing a sinistral sense of shear along the MTL. The zircon U-Pb ages are ca. 80 Ma, which is close to the age of one of the three previously reported magmatic pulses of the Ryoke granitoids in the Mikawa area. The quartz c-axis fabric has an intense Y-maximum with a weak cross-girdle pattern, which is formed at the median temperature during deformation. In combination with the plagioclase-quartz microstructures, mylonite is likely to have been formed by plastic shear deformation during progressive retrogression from high-T to low-T conditions.
We discuss the tectonic conditions of the mylonite formation in this area in comparison with other regions.
In this study, microstructural observations using microscopy, quartz c-axis CPOs using the SEM-EBSD system, and zircon U-Pb geochronology of mylonite collected from outcrops along the MTL in the Shinshiro area, Aichi Prefecture, were performed to investigate the deformation temperature, shear sense, and protolith. The mylonite is tonalitic and has an asymmetric porphyroclastic texture, consisting mainly of coarse-grained plagioclase, indicating dextral shear in this area, in contrast to the other mylonite showing a sinistral sense of shear along the MTL. The zircon U-Pb ages are ca. 80 Ma, which is close to the age of one of the three previously reported magmatic pulses of the Ryoke granitoids in the Mikawa area. The quartz c-axis fabric has an intense Y-maximum with a weak cross-girdle pattern, which is formed at the median temperature during deformation. In combination with the plagioclase-quartz microstructures, mylonite is likely to have been formed by plastic shear deformation during progressive retrogression from high-T to low-T conditions.
We discuss the tectonic conditions of the mylonite formation in this area in comparison with other regions.