[SCG63-P18] 三重県中央構造線に沿って分布するマイロナイト帯の発展史
キーワード：中央構造線、歪局所化、マイロナイト、石英C軸ファブリックパターン、石英動的再結晶
The Median Tectonic Line (MTL) is the longest fault in Japan over 800 km and is an excellent place to study rock deformation. Both the brittle and plastic deformation associated with the fault zone is crucial for the understanding of fault development. A ductile shear zone with sinistral sense was first developed at the end of the Cretaceous period (74-67 Ma, Shimada, et al., 1998) at the north block of the Median Tectonic Line where the Ryoke granite is located. During Early Paleogene (63-58 Ma, Kubota and Takeshita, 2008), it joined together with the Sanbagawa belt located in the southwestern outer zone, forming the current of MTL. In this study, the localization of deformation was described for the plastically deformed Ryoke granites, and their development is also discussed.
In recent years, studies on the plastic deformation of mylonites at the Median tectonic line has been actively conducted. According to Dong (2019), based on the c-axis fabric of quartz microstructure, the mylonites could be categorized as tonalitic protomylonite zone (distance from Median tectonic line, 0-350m), granitic mylonite zone with S-type quartz texture (350-500m) and granitic mylonite zone with P-type quartz texture (500-800m). The definition of the S-type and P-type quartz microstructures are based on Masuda and Fujimura (1981). In the S-type mylonite zone, a Y-maxima c-axis fabric (estimated deformation temperature: 400-500 degrees, e.g. Takeshita, 1996) and a type II cross girdle c-axis fabric(300-400 degrees) was developed. By comparing the mylonite zone with P-type quartz microstructure with the mylonite zone with S-type quartz microstructure, the grain size of quartz grains has been reduced from 140 microns to 20-30 microns, where the quartz grains are becoming finer. In the mylonite zone with a P-type quartz microstructure, only the Y-maximum c-axis fabric develops as it experiences a relatively higher deformational temperature. Based on the above-mentioned results, first, a mylonite zone with a P-type quartz microstructure was formed, and then the deformation stopped at a distance of more than 500 m north of the MTL. The deformation continued at a distance of 350-500 m as the temperature decreased. Thus, a localization model for the deformation that forms the S-type mylonite zone was proposed (Dong, 2019).
In this study, we conducted field surveys and analyzed the microstructure of the Tsukide region over an area of 2km width and 1km north from the MTL. To analyze the deformational characteristics of K-feldspar, the K-feldspars are stained with sodium cobaltinitrite. The following new findings were obtained from the study. (1) The K-feldspar phenocrysts in the mylonite zone with S-type quartz microfabric located at 350-500 m are weakly deformed and not damaged. The K-feldspar is plastically deformed in the mylonite zone with P-type quartz microfabric located at 500-900 m. (2) The K-feldspar in the mylonite zone with S-type quartz microfabric has a smaller grain size, and it was observed to be associated with the development of the EW trending fault. Grain size reduction was seen at the northern block of the fault, but it was not observable in the southern block.
Based on the above results, the following considerations were made on the development of the MTL structure. In the mylonite zone with S-type quartz microfabric, K-feldspar is not altered and maintains the original phenocryst texture, therefore it cannot be fully interpreted by the progressive the deformational process as proposed by Dong (2019). One of the possible explanations is that the mylonite zones with P-type and S-type quartz microfabric have different source rocks and are formed under different conditions, and they may have been juxtaposed together by the possible presence of faults.
In recent years, studies on the plastic deformation of mylonites at the Median tectonic line has been actively conducted. According to Dong (2019), based on the c-axis fabric of quartz microstructure, the mylonites could be categorized as tonalitic protomylonite zone (distance from Median tectonic line, 0-350m), granitic mylonite zone with S-type quartz texture (350-500m) and granitic mylonite zone with P-type quartz texture (500-800m). The definition of the S-type and P-type quartz microstructures are based on Masuda and Fujimura (1981). In the S-type mylonite zone, a Y-maxima c-axis fabric (estimated deformation temperature: 400-500 degrees, e.g. Takeshita, 1996) and a type II cross girdle c-axis fabric(300-400 degrees) was developed. By comparing the mylonite zone with P-type quartz microstructure with the mylonite zone with S-type quartz microstructure, the grain size of quartz grains has been reduced from 140 microns to 20-30 microns, where the quartz grains are becoming finer. In the mylonite zone with a P-type quartz microstructure, only the Y-maximum c-axis fabric develops as it experiences a relatively higher deformational temperature. Based on the above-mentioned results, first, a mylonite zone with a P-type quartz microstructure was formed, and then the deformation stopped at a distance of more than 500 m north of the MTL. The deformation continued at a distance of 350-500 m as the temperature decreased. Thus, a localization model for the deformation that forms the S-type mylonite zone was proposed (Dong, 2019).
In this study, we conducted field surveys and analyzed the microstructure of the Tsukide region over an area of 2km width and 1km north from the MTL. To analyze the deformational characteristics of K-feldspar, the K-feldspars are stained with sodium cobaltinitrite. The following new findings were obtained from the study. (1) The K-feldspar phenocrysts in the mylonite zone with S-type quartz microfabric located at 350-500 m are weakly deformed and not damaged. The K-feldspar is plastically deformed in the mylonite zone with P-type quartz microfabric located at 500-900 m. (2) The K-feldspar in the mylonite zone with S-type quartz microfabric has a smaller grain size, and it was observed to be associated with the development of the EW trending fault. Grain size reduction was seen at the northern block of the fault, but it was not observable in the southern block.
Based on the above results, the following considerations were made on the development of the MTL structure. In the mylonite zone with S-type quartz microfabric, K-feldspar is not altered and maintains the original phenocryst texture, therefore it cannot be fully interpreted by the progressive the deformational process as proposed by Dong (2019). One of the possible explanations is that the mylonite zones with P-type and S-type quartz microfabric have different source rocks and are formed under different conditions, and they may have been juxtaposed together by the possible presence of faults.