5:15 PM - 6:30 PM
[SCG44-P04] Detailed characteristics of Raman spectra of carbonaceous materials in fault rocks nearby the Median Tectonic Line
Keywords:Raman spectral analysis, carbonaceous material, Median Tectonic Line, Sanbagawa belt, Ryoke belt, fault rock
Based on sensitivity of Raman spectra of carbonaceous material (CM) to the degree of graphitization of CM and the close relationship between the graphitization and metamorphic temperature, the Raman CM geothermometry has been developed and well used to estimate the maximum attained temperature in sedimentary and metamorphic rocks. In contrast, some studies pointed out that the degree of graphitization of CM is modified by brittle shearing. Therefore, to obtain the reliable maximum temperatures from fault rocks by this method, it is important to clarify the effects of brittle shearing on the Raman spectra of CM and here we conducted the Raman spectral analyses of CM in pelitic fault rocks nearby the Median Tectonic Line (MTL)—which is the largest on-land fault in Japan—in the Mizoguchi outcrop, Nagano Prefecture, central Japan.
In the Mizoguchi outcrop, the MTL is well exposed and recognized as the geological boundary separating the high-P/T Sanbagawa and low-P/T Ryoke belts. Five pelitic samples within 13 m from the MTL in the Sanbagawa side and three pelitic samples within 3.5 m in the Ryoke side were collected. As microstructural characteristics, brittle deformation structures are well developed in all samples and CM are contained in highly sheared matrix parts and also as inclusions in rock fragments or in porphyroblasts. In addition, the intensity of brittle shearing increases towards the MTL. To compare these structural characteristics, we respectively analyzed CM of matrix parts (matrix-CM) and of inclusion parts (inclusion-CM) and estimated temperature for each Raman spectrum by using the Raman CM geothermometry to use this value as an index of the degree of graphitization.
The average temperatures of inclusion-CM are ~370–430 ºC in the Sanbagawa belt and ~400–520 ºC in the Ryoke belt while those of matrix-CM are ~360–380 ºC in the Sanbagawa belt and ~370–410 ºC in the Ryoke belt. Comparison the data between inclusion- and matrix-CM in same samples implies lower average temperatures and smaller heterogeneities of matrix samples than those of inclusion-CM. In addition, as the special trends of the average temperatures, inclusion-CM show decreasing of temperature towards the MTL on both sides of the MTL while those of matrix-CM show no clear change. Based on a characteristic that temperature estimated by the Raman CM geothermometry increases as increasing the degree of graphitization of CM and the observed close relationship between microstructural characteristic and estimated temperature, effects of secondary brittle shearing result in the lower values of matrix-CM, while inclusion-CM preserve metamorphic conditions recorded before brittle shearing although some parts of the inclusion-CM (especially in samples within ~1m from the MTL) suffered brittle shearing. These suggest not only that the Raman spectral analysis of CM in fault rock is a useful tool to extract the metamorphic temperature recorded before faulting also that importance of careful selection of CM based on comparing microstructures of the brittle deformation.
In the Mizoguchi outcrop, the MTL is well exposed and recognized as the geological boundary separating the high-P/T Sanbagawa and low-P/T Ryoke belts. Five pelitic samples within 13 m from the MTL in the Sanbagawa side and three pelitic samples within 3.5 m in the Ryoke side were collected. As microstructural characteristics, brittle deformation structures are well developed in all samples and CM are contained in highly sheared matrix parts and also as inclusions in rock fragments or in porphyroblasts. In addition, the intensity of brittle shearing increases towards the MTL. To compare these structural characteristics, we respectively analyzed CM of matrix parts (matrix-CM) and of inclusion parts (inclusion-CM) and estimated temperature for each Raman spectrum by using the Raman CM geothermometry to use this value as an index of the degree of graphitization.
The average temperatures of inclusion-CM are ~370–430 ºC in the Sanbagawa belt and ~400–520 ºC in the Ryoke belt while those of matrix-CM are ~360–380 ºC in the Sanbagawa belt and ~370–410 ºC in the Ryoke belt. Comparison the data between inclusion- and matrix-CM in same samples implies lower average temperatures and smaller heterogeneities of matrix samples than those of inclusion-CM. In addition, as the special trends of the average temperatures, inclusion-CM show decreasing of temperature towards the MTL on both sides of the MTL while those of matrix-CM show no clear change. Based on a characteristic that temperature estimated by the Raman CM geothermometry increases as increasing the degree of graphitization of CM and the observed close relationship between microstructural characteristic and estimated temperature, effects of secondary brittle shearing result in the lower values of matrix-CM, while inclusion-CM preserve metamorphic conditions recorded before brittle shearing although some parts of the inclusion-CM (especially in samples within ~1m from the MTL) suffered brittle shearing. These suggest not only that the Raman spectral analysis of CM in fault rock is a useful tool to extract the metamorphic temperature recorded before faulting also that importance of careful selection of CM based on comparing microstructures of the brittle deformation.