10:45 〜 11:00
[SMP25-07] Rheological and metamorphic evolutions of high-grade Shimanto accretionary complex in Kerama islands, Ryukyu
The Kerama Formation in the Ryukyu Arc is located at the southwestern edge of the Shimanto Belt, made up of repetition of greenstone, metasandstone, and pelitic schist. Sedimentation and metamorphic ages were estimated to be ca. 105 and ca. 91 Ma, respectively, based on zircon U-Pb and muscovite K-Ar ages (Yamamoto et al, submitted). Occurrence of terrigenous metasediment and N-MORB type greenstone contacts are intrusive in most places, suggesting ocean floor spreading occurred near the trench (Chinen et al., 2004; Yamamoto et al., submitted).
Although metamorphic mineral assemblage of greenstone (epidote + actinolite; Hashimoto, 1978) and the existence of mylonite originated from conglomerate (containing quartz, plagioclase, K-feldspar, muscovite and chlorite; Yamamoto et al., submitted) suggest metamorphism and deformation occurred at high temperature conditions, there have been few reports on the detailed metamorphic temperature estimations and deformation microstructures of the Kerara Formation before. In this study, we present results of Raman spectroscopy of carbonaceous material (RSCM) thermometry and electron backscatter diffraction (EBSD) analyses of the rocks of the Kerama Formation to constrain peak metamorphic temperature and rheological condition related to subduction of spreading center.
RSCM analyses were performed using a Raman spectrometer with a 532 nm Nd-YAG laser (system installed at Nagoya University). Peak metamorphic temperature of pelitic schist and conglomerate mylonite (originated from gravel sandstone) was estimated to be 470–525°C, which is consistent with the temperature range of metamorphic facies proposed in the greenstone.
Measurement of grain size and crystallographic preferred orientations (CPO) in quartz were conducted by SEM-EBSD-EDS system in GSJ-Lab (HITACHI SU3500 SEM mounted with Oxford EBSD detector). Recrystallized quartz region from a sample of conglomerate mylonite shows type II crossed girdle CPO. According to the quartz c-axis fabric opening-angle thermometer of Faleiros et al (2017), it is suggested that the recrystallized quartz region from the sample of conglomerate mylonite was deformed under ~530°C, which corresponds to the peak metamorphic temperature estimated from RSCM thermometry. The mean grain size of quartz in the mylonite is ~80 µm, which suggests the differential stress magnitudes of ~30 MPa (Cross et al. 2017).
In contrast to common occurrence of mylonite along fossil fault zones in continental lithosphere, mylonite in subduction setting is rare: no mylonite has reported from the Shimanto Belt so far. The geology of Kerama islands would be a strong constrain for rheology of subduction plate interface at high temperature condition.
Although metamorphic mineral assemblage of greenstone (epidote + actinolite; Hashimoto, 1978) and the existence of mylonite originated from conglomerate (containing quartz, plagioclase, K-feldspar, muscovite and chlorite; Yamamoto et al., submitted) suggest metamorphism and deformation occurred at high temperature conditions, there have been few reports on the detailed metamorphic temperature estimations and deformation microstructures of the Kerara Formation before. In this study, we present results of Raman spectroscopy of carbonaceous material (RSCM) thermometry and electron backscatter diffraction (EBSD) analyses of the rocks of the Kerama Formation to constrain peak metamorphic temperature and rheological condition related to subduction of spreading center.
RSCM analyses were performed using a Raman spectrometer with a 532 nm Nd-YAG laser (system installed at Nagoya University). Peak metamorphic temperature of pelitic schist and conglomerate mylonite (originated from gravel sandstone) was estimated to be 470–525°C, which is consistent with the temperature range of metamorphic facies proposed in the greenstone.
Measurement of grain size and crystallographic preferred orientations (CPO) in quartz were conducted by SEM-EBSD-EDS system in GSJ-Lab (HITACHI SU3500 SEM mounted with Oxford EBSD detector). Recrystallized quartz region from a sample of conglomerate mylonite shows type II crossed girdle CPO. According to the quartz c-axis fabric opening-angle thermometer of Faleiros et al (2017), it is suggested that the recrystallized quartz region from the sample of conglomerate mylonite was deformed under ~530°C, which corresponds to the peak metamorphic temperature estimated from RSCM thermometry. The mean grain size of quartz in the mylonite is ~80 µm, which suggests the differential stress magnitudes of ~30 MPa (Cross et al. 2017).
In contrast to common occurrence of mylonite along fossil fault zones in continental lithosphere, mylonite in subduction setting is rare: no mylonite has reported from the Shimanto Belt so far. The geology of Kerama islands would be a strong constrain for rheology of subduction plate interface at high temperature condition.