5:15 PM - 6:30 PM
[SMP25-P16] P-T estimations of Ryoke metamorphic rocks in the Takato-Ohshika area, Nagano, Japan
Keywords:Ryoke metamorphic rocks, MTL, Low-P/T type metamorphism
Low–P/T type metamorphism is closely related to the arc magmatism and the detailed P–T–t–D estimations of metamorphic rocks associated with granitoids lead to be understanding of the tectonic evolution of active convergent margin. Recently, a lot of U-Pb zircon ages from Ryoke granitoids and metamorphic rocks in Yanai and Mikawa regions were reported (e.g., Skrzypek et al. 2016; Takatsuka et al. 2018). These age data imply that the reconsideration of the classical intrusive relationship between gneissose older and massive younger Ryoke granitoids closely related to the low-P/T type metamorphism is required. Hence, we attempted to assess the detailed P–T–D estimations of mylonitic metasediments along the MTL in the Takato-Oshika area. The area is located in the eastern most part of the Ryoke metamorphic belt, where is suitable for comparison with other areas previously reported.
In the study area, the gneissose Hbl–Bt tonalite (the so-called Hiji tonalite) is widely exposed along the MTL, but there have been few reports on the Ryoke metamorphic rocks before. In this study, we conducted a detailed geologic field mapping survey, during which meter- and centimeter-scale blocks of Ryoke metamorphic rocks within the Ryoke granitic mylonite were successfully sampled. The mylonitic metasediments are locally associated with C’-type shear bands, suggesting sinistral shearing. The representative mineral assemblage of the mylonitic metasediments comprises garnet, biotite, muscovite, cordierite pseudomorphs (muscovite aggregates), plagioclase, and quartz, with minor amounts of apatite, zircon, tourmaline, sillimanite, and platy graphite. The highest-grade metamorphic rocks in the Grt–Crd zone of the Ryoke metamorphic belt are exposed at distances of ~1800 m from the MTL. Detailed petrological analyses coupled with conventional thermobarometers and P–T pseudosection modeling give estimated peak P–T conditions of 760–790 °C at 4.6–5.3 kbar. Likewise, the surrounding gneissose Ryoke granitoids were emplaced at around 680–710 °C and 4.6–5.8 kbar by Al in hornblende barometry and Hbl–Pl thermometry.
On the other hand, peak P–T estimations of mylonitic metasediments have been estimated to be 560 to 710 °C at 2.0–5.1 kbar, suggesting the decrease in peak P–T conditions with approaching the MTL. Such rocks correspond to the Kfs–Sill to Bt zone of the Ryoke metamorphic belt and are structurally upper unit to the highest-grade Ryoke metamorphic rocks. However, the present Ryoke metamorphic rocks and granitoids show a monoclinal structure with a dip of about 50–70°W and the structurally upper unit in the Bt and Kfs–Sill zones of Ryoke belts were stacked at the lower portion of the highest-grade metamorphic rocks in the Grt–Crd zone. This suggests that the primary metamorphic zonation during the low-P/T type metamorphism has a tight asymmetric anticline at around the MTL. In addition, we revealed that a large pressure difference of >10 km was inferred from peak P–T conditions between the highest-grade metamorphic rocks and mylonitic rocks in a width of only ~1500m at around the MTL. These data suggest that a considerable amount of geologic mass in the middle crust was shortened or stretched, and upper units of Ryoke metamorphic rocks in the Bt and Kfs–Sill zones were selectively dragged into the deeper crust by ductile deformation related to the MTL.
[Reference]: Skrzypek et al. (2016), Lithos, 260, 9-27; Takatsuka et al. (2018), Lithos, 308-309, 428-445.
In the study area, the gneissose Hbl–Bt tonalite (the so-called Hiji tonalite) is widely exposed along the MTL, but there have been few reports on the Ryoke metamorphic rocks before. In this study, we conducted a detailed geologic field mapping survey, during which meter- and centimeter-scale blocks of Ryoke metamorphic rocks within the Ryoke granitic mylonite were successfully sampled. The mylonitic metasediments are locally associated with C’-type shear bands, suggesting sinistral shearing. The representative mineral assemblage of the mylonitic metasediments comprises garnet, biotite, muscovite, cordierite pseudomorphs (muscovite aggregates), plagioclase, and quartz, with minor amounts of apatite, zircon, tourmaline, sillimanite, and platy graphite. The highest-grade metamorphic rocks in the Grt–Crd zone of the Ryoke metamorphic belt are exposed at distances of ~1800 m from the MTL. Detailed petrological analyses coupled with conventional thermobarometers and P–T pseudosection modeling give estimated peak P–T conditions of 760–790 °C at 4.6–5.3 kbar. Likewise, the surrounding gneissose Ryoke granitoids were emplaced at around 680–710 °C and 4.6–5.8 kbar by Al in hornblende barometry and Hbl–Pl thermometry.
On the other hand, peak P–T estimations of mylonitic metasediments have been estimated to be 560 to 710 °C at 2.0–5.1 kbar, suggesting the decrease in peak P–T conditions with approaching the MTL. Such rocks correspond to the Kfs–Sill to Bt zone of the Ryoke metamorphic belt and are structurally upper unit to the highest-grade Ryoke metamorphic rocks. However, the present Ryoke metamorphic rocks and granitoids show a monoclinal structure with a dip of about 50–70°W and the structurally upper unit in the Bt and Kfs–Sill zones of Ryoke belts were stacked at the lower portion of the highest-grade metamorphic rocks in the Grt–Crd zone. This suggests that the primary metamorphic zonation during the low-P/T type metamorphism has a tight asymmetric anticline at around the MTL. In addition, we revealed that a large pressure difference of >10 km was inferred from peak P–T conditions between the highest-grade metamorphic rocks and mylonitic rocks in a width of only ~1500m at around the MTL. These data suggest that a considerable amount of geologic mass in the middle crust was shortened or stretched, and upper units of Ryoke metamorphic rocks in the Bt and Kfs–Sill zones were selectively dragged into the deeper crust by ductile deformation related to the MTL.
[Reference]: Skrzypek et al. (2016), Lithos, 260, 9-27; Takatsuka et al. (2018), Lithos, 308-309, 428-445.