Raman constraints on peak metamorphic conditions of the Cretaceous Nagasaki metamorphic rocks in the Nomo Peninsula, western Kyushu, Japan

The Nomo metamorphic rocks in western Kyushu, Japan, are distributed in the southwesternmost part of the Sanbagawa metamorphic belt; yet quantitative pressure–temperature (P–T) constraints have remained limited. Here, we apply Raman spectroscopy of carbonaceous material (RSCM) thermometry and quartz-in-garnet Raman barometry to pelitic schists from the biotite zone of the Sanwa Formation (metamorphic ages of 90–70 Ma) to constrain peak P–T conditions. RSCM thermometry of three samples yields peak metamorphic temperatures of 463–471 °C. Raman spectra of 92 quartz inclusions in garnet from three samples provide residual pressures of 0.46–0.57 GPa, which correspond to quartz entrapment pressures of 1.03–1.28 GPa within the garnet growth temperature range of ∼400–470 °C. These results suggest that the Sanwa Formation experienced peak P–T conditions of ∼460–470 °C at ∼1.0–1.3 GPa. The estimated temperatures overlap those reported for the garnet zone of the Sanbagawa belt in central Shikoku, whereas the inclusion-based pressures are higher than thermobarometric estimates for that zone. This discrepancy suggests that quartz inclusions in garnet may record conditions during prograde subduction along a Sanbagawa-type subduction-zone geotherm prior to exhumation.