Metamorphism of the Wakasa blueschist, Renge belt, SW Japan: metamorphic phase equilibria and P-T path

摘要

Blueschists exposed in the Wakasa area, Renge metamorphic belt record three-stages of metamorphism, i.e. prograde, peak and retrograde metamorphic stages. The prograde stage is defined by the minerals included in the schistosity-forming minerals, i.e. chlorite,epidote, phengite, albite, winchite/glaucophane, titanite, hematite and quartz. The peak metamorphic stage is defined by schistosity-forming minerals of amphibole (glaucophane, magnesioriebeckite), epidote, phengite, chlorite, titanite, hematite and quartz. The metamorphic pressure-temperature (P-T) conditions are well-constrained using phase equilibrium modelling (pseudosection modelling) in the Na_2O-CaO-K_2O-FeO-MgO-Al_2O_3-SiO_2-H_2O-O_2 (NCKFMASHO) system. P-T estimates based on the compositional isopleths of phengite and chlorite and the modelled paragenetic assemblage indicate a prograde metamorphism of 300-320°C and 0.5 GPa and the peak metamorphism of 350-390°C and 1.05-1.2 GPa in the lawsonite- or epidote-glaucophane schist facies conditions. Winchite to actinolite in the rim of the schistosity-forming amphibole, matrix albite, and chlorite and K-feldspar replacing the peak metamorphic minerals of glaucophane, phengite and epidote, suggest the conditions of retrograde stage, 360-400°C and 0.4-0.5 GPa. The P-T conditions for these metamorphic stages suggest a clockwise P-T path with isothermal decompression at the initial phase of the exhumation, that is similar to the P-T path reported from the Gotsu blueschist, Suo metamorphic belt, although the metamorphic temperature and pressure at the peak are considerably higher than the Wakasa blueschist.