Petrology and possible significance of barroisite-bearing metabasite from the Kebara Formation in NW Kii Peninsula

Abstract

We report the new finding of barroisite (Brs)-bearing metabasites within a metabasite layer from the Kebara Formation, a unit exposed between the Sanbagawa and the Chichibu belts in NW Kii Peninsula. The dominant lithotype of the metabasite layer shows pale-green colors and is mainly composed of sodium amphibole, actinolite, pumpellyite and epidote. It is in agreement with reported mineral assemblages in the Kebara Formation which document pumpellyite-actinolite (PA) or pumpellyite-blueschist (PBS) facies conditions (<340 °C and 0.8 GPa), and with geothermometry based on the Raman analysis of carbonaceous material from metapelite samples which give peak metamorphic temperatures of 300-340 °C. Brs grains are identified from metabasites with dark-green color in the layer, and are closely associated with epidote, chlorite, white mica, albite and quartz, but not with pumpellyite. Brs grains are replaced by sodium amphibole and/or winchite at the rim with a distinct compositional gap. Thermodynamic calculation suggests that the Brs + epidote + chlorite + albite + quartz assemblage is stable at P-T conditions higher than 450 °C and 0.4 GPa. The abovementioned data suggest that the Brs-bearing metabasites suffered an early higher temperature (>450 °C) metamorphism and then overprinted by PA or PBS facies metamorphism along with the main constituents of the Kebara Formation. In the Besshi area of the Sanbagawa belt, the earlier subducted higher grade rocks are considered to juxtapose to the newly subducted rocks and overprinted retrograde metamorphism during their exhumation stage. Our new finding suggests that the similar phenomenon was took place in the lower grade part of the Sanbagawa belt.