Prograde pressure-temperature paths in the pelitic schists of the Sambagawa metamorphic belt, SW Japan

Prograde P-T paths recorded by the chemistry of minerals of subduction-related metamorphic rocks allow inference of tectonic processes at convergent margins. This paper elucidates the changing P-T conditions during garnet growth in pelitic schists of the Sambagawa metamorphic belt, which is a subduction related metamorphic belt in the south-western part of Japan. Three types of chemical zoning patterns were observed in garnet: Ca-rich normal zoning, Ca-poor normal zoning and intrasectoral zoning. Petrological studies indicate that normally-zoned garnet grains grew keeping surface chemical equilibrium with the matrix, in the stable mineral assemblage of garnet + muscovite + chlorite + plagioclase + paragonite + epidote + quartz +/- biotite. Pressure and temperature histories were inversely calculated from the normally-zoned garnet in this assemblage, applying the differential thermodynamic method (Gibbs' method) with the latest available thermodynamic data set for minerals. The deduced P-T paths indicate slight increase of temperature with increasing pressure throughout garnet growth, having an average dP /dT of 0.4-0.5 GPa/100 degreesC. Garnet started growing at around 470 degreesC and 0.6 GPa to achieve the thermal and baric peak condition near the rim (520 degreesC, 0.9 GPa). The high-temperature condition at relatively low pressure (for subduction related metamorphism) suggests that heating occurred before or simultaneously with subduction.