Pressure-temperature path of Sanbagawa prograde metamorphism deduced from grossular zoning of garnet

The zonal structure of prograde garnet in pelitic schists from the medium-grade garnet zone and the higher-grade albite biotite zone was, examined to investigate the evolution of prograde P-T paths of the Sanbagawa metamorphism. The garnet studied shows a bell-shaped chemical zoning of the spessartine component, which decreases in abundance from the core towards the rim. Almandine and pyrope contents and X-Mg [= Mg/(Mg + Fe2+)] increase monotonously outwards. The general scheme of the zonal structure for grossular content [X-Grs = Ca/(Fe2+ + Mn + Mg + Ca)] can be summarized as: (1) X-Grs increases outwards (inner segment) and reaches a maximum at an intermediate position between the crystal core and the rim, then decreases towards the outermost rim (outer segment) (2) the inner segment of garnet in the garnet zone samples tends to have a higher X-Grs/X-Sps values for a given X-Sps than those in the albite-biotite zone samples (3) average X-Sps at the maximum X-Grs position in the albite-biotite zone samples ranges from 0.02 to 0.12 and is lower than that in the garnet zone samples (0.13-0.32) (4) the maximum X-Grs in the albite-biotite zone samples (0.34-0.39 on average) lends to be higher then that in the garnet zone samples (0.26-0.36), and (5) differences of X-Grs between the maximum and rim in the albite-biotite zone samples are between 0.10 and 0.14 and higher than those in the garnet zone samples (< 0.11). These facts imply that albite-biotite zone materials (a) were recrystallized under lower dP/dT conditions at an early stage of the prograde metamorphism (b) began their exhumation under higher P-T conditions and (c) have been continuously heated during exhumation fbr a longer duration than the garnet zone materials. The systematic changes of prograde P-T paths can be interpreted as documenting the evolution of the Sanbagawa subduction zone.