Transition from dehydration to hydration during exhumation of the Sanbagawa metamorphic belt, Japan, revealed by the continuous P–T path recorded in garnet and amphibole zoning

To investigate the water budget in deep accretionary prisms, we undertook a detailed petrologic analysis of garnet-bearing amphibolites in the subduction-origin high-pressure Sanbagawa metamorphic belt, Japan. We obtained a continuous P–T path of exhumation by the application of differential thermodynamics to compositional zoning in both garnet and amphibole. The P–T path is characterized by three stages of decompression: (1) heating decompression from 1.5 GPa and 550 °C to 1.1 GPa and 600 °C; (2) isothermal decompression toward 0.8 GPa and 580 °C; and (3) cooling decompression to 0.3 GPa and 400 °C. Comparison with the results of numerical modeling of the thermal structures of subduction zones suggests that the peak-pressure condition is warmer than the slab surface temperature in a typical subduction zone, but is consistent with subduction of a young slab during the Sanbagawa metamorphism. The heating decompression just after the peak-pressure condition is explained by heating from the mantle wedge during ascent of the metamorphic body in the accretionary prism; heating stops when the metamorphic body reaches crustal depths. Mass balance calculations for the mineral reactions in an amphibolite reveal that the dehydration associated with the growth of Ca-poor garnet takes place during heating decompression, whereas hydration takes place during cooling decompression by the breakdown of garnet and pargasite to form chlorite and actinolite. Such a transition from dehydration to hydration in the exhuming metabasite body in an accretionary prism generates fluids between the continental Moho and the slab surface and may explain the seismic activity above the subduction plate boundary, such as supra-slab earthquakes.