Metamorphic P-T-time history of the Sanbagawa belt in central Shikoku, Japan and implications for retrograde metamorphism during exhumation

The peak metamorphic conditions and processes of the Cretaceous Sanbagawa HP metamorphic belt, central Shikoku, Japan, were examined based on petrological, thermodynamic and geochronological studies in the Asemi-gawa area, where garnetite (garnet > 50 wt.%) is included in mafic schists of the oligoclase-biotite zone. The representative mineral assemblage in the garnetite is Grt + Qtz + Phe + Ep + Chl + Rt. The peak and retrograde pressure-temperature (P-T) conditions were estimated by thermodynamic calculations using THERMOCALC ver. 3.31 (Holland and Powell, 1998) and THERIAK-DOMINO ver. 15.05.08 (De Capitani, 1994). The estimated peak and retrograde P-T conditions of the garnetite are in the range of P = 15-19 kbar and T = 500-520 °C, and P = 7-11 kbar and T = 460-510 °C, respectively. The P-T data indicate that the whole package of the oligoclase-biotite zone in the Asemi-gawa area once reached eclogite facies (ca. 48-60 km depth) and underwent retrograde metamorphism under epidote-amphibolite facies conditions (ca. 22-35 km depth) during the exhumation. A zircon U-Pb age was determined using Nano-SIMS for zircons extracted from pelitic schist of the oligoclase-biotite zone. Only zircon rims yielded a concordia age of 85.6 ±3.0 Ma (weighted mean age) that is considered to be the time of the retrograde recrystallization. These results imply that the eclogite-facies rocks in the Sanbagawa belt, which have been considered to be tectonic blocks or nappes, are an integral part of the Sanbagawa belt, and formed together with the surrounding schists under a specific thermal gradient at depth in the subduction zone.