Phase equilibria modelling of blueschist and eclogite from the Sanbagawa metamorphic belt of southwest Japan reveals along-strike consistency in tectonothermal architecture

摘要

The Sanbagawa metamorphic belt of southwest Japan is one of the type localities of subduction-related high-P metamorphism. However, variable pressure-temperature (P-T) paths and metabasic assemblages have been reported for eclogite units in the region, leading to uncertainty about the subduction zone paleo-thermal structure and associated tectonometamorphic conditions. To analyse this variation, phase equilibria modelling was applied to the three main high-P metabasic rock types documented in the region - glaucophane eclogite, barroisite eclogite and garnet blueschist - with modelling performed over a range of P, T, bulk rock H2O and bulk rock ferric iron conditions using thermocalc. All samples are calculated to share a common steep prograde P-T path to similar peak conditions of approximate to 16-20kbar and 560-610 degrees C. The results establish that regional assemblage variation is systematic, with the alternation in peak amphibole phase due to peak conditions overlapping the glaucophane-barroisite solvus, and bulk composition effects stabilizing blueschist v. eclogite facies assemblages at similar P-T conditions. Furthermore, the results reveal that a steep prograde P-T path is common to all eclogite units in the Sanbagawa belt, indicating that metamorphic conditions were consistent along strike. All localities are compatible with predictions made by a ridge approach model, which attributes eclogite facies metamorphism and exhumation of the Sanbagawa belt to the approach of a spreading ridge.