Thermal maturity in the Shimanto accretionary prism, southwest Japan, with the thermal change of the subducting slab: fluid inclusion and vitrinite reflectance study

The P–T path of the Cretaceous Shimanto accretionary complex, western Shikoku, southwest Japan, was determined by a combination of vitrinite reflectance data and homogenization temperatures of fluid inclusion analysis. Within the complex, the P–T paths of the Upper Cretaceous Yokonami Melange and surrounding coherent strata were compared. No P–T difference was found between the Yokonami Melange and surrounding strata. At least two types of fluids: methane-rich and water-rich fluids, migrated at different stages during metamorphism within the complex. The methane-rich fluid was initially trapped in vein quartz under pressures of ~260 MPa and a geothermal gradient of ~24°C/km. The water-rich fluid was included in calcite at 95–125 MPa and a geothermal gradient of 50°C/km. The complex sequence of occurrence of the vein minerals and radiometric age data suggest that the water-rich fluid was trapped during the Kula–Pacific ridge subduction into the Shimanto accretionary complex. The paleoheat flow values for the two stages of subduction are consistent with the changes in the inferred slab age. The metamorphism of the Shimanto accretionary complex was predominantly controlled by the thermal condition of the subducting plate.