Thermal evolution of the Ryoke metamorphic belt, southwestern Japan: Tectonic and numerical modeling

Abstract The Ryoke metamorphic belt of southwestern Japan is composed of Cretaceous Ryoke granitoids and associated metamorphic rocks of low-pressure facies series. The Ryoke granitoids are divided into sheet-like bodies (e.g. Gamano granodiorite) and stock-like bodies. The Gamano granodiorite intruded concordantly into the high-grade metamorphic rocks without development of a contact metamorphic aureole, and the intrusion ages of the granodiorite are similar to the ages of thermal peak of the low pressure (low-P) metamorphism. It is suggested that the low-P Ryoke metamorphism resulted from the intrusion of the Gamano granodiorite. In this study, a simple 1-D numerical model of conductive heat transfer was used to evaluate the thermal effects of emplacement of the Gamano granodiorite. Calculated temperature-time ( T-t ) paths are characterized by a rapid increase of metamorphic temperature and a relatively short-lived period of high temperature. For example, the T-t path at the 15-km depth is characterized by a rapid average increase in temperature of 1.4 × 10-³°C/year and high temperatures for < ca 0.5 Ma. The calculated peak temperature for each depth is nearly equal to the petrologically estimated value for each correlated metamorphic zone. The results suggest that the magma-intrusion model is one possible thermal model for low-pressure facies series metamorphism.