Low-pressure Metamorphism in the Ryoke Metamorphic Belt in the Yanai District, Southwest Japan

The Ryoke metamorphic belt is one of the typical low-pressure type metamorphic belts in the world. It is composed of granitoids (Older and Younger Ryoke granitoids) and associated metamorphic complex (Ryoke metamorphic rocks) of Cretaceous age. The Ryoke metamorphic rocks in the Yanai district, southwest Japan, show three different phases of ductile deformation. During the first phase (D1), a distinct foliation parallel to lithologic layering was formed under the thermal peak conditions of the low-pressure facies series metamorphism. The second phase deformation (D2) led to the formation of mylonitic shear zones and nappes. Deformation of the third phase (D3) was responsible for the formation of the upright folds with E-W trending axes. The movement picture of D1 during and immediately before the intrusion of the Older Ryoke granitoids was of extension tectonics. After D1, the nappes and upright folds of the metamorphic rocks and granitoids were formed during D2 and D3 probably under compressional stress field. The regional Ryoke metamorphism has been divided into two phases, M0 and Ml. The metamorphism of M0 was of nearly medium-pressure facies series (ca. 30°C /km) and that of M1 was of low-pressure facies series (ca. 40 ～ 50°C / km). On the basis of the mineral assemblages crystallized under M1, the Ryoke metamorphic rocks are divided into four metamorphic zones: biotite zone, cordierite zone (460 ～ 590°C, 2.5 ～ 3.5 kbar), sillimanite zone (630 ～ 690°C, 3 ～ 5 kbar), and garnet zone (730 ～ 770°C, 5.5 ～ 6.5 kbar). Because the intrusion of the Older Ryoke granitoids has a strong time and spatial association with M1, it is suggested that the heat sources of M1 are the emplacement of the Older Ryoke granitoids. By using 1-D numerical simulation, the thermal model for M1 was developed by heat conduction with fluid advection caused by intrusion of a granodiorite sheet at intermediate crustal levels. The results of the thermal model nearly consist with the petrologically estimated highest metamorphic temperatures during M1. Garnet crystals from the sillimanite zone are chemically zoned and show several kinds of zoning patterns. The observed overall zoning patterns in the garnets with different radii are well reproduced by the numerical analysis. These results suggest that the temperature-time path gives a good explanation for M1. Therefore, it can be said that the sheet-like Older Ryoke granitoids intruded at intermediate crustal levels(≈ 15-km-depth) are a heat source of M1. In conclusion, the Ryoke metamorphic rocks firstly were heated under medium-pressure facies conditions, and then they were further heated under low-pressure facies conditions caused by the intrusion of the Older Ryoke granitoids.