Low-pressure metamorphism in the Ryoke metamorphic belt in the Yanai district, southwest Japan

西南日本柳井地域領家変成帯における低圧型変成作用

奥平 敬元

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 their 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 fades series metamorphism, which is probably ascribed to the sheet-like intrusion of the Older Ryoke granitoids. 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. In the metamorphic rocks of the Tsuzu area, which is placed in the northern part of the Yanai district, there are many melt-filled fractures of minor scales, which cut across their foliation. The deformation related to the formation of these melt-filled fractures resulted commonly from foliation parallel shearing under extensional stress field. The overall movement picture inferred from the melt-filled fractures appears to be top to the N sense of shear, and the deformation related to the formation of the melt-filled fractures was responsible for the formation of the normal fault zones, along which the intrusion of the Older Ryoke granitoids occurred. Asymmetric textures such as extensional crenulation cleavage and rotation of porphyroblasts, which grew under the thermal peak of M1, are also formed in the metapelites. The shear sense read from the asymmetric textures is the top to the north. This is harmonic with the overall movement picture inferred from the melt-filled fractures. Therefore it can be said that the overall movement picture of D1 during and immediately before the intrusion of the Older Ryoke granitoids was of extension tectonics.

As inferred from the dislocation densities in quartz grains deformed during D1, strain rate for D1 appears to be high (≈ 10-10 ~ 10-7 s-1). 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 M1. 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 intnision of a granodiorite sheet at intermediate crustal levels. The calculated temperature-time path (T-t path) for the sillimanite zone during M1 is characterized by a rapid increase of temperature, 0.0017°C/year on average, and a short-period of high-temperature condition (> 600°C), shorter than 0.5 Ma. 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 patterns systematically vary with grain size, which are between ca. 0.1 and 0.5 mm in radius. Large grains (> ca. 0.4 mm) show normal zoning and small grains (< ca. 0.4 mm) show unzoned or reversely zoned profile in their cores. Observations of the chemical zoning and of the spatial and size distributions of the garnets between ca. 0.1 and 0.5 mm in radius suggest that the garnets have been formed by continuous nucleation and diffusion-controlled growth. To examine the validity of the T-t path for M1, the chemical zonings of garnets with different radii are simulated for the T-t path using a numerical model of continuous nucleation and diffusion-controlled growth, in combination with intracrystalline diffusion, and are compared with the observed ones. The observed overall zoning patterns in the garnets with different radii are well reproduced by the numerical model, in spite of the fact that the simulated zoning patterns greatly change responding to the subtle changes in the T-t history. Therefore, these results suggest that the T-t 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 facics conditions caused by the intrusion of the Older Ryoke granitoids.

地球科学・地学・地質学 [ 450 ]

英語

Copyright(c) by Author

広島大学