TEMPERATURE-TIME PATH FOR THE LOW-PRESSURE RYOKE METAMORPHISM, JAPAN,BASED ON CHEMICAL ZONING IN GARNET

Garnet crystals from low-plessure/high-temperature (LPHT) Ryoke metamo rphic locks in the Yanai district, south-western Japan, show several k inds of chemical zoning patterns that systematically vary with grain r adius between c. 0.1 and 0.5 mm. Large grains (> c. 0.4 mm) show norma l zoning and small grains (<c. 0.4 mm) show unzoned or reversely zoned cores. Observations of the chemical zoning and of the spatial and siz e distributions of the garnet grains between c. 0.1 and 0.5 mm in radi us suggest that they were formed by continuous nucleation and diffusio n-controlled growth. A previously estimated temperature-time path (T-t path) for the Ryoke metamorphism, using 1-D numerical simulation, is characterized by a rapid increase in temperature, 0.0017 degrees C yr( -1) on average, and a period of high temperature (>600 degrees C) shor ter than 0.5 Myr, which was presumably caused by the intrusion of a gr anodiorite sheet. Chemical zoning of garnet grains with different radi i simulated for the T-t path using a numerical model of continuous nuc leation and diffusion-controlled growth, in combination with intracrys talline diffusion, compares well with the observed zoning patterns in garnet grains with different radii. This is in spite of the fact that the simulated zoning patterns vary greatly, depending on subtle differ ences in the T-t history. Therefore, they suggest that the T-t path gi ves a good explanation for the LPHT Ryoke metamorphism. Although this study only refers to the Ryoke metamorphism, the technique map be appl icable to thermal modelling of other metamorphic terranes.