Temperature–time path for the low‐pressure Ryoke metamorphism, Japan, based on chemical zoning in garnet

Garnet crystals from low‐pressure/high‐temperature (LPHT) Ryoke metamorphic rocks in the Yanai district, south‐western Japan, show several kinds of chemical zoning patterns that systematically vary with grain radius between c. 0.1 and 0.5 mm. Large grains (>c. 0.4 mm) show normal zoning and small grains (<c. 0.4 mm) show unzoned or reversely zoned cores. Observations of the chemical zoning and of the spatial and size distributions of the garnet grains between c. 0.1 and 0.5 mm in radius suggest that they were formed by continuous nucleation and diffusion‐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 °C yr−1 on average, and a period of high temperature (>600 °C) shorter than 0.5 Myr, which was presumably caused by the intrusion of a granodiorite sheet. Chemical zoning of garnet grains with different radii simulated for the T –t path using a numerical model of continuous nucleation and diffusion‐controlled growth, in combination with intracrystalline 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 differences in the T–t history. Therefore, they suggest that the T–tpath gives a good explanation for the LPHT Ryoke metamorphism. Although this study only refers to the Ryoke metamorphism, the technique may be applicable to thermal modelling of other metamorphic terranes.