The Microarea Composition of Garnet and Biotite from the Ji'an Group in Tonghua District and Its Significance as an Indicator of Metamorphic P-T-t Path

摘要

The garnet (Grt) and biotite (Bt) from gneisses of the Ji’ an Group are characterized by diffusion zoning at the rim, but equilibrium composition of metamorphic peak is usually remained in extensive interior area. Garnet with growth zoning is also found in the kyanite zone.In the light of microarea compositional variation of Grt and Bt, the temperatuIe and pressuIe at the progressive, peak and post peak metamorphic stages are determined by correctly using GrtBt thermometer and GASP barometer. On this basis, a counterclockwise P-T-t path can be constructed, which reflects the closing process of an ensialic rift belt in this region during the Early Proterozoic.