Microstructural records of multiple retrograde local fluid infiltrations in the pelitic gneiss

Abstract

The alkali-feldspar (Akfs) and biotite in the pelitic gneiss preserves characteristic microstructural evidence of multi-stage H2O supplement during the retrograde metamorphism. The first one is ‘zoned feldspar’, in which mesoperthitic, anti-perthitic and lamella-free plagioclase zones coexist within single crystal. It is occasionally found next to biotite, and is always depleted in orthoclase component toward biotite. The second one is the serrate boundary between Akfs and biotite. These microstructures formed as the biotite grew by consuming potash in Akfs when H2O-bearing fluid passed through the grain boundaries. The former was formed at 825-900 oC. The former will be reported in detail.