Diffusion metasomatism in silica-undersaturated sapphirine-bearing granulite from Rumdoodle Peak, Framnes Mountains, east Antarctica

At Rumdoodle Peak, near Mawson Base, east Antarctica, an enclave of metased imentary granulite is enclosed in the Mawson Charnockite, an extensive c. 9 60 Ma intrusion. The enclave contains a disrupted layer of black spinel-ort hopyroxene-phlogopite gneiss, which was truncated by a quartz-rich vein. A reaction band that developed between these units is composed of a sequence of mineral zones that contain spinel, orthopyroxene, sapphirine, cordierite and plagioclase. The sequence of mineral zones approximately matches that predicted by a model of closed system diffusion metasomatism, involving the exchange of Si for Fe and ME. The reaction bands differ from the model in the presence of microscale disequilibrium textures, that include "double co ronas" composed of cordierite surrounding sapphirine and sapphirine surroun ding spinel. The growth of the reaction band was controlled by diffusion al ong intergranular pathways, with local equilibrium maintained adjacent to g rain boundaries. The presence of corona textures is a result of slow reacti on rates, due to limited diffusive exchange of Si and Al across mineral gra ins.