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

Abstract

At Rumdoodle Peak, near Mawson Base, east Antarctica, an enclave of metasedimentary granulite is enclosed in the Mawson Charnockite, an extensive c. 960 Ma intrusion. The enclave contains a disrupted layer of black spinel-orthopyroxene-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 Mg. The reaction bands differ from the model in the presence of micro-scale disequilibrium textures, that include “double coronas” composed of cordierite surrounding sapphirine and sapphirine surrounding spinel. The growth of the reaction band was controlled by diffusion along intergranular pathways, with local equilibrium maintained adjacent to grain boundaries. The presence of corona textures is a result of slow reaction rates, due to limited diffusive exchange of Si and Al across mineral grains.