Sapphirine in the high-grade quartzofeldspathic gneiss of the Larsemann Hills,East Antarctica

Study of sapphirine and related mineral association in the high grade region of the Larsemann Hills,East Antarctica,shows that sapphirine of the area is characterized by its magnesio ,iron and aluminum rich,but silica poor feature,and the obvious intra and intergrain changes in compositions.The change is mainly manifested as the Tschermark substitution (Mg,Fe) Si=2Al.In the high grade metamorphism and anatexis process the multistage crystallization of minerals occurred and resulted in the complexity of the mineral association,such as the differentiation of leuco and melano components.Among them,themafic rich minerals formed earlier,and the differentiation of magnesio and iron components is responsible for the earlier presence of iron rich minerals and later crystallization of magnesio rich minerals,thus the successive associations of multistage occurred.The rock composition is an important but not critical factor to the occurrence of sapphirine.It is the mobilization of components that accounts for the formation of sapphirine.The multistage evolution of mineral association to some degree reflects the changing composition and opening of the setting.It is therefore deduced that the protolith from which sapphirine is derived is not necessarily magnesio rich pelite.The heterogeneity of sapphirine composition is resulted from the various media,not the PT changes. Sapphirine formed at 840～880 ℃,not the so called ultrahigh temperature condition(>1000 ℃).Its formation is related to both the filtration and diffusion processesin high grade metamorphism and anatexis.