Chemical zoning and crystal size distribution of garnet in a quartzo-feldspathic gneiss from the Lützow-Holm Complex at Skallen, East Antarctica: Implication for geothermometry

Abstract

Studies of chemical compositions and grain size of garnet in a quartzo-feldspathic gneiss from the Lützow-Holm Complex at Skallen, East Antarctica, form the basis of an interpretation of timing of garnet homogenization. The gneiss contains only garnet as mafic minerals except sporadic biotite inclusions in the garnet. The garnet represents approximately constant Mg/(Fe + Mg) within a grain. The values of small grains are higher than those of large grains, while the grossular content at their periphery is similar. The grain size shows lognormal distribution. These features indicate that the grains were mutually not in equilibrium and were poorly annealed, which implies insufficient grain-boundary diffusion. The consideration based on a qualitative phase diagram suggests that the Mg/(Fe + Mg) of the small grains and that of the large grains were homogenized before and after cessation of the grain-boundary diffusion, respectively. The small grains preserve the composition that was in equilibrium with biotite. In contrast, the large grains represent their bulk compositions of chemical zoning profiles that were formed during fractional growth. The homogeneous interior of large grains is, therefore, not always appropriate for geothermometry. The highest Mg/(Fe + Mg) among small grains would be suitable when the grain-boundary diffusion ceased before homogenization. The erroneous use of large grains causes depression of the estimated temperature by 60-80 °C, even though the compositional difference corresponds to only ∼ 10 °C in the phase diagram.