USING CALCULATED CHEMICAL POTENTIAL RELATIONSHIPS TO ACCOUNT FOR CORONA AND SYMPLECTITE TEXTURES IN GRANULITES

Stable mineral assemblages, proportions and compositions in metamorphic rocks evolve in response to changing pressure-temperature conditions. These changes occur in part due to diffusion of components between minerals, fluid, and melt, with the driving force for diffusion being gradients in chemical potential of the components developed spatially within a rock. This study uses recent advances in the software THERMOCALC to quantitatively investigate chemical potential relationships in granulite facies rocks. Phase diagrams with chemical potential components as axes are used to explore the spatial arrangement of components and resulting textures during the metamorphic evolution of rocks. Calculated mineral equilibria are used to account for textures in two specific granulite occurrences: felsic granulites associated with eclogites (Snowbird Zone, Canada) and spinel-quartz UHT granulites (Anapolis-Itaucu Complex, Brazil). In the felsic granulites, symplectite textures involving sapphirine-spinel-corundum-anorthite surround kyanite and opx-pl coronas surround garnet. These textures reflect the instability of peak porphyroblasts during marked decompression. The gradients set up as a consequence drive the texture development, with zoning in plagioclase reflecting these gradients. In a local equilibrium model, the mineral and mineral compositional spatial relationships are shown to correspond to paths in chemical potential gradients in the model system NCKFMAS. The replacement of kyanite by sa-an or sp-an appears to be metastable with respect to cor-an, presumably due to the difficulty in nucleating corundum. A second example is provided by UHT granulites that contain the equilibrium assemblage garnet-spinel-sillimanite-quartz. Calculated chemical potential relationships are explored in the NCKFMASHTO model system where slight decompression and cooling results in the adjustment of modes of garnet and spinel to produce symplectite and corona textures. Garnet porphyroblasts are replaced by symplectites of spinel-plagioclase with new garnet in turn replacing the symplectite. Corona textures occur where spinel, corundum, and ilmenite form a core surrounded by consecutive layers of sillimanite and garnet, and matrix quartz.