Using calculated chemical potential relationships to account for replacement of kyanite by symplectite in high pressure granulites

Calculated mineral equilibria are used to account for the formation of sapphirine-plagioclase, spinel-plagioclase and corundum-plagioclase symplectites replacing kyanite in quartz-plagioclase-garnet-kyanite granulite facies gneisses from the Southern Domain of the Athabasca granulite terrane, a segment of the Snowbird tectonic zone in northern Saskatchewan, Canada. Metamorphic conditions of >14 kbar and 800 °C are established for the high pressure, garnet-kyanite assemblage using constraints from P-T pseudosections and Zr-in-rutile thermometry. Replacement of kyanite by symplectites reflects the reaction of kyanite with the matrix following near-isothermal decompression to <10 kbar. The chemical potential gradients developed between the kyanite and the matrix led to diffusion that attempted to flatten the gradients, kyanite persisting as a stable phase while it is consumed by symplectite from its edge. In this local equilibrium model, the mineral and mineral compositional spatial relationships are shown to correspond to paths in μ(Na₂O)-μ(CaO)-μ(K₂O)-μ(FeO)-μ(MgO) in the model chemical system, Na₂O-CaO-K₂O-FeO-MgO-Al₂O₃-SiO₂ (NCKFMAS), with SiO₂ and Al₂O₃ taken to be completely immobile. The values of μ(Na₂O) and μ(CaO) are constrained by fixing P-T conditions and choosing appropriate μ(Na₂O) and μ(CaO) values that correspond to the observed plagioclase compositions. μ(FeO)-μ(MgO) diagrams show the corresponding spatial relationships with kyanite and the symplectite phases. These results demonstrate that the replacement of kyanite by sapphirine-plagioclase and spinel-plagioclase appears to be metastable with respect to replacement by corundum-plagioclase. Replacement by corundum-plagioclase does also occur, apparently overprinting pre-existing symplectite and also kyanite. Ignoring corundum, the resulting diagrams account for the spatial relationships and compositions observed in the spinel-plagioclase and sapphirine-plagioclase symplectites. They are predicted to occur over both a wide range of P-T conditions (6-11 kbar, 650-850 °C) and plagioclase compositions (X_An = 0.5-0.9). The wide range of P-T conditions that may result in identical spatial and compositional relationships suggests that such reaction textures may be of limited use in accurately quantifying the P-T conditions of retrograde metamorphism.