Thermodynamic analysis of phase relations in a high-grade gneissic xenolith from the anatectic region of the late Archean Qôrqut Granite Complex indicates that fluids, probably derived from the granite complex, caused partial replacement of primary plagioclase, corundum, and biotite in the xenolith by epidote, margarite, and muscovite. Compositional relations within the metamorphic mineral assemblage, together with logarithmic activity and fugacity phase diagrams in the system Na2O-K2O-CaO-FeO-MgO-Fe2O3-Al2O3-SiO2-H2O, define local equilibrium constraints on temperature, oxygen fugacity, activity of aqueous silica, and cation activity ratios in the fluid phase during hydration of the xenolith.

The inferred lithostatic pressure at the time of intrusion of the granite complex is ∼5 kbar. At this pressure and ∼580°C, plagioclase and corundum react with water to form margarite. Equilibrium among plagioclase, corundum, margarite, biotite, epidote, muscovite, and an aqueous solution in the xenolith at this temperature and pressure requires an oxygen fugacity of ∼10-17 bars. At temperatures less than 580°C, the assemblage biotite + muscovite + margarite + epidote is stable over a wide range of cation to H+ activity ratios and activity of silica in the fluid phase, but defines a narrow range of oxygen fugac-ities at any given temperature. The calculated oxygen fugacities for the xenolith are close to the maximum values estimated from Fe-Ti oxide phase relations in some granites and pegmatites reported in the literature. Hydration of plagioclase and corundum and the oxidation of biotite in the xenolith to form epidote, margarite, and muscovite require an influx of H2O and possibly SiO2 from the surrounding granite.

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