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Sapphirine + quartz-bearing pelitic gneisses from Wilson Lake, in the Grenville Province of central Labrador, and from other granulite-facies terranes, are well known for their reaction rims and nonequilibrium textures. However, some corundum-bearing gneisses from the Red Wine Mountains massif in the Wilson Lake area have a low variance assemblage that appears to record equilibrium conditions of regional metamorphism. The silica-undersaturated assemblage sapphirine (Spr) + ortho-pyroxene (Opx) + sillimanite (Sil) + garnet (Grt) + spinel (Spl) + corundum (Crn) + magnetite + titanhematite (+ plagioclase + biotite) approaches invariance in the six-component system FeO-MgO-Al2O3-SiO2-Fe2O3-TiO2. The resulting cordierite [Crd]-absent invariant point appears to be stable near 900 °C and 1000 MPa, and at an oxygen fugacity (fO2) defined by coexisting pure magnetite and titanhematite (after integration of exsolved ferrian ilmenite). Further evidence of high fO2 is exsolved titanhematite (XFe2O3 = 0.73) in orthopyroxene, exsolved hematite in sillimanite, and high dissolved Fe3+ in sillimanite and corundum. Systematic partitioning of Fe3+ in the coexisting silicate and oxide phases documents their mutual equilibrium.

The P-T-fO2 conditions represented by the [Crd]-absent invariant point in corundum-bearing rocks are consistent with the stability of spinel + quartz. This would not be the case if XMg(Grt) < XMg(Spl). The phase relationships presented here and the absence of garnet + cordierite assemblages in this area further suggests that the [Crd]-absent invariant point is stable in the field of sapphirine + quartz. The partitioning sequence XFe3+ (Spr) > XFe3+ (Spl) > XFe3+ (Opx) > XFe3+ (Grt) we estimate from microprobe analyses extends the stabilities of sapphirine and spinel relative to orthopyroxene and garnet, shifting the [Crd]-absent invariant point to a lower temperature.

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