LOW VARIANCE SAPPHIRINE-BEARING ASSEMBLAGES FROM WILSON LAKE, LABRADOR

Sapphirine + quartz-bearing pelitic gneisses from Wilson Lake, Central Labrador and from other granulite-facies terranes are well known for their reaction rims and non-equilibrium textures. Some corundum-bearing gneisses from the Wilson Lake allochthon, however, have a low variance assemblage that appears to record equilibrium conditions of regional metamorphism. The phases sapphirine (sa), orthopyroxene (px), garnet (gt), spinel (sp), sillimanite (si), magnetite (mt), titanhematite (hm), biotite, and plagioclase are all closely associated within single thin sections. Cordierite (Cd) is stable but only for more Mg-rich bulk compositions. The coexisting phases from Wilson Lake compose an assemblage that approaches the [Cd]-missing invariant point in the FMAS system at relatively high fO₂ conditions. Microprobe analyses of these coexisting phases show that X_Mg(px) » X_Mg(sa) > X_Mg(gt) > X_Mg(sp). The spinel contains 4-7% exsolved magnetite as determined from BSE images. When integrated, the resulting spinel is less magnesian than coexisting garnets, contrary to the partitioning usually invoked for FMAS petrogenetic grids. Oxygen fugacity at near peak conditions was sufficient to stabilize titanhematite (X_Fe2O3 » 0.57 after BSE integration of exsolved ferrian ilmenite) in the presence of magnetite. Exsolved titanhematite (X_Fe2O3 » 0.73) in orthopyroxene, exsolved hematite in sillimanite, and high dissolved Fe³⁺ in sillimanite and corundum also indicate high fO₂ conditions. The titanhematite exsolution in the orthopyroxene occur as parallel sheet-like lamellae less than 1 mm in thickness. Microprobe spot analyses of varying proportions of orthopyroxene host and titanhematite lamellae were extrapolated to a Si-free composition to obtain the lamellae composition. Samples containing sillimanite without exsolved hematite have high dissolved Fe³⁺ (X_Fe2SiO5 » 0.014), whereas samples containing sillimanite with exsolved hematite have even greater amounts of dissolved Fe³⁺ (X_Fe2SiO5 » 0.018). Systematic partitioning of Fe³⁺ in the coexisting silicate and oxide phases further demonstrates that these rocks reached equilibrium under high fO₂ conditions.