Stability of sapphirine plus quartz in the oxidized rocks of the Wilson Lake terrane, Labrador: calculated equilibria in NCKFMASHTO

The equilibrium coexistence of sapphirine + quartz is inferred to record temperatures in excess of 980 degrees C, based on the stability of this assemblage in the simplified chemical system FeOMgOAl2O3SiO2 (FMAS) system. However, the potential for sapphirine to contain significant Fe3+ suggests that the stability of sapphirine + quartz could extend to lower temperatures than those constrained in this ideal system. The Wilson Lake terrane in the Grenville Province of central Labrador preserves sapphirine + quartz-bearing assemblages in highly oxidized bulk compositions, and provides an opportunity to explore the stability of sapphirine + quartz in such rock compositions within the Na2OCaOK2OFeOMgOAl2O3SiO2H2OTiO2O (NCKFMASHTO) chemical system. Starting with the phase equilibria in FeOMgOAl2O3SiO2TiO2O (FMASTO), expansion into K2OFeOMgOAl2O3SiO2H2OTiO2O (KFMASHTO) allows the effect of the stability of the additional phases, biotite, K-feldspar and melt, on the stability of sapphirine + quartz to be assessed. These phase relations are evaluated generally using PT projections, and the ultimate extension into NCKFMASHTO is done with pseudosections. Conditions of peak metamorphism in the Wilson Lake terrane are constrained using PT pseudosections, and the appropriate H2O and O contents to use in the modelled compositions are investigated using TMH2O and TMO pseudosections. The peak PT estimates from a sapphirine + quartz-bearing sample are similar to 960 to 935 degrees C at similar to 10 to 8.6 kbar, similar to estimates from orthopyroxene + sillimanite + quartz +/- garnet-bearing samples. Whereas the sapphirine + quartz-bearing sample is more Fe-rich than the orthopyroxene + sillimanite-bearing sample on an all-Fe-as-FeO basis, once the oxidation state is taken into account, the former is effectively more magnesian than the latter, accounting for the sapphirine occurrence.