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

Abstract

The equilibrium coexistence of sapphirine+quartz is inferred to record temperatures in excess of 980°C, based on the stability of this assemblage in the simplified chemical system FeO-MgO-Al 2O 3-SiO 2 (FMAS) system. However, the potential for sapphirine to contain significant Fe 3+ 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 Na 2O-CaO-K 2O-FeO-MgO-Al 2O 3-SiO 2-H 2O-TiO 2-O (NCKFMASHTO) chemical system. Starting with the phase equilibria in FeO-MgO-Al 2O 3-SiO 2-TiO 2-O (FMASTO), expansion into K 2O-FeO-MgO-Al 2O 3-SiO 2-H 2O-TiO 2-O (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 P-T projections, and the ultimate extension into NCKFMASHTO is done with pseudosections. Conditions of peak metamorphism in the Wilson Lake terrane are constrained using P-T pseudosections, and the appropriate H 2O and O contents to use in the modelled compositions are investigated using T-M H2O and T-M O pseudosections. The peak P-T estimates from a sapphirine+quartz-bearing sample are ~960 to 935°C at ~10 to 8.6kbar, 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. © 2011 Blackwell Publishing Ltd.