Coronas in Anorthositic Rocks of the Baie-Comeau Complex, Grenville Province, Quebec

The Baie-Comeau Anorthosite Complex (BCAC), Quebec, comprises a complex suite of deformed and metamorphosed anorthosites, leuconorites, and leucotroctolites, some of which contain spectacular coronas around orthopyroxene and olivine. In the leuconorites, orthopyroxene (En₅₈, Al₂O₃=2.4 wt%) is rimmed by hornblende (X_Mg=0.41, Na₂O=2.0, Al₂O₃=15.9 wt%), spinel (X_Mg=0.11), and biotite (X_Mg=0.51). In the leucotroctolites, olivine (Fo₅₆) is surrounded by orthopyroxene (En₄₁, 5.0 wt% Al₂O₃) followed by discontinuous patches of biotite (X_Mg=0.65) and spinel (X_Mg=0.25). In both corona types, plagioclase shows a dramatic shift in composition from ~An₅₅ in the matrix to ~An₈₀ in the coronas. Coronas around magnetite occur in both leucotroctolite and leuconorite, and contain rims of radiating plates of Ti-rich biotite (X_Mg=0.47, up to 6 wt% TiO₂). It is possible that the coronas developed isochemically, whereby the constituents of the product phases were provided by the reactants (with the exception of H₂O). This scenario can account for the development of orthopyroxene, spinel, and hornblende, but it is difficult to explain the presence of biotite. Although it is possible that feldspar provided the necessary K, it is more likely that either K was derived externally or that a primary biotite was available during reaction.

The sporadic distribution of coronas in the BCAC remains problematic. The majority of samples are corona-free, consisting of plagioclase with scattered grains of hornblende plus minor biotite and Fe-Ti oxide. The normative compositions of these corona-free rocks appear the same as those with coronas; hence, it is difficult to appeal to bulk-compositional effects. Regional variations in P-T conditions or localized access to fluids are also possible, but it is equally likely that the corona-forming reactions went to completion, eliminating petrographic evidence for the former presence of olivine and/or orthopyroxene. The timing, exact P-T conditions, and details of the corona-forming reactions remain the subject of ongoing investigations.