Bulk Compositional Influence on Diverse Metapelitic Mineral Assemblages in the Whetstone Lake Area, Ontario.

This article investigates the influence of bulk rock composition on the diversity of metapelitic mineral assemblages within the kyanite and lower sillimanite metamorphic zones of the Whetstone Lake area in southeastern Ontario, where metamorphic grade varies only modestly. Using petrographic observations, whole-rock geochemistry, mineral proportions, and compositions from 32 samples, the study demonstrates that variations in molar Mg/(Mg+Fe) (XMg*) and ferric iron ratio (XFe³⁺) primarily control mineral assemblage development, with K₂O, Al₂O₃, and MnO exerting secondary effects. Phase equilibrium modeling employing thermodynamic data set 6.2 and associated Fe³⁺-bearing solution models broadly reproduces observed mineral assemblages and modal abundances, especially when using carefully derived local bulk compositions rather than whole-rock analyses, with predicted peak metamorphic conditions between 620–675°C and 6.5–7.5 kbar. However, significant discrepancies remain between predicted and observed mineral compositions, including overestimation of Mg in garnet, staurolite, and cordierite, overestimation of Ti in staurolite and biotite, underestimation of Si in biotite, and misestimation of Al and Fe oxidation states in muscovite, indicating areas for refinement in thermodynamic models. The Whetstone Lake metapelite suite thus provides a valuable natural testbed for evaluating and improving predictive capabilities of metamorphic phase equilibrium modeling.