BARROVIAN-TYPE METAMORPHISM IN THE WHETSTONE LAKE REGION, SE ONTARIO: BULK COMPOSITIONAL CONTROL ON PELITIC MINERAL ASSEMBLAGES

Some fifty years ago, Dugald Carmichael (1969, CMP; 1970, JPet) detailed the mineral assemblages and reaction textures from across the regional garnet-staurolite-kyanite-sillimanite metamorphic sequence of the Whetstone Lake region, SE Ontario. These studies, followed by an adventurous canoe-based field trip led by Dugald (and much enjoyed by Peter Robinson) at the conjoined GSA-GAC-MAC conference in Toronto, made Whetstone Lake a familiar place name in metamorphic geology. Here, we re-examine the rocks of this region, and present a dataset of petrographic observations, mineral assemblages, phase proportions, bulk compositions and mineral compositions. Of particular interest is the wide variety of mineral assemblages that occur in close proximity to each other. For example, several different assemblages are found within the metasediments of the kyanite zone alone: Ky-St-Ms-Bt-Ilm, St-Ms-Bt-Ilm, Grt-St-Ms-Bt-Ilm, Ky-Ms-Bt-Hem-Mag, Ky-Crd-Ms-Bt-Hem and Grt-St-Crd-Ged-Ilm; whilst in the sillimanite zone, the following assemblages are observed: Sil-St-Ms-Bt-Ilm, Grt-St-Ms-Bt-Ilm, Grt-Sil-St-Ms-Bt-Ilm, Grt-Sil-Ky-St-Ms-Bt-Ilm, Ky-St-Ms-Hem-Mag (all include Qz-Pl). We assess the bulk compositional differences that led to these disparate assemblages occurring at equivalent metamorphic grades, paying particular attention to Al₂O₃ and K₂O concentrations, as well as X_Mg and X_Fe3+ ratios. Combining these observations, we have elucidated the nature of metamorphic reactions in the differing bulk compositions, allowing a comparison with predictions from (1) KFMASH petrogenetic grids and (2) phase equilibrium modelling in 10 or 11 component systems using recent thermodynamic databases and a-X relations. This work emphasises the control of bulk composition on the reaction history of metapelitic rocks, whilst also identifying discrepancies between thermodynamic predictions and natural observations.