Intersecting Isograds in the Whetstone Lake Area, Ontario

Abstract

Pelitic and calcareous rocks in the Whetstone Lake area have an unusually wide range of chemical composition. Metamorphic reactions have been deduced that represent the observed ‘discontinuities’ in compatible mineral assemblages, and by plotting the reactant and the product assemblage of each reaction on a map, metamorphic isograds have been delincated ‘from both sides’. For the pelitic rocks, successively higher-grade isograds are based on the following reactions: (1)chlorite+muscovite+garnet⇌staurolite+biotite+quartz+water; (2) chlorite+muscovite+staurolite+quartz⇌ kyanite+biotite+water; (3) kyanite⇌sillimanite; (4)staurolite+museovite+quartz⇌sillimanite+garnet+biotite+water. A fifth isograd, based on the reaction (5) biotite+calcite+quartz⇌Ca-amphibole+K-feldspar+carbon dioxide+water intersects the isograds based on reactions (2), (3), and (4) in such a manner as to indicate that the H2O/CO2 fugacity ratio was significantly higher in the vicinity of a granite pluton than in the metasedimentary rocks remote from the pluton. Chemical analyses of the coexisting minerals in reaction (5) indicate that the real reaction may involve plagioclase, epidote, sphene, and Fe-Ti oxides as well.