Petrology of the Cormacks Lake Complex, Newfoundland: decompressional reaction relations in cordierite plus orthoamphibole-bearing gneisses and associated rocks

Cordierite+orthoamphibole (Crd+Oam)-bearing gneisses in the Cormacks Lake c omplex are regionally associated With metapelites containing prismatic sill imanite and K-feldspar, metabasites that locally contain metamorphic orthop yroxene, and other high-grade rocks in the Central Gneiss (Dashwoods) subzo ne, in the southwestern Newfoundland Appalachians. Retrograde features form ed at the expense of the granulite-facies assemblages are ubiquitous. For e xample, in some migmatitic rocks, garnet is resorbed by Crd+Oam, and in met apelites, cordierite separates corroded garnet and sillimanite. Mineral the rmobarometry suggests that, following granulite-facies metamorphism (T<785 degrees C, P<7.5 kbar), retrogression occurred as the Cormacks Lake gneisse s cooled through Mg-Fe diffusional blocking temperatures as they decompress ed to a pressure of similar to 3-4 kbar. Given the absence of Barrovian (or higher pressure) mineral assemblages in the metapelites, regional tectonic reconstructions involving the thrusting of a neighbouring terrane (Notre: Dame subzone) over the Central Gneiss subzone appear to be supported only b y the model ate pressure determined for the granulite facies event. Althoug h scarcely discernible given re-equilibration effects and the imprecision o f thermobarometers, subsequent decompression nonetheless had a marked impac t on the mineralogy of the gneisses.