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

¹ Department of Geology, Saint Mary’s University, Halifax, Nova Scotia, Canada B3H 3C3
² Department of Geological Sciences, Okanagan University College, 3333 College Way, Kolowna, British Columbia, Canada V1V 1V7

^* E-mail: victor.owen{at}stmarys.ca

Cordierite+orthoamphibole (Crd+Oam)-bearing gneisses in the Cormacks Lake complex are regionally associated with metapelites containing prismatic sillimanite and K-feldspar, metabasites that locally contain metamorphic orthopyroxene, and other high-grade rocks in the Central Gneiss (Dashwoods) subzone, in the southwestern Newfoundland Appalachians. Retrograde features formed at the expense of the granulite-facies assemblages are ubiquitous. For example, in some migmatitic rocks, garnet is resorbed by Crd+Oam, and in metapelites, cordierite separates corroded garnet and sillimanite. Mineral thermobarometry suggests that, following granulite-facies metamorphism (T<785°C, P<7.5 kbar), retrogression occurred as the Cormacks Lake gneisses cooled through Mg-Fe diffusional blocking temperatures as they decompressed to a pressure of ~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 by the moderate pressure determined for the granulite facies event. Although scarcely discernible given re-equilibration effects and the imprecision of thermobarometers, subsequent decompression nonetheless had a marked impact on the mineralogy of the gneisses.

KEYWORDS: cordierite-orthoamphibole-bearing gneiss, Cormacks Lake Complex, Newfoundland

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