Petrology of cordierite + gedrite-bearing sodic granulite from the Grenvillian Long Range Inlier, Newfoundland

Abstract

Sodium-rich, potassium-poor granulite in the Long Range Inlier contains Ab + Qtz + Crd + Bt + Opx + Ilm and either gedrite or garnet, but rarely both. The distribution of garnet in the sodic gneiss was influenced by bulk compositional controls (e.g., higher Al2O3/SiO2). Textural evidence indicates that gedrite was metastable with respect to Crd + Opx during granulite-grade metamorphism. Peak metamorphic conditions are estimated to have been ~700 °C at 600 MPa; the metamorphic fluid was CO2 rich. The preservation of pelitic (Sil + Crd + Bt + Grt + Qtz + Mc + And) layers in which K2O > Na2O suggests a sedimentary precursor for the sodic gneiss, but the granulites are more sodic and less magnesian than primary sediments recently identified as possible precursors to cordierite–orthoamphibole rocks (e.g., ultramafic-derived greywackes; evaporitic mudstones). Leucosomes tend to be more sodic (mean Na2O/K2O = 23.5) than mesosomes (mean Na2O/K2O = 17.8 m garnet-free gneiss and 1.4 m garnetiferous gneiss), so, unless migmatitic melts were fractionated, the unusual composition of the sodic gneiss cannot be attributed solely to partial melting processes. The gneiss is interpreted to be derived from a weathered greywacke.