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

• Published in: Canadian Journal of Earth Sciences
• Main Authors: Owen, J.V., Greenough, J.D.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 1995
• Subjects: General Earth and Planetary Sciences; Newfoundland
• Online Access: http://dx.doi.org/10.1139/e95-085; http://www.nrcresearchpress.com/doi/pdf/10.1139/e95-085

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 Al 2 O 3 /SiO 2 ). 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 CO 2 rich. The preservation of pelitic (Sil + Crd + Bt + Grt + Qtz + Mc + And) layers in which K 2 O > Na 2 O 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 Na 2 O/K 2 O = 23.5) than mesosomes (mean Na 2 O/K 2 O = 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.