Recrystallization of quartzofeldspathic rocks in a paleozoic thrust belt, Grand Lake, Newfoundland: Implications for garnet-biotite thermometry

Abstract

Migmatitic granitic gneiss and associated garnetiferous granite at Grand Lake are tectonically interleaved with high-pressure ( ∼ 9 kbar) metapelites that contain Barrovian (Ky-St-Grt-Bt-Ms-Rt) mineral assemblages. The migmatites contain metabasites that are compositionally similar to the latest Proterozoic Long Range dykes, suggesting that the gneisses correlate to Grenvillian inliers in western Newfoundland. In the metapelites, the Barrovian porphyroblastic assemblage overprints microfolds and is itself overprinted by greenschist-facies assemblages in thrust-related high-strain zones. Evidence that the quartzofeldspathic rocks also experienced Barrovian metamorphism is provided by low Ti >Al amphiboles in metabasites in the gneiss, and, despite the migmatitic character of these rocks, the presence of growth-zoned garnets with bell-shaped compositional profiles (e.g., rimward increase in X Prp and decrease in X Sps) similar to those exhibited by garnet in the metapelite. The range of garnet-biotite paleotemperatures for the quartzofeldspathic rocks is only marginally higher ( T max=530–660°C) than that determined for the metapelites ( T max=500–615°C). There are no systematic differences in Grt-Bt temperatures between (proto) mylonitic rocks and nearby precursors. These results indicate that the quartzofeldspathic rocks and metapelites share a common, post-migmatization (of the gneisses) metamorphic history; they may well have originally been linked by a basement/cover relationship. Mineralogical evidence for the earlier, higher grade metamorphic history of the migmatites has virtually been eradicated even outside the high-strain zones. In this area, there is no clear relation between qualitatively-estimated superimposed strain and the degree of resetting of the Grt-Bt geothermometer.