RECRYSTALLIZATION OF QUARTZOFELDSPATHIC ROCKS IN A PALEOZOIC THRUST BELT, GRAND LAKE, NEWFOUNDLAND - IMPLICATIONS FOR GARNET BIOTITE THERMOMETRY

Migmatitic granitic gneiss and associated garnetiferous granite at Gra nd Lake are tectonically interleaved with high-pressure (similar to 9 kbar) metapelites that contain Barrovian (Ky-St-Grt-Bt-Ms-Rt) mineral assemblages. The migmatites contain metabasites that are compositional ly 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 overprin ts microfolds and is itself overprinted by greenschist-facies assembla ges in thrust-related high-strain zones. Evidence that the quartzofeld spathic rocks also experienced Barrovian metamorphism is provided by l ow Ti/Al amphiboles in metabasites in the gneiss, and, despite the mig matitic 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 t he metapelite. The range of garnet-biotite paleotemperatures for the q uartzofeldspathic rocks is only marginally higher (T-max = 530-660 deg rees C) than that determined for the metapelites (T-max = 500-615 degr ees C), There are no systematic differences in Grt-Bt temperatures bet ween (proto)mylonitic rocks and nearby precursors. These results indic ate that the quartzofeldspathic rocks and metapelites share a common, post-migmatization (of the gneisses) metamorphic history; they may wel l have originally been linked by a basement/cover relationship. Minera logical evidence for the earlier, higher grade metamorphic history of the migmatites has virtually been eradicated even outside the high-str ain zones. In this area, there is no clear relation between qualitativ ely-estimated superimposed strain and the degree of resetting of the G rt-Bt geothermometer.