Tectono-metamorphic evolution of the Grenville Front Zone, Smokey Archipelago, Labrador

• Main Author: Owen, John Victor
• Format: Thesis
• Language: English
• Published: Memorial University of Newfoundland 1985
• Subjects: Makkovik; Benedict; The Bluff
• Online Access: https://research.library.mun.ca/6920/; https://research.library.mun.ca/6920/1/JohnVictorOwen.pdf; https://research.library.mun.ca/6920/3/JohnVictorOwen.pdf

The Grenville Front in eastern Labrador coincides with the Benedict Fault, a south-dipping zone of heterogeneously deformed rocks. In the Smokey archipelago, the Benedict Fault transects Paleohelikian plutonic rocks of the Benedict Mountains Intrusive Suite (BMIS). Various members of the BMIS locally contain a north- to northeast-trending pre-Grenvillian planar fabric known as the Makkovik trend. Makkovik trend fabrics are defined by amphibolite-facies mineral assemblages, and their presence characterizes the BMIS as a pre-Grenvillian lithostructural domain, one of three identified in the area. The age of the Makkovik trend is constrained by Rb-Sr (whole rock) dates determined for granulite-facies gneisses (≥ ca. 1.9Ga) intruded by the BMIS, and by a 1676 ± 77Ma age determined for ferrodiorite-ferrosyenite which intrudes the BMIS. -- The White Bear Islands Granulite Complex (WBIGC), a second lithostructural domain, comprises high-grade ortho- and paragneisses, which show evidence of a period of passive retrogression to amphibolite-facies predating the development of Grenvillian fabrics. Relatively high PT-estimates of 830-860 ± 75ﾟC and 7-8 ± 1Kbar were determined for the granulite-facies event using the two-pyroxene geothermometer and the Al content of orthopyroxene coexisting with garnet. Relatively low PT-estimates of ca. 685ﾟC and 4.6Kbar were derived using various calibrations of the garnet-orthopyroxene (-plagioclase-quartz) geothermobarometer. -- Conditions of pre-Grenvillian retrogression of the WBIGC are estimated at 650 ± 50ﾟC and ca. 5.5 ± 1.5Kbar (garnet-cordierite geothermobarometry) assuming P(H2O) ~ 0.4P(total). The garnet-biotite and two-feldspar geothermometers give variable results which are interpreted to record points on the cooling curve of the WBIGC. -- The third lithostructural domain, known as the Bluff Head orthogneiss, consists predominantly of amphibolite-facies granodioritic gneisses typical of the northern Groswater Bay Terrane. Age relations of these gneisses with the BMIS and the ...