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

Owen, John Victor (1985) Tectono-metamorphic evolution of the Grenville Front Zone, Smokey Archipelago, Labrador. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

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 WBIGC are uncertain. Both gneissic domains, however, locally contain pre-migmatitic metabasites bearing clinopyroxenes of similar composition, suggesting equilibration under broadly similar metamorphic conditions. The WBIGC and the Bluff Head orthogneiss are thus inferred to have shared a common, early tectono-metamorphic history prior to the emplacement of the BMIS. Pre-Grenvillian retrogression of the WBIGC is most pronounced in the western portion of the study area, and may be transitional into the amphibolite-facies assemblages near Bluff Head. Alternatively, the Bluff Head orthogneiss may record the effects of significant recrystallization during the Grenvillian orogeny; garnet-biotite and two-feldspar temperature estimates for the gneisses are consistent with conditions determined for Grenvillian metamorphism in the area. -- A zonal distribution of structural and metamorphic features attributed to the Grenvillian orogeny is apparent from north to south across the Smokey archipelago. Three east-west trending Grenvillian structural domains, separated by major Grenvillian high-strain zones, have been recognized on the basis of contrasting fabric development and metamorphic grade. North of the Benedict Fault, Makkovik trend fabrics are locally retrograded to greenschist-facies assemblages. Greenschist- to lower amphibolite-facies assemblages characterize south-dipping Grenvillian fabrics in a ~3km wide transitional domain to the south of the Benedict Fault. The Cut Throat Island Fault (CTIF), a south-dipping zone of mylonites, defines the southern extent of the transitional domain. Garnetiferous assemblages of the epidote amphibolite- to lower amphibolite-facies characterize Grenvillian fabrics south of the CTIF. Grenvillian L-S fabrics are most extensively developed in this domain and formed during the second of three phases of folding attributed to the Grenvillian orogeny. Temperatures of Grenvillian metamorphism south of the CTIF are best estimated by low Mn-Ca garnet-bearing mineral assemblages at ca. 550 ± 30ﾟC (garnet-biotite, garnet-amphibole thermometery). Mineral assemblages suitable for pressure determinations are lacking; however the composition of amphiboles together with the presence of garnet in domain G indicate a southerly increase in lithostatic pressure in the map area. -- The Grenville Front zone on the Labrador coast is thus characterized by a southward-increasing Grenvillian metamorphic gradient which has been telescoped across major thrust (or high angle reverse) faults, and by the heterogeneous development of L-S fabrics which overprint pre-Grenvillian structural and metamorphic features. Similar features characterize the Grenville Front zone elsewhere in Labrador, and are consistent with the characterization of this portion of the Grenville Province as a region of significant crustal thickening about 1Ga ago.

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