Geology of the Precambrian Ramah Group and basement rocks in the Nachvak Fiord-Saglek Fiord area, North Labrador

This report describes an area, in the Torngat Mountains, straddling the NainChurchill Structural Province boundary. Nain Province to the east, part of the North Atlantic Archean craton, is chiefly amphibolite facies rocks with minor areas of granulite facies. Granitic gneisses and migmatites predominate and contain horizons of basic gneiss, paragneiss, iron formation and ultramafic rocks. A quartz monzonitic orthogneiss body is located in the north. These rocks, deformed and metamorphosed during the Kenoran Orogeny, form a steeply inclined, linear, north-trending zone, cut by granite veins and by younger, early Aphebian, west-trending diabase dyke swarms. Uplift and erosion resulted in peneplanation of the craton. The Ramah Group forms 0 linear, north-trending Hudsonian fold-belt within Churchill Province. This supracrustal Aphebian group lies unconformably on peneplaned Archean basement rocks of Nain Province and passes from slightly recrystallized platform deposits into highly deformed and metamorphosed equivalents in the west. Although metamorphic grade is Chiefly greenschist and low amphibolite facies, a southern extension, traced for 26 miles from Saglek Fiord to Hebron Fiord, is high amphibolite facies. Ramah Group rocks are preserved in an asymmetrical synclinorium, bounded on the west by a west-dipping thrust and by faults. The 5000 feet of stratified rocks, predominantly of sedimentary origin with one altered tholeiitic basalt flow and several amphibolitized diabase sills, overlie a pre-Ramah regolith. Six unnamed formations mapped are, in ascending order, two lower quartzite units, two slate units separated by a dolomite unit, and a greywacke unit. Paleocurrent data indicate deposition by dominantly westward directed currents. Miogeosynclinal rocks forming the bulk of the group were probably derived from a North Atlantic Archean craton to the east. Archean basement rocks, reworked by the Hudsonian Orogeny, form a mobile belt in Churchill Province west of the Ramah Group. The reworked zone contains lithologies similar to the Archean but is Chiefly granulite facies. Fresh pre-Ramah diabase dykes that cut the Archean are metamorphosed and deformed in the mobile belt. An important cataclastic zone with extensive faults, thrusts, regional mylonites, blastomylonites, and pseudotachylyte characterizes the belt. The boundary between the Hudsonian mobile belt and stable Archean craton is obscured in the basement by the overlying Ramah Group, but represents a Labrador continuation of the Greenland Nagssugtoquidian boundary. Overthrusting of the mobile belt over the Nain Archean craton to the east and transcurrent sinistral movement took place in the boundary zone. Although no mineral occurrences of economic importance were discovered in lowgrade iron, copper, lead and zinc mineralization is present in places.