The geology and geochemistry of the McKay River Area volcanic rocks, western Labrador

Noel, Nathaniel Thomas (1992) The geology and geochemistry of the McKay River Area volcanic rocks, western Labrador. Masters thesis, Memorial University of Newfoundland.

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Abstract

The McKay River area is situated in the south eastern section of the Labrador Trough on the boundary between the Grenville and Churchill tectonic provinces. Major lithologic subdivisions identified in the area consist of Archean basement gneisses (2500 ma), a sequence of Aphebian supracrustal rocks (2500-1800 ma), composed primarily of the Knob Lake Group, and some Helikian intrusive, extrusive, and sedimentary rocks (1800-1300 ma). -- Unique to this region of the Labrador Trough is the occurrence of three distinct mafic volcanic sequences. The first, tentatively named the sub-eruptive suite consists of high level basaltic dykes and associated tuffaceous rocks which are intimately associated with the Sokoman Formation. These rocks are chemically similar to the volcanic rocks of the Nimish Sub Group and formed as a direct result of rifting associated with the development of the Labrador Trough. This is the first time this unit has been recognized in the McKay River area, which has subsequently led to the discovery of new information concerning the tectonic and structural history of the area. -- The other two volcanic sequences are separated from the rocks of the Knob Lake Group by a tectonic boundary and are considered to be allocthonous. The first of these, consists of alkali basalts and associated pyroclastic rocks and is characterized by higher than average TiO2 and CaO. It has been tentatively named the McKay River formation. -- The second sequence, which stratigraphically overlies the McKay River formation has been tentatively named the Rose Bay formation. It consists of a suite of alkaline ultrabasic lavas and associated fragmental and volcanoclastic sedimentary rocks. The petrochemistry of these rocks indicate that they represent primary or near primary magma compositions and exhibit strong chemical affinities with a rare rock type known as olivine melilitite. Similarities are also evident between these rocks and some lamprophyres and carbonatites in the northern Labrador Trough. -- Petrochemical and structural evidence suggests that while the McKay River and Rose Bay formations may be synchronous with the Sokoman Formation, they were deposited at some distance from the main rift system and south of their present location. They were transported northward to their present location as a result of the Grenville Orogeny, circa 1000 ma.

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