Tectonic and regional relationships of the Seal Lake and Bruce River magmatic provinces

Abstract

The Bruce River and Seal Lake magmatic provinces represent mid-Helikian, anorogenic igneous events that occur near the junction of three overlapping orogenic be its; Archean, Hudsonian and Grenvillian. The Grevillian belt flanks both provinces on the south and is responsible for imposing low to subgreenschist facies metamorphism on the otherwise undeformed sequences of volcanic and subvolcanic rocks. Both igneous associations are predominantly subaerial and are related to north-northeast - trending extensional fractures but are of greatly contrasting types: the earlier Bruce River volcanics are a bimodal suite dominated by basalt flows and rhyoliticash flows; the Seal Lake assemblage is composed of plateau basalts and ·comagmatic sills. They are separated by an unconformity and differ in age by about 200 Ma (Bruce River--1 526, Seal Lake --1323). The Bruce River volcanics form the upper division of the Bruce River (formerly Croteau) Group which includes, in addition, sandstones and conglomerates. The volcanic sequence is about 6000 m thick and is subdivisible into nine units composed variously of flows, tuffs, breccias and ignimbrites. Although bimodality is the dominant characterisitic of the suite, the composition does range from basalt to rhyolite and shows a broad lycalc-alkaline trend. Its principal chemical distinctions in comparison with world ave rages are higher contents of K20 and, to lesser degree Ba, Sr and Zr. The variation in composition of Bruce River volcanics could be der ived by fractional crystallization of the phenocrysts present, but the proportions would not be consistent with the bimodal distribution observed and an origin by partial melting from two sources-- crust and mantle--is proposed instead. A probable correlative of the Bruce River Group is the Letitia Lake Group of generally similar petrography and composition to the Bruce River felsic fraction but with distortions in composition attributab le to alteration. Exceptional enrichment in Y, Zr and rare-earths are a striking feature of the Letitia Lake rocks, possibly resulting from metasomatism induced by intrusion of the Red Wine Alka line Complexes. In the Seal Lake Group, a lower division of predominantly plateau basalts, 2200 to 4700m thick , is succeeded by a thick (3900 - 7000 m) assemblage of red sands tone and shale with minor interbedded basalt near its top . The sediments are profusely intruded by dolerite sills. The sequence can be interpreted as signifying· an initial phase of rapid extrusion, followed by a long dormant period accompanied by subsidence and sedimentation; then a renewal of magmatism with a consequent loading of the sedimentary column with sills and eventual extrusion again at surface. The magmatic rocks are olivine basalts and dolerites transitional between tholeiitic and alkaline basaltic magma types. They show almost no variation in composition through a stratigraphic range that, with the dolerites, would be equivalent to 3700 to 6500 m; a feature that could be consistent with a lack of magma chambers at intermediate depths. Of similar age and possibly related to the Seal Lake magmatic province are the Red Wine Alkaline Complexes, intrusive into the adjoining terrane. Copper deposits associated with the upper lavas of the Seal Lake Group are believed to have had t heir source in the volcanic rocks and to have been concentrated du ring deformation accompanying the Grenvillian Orogeny. The Seal Lake and Bruce River provinces can both be related to widespread magmatic episodes. The Bruce River volcanics appear to be part of a belt of roughly contemporaneous, anorogenic igneous associations that extend from Labrador to Texas and include the anorthosite suite and the St. Francois Mountains group of rhyolite-granite complexes. In Canada these have been referred to the Elsonian 'Orogeny', but there is no evidence of accompanying deformation, and a thermal event may be all that is involved. The Seal Lake Group and accompanying Red Wine Alkaline Complexes are matched in Greenland by the plateau basalts and alkaline intrusions of the contemporaneous Gardar Province, and both may be linked tectonically to the Keweenawan Province of Lake Superior.