The Labrador Trough is also known as the “New Quebec Orogen” in many papers and reports covering its geology. It is named after the Labrador territory, which is the main land mass of the province of Newfoundland and Labrador. The Labrador Trough is also commonly referred to as the “Trough” by the many geologists who work or have worked in the area.
Expeditions of Father Louis Babel to the northern Gulf of St. Lawrence in the 1860s likely led to early references to the presence of ferriferous rocks in the Labrador Trough (Clark and Wares, 2004). Between 1892 and 1895, explorer and geologist Albert Peter Low made the first geological observations for the Geological Survey of Canada. Since 1929, research and discovery of iron deposits have led to numerous geological surveys on this territory. Most of the Labrador Trough was mapped at 1:50,000 and 1:63,360 scales by geological services in Quebec, Newfoundland and Labrador and Canada. These maps are supplemented by compilations of selected areas at 1:100,000 (Dimroth, 1978; Wardle, 1982) and 1:250,000 (Taylor, 1979) scales. The geology of the Trough and the southern part of its hinterland in Quebec was also compiled at a scale of 1:250,000 by Clark et al. (1990). Other areas of economic interest have also been the subject of extensive work by mining companies, including IOC, Labrador Mining and Exploration, and Hollinger North Shore Exploration.
The first stratigraphic and structural syntheses of the Labrador Trough were produced by Dimroth (1970a, 1972, 1978, 1981), Dimroth et al. (1970), Dimroth and Dressler (1978), Harrison et al. (1970), and Wardle and Bailey (1981). The Trough was subdivided into three segments of supracrustal rocks: the miogeosynclinal western strip, the eugeosynclinal central strip and the hinterland eastern strip. Dimroth (1970a) created the first image of the New Quebec Orogen structural style. Its interpretations were later clarified or modified by work in the central and northern parts of the orogen (e.g., Dressler, 1979; Clark, 1988; Brouillette, 1989; Boone and Hynes, 1990; Wares and Goutier, 1989, 1990; Goulet, 1995; Madore and Larbi, 2000). In their lithotectonic and metallogenic synthesis of the New Quebec Orogen, Clark and Wares (2004) standardized the stratigraphic nomenclature of the Quebec portion of the Labrador Trough and divided the Kaniapiskau Supergroup (Frarey and Duffell 1964) into three cycles, two volcano-sedimentary cycles at the base, overlain by a cycle of molasse-type metasedimentary rocks. They also subdivided the orogen into eleven lithotectonic zones, most of which are allochtonous and limited by thrust faults or erosional discordances.
Since 2011, a review of Labrador Trough geological maps using high resolution magnetic maps and RapidEye satellite images has been underway at Géologie Québec. Preliminary observations and interpretations from the review of airborne geophysical survey data were made by Clark and Amours (2012). In the summer of 2017, a new 1:50,000 scale geological survey was conducted in the Kangirsuk area covering the northern end of the Labrador Trough and part of the Archean Minto Subprovince. The work has improved the historic geological map and enhanced the mineral potential of this region (Bilodeau and Caron-Côté, 2017).
The Labrador Trough is the foreland (western part) of the New Quebec Orogen, which is divided into two major lithotectonic domains, the Labrador Trough in the west and the Rachel-Laporte Zone in the east. The Trough is also an important part of the Trans-Hudsonian Orogen, which is a large orogenic belt extending northeast from North Dakota to Greenland, and which corresponds to the Churchill Province in its Quebec portion. Rocks of the Labrador Trough extend approximately 850 km from the Grenville Front in the south to the western shore of Ungava Bay in the north. The Trough is limited to the west and north by the Superior Province, to the south by the Grenville Front and to the east by the Rachel-Laporte Zone.
The Labrador Trough is a southwest-verging volcano-sedimentary belt that was folded and transported over rocks of the northeastern margin of the Superior Province during the Paleoproterozoic New Quebec Orogenesis (Clark, 1994; Hoffman, 1990). It primarily consists of greenschist and amphibolite facies sedimentary and igneous rocks that range in age from 2.17 to 1.87 Ga (Rohon et al., 1993; Machado et al., 1997). These rocks have formed in a wide variety of environments, including rifts, continental platforms and slopes, ocean basins and river environments (Clark, 1994). This has resulted in a wide variety of sedimentary rocks, the best known being iron formations of the Schefferville area. The Trough also includes several horizons of basaltic lava, numerous mafic and ultramafic sills of tholeitic affinity, as well as mafic to felsic units of alkaline affinity. Finally, there is the presence of a 15 km-long carbonatite complex, the Le Moyne Carbonatite, hosted in a sequence of rhyodacite flows, highly potassic volcanics and basalts associated with dolomitic and pelitic sedimentary rocks.
The Labrador Trough comprises two large sedimentation and volcanism cycles and a third sedimentation cycle that consists of the Kaniapiskau Supergroup (Frarey and Duffell, 1964) (simplified stratigraphic section). The first cycle consists of an intracratonic basin (rift) sequence overlain by a passive continental margin sequence. First-cycle deposits consist of sandstone and conglomerate of the Seward Group lying on the Archean basement (Superior Province) and representing an immature continental rift sequence. This sequence would have been deposited along the northeastern margin of the Superior Province 2.2 Ga ago (e.g. Hoffman, 1988; Wardle et al., 2002). Volcanic activity was contemporary to sedimentation (Clark and Wares, 2004). The Seward Group is overlain by sandstone and dolomite of the Pistolet Group that have been deposited on a passive margin platform. This platform eventually collapsed and the Swampy Bay Group basalt and flysch sequence was deposited in a marine environment. The end of the first cycle is characterized by marine regression during which the Attikamagen Group dolomitic reef complex was deposited.
The second cycle, which ranges in age from 1.88 to 187 Ga, begins with a marine transgression characterized by sediments of the Ferriman Group, from base to top: quartzitic sandstone and arenite of the Wishart Formation; mudstone, siltstone and ferriferous sandstone of the Ruth Formation, ferriferous and cherty sedimentary rocks of the Sokoman Formation, and sandstone, mudstone and turbidite of the Menihek Formation (Clark and Wares, 2004; Dimroth, 1978). These rocks unconformably lie on the Superior Province and rocks of the first cycle (Dimroth, 1978). In the south-central part of the Trough, the sequence formed by the Wishart-Sokoman-Menihek formations of the Ferriman Group are time-stratigraphic with basalt and flysch of the Double Group (Clark and Wares, 2004). The northern part of the Trough is formed by the Koksoak Group sequence consisting of turbiditic and ferriferous units of the Baby Formation, at the base, overlain by basalt of the Hellancourt Formation, at the top. The end of the second cycle includes conglomerate and dolomite of the Le Moyne Group that characterize the north-central part of the Trough. Many mafic and ultramafic sills of tholeiitic affinity associated with the Montagnais Intrusive Supersuite cut all of the first and second-cycle sequences. In addition, a significant carbonatite intrusion (Le Moyne Complex) occurred near the end of the second cycle. Second-cycle platform rocks are unconformably overlain by a third and last sedimentary cycle synorogenic molasse sequence.
Clark and Wares (2004) divide the Labrador Trough into eleven lithotectonic zones based on distinctive criteria such as lithological assemblages, structural style and mineral deposit distribution. Three zones correspond to autochtonous and parautochtonous sedimentary rocks (Berard, Cambrien and Tamarack zones), three zones to allochtonous sedimentary rocks (Meleye, Schefferville and Wheeler zones), and five zones to first-cycle (Howse Zone), second-cycle (Payne, Gerido and Retty zones) or first two cycles (Hurst Zone) volcano-sedimentary sequences.
Several models of tectonic evolution have been proposed to explain the origin of the Labrador Trough and its hinterland (Wardle and Bailey, 1981; Hoffman, 1987, 1990; Wares et al., 1988; Van der Leeden et al., 1990; St. Seymour et al., 1991; Wares and Skulski, 1992; Skulski et al., 1994). In their synthesis, Clark and Wares (2004) laid the groundwork for the evolution of the Labrador Trough based on previous models and available geochronological data.
The geological history of the New Quebec Orogen spans several tens of millions of years ranging from ~2.2 to 1.74 Ga (New Quebec Orogen evolution chronology):
- About 2.2 Ga ago, rocks of the western part of the New Quebec Orogen (Labrador Trough) were deposited along the Archean margin of the Superior Province following rifting. This period, which corresponds to the onset of first-cycle sedimentation, is characterized by deposition of immature sediments, slightly alkaline volcanics, and intrusion of mafic dykes;
- Between approximately 2.17 and 2.14 Ga, deposition of passive margin sediments, MORB-like mafic volcanism and intrusion of mafic sills characterize most of the first cycle. The end of cycle (<2.06 Ga) is marked by deposition of dolomite and chert on a restored platform;
- From 1.88 to 1.87 Ga, second-cycle platform and basin sedimentation is associated with a new rifting episode or development of a fore-trough basin. This period is characterized by meimechite and carbonatite deposition, MORB-like mafic volcanism corresponding to the formation of a transitional continental-oceanic crust and intrusion of mafic-ultramafic sills;
- From 1.84 to 1.83 Ga, a deformation and high-grade metamorphism phase occurs in the New Quebec Orogen hinterland near Kuujjuaq. During the same period up to 1.81 Ga, a regional-scale granitic and charnockitic intrusion, the De Pas Supersuite (formerly De Pas Batholith), was emplaced. This supersuite is interpreted by several authors as associated with a Proterozoic magmatic arc environment connected to a subduction zone developed during the New Quebec Orogenesis (Dunphy and Skulsky, 1996; Martelain et al., 1998). According to Wardle et al. (1990, 2002), this supersuite may also be associated with a syncollisional component in the New Quebec Orogen hinterland;
- From 1.82 to 1.77 Ga, there would have been an oblique collision between the Superior craton and the Core Zone of the Churchill Province during the New Quebec Orogenesis. This event resulted in transpressure-type deformation and the formation of a western-verging thrust and fold belt, the Labrador Trough. During this period, molasse-type sediments are deposited on the Superior Province margin in the third cycle;
- Around 1.81 Ga, undeformed and likely post-tectonic small intrusions of monzonite occur in the Labrador Trough;
- Finally, from 1.77 to 1.74 Ga, the hinterland near Kuujjuaq is characterized by pegmatite intrusion and hydrothermal activity followed by a cooling period.