The Lake Harbour Group was introduced by Jackson and Taylor (1972) to designate a sequence of Paleoproterozoic volcano-sedimentary rocks located on Baffin Island, Nunavut, and northeastern Quebec and Labrador (“Dorset Fold Belt”). Taylor (1979) repeated the term, but reduced the unit to the rank of formation in order, in his view, to comply with the North American Stratigraphic Code at the time. Hoffman (1988) did, however, use the informal term “Koroc River sediments” while acknowledging its similarity to the Lake Harbour Group. For priority reasons, Verpaelst et al. (2000) and Simard et al. (2013) maintained the term Lake Harbour Group in the Koroc River and Kuujjuaq areas. Although the correlation with Baffin Island units remains uncertain, Lafrance et al. (2015, 2016) and Mathieu et al. (2018) also assigned volcano-sedimentary rock sequences located in the eastern part of the Core Zone to the Lake Harbour Group to provide some continuity with the units recognized further north.

The degree of metamorphism in rocks of the Lake Harbour Group in southeastern Churchill Province (SECP), including the loss of primary structures, makes use of the term group inappropriate. Since this is a well-established name in the literature, the term Lake Harbour Group has been maintained. However, the unit is classified as a complex as it combines rocks of different genetic classes.

The Lake Harbour Group consists of four units: 1) a paragneiss unit (pPhb1); 2) a quartzite unit (pPhb2); 3) a calcosilicate rock and marble unit (pPhb3); and 4) an amphibolite unit (pPhb4). In the Koroc River area, Verpaelst et al. (2000) describe a sedimentary sequence with apparent thickness of the order of one kilometer. Stratigraphic boundaries are not exposed and sedimentary polarity remains uncertain. The lower contact, observed near the Baudan River, is a ductile shear zone or even a decollement zone. However, the marble at the « base » of the sequence at this location may be a more ductile horizon in which deformation has been concentrated. Elsewhere, the lower contact is a fault in a ductile shear zone. The upper contact was not observed. In several places, pink pegmatite is found ranging from one to three metres thick in contact with Archean gneiss and marbles.

The Lake Harbour Group is characterized by the development of amphibolite facies metamorphic assemblages and by evidence of more or less advanced partial melting. It has a biotite-plagioclase-K- feldspar-quartz-garnet-sillimanite-muscovite-amphibole paragenesis. The degree of partial melting is also highly variable, ranging from paragneiss containing 2 to 30% leucosome to stromatic metatexite, to ultrametamorphic granite. The presence of leucosome indicates that metamorphic conditions reached exceeded the melting conditions resulting from dehydration of mica. Likewise, the presence of sillimanite shows that the stability field of this mineral has been reached. These observations suggest an amphibolite facies metamorphism corroborated by the geothermobarometric calculations of Charette (2016) in the Henrietta Lake area, and indicate P-T conditions of 5.8 kbar at 645°C and 6.5 kbar at 771°C in the Lake Harbour Group metasediments. 

Paragneiss is the predominant unit of the Lake Harbour Group. It usually has a banded or stromatic texture materialised by the presence of 5 to 25% millimetric to centimetric white leucosome bands. The rock composition varies depending on the amount of K-feldspar (1-30%), quartz (5-30%) and mafic minerals (biotite and garnet, 10-25%). Its colour varies from medium grey to dark grey in fresh exposure with a distinctive rusty brown or brownish patina. Paragneiss is fine to medium grained, well foliated and has a well developed granoblastic texture. Biotite and garnet are locally associated with sillimanite. Garnet occurs as small disseminated grains or as centimetric poikiloblasts containing inclusions of other minerals observed in the paragneiss. Sillimanite occurs in several forms: 1) small disseminated prisms; 2) fibrous, associated with mica (muscovite and biotite); 3) millimetric to centimetric nodules; and 4) flexuous clusters surrounding feldspar phenocrystals. Muscovite, cordierite, zircon, apatite, epidote, graphite, allanite, tourmaline, magnetite, rutile and sulphides are the most common secondary minerals.

In the Falcoz Shear Zone, paragneiss shows a well-defined quartz band protomylonitic texture. In outcrop, there are many decimetre to decametre-wide rusted zones, as well as paragneiss horizons that have undergone severe surface alterations and where the rock is reduced to sand. In these horizons, paragneiss is rich in K-feldspar and contains 5 to 20% graphite. More locally, orthopyroxene is observed in paragneiss. The latter then takes on a brown sugar alteration colour characteristic of orthopyroxene rocks. In these rocks, biotite (2-8%) is dark brown, almost reddish, indicating high temperature biotite. There is also green hornblende (3-7%), garnet (5-15%) and augite (<5%).

Decimetric to decametric horizons of impure quartzite, calcosilicate rocks and amphibolite are interbedded in the pPhb1 paragneiss. Grenatites were also observed on three outcrops in the Koroc River area. Two of them contain diopside and sphene with or without calcite, plagioclase, epidote, magnetite and sulphides. The third outcrop, located in the Falcoz Shear Zone, also contains quartz, apatite, sulphides, red biotite, magnetite, monazite and harrisonite.

Lake Harbour Group 1a (pPhb1a): Diatexite Derived from Melting of Paragneiss

In some areas, the more important melting of paragneiss resulted in diatexites assigned to subunit pPhb1a. These diatexites contain 40 to 65% mobilisate in the form of millimetric to centimetric whitish bands. In outcrop, there are numerous zones of leucosome accumulation, as well as enclaves and decimatric to metric horizons of preserved paragneiss. Diatexite is heterogranular, grey in altered surface and brownish white in fresh exposure. Its composition is variable due to unequal distribution of K-feldspar (0-20%) in the rock. It contains 10 to 20% brown to red biotite forming schlierens that mould quartzofeldspathic clusters or zones. Biotite is regularly associated with muscovite flakes and, more locally, sillimanite. The main accessory minerals are garnet (<5%), sphene, hornblende, apatite, zircon, allanite, epidote and graphite.