Background

Description

Koroc River Suite 1 (pPko1): Hypersthene Amphibolite

Koroc River Suite 2 (pPko2): Paragneiss and Leucogneiss

The pPko2 unit consists of horizons of variable width, extending over several kilometres in length in the Lomier Complex. Their rusty patina and gossan strings that are easily spotted in the field make them a lithological marker for defining the structural grain of the region. Girard (1990) describes several lithofacies that are entangled in this unit and that are not distinguished on the map; aluminous paragneiss, ferro-aluminous paragneiss, quartzite and leucogneiss. The paragneiss are variably migmatized and locally contain leucosome in clusters and diffuse ribbons.

Aluminous paragneiss has a predominantly quartzofeldspathic paragenesis with purple poikiloblastic garnet, fibrous sillimanite, brown biotite and pyrrhotite. Quartz is a significant proportion (40-80%) and forms rods with undulatory extinction in a microcline and plagioclase matrix with indented grain edges. Orthopyroxene is observed locally.

Ferro-aluminous paragneiss has <50% quartz and feldspar, and is dominated by cordierite accompanied by garnet, prismatic and locally poikiloblastic sillimanite, brown biotite and pyrrhotite. Zircon, apatite (scarce), ouvarovite, picotite and hercynite (especially in the core of garnet and sillimanite), as well as scarce grains of hypersthene and augite, compose the accessory mineral assemblage. Scapolite, sphene and allanite with epidote core are the secondary minerals. Garnet is granular to globulous, rich in inclusions (biotite, pyrrhotite, sillimanite, quartz and feldspar), rounded, flattened or stretched in the tectonic grain. It forms polymineralic aggregates with sillimanite, biotite and accessory minerals. Quartz is variable in proportion (locally very low) and forms zones and bands as well as droplets and scarce myrmekites in feldspar. K-feldspar can be either absent or abundant in the rock. All minerals show a granoblastic texture. However, late deformation commonly makes intergrowing or granulated grain edges, polydomanial or undulatory extinctions, and deformed twin networks. Quarzite horizons generally contain finely disseminated sulphides and graphite. Although evidence of partial melting is not readily distinguishable in outcrop, Charette et al. (2021) mention that garnet may contain optically continuous inclusions with edges along its crystal faces, suggesting that they correspond to pseudomorphs of anatectic fluid and that garnet growth therefore occurred in suprasolidus conditions.

Leucogneiss is poor in ferromagnesian minerals, which consist of biotite (<2%), garnet and opaque minerals. This leucogneiss is located mainly in the eastern portion of the Lomier Complex. It is granoblastic to saccharoidal. Paragneiss contains 20-60% decimetric to metric subconformable horizons of whitish trondhjemite (leucogranitoid), which could be derived from partial melting of paragneiss (Girard, 1990). These horizons are locally pegmatitic but usually medium grained. They contain ~30% quartz, 55% plagioclase and <5% biotite. Antiperthites develop in plagioclase for a total of 5% except within grains that have undergone dynamic recrystallization. Deformation is generally moderate and the intrusive appearance is preserved.

Koroc River Suite 3 (pPko3): Marble and Calcosilicate Rocks

Unit pPko3 occurs as metric horizons measured over several kilometres along tectonic grain. These rocks are stretched in deformation corridors, in association with gneiss of the Lomier Complex. They are more abundant and less deformed in sheet 14E13. Marble, which exhibits marked differential erosion, is generally exposed in valley troughs and on cliff sides. Foliation, which is very circumvented, is defined by alternating carbonate and calcosilicate horizons. The marble is granoblastic and is composed of calcite, scapolite, vesuvianite and diopside with, in lesser proportions, quartz, feldspar, biotite and sphene. Brecciated textures are common and are characterized by rounded calcosilicate aggregates floating in a matrix of fine-grained carbonates. Calcosilicate facies with diopside and plagioclase are associated with these breccias.

Koroc River Suite 4 (pPko4): Sulphide-Facies Iron Formation

Unit pPko4 represents a single area (sheet 14E12) consisting of small sulphide-facies iron formation lenses in amphibolites and paragneisses of the Koroc River Group. Iron formation has a brecciated to nebulitic texture in which rounded silicate fragments, millimetric to centimetric, float in a matrix of massive and granoblastic pyrrhotite (<40%). Fragments consist mainly of granoblastic plagioclase but also adjacent quartzofeldspathic rocks. Biotite grows at the interface between fragments and sulphides. There is strong surface alteration of bird’s eye pyrrhotite composed of marcasite and pyrite. Trace chalcopyrite shows mutual relationships on the edge of pyrrhotite grains or is remobilized in thin fractures within silicate phases. Pentlandite forms rare exsolutions in pyrrhotite while isolated sphalerite grains are dispersed in silicates.

Thickness and Distribution

The Koroc River Suite forms continuous horizons over kilometres, of variable width, but generally hectometre wide, in the Lomier Complex. It covers a limited area of ~84 km² in the eastern part of the Falcoz Lithotectonic Domain, with the pPko2 unit (~63 km²) being the largest. The Koroc River Suite is bounded by the ZCpil to the west and the ZCabl to the east. In the northern part of the Falcoz Lithotectonic Domain, the ZCpil meets the ZCabl, which bifurcates in a NW-SE orientation (rather than N-S in the rest of the domain). In this area, the Koroc River Suite is thus bounded to the southwest by the ZCabl.

Dating

Three detrital zircons from the Koroc River Suite paragneiss (outcrop 2013-BC-6210) were analyzed. The ages range from 2447 Ma to 2039 Ma (Charette et al., 2021), the latter corresponding to the deposition maximum age. Zircons dated around 1845 Ma are interpreted by Charette et al. (2021) as having recorded anatectic liquid crystallization. These authors also dated garnet by the Lu-Hf method at ~1956 Ma, which represents the maximum age of garnet growth.

Stratigraphic Relationship(s)

The transitional and diffuse contact of the Courdon and Koroc River suites with the Lomier Complex gneisses led Girard (1990) to consider the gneisses as more deformed areas of the units of these two suites.

The origin of the Lomier Complex is currently enigmatic (Wardle et al., 1990 and 2002; Lafrance et al., 2016; Corrigan et al., 2018; Charette et al., 2021), representing a significant gap in the geodynamic evolution of the region (Charette, 2016). It could be an equivalent of the Cumberland batholith on Baffin Island (St-Onge et al., 2009; Wardle et al., 2002), and thus be interpreted as the root of a magmatic arc emplaced at the eastern margin of the Falcoz lithotectonic Domain (Ermanovics and Van Kranendonk, 1998). However, the most accepted hypothesis is that it represents a continental arc related to eastward subduction of the Falcoz Domain beneath the Nain Province (Van Kranendonk and Wardle, 1996; Wardle et al., 2002).

The Koroc River Suite units also form dismembered horizons in gneisses of the Lomier Complex. Although the amphibolite sequences (pPko1) are commonly associated with the paragneisses of the pPko2 unit, suggesting a supracrustal origin, Girard (1990) mentions that they may also have a link to mafic rocks of the Courdon Intrusive Suite (pPcou2) with which they show gradual contacts locally. In other places, supracrustal rocks of the Koroc River Suite are cut by rocks of the Courdon Intrusive Suite (Girard, 1990).

Paleontology

Does not apply.

References

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