The Qimussinguat Complex was introduced by Madore et al. (1999, 2000) in the Peters Lake area (sheet 24M) to describe lithological assemblages corresponding to a strong subcircular magnetic anomaly within the Douglas Harbour Domain. It takes its name from Qimussinguat Lakes about 40 km away to the NW of Peters Lake. The complex then consisted mainly of orthopyroxene intrusive and metamorphic rocks (gneissic orthopyroxene-clinopyroxene tonalite [Aqim4], and hornblende gabbronorite and biotite gabbronorite [Aqim6]), but also included migmatitic granite (Aqim5), granodiorite and granite intrusions (Aqim5a), small ultramafic bodies (Aqim3a), klippes of granulitic supracrustal rocks (mafic gneiss/granulitic metavolcanics [Aqim3] and biotite-garnet paragneiss [Aqim2]), and iron formation layers (Aqim1). The Qimussinguat Complex thus defined had been extended in the Arnaud River (sheet 25D; Madore and Larbie, 2000, 2001) and Klotz Lake (sheet 35A; Madore et al., 2001, 2002) areas. Simard et al. (2008) redefined the Qimussinguat Complex to retain only orthopyroxene rocks within the unit. Supracrustal rocks, ultramafic rocks and iron formations were assigned to the Arnaud Complex, whereas granodiorites and granites were assigned to the Leridon Suite.

The Qimussinguat Complex consists of orthopyroxene orthogneiss and charnockitic intrusions mainly of tonalitic composition and more rarely of granodioritic, granitic and dioritic composition (Madore et al., 1999, 2000; Madore and Larbie, 2000, 2001; Madore et al., 2001, 20002; Simard, 2008; Simard et al., 2008). It has been divided into two informal units (Simard, 2002a, b; MNRF, 2008): gneissic or foliated orthopyroxene-clinopyroxene tonalite (nAqim1, formerly Aqim4 [Madore et al., 1999, 2000; Madore and Larbie, 2000, 2001; Madore et al., 2001, 2002]), and hornblende gabbronorite and biotite gabbronorite (nAqim2, formerly Aqim6 [Madore et al., 1999, 2000; Madore and Larbie, 2000, 2001]).

Gneissic or foliated tonalite (granulitic orthogneiss) characterized by the presence of two pyroxenes (clinopyroxene and orthopyroxene) is the main lithology of the Qimussinguat Complex. It is inserted between centimetre-thick dioritic layers that represent 1-25% of the rock. This type of gneiss constitutes ~70% of the unit. Tonalite also contains trondhjemitic, granodioritic and granitic phases. It contains 1-10% small mafic enclaves (<2 m²) consisting of metagabbro, amphibolite or mafic gneiss. In outcrops, the rock is heterogeneous, migmatized and made up of 10-60% felsic mobilisate, which is present as centimetre-to-decametric-thick discontinuous veins.

In thin sections, gneissic or foliated tonalite and its associated phases are granoblastic and medium grained (0.5-2.0 mm). Despite recrystallization, these rocks locally exhibit a more massive and coarser-grained (1-4 mm) relic texture of magmatic origin. In gneissic facies, ferromagnesian minerals tend to concentrate as porphyroblasts in dioritic bands. These ferromagnesian minerals are reddish biotite, clinopyroxene, orthopyroxene and green hornblende. Garnet porphyroblasts (<2%) are observed in a few places. Between 1 and 10% of opaque minerals, mostly magnetite, are disseminated in the rock. The most common secondary minerals are apatite and zircon.

Qimussinguat Complex 2 (nAqim2) : Hornblende Gabbronorite and Biotite Gabbronorite

A few small gabbronorite intrusive bodies, not exceeding 20 km in diameter, are found mainly inside gneissic or foliated tonalite (unit nAqim1) as metric enclaves surrounded by felsic mobilisate, thus defining an agmatitic texture. These gabbronorite bodies are also found as enclaves in granulitic rocks. Based on petrographic and geochemical observations, two types of gabbronorite can be identified: hornblende gabbronorite and biotite gabbronorite. In thin sections, these rocks display a heterogranular granoblastic texture with grain sizes ranging from 1 to 5 mm. Locally, remnants of subophitic textures are observed, but primary igneous textures are usually replaced by recrystallization textures.

Both types of gabbronorite are mainly composed of clinopyroxene (10-35%), orthopyroxene (5-30%) and plagioclase (40-55%). Hornblende gabbronorite contains 15-30% green hornblende partially replacing pyroxenes and a small amount of biotite (1-5%). This first type of gabbronorite is usually deformed and displays a well-developed foliation. Biotite gabbronorite contains 5-15% biotite and <5% hornblende. This second type of gabbronorite is less deformed than the previous and locally forms dykesdisplaying  an igneous porphyraceous texture. Secondary minerals observed in both types of gabbronorite are apatite (1-3%) and quartz (1-2%). These rocks contain 5-15% disseminated opaque minerals, mainly magnetite and ilmenite.

Thickness and distribution

The Qimussinguat Complex is a subcircular unit that covers a large area, corresponding to a magnetic high, in the Douglas Harbour Domain (Madore et al., 1999, 2000; Simard et al., 2008; Simard, 2008). It forms a strip 45-50 km wide at the ends to 90 km in the centre which begins in the northern part of sheet 24M and continues towards the NNW over 150 km.

Dating

U-Pb dating on zircons of a massive granulitic charnockitic gneiss from the Peters Lake area yielded crystallization ages of 2734 ±3 and 2738 ±7 Ma, as well as an inherited age of 2798 ±15 Ma (David et al., 2009).