|Type area:||Courdon Lake area (NTS sheet 14E12)|
|Geological province:||Churchill Province|
|Geological subdivision:||Torngat Orogen|
Table des matières
The Lomier Complex was defined by Girard (1990) in the Courdon Lake area (NTS sheet 14E12) to group granulitic gneisses of varying compositions characterized by a strong aeromagnetic signature and a N-S tectonic grain along the Quebec-Labrador border. This complex has also been described in the Henrietta Lake area (Lafrance et al., 2015) and extended northward in the Koroc River and Hébron area (Verpaelst et al., 2000) and in the Point le Droit area (Mathieu et al., 2018), where it pinches on the Abloviak Shear Zone (ASZ) that is shifting to the WNW in this area.
The Lomier Complex consists of two closely related packages in the field, a supracrustal sequence related to the Koroc River Group (pPko), which is cut by the Courdon Intrusive Suite (pPcou). Girard (1990) grouped rocks whose protoliths are no longer recognizable due to their heterogeneity, as well as those that have developed intense tectonic banding in gneiss units of the Lomier Complex (pPlom). This author considers rocks of the Courdon Intrusive Suite and the Koroc River Group to be the protoliths of these gneisses, either by alternating units or by the presence of numerous subconformable granitic intrusions accentuating the gneissic appearance. The Lomier Complex consists of two units: a granulitic gneiss unit (pPlom1) and a granitic gneiss unit (pPlom2).
Gneisses of the Lomier Complex are considered to be alternating millimetric to decametric horizons of Koroc River Group and Courdon Intrusive Suite rocks in deformation zones. They regularly contain decimetric horizons that are better preserved and in gradual contact with these units. Gneiss as a whole shows well-developed foliation and lineation, often mylonitic with formation of quartz bands and rods. According to Girard (1990), gneiss represents zones of mixing between different pre-tectonic to syntectonic lithologies. The enderbite protolith dominates (60%) and is intertwined with amphibolites (30%) and paragneiss (10%). Banding occurs at all scales, from decametre to millimetre, with bands continuous hectometrically or in regular or heterogeneous klippes. Contacts are paralleled and transposed into foliation. The gneiss mineralogy is similar to that of their protoliths (Courdon Intrusive Suite and Koroc River Group).
Gneiss of the Lomier Complex has sharp banding with alternating millimetric to decamatric bands and horizons of enderbite, diorite, charnockite and opdalite, all of which are fine to very fine grained and highly magnetic. Rocks are affected by a reduction in grain size of variable intensity, marked by fine-grained corridors between quartzofeldspathic porphyroclasts (Charette and Guilmette, 2014). Mineral phases have undulatory extinction, deformed twins, and indented crystalline edges, indicating intense deformation. Gneissosity is defined by the concentration of orthopyroxene in laminae and the presence of quartz bands with subgrain extinction. Locally, it was possible to distinguish predominantly enderbitic (pPlom1a) or predominantly charnockitic (pPlom1b) gneiss.
Lomier Complex 1a (pPlom1a): Enderbitic gneiss
Subunit pPlom1a is characterized by predominant enderbitic horizons in gneiss (50-65%). These horizons are leucocratic (5-12% mafic minerals), brown sugar coloured or greenish and include orthopyroxene as well as apatite and zircon as accessory minerals. Gneiss also comprises 20 to 35% intermediate and mafic horizons (hypersthene diorite and gabbronorite), generally slightly finer than felsic horizons. These contain 35 to 55% mafic minerals, dominated by pyroxenes (orthopyroxene and clinopyroxene). Enderbitic gneiss also contains diffuse centimetric bands of slightly coarser-grained enderbite and charnockite, which could represent leucosome. Accessory minerals observed in the different phases of the gneiss are apatite, zircon, opaque minerals and epidote. Orthopyroxene is variable retrograded to hornblende and biotite. Iddingsite is also observed in the edges and fractures of orthopyroxene.
Lomier Complex 1b (pPlom1b): Charnockitic gneiss
Charnockitic gneiss has a light brown sugar colour and contains 4 to 10% mafic minerals, mainly pyroxenes. Stretching of minerals is often more developed than banding and foliation. Banding, which is little visible in outcrop, is caused by the change in the percentage of K-feldspar (charnockite to opdalite to farsundite) and in grain size (very fine to fine), which can be easily observed on stained samples. Between 5% and 15% diffuse bands or intrusions of leucocratic and coarse-grained charnockite are paralleled in deformation. K-feldspar is perthitic and often rimmed with myrmekites. Mafic minerals of charnockitic gneiss are dominated by broken orthopyroxene, not altered or partially replaced by a mixture of iddingiste and carbonates. Gneiss also contains thin biotite flakes and up to 2% finely disseminated magnetite. The main accessory minerals are apatite, zircon, amphibole (replacing pyroxene) and interstitial carbonates. Clinopyroxene and garnet are also observed sporadically.
The granitic gneiss unit forms a 3 to 5 km wide strip on the western edge of the Lomier Complex and probably represents an area of retrograde metamorphism linked to the Lac Pilliamet Shear Zone. This unit consists of two deformed alternating phases in decimetric to metric horizons: a dioritic grey phase (locally granitic) and a coarser-grained pink granite phase. The granitic phase is generally dominant. Diorite is granoblastic and contains between 15 and 35% mafic minerals, consisting of green hornblende and chloritized brown biotite. Pink granite is hololeucocratic, not granoblastic. It is slightly foliated to ultramylonitic and hematitized along faults. According to Girard (1990), the granitic phase represents veins and lenses of leucosome with sharp contacts with gneiss (paleosome). A thin biotitic melanosome locally forms a millimetric rim at the interface between granite and host gneiss. Granite is dominated by partially recrystallized microcline-rich lenses alternating with quartz bands with undulatory extinction. It contains some biotite, brown or green, with zircon and opaque inclusions, as well as muscovite and chlorite.
The Lomier Complex is a 10 to 15 km wide N-S strip spanning more than 200 km long along the Quebec-Labrador border. It is limited by the Lac Pilliamet Shear Zone (LPSZ) in the west and the Abloviak Shear Zone (ASZ) in the east. In the northern part of the southeastern Churchill Province, the LPSZ converges to the ASZ, which then turns WNW in that area. The Lomier Complex then pinches at the junction of the two faults.
An age of approximately 1877 Ma was obtained in Labrador in an enderbitic gneiss located on the eastern edge of the Lomier Complex (Bertrand et al., 1993).
|Isotopic System||Mineral||Crystallization Age (Ma)||(+)||(-)||Metamorphic Age (Ma)||(+)||(-)||Reference(s)|
|U-Pb||Zircon||1876.9||1||1||1822.5||1.1||1.1||Bertrand et al., 1993|
The Lomier Complex is in tectonic contact with the Sukaliuk Complex in the west (LPSZ) and the Tasiuyak Complex in the east (ASZ). Transitional and diffuse contact between the Courdon Intrusive Suite and the Lomier Complex gneiss led Girard (1990) to consider the gneiss as more deformed zones of units of the Courdon Intrusive Suite and the Koroc River Group. According to Girard (1990), the Lomier Complex could be the equivalent of the Eastern Basement Complex defined further south by Ryan et al. (1988) and Wardle et al. (1990).
Does not apply.