|Author:||Lucas and St-Onge, 1991|
|Geological province:||Churchill Province|
|Geological subdivision:||Ungava Orogen / Narsajuaq Arc|
|Lithology:||Plutonic and metasedimentary rocks|
Table des matières
The Narsajuaq Complex was introduced by Charette and Beaudette (2018) to describe geological units mapped during previous work. These units were previously included in the Narsajuaq Arc, which was defined as a cartographic unit in the synthesis of the Ungava Orogen (Lamothe, 2007). However, Charette and Beaudette (2018) have abandoned this definition of the Narsajuaq Arc, considering it more of a lithotectonic domain. The “Narsajuaq Complex” stratigraphic unit was therefore introduced to comply with the North American Stratigraphic Code.
In addition, in the Cape Wolstenholme region mapped at a 1:100,000 scale by the Ministère in summer 2017, lithologies of the western portion of the arc were instead divided into complexes and suites, the main ones being the Pingasualuit Complex (pPpgs), Suluraaq Suite (pPslq), Nallujaq Suite (pPnal), Tasialuk Allipaaq Complex (pPali) and Sanningajualuk Suite (pPsnn).
The Narsajuaq Arc consists of an assemblage of highly metamorphosed plutonic rocks and a small amount of sedimentary rocks that have been accreted to the margin of the Superior Province around 1.80 Ga (Lucas and St-Onge, 1992). Located in the northern part of the Ungava Orogen, the Narsajuaq Complex consists essentially of three intrusive suites that account for more than 90% of the arc. These three suites consist of: 1) an older plutonic suite (1863-1844 Ma, St-Onge et al., 1992) composed of tonalite gneiss, granodiorite and quartz diorite (pPnaq1); 2) a younger intrusive suite (1836-1821 Ma, St-Onge et al., 1992; Parrish, 1989) composed of quartz diorite (pPnaq2), tonalite and granodiorite (pPnaq3) and monzogranite or syenogranite (pPnaq4); and 3) a late anatectic suite (1803-1800 Ma; Dunphy et al., 1995; Parrish, 1989) composed of syenogranite (pPnaq5) and monzogranite (pPnaq6). With the exception of the late suite with relatively undeformed units, intrusive rocks of the Narsajuaq Complex are generally gneissic or foliated.
The main lithology of the older suite, and of the Narsajuaq Complex as a whole, is a well-banded sequence of tonalite and quartz diorite in which conformable and unconformable granitic veins are found (St-Onge et al., 1992). Units in the older suite have a very homogeneous composition and texture over 250 km. A penetrative tectonic foliation at the granulite facies, parallel to compositional banding at the outcrop scale, characterizes most plutonic rocks of the Narsajuaq Complex (Lucas and St-Onge, 1995).
The older plutonic suite consists of the gneiss unit pPnaq1, a banded assemblage of tonalite and quartz diorite cut by felsic veins whose composition is mostly monzogranitic. Gneiss consists of 70 to 80% biotite-hornblende tonalite. Locally, there is a dominance of hornblende-biotite quartz diorite. Bands are decametre to centimetre thick. Diorite horizons are usually boudinaged and surrounded by tonalite. Decimetric to metric horizons of biotite granodiorite, amphibolite, pyroxenite and peridotite are observed in places. Mafic-ultramafic rocks are in places found as enclaves (St-Onge et al., 1992; Lucas and St-Onge, 1997).
The younger plutonic suite cuts the older plutonic suite as well as metasedimentary rocks of the Sugluk Group. Unlike the older plutonic suite, they consist of kilometric plutons, isolated, usually tabular and occurring as sheets. This younger suite contains quartz diorite, monzodiorite, tonalite and monzogranite with proportions of 50% monzogranite, 35% mafic plutons (diorite and monzodiorite) and 15% tonalite. Plutons in the younger suite show varying degrees of deformation from massive to very foliated. In addition, plutons are composed of various enclaves from the older suite, the Sugluk Group, deep crust lithologies and comagmatic rocks (pyroxenite, diorite, tonalite). Based on the size and degree of homogeneity of the intrusions of the younger plutonic suite, it is believed to have developed in shallow levels of the crust (Lucas and St-Onge, 1997).
Diorite intrusions are primarily located in the southern portion of the Narsajuaq Arc near the tectonic contact with the Cape Smith Belt. The degree of deformation varies from massive to highly deformed, depending on the proximity of collision faults (Lucas and St-Onge, 1997). Quartz diorite intrusions contain pyroxenite, diorite and tonalite enclaves (St-Onge et al., 1992).
Hornblende-biotite tonalite kilometric plutons have intruded in the northern Narsajuaq Arc, north of Sugluk Inlet and Charles Island. These are tabular, foliated and parallel to banding of the older suite’s tonalite and quartz diorite. Tonalite is usually medium grained, even grained and contain abundant centimetric to metric enclaves of various compositions (Sugluk Group, gneiss of the older suite, pyroxenite, amphibolite and anorthosite). Biotite monzogranite veins, oriented parallel to or intersecting foliation, cut tonalitic masses (Lucas and St-Onge, 1997).
Intrusive bodies of monzogranite form injections that alternate with gneiss of the older suite (pPnaq1) and tonalite of the younger suite (pPnaq3). Monzogranite is composed of hornblende, biotite and clinopyroxene, which are even grained and medium grained. The texture can be megacrystalline towards the centre of intrusions with an abundance of K-feldspar phenocrystals. Monzodioritic intrusions have smaller dimensions (~2 km) along the Narsajuaq Valley, in the southern sector of Sugluk Inlet (Lucas and St-Onge, 1997).
Plutonic rocks in this suite consist of small granitic plutons and late-tectonic to post-tectonic, even-grained to pegmatitic veins or dykes. This suite is generally not deformed and not metamorphosed (Lucas and St-Onge, 1997).
The late suite consists of pegmatitic muscovite ± garnet syenogranite. Syenogranite occurs as undeformed dykes and sills that cut surrounding rocks. The dykes show a conjugate series steeply dipping and oriented NE to NW (St-Onge et al., 1992). A dyke from the Sugluk Inlet region cutting a tonalite from the younger plutonic suite was dated 1758 ±1 Ma (Parrish, 1989).
In the late suite, one of the monzogranite plutons in the Duquet Lake area, about 2.5 km in diameter, is egg-shaped, undeformed and non-metamorphosed. The pluton zircon U-Pb dating indicates a crystallization age of 1742.2 ±1.3 Ma (Dunphy et al., 1995). It is potassium-rich, hyperaluminous and has geochemical characteristics related to continental crust-derived granites (Lucas and St-Onge, 1997).
Refer to the above section for information.
Dunphy and Ludden (1998) summarize the ages for each group as follows: 1) the older plutonic suite between 1863 and1844 Ma, 2) the younger plutonic suite between 1836 and1821 Ma, and 3) the late plutonic suite between 1803 and1800 Ma.
The Narsajuaq Arc units overthrust the Archean basement (Kovik Antiform). The south front of the Narsajuaq Arc overthrusts the Watts Group units in the Nord Domain. St-Onge et al. (1992) and Dunphy and Ludden (1998) suggest a correlation between the older and younger plutonic suites of the Narsajuaq Complex and the intrusions of the Cape Smith Suite of the Nord Domain. Intrusion ages are grouped in two distinct periods contemporary to the Narsajuaq suites. Geochemical characteristics and similarity in age of the Parent Group volcanics and the Narsajuaq Complex plutonic rocks suggest that the Parent Group could represent a volcanic equivalent of the Narsajuaq magmatic arc, but with less crustal contamination (Picard et al., 1990; St-Onge et al., 1992; Dunphy et al., 1995; Dunphy and Ludden, 1998).
Does not apply.