|Author:||Charette and Beaudette, 2018|
|Type area:||Erik Cove area (NTS sheet 35K11)|
|Geological province:||Churchill Province|
|Geological subdivision:||Ungava Orogen / Narsajuaq Arc|
|Lithology:||Metasedimentary and felsic intrusive rocks|
Table des matières
The Erik Cove Complex was introduced by Charette and Beaudette (2018) to describe the klippes and kilometre-long paragneiss and migmatite sequences distributed within the Narsajuaq Arc. Prior to the work of these authors, paragneiss was assigned to the Sugluk Group (Lucas and St-Onge, 1991). However, the high degree of metamorphism of metasedimentary rocks present in the Narsajuaq Arc (characterized in particular by the loss of primary structures), paragenesis of the amphibolite facies and, (locally granulite facies), and presence of partial melting evidence, makes the use of the group rank inappropriate in the context of the North American Stratigraphic Code (NACSN, 1983, 2005; MERQ, 1986). Furthermore, Charette and Beaudette (2018) considered it appropriate to abandon the term Sugluk, which already refers to many tectonic entities in the region (e.g., Sugluk suture, Sugluk block, Sugluk terrane, etc.), in favour of the “Erik Cove Complex” which includes not only paragneiss, but also migmatite considered derived from its melting and their ultrametamorphic granite-type mobilisate. It should be noted that to date, rocks assigned to the Sugluk Group remain even further east; future geological mapping campaigns planned by the Ministère in these areas should lead to an update.
The Erik Cove Complex consist of variably migmatitized metasedimentary klippes that have been divided according to their partial melting rate and their migmatitic texture. The main unit is a weakly migmatitized biotite ± garnet banded paragneiss (pPecv1), including locally iron formations (pPecv1a) and forming large kilometric stripes of more sandy composition in the northern part of the arc (pPecv1b). Units pPecv2 and pPecv3 consist of paragneiss with higher partial melting rate and are described as metatexite and diatexite respectively. Areas with significant amounts of mobilisate occurring as zones and injections in paragneiss are grouped into unit pPecv4.
Unit pPecv1 is Erik Cove Complex’s dominant metasedimentary sequence facies. Paragneiss of this unit is generally not very migmatitized, although it may contain some decimetric to metric horizons or zones of metatexite or diatexite. On some outcrops, compositional banding is characterized by variations in proportions of ferromagnesian minerals and quartz, as well as the presence of garnet, aluminosilicate, graphite or sulphides. In some places, banding is also characterized by variations in grain size or by the presence of centimetric to decametric horizons of quartzite or quartz-rich metasediment. Horizons of dioritic or gabbroic composition are also locally present in sharp contact within paragneiss, or boudinaged.
Paragneiss of unit pPecv1 contains between 5% and 20% whitish leucosome as bands or millimetric to centimetric clusters in diffuse contact. These are generally coarser-grained than the matrix, ranging from fine to medium-grained, and include garnet porphyroblast spots and thin horizons or clusters of biotite. According to the petrographic study of unit pPecv1, the composition of paragneiss is generally low in potassium and varies from psammitic to semi-pelitic. Horizons of pelitic composition are present locally. Biotite is the primary ferromagnesian mineral and is showing variable proportions (15-40%). In places, this mica is brownish red indicating high temperature conditions have been reached. When present, garnet occurs as porphyroclasts, locally fractured or elongated in foliation. Garnet proportions are generally less than 10% but can reach 30% in some horizons. The quartzofeldspathic matrix is well recrystallized and, close to faults, has a preferential orientation as well as quartz bands. Locally some amphibole is present in paragneiss. Also, orthopyroxene is locally observed in the mineral assemblage and indicates granulite facies metamorphic conditions. Accessory minerals include muscovite, chlorite, carbonates, sulphides, graphite, while trace minerals are apatite, allanite, zircon, and monazite.
Erik Cove Complex 1a (pPecv1a): Banded Paragneiss and Migmatitized Paragneiss with Iron Formation Horizons
Subunit pPecv1a was introduced to distinguish areas where pPecv1 paragneiss contains horizons of magnetite-rich iron formation or paragneiss. In general, these are silicate or oxide iron formation horizons a few metres thick (2-10 m), in sharp contact within weakly migmatitized paragneiss.Oxide iron formation banding is regular, millimetric to centimetric, while silicate iron formation does not show banding or anastomosed banding produced by quartz bands.
Erik Cove Complex 1b (pPecv1b): Sandy Metasediment, Rusty Metasediment and Mobilisate-Type Tonalitic Injections
In the northwest sector of the Narsajuaq Arc, two broad kilometric strips of metasediment are distinguished from paragneiss klippes within the Narsajuaq Arc units (pPecv1) by their greater thickness, the presence of rusty to purplish graphite-pyrite-pyrrhotite horizons, and their locally potassic and usually more psammitic to sandy composition. These have been assigned to subunit pPecv1b. Metasedimentary sequences of this subunit are cut by numerous whitish leucoratic injections locally showing lilac garnet porphyroblasts. The latter generally account for 10 to 40% of outcrops and are subconformable with or cut the metasediment fabric.
Within the broad strips of subunit pPecv1b, metasediments are generally light grey to white in fresh exposure and rusty orange in altered patina. Quartzite beds centimetre to decimeter thick are more commonly observed than in pPecv1 paragneiss. The rusty to purplish graphite-pyrite-pyrrhotite horizons mark a decimetric to metric banding in metasediments. These horizons are sometimes boudinaged or folded and are generally observed along the shore of Hudson Strait in the Salluit and Cape Wolstenholme areas. Also, metatexite banded horizons are locally present in these metasedimentary sequences. The pPecv1b subunit usually consists of biotite or biotite-garnet metasedimentary rocks. Sillimanite and cordierite are observed locally.
Unit pPecv2 includes areas where paragneiss is more migmatitized (metatexite). In these areas, lithologies comprise between 20% and 35% leucosome, which is characterized by diffuse white millimetric to centimetric bands. Typically, this unit is distinguished from pPecv1 paragneiss by a banded appearance which produces locally a stromatic texture. These bands are folded with the metatexite foliation and are locally boudinaged. The leucosome is also concentrated in decimetric horizons or zones that are conformable with or cut foliation. They are usually associated with pinkish to lilac garnet porphyroblasts (<20%) and biotite clusters or schlierens. Garnet is rimmed by biotite locally. Like pPecv1 paragneiss, compositional variations as well as intermediate to mafic horizons are observed in pPecv2 metatexite.
Diatexite is generally of the paleosome raft diatexite or schlieren diatexite type as classified by Sawyer (2008). It is whitish to rusty brown in altered surface with a variable proportion of biotite-garnet schlierens (5-20%). Sillimanite is also observed locally. Better preserved centimetric to decimetric paragneiss rafts are present in diffuse contact and in varying proportions within diatexite.
Unit pPecv4 includes a significant number of whitish intrusive material interpreted as ultrametamorphic granite-type mobilisate. This mobilisate occurs as whitish zones and injections that can occupy 50 to 75% of the outcrop surface. Pink to lilac garnet is visible locally. Paragneiss occurs in places as diffuse horizons or enclaves within ultrametamorphic granite.
Paragneiss and migmatitized paragneiss of the Erik Cove Complex form hectometric to kilometric klippes within the Narsajuaq Arc. The thickest klippes are located near large deformation zones (i.e., the Sugluk Suture and the Naujaat Shear Zone). Units pPecv2 and pPecv3 are usually associated with pPecv1 paragneiss. The latter is in lateral continuity or occurs as rafts within these migmatitized units.
Several detrital zircons and rutile were dated using U-Pb method in a quartzite sample taken north of Salluit Bay and belonging to the Sugluk Group (sample Sugluk-4-87; Parrish, 1989). The oldest detrital zircon was dated to 2545 Ma, while the youngest zircon gave an age of 1830 Ma (Parrish, 1989). These ages indicate the source of metasedimentary rocks of the Narsajuaq Arc is diverse (Archean and Paleoproterozoic) and the maximum deposition age is 1830Ma. However, Parrish (1989) and St-Onge et al. (1992) points to the inconsistency of this date with events of late intrusion and granulite facies metamorphismthat affected the area between 1835 and 1820 Ma. This geochronologic data suggests that rapid burial of sediments occurred, likely as a result of overthrusting of the various intrusive suites of the arc around 1830 Ma (Parrish, 1989; St-Onge et al., 1992). A cooling age of 1719 Ma was obtained by Bracciali et al. (2013) on rutile grains from the same quartzite sample. This age suggests a long cooling period following the collisional metamorphism of the Ungava orogenesis.
Sulphur isotope analyses were conducted by Bui et al. (in preparation) on nine paragneiss samples from unit pPecv1 and subunit pPecv1b. A sample (17-MB-3025-A) gave results indicating fractionation of characteristic sulphur isotopes of sedimentary cycles in the Archean. Therefore, the possibility of the Erik Cove Complex including Archean sedimentary rocks is not precluded, but requires further sampling and diversification of analytical methods to be confirmed.
Paragneiss of the Erik Cove Complex are probably one of the oldest lithologies of the Narsajuaq Arc. They are incorporated into rocks of the arc as klippes. St-Onge et al. (1992) interpreted these metasedimentary rocks as deep-water deposits emplaced on the edge of an island arc complex, with likely rapid burial (Parrish, 1989; St-Onge et al., 1992).
Does not apply.