|Author:||Verpaelst et al., 2000|
|Type area:||Koroc River area (NTS sheet 24I)|
|Geological province:||Churchill Province|
|Geological subdivision:||Core Zone|
|Lithology:||Mafic and ultramafic intrusions|
Table des matières
The Nuvulialuk Suite was introduced by Verpaelst et al. (2000) in the Koroc River area (NTS sheet 24I) to group bands of mafic and ultramafic intrusive rocks. It was continued southward in the Henrietta Lake area (Lafrance et al., 2015) and northward in the Pointe Le Droit area (Mathieu et al., 2018).
The Nuvulialuk Suite consists of two units, a gabbro and gabbronorite unit (pPnuv1) and an ultramafic rock unit (pPnuv2). Rocks of the Nuvulialuk Suite frequently contain decimetric horizons of paragneiss and quartzite of the Lake Harbour Group (pPhb).
Unit pPnuv1 consists of gabbro and gabbronorite as well as some diorite, orthopyroxene diorite and norite, all even-grained, medium to fine-grained, black-and-white speckled and of massive to foliated appearance. In places, mafic intrusions have experienced some partial melting evidenced by the presence of neosome in diffuse clusters (migmatite patch). This neosome is coarser grained and more heterogeneous than the paleosome. Plagioclase is granoblastic, locally sericitized and occurs between the coarser-grained mafic minerals. It locally contains rounded quartz inclusions.
Gabbro and diorite are generally non-magnetic and contain 30 to 65% mafic minerals, mainly green hornblende and actinolite with clinopyroxene (<25%). Quartz (<5%) and brown biotite are common. The latter typically represents less than 5% of the mineral phases, however, it may be equivalent to hornblende in some samples. In deformed zones, amphiboles and biotite are aligned in foliation. Locally, clinopyroxene is better preserved and represents the principal mafic mineral. It then forms large poikilitic crystals partially uralitized or small granoblastic grains. Gabbronorite, orthopyroxene diorite and norite are magnetic and slightly richer in mafic minerals. They contain, in addition to the mafic minerals mentioned above, orthopyroxene. Intergrowths between pyroxenes and quartz are observed in thin section.
The main accessory minerals are opaque minerals (2-5%; ilmenite, magnetite ± sulphides), apatite, sphene, epidote, zircon and, more locally, carbonates, allanite, muscovite, tourmaline and garnet. Pyroxene relics are more common in the core of intrusions. Near contacts, where circulation of fluids has been more pronounced, rocks are completely amphibolitized. In the Keglo Bay area (sheet 24P04), an oxide-rich decimetric to metric horizon was observed in a gabbronorite assigned to the Nuvulialuk Suite.
In the field, ultramafic rocks of the Nuvulialuk Suite are easily identifiable since they form fairly well-aligned hills, with buff-coloured or orange-brown patina (lichen present) and elephant skin texture caused by the differential erosion of the polygonal joint network on the surface of the rock. In the Koroc River area, ultramafic rocks are generally more severely broken up into sand. Despite the preservation of some magmatic textures, ultramafic rocks are generally foliated.
Verpaelst et al. (2000) describe four types of ultramafic rocks in unit pPnuv2: dunite, peridotite, pyroxenite and hornblendite. However, the work of Pedreira Pérez (2017) did not demonstrate the presence of dunite. The large-scale intrusions are differentiated and show well-preserved magmatic bedding and primary mineral assemblage. There are different rock facies, gradually moving from cumulate-rich harzburgite and lherzolite at the base to olivine websterite, then gabbro at the top. These rocks are homogeneous and of fine to medium-grained. There are magnetite and serpentine veins in positive relief as well as green or brown spinel (hercynite and picotite; <5%), ilmenite (<5%), chromite (<3%) and finely disseminated magnetite (<5%) and sulphides (<5%). These are disseminated as anhedral grains of less than 1 mm. The most commonly observed assemblage is pyrrhotite-pentlandite ± chalcopyrite ± pyrite. Other accesory minerals include phlogopite, apatite and rutile. In the Koroc River area, anorthosite was observed locally in association with ultramafic rocks.
Harzburgite is mainly composed of serpentine (40-85%), with or without relics of olivine, orthopyroxene (7-50%), amphiboles (1-15%), clinochlore (1-12%), chromite (1-3%), magnetite and ilmenite (<1%). Orthopyroxene forms aggregates of subhedral to anhedral millimetric grains with undularoty extinction. Lherzolite is essentially composed of the same mineral phases, but in different proportions, with clinopyroxene, orthopyroxene and amphiboles being more abundant between serpentinized olivine crystals. Pyroxenes also form subhedral and anhedral grains up to locally 15 mm. Olivine relics in peridotite are between 1 mm and 2 cm in diameter and are rimmed by serpentine or orthopyroxene. Chromite grains have a zoned appearance due to the presence of hercynite rim. Deformation and metamorphism result in serpentinization and iddingsitization of olivine to varying degrees and formation of tremolite and clinochlore acicular crystals randomly oriented. In rare cases, epidote develops on amphibole and chlorite. Locally, late fractures in the rock contain carbonates, prehnite and epidote. In some thin sections of carbonate rocks associated with ultramafics, there is an abundance of olivine, pyroxene and spinel crystals associated with carbonates. It could be zones of alteration and metamorphism of listwaenite-type ultramafic rocks.
Olivine websterite contains mostly pyroxenes, in the form of large crystals or fine granoblastic matrix, and 5 to 25% olivine cumulates variably serpentinized (antigorite and iddingsite). There are also small mottled phlogopite and clinochlore, plagioclase (1-5%), actinolite, magnetite and spinel. During deformation and metamorphism, some of these pyroxenes became granoblastic aggregates of pyroxenes and amphiboles in varying proportions, accompanied by some clinochlore. Spinel appears at the triple points within these aggregates. Even large pyroxene crystals turn into amphibole as spots on or around the grain. In zones of intense deformation, the rock becomes a tremolite-chlorite schist with a nematoblastic texture. Locally, pyroxenes are completely transformed into a mixture of hornblende and actinolite.
Amphibolite is fine to coarse grained, even grained and composed of hornblende, tremolite, actinolite, phlogopite, chlorite and locally some interstitial plagioclase. Amphiboles are granoblastic or form large tangled or lepidoblastic crystals. Serpentinized orthopyroxene relics were observed in some thin sections. The main accessory minerals are opaque minerals (<5%), apatite, magnetite, hercynite, ilmenite, chlorite and sulphides.
Intrusions of the Nuvulialuk Suite typically are less than 1 km wide by 1 to 5 km long. The largest intrusions are in the areas of Mount Nuvulialuk (sheet 24I06) and Ijurvik Lake (sheet 24I03), where they are 2 to 3 km wide and nearly 20 km long. The Nuvulialuk Suite is located in the eastern part of the Core Zone and is limited by the Moonbase Shear Zone (MSZ) in the west and the Blumath Deformation Corridor (BDC) in the east.
The age of 1863 Ma, obtained in a decametric amphibolite horizon on outcrop 98-DB-3099, is interpreted as the age of implementation of this lithology. Amphibolite is foliated and shows a lesser degree of deformation than the tonalitic gneiss in which it is interbedded. According to this age, amphibolite was likely tectonically juxtaposed against tonalitic gneiss during the Paleoproterozoic deformation episode. According to Verpaelst et al. (2000), this could thus be a dyke from the Nuvulialuk Suite. The Archean age is interpreted as an inherited age of zircons that underwent remobilization during trans-hudsonian metamorphism.
An age around 1840 Ma, considered to be crystallization, was obtained from a sample taken by the Geological Survey of Canada in the summer of 2014 in a mafic intrusion of the Nuvulialuk Suite (pPnuv1). The sample was taken from the northeast portion of sheet 24H10.
|Isotopic System||Mineral||Crystallization Age (Ma)||(+)||(-)||Inherited Age (Ma)||(+)||(-)||Reference(s)|
|U-Pb||Zircon||1840||14||14||–||–||–||Corrigan et al., 2018|
|U-Pb||Zircon||1863||7||7||2498||12||12||David et al., 2008 (98DB3099)|
Ultramafic intrusive and metamorphosed rocks of the Nuvulialuk Suite are regularly spatially associated with metasedimentary rocks of the Lake Harbour Group. Contacts with paragneiss and quartzite are marked by significant alteration suggesting fluid circulation. Verpaelst et al. (2000) interpret these intrusive rocks as sills and dykes a few kilometres long that have developed within the Lake Harbour Group volcano-sedimentary sequences. They also form imbricate horizons within Archean gneissic complexes.
The work of Pedreira Pérez (2017) suggests that some of the intrusions assigned to the Nuvulialuk Suite, located near the Blumath Deformation Corridor, correspond to an ophiolitic sequence. However, the work of Charette (2016) indicates that metasedimentary rocks from the area began to melt as of 1885 Ma (start of compressive regime and burial). The age of 1840 Ma obtained in the Nuvulialuk Suite therefore seems to exclude the possibility that it is ophiolite but rather differentiated intrusions.
Does not apply.