|Type area:||Nachvak Fjord area, Labrador (NTS sheet 14M04)|
|Geological province:||Nain Province|
|Geological subdivision:||Burwell Lithotectonic Domain|
|Lithology:||Metamorphosed and migmatitized sedimentary rocks|
Table des matières
Wardle (1983) designated the “Tasiuyak gneiss” after a branch of the Nachvak Fjord of the same name, a strip originally mapped by Taylor (1979) in the Saglek Fjord area of Labrador. This unit is initially described as a homogeneous protomylonitic quartzofeldspathic gneiss characterized by white patina and lilac garnet. In the ensuing years, geologists at the Geological Survey of Canada (GSC) conducted a regional mapping campaign covering the entire Torngat Mountains that helped to better identify Tasiuyak Gneiss (Van Kranendonk, 1993, 1994a, 1994b; Van Kranendonk et al., 1994a, 1994b, 1995; Van Kranendonk and Wardle, 1995). Verpaelst et al. (2000) mapped the Tasiuyak Gneiss in the Hebron area (NTS sheet 14L), at the foot of the Iberville Mountain, using Wardle’s terminology. The name “Tasiuyak Complex” was introduced by Mathieu et al. (2018) as a result of the Ministère’s 2017 regional mapping work in the Point Le Droit area (sheet 24P) to comply with the North American Stratigraphic Code.
The Tasiuyak Complex consists of lithologies derived from sedimentary protoliths. The high metamorphic grade, partial melting and intensity of deformation complicate the distinction between paragneiss, metatexites and other diatexites. As early as 1983, Wardle was reluctant to quantify the degree of partial melting and to identify the exact composition of the protolith. However, he proposed as a protolith an immature sedimentary rock composed of horizons of pelite and quartz arenite. Only the late facies of the complex is clearly identified as ultrametamorphic granite. The Tasiuyak Complex is predominantly composed of a paragneiss, metatexite and diatexite unit characterized by a very light beige patina, lilac garnet and a small proportion of hydrated phases such as biotite. The other metasedimentary units represent minor components that are commonly interstratified at the outcrop scale. The Tasiuyak Complex is subdivided into: 1) a unit of garnet-sillimanite-rutile paragneiss, metatexite and diatexite associated with minor amounts of biotite (pPtas1); 2) a quartzite unit (pPtas2); 3) a marble and calcosilicate rock unit (pPtas3); and 4) a white ultrametamorphic granite unit (pPtas4).
Rocks of the Tasiuyak Complex are characterized by a low to very low magnetic susceptibility, which makes them easily recognizable on aeromagnetic maps (GSC, 1985). Similarly, the distinctive light beige patina and rusty horizons allow the unit to be followed on satellite images. The mineral assemblages described in the Tasiuyak Complex samples are predominantly anhydrous (garnet-sillimanite-biotite-oligoclase-perthite-rutile-graphite). They are typical of the granulite facies. In addition, some horizons contain orthopyroxene. Tettelaar and Indares (2007) studied in more detail the Paleoproterozoic regional metamorphism to the granulite facies of a metasedimentary strip located in the southern part of the complex, west of the Nain community. This episode led to partial melting of metapelites. These authors propose a continuous biotite + sillimanite + plagioclase + quartz = garnet + K-feldspar + liquid reaction. The metamorphic peak conditions were about 8 to 10 kbars for a temperature of 870°C in this area.
The dominant unit pPtas1 consists of a rock with very light beige patina, commonly yellowish, and a bright white fresh surface. It shows compositional and granulometric banding of centimetric to decimetric scale. Bands are heterogeneous, very fine grained (recrystallization) to medium grained. The major mineral phases are milky quartz (20-75%), K-feldspar (1-30%), usually microcline but also orthose and perthites, and white plagioclase (5-60%). The plagioclase composition was optically determined by Ermanovics and Van Kranendonk (1998) at An25-37 (oligoclase). Mafic or aluminous minerals (5-40%) are represented by garnet (10-25%), sillimanite and red biotite (<5%). Accessory phases are graphite, sulphide, rutile, zircon and apatite.
Garnet has a typical lilac colour. It occurs as rounded poikiloblastic crystals (up to 5 cm in diameter) with rotational textures, which suggests syntectonic crystallization. A second generation of subhedral crystals has inclusions and is therefore interpreted as late-tectonic. These two habits commonly observed in the same outcrop imply that the growth of garnet continued after the deformation episode. Garnet is rich in quartz and sillimanite inclusions. Its composition is almandine-pyrope with a Mg proportion decreasing from the core to the grain edge (Ermanovics and Van Kranendonk, 1998).
Sillimanite is nematoblastic. Rods are oriented parallel to foliation and follow the contour of garnets, which suggests pre-kinematic growth, but have a random orientation in the foliation plane. Rutile occurs as small, elongated prisms, but also as sphene pseudomorph, from which it preserves the wedge-shaped habit. Mafic minerals are concentrated in thinner bands that could be considered relics of more refractory material. They are found in smaller quantities in coarser quartzofeldspathic bands interpreted as mobilisate.
Nebulatic, stromatic and, more scarcely, mesh migmatitic textures are reported, but the intensity of the deformation makes it difficult to recognize these different facies. Some authors (e.g., Van Kranendonk et al., 1994a) interpret the compositional banding as remnants of primary bedding transposed to an immature sediment. They report the presence of millimetric to metric bands of paragneiss derived from quartz arenite, semi-pelite and petite. The chemical composition of these rocks reaches 26% Al2O3 (Ermanovics and Van Kranendonk, 1998).
Most of the Tasiuyak Complex is affected by the Abloviak Shear Zone (ASZ) and has protomylonitic texture. Quartz with undulatory extinction forms rods or lenses stretched due to dynamic recrystallization. K-feldspar and plagioclase have undulatory extinction and curved twins. Textures associated with deformation such as mortar structure and grain edge migration are observed everywhere in thin section. In the western area, which is the most deformed, poikiloblastic garnets are associated with recrystallization tails.
Bands of mafic to ultramafic rocks with garnet and spinel have been reported by several authors (Verpaelst et al., 2000; Ermanovics and Van Kranendonk, 1998). These bands are not mapped to the work scale as they are less than 10 m wide. Their mineralogical composition includes garnet, orthopyroxene, clinopyroxene, hornblende, plagioclase, hercynite, oxides and quartz inclusions in garnet. These rocks are granoblastic; garnet and pyroxene are poikiloblastic. Ermanovics and Van Kranendonk (1998) also report the presence of low silica paragneiss with sapphirine and orthopyroxene in the south of the Tasiuyak Complex.
The main pPtas1 unit incorporates four subunits: 1) an orthopyroxene-garnet-rutile ± sillimanite ± biotite paragneiss, metatexite and diatexite subunit (pPtas1a); 2) a rusty paragneiss subunit (pPtas1b); 3) a subunit of biotite-garnet paragneiss, metatexite and diatxite poor in sillimanite (pPtas1c); and 4) a cordierite-garnet-rutile ± sillimanite ± biotite paragneiss, metatexite and diatxite subunit (pPtas1d).
Tasiuyak Complex 1a (pPtas1a): Orthopyroxene-Garnet-Rutile ± Sillimanite ± Biotite Paragneiss, Metatexite and Diatexite
Subunit pPtas1a shows the same characteristics, both photographic and petrological, as the main unit, except for the presence of orthopyroxene visible in thin section in the paleosome or as a peritectic phase in the mobilisate. Hypersthene is subhedral, corroded and regularly shows partial replacement by iddingsite. Locally, the subunit has a brown sugar colour characteristic of orthopyroxene rocks, but this is not always the case. This subunit is weakly magnetic.
Tasiuyak Complex 1b (pPtas1b): Rusty Paragneiss
Rusty paragneiss with graphite and disseminated sulphides (pPtas1b) was first described by Van Kranendonk (1994a, b). This subunit has the same petrological and petrographic characteristics as the main pPtas1 unit. The main differences are finer grain size, with quartz, plagioclase and K-feldspar grains smaller than one millimetre, and garnet rarity (<2%). Graphite occurs as millimetric flakes that commonly form clusters aligned in foliation in association with red biotite flakes. Analyses indicate an δ13C isotopic ratio of -34.37‰, which suggests a biogenic origin for carbon (Bodycomb, 1994). Bodycomb proposes that these horizons come from metamorphism of black shales. Pyrrhotite is fine, subhedral and disseminated. Less sphalerite, pyrite, magnetite and gahnite are also reported by this author. The proportions of graphite and sulphides vary from a few percent to 40% (semi-massive mineralization). The rusty strips are easily located due to the colour contrast with the paragneiss characterized by a dominant beige patina. These units are variable in size and form lenses or metric boudins up to kilometric strips a few metres to a few tens of metres wide. pPtas1b horizons are usually boudinaged and include 2 cm to 2 m thick quartzite bands. This subunit is slightly magnetic.
Tasiuyak Complex 1c (pPtas1c): Biotite-Garnet Paragneiss, Metatexite and Diatexite Poor in Sillimanite
Subunit pPtas1c is characterized by the absence of sillimanite and relative abundance of red biotite (Van Kranendok, 1994b). Its patina is beige grey, darker than the main unit, and it outcrops mostly on the eastern and northern edges of the complex, close to contact with the Killinek Suite. It has the same characteristics as the dominant unit, but incorporates homogeneous metric horizons rich in biotite and fine-grained garnet.
Tasiuyak Complex 1d (pPtas1d): Cordierite-Garnet-Rutile ± Sillimanite ± Biotite Paragneiss, Metatexite and Diatexite
This unit outcrops occasionally. It shows all the characteristics of the main unit, the difference being the presence of retrograde cordierite. Cordierite is the result of pseudomorphic replacement of sillimanite. Locally, it forms perfectly crystallized crystals. It was also observed as inclusion in garnet. The presence of cordierite is accompanied by a second generation of garnet that forms symplectites with quartz. Cordierite is partially replaced by pinite.
The unit pPtas2 quartzite is described by Van Kranendonk (1994b) as a fine-grained grey rock with graphite and garnet. Metric horizons of siliceous paragneiss and quartzite were also mapped in 2017 in the Point Le Droit area (Mathieu et al., 2018). They are interbedded with the main pPtas1 unit and are rarely mapped to the work scale. The biggest horizons are 400 m thick. The patina is very light brown to grey and the fresh surface is whitish. Rocks are characterized by medium grain size and centimetric compositional banding defined by variations in the proportion of quartz (70-90%). Late lilac garnet (<10%) is fine grained (1-2 mm) and evenly distributed in the rock. It locally forms porphyroblasts up to 5 mm in diameter. Quartz forms rods or stretched lenses with undulatory and subgrain extinction. Quarzite also contains plagioclase and K-feldspars finely recrystallized, graphite flakes and rutile prisms.
Van Kranendonk et al. (1993) report the existence of thin horizons of marble and calcosilicate rocks (<15 m thick) interstratified with metasedimentary rocks of the main pPtas1 unit. These authors describe impure diopside-olivine-calcite marbles interbedded with light-green diopside calcosilicate horizons 1 to 10 m thick, cut locally by many conformable or unconformable white quartz veins. White marble in fresh brittle has a buff-coloured patina. It is coarse grained, banded and contains more than 40% calcosilicate bands in positive relief. Calcosilicate horizons in the Point Le Droit area (Mathieu et al., 2018) are finely to moderately recrystallized, very light brown and light green in fresh exposure with a grey-green to brown patina. They consist of a heterogeneous assemblage of diopside, orthopyroxene, plagioclase replaced in places by scapolite, quartz and phlogopite (up to 10%). Accessory phases are actinolite, sphene, olivine and interstitial carbonates.
This unit was introduced to designate the most evolved lithological variety in terms of partial melting. It consists mainly of K-feldspar granite and syenogranite, which form discontinuous and diffuse metric clusters in metatexite outcrops (pPtas1), but also larger masses up to 1 to 3 km in diameter. The patina is very light beige. Granite is generally homogeneous with little textural variation such as banding observed locally. It also contains enclaves and horizons of paragneiss and metatexite in sharp contact. On the other hand, it shows diffuse contact with diatexite. Granite is characterized by coarse-grained perthitic microcline. Fine to medium-grained quartz is interstitial and forms anhedral crystal clusters with undulatory extinction. Myrmekites develop along the edges of plagioclase crystals (10%). Mafic and aluminous minerals do not exceed 10% of the mode. They are represented by garnet, biotite and sillimanite. Accessory phases are graphite, epidote, zircon and opaque minerals.
Tasiuyak Complex 4a (pPtas4a): White Garnet-Cordierite Granite
This subunit shows a slightly yellow alteration patina and a bluish light grey fresh surface. Blue cordierite crystals are clearly visible in the samples. In thin section, cordierite is partially replaced by pinite.
Tasiuyak Complex 4b (pPtas4b): White Garnet Tonalite
Subunit pPtas4b consists of a coarse-grained, homogeneous, white tonalite poor in mafic minerals (<5%). It is impossible in the field to differentiate it from the granite facies. Mafic minerals are garnet, biotite and cordierite. Plagioclase is weakly sericitized and shows twins slightly bent by deformation.
The Tasiuyak Complex features a 400 km-long, NNW-oriented flared strip that stretches from western Saglek Fjord in Labrador, curves to the NW in the Jacques Rousseau Mountain area to reach the Abloviak Fjord in Quebec. The complex also outcrops on Baffin Island. Its width exceeds 60 km in its southern part and is reduced to 15 km in its northern part. Klippes a few decametres to a few kilometres long by a few metres wide are incorporated into gneiss of the Noodleook Complex (ApPnoo) and intrusions of the Killinek Suite (pPkil).
Bertrand et al. (1993) dated zircons from a sample described as a garnet-biotite ultrametamorphic granite syntectonic to the Tasiuyak Complex (pPtas4). The age obtained around 1844 Ma also corresponds to the age of formation of the Abloviak Shear Zone. The sample also provided inherited ages between 1886 and 2087 Ma.
Scott and Gauthier (1996) obtained ages ranging from 1940 ±2 to 1895 ±2 Ma for detrital zircons from a Tasiuyak metasedimentary rock sample sampled in northern Labrador. The age of the youngest zircon obtained for this sample (1895 Ma) is the maximum age of sediment deposition. A sample of orthopyroxene quartz diorite from the Killinek Suite (pPkil4) cutting paragneiss was dated 1895 Ma by Scott (1995). The ages of detrital zircons below 2.1 Ga imply that the Tasiuyak cannot be the result of erosion of Archean rocks of the Nain Province, since they are too old, nor those of the younger Churchill Province.
|Isotopic System||Mineral||Crystallization Age (Ma)||(+)||(-)||Reference(s)|
|U-Pb||Zircon||1843.9||3.6||3.6||Bertrand et al., 1993|
Metasedimentary rocks of the Tasiuyak Complex show transposed contact with intrusions of the Killinek Suite (pPkil) and gneiss of the Noodleook Complex (ApPnoo). In this very heavily deformed area, it is not possible to define the nature of these contacts. Nevertheless, a crosscutting relationship of paragneiss (pPtas1) by Killinek orthopyroxene quartz diorite (pPkil4) is in places observed (Scott, 1995). Very locally, white garnet granite (pPtas4) intrudes into Killinek orthopyroxene quartz diorite (pPkil2), suggesting that partial melting occurred after the Killinek Suite was emplaced.
Does not apply.