|Author:||Charette and Beaudette, 2018|
|Geological province:||Churchill Province|
|Geological subdivision:||Ungava Orogen / Kovik Antiform|
|Lithology:||Foliated to gneissic plutonic rock assemblage|
Table des matières
Rocks of the Kovik Complex were first mapped by Taylor (1982) who assigned the Aphebian age to all plutonic rocks lying north of the Sud Domain (also known as the Cape Smith Belt or Ungava Trough). Doig (1983, 1987) has shown through Rb/Sr datings ranging from 2934 to 2569 Ma, that rocks of the Déception Bay, which he calls « Déception Gneiss », are Archean. In most of the Ministère’s work, the term « Déception Group » has been used to describe the rocks forming the internal northern bedrock (Lamothe et al., 1984; Tremblay, 1991; Barrette, 1990a, 1990b). This term was abandoned by Lamothe (2007) who introduced the name “Kovik Antiform”, as suggested by Hocq (unpublished), as a lithodemic unit. However, this name does not comply with the rules of the North American Stratigraphic Code (1983, 2005). As a result, Charette and Beaudette (2018) preferred to transform the Kovik Antiform into a lithotectonic domain and introduced the “Kovik Complex” to describe geological units of this area.
The Kovik Complex stratigraphic subdivision is based on Lamothe (2007). Charette and Beaudette (2018) extended the Kovik Complex from 5 to 20 km north in the western part of the Kovik Antiform. These authors also introduced units of gabbro and banded diorite, with garnet locally (ApPkvk2a), metatexite and diatexite derived from partial melting of metasedimentary rocks (ApPkvk3a and 3b, respectively) and migmatitized quartz diorite (ApPkvk6a).
The Kovik Antiform is interpreted as a window of Archean rocks, equivalent to those of the Superior craton, within the Ungava Orogen. Although St-Onge and Lucas (1990a) highlight the complete and continuous transition of Archean rocks between the Superior Province and the Kovik Antiform, it was considered preferable to retain the lithological divisions proposed by St-Onge et al. (2006) and Lamothe (2007) and not extend the subdivisions proposed by Simard (2008) for the southern bedrock (Superior Province) to the north bedrock (Kovik Antiform). Also, Baragar (2015) proposes an alternative lithological division for the northern portion of the west Kovik Antiform. However, it was not included in this description of the Kovik Complex.
The Kovik Complex consists of an Archean rock assemblage of the parautochtonous bedrock underlying the Ungava Orogen allochtonous units. This assemblage is composed principally of biotite ± hornblende ± epidote ± titanite ± allanite tonalite or granodiorite (ApPkvk4, ApPkvk5 and ApPkvk6) with varying proportions of mafic and metasedimentary rock enclaves (Taylor, 1982; St-Onge and Lucas, 1990, 1992). Scarce klippes of siliciclastic metasedimentary rock (ApPkvk3) and extrusive or intrusive mafic rock (ApPkvk1) are preserved within or on the edges of tonalitic intrusions. Late metric to kilometric intrusions, varying from tonalitic to syenogranitic (ApPkvk7 to ApPkvk10), intrude into tonalite and granodiorite (St-Onge and Lucas, 1992).
Little information is available for unit ApPkvk1 that was only described by Moorhead (1996) and Charette and Beaudette (2018). This fine-grained rock is black in altered surface and green in fresh exposure. The appearance of amphibolite varies from massive to banded. When present, a compositional variation is marked by milimetric to centimetric melanocratic bands containing clinopyroxene and hornblende as well as decimetric to metric massive ultramafic horizons. In places, up to 7% discontinuous millimetric bands of leucosome are parallel to foliation. In hornblendite, hornblende forms up to 90% of the rock and is accompanied by diminishing proportions of quartz, garnet, chlorite, sphene and ilmenite, which is sometimes coarser-grained. Hornblende is very fresh and shows only partial chloritization and biotitization when in contact with garnet.
Mafic and ultramafic rocks of unit ApPkvk2 form klippes and masses of kilometre dimensions between the vast tonalite massifs and granitic plutons, as well as enclaves digested by these clusters. At the outcrop scale, these rocks are associated with tonalite in two ways: (1) interstratification of diorite and tonalite, apparently comagmatic; and 2) angular, low-elongation enclaves of diorite, amphibolite, pyroxenite and peridotite in tonalite (agmatitic texture).
Scarce late diorite plutons cuts the tonalite plutons and form bodies hundreds of metres to several kilometres in diameter, even-grained and medium-grained. A small mafic intrusive rock composite complex intruding into tonalite was mapped in more detail on the northeast side of the Deception Bay. This complex mainly consists of hornblende-biotite quartz diorite, which changes to hornblende-biotite tonalite towards the interior of the pluton. Quartz diorite contains inclusions of pyroxenite, diorite and tonalite (Lucas and St-Onge, 1997). There is a phase of mafic diorite associated with quartz diorite in which large ovoid plagioclase phenocrystals up to 3 cm in length dominate. Plagioclase phenocrystals have a hornblende core, and garnet inclusions are present locally. Clinopyroxene-hornblende-biotite pyroxenite forms a distinct second intrusive phase associated with quartz diorite and surrounding it. All units of the intrusive complex are cut by monzogranite veins. Foliated borders and a massive core with well-defined shear zones characterize the complex (Lucas and St-Onge, 1997).
Kovik Complex 2a (ApPkvk2a): Gabbro and Diorite with Banded Gabbro Horizons
Subunit ApPkvk2a was introduced to distinguish mafic rocks that are commonly banded, but also occur as foliated to massive horizons. Banded sequences are characterized by a compositional variation from diorite to gabbro. In several places, gabbro includes horizons rich in garnet porphyroblasts (0.5 to 1 cm) rimmed by plagioclase. A melanogabbro or pyroxenite is in transitional contact with gabbro or forms centimetric to metric boudinaged horizons. Banding is accentuated by leucosomes (5-15%) in bands or in discontinuous millimetric to centimetric clusters. These include garnet and clinopyroxene in places. A gneissic texture is observed locally. In places, metric horizons of metasedimentary rock are interbedded in the banded mafic sequence. This banded facies is granoblastic and fine-grained, while the foliated to massive facies is medium-grained. The foliated to massive facies occurs as horizons in banded rocks of subunit ApPkvk2a. When massive, diorite and gabbro are homogeneous and locally show a cumulate or speckled texture. Cluster leucosomes make up less than 10% of foliated gabbro. The main minerals of subunit ApPkvk2a rocks are hornblende and clinopyroxene, and the main accessory minerals are sphene, calcite, apatite, actinolite and epidote. In thin section, hornblende replaces pyroxene, which is usually observed as a relic in the core of hornblende crystals.
Clastic fine-grained metasedimentary rocks have been preserved as large klippes in the tonalite and monzogranite plutons. They extend mainly to the east of Deception Bay. The dominant composition is semi-pelite. It has a rusty patina and is, in places, interstratified with pelitic and amphibolitic bands less than one metre thick. A marked stratification is characteristic of semi-pelite and pelite. These lithologies include local horizons of conglomerate, marble and amphibolite. In the Wakeham Bay area, tonalite and monzogranite massifs contain small lenses (0.5 m thick) of limestone marble (Lucas and St-Onge, 1997).
In the tectonic window of the western Archean basement, metasedimentary rocks are smaller in size within the tonalite of unit ApPkvk4 and form only locally 1:100,000 mappable strips of significant length. In this area, the metasedimentary sequence mostly consists of pelite and semi-pelite, and includes scarce quartzite strata, indicating relatively deep water sedimentation. Granitoid veins from adjacent plutonic massifs or derived from local partial melting cut the sequence (Lucas and St-Onge, 1997). Charette and Beaudette (2018) recognized significant partial melting within metasedimentary klippes and divided unit ApPkvk3 into two subunits according to the degree of migmatitization: metatexite (ApPkvk3a) and diatexite (ApPkvk3b).
Kovik Complex 3a (ApPkvk3a): Migmatitized Paragneiss and Metatexite
Migmatized paragneiss of subunit ApPkvk3a has a grey beige to rusty patina and characteristic migmatitic banding marked by biotite schlierens and mobilisates in varying proportions (10-25% in general). Migmatitic banding produces a stromatic structure in outcrop. Mobilisates are millimetric to centimetric, white, parallel to foliation, medium-grained and locally folded. Their composition varies from granodioritic to granitic. They have biotite rims and include 2 to 10% pink or lilac garnet. Klippes of subunit ApPkvk3a have deformation that varies in intensity. Generally, paragneiss are crenulated and, in places, show a mylonitic texture. Mylonitization produces discontinuous boudinaged leucosomes and plagioclase or K-feldspar porphyroclasts. The subunit shows compositional variations marked, in outcrop, by a change in the content of quartz and mafic minerals (15-35%). Also, petrographic studies indicate a 0 to 30% change in the proportion of K-feldspar. Migmatitized paragneiss contains better preserved paragneiss horizons, decimetric diatexite horizons and, in places, usually boudinaged centimetric to metric gabbroic horizons. In terms of mineralogy, migmatitized paragneiss of subunit ApPkvk3a is composed of red biotite, garnet, sillimanite, muscovite and graphite. Garnet is locally rimmed with plagioclase.
Kovik Complex 3b (ApPkvk3b): Diatexite
Diatexite of subunit ApPkvk3b is heterogeneous and contains locally better preserved horizons of migmatitized paragneiss and paragneiss. The heterogeneity of this medium to coarse-grained whitish rock is produced by the distribution of mafic minerals in many schlierens and millimetric clusters, as well as by the presence of bands, zones or injections of white leucocatic material akin to ultrametamorphic granite. Diatexite is, in places, affected by folding or crenulation. Garnet (5 mm-1 cm) and biotite are concentrated in discontinuous clusters and in numerous schlierens.
Unit ApPkvk4 is the dominant plutonic rock unit of the Kovik Complex. Its foliated to gneissic texture and altered grey surface are characteristic. Within this tonalite, banding is marked by a compositional variation ranging from tonalite to quartz diorite and is accentuated by 5 to 20% millimetric to centimetric, whitish or pink leucosomes. These are granitic to tonalitic, medium to coarse-grained and sometimes folded. A small proportion (1-5%) of decimetric diorite horizons and enclaves are in sharp or transitional contact in tonalite. More heavily migmatitized tonalite horizons, with up to 40% leucosome bands, are present locally. Petrographic studies show that the distribution of K-feldspar is generally heterogeneous and that it is concentrated in diffuse bands in the rock. Mafic minerals are disseminated or as millimetric laminas that make up 7 to 25% of the mineralogy. Biotite is the main mafic mineral, followed by hornblende. Accessory minerals observed in thin section include apatite, zircon, monazite and, locally, sphene, epidote and magnetite.
Kovik Complex 4a (ApPkvk4a): Banded Heterogeneous Migmatitized Tonalite and Biotite Schlieren Tonalite
Subunit ApPkvk4a represents areas where ApPkvk4 tonalite is strongly migmatitized. When the contact is observed, the migmatitic facies is transitional with ApPkvk4 tonalite. Subunit ApPkvk4a is characterized by diffuse banding formed by millimetric to centimetric coarse-grained leucosomes conformable with foliation of the tonalite. Other leucosomes cut foliation. These are usually in sharp contact with the migmatitized tonalite and could represent migration zones. Subunit ApPkvk4a is medium to coarse-grained, contains 10-15% biotite schlierens or discontiuous laminas and 5-10% centimetric diorite enclaves. In some places, finer-grained, less-migmatitized tonalite klippes could represent paleosome rafts. Biotite is the principal mafic mineral.
Kovik Complex 4b (ApPkvk4b): Biotite Schlieren Heterogeneous Migmatite
Migmatite of subunit ApPkvk4b is derived from partial melting of ApPkvk4 tonalite and is recognized by its heterogeneous appearance. In some places, it looks like a heterogranular speckled intrusive rock with biotite clusters and in other places it has chaotic banding. A high proportion of whitish leucocratic ultrametamorphic granite is commonly observed in outcrop. This subunit contains centimetric bands or partially digested metric enclaves of dioritic composition that may resemble restites. Subunit ApPkvk4b contains a high proportion of millimetric to centimetric leucosomes (20-40%) that form ptygmatic folds. In banded areas, mobilisates are generally conformable with foliation, although in some places they cut it, which makes it anastomosed. At the edge of these mobilisates, mafic minerals are coarser-grained. Mafic minerals account for between 5 and 15% of the mineralogy of migmatite and their distribution is heterogeneous, even though they are more concentrated in schlierens. This subunit shows local chloritization and epidotization. In thin section, epidote is coarse-grained and accounts for 5 to 10% of mineralogy. Chlorite, on the other hand, forms small crystals associated with epidote.
Unit ApPkvk5 was described by St-Onge and Lucas (1992) and St-Onge et al. (1992) to distinguish areas where tonalite and granodiorite of the Kovik Complex contain few mafic rock enclaves, unlike unit ApPkvk4 which contains a lot. This informal unit also has distinct mineralogy that includes hornblende as the main mafic mineral, unlike unit ApPkvk4 where biotite is the main mafic mineral.
Unit ApPkvk6 is poorer in K-feldspar than units ApPkvk4 and ApPkvk5. Tonalite banding is marked by quartz diorite bands or strata. Like unit ApPkvk5, the main mafic mineral is hornblende, followed by biotite.
Kovik Complex 6a (ApPkvk6a): Quartz Diorite and Migmatitized Tonalite
Subunit ApPkvk6a consists of lithologies varying from quartz diorite to tonalite. They are homogeneous, well-foliated and show small, recrystallized plagioclase phenocrystals. This subunit is commonly observed in transitional contact with ApPkvk4 tonalite. Within subunit ApPkvk6a, millimetric to centimetric discontinuous leucosome bands account for 5 to 10% of the lithology. In places, quartz diorite horizons have a gneissic or banded appearance where the proportion of leucocratic bands reaches 20 to 45% of the lithology. Local centimetric to decimetric gabbro horizons are also observed. These mafic horizons are boudinaged in places and generally account for less than 20% of the outcrop. In subunit ApPkvk6a, the proportion of quartz varies between 0 and 20% and plagioclase form small polycristalline clusters (recrystallized phenocrystals) of 0.5 to 1 cm. Mafic minerals account for 17 to 35% of lithology and are distributed homogeneously or in small clusters. The main mafic mineral is hornblende, followed by biotite. Both are locally replaced by chlorite. The main accessory minerals include chlorite, apatite, zircon, sphene and epidote.
Southwest of Foul Bay, two hornblende-biotite tonalitic to granodioritic plutons intruded into the older tonalite. This younger tonalite is fine-grained, foliated and even-grained. Both plutons contain numerous amphibolite and quartz diorite enclaves.
Granite plutons cut tonalite and are generally characterized by mineral assemblages of the granulite facies and by tectonic foliation of varying intensity. Some plutons show a massive texture as a whole, others in their cores alone. These granitic plutons typically contain centimetric to kilometric xenotliths of tonalite, quartz diorite, amphibolite and pyroxenite (St-Onge et al., 1992). Granite plutons, whether foliated or massive, are tabular and oriented E-W. They appear to have developed parallel to the E-Wtectonic foliation in tonalite. Biotite is the principal mafic mineral. Muscovite, hornblende and epidote are also observed in thin section and make up less than 1% of the rock.
Of the granitic rocks cutting ancient tonalite, monzogranite is most abundant. Pluton composition, however, can vary from granodiorite on the edge to syenogranite near the centre. Corollarily, grain size varies from fine to coarse. In addition, plutons are homogeneous, white to pinkish, foliated or massive and usually contain centimetric to kilometric enclaves of tonalite, quartz diorite, amphibolite and pyroxenite. However, petrographic studies indicate an heterogeneity in diffuse phases that is produced by the variation in K-feldspar content, which can range from 10% to 75%. Unit ApPkvk9 also includes a monzonite, sometimes gneissic, which contains centimetric K-feldspar phenocrystals. The rock is composed of 10-30% orthoclase, 25-40% plagioclase, 10-25% quartz and 5-20% mafic minerals, mainly biotite. In places, hornblende is also present, but in smaller proportions. Magnetite, muscovite and epidote are locally observed. In some places, apatite forms millimetric crystals that can account for up to 3% of the rock.
Syenogranite of unit ApPkvk10 contains mainly biotite as a mafic mineral. However, hornblende is locally observed. It may have a pegmatitic texture. Little information is available for this unit.
The Kovik Complex is located in the central portion of the Ungava Orogen and extends east to the New Quebec Orogen. The Kovik Complex is currently the only lithodemic unit in the Kovik Antiform domain. This lithotectonic domain is bordered to the south by the Sud Domain. An axial trough zone in the centre of the antiform is occupied by rocks of the Narsajuaq Arc, interpreted as an allochtonous calc-alkaline magmatic terrane overthrusting on the parautochtonous bedrock and on rocks of the Nord Domain (Lucas and St-Onge, 1992), separating the northern basement into two blocks (east and west antiform). In the west Kovik Antiform, the Sugluk Suture marks the boundary between the northern part of the domain and the Narsajuaq Arc.
Unit ApPkvk4 forms most of the Kovik Complex. Late tonalite, granodiorite and monzogranite of units ApPkvk7, ApPkvk8 and AKpPkvk9 intrude as small plutons into ApPkvk4 tonalite and granodiorite. ApPkvk9 monzogranite is commonly found as veins and intrusions in tonalite (ApPkvk4) and quartz diorite (ApPkvk6a) and can account for up to 40% of outcrops. The only syenogranite and pegmatite mapped pluton of unit ApPkvk10 is located in the eastern portion of the Kovik Antiform.
Two U-Pb datings on zircons in biotite ± hornblende tonalite (ApPkvk5) indicated protolith ages of 2882 +44/-28 Ma (Parrish, 1989) and 2737 ±2 Ma (Scott and St-Onge, 1995). A sheared tonalite klippe incorporated into the basal decollement between the Kovik Antiform and sedimentary rocks of the Povungnituk Group returned a U-Pb age on zircons of 2780 ±4 Ma (Parrish, 1989). A small composite complex of mafic intrusive rocks (ApPkvk2) intruding tonalite was mapped on the northeast side of Deception Bay and dated by the U-Pb zircon method at 2740 ±10 Ma (Parrish, personal communication, 1992). Four Rb-Sr ages of total rocks were obtained on paragneiss and orthogneiss of the Kovik Complex. These indicate crystallization ages between 2569 ±83 Ma and 2934 ±77 Ma (Doig, 1983).
The Kovik Complex is currently the only stratigraphic unit in the Kovik Antiform. This lithotectonic domain represents a window on the parautochtonous Archean basement through allochtonous rocks of the Ungava Orogen. This window is interpreted as the result of rock uplift during post-accretion crustal deformations D3 and D4. Tremblay (1991) and Lamothe (1994) indicate that the contact between the Archean basement which forms the antiform and allochtonous rocks of the orogen is generally marked by a decollement fault, except in very rare places where basal conglomerate metric lenses are preserved. Charette and Beaudette (2018) propose that contact between the Kovik Complex and the Narsajuaq Arc units correspond to an important tectonic structure, the Sugluk Suture.
Units ApPkvk2 to ApPkvk 6 consist of ancient plutons forming the Archean basement from which granitic veins were injected. Migmatites of subunits ApPkvk4a and ApPkvk4b are in transitional contact with ApPkvk4 tonalite and are interpreted as derived from melting of this tonalite. Migmatites occur as metric horizons in tonalite and form elongated kilometric masses that are oriented along the east-west structural grain. Different lithologies are found as enclaves in the migmatites, namely ApPkvk2a gabbro and diorite, ApPkvk3 migmatitized paragneiss and ApPkvk6a quartz diorite. ApPkvk6a quartz diorites are commonly observed in transitional contact with ApPkvk4 tonalite. Because of its intermediate composition, quartz diorite present in the western portion of the antiform was assigned to unit ApPkvk6, which had been defined by St-Onge et al. (1992), but appear to be comagmatic with ApPkvk4 tonalite. Metasedimentary klippes of unit ApPkvk3 are commonly heavily deformed and located near a deformation zone. ApPkvk1 amphibolite is observed near or in horizons within paragneiss.
In all older cartographic units, granitic veins show varying degrees of deformation; these veins appear to be related to the episode of voluminous granitic plutonism, contemporary to deformation (Lucas and St-Onge, 1995). This hypothesis is corroborated by the relative absence of granitic veins in granitic massifs and the overall variable deformation states of both veins and granitic plutons. Granitic rocks that cut ancient ApPkvk4 tonalite group rocks vary from granodiorite (ApPkvk8) to syenogranite (ApPkvk10), although monzogranite (ApPkvk9) is arguably the most abundant type. Based on pluton geometry and deformation, it appears that the oldest units are granodiorite and monzogranite and that the most recent pluton is syenogranite (ApPkvk10).
Does not apply.