Southeastern Churchill Province, Nunavik, Quebec, Canada: Geological Synthesis

Since 2009, the Ministère‘s work, in collaboration with its partners, the Geological Survey of Canada (GSC), Department of Mines and Energy of Newfoundland and Labrador and several universities, has identified six distinct lithotectonic domains in the Southeastern Churchill Province. Five of these domains are or will each be the subject of a Geological Bulletin and stratigraphic record. These include the Rachel-Laporte, Baleine, George (Charette et al., 2018), Mistinibi-Raude (Charette et al., 2019) and Falcoz domains. The sixth domain, the Labrador Trough, has already been synthesized (Clark and Wares, 2004). A seventh domain in the Nain Province, the Burwell, was also described during this work.

Geological Bulletins for each lithotectonic domain present different sections on lithostratigraphy (including a stratigraphic diagram), lithogeochemistry, metamorphism, structure (including a structural cross-section) and regional tectonics of the relevant domain. The main prospective environments for exploration and types of mineralization are also described. All SECP geological units now have a stratigraphic record, and large deformation zones are presented in the Structural Lexicon of Quebec.

Methodology

Previous Work

Several geological surveys have been conducted in the SECP at different scales by the Ministère. The main mapping campaigns took place in the northern and southern parts since access is easier. This work took place primarily in the 1950s and 1980s and include several master’s and doctoral theses. During the 1960s and 1970s, the Geological Survey of Canada also conducted several regional geological mapping projects, including 1:250 000-scale mapping that covered the entire SECP in Quebec and Labrador (Taylor, 1979). The complete list of this work is presented in the “Previous Work” sections of the Geological Bulletins of the different lithotectonic domains involved in this work (Rachel-Laporte, Baleine, George, Mistinibi-Raude and Falcoz).

During the 1980s, the concerted efforts of several geologists working for various geological surveys and departments across Canada resulted in the development of tectonic models for the Trans-Hudsonian Orogen (Hoffman, 1988, 1990b). This work has led to numerous papers in a special volume published by the Geological Association of Canada (1990, Special Paper 37), one of which deals specifically with the SECP. During the same period, another series of papers on the SECP was published in a special issue of Geoscience Canada (1990, Volume 17, Number 4). In the early 1990s, the completion of a deep seismic survey in Ungava Bay and the Labrador Sea (Lithoprobe Program, ECSOOT transect) coupled with work on isotopic geochemistry, geochronology and structural geology resulted in a better general interpretation of the SECP. Among other things, this work helped link the evolution of the Canadian Shield with that of Greenland. The latest interpretations of this work have been published as several papers in a full issue of the Canadian Journal of Earth Sciences (2002a, Volume 39, Number 5).

Geological Context

The Southeastern Churchill Province (SECP) is interpreted as a branch of the Trans-Hudsonian Orogen, a large Paleoproterozoic orogenic belt that extends from North Dakota to Greenland. This large orogenic system is part of a long period of global convergence that led to the formation of the Laurentia Craton (1.97-1.82 Ga; Hoffman, 1990a) and later to the Columbia Supercontinent (Nuna).

The Trans-Hudsonian Orogen was named by Hoffman (1981) to define the Paleoproterozoic collision between the Superior and Archean domains of the NW part of the Churchill. The type area is located in northern Manitoba and Saskatchewan and includes both external and internal zones, represented respectively by Archean continental margins and a Proterozoic juvenile crust. A broader definition has been adopted by Lewry et al. (1985) and Lewry and Collerson (1990), which includes not only Paleoproterozoic metamorphic rocks associated with this collision, but also the large metamorphic and structural imprint area affecting Archean to Paleoproterozoic Churchill rocks. This more inclusive definition has been retained in the literature. Corrigan et al. (2009) notes that it provides a better understanding of the orogenic system and the near-continuous tectonothermal imprint formed during the collision between the Superior Craton, the Reindeer Zone and a continental amalgam comprising the Slave, Rae and Hearne cratons. The latter two, limited by the Snowbird Tectonic Zone, represent the western portion of the Churchill Province.

Rocks affected by the Trans-Hudsonian Orogen are divided into three main branches (Hoffman, 1988): 1) the SW branch, west of Hudson Bay; 2) the NE branch, in the central section, north of Hudson Strait; and 3) the SE branch, south of Ungava Bay. In its centre, the Trans-Hudsonian Orogen is masked by sedimentary rocks of the Hudson Bay Paleozoic basin. The SE branch, which separates the Superior and North Atlantic cratons, is commonly interpreted as the Quebec portion of the Churchill Province (hence its SECP acronym). However, the correlation between these two parts is still under discussion (Bourlon et al., 2002; St-Onge et al., 2002; Wardle et al., 2002a). This large orogenic ensemble is also divided into three tectonic divisions by Corrigan et al. (2009): the Churchill margin (peri-Churchill), the Reindeer Zone and the Superior margin (peri-Superior). The peri-Churchill domain would have experienced progressive continental growth prior to the collision with the Superior Craton. This growth is marked by the accretion of microcontinents (Hearne, Meta Incognita/Core Zone, Sugluk) and arc terranes (La Ronge-Lynn Lake) to the Slave-Rae Craton. The Reindeer Zone includes elements that are exotic to the Churchill and Superior and present in the Manikewan Ocean: primitive to evolved oceanic arcs, back-arc basins, crust and oceanic plateaus, and Archean fragments such as the Sask Craton. These ensembles would have amalgamated with the peri-Superior domain during the final collision phase of the Trans-Hudsonian Orogen.

The SECP represents an area of this vast compression where the North Atlantic Craton (mainly exposed in Labrador and Greenland) and exotic fragments collided with the eastern margin of the Superior. This area is interpreted by several authors to be correlated with the lithologies and geological context observed on Baffin Island and in the Ungava Peninsula (St-Onge et al., 2009; Wardle et al., 2002a and references cited; correlative charter of Wardle et al., 2002b).

In Quebec, the SECP is limited by the Archean Superior Craton to the west, and by the Nain Province (North Atlantic craton and Burwell Lithotectonic Domain) and Makkovik Province to the east. Its contact with the Nain Province is partly masked by large Mesoproterozoic intrusions. The SECP is cut to the south by the Grenville Province.

Lithotectonic Subdivisions

The SECP has long been considered to have a tripartite structure (Hoffman, 1990a; van der Leeden et al., 1990; Wardle et al., 1990a) consisting of the Archean Core Zone remobilized in the Paleoproterozoic, bordered by two orogens:

• the Torngat Orogen (1.89-1.81 Ga) in the east, which corresponds to the suture between the Churchill and Nain provinces;
• the New Quebec Orogen (1.82-1.77 Ga) to the west, representing the suture between the Churchill and Superior provinces (Hoffman, 1990a).

The compression generated by these collisions is responsible for the regional NW-SE to N-S structural grain which affects most of the SECP. However, recent work of the Ministère (Verpaelst et al., 2000; Hammouche et al., 2011, 2012; Simard et al., 2013; Lafrance et al., 2014, 2015, 2016; Charette et al., 2016; Mathieu et al., 2018), the GSC (Corrigan et al., 2015a, b, 2018) and the academic community (Charette, 2016; Vanier et al., 2017, 2018; Godet et al., in preparation) reveal that this three-part representation of the SECP is inadequate.

The new results propose, as part of the SECP synthesis, a division into six distinct lithotectonic domains that are, from west to east: the Labrador Trough and the Rachel-Laporte, Baleine, George, Mistinibi-Raude and Falcoz domains. The Labrador Trough has not been the subject of this bulletin because a lithotectonic and metallogenic synthesis is already available (Clark and Wares, 2004). The other five domains are described in detail in their respective Geological Bulletin.

According to Wardle et al. (2002a), the Core Zone represents an Archean microcontinent characterized by a 35 to 40 km thick crust. It is attached to a fragment of the Archean Superior Province (Kuujjuaq Domain of Wardle) separated during a period of rift opening that led to the formation of the Labrador Trough at the beginning of the Paleoproterozoic (2.17-1.89 Ga). Recent field observations, isotopic geochemistry and geochronology have defined four different lithotectonic domains forming the amalgamated Core Zone-Kuujjuaq Domain block of Wardle et al. (2002a), i.e. the Baleine, George, Mistinibi-Raude and the western part of the Falcoz Domain. These domains with distinct lithological characteristics, limited by major shear zones, also contrast by their tectonometamorphic evolution and their crystallization age range. The term “Core Zone”, used in the text below for easy reading, represents a block composed of lithotectonic domains of different origins amalgamated by tectonic processes. We believe that the boundaries of this block evolved throughout its amalgamation. In its final state, it includes all lithologies of lithotectonic domains located east of the Labrador Trough up to the Nain Province, which is represented in Quebec by the Burwell Lithotectonic Domain. It also includes the Rachel-Laporte and Falcoz domains, previously assigned to the New Quebec and Torngat orogens respectively. The terms “Torngat Orogen” and “New Quebec Orogen” are no longer used as lithotectonic divisions. Rather, we consider that the imprint of these orogeneses is not limited to domains previously included in these divisions, but continues within the different lithotectonic domains previously grouped under the term “Core Zone”.

The Torngat Orogen is the result of a series of tectonometamorphic events that took place in the Paleoproterozoic (1.89-1.81 Ga; Van Kranendonk and Ermanovics, 1990; Van Kranendonk, 1996; Wardle and Van Kranendonk, 1996; Ermanovics and Van Kranendonk, 1998; Wardle et al., 2002a; Charette, 2016). It begins with the oblique collision between the Core Zone and the Archean North Atlantic Craton, which doubles the thickness of the crust between 1890 Ma and 1850 Ma, and develops a double-verging transpressive structure characterized by a subvertical central axis (Rivers et al., 1996; Wardle et al., 2002a). This process generated upper amphibolite to granulite facies metamorphic conditions. This event is followed by strike-slip or transpressure deformation between 1845 Ma and 1822 Ma, and then by cooling allowing leucosome to crystallize around 1810 Ma. A reactivation and uplift event around 1790-1780 Ma was identified at the eastern end of the orogen (Rivers et al., 1996; Van Kranendonk, 1996). In Quebec, Verpaelst et al. (2000) limited the Torngat Orogen to an assemblage of granulitic gneiss of various origins, completely remobilized during the orogenesis and dominated by a large system of sinistral shear zones. This area now corresponds to the eastern part of the Falcoz Lithotectonic Domain. However, recent tectonometamorphic studies (Charette, 2016; Godet et al., in preparation) indicate that the tectonothermal imprint of the Torngat Orogen is not limited to granulitic rocks, but extends westward to the Baleine Lithotectonic Domain. Only the Mistinibi-Raude Domain appears to have escaped the metamorphic imprint of this orogenesis.

The amalgamated block, consisting of the Core Zone and the North Atlantic Craton, subsequently collided obliquely with the Superior Craton during the New Quebec Orogenesis. On the eastern margin of the Superior Craton, the volcano-sedimentary belt of the Labrador Trough (and its hinterland) was folded and thrust toward the WSW on the craton. It resulted is duplex structures and exposure of the Archean basement by structural domes (thick-skinned tectonics), resulting in a burial and greenschist to amphibolite facies metamorphism of volcano-sedimentary sequences of the Labrador Trough and Rachel-Laporte Domain. The eastern boundary of the New Quebec Orogen was previously marked by the Kuujjuaq Domain (also known as Kuujjuaq Terrane; Wardle et al., 1990a; Perrault and Hynes, 1990). It is now included in the Baleine Lithotectonic Domain, which extends eastward to the Lac Tudor Shear Zone. Clark and Wares (2004) standardized stratigraphic nomenclature of the Quebec portion of the Labrador Trough and divided the Kaniapiskau Supergroup, which consists of all the groups and formations of the Labrador Trough, into three cycles: two volcano-sedimentary cycles at the base overlain by a cycle of molasse-type metasedimentary rocks.

The New Quebec Orogen contrasts greatly in deformation style and metamorphic grade with the high-temperature deformed granulitic units of the Torngat Orogen. The Core Zone is dissected by large NNW-SSE to N-S mainly dextral shear zones such as the Rivière George and Lac Tudor shear zones. Approaching the Torngat Orogen, sinistral NW-SE shear zones, such as the Falcoz and Moonbase shear zones, could represent shear networks coupled with dextral shearing (Vanier et al., 2018).

New data acquired in the SECP showed that the tectonometamorphic evolution of this geological province in the Paleoproterozoic was not simply the result of the development of orogenic belts located on both sides of the Core Zone, but that the latter was in fact a vast suprasolidus domain. Field observations, geochronology work and tectonic studies made in recent years show that the Archean gneissic basement and sedimentary cover have been migmatized in the Paleoproterozoic. In fact, evolved partial melting products have crystallization ages generally between 1820 Ma and 1810 Ma. Also, several evidence of metamorphic events before collision with the Superior Craton was found in the SECP, suggesting that the evolution of the SECP is more complex than previously thought.

Models

Dewey and Burke (1973) were the first to propose an interpretation of the Churchill using plate tectonics. They propose that the Churchill represents the Archean hinterland remobilized in the Hudsonian (Tibetan plateau-like collisional environment) that extends to the Slave Province (NW), Baffin Island (N) and Nain Province (NE). The Slave-Churchill and Nain-Churchill borders were then interpreted as intraplate crustal thickening limits (single collision zones) rather than as Hudsonian sutures. Work in recent decades has highlighted the existence of sutures zones, suggesting that the northern part of the Churchill represents a tectonic collage of microcontinents amalgamated in the Paleoproterozoic (Gibb et al., 1978, 1983; Sharpton et al., 1987; Hoffman,1990a). 

Hoffman (1990b) envisions a model similar to the evolution of SE Asia that involves seven ancient microcontinents (in this case Nain, Slave, Superior, Wyoming, Burwell, Hearne and Rae) and multiple collision zones. In this model, the Labrador Trough is considered to be a pull-apart basin that would have been overthrusted on the margin of the Superior Craton during the New Quebec Orgenesis, very early in the history of Trans-Hudsonian compression. It is then uncertain whether the Burwell Domain would subsequently collide with the North Atlantic Craton before the Torngat Orogenesis, or whether collisions of the Core Zone, Burwell and North Atlantic blocks would be concurrent. This is still a matter for debate.

Wardle et al. (2002a) suggested a model of transpressive orogen and corrected the erroneous chronology of orogenic events proposed by Hoffman (1990b). According to this model, the SECP is the result of successive oblique convergences between the North Atlantic Craton with an Archean block represented by the Core Zone, and then between the latter and the Superior Craton. The development of transpressure is recorded on the sides of the North Atlantic and Superior cratons as they moved northward during the Trans-Hudsonian Orogenesis. Wardle et al. (2002a) present the tectonic development of this region in five main stages shown here.

The gravimetric and magnetic data extended the tectonic domains identified on land across Ungava Bay and Hudson Strait (Bourlon et al., 2002). The magnetic signature of the Superior Province reappears east of volcano-sedimentary basins of the Labrador Trough and Rachel-Laporte Domain, in the Baleine Lithotectonic Domain (Wardle et al., 2002a). The age and type of lithological units present in the northern portion of the Baleine Lithotectonic Domain also suggest an affinity between these two areas (Simard et al., 2013; Lafrance et al., 2014). Corrigan et al. (2009) interpret the western portion of the Core Zone (namely the George Lithotectonic Domain) as the extension of the Meta Incognita microcontinent south of Ungava Bay. Based on preliminary isotopic data, Isnard et al. (1998) suggest that rocks on either side of the Rivière George Shear Zone (ZCrge) share a similar geological history. However, recent tectonometamorphic studies (Charette, 2016; Godet et al., in preparation), supported by numerous geochronological data (which are listed in the various SECP stratigraphic records) show the opposite.

New results from the Ministère and its academic partners have shown that the tectonometamorphic evolution of the SECP is responsible for the burial of a major portion of the SECP in the Paleoproterozoic following the collision with the North Atlantic Craton, and that the high thermal state of the SECP has long persisted (20-80 Ma; Charette, 2016). In the Falcoz and Mistinibi-Raude lithotectonic domains, east of the ZCrge, no evidence of tectonically accelerated exhumation processes was found, both by the nature of observed structures and by pressure-temperature-time (PTt) paths. Instead, metamorphic and geochronological data suggest a long period of cooling under a high geothermal gradient, as would have been produced by erosion (Charette, 2016). In contrast, in the Baleine Lithotectonic Domain, a PTt path showing isothermal decompression suggests tectonic exhumation. This may be consistent with westward thrusting of the Baleine Domain on the Rachel-Laporte Domain during collision with the Superior Craton (Godet et al., in preparation). An orogenic boundary is located in the George Lithotectonic Domain, or at the boundary with the Baleine Lithotectonic Domain. The origin of this boundary remains enigmatic.

The importance of the George Lithotectonic Domain in the SECP tectonic evolution had already been raised by some authors. Goulet and Ciesielski (1990) proposed dividing the Core Zone into two large tectonic segments separated by the ZCrge, that is Kuujjuaq in the west and George River in the east. Wardle et al. (2002) did not retain the Goulet and Ciesielski divisions (1990) but defined the Lac Tudor Shear Zone (formerly limiting the Kuujjuaq Domain, as discussed above) as representing the eastern boundary of the New Quebec Orogen.

Acknowledgements:

This Geological Bulletin was made possible through the cooperation of many people who have been actively involved in the various stages of the project, including geologists and students who participated in field campaigns between 2009 and 2017, professors and students who have dealt with specific and regional issues in university studies, as well as our partners at the Geological Survey of Canada, Geotop and Jack Satterly Geochronology Laboratory.

References

Publications Available Through Sigéom Examine

Other Publications