Background

Description

Geology

The current geological knowledge of the Minto Subprovince is derived from the entire new data acquired through the Programme Grand Nord. The Minto Subprovince is predominantly composed of Archean plutonic rocks. These igneous rocks, of felsic to ultramafic composition, are more or less foliated and have variably preserved their original textures. Felsic plutonic rocks are the most common and widespread. They are composed of tonalite (40%), granite (20%), granodiorite (15%), charnockitic rocks (15%) and, more rarely, monzonite and syenite (<1%). They usually appear in elongated sheets defining the predominant structural style of the region. Small, limited plutons of dioritic, gabbroic and ultramafic rocks account for less than 2% of the bedrock. Metalvolcanic and metasedimentary rocks account for less than 10% of all Archean rocks. These rocks form isolated lenticular units or discontinuous dismembered belts within the intrusive masses. The best preserved belts can be up to 10 km wide and 120 km long (Leclair, 2008). Geochronological data collected on Archean rocks enabled the development of a stratigraphic model and thus positioning of stratigraphic units in a temporal evolution model for the Archean period between 3830 and 2630 Ma (period before 2740 Ma; period between 2740 and 2630 Ma). In addition, a unified stratigraphy for all Archean units of the northeast Superior Province has been proposed in a stratigraphic lexicon that mainly describes lithodemic units that group intrusive and metamorphic rocks (Simard, 2008).

Paleoproterozoic sedimentary rocks of the Sakami Formation lie unconformably on the rocks of the Minto Subprovince and form small isolated klippes, scattered along a mainly EW axis at the 56^th parallel (Eade, 1966; Clark, 1984). In the La Grande Subprovince, cross-cutting relationships involving diabase dykes were used to assess the age of the Sakami Formation between 2500 and 2216 Ma (Goutier et al., 2001). A K-Ar dating obtained from micas contained in a Sakami sandstone (Clark, 1984) indicates a sedimentation age prior to 2230 Ma. There are also a dozen Proterozoic dyke swarms of different orientation cutting Minto Archean rocks (Buchan and Ernst, 2004; Maurice, 2008). These diabase dykes, which are generally less than 100 m thick, can be tracked over several kilometres. They are not deformed or metamorphosed, except in certain areas near the Proterozoic orogens. In addition, a swarm of about 100 lamprophyre and carbonatite dykes occupies the central part of the Aigneau Lake region. Some dykes were also observed in the northeast region of Gayot Lake. They are centimetric to metric dykes with orientation ranging from NW-SE to NS. Carbonatite dykes are spatially associated with carbonate lamprophyre dykes. Carbonatite is a homogeneous rock, fine to coarse-grained, characterized by an orange to rusty-brown colour in altered surface. Dating made on two carbonatite dykes gave ages of 1932 Ma and 1941 Ma (Simard et al., 2008).

Two diatremes were identified along the Ungava Trough in the Couture Lake area. These rocks had been recognized by Moorhead (1989) who included them in the Povungnituk Group (Ungava Orogen). The relationship between the diatremes and the supracrustal sequence of the Povungnituk Group has not been established, Madore et al. (2002) grouped the two intrusions under the term Kuuvvaluk Diatremes. These are small, unbroken circular intrusions of approximately 70 m in diameter, composed of ultramafic phlogopite lamprophyre with pyroxene and olivine relics showing well-preserved igneous textures. These intrusions were not dated, but likely belong to the Proterozoic. Two diatreme breccias were also observed in the Lake à l’Eau Claire area but were not assigned to any stratigraphic units. Field observations suggest that these breccias are associated with the intrusion of Proterozoic diabase dykes in this area. These are 15 to 20 m thick brecciated dykes containing fragments of sedimentary rocks and Proterozoic volcanic glass as well as fragments of Archean granitoids. One of these dykes was interpreted as a clastic sedimentary dyke by Chandler (1988).

Pennsylvanian-aged impactites (Bostock, 1969; Rondot et al., 1993) containing Ordovician limestone enclaves are found on the ring of islands in the centre of Lake à l’Eau Claire. The two rounded depressions at Lake à l’Eau Claire were the result of a double meteoritic impact (Beals et al., 1956, 1960). The impact rocks were dated by K-Ar (Bostock, 1969), Rb-Sr (Reimold et al., 1981) and Ar-Ar (Bottomley et al., 1990) methods. The ages obtained, approximately 280 Ma, indicate that a cover of Pennsylvanian rocks was present at the time of impact. Silurian-aged impactites (Bottomley et al., 1990) were also observed in Quaternary soft sediments on the edge of the Couture Lake astroblem (Madore et al., 2002). Finally, the Pingaluit crater, formerly known as the New Quebec crater, is located just north of the Nantais Belt. It is a perfectly round bowl with a sharp rim that houses a lake about 3 km in diameter. Twenty-one impactite fragments were found near the crater. ⁴⁰Ar/³⁹Ar dating of these fragments estimated that the impact occurred approximately 1.3 Ma ago (Grieve et al., 1989).

The Minto Subprovince is subdivided into seven major lithotectonic domains whose nature and boundaries were clarified during the Programme Grand Nord work (Minto subdivision map):

• Tikkerutuk Domain;
• Bienville Domain;
• Lac Minto Domain;
• Goudalie Domain;
• Qalluviartuuq Domain;
• Utsalik Domain;
• Douglas Harbour Domain.

Isotopic and geochronological data, as well as the distribution of stratigraphic units, show that the different magmatic events that have affected the entire Minto Subprovince at different times of the Archean transcend the boundary of the lithotectonic domains. Therefore, domains must be seen as the product of several overlapping magmatic events. They are characterized by stratigraphic units and geochronological and isotopic data that reflect the best preserved events (Leclerc, 2008).

Geochronological, isotopic and stratigraphic data have also revealed two large terranes within the northeast Superior Province that have played an important role in the evolution of the Archean crust:

• Hudson Bay Terrane;
• Rivière Arnaud Terrane.

The Hudson Bay Terrane, to the south and west, shows evidence of a crust formed from the Eoarchean (>3600 Ma), whereas the history of the Rivière Arnaud Terrane, to the north, began after 2900 Ma during the Mesoarchean era. The two terranes had a different evolution until about 2740 Ma. Prior to this date, the two terranes were affected by different magmatic events marked mainly by mafic volcanism and tonalitic plutonism. After 2740 Ma, potassium and enderbitic magmatism spread throughout the northeast Superior Province affecting both terranes in a similar way.

Finally, the Minto Subprovince is also characterized by the presence of a structural complex, the Diana Structural Complex, which is a transition zone between rocks of the Superior Province in the west and those of the Churchill Province in the east. The northeast part of Douglas Harbour Domain and the Diana Complex have been affected by the Trans-Hudsonian Paleoproterozoic Orogenesis.

Geological Evolution

The geological evolution of the northeast Superior Province spans more than 1.2 Ga. It is characterized by a complex history of volcanism, sedimentation, plutonism, deformation and polyphased metamorphism that demonstrates episodic growth and a repeated reworking of the continental crust between 3.82 and 2.62 Ga.

The model presented here is the one that results from the work of the Programme Grand Nord. This model involves the formation of ancient cratonic nuclei (3.9 to 2.9 Ga; Hudson Bay Terrane) and domains of volcanic and intrusive rocks derived from a younger juvenile crustal source (<3.0 Ga; Rivière Arnaud Terrane). The Hudson Bay Terrane was formed from the remnants of an evolved Eoarchean to Mesoarchean sial crust, while the Rivière Arnaud Terrane was formed from a primitive Mesoarchean crust in the ocean. The two terranes had a different evolution until about 2700 Ma. Prior to this date, they were affected by different magmatic events marked mainly by mafic volcanism and tonalitic plutonism. After 2700 Ma, potassium (granite and granodiorite units) and charnockitic (pyroxene granitoids) plutonism spread throughout the northeast of the Superior Province, affecting both terranes in a similar way. This plutonism, after 2.74 Ga, coincides with the main deformation phase responsible for the NW-SE to N-S structural grain (Leclair, 2008).

Four major stages of growth and reworking of the crust are recognized, each of which can contain up to three distinct geological events (pattern of geological stages and events):

1. The first stage of evolution, between 3.9 and 2.9 Ga, is characterized by the formation of paleoecratons and encompasses two magmatic Eoarchean events (EA-1 and EA-2) and one Mesoarchean event (MA-1) that remains cryptic;
2. The second stage, between 2.9 and 2.74 Ga, is essentially the deposition of sequences of tholeiitic basalt, komatite and sedimentary rocks and the intrusion of large tonalitic rock suites. This evolutionary stage consists essentially of two Mesoarchean magmatic events (MA-2 and MA-3) as well as Neoarchean tonalitic magmatism that has developed diachronically at the regional level (NA-1).
3. The third stage, between 2.74 and 2.68 Ga, marks a major change in the type of magmatism that resulted in syntectonic to post-tectonic intrusions of granodiorite, granite and pyroxene granitoids. The two main plutonic events (NA-2 and NA-3) are widespread in most of the region.
4. The fourth and final stage of evolution, between 2.68 and 2.62 Ga, comprises a single event (NA-4) characterized by the emplacement of small local intrusions of granite, syenite and carbonatite, and by hydrothermal activity concentrated mainly along ductile and fragile permeable areas. This stage also corresponds to intense magmatic activity in the Ashuanipi Subprovince.

References