Background

Description

Geology

The Opinaca subprovince comprises a wide range of paragneiss and migmatites derived from feldspathic wacke. This metasedimentary complex, which can be a few kilometres thick, forms the Laguiche Complex and covers most of the area of the Opinaca Subprovince. The Laguiche Complex comprises several lithological assemblages, some of which have been divided according to the percentage of mobilisate present in the rock. Metasedimentary rocks of the Laguiche Complex mainly consist of paragneiss, metatexite and diatexite. In the Opinaca Reservoir area, paragneiss occur as decametric to kilometric strips extending over tens of kilometres. These rocks contain 20 to 50% tonalitic or granitic mobilisate and are characterized by well-developed migmatitic banding (Bandyayera and Fliszar, 2008). The eastern end of the Opinaca Subprovince is characterized by an assemblage of supracrustal rocks that host intrusive rocks. Supracrustal rocks belonging to the Hublet Group (Mercator Formation) consist of early gneiss that are likely derived from grauwacke and that contain volcanic rocks (Lamothe et al., 1998). These early gneiss resemble those described elsewhere in the Ashuanipi Subprovince (Percival et al., 1992; James, 1997). According to Lamothe et al. (2000), sedimentary rocks of the Hublet Group are likely equivalent to those of the Laguiche Complex, which form most of the western part of the Opinaca Subprovince.

Metasedimentary rocks of the Opinaca are intruded by multiple post-tectonic to late-tectonic intrusions consisting of granodiorite, pegmatitic granite and tonalite cutting paragneiss of the Laguiche Complex. These intrusions are peraluminous and type S (Moukhsil et al., 2003). Some small ultramafic intrusions are also present on the edge of the Opinaca Subprovince. However, their relationship with metasedimentary rocks of the Laguiche Complex is unknown. Finally, all Archean units of the Opinaca Subprovince are cut by Proterozoic diabase dykes belonging to the Mistassini, Senneterre and Matachewan swarms.

The magnetic signature of the Opinaca Subprovince is relatively weak and punctuated by intense linear anomalies, many of which correspond to strips of metavolcanic rocks and iron formations (Sharma, 1978; Chartrand and Gauthier, 1995; Gauthier, 1996). In the western part of the Opinaca Subprovince, the sedimentary assemblage formed by the Laguiche Complex and the Hublet Group forms the axial part of a large regional antiform with shallow dip to the NE, bordered to the east and north by the southern portion of the Ashuanipi Subprovince (Lamothe et al., 2000). In the Opinaca Reservoir area, rocks of the Opinaca Subprovince show a deformation pattern in domes and basins; the core of the domes is occupied by late granitic intrusions (Bandyayera et al., 2010). In general, the Opinaca Subprovince is characterized by increasing metamorphism towards the south from its contact with the La Grande Subprovince. Metamorphism changes from mid to upper amphibolite facies and reaches the granulite facies with the occurence of hypersthene in most detrital rocks.

Geological Evolution

The geological history of the Opinaca Subprovince is comparable to that of the Nemiscau and other sedimentary subprovinces that make up part of the Superior Province. The Opinaca Subprovince corresonds to the remains of a large sedimentary basin. The sedimentation age remains poorly known. A maximum age for sediment deposition of 2721 Ma was obtained on detrital zircons from the Mercator Formation (Wodicka et al., 2009). The deposition age of the Opinaca Subprovince would be between 2721 and 2671 Ma (David et al., 2011). This age range would also correspond to the deposition age of two sedimentary basins in the La Grande Subprovince, dated <2686 ±3 Ma and <2714 Ma (Ravenelle et al., 2008, 2010), north of the Opinaca Reservoir. These data suggest that the Opinaca Subprovince sedimentary rocks would not be younger than those of the La Grande Subprovince, as proposed by Goutier et al. (2002). Sedimentary units of the Opinaca and La Grande subprovinces could be contemporary or form several separate basins (Bandyayera et al., 2010). In addition, the Frégate Pluton is intrusive in the Laguiche Complex, thus sedimentary deposition responsible for paragneiss of the Laguiche Complex in the Richardie Lake area predates this intrusion dated 2710.4 ±2.4 Ma (Augland et al., 2016). Furthermore, the presence of mega-enclaves of migmatitized paragneiss from the Laguiche Complex in the Richardie Suite, dated 2693 ±2.4 Ma (Pichi Pluton), indicates that sediment deposition and initiation of the migmatitization episode began before 2693 Ma (Gigon and Goutier, 2017).

As with the Nemiscau Subprovince, the Opinaca Subprovince could represent the eastern extension of the English River Subprovince (Card, 1990). Other authors rather support the hypothesis of an eastern extension of the Quetico Basin. Percival (1989) and Percival and Williams (1989) propose that the Quetico, Nemiscau and Opinaca subprovinces, as well as the Ashuanipi Subprovince, could be a broad accretionary prism. The work of Doyon (2004) also suggests that the Quetico, Nemiscau and Opinaca basins are geochemically similar. Models proposed by Moukshil et al. (2003) and Percival (1989) argue that the Nemiscau, Opinaca and Quetico basins are considered to be back-arc basins due to the presence of sediments, mid-ocean ridge tholeiitic basalts (MORB), potassium-depleted tholeiitic rocks and syntectonic and synvolcanic plutons. Some hypotheses were put forward for understanding the geodynamic context of the formation of sedimentary sequences of the Laguiche Complex and its evolution during deformation and metamorphism episodes. According to Lapointe (2008), the structural signature of the Eeyou Istchee James Bay region is compatible with a central metamorphic complex centred on the Opinaca Basin (Laguiche Complex). Gold mineralization distributed on the periphery of the Opinaca Subprovince may be associated with decollement structures of such a complex. Bandyayera et al. (2010) showed that the Opinaca Subprovince is characterized by dome and basin structures that are locally stretched and dismembered. Mobilisate injections would preferentially be aligned with domes and the centre of basins would be occupied by late intrusions.

In its tectonometamorphic interpretation of the northern Superior Province, Cadéron (2003) established the first geothermobarometric data characterizing metasedimentary rocks of the Opinaca Subprovince. The Opinaca Subprovince is characterized by a polymetamorphic evolution over a period of 65 Ma, and structural data coupled with geothermobarometric results allowed for the proposition of a tectonometamorphic model of the Opinaca evolution. In this model, the Opinaca Subprovince represents an Archean intracontinental basin formed during an extension period. An ocean plate plunging towards the NW causes crustal thinning and extension, and formation of a graben that separates an Archean continental crust represented by the La Grande and Opatica subprovinces. Metasedimentary rocks of the Opinaca Subprovince would therefore come from erosion of the La Grande and Opatica subprovinces. A period of compression begins around 2705 Ma and continues until 2680 Ma, corresponding to emplacement of diatxites of the Ashuanipi Complex. Beginning of the closure of the Opinaca Basin would be contemporary with regional deformation phase D₁ (2705-2680 Ma). During basin closure, the Opinaca overthrusts the La Grande Subprovince to the north and Opatica Subprovince to the southwest and would represent a double-verging structure. Ancient normal faults appear to have undergone recurrence of reverse movement in this process. Subsequently, the Ashuanipi (2680-2650 Ma) was emplaced and the compression phase (around 2645 Ma) was completed. During regional deformation phase D₃, the Opinaca was folded and formed a large southward-overfold antiform structure.

References