The name “Ashuanipi” was introduced in the 1940s by the Labrador Mining and Exploration Company geologists who referred to the informal term “Ashuanipi series” as ancient metamorphosed rocks located west of the Labrador Trough in the Schefferville area (Frarey and Duffell, 1964). Harrison (1952) proposed the term “Ashuanipi Complex” for these rocks, while Frarey (1961) used the term “Ashuanipi Group” and Baragar (1967) used the term “Basement Complex”. Subsequently, the term “Ashuanipi Complex” was used extensively in regional geological mapping projects (Fahrig, 1967; Rivers, 1985a, b; Dimroth, 1978; Clark, 1984). In their new subdivision of the Superior Province, Card and Ciesielski (1986) elevated the Ashuanipi to the rank of a subprovince by defining it as a high-grade metamorphic gneissic subprovince. Since then, the term “Ashuanipi Subprovince” has been accepted in Quebec and used by the majority of authors who carried out work there (Hocq, 1994; Lamothe et al., 1998, 2000; Chevé and Brouillette, 1995; Gosselin and Simard, 2000; Simard et al., 2008, 2009a, b; Parent, 2011; Guermina and Sawyer, 2003; Morfin et al., 2013, 2014). In contrast, the term “Ashuanipi Complex” was preferred over “Ashuanipi Subprovince” in Labrador (James, 1997; van Nostrand and Bradford, 2014) and in several scientific papers (Percival, 1987, 1990, 1991a, b, 1993; Percival and Girard, 1988; Percival et al., 1992, 2003; Mortensen and Percival, 1987).
The first geological observations of the Ashuanipi Subprovince were made by Low (1896a,b, c) along the Caniapiscau River. He was able to identify contact between Archean gneiss and rocks of the Labrador Trough, to which he attributed a Cambrian age (Low, 1896a). Subsequently, two large influxes led to 28 geological surveys covering the entire Ashuanipi Subprovince. The first regional reconnaissance campaigns were conducted by the Geological Survey of Canada in the 1950s and 1960s. The second influx of mapping was largely carried out by the Government of Québec after 1985, in the mid-1990s with systematic geological mapping at the 1:250 000 scale. This work was part of the Mid-North and Far North programs to open new territories for mineral exploration and to acquire new geoscience data in the vast unknown regions of northern Quebec. These mapping programs were completed in the summer of 2008.
Early work in the Ashuanipi Subprovince defined the major regional lithological divisions and delineated the boundaries of this subprovince with the Labrador Trough in the east and with the Grenville Province in the southeast. The first major stratigraphic divisions were proposed by Chevé and Brouillette (1995) in the NE Ashuanipi Subprovince. These authors defined two large complexes, the “Canyon Eaton Complex” and the “Rivière du Sable Complex”, two terms that have now been abandoned. Units of these two complexes were reassigned respectively to the La Grande Subprovince (Simard et al., 2009a) and various units of the Ashuanipi Subprovince (Simard et al., 2015). Subsequent 1:250 000 geological mapping projects have led to the definition of many stratigraphic units in the Ashuanipi Subprovince.
The Ashuanipi Subprovince is located at the eastern end of the Superior Province. Roughly circular, it covers an area of >90 000 km2 and is mostly located in Quebec and extends eastward into Labrador. The Ashuanipi Subprovince is bordered to the east by Paleoproterozoic rocks of the Labrador Trough (Southeastern Churchill Province) and to the southeast by the Grenville Province. Within the Superior Province, the Ashuanipi Subprovince is in contact with the Minto Subprovince to the north, the La Grande Subrpovince to the west and northwest, and the Opinaca and Opatica subprovinces to the south.
The Ashuanipi Subprovince is an Archean high-grade metamorphic terrain consisting of ancient units (<2725-2680 Ma) of metasedimentary rocks, volcanic rocks, tonalitic and granodioritic intrusions and, to a lesser extent, mafic to ultramafic rocks. Ancient units are cut by large diatexite masses (2680-2660 Ma) that characterize the Ashuanipi Subprovince. Diatexites result from partial melting of older units, mainly paragneiss, which were the dominant lithological assemblage of this subprovince prior to the melting event. All these units are cut by a series of later intrusions of granite, granodiorite and tonalite (2650-2635 Ma) as well as leucogranites (<2635 Ma), small syenite masses (2625 Ma) and subcircular plutons of anorogenic fluorine granite (2570 Ma) (Simard et al., 2015).
The age of sedimentation, albeit poorly defined, was estimated at ~2700 Ma based on the youngest detrital zircons obtained in a single paragneiss sample from the Schefferville area (Percival et al., 1992). However, Wodicka et al. (2009) interpreted a maximum age of 2726 Ma for the deposition of sediments from southern Ashuanipi. In this area, volcanic rocks and synvolcanic intrusions dated between ~2709 and 2704 Ma represent the remains of bimodal volcanic activity contemporaneous to sedimentation (Parent, 1998). These volcanic sequences contain several mineralized zones of volcanogenic sulphides. The age of volcanism and associated magmatism is comparable to some similar events in the Opatica Subprovince and in the Eastmain River Volcanic Band (ERVB) of the La Grande Subprovince (Simard et al., 2015).
Rocks of the Ashuanipi Subprovince have been affected by a granulitic, continuous and polyphase tectonometamorphic event that helped reshape the Archean crust over a period of ~70 million years (2700-2632 Ma). This event is responsible for the recrystallization and partial melting of older units, which culminated in the production of a significant volume of melting material that produced large diatexite units (2680-2660 Ma). Granitic and charnockitic units emplaced subsequently (2650-2625 Ma) would be fractionated phases related to the same major regional melting event (Guernina, 2007). Mineral parageneses or metamorphic assemblages, mainly the widespread presence of orthopyroxene, as well as U-Pb ages, indicate that granulitic facies conditions prevailed over a long period of time. However, some areas in the central and northern parts of the Ashuanipi show assemblages of the amphibolite facies. Later thermal phenomena appear to have succeeded regional granulitic metamorphism, as suggested by the presence of titanite and monazite dated between 2620 and 2598 Ma. Finally, Ashuanipi rocks were locally retromorphosed to the greenschist facies along late brittle faults, and to the amphibolite facies in the Grenville Front area (Simard et al., 2015).
The structural grain of the eastern portion of the Superior Province has two predominant orientations: an E-W orientation linked to the La Grande and Opinaca subprovinces, and an NNW-SSE orientation corresponding to the Minto Subprovince. The Ashuanipi Subprovince is at the junction of these two major structural trends. It is characterized by a structural grain whose orientation varies considerably on a regional scale. The structural scheme of the Ashuanipi Subprovince reveals a complex tectonometamorphic history resulting from five Archean deformation phases: four ductile (D1-D4) and one brittle (D5). The complexity of this structural scheme is due in large part to the predominant deformation D2 generated in the magmatic state and closely associated with diatexite emplacement (Simard et al., 2015).
The geological evolution of the Ashuanipi Subprovince spans over >150 Ma (2725-2570 Ma). Its history is marked by several periods of crustal growth, remodeling, recycling and various tectonic phases. Stratigraphic, geochronological, metamorphic and structural data enable the identification of four major events in the evolution of the Ashuanipi (Simard et al., 2015).
The first event (2725-2680 Ma) involves the formation of one or more large sedimentary basins where isolated volcanic centres have developed. These volcano-sedimentary rocks were cut by tonalitic and granodioritic intrusions and by small intrusions of mafic to ultramafic rocks.
All these ancient rocks were affected by a major melting event deep in the crust under granulitic facies conditions. This second event took place on >70 Ma and resulted in partial melting of much of the crust to form large regional diatexite units (2680-2660 Ma). Melting occurred in an active tectonic environment that favoured the development of a migmatitic foliation marking the regional structural grain.
The formation of diatexite was followed, in the third event, by the emplacement of large regional porphyraceous granitoid intrusions and charnockitic intrusions (2650-2635 Ma), as well as leucogranite intrusions (<2635 Ma) and small syenite masses (<2625 Ma). All of these intrusive units could be evolved fractionation products related to regional melting.
Following craton stabilization, a final Archean magmatic event consists of the intrusion of a series of subcircular plutons of anorogenic fluorine granite (2575 Ma) located in the SW portion of the Ashuanipi Subprovince.
The eastern and southeastern parts of the Ashuanipi Subprovince have also been affected by two Proterozoic events. During the Paleoproterozoic New Quebec Orogenesis, a NE-SW fault system developed and affected the eastern Ashuanipi. This system controlled the emplacement of the Senneterre diabase Dykes (2.2 Ma). The southeast portion of the Ashuanipi has been affected by thrust faults associated with the Grenville Front that border the Mesoproterozoic Grenvillian Orogen (~1.0 Ga).
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