|Type area:||Bohier Island, Lérand and Cadieux lakes areas (NTS sheets 33A01, 33A07, 33A08, 23D04, 23D05, 23D11 and 23D12)|
|Geological province:||Superior Province|
|Geological subdivision:||Opatica Subprovince|
Table des matières
The Upper Eastmain Green Rock Belt was first mapped by geologists from the Geological Survey of Canada. The geological survey of Eade (1966), covering a large area of the Eeyou Istchee James Bay region, identified a strip of greenstones north of the Otish Mountains, but did not really define its extensions. The Upper Estmain Greenstone Belt was better circumscribed in the 1980s following the work of Hocq (1985), Roy (1985, 1988), Couture (1986, 1987a, 1987b, 1987c, 1993) and Couture and Guha (1990). Couture (1986) divided the rocks of the belt into two lithostratigraphic groups: the Bohier Group, which is mainly composed of sedimentary rocks, and the René Group, which comprises mostly volcanic rocks (Couture 1987a and Roy 1988). Beauchamp and Massei (2018) subdivided René Group volcanic rocks into four formations based on geochemistry and geochronology data from their 2017 mapping work.
The unit name is from Lac René located in NTS sheet 33A08.
The René Group comprises volcanic rocks belonging to the Upper Eastmain Greenstone Belt (Opatica Subprovince). The stratigraphy of the René Group units is based primarily on the study of the belt southern branch outcrops that are better preserved.
The René Group includes at least three volcanic events. The first event, which is mostly mafic, is of Mesoarchean age and corresponds to the Érasme Formation (2800 ±6 Ma; Davis, personal communication, 2018). The Roman Formation represents the second volcanic event. In the southern branch, this unit consists of felsic and intermediate volcaniclastics and volcanics dated 2770 ±3 Ma (Davis, personal communication, 2018). The Roman Formation is lithologically more varied in the eastern branch of the belt than in the southern branch. At this site, it includes mafic volcanic rocks (2774 ±6 Ma; Talla Takam et al., in preparation) and minor amounts of iron formations and sedimentary rocks. The 30 Ma gap between emplacement of the Érasme and Roman formations could be explained by the presence of a hiatus or a fault. Finally, the last event corresponds to the Clément Formation, which consists essentially of mafic volcanic rocks, sills and ultramafic flows. This last unit has not been dated. Several stratigraphic polarity indicators (pillow peduncles facing south and SSW) indicate that the Clément Formation is at the top of the volcanic pile. Since the stratigraphy of the René Group is overturned, the younger Clément Formation is structurally under the older rocks of the Roman and Érasme formations.
The stratigraphic position of the Dolent Formation, a mostly basalt unit, is not known at this time. Future mapping and geochronology work will likely clarify the stratigraphic position of this formation.
Rocks of the René Group are generally metamorphosed to amphibolite facies (Couture, 1987a; Roy, 1988; Talla Takam et al., in preparation) and have undergone local retrograde metamorphism to green schist facies (Dejou, 1987; Couture, 1993; Talla Takam et al., in preparation). Within the René Group, the intensity of regional metamorphism is not uniform. In general, it is higher in the central part of the south branch, in the area of Lac René (sheet 33A08; Couture, 1993; Beauchamp and Massei, 2018).
The René Group is located in the Opatica Subprovince on sheets 33A01, 33A07, 33A08, 23D04, 23D05, 23D11 and 23D12. This group is the main lithostratigraphic unit of the Upper Eastmain Greenstone Belt. In the east and west branches of the belt, the René Group is oriented NE-SW to EW and extends from Wahemen Lake (23D11) to Cadieux Lake (33A07) for a distance of approximately 100 km. The René Group is also present in the south branch of the belt. It was mapped from Bohier Island (sheet 33A08) to north of Laparre Lake (sheet 23D04) for a distance of 35 km from north to SW. The René Group is also present in the north branch of the belt, from north of the Eastmain River to Bohier Island. It ranges in thickness from 500 m to 9 km.
Rocks of the Érasme Formation, at the base of the René Group, are dated 2800 ±6 Ma (Davis, personal communication, 2018). Two ages were obtained in rocks of the Roman Formation corresponding to the second volcanic event. They were dated 2770 ±3 Ma (Davis, personal communication, 2018) and 2774 ±6 Ma (Talla Takam et al., in preparation).
Rocks of the René Group represent the remains of a supracrustal cover that rests on a Mesoarchean gneissic basement. The age of the basement was estimated at 2822 Ma (Talla Takam et al., in preparation) from inherited zircons of the Misasque Pluton which was contaminated by the basement during its intrusion.
The base of the Érasme Formation, in the lower part of the René Group, corresponds to intrusive contacts. The Bohier Island Pluton (2771 ±6 Ma; Davis, personal communication) is contemporary to the Roman Formation and cuts the Érasme Formation. The Chiyaaskw (2746 ±3 Ma; Talla Takam et al., in preparation), Misasque (2695 ±8 Ma; Talla Takam et al., in preparation) and Cadieux plutons, and the Wahemen Granite (2640 ±10 Ma; Talla Takam et al., in preparation) also cut volcanic rocks of the René Group.
The René Group top contact is generally characterized by the presence of a thrust fault that masks the discordance with the Bohier Group sedimentary rocks. Volcanic rocks of the René Group of the Opatica Subprovince overlap paragneisses of the Opinaca Subprovince (Laguiche Complex). the Bart Thrust Fault is clearly marked on the aeromagnetic map (D’Amours, 2011; Benahmed and Intissar, 2016). The magnetic lineaments of the Opinaca Subprovince are straight, oriented ENE-WSW and are truncated by the Bart Thrust Fault.
Finally, Palaeoprotezoic sedimentary rocks of the Otish Basin (Otish Supergroup) unconformably lie on the René Group volcanics.
Does not apply.