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|Type area:||Doublet Lake area, ~60 km NE of Schefferville, in the eastern part of the Labrador Trough (NTS sheet 23O01)|
|Geological province:||Churchill Province|
|Geological subdivision:||New Quebec Orogen (Labrador Trough) / Gerido et Letty lithotectonic zones|
|Lithology:||Pyroclastic rocks, iron formation, slate, claystone, sandstone, basalt|
In 1949, geologists from Labrador Mining and Exploration divided the volcano-sedimentary sequence of the eastern belt of the southern Labrador Trough into three series called, from SW to NE, Howse, Murdoch and Doublet (see Frarey and Duffell, 1964). The informal name Doublet Group was introduced by Harrison (1952) to refer to a sequence of mafic lavas and tuffs, quartzite, mudstone and slate near the eastern edge of the Labrador Trough (NE section of the Burnt Creek strip, NTS sheet 23J). The name comes from Doublet Lake, located ~4.5 km SE of Willbob Lake (sheet 23O01). The Howse and Murdoch (Murdock in some publications) groups completed the Proterozoic succession in the Trough. These groups were placed above the Knob Lake Group in the stratigraphic column (Harrison, 1952). Subsequently, Baragar (1958, 1960) informally relegated the Murdoch Group to formation rank. The name Doublet Group was formalized by Frarey and Duffell (1964) who divided the Doublet volcano-sedimentary sequence into a lower sedimentary unit, the Thompson Lake Formation, and an upper volcanic unit, the Willbob Formation. Frarey and Duffell (1964) also formalized the name Murdoch Formation and included it in the Doublet Group. From bottom to top, this group consisted of the Murdoch, Thompson Lake and Willbob formations. Finally, Frarey and Duffell (1964) proposed to abandon the Howse group as being equivalent to the Menihek Formation (Harrison, 1952).
Baragar (1967) and Frarey (1952, 1967) mapped the Doublet Group in the Murdoch and Ahr lakes area (sheets 23O07 and 23O10), Thompson Lake area (sheet 23O08) and Willbob Lake area (sheet 23O01). Further east, the Doublet Group was mapped by Girard (1995) in the Deborah Lake area (sheet 23P05) and by Fahrig (1952, 1964) in the Griffis Lake area (sheet 23P04). It was followed to the SE by Donaldson (1966) in the Marion Lake area (sheet 23I13) and by Wardle (1979, 1982) on the Labrador side. Volcanic rocks of the Doublet Group have been mapped by Dimroth (1972, 1978) northward to latitude 56°30′N. Further north, only the Murdoch Formation has been recognized by Dressler (1979) between latitudes 56°30′N and 57°15′N, and by Clark (1978) west of Herodier Lake (sheet 24F07). Dressler (1979) included two other formations in the Doublet Group in his regional mapping, the Aulneau and Nachicapau formations. Clark and Wares (2004) subsequently reassigned the Aulneau Formation to the Le Moyne Group and proposed abandoning the Nachicapau Formation as partly equivalent to the Bacchus (Swampy Bay Group) and Denault (Attikamagen Group) formations. Thus, in the area north of latitude 56° 30′ N, the Doublet Group consists solely of the Murdoch Formation.
The Doublet Group is an allochthonous volcano-sedimentary unit belonging to the second cycle of the Labrador Trough (Clark and Wares, 2004). It comprises a suite of mafic volcanic rocks interstratified with lesser amounts of clastic sedimentary rocks (Harrison, 1952). The Doublet Group comprises three formations: the Murdoch Formation at the base, the Thompson Lake Formation and the Willbob Formation at the top. The Murdoch Formation consists of mafic pyroclastic rocks. In the Retty Lithotectonic Zone, these rocks are overlain by locally sulphidic rhythmites, a local oxide facies iron formation and a basalt layer belonging to the Thompson Lake Formation. The Willbob Formation consists of a thick sequence of tholeiitic basalt and interstratified sedimentary layers (Baragar, 1967; Frarey, 1967; Clark and Wares, 2004).
The Doublet Group is thought to have been deposited in a basin east of the Ferriman Group platform sequence, which was progressively deepening from west to east (Dimroth, 1981; Wardle and Bailey, 1981; Le Gallais and Lavoie, 1982; Clark and Wares, 2004). The Doublet Group is associated with a new rifting episode in the Labrador Trough. This episode began with a period of explosive volcanic activity (Murdoch Formation), followed by a hiatus (Thompson Lake Formation) and then extensional mafic volcanism (Willbob Formation) (Wardle and Bailey, 1981; Rohon et al., 1993; Skulski et al., 1993). Skulski et al. (1993) also suggested that mafic volcanism developed in pull apart ocean basins filled with a large amount of sediments. The rocks of the Doublet Group are metamorphosed to the greenschist facies west of the Lac Keato and Lac Herodier faults. They reach the lower amphibolite facies in the area SW of Deborah Lake (Dimroth and Dressler, 1978; Fraser et al., 1978; Girard, 1995). The boundary was arbitrarily placed by Girard (1995) along the Lac Du Chien Fault.
The economic potential of the Doublet Group is associated with the presence of mafic and mafic-ultramafic sills intruding the Murdoch-Thompson Lake-Willbob sequence of the Retty Lithotectonic Zone. The peridotite-gabbro sill at the Thompson Lake-Willbob contact is of great metallogenic significance, containing abundant Cu-Ni ± PGE mineralization (Clark and Wares, 2004).
Thickness and distribution
The Doublet Group belongs to the allochthonous Gerido and Retty lithotectonic zones, as defined by Clark and Wares (2004). It extends for ~375 km along the eastern margin of the Labrador Trough from the area west of Herodier Lake (sheets 24F06 and 24F07) to André Lake, in Labrador (sheet 23I12). In the southern part of the Trough, the Doublet Group extends laterally over a maximum width of 35 km and narrows towards the north. South of latitude 56°N, the Doublet Group includes the Murdoch, Thompson Lake and Willbob formations, while north of this latitude, it consists only of the Murdoch Formation.
South of latitude 56° N, the thickness of the Doublet Group is estimated at 5000 m by Wardle and Bailey (1981) and 6000 m by Baragar (1967). According to Dimroth (1978), neither the top nor the total thickness of the Willbob Formation, the top unit of the Doublet, could be determined. Consequently, the maximum thickness of the Doublet Group is not known. According to Frarey (1967), its minimum thickness is estimated at 1300 m near latitude 56°N. North of this latitude, the thickness of the Doublet Group (Murdoch Formation) is estimated at >1200 m (Frarey, 1967) and can reach up to ~2000 m (Dressler, 1979).
The Doublet Group represents the deep-water equivalent of the Ferriman Group in the central Labrador Trough. The Doublet Group was thrusted over the Ferriman Group during the Hudson Orogenesis. The base of the Doublet Group (Murdoch Formation) structurally overlies the top of the Ferriman Group (Menihek Formation) along the Lac Walsh Fault (Baragar, 1967; Frarey, 1967; Wardle and Bailey, 1981; Findlay et al., 1995). This fault represents the western limit of the Doublet Group south of latitude 56°N. However, in the vicinity of the Wheeler Dome, this fault disappears and the base of the Doublet Group conformably overlies the Menihek Formation (Wardle and Bailey, 1981). Locally, Baragar (1967) and Dimroth (1978) have described a possible conformable relationship between the Murdoch and Menihek formations. Basalts of the Willbob Formation form the upper part of the Doublet Group (Baragar, 1967; Dimroth, 1978). Near latitude 57° N, the Doublet Group is interdigitated with the Koksoak Group. The package is locally overlain by a carbonatite intrusion accompanied by volcano-sedimentary rocks belonging to the Le Moyne Group (Birkett and Clark, 1991; Machado et al., 1997; Clark and Wares, 2004). The Doublet Group is generally bounded to the east by major thrust faults, namely the Lac Hérodier, Lac Keato and Gill Lake faults, separating it from the Laporte Supersuite (Clark and Wares, 2004). The latter is possibly partly equivalent to the Doublet Group (Girard, 1995).
Based on geochronological results, the Doublet Group can be partially correlated with the Ferriman Group (Menihek Formation) in the southern part of the orogen, and with the Koksoak and Le Moyne Groups in the north-central and northern parts of the orogen (Clark, 1988; Clark and Thorpe, 1990; Rohon et al., 1993; Findlay et al., 1995; Clark and Wares, 2004). Earlier work (Dimroth, 1970; Dimroth et al., 1970) suggested that the Doublet Group was part of the third volcano-sedimentary cycle of the Labrador Trough. More recent stratigraphic interpretations have shown that it belongs to the second volcano-sedimentary cycle of the Trough and was deposited between 1.88 and 1.87 Ga (Clark and Wares, 2004). Numerous mafic-ultramafic sills (Gerido Intrusive Suite) intrude into the Doublet Group (Frarey and Duffell 1964; Baragar 1967; Clark and Wares, 2004).
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Publications Available Through SIGÉOM Examine
CLARK, T., 1978. REGION DU LAC HERODIER ( NOUVEAU-QUEBEC) – RAPPORT PRELIMINAIRE. MRN; DPV 568, 48 pages, 2 plans.
CLARK, T., 1987. STRATIGRAPHIE, PETROGRAPHIE ET PETROCHIMIE DE LA FORMATION DE FER DE BABY DANS LA REGION DU LAC HERODIER (FOSSE DU LABRADOR). MRN; ET 87-13, 44 pages.
CLARK, T., WARES, R., 2004. SYNTHESE LITHOTECTONIQUE ET METALLOGENIQUE DE L’OROGENE DU NOUVEAU-QUEBEC (FOSSE DU LABRADOR). MRNFP; MM 2004-01, 182 pages, 1 plan.
DIMROTH, E., 1972. STRATIGRAPHY OF PART OF THE CENTRAL LABRADOR TROUGH. MRN; DP 154, 304 pages, 6 plans.
DIMROTH, E., 1978. Région de la fosse du Labrador entre les latitudes 54° 30′ et 56° 30′. MRN; RG 193, 417 pages, 16 plans.
DRESSLER, B., CIESIELSKI, A., 1979. Région de la fosse du Labrador. MRN; RG 195, 136 pages, 14 plans.
GIRARD, R., 1995. GEOLOGIE DE LA REGION DU LAC DEBORAH, TERRITOIRE-DU-NOUVEAU-QUEBEC. MRN; MB 95-20, 186 pages, 3 plans.
BARAGAR, W.R.A. 1958. Ahr Lake map-area, New Quebec (23O/10). Geological Survey of Canada; Paper 57-7, 9 pages. https://doi.org/10.4095/104037
BARAGAR, W.R.A. 1960. Petrology of basaltic rocks in part of the Labrador Trough. Bulletin of the Geological Society of America; volume 71, pages 1589-1643.
BARAGAR, W.R.A. 1967. Wakuach Lake map-area, Quebec-Labrador (23O). Geological Survey of Canada; Memoir 344, 174 pages. https://doi.org/10.4095/123960
BIRKETT, T.C., CLARK, T. 1991. Géologie et potentiel métallifère de la carbonatite protérozoïque du lac LeMoyne dans le nord du Québec. Geological Survey of Canada; Work in Progress Forum, Programme and Abstracts, page 20. https://doi.org/10.4095/311251
CLARK, T., THORPE, R.I. 1990. Model lead ages from the Labrador Trough and their stratigraphic implications. In The Early Proterozoic Trans-Hudson Orogen of North America: Lithotectonic Correlations and Evolution (J.F. Lewry, J.F. and M.R. Stauffer, editors). Geological Association of Canada; Special Paper 37, pages 413-432.
DIMROTH, E.1981. Labrador Geosyncline: type example of early Proterozoic cratonic reactivation. In Precambrian Plate Tectonics (A. Kroner, editor). Developments in Precambrian Geology 4, Elsevier Scientific, Amsterdam; pages 331-352.
DIMROTH, E., DRESSLER, B. 1978. Metamorphism of the Labrador Trough. In Metamorphism in the Canadian Shield. Geological Survey of Canada; Paper 78-10, pages 215-236. https://doi.org/10.4095/104534
DONALDSON, J.A.1966. Marion Lake map-area, Quebec-Newfoundland (23I/13). Geological Survey of Canada; Memoir 338, 85 pages. https://doi.org/10.4095/123900
FAHRIG, W.F. 1952. Griffis Lake, territory of New Quebec, Quebec. Geological Survey of Canada; Paper 51-23, 1 page. https://doi.org/10.4095/108372
FAHRIG, W.F. 1964. Géologie, Griffis Lake, Nouveau-Québec. Geological Survey of Canada; Map 1121A. https://doi.org/10.4095/107788
FINDLAY, J.M., PARRISH, R.R., BIRKETT, T., WATANABE D.H. 1995. U-Pb ages from the Nimish Formation and Montagnais glomeroporphyritic gabbro of the central New Québec Orogen, Canada. Canadian Journal of Earth Sciences; volume 32, pages 1208-1220. https://doi.org/10.1139/e95-099
FRAREY, M.J. 1952. Preliminary map Willbob Lake, Quebec and Newfoundland. Geological Survey of Canada; Paper 52-16, 10 pages. https://doi.org/10.4095/124122
FRAREY, M.J. 1967. Willbob Lake and Thompson Lake map-areas, Quebec and Newfoundland (23 O/1 and 23 O/8). Geological Survey of Canada; Memoir 348, 73 pages. https://doi.org/10.4095/123896
FRAREY, M.J., DUFFELL, S. 1964. Revised stratigraphic nomenclature for the central part of the Labrador Trough. Geological Survey of Canada; Paper 64-25, 13 pages. https://doi.org/10.4095/123909
FRASER, J.A., HEYWOOD, W.W., MAZURSKI, M.A. 1978. Carte métamorphique du Bouclier Canadien. Geological Survey of Canada; Map 1475A. https://doi.org/10.4095/133909
HARRISON, J.M. 1952. The Quebec-Labrador iron belt, Quebec and Newfoundland. Geological Survey of Canada;Paper 52-20, 21 pages. https://doi.org/10.4095/123923
LE GALLAIS, C.J., LAVOIE, S. 1982. Basin evolution of the Lower Proterozoic Kaniapiskau Supergroup, central Labrador Miogeocline (Trough), Quebec. Bulletin of Canadian Petroleum Geology; volume 30, pages 150-166. https://doi.org/10.35767/gscpgbull.30.2.150
MACHADO, N., CLARK, T., DAVID, J., GOULET, N. 1997. U-Pb ages for magmatism and deformation in the New Quebec Orogen. Canadian Journal of Earth Sciences; Volume 34, pages 716-723. https://doi.org/10.1139/e17-058
ROHON, M.-L., VIALETTE, Y., CLARK, T., ROGER, G., OHNENSTETTER, D., VIDAL, P. 1993. Aphebian mafic-ultramafic magmatism in the Labrador Trough (New Quebec): its age and the nature of its mantle source. Canadian Journal of Earth Sciences; volume 30, pages 1582-1593. https://doi.org/10.1139/e93-136
WARDLE, R.J. 1979. Geology of the eastern margin of the Labrador Trough. Department of Mines and Energy, Government of Newfoundland and Labrador; Report 78-9, 22 pages. https://gis.geosurv.gov.nl.ca/geofilePDFS/WBox040/LAB_0415.pdf
WARDLE, R.J. 1982. Geology of the south-central Labrador Trough. Government of Newfoundland and Labrador, Department of Mines and Energy, Mineral Development Division, Map 82-005.
WARDLE, R.J., BAILEY, D.G. 1981. Early Proterozoic sequences in Labrador. In Proterozoic basins of Canada (F.H.A. Campbell, editor). Geological Survey of Canada; Paper 81-10, pages 331-359. https://doi.org/10.4095/124192
Ministère de l’Énergie et des Ressources naturelles (MERN). Doublet Group. Quebec Stratigraphic Lexicon. https://gq.mines.gouv.qc.ca/lexique-stratigraphique/province-de-churchill/groupe-de-doublet_en [accessed on Day Month Year].
Charles St-Hilaire, GIT, M.Sc. firstname.lastname@example.org; Thomas Clark, P. Geo., Ph.D. (redaction)
Mehdi A. Guemache, P. Geo., Ph.D. (coordination); anonymous (critical review); Simon Auclair, P. Geo., M.Sc. (editing); Céline Dupuis, P. Geo., Ph.D. (English version); André Tremblay (HTML editing).