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 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).

Dating

A Pb-Pb age of 1885 ±67 Ma was obtained for basalts of the Willbob Formation (pPwl1), the upper part of the Doublet Group (Rohon et al., 1993).