The rocks of this unit were first recognized by Retty (1937) during mapping in the Dyke and Astray lakes area (sheets 23J09 and 23J10). Fahrig (1949) (see Labrador and Mining Exploration Company, 1949, in Frarey and Duffell, 1964) used the term « Nimish volcanics » to describe a thick sequence of volcanic and pyroclastic rocks interbedded with slate and iron formation units in the Astray Lake and Birch Lake areas (sheet 23I04). Nimish volcanic rocks were variously referred to as the « Nimish Formation » (Melihercsik, 1952); « Nimish volcanic rocks » (Kavanagh, 1952; Perrault, 1952, 1955; Stevenson, 1952); and « Nimish Group » (Sauvé, 1953; Usher, 1953). Frarey and Duffell (1964) subsequently included these volcanic rocks as informal subdivisions of the Wishart and Sokoman formations of the Knob Lake Group.

In his mapping work in the Astray and Dykes lakes area (sheet 23J09) on the Labrador side, Evans (1978) provided a detailed description of the stratigraphy, petrology and geochemistry of Nimish volcanic rocks. He demonstrated that they occur at two stratigraphic levels below and above the Sokoman Formation, respectively. Evans (1978) divided them into two informal units and proposed the term Petitsikapau Lake formation for rocks between the Wishart and Sokoman formations and the term Astray Lake formation for those between the Sokoman and Menihek formations. The Petitsikapau Lake and Astray Lake formations were grouped together as the Nimish Subgroup to maintain unity as a subdivision of the Knob Lake Group. The use of the term « subgroup » is, however, inconsistent with the division of the Knob Lake Group into subgroups (Seward, Pistolet, Swampy Bay, Attikamagen and Ferriman) (Dimroth, 1978), the « Nimish » being part of the « Ferriman Subgroup ». Dimroth (1978) does not assign a specific rank to Nimish rocks, but rather describes them as accessory volcanic rocks occurring in the Wishart and Ruth formations in the Schefferville area. In his compilation map, Wardle (1982) reclassified the Nimish Subgroup as a formation and extended the unit into the Schefferville area beyond the area mapped by Evans (1978). Clark and Wares (2004) subsequently reclassified all Dimroth (1978) subgroups as groups and the Knob Lake Group was abandoned.

According to the North American Stratigraphic Code (NACSN, 1983, 2005), a formal stratigraphic unit must be bounded by a single lower and upper surface, so that its name is not repeated in the stratigraphic column. Here, because the « Nimish » represents a diverse package of volcanic and associated intrusive rocks (see Description section), the unit is referred to as the Nimish Volcanic Complex. This designation is consistent with the use of the term « volcanic complex » as formulated by the North American Stratigraphic Code (NACSN, 1983, 2005).

In the Schefferville area, the Nimish Volcanic Complex consists mainly of tuff and tuffaceous sandstone and siltstone commonly associated with massive or pillow basalt flows (Gross, 1968; Zajac, 1974; Dimroth et al., 1970; Dimroth, 1978). Tuff and tuffaceous rocks occur in beds several centimetres to several metres thick and locally display well-developed cross bedding. These rocks are dark olive green and composed of rounded, amygdaloidal, greenish grey volcanic fragments hosted in a chloritic matrix. The size of the fragments varies from <1 mm to 1 cm. Some fragments of black shale, dark grey chert and iron formation are also observed (Zajac, 1974). K-feldspar is a common constituent of the Nimish volcanics, which may contain up to 20-30% K-feldspar in the Schefferville area (Zajac, 1974). Volcanic fragments contain abundant K-feldspar as elongated crystals in a chloritized matrix. K-feldspar is usually white to colourless and very finely granular. According to Zajac (1974), adularia (orthoclase) is probably the most abundant feldspar. In the Schefferville area, basalt flows are mostly pillowed. The rock is greenish grey, fine grained, amygdaloidal and composed mainly of plagioclase and a minor amount of augite in a matrix of chlorite, sericite, sphene, leucoxene and carbonate. Basalt also contains ilmenite and locally pyrite or magnetite (Zajac, 1974).

Geochemically, the Nimish volcanic rocks are also characterized by high potassium content and high light rare earth enrichment (Evans, 1978; Findlay et al., 1995; Watanabe, 1998). In the Schefferville area, values of 3.88%, 5.94% and 7.86% K₂O were obtained by Dimroth (1978) for Nimish tuffs.

In Labrador, the rocks of the Nimish Volcanic Complex were described mainly by Gross (1968), Zajac (1974), Wardle (1979, 1982), Wardle and Bailey (1981), Findlay et al. (1995) and Watanabe (1998). They consist mainly of mafic volcanic rocks and locally include minor amounts of felsic volcanic rocks of alkaline affinity (rhyolite, trachyte, rhyodacite), conglomerate and breccia (flow breccia, pillow breccia). According to Evans (1978), felsic volcanic rocks are spatially associated with local volcanic centres in the Dyke Lake area. Conglomerates were interpreted as alluvial conglomerates (fanglomerates) surrounding volcanic centres. In the vicinity of Jasper Mountain, in the Dyke Lake area, a conglomerate layer contains well-rounded pebbles of quartz syenite, basalt, rhyolite and iron formation. According to Findlay et al. (1995), fragments that make up conglomerate are of local origin. Volcanic fragments are thought to be from the Nimish Volcanic Complex whereas iron formation fragments are thought to be derived from the Sokoman Formation. Although no syenite intrusions were recognized to date in the Dyke Lake area, possibly due to the lack of outcrop, syenite pebbles are also considered to be of local origin. Syenite is interpreted as the intrusive equivalent of Nimish felsic volcanics (Findlay et al., 1995; Watanabe, 1998).

The Nimish Volcanic Complex is locally the result of extensional magmatism near the margin of the Superior Province in the southern part of the Trough (Watanabe, 1998). According to Sauvé (1953) and Evans (1978), volcanic rocks were derived from the erosion of volcanic islands in the Dyke Lake area and, further SE, in Point Lake (sheet 23J08). All Nimish rocks were metamorphosed to the greenschist facies or lower (Evans, 1978).

Thickness and distribution

The Nimish Volcanic Complex belongs to the allochthonous Schefferville Lithotectonic Zone as defined by Clark and Wares (2004). This unit is not very common in Quebec. It is found in the area east of Schefferville, where it forms a thin strip extending SE for ~8 km east of Dolly and Barry lakes (sheet 23J15). It is also present east of Houston Lake, SE of Schefferville (sheet 23J10). On the Labrador side, Nimish volcanic rocks extend SE of the Schefferville area into the Giasson and Sims lakes area (sheet 23I04). They are also present in the area between Dyke and Snelgrove lakes (sheets 23I12 and 23J09) (Gross, 1968; Evans, 1978; Wardle, 1979, 1982). The maximum thickness of the complex is estimated to be 1700 m (Evans, 1978).

Dating

A quartz syenite pebble from a conglomerate interbedded in Nimish felsic volcanic rocks was dated 1878 ±1 Ma (Findlay et al., 1995). An approximate age of 1880 Ma was previously obtained from zircons (Tyson C. Birkett, pers. comm., 1989, in Clark, 1994, p. 50).