|Reference section:||The construction of Road 167 west of Cummings Lake in the early 2000s revealed a 1.5 km continuous outcrop section (UTM NAD83, zone 18 : 555395 mE, 5533658 mN). Mapping of this section refined the stratigraphy of the Cummings Intrusive Suite. Available in Bédard et al. (2009).|
|Type area:||Chibougamau area (NTS sheet 32G16)|
|Geological province:||Superior Province|
|Geological subdivision:||Abitibi Subprovince|
|Lithology:||Mafic to ultramafic intrusive rocks|
Duquette (1970) was the first to mention the Cummings Complex in its work. The Cummings Complex stratigraphy was established from underground work on the Opémiska property (McMillan, 1972), during surface mapping by Duquette (1982) and drilling in the Barlow Lake area (Poitras, 1984). Daigneault and Allard (1990) suggest modifying the original Cummings Complex name for Cummings Sills to meet the standards of the North American Stratigraphic Code (DV 86-02, sections 31 to 41). Leclerc et al. (2017) rather propose to change the name for the Cummings Intrusive Suite to include the following three formal lithodemes: (a) Roberge Sill, (b) Ventures Sill, and (c) Bourbeau Sill.
The Cummings Intrusive Suite consists of three differentiated sills that always appear in the same stratigraphic order (Duquette, 1982; Poitras, 1984; Bédard et al., 2009):
The Roberge Sill (nArob) includes a pyroxenitic zone with a gabbro chilled margin, a rhythmic zone composed of dunite, werhlite and clinopyroxenite with sheared, mylonitized and serpentinized contacts and a sommital dunitic zone in direct contact with the host rock, without showing a chilled margin.
The Ventures Sill (nAven) includes a zone of dark green clinopyroxenite with mesocumulate-adcumulate texture, a zone of foliated gabbro and ophitic gabbro, and a granophyre zone. Granophyre is characterized by a dominant myrmekitic texture with a dioritic border in the first few metres.
The Bourbeau Sill (nAbou) consists of a thin layer of basal bronzite-clinopyroxene pyroxenite, overlaid by massive subophitic leucogabbro and a large amount of ferrodiorite and quartz ferrogabbro.
The Roberge (up to 500 m thick), Ventures (250 m to 1000 m thick) and Bourbeau (450 m to 750 m thick) sills were recognized on both sides of the Chibougamau Anticline, over a distance of more than 85 km.
A sample of quartz ferrodiorite from the Bourbeau Sill was dated 2716.7 +1.0/-0.4 Ma (Mortensen, 1993).
|Isotopic System||Mineral||Crystallization Age (Ma)||(+)||(-)||Reference(s)|
Rocks of the Cummings Intrusive Suite cut volcanic rocks of the Roy Group’s second volcano-sedimentary cycle (Duquette, 1970, 1982).
Does not apply.