The Chenaux Intrusive Suite was introduced by Bilodeau (2022) following a geological survey carried out in the Île-du-Grand-Calumet area (sheet 31F10) in 2021. The unit includes the « Chenaux Gabbro », a pluton named by the Ontario Geological Survey (Lumbers, 1982). This name was used in subsequent geological surveys (Abdurahman, 1989; Azar and Easton, 2015). Part of the unit was described in the area of the Bryson Municipality, in Québec, following various geological mapping campaigns carried out by the Ministère (Osborne, 1944; Katz, 1976; Madore et al., 1994).

Chenaux Intrusive Suite 1 (mPchn1): Subophitic and mesocratic gabbro, layers of fine-grained melanocratic gabbro

Unit mPchn1 is a gabbro pluton located in the Bryson municipality. It consists mainly of fine- to medium-grained, massive and homogeneous mesogabbro. The edges of the intrusion locally contain a subfacies of fine-grained melanocratic metagabbro. Certain contacts of the intrusion are affected by deformation, giving this subfacies the appearance of banded amphibolite. Unit mPchn1 is locally intruded by dykes and sills of coarse-grained hololeucocratic granite, tonalite and quartz diorite, particularly at the edge of the intrusion.

Main Mesocratic Facies

The mesocratic facies typically has a dark to very dark grey alteration patina and a very dark grey to greenish-grey fresh surface. This facies is composed of 40% to 50% unaltered euhedral plagioclase without preferential orientation and ~50% clinopyroxene with preserved cores and uralitized rims. Orthopyroxene crystals were observed in very small proportions in a few samples. Ferromagnesian minerals fill the space between the plagioclase crystals, giving this facies a characteristic subophitic texture. A later generation of euhedral hornblende of medium and locally coarse grain size is observed. Accessory minerals include garnet, orthopyroxene, olivine, titanite, apatite, magnetite, ilmenite, spinel and sulphides (Madore et al., 1994). The latter represent up to 2% of the volume of the rock in the form of small pyrite clusters as well as pyrite and pyrrhotite veinlets or isolated cubic crystals.

Melanocratic Subfacies

The melanocratic parts of the intrusion’s edge are black on the weathered and fresh surfaces. The plagioclase is locally recrystallized, slightly damouritized and present as small clusters of heterogranular crystals forming triple junction between them. The light-coloured altered minerals are beige. This facies is composed of 40% to 50% fine granoblastic hornblende homogeneously filling the interstices of plagioclase crystals. Up to 5% unaltered clinopyroxene and 3% unaltered orthopyroxene are found in fairly homogeneously distributed crystals associated with hornblende as well as up to 12% epidote in the intrusion border zones. This facies generally has a lower magnetic susceptibility than the mesocratic facies.

Unit mPchn2 represents a second major gabbro pluton in the Île-du-Grand-Calumet area, almost all of which outcrops in Ontario. Works by Azar and Easton (2015) resulted in the subdivision of the unit into four distinct facies: 1) a main facies of alkaline metagabbro, 2) a group of olivine leucogabbro, leucotroctolite and gabbronorite, 3) a highly magnetic oxides-bearing mesogabbro, locally brecciated or pegmatitic, and 4) a facies of gabbro-derived hornblendite and magnetic amphibolite.

The small part of the intrusion located in Québec consists of a massive, homogeneous, subophitic, mesocratic gabbro, probably corresponding to the oxides-bearing mesogabbro facies defined by Azar and Easton (2015). The magnetic susceptibility of the samples varies from very weak to moderate. The rock is generally fine to medium grained. However, in places it contains partings in sharp or diffuse contacts with a gabbro of similar composition, fine-grained and melanocratic in appearance. Alternating centimetric to decimetric layers of these two facies have been interpreted as magmatic bedding preserved in the part of the intrusion located in Ontario (Azar and Easton, 2015). The alteration patina of the mesocratic facies is very dark grey to very dark brownish-grey and the fresh surface is very dark grey. The finer-grained parts are very dark grey to black on the fresh and weathered surfaces. The rock is composed of 50% euhedral plagioclase in large laths and stocky crystals containing slightly deformed twins The plagioclase is generally unaltered, but displays slight saussuritization associated with microfractures. The gabbro is composed of a maximum of 50% clinopyroxene slightly uralitized into homogeneously distributed medium-grained amphibole. Some samples contain 40% to 50% clinopyroxene relicts completely replaced by fine to medium hornblende. The rock contains 2% to 6% of fine to medium, homogeneously distributed iron oxides and sulphides, generally associated with ferromagnesian minerals. Centimetric clusters of pyrite and marcasite have been observed in outcrops punctually. Accessory minerals include magnetite, ilmenite, olivine, serpentine, titanite and apatite.

Thickness and Distribution

The Chenaux Intrusive Suite comprises three plutons mapped in the Île-du-Grand-Calumet area. The intrusion in the vicinity of the Bryson Municipality (mPchn1) is approximately 10 km long by 7 km wide; the intrusion in the Portage-du-Fort area (mPchn1a) is approximately 7 km long by 4 km wide; the part of the intrusion located in Quebec (mPchn2) is only 2.5 km long by 0.5 km wide. Most of this pluton outcrops in Ontario. These three gabbro intrusions are the only known Chenaux units in the Québec part of the Grenville.

Dating

A sample of leucogabbro from unit mPchn2 was dated using the U-Pb method (Pehrsson et al., 1996). The zircons returned a crystallization age of 1231 ±2 Ma.