|Author:||Bandyayera and Daoudene, in preparation|
|Age:||Precambrian / Archean / Neoarchean|
|Type area:||The most representative outcrops are located northwest of the Goulde Lake area (NTS sheet 32O11).|
|Geological province:||Superior Province|
|Geological subdivision:||Nemiscau Subprovince|
Table des matières
The Lac des Montagnes Group was introduced in the early 2000s when the Nemiscau and La Grande subprovinces were compiled. It was defined to describe a package of volcano-sedimentary rocks observed south of the Champion Complex along the Rupert River. Valiquette (1963) first described these rocks in the Lake des Montagnes area. Subdivisions and descriptions of this unit are based on the work of Valiquette (1963, 1975).
The Lac des Montagnes Group consists mainly of volcano-sedimentary rocks. It is divided into four units: 1) amphibolitized basalt and amphibolite (nAmo1); 2) intermediate volcanics (Amo2); 3) felsic volcanoclastics (nAmo3); and 4) iron formation (nAmo4). Bandyayera and Daoudene (in preparation) have joined to the Rupert Group the variably migmatized biotite-hornblende ± staurotide ± garnet paragneiss units originally attached to the Lac des Montagnes Group. In fact, these paragneiss form a homogeneous unit of strong lateral extension that has the same petrographic and structural characteristics.
This unit represents about 80% of the area of the Lac des Montagnes Group. Representative outcrops are located northwest and north of Senay Lake (NTS sheet 32O1; Valiquette, 1975).
The basalt and amphibolite derived from basalt in the nAmo1 unit are fine to medium-grained, green-grey in fresh exposure and dark green to black, locally light-grey in altered patina. They form massive to pillow flows locally associated with flow breccias. Pillows are usually elongated, stretched and flattened and are surrounded by well-preserved black hyaloclastic material. Locally, deformed quartz chambers appear to indicate stratigraphic polarity to the south. The margins of the pillows are generally richer in hornblende than the rest of the rock and show epidote and locally silica alteration. These areas may also contain 5-10% garnet, 5% biotite and up to 15% disseminated sulphides. The spaces between pillows, the centre of pillows or the massive lava flows show in places a replacement with felsic material or saussuritized nuclei (“epidote balls”). These nuclei are widespread in these basaltic rocks.
In thin section, metavolcanic rocks of the nAmo1 unit show fine to very fine grain size and a generally granonematoblastic texture. They are mainly composed of amphibole (hornblende, actinolite-tremolite), for over 50 modal%, and plagioclase (40%) associated with minor amounts of quartz and biotite. Hornblende is partially replaced by chlorite and epidote. These mafic rocks metamorphosed to the upper greenschist and amphibolite facies also contain sphene, sulphides, calcite and sericite as accessory or alteration minerals.
Unit nAmo1 contains in places decametric horizons of massive to pillow ultramafic lava. The tremolite-rich foliated rock is magnetic, fine to medium-grained and is dark grey to brownish in altered surface and medium to dark greenish grey in fresh exposure. Ultramafic horizons are characterized by tourmaline alteration combined with enrichment in tremolite-actinolite. Tourmaline (5-15%) is in the form of clusters, or isolated crystals up to 3 cm, aligned with foliation.
Lithogeochemical analyses show that the nAmo1 unit is composed of tholeitic basalt and locally andesitic basalt of transitional to calc-alkaline affinity. In the Mezière and Nemiscau lakes region (sheets 32N06 and 32N07) to the west, most tholeitic basalts, especially magnesian and komatiitic basalts, are characterized by heavy rare earth enrichment compared to light rare earths. A number of trace element ratios clearly show a geotectonic setting environment for these rocks similar to that of N-MORB (Pearce, 2008). Further east, in the Lake de la Sicotière area (sheet 32N08), tholeitic basalts are instead characterized by generally flat rare earth profile. Their ratios of trace elements suggest a geodynamic environment similar to that of island-arc tholeiite, or ocean floor basalt contaminated by continental crust (Pearce, 2008; Wood, 1983).
Lac des Montagnes Group 1a (nAmo1a): Metatexite Derived From Amphibolite, Containing 20 to 50% Mobilisate
Subunit nAmo1a consists of metatexite from partial fusion of a banded amphibolite of probable volcanic origin. The non melted rock is fine to medium-grained, granoblastic, showing a flat profile of chondrite-normalized rare earths. Orthopyroxene, clinopyroxene and garnet are common in melted amphibolite and mobilisate. The subunit contains subangular centimetric rafts of banded amphibolite in the leucosome. This arrangement locally defines an agmatitic texture.
The intermediate volcanic unit is the least known of the Lac des Montagnes Group. These rocks are generally described in outcrops as amphibolitized basalts. Therefore, they were renamed based on lithogeochemical analysis results. Intermediate volcanics are dark grey in altered surface, medium grey to greenish grey in fresh exposure. The rock is fine to medium-grained, homogeneous, foliated and locally magnetic. Locally observed structures are interpreted as stretched and flattened isolated pillows, amphibolitized flow margins or pillow breccias. The rock contains 60% hornblende 0.5 to 5 mm and 1 to 2% magnetite 0.2 to 0.5 mm. In places, silicified and garnet-rich (up to 30%) horizons are visible. Chemically, these rocks are calc-alkaline affinity andesites, richer in SiO2, TiO2 and light rare earth elements than basalts of the nAmo1 unit.
Felsic volcanic rocks make up a small proportion of the Lac des Montagnes Group. Unit nAmo3 consists of felsic volcanoclastics that form decametric to hectometric horizons, interstratified in the basaltic sequence of the nAmo1 unit. The best preserved sequences are bedded felsic tuffs characterized by alternating centimetric to metric beds of lapilli tuffs, fine-grained massive ash tuffs and conchoidal-fractured fine-grained tuffs. The rock is whitish in altered surface and greenish grey in fresh exposure. The matrix is sericitized and chloritized.
In thin section, felsic tuffs are granoblastic and mainly composed of quartz and plagioclase grains included in a fine-grained recrystallized matrix composed of chlorite, sericite, epidote, quartz and plagioclase. Epidote, apatite, sphene and sulphides are present as accessory minerals. Lithogeochemical analyses show that the felsic tuff of unit nAmo3 is of rhyodactic composition and calc-alkaline affinity.
The nAmo4 unit consists of several horizons of iron formation less than 150 m thick interstratified in the metabasalt unit (nAmo1). It is clearly visible on aeromagnetic maps. Iron formations at the silicate and oxide facies are massive to banded. They are composed of ribbons and laminae of magnetite that alternate with amphibole-rich magnetic bands and magnetite paragneiss bands. Massive magnetite bands are black to blue grey and are at places of metric thickness. Amphibole-rich bands are greenish grey and are very limited in extent. Beige-grey magnetite paragneiss makes up about one third of the total thickness of the iron formation. The entire sequence is affected by numerous tight isoclinal folds that have axial planes parallel to regional schistosity.
Horizons of silicate iron formation were also observed in volcano-sedimentary rock outliers of the Champion Complex and were linked to the Lac des Montagnes Group. In thin section, the rock contains up to 75% cummingtonite and 15% garnet poekiloblasts.
The Lac des Montagnes Group extends EW to NE over a distance of more than 150 km, and its width rarely exceeds 1 km.
A sample of felsic volcanoclastics belonging to the nAmo3 unit gave a crystallization age of 2712.6 ±5 Ma. The statistical modelling of the geochronological dataset of this sample indicates three modes, at 2706.6 ±5.1 Ma, 2755 ±7.7 Ma and 2810 ±7.6 Ma. The youngest age corresponds, taking errors into account, with the crystallization age of 2712.6 ±5 Ma. The other two ages correspond to older volcanic episodes.
|Isotopic System||Mineral||Crystallization Age (Ma)||(+)||(-)||Reference(s)|
|U-Pb||Zircon||2712.6||5||5||David et al., personal communication|
The Lac des Montagnes Group consists mainly of volcano-sedimentary rocks located south of the Rivière Rupert Shear Zone, between the Champion Complex (La Grande Subprovince) to the north, and migmatized rocks of the Nemiscau Subprovince to the south. The volcano-sedimentary rock belt located south of the Rivière Nemiscau Shear Zone has also been joined to the Lac des Montagnes Group because it shows the same lithological characteristics and associations. The contact between the Lac des Montagnes Group and the Rupert Complex is generally sheared off, but may represent an old discordance. The Rupert Group contains, in places, variably migmatized horizons of volcanic amphibolite that are likely from the Lac des Montagnes Group.
Does not apply.