The Rouvray Gneiss Complex (mProu) was named by Hébert and Cadieux (2003) when mapping the area of the Portneuf and Maria-Chapdelaine Lakes (sheets 22E01 and 22E02). This refers to the continuity of the gneissic complex described by Kehlenbeck (1977) in the Rouvray Lake area (sheet 22E07). It was subsequently extended to the orthogneissic rocks and banded gneisses of the northern part of the Lac-Saint-Jean Anorthositic Suite by Hébert et al. (2009) as part of their synthesis of the Pipmuacan Reservoir area (sheet 22E). They subdivided this complex into three units: mProu1, mProu2 and mProu2a. However, the latter subunit, consisting of two small lenses of upright gneiss at the eastern border of the region, near the Pipmuacan Deformation Zone (sheet 22E16), was assigned to the Hulot Complex (mPulo1) by Gobeil et al. (2002).

Rouvray Gneissic Complex 1 (mProu1): Granulitic orthogneiss (charnockite, mangerite) with dioritic and gabbronorite components, granitic gneiss, quartzofeldspathic gneiss; quartzite, paragneiss and amphibolite klippes

Unit mProu1 consists of granulitic orthogneiss (charnockite and mangerite), diorite, granitic gneiss and quartzofeldspathic gneiss. There are also klippes belonging to the Saint-Yves Supracrustal Sequence as well as levels of amphibolitized hypersthene diorite, gabbronorite, gabbro and thin levels of anorthosite. The main levels of hypersthene diorite and gabbronorite are concentrated between the Rouvray and Bergeron Lakes (sheets 22E07 and 22E02).

Granulitic orthogneisses are a white to very pale pink colour in altered surface. In fresh surface, syeno-granitic orthogneiss is pink, charnockitic orthogneiss is beige, while orthogneiss of mangeritic composition is green. The thickness of the orthogneiss levels varies from centimetres to metres. The most common facies (>85%) are mangeritic and charnockitic gneiss. These rocks are generally gneissic, but locally banded. In many places these rocks have retained their igneous texture and in some cases, it is possible to recognize the rapakivic texture. The average composition is 30% quartz, 25% potassium feldspar, 40% plagioclase (oligoclase) and 5% an almost equal composition of hornblende, biotite, orthopyroxene and clinopyroxene. When the feldspar is recrystallized, it forms crystals that give the rock a porphyroclastic texture.

The diorite is dark green to black in alteration patina, fine to coarse grained, massive and locally ophitic in structure. In the field, it seems to be comagmatic with the granulitic orthogneiss and form levels that are often boudinaged. This lithology is in positive relief in relation to the hosting gneiss. The diorite contains orthopyroxene, generally retrograded into hornblende and biotite. The exclusive association of these diorites within facies mProu1 indicates that they are older rocks than those of the adjacent anorthositic suites. They may represent swarms of dykes that were folded and dismembered after their emplacement. These diorites have also been observed in the Cap à l’Est Complex (Hébert and Lacoste, 1998b, c, d) in the south of the region, and in the Hulot Complex in the adjacent sheet to the NE (Gobeil et al., 2003). However, these authors used the general term ‘amphibolite’ to refer to these rocks.

Granitic gneiss is pink in alteration patina, very fine to medium grained and homogeneous. It contains >15% biotite and 50% potassium feldspar. It forms discontinuous lenses up to 10 m thick.

Quartzofeldspathic gneiss is light grey in alteration patina, medium to fine grained and strongly laminated. It generally contains leucosomes and melanosomes, and is commonly associated with granitic gneiss.

Gabbronorite is dark grey, fine to coarse grained, well foliated and recrystallized. It usually shows a coronitic texture where orthopyroxene cores are surrounded by almandine garnet. Very thin metric levels of anorthosite are observed and generally accompany the gabbronorite. This rock consists of pink labradorite-type plagioclase, suggesting that the gabbronorite and anorthosite levels identified in this area may be associated with the Vanel Anorthosite. Gabbro levels with and without olivine have been mapped within unit mProu1 by Kehlenbeck (1977), in the area surrounding Rouvray Lake.

Only two outcrops of supracrustal rocks (Saint-Yves Supracrustal Sequence) were observed north of Maria-Chapdelaine Lake (sheet 22E02). In both cases, they are in the form of large enclaves a few dozen metres long and up to 15 m thick. These enclaves are composed of quartzite, garnet paragneiss and amphibolite.

The quartzite is massive and in layers ~2 to 3 m thick. Two facies are observed: a first one white and homogeneous, and a second one with alternating white and pink-red laminae. The colour of the pink-red laminae is attributable to the presence of garnet in variable proportions. In thin sections, small ovoid structures can be observed containing completely recrystallized feldspar, which is in very clear contact with the surrounding quartz. It resembles small grains of sand. A little muscovite is also noted.

The grey-coloured paragneiss is massive, while the rust-coloured paragneiss is strongly foliated and friable. The grain size varies from fine to medium. Garnet, which is lilac-coloured and medium to fine grained, makes up 15-20% of the rock. The proportion of quartz exceeds 25%, that of plagioclase is ~50%, and the rest of the rock is composed of biotite with some chlorite in places.

Amphibolite is black and fine grained. It contains >50% amphibole. Biotite (25-30%) is the other abundant ferromagnesian mineral, whereas the rest of the rock consists of plagioclase.

Unit mProu2 consists of greyish gneiss in both alteration patina and fresh surface, more or less banded, of granodioritic to tonalitic composition as well as amphibolite and granitic gneiss. There are rare klippes of hornblende gneiss and supracrustal rocks (garnet and sillimanite gneiss, quartzite and amphibolite) probably belonging to the Saint-Yves Supracrustal Sequence (Hébert et al., 1999c; Hébert, 2002). Gabbro levels are also found in the Hirondelles Bay area (Hocq, 1978; sheet 22E07). This unit includes the biotite augen gneiss described by Kehlenbeck (1977).

Banded gneiss is medium to fine grained and well foliated. Typical outcrops are pink in fresh surface, but alter to buff or light grey in places. Grey layers, in lesser proportions, outcrop abundantly throughout the unit. Darker levels of hornblende of lesser importance are generally present. Fusiform aggregates of feldspar, small veinlets of ferromagnesian minerals and elongated grains of quartz give the rock a banded aspect. Under the microscope, the rock shows a very marked banding similar to that observed in the sample. The quartz is present in long, highly deformed crystals with undulose extinction. The pink layers are made up of small microcline crystals and large microperthite crystals. In the grey layers, weakly twinned plagioclase (oligoclase) is the dominant feldspar. Microcline is rare and the potassium feldspar is usually microperthite or homogeneous monoclinic feldspar. The ferromagnesian minerals, green hornblende and brown biotite, usually occur together. In some samples, pennine, orthopyroxene and clinopyroxene are associated with hornblende and biotite. Accessory minerals include apatite, titanite, zircon and magnetite. Modal analyses of typical pink layers show on average 35% quartz, 35% potassium feldspar, 25% oligoclase and 5% hornblende and biotite. Representative samples from grey layers contain 35% quartz, 15% potassium feldspar, 45% oligoclase and 5% biotite and hornblende.

Outcrops of augen gneiss are present in the SW corner of sheet 22E07 and extend northward along the eastern shore of the Pipmuacan Reservoir. They are typically well foliated with pink feldspar eyes surrounded by quartz and elongated dark minerals. Under the microscope, augen gneiss has a characteristic appearance with large mesoperthite porphyroblasts and highly deformed quartz lenses intercalated in a matrix of microcline, oligoclase and fine-grained quartz. Small grains of weakly twinned oligoclase occur in lenticular aggregates partially enveloping potassium feldspar porphyroblasts. The porphyroblasts are usually recrystallized into ovoid aggregates of numerous small feldspar grains. Ferromagnesian minerals and elongated quartz always surround these aggregates. The main ferromagnesian minerals, green-brown hornblende and brown biotite, are usually associated. Pennine, with its characteristic Berlin blue hue in cross nicols prisms, is the only ferromagnesian mineral in two thin sections. No pyroxene is visible in the augen gneiss samples examined. These rocks contain on average 23% quartz, 60% potassium feldspar, 12% oligoclase and 5% hornblende and biotite.

Thickness and distribution

Unit mProu1 occupies the Rouvray Lake area (centre of sheet 22E07) and a significant portion of the central part further north (sheet 22E02). It also covers a good part of sheets 22E12 to 22E15 and 22L01. Unit mProu2 is found in the central northern and NE parts and in the vicinity of Pamouscachiou Lake (sheet 22E07) and to the NW (sheets 22E06 and 22E11). It also occupies most of sheet 22E16 and extends into the adjacent sheets (sheets 22L01, 22E09, and 22E11 to 22E15).

Dating

An age of 1484 ±30 Ma was obtained from a sample of mangerite orthogneiss in the central part of the Pipmuacan Reservoir area (sheet 22E07; van Breemen, 2009). In the adjacent NE sector, Gobeil et al. (2002) obtained a comparable age of 1434 +64/-28 Ma from a tonalitic gneiss (enderbite) of the Hulot Gneissic Complex. Hébert et al. (2009) believe that this gneiss is equivalent to that observed in the mProu2 unit of the Rouvray Gneissic Complex.