The Saint-Méthode Plutonic Suite was defined by Moukhsil and El Bourki (2020) during mapping work in the Normandin-Roberval-Saint-Félicien area of the Saguenay-Lac-Saint-Jean region (sheets 32A09, 32A10, 32A15 and 32A16). Previously, Morfin et al. (2015) had not individualized this suite on their geological map of sheets 32A09 and 32A10, although it is very magnetic and easily identifiable on aeromagnetic maps (Intissar and Benahmed, 2015).

The Saint-Méthode Plutonic Suite corresponds to a large polyphase batholith consisting mainly of alkali feldspar granite, mangerite and quartz syenite. It also includes minor amounts of gabbronorite, hypersthene syenite and paragneiss enclaves of the Barrois Complex.

Alkali feldspar granite is foliated and salmon-pink in fresh and altered surfaces. It is medium to coarse grained and is composed of microcline, large zones of quartz with undulatory extinction, locally chloritized biotite, titanite, and lesser amounts of orthopyroxene and accessory minerals (zircon, epidote, apatite). Some outcrops of this facies are K-feldspar porphyritic (phenocrystals 1-2 cm thick). In thin section, alkali feldspar granite locally contains millimetric myrmekite grains, titanite with opaque mineral cores (magnetite), sericitized plagioclase crystals and some perthitic K-feldspar. In general, the granite’s texture is magmatic; however, at the edge of the intrusion, granite is deformed and exhibits crumbled and stretched quartz grains.

Mangerite is whitish to greyish in altered patina and pinkish to greenish in fresh exposure. It is composed of quartz forming large quartz zones with undulatory extinction, in places myrmekitic, sericitized plagioclase, K-feldspar having a mesh texture (microcline), ferromagnesian minerals (orthopyroxene, clinopyroxene, green hornblende and biotite) whose arrangement in elongated clusters marks foliation, opaque minerals (magnetite and pyrrhotite), and accessory minerals such as apatite, zircon and titanite. It is mainly K-feldspar porphyritic, and although its texture is generally magmatic, several outcrops display significant deformation at faults and shear zones.

Quartz syenite is greyish or salmon-pink in fresh and altered surfaces (outcrop 19-CD-10100), and coarse grained. It is K-fledspar porphyritic. It is perthitic and displays Carlsbad twinning locally. It is also composed of quartz forming large zones with undulatory extinction, locally myrmekitic, and smaller amounts of plagioclase (sericitized), ferromagnesian minerals (green hornblende and biotite), opaque minerals (magnetite), and accessory minerals such as apatite and zircon. Quartz syenite contains centimetric enclaves of fine-grained gabbronorite. Hypersthene syenite has the same composition as quartz syenite, but contains 5-8% orthopyroxene. In the field, a few outcrops of syenite display a coronitic texture with hypersthene in the centre and clinopyroxene at the edge, the latter being in turn lined with magnetite.

Gabbronorite outcrops at several points of the intrusion. It is black in fresh exposure and rusty black in altered patina. The rock is fine-grained, locally plagioclase porphyritic and foliated. It is composed of orthopyroxene, clinopyroxene, amphibole, plagioclase, biotite and magnetite. Gabbronorite also contains K-feldspar xenocrystals from the syenite of the same unit; outcrop 19-DJ-8136 displays magmatic mixture and diffuse contact between the two facies. This also indicates that these facies are probably cogenetic. In places, injections of the syenitic facies into gabbronorite are also observed.

Thickness and Distribution

The Saint-Méthode Plutonic Suite consists of a polyphase batholith which is ~32 km long and ~19 km wide (608 km²). It outcrops mainly in sheets 32A09 and 32A10. The interpretation of aeromagnetic maps of the area (Intissar and Benahmed, 2015) has allowed for extending the unit to the north (sheets 32A15 and 32A16).

Datation

The emplacement age of the suite, i.e. 1021 ±15 Ma, obtained on a mangerite sample (outcrop 2019-AM-0170), coincides with the magmatic event of the middle Grenville (1050-1018 Ma; Gower and Krogh, 2002), which corresponds to a thickening of the crust during the Grenvillian collisional period (e.g. Moukhsil et al., 2017).

Dating

None.