The Allegrin Plutonic Suite was defined by Moukhsil and El Bourki (2020) during geological mapping work in the Normandin-Roberval-Saint-Félicien area of the Saguenay-Lac-Saint-Jean region (sheets 32A09, 32A10, 32A15 and 32A16). It is highly magnetic and easily identifiable on aeromagnetic maps (Intissar and Benahmed, 2015) in the form of two distinct batholiths.

The Allegrin Plutonic Suite corresponds to two large polyphase batholiths consisting of quartz syenite, alkali feldspar granite, hypersthene granite (charnockite) and gabbronorite. Granitic gneiss also occurs along the batholiths’ edges.

Quartz syenite is pink-grey in fresh exposure and grey in altered patina, coarse grained and foliated. The rock contains quartz in large zones, plagioclase (sericitized), K-feldspar (microcline), green hornblende in foliation-oriented clusters, opaque minerals (magnetite) and accessory minerals such as apatite and zircon. Centimetric enclaves of fine to medium-grained blackish pyroxene-garnet gabbronorite are present. Charnockite is white-grey in fresh exposure, rust brown in altered patina, fine to medium grained and granoblastic. It is composed of quartz, orthopyroxene, K-feldspar, plagioclase and biotite. Locally, charnockite displays a mylonitic foliation and isoclinal folds of NNW-SSE orientation parallel to regional foliation.

Alkali feldspar granite is pink in fresh exposure, pink-grey in altered patina, foliated to gneissic and perthitic K-feldspar porphyroclastic. When deformed, the rock is fine to medium grained. In thin section, the rock contains quartz as subgrains, green hornblende, biotite in variable amounts, opaque minerals (magnetite) and accessory minerals (apatite and zircon). Biotite and green hornblende occur mainly as grain clusters.

Gabbronorite is grey-black in fresh and altered surfaces, coarse grained, locally plagioclase porphyritic and subophitic. In thin sections, when the rock is deformed, plagioclase locally has numerous narrow polysynthetic twins. These are more or less curved and pinching out towards mineral edges. Gabbronorite also contains large crystals of green hornblende, orthopyroxene, clinopyroxene, magnetite and a smaller amount of apatite. Hornblende, more abundant than pyroxene, is commonly associated with orthopyroxene (uralitization). In the field, gabbronorite also occurs as centimetric to metric boudins. It is also intruded by granitic and syenitic pegmatite dykes.

Granitic gneiss is pink-grey in fresh exposure and pink in altered patina. The rock is medium grained and composed of quartz, K-feldspar, plagioclase, biotite, hornblende, and accessory minerals such as apatite and zircon. This gneiss is mainly observed at the edge of the suite and cut by pink granitic veins and veinlets.

Thickness and Distribution

The Allegrin Plutonic Suite outcrops in the SW and NW corners of sheet 32A15. It has also been mapped in the NW corner of sheet 32A10. Interpretation of aeromagnetic maps allows it to be extended northwards (sheet 32H02) and westwards (sheets 32A11 and 32A14). Aeromagnetic maps also make it possible to evaluate its distribution further south in two scattered batholiths, one of 460 km² located further south (sheets 32A10, 32A11, 32A14 and 32A15), and the other of 64 km² located further north (sheets 32A14, 32A15 and 32H02).

Dating

None.