The Casterne Pluton is a formal unit of the Féron Suite established by Côté-Roberge and Goutier (2019). The name refers to Casterne Lake, located less than 5 km from the type outcrop used for isotopic dating (2018-MY-6315).

The composition of the Casterne Pluton is intermediate to felsic. The most commonly observed lithology is a porphyritic quartz monzodiorite. There are also smaller amounts of monzonite, quartz monzonite, monzodiorite, quartz diorite and granodiorite. These lithologies are grey in fresh exposure and commonly intruded with coarse-grained pink granite.

The intensity of deformation varies from low to medium. A south-dipping E-W to NE-SW foliation can be measured on most outcrops. In places, rocks show a L/S tectonite fabric with secondary and tectonometamorphic mineral lineations defined by the alignment of mafic minerals shallowly dipping towards the ENE to NE.

K-feldspar phenocrystals make up 1-10% of the rock and are 1-2 cm long, five times coarser than the medium-grained matrix. Petrographic observations indicate that these phenocrystals are orthose, although microcline is abundant in the matrix. The pluton’s mineralogy is characterized by hornblende (5-15%), biotite (3-15%) and trace magnetite. Sphene and epidote are observed in a few places.