The Terriault Complex was defined in the Brisson Lake area (Lafrance et al., 2016) and consists of a very complex package of several lithologies of different composition and age.

The Terriault Complex is dominated by felsic intrusive rocks containing 5-25% centimetric to metric enclaves of deformed rocks. This complex is associated with a strong positive aeromagnetic anomaly characterized by disturbances and wrinkles, suggesting unit drag along divergent fault systems bordering the unit. Despite the strong magnetic anomaly associated with the Terriault Complex, only the few felsic gneiss enclaves are magnetic. Intermediate to felsic intrusive phases, as well as mafic to intermediate enclaves, are generally not very magnetic.

The Terriault Complex also contains injections of medium-grained to pegmatitic pink granite poor in ferromagnesian minerals. These injections form between 5% and 30% of outcrops and occur as clusters, dykes or subconformable millimetric to decimetric bands. The high proportions of enclaves and injections give outcrops of the Terriault Complex a very heterogeneous appearance, although each phase in itself is relatively homogeneous.

This complex has been divided into two units: heterogeneous and enclave-rich granodiorite and tonalite (ApPter1), and enderbite (ApPter2).

The average intrusive rock composition of unit ApPter1 is at the boundary between granodiorite and tonalite. Locally, there is quartz monzodiorite and monzogranite. These rocks are grey with a pinkish or purplish shade, massive to slightly foliated, medium grained and even grained. They contain <20 to 30% slightly dark purplish-grey quartz, 10 to 20% ferromagnesian minerals and 2 to 15% locally coarse K-feldspar crystals (≤1 cm) distributed irregularly. Ferromagnesian minerals occur as millimetric phenocrystals of green hornblende, as well as millimetric to centimetric clusters of hornblende and brown biotite. These clusters are locally magnetic and also contain accessory minerals, including epidote, apatite, opaque minerals and allanite with, in places, sphene, zircon and muscovite. In thin sections, these rocks display magmatic or partial recrystallization textures, particularly for quartz, which forms subgrains with undulatory extinction around feldspar crystals.

Enclaves consist of diorite, quartz diorite, leucogabbro, tonalite and monzodiorite. They are well foliated, fine to medium grained, granoblastic, and commonly gneissic and banded. The proportions of quartz (2-18%) and ferromagnesian minerals (10-30%) vary by rock types and K-feldspar is sparse. Ferromagnesian minerals consist of green hornblende associated with small amounts of chloritized biotite. Plagioclase is slightly sericitized. Epidote, opaque minerals, apatite and allanite are observed as accessory phases. In the central part of the complex, mafic rock enclaves are more abundant (~40%). There is also an outcrop consisting entirely of foliated leucogabbro and another of completely recrystallized fine-grained anorthosite.