The Marjoulet Gneiss was first described following geological mapping work in 2015 (Gigon et al., 2020), where a foliated to banded biotite ± hornblende tonalitic gneiss was observed in the central portion of sheet 33F01. The name refers to Marjoulet Lake located in this area. The 2016 and 2018 summer mapping work extended the unit east and southeast, with new strips of gneiss identified in sheets 33G04 and 33G03, and 33B14.

The Marjoulet Gneiss consists mainly of biotite ± hornblende tonalite and granodiorite with a foliated to banded structure. The amount of biotite generally ranges from 2 to 10% and reaches locally 25%. Magnetite (<1%) and hornblende (<8%) are also present. Grain size varies from fine to medium. Foliation is highlighted by biotite schlierens. Banding is formed by compositional variations defined by more felsic centimetric bands, more ferromagnesian bands and dioritic horizons, which are usually boudinaged. Enclaves (15 cm to 2 m) and centimetric to metric horizons of amphibolite, paragneiss or ultramafic rock are also observed. A large volume of granitic intrusions is also present in this unit (Gigon et al., 2020).

In the field, the gneissic structure has not always been listed since these rocks’ compositional banding have sometimes been associated with ultrametamorphic granite, banded granite or a foliated tonalite intruded by granite. Furthermore, rock heterogeneity and the presence of biotite schlierens make mineral percentages highly variable and difficult to assess. It is therefore likely that the Marjoulet Gneiss is more widespread than currently known.