The De Pas Granitic Suite was defined by Danis (1991) and subdivided into informal units based on the dominant lithology. This suite, like the De Pas Charnockitic Suite, is part of the De Pas Supersuite, a polyphase syntectonic intrusion that extends >440 km in a N-S to NNW-SSE orientation in the George Lithotectonic Domain (Charette et al., 2018).

A regional compilation by geologists of the Ministère in 2003 (MRNF, 2010a-c) subdivided the De Pas Granitic Suite, then referred to as the “De Pas Batholith”, into six informal units (pPdep1 to pPdep6). Hammouche et al. (2011) subsequently redefined several units and introduced a seventh (pPdep7). Simard et al. (2013) introduced an eighth unit (pPdep8) of small syenite and monzonite intrusions. Only informal units pPdep3, pPdep4, pPdep5, pPdep7 and pPdep8 were recognized in the 1:250 000 mapping work conducted by the Ministère between 2011 and 2016. As part of the Southeastern Churchill Province regional synthesis (SECP; Lafrance et al., 2018), the eight known informal units were reduced to five (pPdep1 to pPdep5), as illustrated in the table below.

The De Pas Granitic Suite, consisting mainly of various biotite ± hornblende potassic intrusive phases, is the main unit of the De Pas Supersuite. The rock is intermediate to felsic in composition, and locally porphyraceous. It is divided into five units: 1) quartz diorite, diorite, monzodiorite and monzonite (pPdep1); 2) porphyraceous granodiorite and quartz monzodiorite (pPdep2); 3) heterogeneous granite containing numerous enclaves (pPdep3); 4) biotite granite (pPdep4); and, 5) syenite and monzonite (pPdep5).

Rocks of unit pPdep2 are intruded to varying degrees by the different phases of unit pPdep4 as subconformable layers and centimetre to metre-wide dykes. Unit pPdep4 also includes decimetric and metric enclaves of unit pPdep2 porphyraceous granodiorite.

The De Pas Granitic Suite rocks are commonly affected by the Rivière George Shear Zone, resulting in the development of mylonitic foliation, K-feldspar porphyroclasts and quartz bands, accompanied by more or less intense hematite, epidote and chlorite alteration in platings and veinlets.

De Pas Granitic Suite 1a (pPdep1a): Homogeneous Quartz Diorite and Diorite

Subunit pPdep1a represents an intrusive phase that varies from quartz diorite to diorite and is characterized by a low phenocrystal content (<5%). It is regularly observed in outcrops with porphyraceous granodiorite and quartz monzodiorite of unit pPdep2. The contact between these different lithologies may be sharp, but it is generally gradual and corresponds to the gradual disappearance of K-feldspar phenocrystals. In other places, subrounded diorite xenoliths containing some microcline phenocrystals (pPdep1a) are also observed in porphyraceous rocks of unit pPdep2 (including subunits pPdep2b and pPdep2c). In some cases, quartz diorite is the dominant phase and may have been drawn on the map (pPdep1a).

Rocks of subunit pPdep1a are homogeneous, medium to fine grained, even grained, grey and massive to weakly foliated. They contain 15 to 25% brown biotite and green hornblende, as well as a significant amount of accessory minerals including sphene, allanite, apatite, zircon and opaque minerals. In some areas, the rock shows moderate chloritization and epidotization.

Quartz diorite is locally intruded by leucocratic felsic to intermediate material, giving the rock a heterogeneous and locally brecciated appearance. The leucocratic material is in sharp contact with quartz diorite, which in some places forms angular to subrounded enclaves. Locally, these injections produce banding parallel to the regional fabric. Danis (1988, 1991) proposes that brecciated zones were formed by the fracturing of quartz diorite resulting from a surge of slightly more felsic magma.

De Pas Granitic Suite 1b (pPdep1b): Melanocratic Monzonite and Monzodiorite

A few small melanocratic monzonite and monzodiorite lenses associated with quartz diorite have been grouped in subunit pPdep1b. The contact between these two lithologies was not observed during 1:250 000 mapping campaigns carried out by the Ministère since the summer of 2011. However, van der Leeden (1995) describes this subunit as transitional and gradual metric to decametric layers in quartz diorite. These rocks, dark grey to black and fine-grained, include rounded mafic aggregates of biotite and hornblende with clinopyroxene cores up to 1.5 cm locally. K-feldspar is in some places concentrated in centimetric bands. Many accessory minerals are observed in thin section, the main percentages of which are epidote and sphene.

Unit pPdep2 groups a variety of phases typically characterized by a positive magnetic signature. It consists largely of granodiorite and quartz monzodiorite, but also includes monzogranite, syenogranite and quartz syenite. Pale to whitish pink microcline occurs as euhedral phenocrystals (1-6 cm, locally up to 10 cm), but is also present in the coarse to medium-grained matrix. Phenocrystals, which account for 5 to 25% of the rock, are locally aligned but generally show a random orientation. The rock is pinkish grey in fresh exposure and pinkish white in alteration patina. It is massive to weakly foliated except in mylonitic deformation zones associated with large NW-SE to N-S shear zones. On outcrops, fine-grained monzodiorite is also present, containing ~5% of 3 to 7 mm microcline phenocrystals. This monzonite is found in partially gradual or irregular contacts, associated with reaction rims, or as enclaves in porphyraceous rocks. These observations seem to indicate a mostly contemporary development of these two lithologies.

Unit pPdep4 consists of several homogeneous potassic intrusive phases intruding into each other, locally giving a heterogeneous appearance to outcrops. There are three main intrusive phases, none of which can be mapped at the scale of our work: 1) monzogranite containing 8 to 15% ferromagnesian minerals; 2) granite containing 5 to 10% biotite; and, 3) white granite containing <5% ferromagnesian minerals. Monzogranite and biotite granite are medium grained and massive to slightly foliated. Brown to greenish biotite is the main ferromagnesian mineral, but green hornblende is also found in small amounts, particularly in monzogranite. These two phases are primarily distinguished by their colour, monzogranite being grey, locally pink and biotite granite light pink to pinkish grey (lighter than monzogranite). In both cases, thin sections reveal a relatively well-preserved igneous texture associated with moderate to high recrystallization at grain edges. There are also myrmekitic and anti-perthitic textures, as well as undulatory extinction of quartz. Feldspar shows some initial alteration with the formation of sericite, epidote and carbonate. Accessory minerals, especially epidote and allanite, are abundant (1-4%) and often accompanied by small clusters of ferromagnesian minerals scattered in the rock. They also include sphene, apatite, opaque minerals, zircon as inclusions in biotite and, more locally, fluorite.

Pink leucogranite is massive, medium grained to pegmatitic, and intrudes as centimetric to decimetric layers and dykes in the other granitic phases of unit pPdep4. At this time, the majority of granitic rocks observed within the De Pas Supersuite envelope have been assigned to unit pPdep4. However, it is likely that some of these rocks, particularly leucogranite, represent a much later unit similar to the Dancelou Suite.

De Pas Granitic Suite 5 (pPdep5): Magnetic Syenite and Monzonite