The Sem Suite was introduced by Gosselin et al. (2001, 2002) in the Loups Marins Lake area (sheet 34A) to describe biotite-rich tonalite units having the appearance of diatexite. This unit has not been recognized in other areas (Simard et al., 2008; Simard, 2008).

The Sem Suite consists of heterogeneous greyish tonalite with a yellowish grey to yellowish brown alteration patina, variable grain size, and a diatexite-like appearance (Gosselin et al., 2001, 2002; Simard et al., 2008; Simard, 2008; MNRF, 2010). The rock is characterized by a high amount of biotite (20-40%) distributed unevenly and concentrated in undulating and discontinuous bands similar to schlierens in diatexites. This uneven distribution of biotite, with its variable size, gives the rock a heterogeneous appearance. In general, small, rounded to subrounded plagioclase phenocrystals (1-5%) appear in positive relief on the altered surface. Tonalite is usually well foliated; this foliation is accentuated by the presence of biotite bands and numerous stretched enclaves of variable composition. The rock is affected by pervasive folding easier to observe in areas rich in stretched enclaves. Numerous centimetric to decimetric granite intrusions, conformable or discordant to regional foliation, also accentuate the banded appearance of the unit.

The most common enclaves have dioritic composition. Their length varies from a few centimetres to more than one metre (Gosselin et al., 2001, 2002). It consists of a dark grey amphibolitized diorite, more or less magnetic, slightly to highly foliated and fine to coarse grained. In some areas, there is a predominance of enclaves and preserved layers of biotite-garnet paragneiss with locally associated iron formation enclaves. These paragneiss commonly have a migmatite appearance. Fine-grained amphibolite enclaves, likely derived from mafic volcanic rocks, as well as ultramafic rock enclaves were also observed locally. In some places, there are also diorite, paragneiss or amphibolite outcrops, which likely represent large enclaves preserved within tonalites.

In thin sections, the tonalite’s heterogeneous grain size is evident and enhanced by the presence of coarse, rounded plagioclase grains (Gosselin et al., 2001, 2002). The rock is composed of 20-30% quartz and 30-60% plagioclase. It also contains an average of 20-25% biotite flakes that are aligned and distributed unevenly (usually in clusters) between coarse plagioclase grains; this content reaches 40% locally. Biotite contains small, locally abundant zircons. Typically, biotite is brownish. However, in areas where metamorphism is higher, biotite is deep red and is accompanied by clinopyroxene and, locally, orthopyroxene grains. This mineralogical variation related to metamorphism was also observed in paragneiss enclaves. Tonalite from zones rich in paragneiss enclaves frequently contains garnet and, more locally, sillimanite and cordierite. The magnetite content is variable (<1% on average). It occurs as small, anhedral grains most commonly bonded to biotite flakes. Apatite is accessory. Small amounts of epidote, green hornblende and K-feldspar were observed in a few samples.

Thickness and distribution

The Sem Suite is only found in the Loups Marins Lake area (sheet 34A). It forms several deformed kilometric bodies, usually elongated in different directions.

Dating

None.