Background and Methodology

The Saint-André-du-Lac-Saint-Jean Structural Domain was defined following a geological mapping survey conducted by Moukhsil and Daoudene (2019) in the Commissaires Lake area (NTS sheets 32A01, 32A02, 32A07 and 32A08), SW of Lake Saint-Jean.

Boundaries and Morphology

Stratigraphic Units Concerned

In sheets 32A01 and 32A08, the stratigraphic units present in the Saint-André-du-Lac-Saint-Jean Structural Domain are:

– Montauban Group (mPmt2);

– La Bostonnais Complex (mPbos4);

– Belley Plutonic Suite (mPbly2);

– Lac-Saint-Jean Anorthositic Suite (mPlsj2c);

– Travers Suite (mPtra1, mPtra2 and mPtra4).

Structural Characteristics

In sheets 32A01 and 32A08, most outcrops of the Saint-André-du-Lac-Saint-Jean Structural Domain show only one planar structural fabric, that is a very diffuse tectonometamorphic foliation. For this reason, and since the trajectory of foliation measurements appears to be roughly continuous from one outcrop to another, almost all measurements are attributed to S_n. It is also assumed that the associated D_n deformation phase is Grenvillian, i.e. aged between ~1.085 Ga and 0.986 Ga. Since the Grenvillian orogenic phase is the most recent of those recorded throughout the Grenville Province (i.e. see Moukhsil and Solgadi, 2018), planar structural fabrics associated with at least one earlier orogenic phase (D_n-1) could be identified later in the Saint-André-du-Lac-Saint-Jean Structural Domain. This is why it was chosen to match the S_n foliation to S₂ in “geofiches” (not S₁).

❯ Main Fabrics

The deformation in the Saint-André-du-Lac-Saint-Jean Structural Domain is very heterogeneous, and the intensity varies greatly from one outcrop to another. Indeed, over a distance of a few hundred metres, some outcrops show intrusive rocks with little or no deformation, while others show the same rocks this time affected by a penetrative S_n foliation moderately to highly developed. Whether it is observed in outcrop or thin section, the S_n foliation is mainly highlighted by the preferential orientation of ferromagnesian mineral clusters (pyroxene, amphibole and biotite), and to a lesser extent by the flattening of quartz and feldspar grains.

Therefore, only the area located in sheets 32A01 and 32A08 was the subject of a structural study. The analysis of S_n foliation measurements collected in this area alone does not define the trend at the domain scale. In any case, the S_n foliation is generally oriented NNE-SSW, in addition to having dips of varying intensity, but generally >60°. Two groups stand out on the stereographic projection of S_n foliation measurements: the first consists of foliation measurements having dips towards the WNW, while the second is characterized by dips towards the ESE. It seems that the first group is the most common.

❯ Other Fabrics

Rocks of the Saint-André-du-Lac-Saint-Jean Structural Domain are locally affected by shear bands commonly discontinuous, the width of which ranges from centimetric to metric. To date, only a few of them have been measured following the mapping survey of Moukhsil and Daoudene (2019), mainly in sheet 32A01. The intensity of deformation generally varies from band to band, but is often marked by a C/S structure and the presence of centimetric bands of mylonite and ultramylonite. Of the ten or so shear bands measured, most are N-S to NW-SE oriented and have moderate to steep dip. The associated stretch lineation is in many cases oriented at a low angle to shear bands’ azimuth, suggesting that the associated kinematics have a predominantly strike-slip component. In all cases where the shear direction could be determined, the strike-slip component is always sinistral. Characteristics of these shear bands suggest that these structures are potentially related to those that are ubiquitous in the structural domain immediately to the west, namely the Saint-François-de-Sales Shear Zone.

❯ Folds

Not observed.

❯ Crosscutting Relationships

In sheet 32A01, the Saint-André-du-Lac-Saint-Jean Structural Domain is cut by the Métabetchouane Shear Zone, which was originally documented by Moukhsil and Côté (2018) in sheet 31P16. This NNE-SSW oriented shear zone is at least 30 km long and ~300 m wide. As it approaches the shear zone, the S_n foliation intensifies. In the Métabetchouane Shear Zone, foliation becomes mylonitic. This foliation carries mineral and stretch lineations whose plunge is generally shallow (8-12°) towards the NE. Kinematic criteria that can be regularly observed along this shear zone clearly indicate a main dextral component, a feature initially observed by Moukhsil and Côté (2018).

Geometric Parameters of Regional Faults Cutting the Saint-André-du-Lac-Saint-Jean Structural Domain:

❯ Kinematics

Does not apply.

Deformation Style

Most of the Saint-André-du-Lac-Saint-Jean Structural Domain appears to be located in sheets 22D04 and 22D05. In this area, geology is still unknown and high-resolution magnetic field imaging is non-existent. Under these conditions, it seems difficult to discuss the possible style of deformation at the domain scale. Structural data from the mapping survey of Moukhsil and Daoudene (2019) nevertheless show that deformation is very heterogeneous in the Saint-André-du-Lac-Saint-Jean Structural Domain.

Metamorphic Characteristics

Thin section observation of about ten samples suggests that, in sheet 32A01, all units of the Saint-André-du-lac-Lac-Saint-Jean Structural Domain were affected by metamorphic conditions that have reached the granulite facies. Intermediate rocks of the Belley Plutonic Suite and Travers Suite commonly show plagioclase-quartz-feldspar-orthopyroxene-clinopyroxene-hornblende-biotite mineralogical assemblages. These quartzofeldspathic rocks usually marked by diffuse foliation have quartz and feldspar grains with usually very rounded contacts. This characteristic suggests a process of high-temperature dynamic recrystallization by grain border migration (Gower and Simpson, 1992). In addition, quartz grains usually show a chessboard texture. The latter indicates combined intracrystalline slippage along crystallographic directions a and c, and is indicative of grain deformation under temperature conditions ≥600°C (Blumenfeld et al., 1986; Gapais and Barbarin, 1986).

Alterations

Does not apply.

Geophysical Characteristics

In sheets 32A01 and 32A08, the magnetic signal of the Saint-André-du-Lac-Saint-Jean Structural Domain appears to be very heterogeneous. In this area, while the central part of the domain shows a smooth and very intense susceptibility zone on magnetic field maps (Intissar and Benahmed, 2015), it is much weaker in the areas immediately north and south, and its texture appears more shagreen.

Chronological Markers

NE of sheet 32A08, the Saint-André-du-Lac-Saint-Jean Structural Domain is unconformably overlain by Ordovician sedimentary units of the Trenton Group.

References

Publications available through Sigéom Examine

Other publications