Background and Methodology

The Lac Rond Structural Domain was defined following the geological survey carried out by Moukhsil and El Bourki (2020). This survey was carried out during the summer of 2019 in the Normandin area, NW of Lake Saint-Jean (sheets 32A09, 32A10, 32A15 and 32A16).

Boundaries and Morphology

Stratigraphic Units Concerned

Stratigraphic units forming part of the Lac Rond Structural Domain are:

• the Sainte-Hedwidge Intrusive Suite (mPshe2);
• the Mimosa Plutonic Suite (mPmim1);
• the Saint-Thomas-Didyme Suite (mPstd1, mPstd2);
• the Allegrin Plutonic Suite (mPalg);
• the Barrois Complex (mPboi1, mPboi4, mPboi4b, mPboi4c);
• the Bardeau Plutonic Suite (mPbad);
• the Claire Gabbronorite (mPclr);
• the Mailles Batholith (mPmas).

Structural Characteristics

The various outcrops of the Lac Rond Structural Domain have a planar structural fabric S_n that is expressed mainly by secondary mineral foliation, mylonitic foliation or banding in intrusive rocks, or by gneissosity or migmatitic banding in metasedimentary rocks of the area.

❯ Main Fabrics

The Lac Rond Structural Domain (DSrnd) is characterized by a fairly heterogeneous trajectory of the planar fabric (S_n) in the various units that outcrop within it. The mean  direction is generally N-S with a mean dip eastward (mean foliation of 359°/39°).

In intrusive rocks of the Sainte-Hedwidge Intrusive Suite (mPshe2), the main planar fabric S_n is generally expressed by secondary tectonometamorphic mineral foliation or by mylonitic banding in fault zones. In metasedimentary rocks of the Barrois Complex (mPboi1, mPboi4, mPboi4b, mPboi4c), the S_n foliation is characterized by migmatitic banding, or gneissosity characterized by alternating millimetric to centimetric leucocratic and mesocratic to melanocratic layers clearly visible in paragneiss. In other intrusive rocks (mPmim1, mPalg, mPbad, mPclr, mPmas and mPstd), foliation is essentially dominated by a tectonometamorphic mineral foliation.

The S_n foliation bears a secondary tectonometamorphic mineral and stretch lineation of variable attitude from one outcrop to the next resulting from several folding of the S_n fabric. The stereographic projection of measurements reflects this variability and allows determining a mean lineation oriented 85° and a mean plunge of 42° eastward. However, two subgroups of measurements can be distinguished: the first is oriented 139° with a plunge of 29° towards the SE, and the second is oriented 21° on average with a mean plunge of 22° towards the NNE.

❯ Other Fabrics

In the Lac Rond Structural Domain, the tectometamorphic foliation S_n+1 was observed on only a few outcrops, where it intersects the S_n foliation. This S_n+1 deformation phase was rarely measured, as it is confused with the main S_n factory. In general, it is characterized by the flattening of quartz grains, feldspar grains, or both, as well as by the preferential orientation of ferromagnesian minerals (biotite) clearly visible in fold hinges.

❯ Folds

Analysis of the distribution of geophysical lineaments interpreted from maps of the total magnetic field and its derivatives (Intissar and Benahmed, 2015) and of the planar fabric, allow distinguishing at least two families of folding in the Lac Rond Structural Domain (DSrnd).

In the eastern part of the domain, the trajectory of the S_n foliation (interpreted from the structural data and magnetic lineaments) marks, in the Sainte-Hedwidge Intrusive Suite (mPshe2), the presence of four regional folds whose axial plane trace is roughly oriented NNE-SSW to N-S. The attitude of magnetic lineaments indicates alternating upright synform and antiform folds.

In the Mailles Batholith located in the centre of the domain, two folding phases were determined by the trajectory of the S_n foliation. The first phase of upright synform attitude, with an axial plane trace of curvilinear appearance oriented NE-SW, is folded by the second phase of upright antiform attitude and NW-SE direction.

In the northern part of sheet 32A15, in the Saint-Thomas-Didyme Suite (mPstd2) and paragneiss of the Barrois Complex (mPboi4 and mPboi4c), there are two regional folds whose axial planes are generally oriented NNE-SSW (antiform attitude for the fold to the east, and synform attitude for the fold to the west). Analysis of the foliation around axial planes indicates that these folds are, in general, overturned towards the NW. At the scale of the outcrop, Z folds with a tight opening angle were observed in migmatitic paragneiss of the Barrois Complex (mPboi4c) with an inclined axial plane of reclined fold oriented 28.81°. This type of fold is interpreted as a second-order « parasitic » fold in a major fold.

In the northern part of the Mimosa Plutonic Suite (mPmim1), the trajectory of the S_n foliation reveals synform attitude folding overturned towards the NE with an axial plane trace oriented NW-SE.

Geometric Parameters of Regional Folds: 

❯ Crosscutting Relationships

The Lac Rond Structural Domain is cut by three families of faults deduced from field observations or maps of the aeromagnetic field and its derivatives (Intissar and Benahmed, 2015).

The N-S family is represented by two normal faults of kilometric extent (respectively 10 km and 30 km long) and a reverse fault ~5 km long.

The NW-SE family is represented by faults inferred from geophysical survey to the east (sheet 32A16) and in the NW part of the domain (sheet 32A15), and by a ~15 km long reverse fault located in the SW part. The latter brings the Bardeau Plutonic Suite and Lachance Mangerite into contact with the Sainte-Hedwidge Intrusive Suite (mPshe2).

The NE-SW family is characterized by faults inferred from geophysical survey in the western part of the domain (sheet 32A15) with an estimated extent of 4 to 20 km.

Geometric Parameters of Regional Faults Cutting the XYZ Structural Domain:

❯ Kinematics

Does not apply.

Deformation Style

The Lac Rond Structural Domain has undergone at least two phases of regional deformation and folding in domes and basins, which are characterized by a heterogeneous planar structure. The S_n foliation marks the early phase of deformation D_n and is generally represented by:

• migmatitic banding or gneissosity in calcosilicate rocks and paragneiss of the Barrois Complex (mPboi4 and mPboi4c);
• tectonometamorphic mineral foliation in intrusive rocks of the Bardeau (mPbar) and Mimosa (mPmas) plutonic suites, Sainte-Hedwidge Intrusive Suite (mPshe2), Mailles Batholith (mPmas) and Saint-Thomas-Didyme Suite (mPstd1 and mPstd2).

The S_n planar structure was folded by a D_n+1 deformation phase producing antiforms and synforms upright and overturned towards the NW with an axial plane trace oriented N-S to NNE-SSW. This phase is probably associated with E-W to ESE-WNW shortening and foliation of the axial plane S_n+1 easily differentiated from S_n in fold hinges, but less widespread in the field.

The D_n+2 deformation, less signfificant, created upright antiforms and synforms overturned towards the NE and generally oriented NW-SE. In places, this phase folded structures of the D_n+1 phase.

Metamorphic Characteristics

About a hundred samples of different lithostratigraphic units of Lac Rond Structural Domain (sheets 32A15 and 32A16) were studied under a polarizing microscope. The objective was to determine the type and distribution of metamorphism in the domain. Mineralogical and petrographic parageneses of these samples show that metamorphic conditions have reached the granulite facies characterized by the presence of orthopyroxene on all lithostratigraphic units, with the exception of a few places at the upper amphibolite facies. For example, in the contact zone between the Allegrin and Mimosa plutonic suites and the Bardeau Plutonic Suite, in the SW part of the domain and within calcosilicate rocks of the Barrois Complex (mPboi4 and mPboi4c; NE part of sheet 32A15), an assemblage of biotite-amphibole (hornblende) ferromagnesian minerals, without pyroxene, is observed. This could be the result of retrograde metamorphism from the granulite facies to the upper amphibolite facies.

Alterations

Not observed.

Geophysical Characteristics

The high-resolution total magnetic field signal (Intissar and Benahmed, 2015) in the Lac Rond Structural Domain indicates a heterogeneous geophysical pattern with highly contrasted magnetic zones. In the eastern part of the domain, rocks of the Sainte-Hedwidge Intrusive Suite (mPshe2) show an isotropic magnetic structure with strong magnetic susceptibility. In rocks of the Mailles Batholith (mPmas), Mimosa Plutonic Suite (mPmim1) and Allegrin Plutonic Suite (mPalg), the magnetic structure is annular and elliptical to circular with medium to high-banding density.

On the other hand, in paragneiss and calcosilicate rocks of the Barrois Complex (mPboi4 and mPboi4c) and intrusive rocks of the Saint-Thomas-Didyme Suite (mPstd2), the magnetic structure is more or less banded and curvilinear with a high magnetic contrast that reflects the alternation of these two units in the field.

Chronological Markers

Rocks of the Mimosa Plutonic Suite (1003 ±18 Ma; Papapavlou, 2019) are affected by the S_n planar fabric, which could mean that the D_n deformation in the Lac Rond Structural Domain is of Grenvillian age (event between 1085 Ma and 986 Ma according to Gower and Krogh, 2002). However, the D_n+1 and D_n+2 deformations that have restructured nits of the domain would be of Trans-Grenvillian age.

References

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