Background and Methodology

The Bois Vert Structural Domain (DSbvt) was defined following the geological survey conducted by Moukhsil and El Bourki (2020) in the summer of 2020, in the Girardville area, NW of Lake Saint-Jean (NTS sheet 32H02).

Boundaries and Morphology

Stratigraphic Units Concerned

Stratigraphic units belonging to the Bois Vert Structural Domain are:

• the Bois Vert Plutonic Suite (mPbvr);
• the Barrois Complex 4 (mPboi4);
• the Saint-Thomas-Didyme Suite 2 (mPstd2);
• the Sainte-Hedwidge Intrusive Suite 2 (mPshe2).

Structural Characteristics

Outcrops of the Bois Vert Structural Domain display a planar fabric that is mainly expressed by secondary mineral foliation, mylonitic foliation or banding in intrusive rocks, or by gneissosity or banding in metasedimentary rocks of the study area. Similarly, a magmatic-type primary mineral foliation was observed at the centre of the Bois Vert Plutonic Suite 1 (mPbvr1) intrusion.

❯ Main Fabrics

The Bois Vert Structural Domain (DSbvt) is characterized by a fairly homogeneous planar fabric trajectory (S_n) over most of the domain’s outcrops. It is mainly oriented N-S to NNE-SSW and has a moderate dip eastward (mean foliation 011°/38°).

Foliation in the Bois Vert Plutonic Suite (mPbvr1, mPbvr2 and mPbvr3) is variably developed. Indeed, some outcrops display continuous, penetrative foliation marked by the orientation of ferromagnesian minerals along a preferential plane. Other outcrops, especially in the centre of the intrusion (mPbvr1), exhibit a very discrete primary (magmatic) mineral foliation, locally emphasized by the alignment of ferromagnesian minerals (orthopyroxene and magnetite), without any preferential orientation of K-feldspar phenocrystals.

Within the Bois Vert Plutonic Suite (mPbvr2), foliation follows the edge of the intrusion, resulting in a concentric shape of the foliation trajectory within the intrusion, and more strongly at its edge.

The various measurements of S_n foliation have allowed calculating a mean plane with an azimuth of 011° and a dip of 38° eastward. However, the stereographic projection of S_n poles indicates two dominant groups, located in the SW and NW quadrants of the grid, respectively.

Mineral lineations borne by these planar structures in this domain are mainly concentrated in the SE quadrant of the grid; they have a dipping to locally oblique component.

❯ Other Fabrics

In the Bois Vert Structural Domain (DSbvt), the S_n+1 foliation has been observed on a few outcrops, where it cuts the S_n foliation. This phase (S_n+1) is less penetrative and characterized by the flattening of quartz and feldspar grains, as well as by the preferential orientation of ferromagnesian minerals (biotite). As for other domains in the area, it is not easy to measure the orientation, let alone the dip of the S_n+1 foliation, since it can easily be confused with S_n.

❯ Folds

No regional folds have been observed in this domain. However, isoclinal or symmetrical-style folding has been observed locally on some outcrops of the Bois Vert Plutonic Suite and in an enclave of the Barrois Complex 4 (mPboi4) (e.g. outcrop 20-AM-0145). 

❯ Crosscutting Relationships

The Bois Vert Structural Domain is cut by three families of faults observed in the field or inferred from interpretations of geophysical maps of the aeromagnetic field and its derivatives (Intissar and Benahmed, 2015). These are:

– the first family N-S, represented by the regional normal fault (crossing the domain over ~9 km) and cutting intrusive rocks of unit mPbvr2 and faults inferred from geophysical survey, which cut units mPbvr3 and mPstd2 (in the centre of the domain) and mPboi4 (in the eastern part);
– the second family NE-SW, represented by two faults inferred from geophysical survey that cut unit mPbvr2 in the SW part of the domain;
– the third family, composed of circular faults that follow geological contacts between units of the plutonic suite and surrounding rocks.

❯ Kinematics

Does not apply.

Deformation Style

The Bois Vert Structural Domain has undergone deformation of low to moderate intensity characterized by a well-developed planar fabric that can be correlated between outcrops. The S_n planar fabric represents the early phase of deformation and is marked by primary or secondary mineral foliation, or even mylonitic banding in intrusive rocks or by gneissosity in metasedimentary rocks of the structural domain.

Metamorphic Characteristics

About 20 samples representative of the lithostratigraphic units of the Bois Vert Structural Domain (DSbvt) were analyzed and studied under a polarizing microscope in order to characterize the type of metamorphism and its distribution in the domain. However, mineralogical assemblages and petrographic characteristics of this domain’s samples indicate that metamorphic conditions mainly reached the granulite facies, including some zones at the upper amphibolite facies. The presence of orthopyroxene indicates temperature conditions typical of the granulite facies. In places, a biotite-hornblende ferromagnesian mineral assemblage (absence of pyroxene) can be explained by amphibolite facies retrograde metamorphism.

Alterations

Not observed.

Geophysical Characteristics

The high-resolution total magnetic field map of the study area (Intissar and Benahmed, 2015) indicates that the geophysical pattern of the Bois Vert Structural Domain (DSbvt) is quite heterogeneous, including areas of highly contrasted magnetic susceptibility. In the western part of the domain, the magnetic pattern displays a circular ring texture with low-density magnetic banding in units mPbvr1 and mPbvr2 of the Bois Vert Plutonic Suite. In the eastern part of the domain, the magnetic pattern is characterized by a lobate texture with medium to intense banding density.

Chronological Markers

Does not apply.

References

Publications Available Through Sigéom Examine

Suggested Citation

Contributors