Background and Methodology

The Jean-Marie Structural Domain (DSjme) was defined from a geological mapping survey conducted by Moukhsil and El Bourki (2020) in the Girardville area (NTS sheets 32H02).

Boundaries and Morphology

Stratigraphic Units Concerned

Stratigraphic units belonging to the Jean-Marie Structural Domain are:

– the Jean-Marie Intrusive Suite (mPijm);
- the Bolduc Intrusive Suite (mPblc);
- the Barrois Complex (mPboi4 and mPboi4c).

Structural Characteristics

The Jean-Marie Structural Domain (DSjme) corresponds to the orientation of the planar fabric in units of the Jean-Marie and Bolduc intrusive suites and in the Barrois Complex (mPboi4 and 4c). The planar structure is represented mainly by secondary tectonometamorphic mineral foliation, mylonitic foliation or banding in intrusive rocks, and by gneissosity in metasedimentary facies of the area.

❯ Main Fabrics

The Jean-Marie structural domain is characterized by a homogeneous planar fabric trajectory (S_n) in the different lithostratigraphic units that form it. This planar fabric has an average N-S orientation and an eastward dip (mean S_n foliation: 04°/55°). In rocks of the Bolduc and Jean-Marie intrusive suites, the S_n planar fabric is mainly expressed by a secondary tectonometamorphic mineral foliation or by mylonitic banding in fault zones. This foliation is characterized by the preferential alignment of minerals and, locally, by flattening and grain size reduction in mylonitic zones of the area. In metasedimentary rocks of the Barrois Complex (mPboi4, mPboi4c), gneissosity is characterized by alternating light layers (containing quartz-feldspar) and dark layers (enriched in biotite, magnetite or orthopyroxene).

In general, mineral lineations borne by these planar fabrics are secondary mineral lineations of tectonometamorphic type. Their average orientation is 135° and they have an oblique component (average L_n: 135°/44°). In stereographic projection, these lineations are distributed in particular in the SE quadrant of the canvas with some points in the SW part.

❯ Other Fabrics

On some outcrops of the Jean-Marie structural domain, the S_n planar fabric is cut by a second mineral foliation S_n+1. This is easily recognizable at fold hinges, where minerals generally have a preferential orientation and quartz grains are stretched.

❯ Folds

Analysis of the distribution of geophysical lineaments, interpreted from maps of the total magnetic field and its derivatives (Intissar and Benahmed, 2015) and the planar fabric, indicates that the Jean-Marie Structural Domain (DSjme) is affected by at least one folding phase having a mean axial plane orientation N-S to NNE-SSW. At the outcrop scale, isoclinal or symmetrical-style folding has been observed in rocks of the Jean-Marie and Bolduc intrusive suites as well as in enclaves of the Barrois Complex 4 (mPboi4). The stereographic projection of axial planes indicates that folds are overturned towards the east or west.

Geometric Parameters of Regional Folds: 

❯ Crosscutting Relationships

The Jean-Marie Structural Domain is cut by two families of faults inferred from high-resolution magnetic maps or observations made on outcrops. The first family is N-S to NNW-SSE oriented and is represented by normal and reverse faults of undetermined movement whose orientation is subparallel to the structural pattern interpreted for this domain and which probably played a role in structuring the intrusions of the area. The second family of faults, generally oriented NE-SW, is represented by faults of undetermined movement and by a reverse fault in the SE part of the domain, in contact with the Bois Vert Structural Domain.

Geometric Parameters of Regional Faults Cutting the Jean-Marie Structural Domain:

❯ Kinematics

Does not apply.

Deformation Style

The Jean-Marie Structural Domain has undergone moderate to strong deformation characterized by a well-developed planar fabric, which is homogeneous between outcrops of each lithostratigraphic unit. The S_n planar fabric represents the early phase of deformation and is marked essentially by secondary mineral foliation or mylonitic banding in intrusive rocks or by gneissosity in metasedimentary rocks of the area. The domain underwent a second phase of deformation (S_n+1) characterized by folding that indicates E-W compression.

Metamorphic Characteristics

About 30 samples representative of the lithostratigraphic units of this domain (DSjme) were studied under a polarizing microscope to determine the type and spatial distribution of metamorphism. Mineralogical associations and petrographic characteristics of the samples indicate that the temperature and pressure conditions within the domain have largely reached the granulite facies, including a few places at the upper amphibolite facies.

The presence of orthopyroxene and perthite (K-feldspar with albitic exsolution) is evidence of temperature conditions typical of the granulite facies (≥600°C). In places, a biotite-hornblende ferromagnesian mineral assemblage (pyroxene absent) is observed, as well orthopyroxene altering to hornblende or biotite (uralitization) that can be explained by amphibolite facies retrograde metamorphism.

Alterations

Not observed.

Geophysical Characteristics

The high-resolution residual magnetic field map (Intissar and Benahmed, 2015) of the Jean-Marie Structural Domain indicates that the geophysical pattern is quite homogeneous, including areas of highly contrasting magnetic susceptibility of high magnetism. In the central part of the domain, the magnetic pattern displays an elliptical ring texture with moderate density magnetic banding in units of the Bolduc (mPblc) and Jean-Marie (mPijm) intrusive suites. Elsewhere, the magnetic pattern is rather banded in the NE part of the domain, but lineaments are curvilinear with an overall moderate to intense magnetic susceptibility.

Chronological Markers

Does not apply.

References

Publications Available Through Sigéom Examine

Suggested Citation

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