Background and Methodology

The Ministic Structural Domain was defined following geological mapping surveys conducted by Moukhsil and Daoudene (2019) in the Commissaires Lake area (NTS sheets 32A01, 32A02, 32A07 and 32A08), southwest of Lake Saint-Jean.

 

Boundaries and Morphology

Stratigraphic Units Concerned

The stratigraphic units present in the Ministic Structural Domain are:

– Pope Plutonic Suite (mPpop2);

– Bardeau Plutonic Suite (mPbad);

– Barrois Complex (mPboi4);

– Wabash Complex (mPwab1 and mPwab2);

– Claire Gabbronorite (mPclr);

– Bonhomme Plutonic Suite (mPboh);

– Roc Suite (mPsro2);

– Léo Plutonic Suite (mPleo);

– Rhéaume Intrusive Suite (mPrhe2).

 

Structural Characteristics

In sheets 32A02 and 32A07, most outcrops of the Ministic Structural Domain show only one type of planar structural fabric expressed by gneissosity and migmatitic banding, particularly in the Pope and Bardeau plutonic suites, as well as in the Barrois and Wabash complexes. Since the measurement trajectory of these fabrics appears to be continuous from one outcrop to another, almost all measurements in the domain are attributed to S_n. The age of the associated D_n deformation phase is 1.2 Ga (see section “Chronological markers”). In the event that structural fabrics associated with at least one earlier deformation phase (i.e. D_n-1) could later be identified in the Ministic Structural Domain, it was agreed to match the S_n foliation to S₂ of “geofiches”.

❯ Main Fabrics

 

The S_n planar fabric dominates the Ministic Structural Domain architecture in sheets 32A02 and 32A07. This fabric is expressed in different ways, but it is generally well developed in the Pope and Bardeau plutonic suites, as well as in the Barrois and Wabash complexes. In most of intrusive rocks of the Pope and Bardeau plutonic suites, gneissosity is visible by alternating pinkish and pink-grey centimetric bands, respectively poor and rich in ferromagnesian minerals. In the Barrois and Wabash complexes, the S_n foliation is underlined by ubiquitous migmatitic banding in paragneiss. This banding consists of alternating whitish or slightly pink mobilisate bands and greyish ones usually composed of mesosome. Mobilisate bands commonly have thin fringes almost exclusively composed of biotite along their edges, one of the characteristics that could be interpreted as evidence of in situ partial melting of the rock.

The foliation’s attitude within the Léo and Bonhomme plutonic suites follows the edges of mangeritic intrusions. In other words, the foliation trajectory in these intrusions is concentric. Regionally, this foliation also appears to be concordant to the S_n foliation of the Bardeau Plutonic Suite (host rock). This implies that measurements from the Léo and Bonhomme plutonic suites are associated with the D_n deformation phase. Nevertheless, nothing excludes that this concentric foliation developed when intrusions were emplaced, which, moreover, could be contemporaneous with the D_n deformation. In any case, the foliation in the Léo and Bonhomme plutonic suites is variably developed. In fact, some outcrops show penetrative and continuous foliation marked by the orientation of ferromagnesian mineral clusters in a preferential plane, while other outcrops show very well-developed gneissosity.

In sheets 32A02 and 32A07, S_n foliation measurements allow for the calculation of an average plane with an azimuth of 36° and a 31° SE dip. Nevertheless, although the stereographic projection of S_n poles highlights a dominant group distributed in the WNW area of the canvas, the variability is significant in both orientation and dip. This can be explained by folding of the S_n foliation at the regional scale, which could be the result of at least two deformation phases. This is supported by the attitude of the S_n foliation map trajectory.

The S_n foliation carries an L_n lineation whose attitude is highly variable and difficult to interpret. In fact, this variability would be the result of folding of the S_n foliation.

 

❯ Other Fabrics

Gneissosity and migmatitic banding are commonly folded and cut by a S_n+1 foliation that appears to be roughly parallel to the P_n+1 antiform axial plane trace (see below). This foliation is relatively discreet and usually confused with S_n when N-S oriented. Nevertheless, in P_n+1 hinges, both fabrics are oblique and therefore clearly identifiable. The S_n+1 foliation is emphasized by directional flattening and elongation of quartz grains and ferromagnesian mineral clusters (biotite and amphibole). A limited number of S_n+1 measurements have been collected, but their stereographic projection shows that the S_n+1 foliation is generally oriented NNE-SSW and its dip is typically >70°.

 

❯ Folds

The S_n foliation trajectory inferred from field data and interpretation of geophysical lineaments derived from the analysis of total magnetic field maps and their derivatives (Intissar and Benahmed, 2015) seems to show that the Ministic Structural Domain is affected by at least two folding phases.

The first phase of folding is marked by the presence of a large P_n+1 antiform, whose axial plane trace is generally oriented N-S. Although it was only drawn to the southwest and northwest of sheets 32A02 and 32A07 respectively, the antiform actually appears to continue across the width of sheets 32A02 and 32A07. The mean axial-plane dip of this antiform can be inferred from the S_n foliation attitude in the surrounding area. This dip seems to be moderate to steep and oriented eastward. The P_n+1 antiform of sheets 32A02 and 32A07 appears to be overturned westward.

In the SW corner of sheet 32A02, the P_n+1 antiform trace is folded by tight P_n+2 folds, of kilometric wavelength and amplitude. These folds are oriented NE-SW and the attitude of S_n foliation measurements, located on either side of axial plane traces, suggests that they are generally overturned westward. The Rhéaume Intrusive Suite occupies the axial part of a P_n+2 antiform. Folds having similar attitude (shape, wavelength and amplitude), interpreted as P_n+2, are also present east of the Bonhomme Plutonic Suite. However, they have an ENE-WSW orientation, slightly oblique to those of the Pope Plutonic Suite, but also seem to be overturned towards the NW.

Geometric Parameters of Regional Folds: 

 

❯ Crosscutting Relationships

In sheets 32A02 and 32A07, the Ministic Structural Domain is cut by a number of faults inferred from high-definition magnetic field maps (Intissar and Benahmed, 2015), which appear to have been divided into two families. One is oriented NE-SW and involves dextral strike-slip faults, interpreted in the NW corner of sheet 32A07; the other is oriented NW-SE and consists of sinistral strike-slip faults, interpreted south of sheet 32A02. This family of faults also cuts the Commissaires Structural Domain.

Geometric Parameters of Regional Faults Cutting the Ministic Structural Domain:

 

❯ Kinematics

Does not apply.

 

Deformation Style

The Ministic Structural Domain shows polyphase deformation and at least two regional folding phases. The S_n foliation, which is expressed by gneissosity in the Pope and Bardeau plutonic suites and migmatitic banding in paragneiss of the Barrois and Wabash complexes, was folded for the first time during a D_n+1 deformation event. This appears to be associated with an E-W shortening phase. Subsequently, the P_n+1 antiform is locally overprinted by kilometric open P_n+2 folds. The overall NE-SW orientation of these P_n+2 folds then appears to indicate a NW-SE shortening phase.

 

Metamorphic Characteristics

Approximately 40 representative rock samples were studied as thin sections to establish the metamorphic characteristics of the Ministic Structural Domain. Mineralogical and structural observations indicate that metamorphic conditions within the domain are distributed between the amphibolite and granulite facies.

Felsic to intermediate intrusive rocks of the Bonhomme and Léo plutonic suites commonly show quartz-plagioclase-K-feldspar-orthopyroxene-hornblende-biotite assemblages. Locally, the presence of garnet may be observed. These rocks, whose diffuse foliation is often weakly expressed and marked by the preferential orientation of ferromagnesian minerals, are characterized by coarse grain size and an interlobed heterogranular texture. Indeed, the border between quartz and feldspar grains is often clearly rounded, suggesting a process of dynamic recrystallization at high temperature (Gower and Simpson, 1992). The frequent presence of K-feldspar porphyroclasts developed near the edges of myrmekite grains, the abundance of flame-like perthite in these clasts, and the chessboard texture of quartz grains indicate temperature conditions of at least 600°C during deformation (Blumenfeld et al., 1986; Gapais and Barbarin, 1986; Simpson and Wintsch, 1989; Pryer, 1993).

Around the Bohomme and Léo plutonic suites, rocks of the Bardeau Plutonic Suite and Barrois and Wabash complexes instead show mineralogical assemblages typical of amphibolite facies metamorphic conditions. In fact, aluminous metasedimentary rocks petrographically studied have biotite-garnet-sillimanite-muscovite assemblages, whereas observed intermediate intrusive rocks show an assemblage of biotite-hornblende ferromagnesian minerals, and the absence of pyroxene.

 

Alterations

Does not apply.

 

Geophysical Characteristics

In sheets 32A02 and 32A07, the high-resolution total magnetic field map (Intissar and Benahmed, 2015) shows an overall pattern dominated by curvilinear magnetic lineaments and generally strong and low susceptibility. In some areas, particularly those where the Pope, Bonhomme and Léo plutonic suites overlap, show lobed or elliptical patterns with overall moderate to intense magnetic susceptibility. Elsewhere, the magnetic pattern is more banded, but lineaments are curvilinear. These areas approximate overlapping zones of the Bardeau Plutonic Suite and Barrois and Wabash complexes.

 

Chronological Markers

In the Wemotaci area, graphite paragneiss of the Wabash Complex provided a maximum depositional age of 1204 ±2 Ma (Moukhsil et al., 2015), implying that the S_n migmatisation and deformation of these rocks are younger than ~1.2 Ga. In addition, a syenite of the Rhéaume Intrusive Suite was dated by U-Pb zircon method at 985.3 ±9.3 Ma (Côté et al., 2018). This intrusive sequence forms an ellipse with a long axis overlapping with a P_n+2 axial plane trace. This suggests that the D_n+2 deformation is either younger or contemporaneous to the Rhéaume Intrusive Suite emplacement.

 

References

Publications available through Sigéom Examine

Other publications