Background and Methodology

The Maingault Structural Domain was defined by Beauchamp (2019) following a mapping survey conducted in the Cadieux Lake area (NTS sheets 33A02 and 33A07) in the summer of 2018. It was extended westwards by Bandyayera et al. (2024) and Chartier-Montreuil and Saint-Laurent (2024) in the Chamic Lake (sheets 32P14 and 32P11) and Caulincourt Lake (sheets 33A03 and 33A06) areas.The following description is based primarily on geological observations gathered during the Ministère‘s field work as well as on the analysis of geophysical imagery from the SIGÉOM.

Boundaries and Morphology

Stratigraphic Units Concerned

The stratigraphic units found in the Maingault Structural Domain are:

– Maingault Complex (nAmai1, nAmai2, nAmai3 and nAmai4);

– Mistamiquechamic Formation (nAmtc1 and nAmtc2);

– Chamic Mafic-Ultramafic Suite (nAcmi1 and nAcmi2);

– Voirdye Formation (nAvrd2 and nAvrd2a);

– Mistassini Dyke Swarm (nAmib).

Structural Characteristics

❯ Main Fabrics

The main fabric observed in the Maingault Structural Domain is mainly materialized by gneissosity and migmatitic banding in the tonalite and granodiorite of the Maingault Complex. The most notable structural characteristic of this domain is a sequence of kilometric upright open folds with a slight plunge that form a series of elongated domes trending NNW-SSE. These folds are manifested by slightly to moderately dipping foliations. In outcrop , the gneissosity is marked by granite and diorite bands concordant or subconcordant with this fabric. These bands can also form dismembered iscoclinal folds that are transposed into the S_n fabric. These bands can also form dismembered isoclinal folds transposed into the S_n fabric. In samples and in thin sections, the regional foliation is marked by the segregation and alignment of clear (quartz, plagioclase, K-feldspar) and ferromagnesian minerals (biotite, hornblende, clinopyroxene, orthopyroxene). The associated L_n lineation has a slight plunge and is highlighted by stretched quartz grains or hornblende crystals.

In the metavolcanic rocks of the Mistamiquechamic Formation, the S_n fabric is mainly highlighted by a mm- to cm-thick banding defined by melanocratic to mesocratic layers. It is marked by hornblende crystals that partially to completely replace clinopyroxene and orthopyroxene porphyroblasts. In some places, the banding is affected by folding to form isoclinal folds, reflecting the intense deformation that characterizes this domain. In rocks that have undergone more advanced melting, the neosome, which is partially or totally segregated, highlights a migmatitic band consisting of a quartz-plagioclase ± clinopyroxene ± orthopyroxene ± garnet ± K-feldspar mobilisate and centimetric layers of hornblende-orthopyroxene-clinopyroxene-magnetite ± garnet restite (rare). Unlike the plutonic rocks of the Maingault Complex, the fabric of the Mistamiquechamic Formation has generally moderate to steep dips. The L_n lineation is marked by the preferential orientation of hornblende crystals.

❯ Other Fabrics

The primary foliation observed in the Maingault Structural Domain corresponds to the magmatic bedding recognized in the ultramafic rocks of the Chamic Mafic-Ultramafic Suite. This bedding is highlighted by layers rich in cumulus orthopyroxene crystals. It is cut by zones of L/S (outcrop 24-GS-2083) or L tectonites (24-SL-4050 and 24-SL-4051). It is generally concordant with the regional foliation, suggesting that these intrusions are pretectonic to syntectonic.

It is also worth noting that some outcrops of orthopyroxene paragneiss of the Voirdye Formation, observed along the NW edge of the Maingault Structural Domain, show a preserved sedimentary bedding and coarse to fine granoclassing suggesting normal polarities (24-DB-1005 and 24-WM-1154). This bedding is folded and forms a series of synclinal and anticlinal folds which display hinges characterized by S_n+1 axial plane schistosity. However, these structural elements are more representative of the La Sicotière Structural Domain, since the orthopyroxene paragneiss of the Voirdye Formation is only found at the edge of the Maingault Structural Domain. A full description of these fabrics is provided in the La Sicotière Structural Domain sheet.

❯ Folds

Field observations and the structure stereogram show that the main fabric S_n is folded by a P_n+1 event. The disposition of these folds, which have decametric to kilometeric wavelengths, defines dome shapes elongated in a NNW-SSE direction.

Geometric Parameters of Regional Folds: 

❯ Crosscutting Relationships

Networks of NW-SE and NE-SW conjugated fragile faults have been interpreted from the aeromagnetic maps. The NW-SE faults have the same orientation as the Neoarchean diabase dykes of the Mistassini Dyke Swarm. In sheet 33A02, one of these faults displays apparent dextral movement (Beauchamp, 2019).

❯ Kinematics

Does not apply.

Deformation Style

The Maingault Structural Domain is characterized by a complex architecture and metamorphic history. The exhumation of deep, highly metamorphosed rocks that make up this domain involves vertical movement along the numerous shear zones that border or cross the Maingault Structural Domain. The Voirdye Formation paragneiss and the Maingault Complex rocks share several structural characteristics, suggesting that the Maingault Complex gneissic rocks form the basement of the supracrustal sequence of the Voirdye and Mistamiquechamic formations. In this respect, the Maingault Structural Domain is similar to the gneissic domes of the La Hutte Complex (nAhue1), which were probably exhumed during the D_n and D_n+1 phases interpreted in the La Sicotière Structural Domain. Thus, the D_n and D_n+1 deformation phases affecting the La Sicotière and Maingault structural domains would be contemporaneous.

Metamorphic Characteristics

The tonalitic gneiss of the Maingault Complex and the volcano-sedimentary rocks formed by the Mistamiquechamic and Voirdye formations show abundant evidence of partial melting and all contain variable proportions of orthopyroxene (1-10%), demonstrating that the metamorphic facies of the Maingault Domain has reached granulite facies.

In the granitic metavolcanites of the Mistamiquechamic Formation, partial melting is manifested by the presence of a partially to non-segregated neosome. The contact between the paleosome and the neosome is diffuse. In the latter, the leucosome tends to form irregular, unconnected pockets or nets. The neosome is commonly zoned with a central part composed of centimetric orthopyroxene and clinopyroxene poikiloblasts containing quartz, plagioclase and magnetite inclusions, while the edges are characterized by the quartz-plagioclase-K-feldspar assemblage. The presence of both types of pyroxene in the leucosome shows that these phases are composed of peritectic minerals. The composition of the neosome is almost identical to that of the paleosome, but the latter contains hornblende crystals that can be distinguished by their jagged edges. The scalloped microstructure of hornblende in the palaeosome and the absence of this mineral in the neosome suggest that reactions involving the dehydration of hornblende, such as hornblende + plagioclase = clinopyroxene + liquid, are at the origin of these peritectic phases (Sawyer, 2008). Interestingly, hornblende is clearly predominant over biotite in the paleosome, suggesting that the latter was completely substituted first as a result of the biotite + quartz + plagioclase = orthopyroxene + magnetite + liquid reaction. A similar reaction is probably responsible for the formation of orthopyroxene in the tonalitic gneiss and tonalite of the Maingault Complex. This reaction would also have contributed to the formation of magnetite, which is abnormally abundant in these rocks (up to 5%) and in the Mistamiquechamic granulitic amphibolite (≤10%). Partial melting reactions requiring high oxygen fugacity, such as biotite + plagioclase + quartz = magnetite + ilmenite + liquid, could explain these high proportions of iron oxide.

The orthopyroxene and clinopyroxene observed in thin sections show evidence of retrograde reactions, including crowns comprised of a preserved core surrounded by micas and amphiboles, pseudomorphic replacement by green hornblende, and symplectitic borders composed of hornblende-quartz or biotite-quartz.

Alterations

Does not apply.

Geophysical Characteristics

The map of the vertical gradient of the total magnetic field indicates that this domain has a shagreened texture with some marked heterogeneity, notably due to strongly magnetic bands oriented NNE-SSW. The magnetic signal of the Maingault Structural Domain is very high, which is explained by the presence of magnetite (1-5%) in the gneiss, granite and granulitic amphibolite.

Chronological Markers

This structural domain has not yet been dated. However, a Neoarchean dyke of the Mistassini Dyke Swarm cuts across the domain.

References

Publications Available Through Sigéom Examine

Other Publications

Suggested Citation

Contributors