Background and Methodology

The Dolent Structural Domain was defined by Beauchamp (2019) following a mapping survey conducted in the Cadieux Lake area in the summer of 2018. It is superimposed on the western branch of the Upper Eastmain Greenstone Belt (UEGB) and on the Cadieux Suite. The following description is based primarily on geological observations collected by Beauchamp (2019), as well as analysis of SIGÉOM regional aeromagnetic survey and detailed magnetic maps of Wade et al. (2014).

Boundaries and Morphology

Stratigraphic Units Concerned

The Dolent Structural Domain is composed of volcanic and sedimentary rocks of the Dolent Formation (nAdln1, nAdln2, nAdln3, nAdln4, nAdln5 and nAdln6 ) and Bohier Group (nAbh2, nAbh3). It also includes intrusive rocks of the Cadieux Suite that follow the southern contact of the UEGB.

Structural Characteristics

❯ Main Fabrics

The dominant planar fabric of the Dolent Structural Domain is foliation S_n, which results from the regional deformation phase D_n. Foliation S_n affects most Archean units of the domain, with the exception of the late granitic intrusive rocks of the Wahemen Suite. This highly penetrative foliation is distinguished by millimetric spacing of flattening planes. Whether in volcanic rocks of the Dolent Formation or in metasedimentary rocks of the Bohier Group, foliation S_n is marked by the preferential orientation of ferromagnesian minerals such as biotite and hornblende. In areas that have accommodated a large amount of deformation, it is also marked by the flattening of quartz and feldspar grains.

In intrusive rocks of the Cadieux Suite, foliation is well developed and defined by the alignment of phyllosilicates (biotite and white mica) and hornblende in spaced flattening planes. As the deformation rate increases, K-feldspar porphyroids are flattened in the S_n direction.

In the eastern part of the domain, foliation S_n is oriented WSW to SW and has a moderate dip towards the NW. Secondary mineral lineations and stretch lineations L_n plunge 55° towards NNE. In the western part of the domain, foliation S_n undulates due to the closure of synformal and antiformal P_n folds.

Deformation of rocks in this area is dominated by flattening and stretching. Sedimentary rocks of the Bohier Group have accommodated deformation better than volcanic rocks and mafic-ultramafic sills of the René Group. The banded appearance of polymictic metaconglomerate is caused by a very high degree of flattening of clasts. In places, the rock forms L-S tectonites. It is so deformed that it is difficult to identify the clasts. Basalt pillows of the Dolent Formation are also very flattened, implying the loss of indicators of stratigraphic polarity. Near the southern contact of the UEGB, pillows are so flattened that they are locally difficult to recognize. They form banns 10 cm to 20 cm thick by 0.5 m to 1 m long. Pillow closures are clearly visible from the side (outcrop 18-AB-1040).

❯ Other Fabrics

In addition to the S_n fabric, stratification S₀ as well as fabrics S_n-1 and S_n+1 were observed in the Dolent Domain.

1) Early Fabrics

Fabric S₀: Stratification S₀ was observed locally in the Dolent Domain. It was identified by the orientation of pillows (peduncles) and quartz chambers in basalt and by compositional banding in conglomerates of the Bohier Group. Indicators of stratigraphic polarity in basalt indicate that the volcanic sequence has been reversed.  At the base of the stratigraphy are mafic and ultramafic (intrusive and volcanic) rocks. They are overlain by basalt. The core of the Branche Ouest Syncline is occupied by polymictic conglomerates of the Bohier Group which unconformably overlie (masked by a fault) volcanic rocks of the René Group.

Fabric S_n-1: Foliation S_n-1 is marked by the preferential orientation of ferromagnesian minerals such as biotite and hornblende. Between Lépante and Digne lakes, stratification S₀ and fabric _n-11 both seem to be transposed into the S_n fabric or at least reoriented almost parallel to S_n. At the western end of the domain, where the UEGB ends, there is an angularity between fabrics S_n-1 and S_n. This angularity exists as the result of the succession of regional fold hinges P_n. Both foliations S_n-1 and S_n are present at outcrops 18-AB-1044 and 18-MP-5127. In the hinge of the regional synform P_n (Oriental Anticline), the S_n-1 fabric varies from 030° to 310°. It is folded by a P_n fold which P_n axial plane fabric is oriented WSW-ENE. In the Bohier Island area, the S_n-1 fabric and stretch lineation L_n-1 have been clearly identified in the centre of Bohier Island, in the nodular paraschist unit (nAbh1a) (Beauchamp et al., 2018).
 

2) Late Fabrics

Fabric S_n+1: Foliation S_n is repeated by a crenulation cleavage S_n-1. The S_n-1 fabric is clearly visible on most conglomerate outcrops and on some basalt outcrops. It consists of crenulation cleavage with flattening planes spaced ~1 cm apart and oriented NNE-SSW. P_n+1 folds having NNE-SSW axial planes form Z-shaped structures (outcrops 18-MP-5030, 18-AB-1084, 18-SG-7023, 18-SG-7024, 18-FM-2018 and 18-LP-6027).

❯ Folds

The western end of the Dolent Structural Domain is affected by a succession of several open to isoclinal regional folds P_n. Reverse polarities in pillow basalts of the Dolent Formation indicate increasingly younger layers on either side of the Oriental Anticline. The Branche Ouest Syncline passes in the middle of sedimentary rocks of the Bohier Group. Magnetic field maps reveal, locally in the domain, geometries that could be the result of at least two folding phases, which is confirmed by the folded trajectories of foliation S_n. P_n folds are folded by P_n+1 folds of NE-SW to NNE-SSW axial plane, subparallel to schistosity S_n+1. At the outcrop scale, P_n+1 folds are chevron folds commonly Z-shaped.

Geometric Parameters of Regional Folds: 

❯ Crosscutting Relationships

The existence of very detailed magnetic maps in the Dolent Structural Domain (Wade et al., 2014) has allowed mapping numerous fragile faults. These families of conjugated faults offset rocks in a dextral or sinistral manner. Field evidence corroborates the presence of faults and indicates that these fractures are characterized by steep dips. Families of conjugated diaclases with displacements of a few centimetres to ~ 1 dm have been observed on several outcrops.

Geometric Parameters of Regional Faults Cutting the Dolent Structural Domain:

❯ Kinematics

Does not apply.

Deformation Style

The Dolent Structural Domain has a complex structural architecture. The first phase of deformation D_n-1, with associated foliation S_n-1, is globally oriented ENE-WSW. This deformation phase seems to have led to the development of domes exposing plutonic rocks and synclines preserving volcano-sedimentary sequences. In the Cadieux Lake area, exhumation of the Misasque Complex and Digne Pluton within a dome would then have led to early folding of the overlying Dolent Formation. Vertical movements during D_n-1 would also have been accommodated by shear zones located at the interface between the domes’ gneisso-plutonic packages and the synclines’ supracrustal units (e.g. the Digne Shear Zone).

Subsequently, during deformation phase D_n, an episode of NNW-SSE shortening would have prolonged the subsidence of the supracrustal sequence in an ENE-WSW syncline, filled by the Bohier Group. At the edge of this basin, ENE-WSW shear zones appear to have contributed to the burial of supracrustal rocks. However, NNW-SSE shortening also resulted in the lateral flow of deeper crustal domains, such as the Misasque dome. This flow would have led to parallelization of P_n-1 fold axes and mineral and stretch lineation L_n in a NE-plunging direction and to the overturning of the lithostratigraphic pile at the SW end of the Misasque dome (see the interpretative model of the Cadieux Lake Geological Bulletin; Beauchamp, 2019).

Finally, deformation phase D_n+1 would have folded previous fabrics, but could represent a late increment related to the main deformation episode D_n.

Metamorphic Characteristics

Supracrustal rocks of the Dolent Structural Domain have reached the amphibolite facies. Mafic units nAdln1 and nAdln2 of the Dolent Formation are composed of a typical HB-PG-EP ± SN assemblage. Actinolite and clinopyroxene are present locally and indicate reaching the lower and upper amphibolite facies respectively. Mafic-ultramafic sills (nAdln3) are mainly composed of amphiboles (± HB ± CG ± AC), plagioclase and several retrograde minerals (EP ± TC ± CL ± BO). Finally, volcanics and ultramafic sills (nAdln4) are characterized by prograde TM-MG ± OV and retrograde ST-TC-CL assemblages.

Alterations

Does not apply.

Geophysical Characteristics

Maps of the vertical gradient of the total magnetic field from SIGÉOM and Wade et al. (2014) indicate that this domain is heterogeneous in terms of magnetic susceptibility. Rocks of the UEGB are characterized by a banded magnetic structure and weak to strong magnetic contrasts. Basalts of the Dolent Formation and conglomerates of the Bohier Group generally do not contain magnetite, and thus correspond to zones of low magnetic susceptibility. Thin strips characterized by positive magnetic anomalies correspond to more or less continuous layers of ultramafic volcanic rocks and mafic-ultramafic sills of the Dolent Formation. Magnetic and topographic lineaments of this domain are clearly superimposed on S_n foliation trajectories. Magnetic lineaments are straight and continuous between Lépante Lake and Digne Lake. They are sharply cut to the south by rocks of the Cadieux Suite which are characterized by a very high magnetic susceptibility. At the western end of the UEGB, lineaments are curved and indicate the closure of a synformal anticlinal fold P_n that folds the S_n-1 fabric.

Chronological Markers

A wacke paragneiss sample belonging to the Laguiche Complex (16-HH-1654-A, sheet 33A10) was dated. Monazite in this sample yielded a uniform age of 2692 ±6 Ma (Davis and Sutcliffe, 2018a), which represents the metamorphic age and the minimum age of deposition of sedimentary rocks. These ages appear to date the metamorphic episode associated with partial melting of sedimentary rocks of the Sedimentary Complex, and could therefore date deformation phase D_n.

Since the MacLeod Batholith (2704 ±2 Ma, Davis, 2019) and the Cadieux Suite (2700 ±3 – 2699 ±5 Ma, Davis, 2019; Davis and Sutcliffe, 2018b) are foliated by the S_n fabric, their emplacement is prior to or synchronous with deformation D_n. Their age of crystallization could date the beginning of deformation phase D_n. Deformation D_n is earlier than 2640 Ma (Davis and Sutcliffe, 2018a), corresponding to the age of the Wahemen Suite, which is unaffected by deformation and metamorphism.

References

Publications Available Through Sigéom Examine

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