Background and Methodology

The Lac Tudor Shear Zone (ZCtud) was first described by van der Leeden (1986) in the Southeastern Churchill Province (SECP) and recognized as a lithotectonic domain called “Lac Tudor Shear Zone Domain” (van der Leeden et al., 1990). These authors then considered that this zone also included the Champdoré Shear Zone, located directly to the west, which was later separated by Hammouche et al. (2011). Based on field checks conducted in the summer of 2016, Charette et al. (2018) consider the Champdoré Shear Zone to be a distinct structural domain in which deformation is not pronounced compared to other SECP ß shear zones (see “Structural Geology” section of Charette et al., 2018).

Generally, the ZCtud is considered to represent the western boundary of the De Pas Supersuite in the southern SECP (Martelain, 1986; van der Leeden et al., 1990; James et al., 1996; James and Dunning, 2000). Some authors note that it delineates the Crossroads and McKenzie River domains (James et al., 1996; James and Dunning, 2000; Kerr et al., 1994; Hammouche et al., 2011). Lafrance et al. (2014) report that the Marralik Shear Zone, located in the Saffray Lake area (NTS sheet 24G) appears to be the northern extension of the ZCtud. Others suggest that it is N-S oriented in the southern part of the SEPC as it gradually curves towards the NNW-SSE to NW-SE in the northern portion, where it joins the Lac Turcotte Fault (Wardle et al., 2002; Corrigan et al., 2018). However, as part of the SECP synthesis (Lafrance et al., 2018), it was not possible to extend the ZCtud northward in any certainty.

Boundaries and Morphology

Stratigraphic Units Concerned

In the southern SECP, the ZCtud affects units located on the western edge of the George Lithotectonic Domain, as well as those located on the eastern edge of the Baleine Lithotectonic Domain. In the George Domain, De Pas Supersuite rocks are commonly mylonitized, but some De Pas potassic intrusions are also later. In the eastern margin of the Baleine Domain, the following units are affected by the ZCtud: False and Winnie suites, and Knox, Flat Point, Qurlutuq and Ungava complexes.

Structural Characteristics

❯ Main Fabrics

The ZCtud is characterized by the reorientation and straightening of foliations, the thinning of lithological units, and the presence of mylonites and upright gneiss bands. Deformed rocks have a mylonitic or subvertical planar fabric and contain a subhorizontal stretch lineation. However, insufficient lineation measurements are available. Sanborn-Barrie (2016) also reports lineations with steeper plunges. The lineation is often sparsely developed compared to the planar fabric and generally corresponds to S>L tectonites. On several outcrops, tectonic banding (upright gneiss) occurs as millimetric to centimetric alternating bands, either quartz, quartzofeldspathic or mafic. In K-feldspar porphyritic units of the De Pas Supersuite, these bands are deformed, realigned, stretched and crushed to form porphyroclasts. Some mylonite bands are cut by pink granite intrusions that are themselves mylonitized and folded in places.

❯ Other Fabrics

Does not apply.

❯ Folds

Tight isoclinal folds and intrafolial folds are commonly observed.

❯ Cross-Cutting Relationships

Not observed.

❯ Kinematics

According to van der Leeden et al. (1990), kinematic indicators are complex, but there is evidence of a predominantly dextral component, as well as contraction involving overthrusting of rocks located in the east over those in the west. In thin section, Vanier et al. (2018) observed fish-scale feldspar inclusions in quartz, also indicating dextral shearing.

Deformation Style

The ZCtud is a heterogeneous mylonite zone displaying dextral lateral movement with a westward thrust component (van der Leeden et al., 1990). The dominant deformation style corresponds to a network of straight-lined and highly parallelized shears. The dominant fabric is flattening. The ZCtud is interpreted as a transpressive shear zone synchronous with upper amphibolite metamorphic facies (van der Leeden et al., 1990; James and Dunning, 2000). Some authors define it as the boundary between detached crustal segments of the Superior Province and exotic Archean blocs of the SECP (Girard, 1990; James and Dunning, 2000; Wardle et al., 2002).

Metamorphic Characteristics

The presence of pre-kinematic, syn-kinematic, and post-kinematic leucosomes in the shear system has led to the conclusion that the strike-slip deformation is synchronous to partial melting (Charette et al., 2016). This conclusion implies that the dextral strike-slip movement, associated with convergence affecting the SECP, occurred while the SECP itself underwent thickening and partial melting.

Thin section observations of Vanier et al. (2018) highlight the widespread presence of chessboard extinction involving a deformation temperature >650°C (Mainprice et al., 1986). In fact, feldspars are even grained and granoblastic or form porphyroclasts whose mantle is recrystallized, suggesting feldspar recrystallization by subgrain rotation. Analysis of quartz c-axes indicates a deformation temperature between 600°C and 700°C (Vanier et al., 2018).

Alterations

Not observed.

Geophysical Characteristics

The ZCtud is marked by strong straight-lined and parallelized magnetic lineaments.

Chronological Markers

The ZCtud affects rocks of the De Pas Supersuite. However, some intrusions assigned to De Pas are not deformed and cut the mylonitic fabric. The minimum age of this unit (1805 Ma) therefore allows shearing to be constrained to an earlier period. A Labrador dating in the ZCtud extension also indicates a minimum age of 1802 +9/-14 Ma (James and Dunning, 2000) for the deformation.

References

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