Background and Methodology

This record is based on aeromagnetic maps’ interpretation (Intissar et al., 2014) and information collected from geological survey carried out in the summer of 2019 in the Sirmiq Lake area (Vanier and Lafrance, 2020).

 

Boundaries and Morphology

Stratigraphic Units Concerned

The DSrou is mainly composed of rocks of the Sainte-Hélène Complex. Some klippes of the Pingasualuit Complex and Kuugaq Suite are also present, as well as late granitic rock masses of the Sannigajualuk Suite.

 

Structural Characteristics

❯ Main Fabrics

 

Linear and planar fabrics of the DSrou have a very variable orientation, reflecting both the domed geometry and the presence of some highly migmatized areas. Overall, the foliation and gneissosity’s dip is shallow to moderate (<50°). The same applies to the inclination of lineations which is generally low, commonly <40°. Generally, foliation and gneissosity are straight and regular.

In banded or migmatitic rocks, foliation is accompanied by stromatic or compositional banding mainly defined by grain size and mineralogical variations. Schlierens and melanosome klippes define a planar fabric more irregular than foliation or gneissosity. In areas where partial melting is more advanced, these structures give a creep appearance to the rock.

 

❯ Other Fabrics

Not observed.

 

❯ Folds

On some outcrops, the foliation is folded by wide (amplitude >10 m) or compressed (decimetric) folds. The former are commonly visible in areas where foliation is subhorizontal. Fold observations do not currently allow any organized pattern to be defined. 

❯ Crosscutting Relationships

None.

 

❯ Kinematics

Does not apply.

 

Deformation Style

The DSrou is a dome consisting mainly of dioritic to tonalitic orthogneiss. The shape of the dome is that of an ellipse whose long axis is N-S oriented. This shape is clearly defined by aeromagnetic lineaments and foliation traces. The intensity of deformation is homogeneous and corresponds to a regular foliation or gneissosity usually consistent between outcrops. However, in areas where foliation is very slightly inclined (<20°), the orientation is variable and disordered in appearance.

 

Metamorphic Characteristics

The dominant unit of the DSrou is the Saint-Hélène Complex, which has abundant textures indicative of partial melting such as stromatic banding and various neosomes. Deformation is usually less significant in rocks that have undergone a high partial melting rate than in rocks that appear to be poorly or not migmatized. This indicates that deformation is concomitant with migmatization, so rocks containing high mobilisate contents have recorded less deformation. Stromatic textures form when deformation is simultaneous with anatectic conditions (Sawyer, 2008). Thus, the upper amphibolite facies metamorphic event that caused migmatization of the Sainte-Hélène Complex is also associated with the main deformation.

Petrographic descriptions support high temperature conditions during deformation. Indeed, quartz commonly developed a lenticular arrangement of interlobed to amoeboid grains as well as pinch textures, which indicate dynamic recrystallization by grain edge migration (Guillope and Poirier, 1979; Urai et al., 1986; Jessel, 1987). This quartz recrystallization mechanism is active at >500°C (Kruhl, 1998; Stipp et al., 2002).

 

Alterations

Not observed.

 

Geophysical Characteristics

At the periphery of the DSrou, alternating positive aeromagnetic ridges define ellipses that allow to visualise the general geometry of the dome (Intissar et al., 2014). In the centre, the DSrou has a diffuse aeromagnetic signature that reflects the shallowly dipping geological structures and contacts, which cause little variation in aeromagnetic properties in the horizontal plane.

 

Chronological Markers

Deformation in the DSrou mainly affects the Sainte-Hélène Complex, which is of Archean age and which underwent a partial melting event concomitant with the main deformation. There is currently no known metamorphic age in the Sainte-Hélène Complex. However, this episode is likely close to the age of other complexes and suites of the Narsajuaq Lithotectonic Domain, from 1880 Ma to 1806 Ma (Davis and Sutcliffe, 2019; Vanier and Lafrance, 2020), this age range is currently the best estimate for a synkinematic age. The best approximation for a minimum age corresponds to the intrusion of granitic dykes related to the Sannigajualuk Suite at ~1800 Ma (Parrish, personal communication, 1994), since these cut the main fabric in the DSrou.

 

References

Publications Available Through Sigéom Examine

Other Publications

 

Suggested Citation

Contributors

First publication

Marc-Antoine Vanier, Jr. Eng., M.Sc. marc-antoine.vanier@mern.gouv.qc.ca; Isabelle Lafrance, P. Geo., M.Sc. isabelle.lafrance@mern.gouv.qc.ca (redaction); Ghyslain Roy, P. Geo. (coordination); James Moorhead, P. Geo., M.Sc. (critical review); Simon Auclair, P. Geo., M.Sc. (editing); André Tremblay (HTML editing); Céline Dupuis, P. Geo., Ph.D. (English version)