Background and Methodology

The Ayr Structural Domain was defined by Beauchamp (2020) following a mapping survey conducted in the Cadieux Lake area in the summer of 2018, and by analyzing high-resolution aeromagnetic survey maps (D’Amours, 2011). Chartier-Montreuil et al. (2024) redefined the domain boundaries using geophysical data processed by Cleven et al. (2020) and mapping work by Massei and Hammouche (2016) in the Emmanuel Lake area.

The Ayr Structural Domain takes its name from the eponymous lake located on the contiguous boundary of NTS sheets 33A07 and 33A10.

Boundaries and Morphology

Stratigraphic Units Concerned

The lithostratigraphic and lithodemic units that make up the Ayr Structural Domain are:

• variably migmatized metasedimentary rocks (units nAlgi2a, nAlgi3a and nAlgi4b) of the Laguiche Complex;
• the Nikos Suite (units nAnik1 and nAnik2).

Note that most of the Opinaca Subprovince rocks belonging to this structural domain have not yet been assigned to a formal unit. They include a variety of lithologies: paragneiss, granite, granodiorite, quartz syenite, monzonite, quartz monzodiorite, diorite, and amphibolite.

Structural Characteristics

❯ Main Fabrics

The main fabrics recognized within the DSayr are the S_n tectonometamorphic foliation and the M_n migmatitic banding. The preferential orientation of euhedral to subhedral biotite flakes emphasizes the S_n foliation in granoblastic paragneiss. Where migmatitic banding is stromatic, mobilisate bands are subparallel to the main S_n foliation. In some diatexites, rafts have an orientation that is consistent with the trace of a regional foliation. At the domain scale, the regional S_n foliation is folded. Foliation poles all line up in a great circle which is oriented NNE, suggesting that it is folded.

❯ Other Fabrics

The S₀ stratification, which corresponds to compositional banding, is observed on some sedimentary rock outcrops of the Laguiche Complex (outcrops 18-FM-2092 and 18-LP-6143). Compositional banding is characterized by alternating beds that are richer in biotite, and which were originally more argillaceous, and beds that are poorer in biotite, which represent sandstone beds. It is commonly folded and generally shows a NNW-SSE orientation. Locally, the difference in the apparent partial melting rate within the same outcrop can be explained, among other things, by the presence of compositional banding. Some layers are completely melted (diatexite and anatectic granite), while others only experienced partial melting (paragneiss and metatexite).

❯ Folds

Folding in the Ayr Structural Domain is very chaotic in style and attitude, which makes it difficult to interpret (migmatitization texture and ptygmatitic folding). Some remains of tight to isoclinal P_n tectonic folds with ENE-WSW axial planes are preserved in the least melted parts of the Laguiche Complex metasedimentary sequence (outcrops 18-MP-5048, 18-LP-6014 and 18-LP-6144). The earliest mobilisate phases in the tectonometamorphic history are affected by P_n folding. The axial plane fabric is commonly obliterated by extensive metamorphic recrystallization. Some Z-shaped drag folds suggest apparent dextral movements.

Geometric Parameters of Regional Folds: 

❯ Crosscutting Relationships

The DSayr is cut by steeply dipping, late NNW-SSE oriented brittle faults. Most of these faults have been inferred from maps of the residual magnetic field, vertical gradient and derivative tilt. At the outcrop scale, the expression of these late structures consists of brittle fractures that are either epidotized, hematitized or both.

❯ Kinematics

Does not apply.

Deformation Style

The Ayr Structural Domain displays a geometry of domes and basins. The morphology of the domain as a whole and its internal architecture demonstrate significant crustal deformation under sub-solidus conditions associated with strong N-S shortening.

Metamorphic Characteristics

Metasedimentary rocks of the Laguiche Complex are commonly highly migmatitized, with mobilisate commonly exceeding 50% of the total volume of the rock. The presence of paragneiss and metatexites in the southern part of the DSayr, evolving towards diatexites and anatectic granites in the northern part, indicates an increase in partial melting and possibly in the temperature gradient. Biotite is ubiquitous in this paragneiss. Garnet and hornblende are present locally, both in mobilisate found in restitic parts as well as in paleosome. A biotite-garnet mineralogical assemblage indicates that paragneiss reached metamorphic conditions typical of the upper amphibolite facies. Orthopyroxene was not observed in the Ayr Domain rocks. However, these are surrounded by rocks that have reached the granulites facies. Given the high proportion of partial melting, it is highly likely that the rocks in the Ayr Domain also reached the same granulites facies conditions.

At least three types of mobilisate are clearly visible in paragneiss, and are thought to have been emplaced during the great continuum of D_n deformation:

1) Thin (mm to cm) in situ leucosomes are subconformable to the S₀ fabric, where present. These leucosomes are boudinaged, discontinuous and commonly folded by P_n folds. This mobilisate seems to have been emplaced during deformation D_n.

2) Large injections in the form of bands of granitic to tonalitic melting material, which are preferentially injected into S_n foliation planes. This generation of mobilisate gives a stromatic texture to the rock. This mobilisate would be synkinematic and its emplacement would correspond to the peak of D_n deformation.

3) The mobilisate that was emplaced late during D_n deformation cuts off the generations mentioned above.

Alterations

Does not apply.

Geophysical Characteristics

Aeromagnetic maps (D’Amours, 2011; Cleven et al., 2020) indicate that the Ayr Structural Domain has a very homogeneous magnetic pattern with a medium to high contrast. The domain boundaries correspond to the limits of the large NW fold axis characterizing the area, which can easily be identified due to the concordant orientation of the majority of magnetic lineaments. Relatively continuous bands characterize this boundary.

Chronological Markers

In the Ayr Structural Domain, a datation was performed on a sample of wacke-derived paragneiss from the Laguiche Complex (16-HH-1654-A, sheet 33A10). The monazite in this sample yielded a uniform age of 2692 ±6 Ma (Davis and Sutcliffe, 2018), which represents a metamorphic age and a minimum age of deposition for sedimentary rocks. This age seems to correspond to the metamorphic episode associated with partial melting of sedimentary rocks in the complex; it could therefore date the D_n deformation phase. Two anatectic granite samples also collected in the Laguiche Complex (16-AS-6666-A, sheet 33A10 and 18-SG-7102-A, sheet 33A07 in the Kaawaischiskau Structural Domain) provided crystallization ages (U-Pb on zircons) of 2674 ±5 Ma (Davis and Sutcliffe, 2018) and 2684 ±3 Ma (Davis, 2019), respectively.

References

Publications Available Through Sigéom Examine

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