Metamorphic Tectonites

The whole range of granitic protomylonites, mylonites and breccias, occasionally cemented by pseudotachylite, marks strain gradients toward the Maybelle River shear zone (MRSZ). The intensity of strain controls the intensity of retrogression. Gneissic banding of alternating quartzo-feldspathic and mafic layers is gradually overprinted by a penetrative mylonitic fabric defined by polycrystalline quartz-feldspar ribbons or streaks, individual quartz and feldspar porphyroclasts in a fine-grained matrix of crushed feldspar, variably chloritized biotite, chlorite, muscovite flakes, sericite, epidote/allanite, opaque minerals, apatite and zircon. Incomplete retrograde re-equilibration of an igneous or medium- to high-grade metamorphic mineral assemblage resulted in preservation of scattered monomineral or composite clasts, including garnet, clinopyroxene and sillimanite. Plagioclase is fractured or with bent twin lamellae and partly altered to sericite and epidote. Garnet may retain strain shadows even after considerable alteration to chlorite, and quartz shows strain patterns and pressure solution at points of grain contact. Distorted streaks of graphite occur locally in the chlorite-rich mylonite. The original textural relationships are mainly distorted by strain and hydration at lower temperature, and thus, it is unclear whether the mineral relics belong to the original igneous paragenesis or were formed during a subsequent tectonometamorphic event. However, spinel, cordierite, sillimanite and garnet-bearing assemblages in the Maybelle River lithologies record tectonism at deep to intermediate crustal levels. Deformation at shallower structural levels is recorded by sillimanite-graphite mylonites, biotite and chlorite-bearing mylonites, whereas hematite and clay-rich mylonites record deformation in the shallowest 3 to 4 km. Upon exhumation, late-stage carbonate-sealed micro-cracks and fractures indicate that carbon-rich fluids percolated the basement.

The dominant stretching lineation in ductile mylonites is strike-parallel to oblique. Textural relationships observed in the metamorphic tectonites recovered from the sub-Athabasca basement suggest progressive, non-coaxial deformation within a ductile to brittle-ductile transcurrent shear zone. Discrete shear surfaces, occasionally sericite coated, contain slickenside striae indicating thrust, normal or strike-slip displacement. These late brittle faults affect both the crystalline basement and the overlying Athabasca Group and have offsets of metres to tens of metres. At one location, the intersection of a post-Athabasca fault with the Maybelle River shear zone is accompanied by uranium-mineralization of the overlying Athabasca Group sandstones. The mineralization extends along the MRSZ, indicating the permeable shear zone constituted the main channelway for mineralizing fluids.

Tectothermal Evolution of the Taltson Magmatic Zone

The geological significance of petrological data obtained from drillcore of the sub-Athabasca basement can only be inferred. This is because of the sparseness of index metamorphic minerals, distortion of original textural relationships by retrograde overprinting and limited data on spatial relationships between rock types. Relics of spinel, cordierite, sillimanite and garnet record metamorphism under high- to medium-grade metamorphic conditions, whereas a sequence of mineral assemblages were formed at consecutive stages along a retrograde path through low- to very low-grade metamorphic conditions within progressively narrowing zones of strain and hydration. The spatial distribution of strain and mineral assemblages within the Maybelle River area, and most evidently in the exposed segment of the Taltson magmatic zone (TMZ), are consistent with a protracted ductile to brittle-ductile deformation during the passage of the presently exposed crustal level from deep to intermediate and shallow structural levels.

Diagram depicting syntectonic progressive exhumation

Geothermobarometric calculations indicate peak metamorphic conditions of 6 to 7 kilobars and temperatures of about 975°C and 940° to 1000°C have been locally preserved in the exposed TMZ. Shear zone deformation was initiated at pressures of approximately 5 to 7 kilobars and temperatures of 800°C to 850°C. Uranium-lead zircon and monazite ages suggest granulite facies metamorphic conditions between about 1.95 and 1.93 billion years before present (Ga), partly contemporaneous with the ‘late' granitoid pulses. Strain was heterogeneously partitioned in zones of high non-coaxial deformation within the Taltson basement complex and the adjacent plutons of the Taltson magmatic zone. The end of the main igneous activity and partial crustal stabilization around 1.95 Ga was followed by transition from widespread granulite facies deformation to more localized deformation along transpressional shear zones. Thus, the 1.93 to 1.91 Ga monazite ages and the 1.92 to 1.85 Ga titanite ages, estimated on variably foliated granitoids, most likely record syn-tectonic uplift above the 700°C and 600°C isotherms, respectively, during the passage from granulite to amphibolite conditions. ⁴⁰Ar/³⁹Ar hornblende data from both sheared and massive diorites indicate cooling above the 525°C isotherm at about 1.90 Ga. ⁴⁰Ar/³⁹Ar dates on muscovite from mylonitic micaceous gneisses and from Charles Lake granitoids record slow cooling through 350° to 325°C between about 1.86 Ga and 1.80 Ga, and probably mark the cessation of deformation under low-grade conditions in the TMZ. The lack of significant isotopic record of tectonism between 1.80 and 1.60 Ga suggests the deposition of the Athabasca Group occurred on a tectonically quiet, peneplenized TMZ.

The shortcomings of the subduction-related tectonomagmatic models at, or distant from TMZ encourage re-evaluation of existing data in light of large-scale sub-continental mantle dynamics. Thus, the Taltson basement complex granulite domain of the Rae crust may have been heated above the stable, steady-state geotherm to follow PTt path “b” in diagram either by magma underplating (model B), or by radioactivity and crustal melting (model C). Uplift due to underplating followed by erosion can explain the dominance of high-grade metamorphic rocks in the surface outcrop of Archean terranes. Our isotope data indicate little, if any, addition of juvenile material during the Taltson tectonomagmatic event, which may be interpreted as underplating restricted to a layer at the crust-mantle interface with limited, if any, mixing with crustal melts.
Alternatively and consistent with all lines of evidence, the TMZ may have initiated in the thermally weakened Archean Rae crust by mantle downwelling, under an overall transpressive far-field stress regime with resultant extensional zones, and evolved into an intra-continental orogen associated with crustal melting and early granitoid emplacement during 1.99 and 1.95 Ga. Conspicuous subhorizontal lineation in the metamorphic tectonites and their variable crosscutting relationships with late granitoid phases suggest that TMZ gradually evolved into a wide zone of oblique crustal stretching that triggered decompression melting and emplacement of late syntectonic granitoid rocks between 1.95 Ga and 1.93 Ga. This phase of transcurrency in the TMZ was contemporaneous with, and possibly triggered by the inferred subduction at the western edge of the Slave-Churchill continent. Transition from widespread granulite facies deformation to localized deformation along greenschist transpressional shear zones record syntectonic uplift and erosion of the TMZ orogen between 1.93 Ga and 1.80 Ga and appears kinematically linked to the evolution of the Great Slave Lake shear zone. The 1.75 Ga Fair Point Formation at the base of the Athabasca Group, as well as the Nanacho and Thelon groups farther north, may represent remnants of the clastic infill of the intracontinental basins initiated within crustal down-warping east of the TMZ.

Idealized PTt paths for the metamorphic tectonites of the westernmost sub-Athabasca basement

There is no record of prograde metamorphic reactions, and thus, the dehydration segment of the PTt path for the TMZ remains enigmatic, as does the geodynamic significance of these metamorphic rocks. TMZ has been interpreted in terms of plate boundary tectonomagmatic processes as a subduction/collision Himalayan-type orogen or as a Tian Shan-type orogen developed in a plate-interior setting, in response to distant plate boundary compression. A subduction/collision tectonic evolution (model A in diagram) would require prograde metamorphic reactions through the kyanite field to high-pressure granulites and/or eclogites and/or blueschists along the clockwise PTt path “a” in diagram.

The lack of

• high-pressure rocks or at least kyanite-bearing assemblages;
• regionally consistent thrust zones;
• remnants of the postulated Rutledge oceanic crust; and
• 2.0-1.9 Ga juvenile material in the TMZ

casts doubts on the validity of a subduction/collision scenario, even though it is well supported by the regional geological context.

The Tian Shan analogy for the TMZ, which requires a Himalayan-type collision between 2.0-1.9 Ga at the distant plate margin west of the TMZ, is inconsistent with available data. Deformation of the Coronation Supergroup in the Wopmay orogen, tentatively ascribed to oblique collision of the Atlantic-type western margin of the Slave Province with an exotic island arc, started as late as about 1.84 Ga, thus postdating the main TMZ tectonomagmatic events occurring between about 1.99-1.93 Ga.

The shortcomings of the subduction-related tectonomagmatic models at, or distant from TMZ encourage re-evaluation of existing data in light of large-scale sub-continental mantle dynamics. Thus, the Taltson basement complex granulite domain of the Rae crust may have been heated above the stable, steady-state geotherm to follow PTt path “b” in diagram either by magma underplating (model B), or by radioactivity and crustal melting (model C). Uplift due to underplating followed by erosion can explain the dominance of high-grade metamorphic rocks in the surface outcrop of Archean terranes. Our isotope data indicate little, if any, addition of juvenile material during the Taltson tectonomagmatic event, which may be interpreted as underplating restricted to a layer at the crust-mantle interface with limited, if any, mixing with crustal melts.

Alternatively and consistent with all lines of evidence, the TMZ may have initiated in the thermally weakened Archean Rae crust by mantle downwelling, under an overall transpressive far-field stress regime with resultant extensional zones, and evolved into an intra-continental orogen associated with crustal melting and early granitoid emplacement during 1.99 and 1.95 Ga. Conspicuous subhorizontal lineation in the metamorphic tectonites and their variable crosscutting relationships with late granitoid phases suggest that TMZ gradually evolved into a wide zone of oblique crustal stretching that triggered decompression melting and emplacement of late syntectonic granitoid rocks between around 1.95 Ga and 1.93 Ga. This phase of transcurrency in the TMZ was contemporaneous with, and possibly triggered by the inferred subduction at the western edge of the Slave-Churchill continent. Transition from widespread granulite facies deformation to localized deformation along greenschist transpressional shear zones record syntectonic uplift and erosion of the TMZ orogen between 1.93 Ga and 1.80 Ga and appears kinematically linked to the evolution of the Great Slave Lake shear zone. The 1.75 Ga Fair Point Formation at the base of the Athabasca Group, as well as the Nanacho and Thelon groups farther north, may represent remnants of the clastic infill of the intracontinental basins initiated within crustal down-warping east of the TMZ.