Deformation and extensional exhumation of 1.9 Ga high-pressure granulites along the Wholdaia Lake shear zone, south Rae craton, Northwest Territories, Canada

Abstract

The origin of high-pressure granulites in the south Rae craton and Snowbird tectonic zone (STZ) is highly enigmatic. Current models for their formation and exhumation envisage continental collision at 2.55 Ga and intracratonic orogenesis at 1.9 Ga, or collision and exhumation at ca. 1.9 Ga. As an attempt to reconcile these disparate models, we conducted a regional and detailed mapping program along a geophysical discontinuity 100 km west of the STZ within the south Rae craton of the Northwest Territories, Canada. This work presents the discovery of a new crustal-scale shear zone, the Wholdaia Lake shear zone (WLsz), which deformed and transposed host rocks into a 20-km-wide and 300-km-long ductile high-strain zone. U-Pb zircon geochronology was utilized to establish host-rock crystallization ages, timing of deposition of metasedimentary rocks, and age constraints of metamorphism and ductile shearing. Hanging-wall metasedimentary rocks have a new depositional range of 1.98-1.93 Ga and contain abundant metamorphic zircon at 1.91 Ga. The protoliths of the footwall mafic granulite orthogneisses crystallized at 2.6 Ga and were metamorphosed at 1.9 Ga, which extends the known footprint of 1.9 Ga metamorphism 100 km west of the STZ. During and after 1.9 Ga metamorphism, the WLsz began progressively exhuming footwall rocks in three distinct stages, associated with (1) normal-oblique shearing at high-pressure granulite-facies conditions, (2) normal-oblique shearing accompanied by mylonitization at amphibolite-facies conditions, and (3) normal-oblique shearing with ultramylonite development at amphibolite-to greenschist-facies conditions. Ductile shearing was waning by 1.86 Ga, based on ages obtained from late syn-to postkinematic crosscutting dikes. Collectively, the WLsz in concert with other regional structures aided both extensional and thrust-sense exhumation of a large high-grade terrane at 1.9 Ga in the south Rae craton.

The distinct metamorphic stages and structural styles of the 1.94–1.86 Ga Snowbird Orogen, Northwest Territories, Canada

“…Zircon U-Pb and trace element data for this sample indicate a maximum deposition age of 1,983 ± 19 Ma (Thiessen et al, 2018). Metamorphic zircon with fir-tree zoning and flattened heavy rare earth element (HREE) signatures overgrow detrital cores at 1,915 ± 4 Ma (MSWD = 2.7, Thiessen, Regis, & Gibson, 2017).…”

“…The Sb Pg exhibits a schistose to gneissic texture with leucosome segregations (Figure 3, Thiessen et al, 2018). It contains a prominent foliation that strikes northwest and dips ~30° to the northeast ( Figure 2b) km-scale folded package whose axial trace trends to the northnortheast, parallel to S 2 documented throughout the domain.…”

“…The Dodge-Snowbird domain is separated from the Train-Firedrake domain to the west by the WLsz (Figure 2a), a 300 km long, up to 20 km wide, greenschist to granulite facies, crustal-scale shear zone that accommodated extensional exhumation (D 3 ) between c. 1.90 and 1.86 Ga (Thiessen et al, 2018). Curvilinear magnetic patterns within bounding wall rock domains have been realigned in a dextral sense into the linear patterns of the WLsz (Thiessen et al, 2018). These observations indicate that S 1 and S 2 of the Dodge-Snowbird domain predate the transposed foliations of the WLsz.…”

“…Zircon U-Pb age components include an igneous crystallization population at 2,592 ± 16 Ma and a recrystallized metamorphic population at 1,890 ± 33 Ma, the latter of which was interpreted to correlate with the timing of S T1 deformation (Thiessen et al, 2018 of 12 ± 1 kbar and >700°C (Thiessen et al, 2019). Garnet separates from the WLsz MG -M 2 domain returned preliminary Lu-Hf data that indicated an age of 1,870 ± 40 Ma, which was then interpreted to constrain the timing of P-T conditions of 8-11 kbar and >750°C (Thiessen et al, 2019).…”

“…Even so, the dykes generally display a lower degree of strain or no strain, indicating emplacement during the latter stages of S T2 ductile deformation. Ages for dyke emplacement and crystallization range between 1,871 ± 5 and 1,864 ± 2 Ma (Thiessen et al, 2018). WLsz Pg contains cm-scale layering of leucosome and melanosome within the S T2 foliation domain (Figure 5a).…”