The Cora Lake shear zone, Athabasca granulite terrane, an intraplate response to far-field orogenic processes during the amalgamation of Laurentia

Abstract

The Cora Lake shear zone (CLsz) is a 4-6 km wide localized high-strain zone that bisects the polydeformed Athabasca granulite terrane, northern Saskatchewan. It also coincides with the geophysical trace of the Snowbird tectonic zone. The CLsz represents a major lithotectonic and thermobarometric discontinuity within the exposure of >20 000 km 2 of high-pressure granulites. Most rocks have a strong mineral lineation plunging moderately to the southwest. The Northwestern subdomain (hangingwall) is characterized by ca. 2.6 Ga plutonic rocks that contain an early, subhorizontal gneissic layering (ca. 2.57 Ga) that was overprinted by large amplitude folds and a partitioned, but pervasive, axial planar, dextral, shear fabric at ca. 1.9 Ga. Thermobarometry suggests metamorphic conditions of ϳ0.9 GPa and ϳ750°C during both of the phases of tectonism. The footwall is predominantly underlain by the ca. 3.3-3.0 Ga Chipman tonalite, layers of intercalated mafic and felsic granulite, and the widespread 1.9 Ga Chipman mafic dyke swarm. Early subhorizontal layering in the footwall was also folded at ca. 1.9 Ga and transposed into a steeply dipping, northeast-striking axial planar shear fabric that corresponds with the metamorphic peak (1.1-1.2 GPa and 800-900°C). These distinct domains were juxtaposed across the CLsz, which contains a gneissic foliation striking 231°and dipping moderately to steeply to the northwest. Abundant sinistral-normal kinematic indicators are consistent with the distinctly lower pressures to the northwest. The shear zone is characterized by very fine grain sizes, despite its hightemperature assemblages including clinopyroxene and garnet. Thermobarometry from the CLsz displays progressive decompression of reworked footwall rocks with increasing mylonitization. In situ monazite geochronology indicates shearing at 1.89-1.87 Ga shortly after the granulite facies metamorphic peak. The anomalous sinistral kinematics of the CLsz, bracketed in time between periods of dextral shearing, can be explained by changing regional stresses during alternating convergent tectonics to the west and to the southeast of the Athabasca granulite terrane.Résumé : La zone de cisaillement de Cora Lake (CLsz) est une zone localisée de fortes déformations; elle a une largeur de 4 à 6 km et elle divise en deux parties le terrane de granulite polydéformé d'Athabasca, dans le nord de la Saskatchewan. Cette zone coïncide aussi avec la trace géophysique de la zone tectonique Snowbird. La CLsz représente une importante discontinuité lithotectonique et thermobarométrique dans l'affleurement de plus de 20 000 km 2 de granulites de haute pression. La plupart des roches ont une forte linéation minérale plongeant modérément vers le SO. Le sous-domaine nord-ouest (l'éponte supérieure) est caractérisé par des roches plutoniques d'environ 2,6 Ga qui contiennent un rubanement gneissique précoce subhorizontal (ϳ2,57 Ga) lequel a été surimprimé par des plis à grande amplitude et une texture de cisaillement envahissante, partitionnée, à axe planaire ...

Dynamics, EPMA Th‐U‐Total Pb Monazite Geochronology and Tectonic Implications of Deformational Fabric in the Lower‐Middle Crustal Rocks: A Case Study of Ambaji Granulite, NW India

“…The Ambaji granulite is characterized by a pervasive subhorizontal fabric (S 1 ) developed at middle-lower crustal conditions (5.5-6.8 kbar/≥850°C; Singh et al, 2010). This was overprinted by vertical to steeply inclined discrete shear fabric consisted of a high-temperature shear fabric followed by a low-temperature shear fabric with retrogression similar to many granulite terranes (e.g., Dumond et al, 2010;Hodges, 2006;Meissner et al, 2006;Regan et al, 2014;Ross et al, 2004;Sandiford & Powell, 1986). The shear fabric was developed during exhumation (S 2 , 4 kbar/600-500°C).…”

The Contact uranium prospect, Kiggavik project, Nunavut (Canada): Tectonic history, structural constraints and timing of mineralization

“…1), a tectonic trend which has given rise to many interpretations (see for example Hoffman, 1988, Hanmer et al, 1995Ross et al, 2000;Berman et al, 2007). Recent field observations by Regan et al (2014) on the Cora Lake Shear Zone lead them to propose an intracontinental crustal-scale structure formed at ca. 2600 Ma, which underwent several later episodes of transpressional reactivation, with sinistral and dextral motions in response to the Thelon and Trans-Hudsonian orogenies from 1900 to 1820 Ma (Sanborn-Barrie et al, 2001;Mills et al, 2007;Regan et al, 2014).…”

“…Recent field observations by Regan et al (2014) on the Cora Lake Shear Zone lead them to propose an intracontinental crustal-scale structure formed at ca. 2600 Ma, which underwent several later episodes of transpressional reactivation, with sinistral and dextral motions in response to the Thelon and Trans-Hudsonian orogenies from 1900 to 1820 Ma (Sanborn-Barrie et al, 2001;Mills et al, 2007;Regan et al, 2014). Aeromagnetic maps show that a major fault trend in the Kiggavik area, the Andrew Lake Fault (ALF) connects to the STZ (Fig.1).…”

Contrasts in sillimanite deformation in felsic tectonites from anhydrous granulite- and hydrous amphibolite-facies shear zones, western Canadian Shield

“…All subdomains consist of 3.2 to 2.6 Ga igneous protoliths ( Fig. 1c) that have been reworked by at least two high-pressure granulite facies tectonometamorphic events: one at c. 2.6e2.55 Ga (M 1 ) and the other at c. 1.88 Ga (M 2 ; Hanmer et al, 1994Hanmer et al, , 1995Snoeyenbos et al, 1995;Williams et al, 1995Williams et al, , 2000Kopf, 1999;Baldwin et al, 2003Baldwin et al, , 2004Baldwin et al, , 2006Flowers et al, 2006;Mahan et al, 2006;Dumond et al, 2008Dumond et al, , 2010Regan et al, 2014). Paleoproterozoic deformation (1.9e1.8 Ga) produced open to isoclinal folds of an early Archean foliation (S 1 ) and more discrete, steeply dipping shear zones (Dumond et al, 2008(Dumond et al, , 2010.…”

“…Well-developed asymmetric porphyroclasts and SeCeC 0 fabrics indicate sinistral strike-slip displacement with a minor west-side down component. Deformation conditions in the Cora Lake shear zone are estimated at 1.0e0.8 GPa, 850e725 C (M 2 ) and the age is constrained by TheU-total Pb monazite geochronology at 1.89e1.87 Ga (Regan et al, 2014).…”