Subhorizontal fabric in exhumed continental lower crust and implications for lower crustal flow: Athabasca granulite terrane, western Canadian Shield

Abstract

The >20,000 km2 Athabasca granulite terrane is one of Earth's largest exposures of continental lower crust. The terrane is underlain by heterogeneous isobarically cooled orthogneisses termed the Mary batholith. A transect across the batholith documents early, penetrative subhorizontal to gently dipping gneissic foliation (S1). Kilometer‐ to meter‐scale domains of S1 contain lineations (L1) defined by ribbons of recrystallized K‐feldspar + plagioclase + quartz + amphibole ± orthopyroxene. L1 coincides with garnet aggregates, elongate mafic enclaves, and core‐and‐mantle structure in feldspar porphyroclasts. Lineations are coaxial with hinges of isoclinally folded layering (F1). L1 is interpreted as a composite mineral lineation with intersection and stretching components. Kinematics are uniformly top‐to‐the‐ESE. Thermobarometry derived from synkinematic phases is compatible with granulite‐grade (∼800°C) ductile lower crustal flow during D1 at ∼0.9 GPa (∼30 km paleodepths). Results from electron probe microanalyzer (EPMA) Th‐U‐total Pb monazite geochronology support Neoarchean (circa 2.60–2.55 Ga) garnet growth and melt‐enhanced flow. Metamorphic reactions accompanying D1 strain were synkinematic, with preferential nucleation of high‐Ca garnet and amphibole in the Na‐rich mantles of recrystallized plagioclase porphyroclasts. Relatively H2O‐poor and/or CO2‐rich conditions during D1 are required by the preservation of fine‐grained microstructures. Subhorizontal tectonites in the Mary batholith may represent an important field‐based analog for lower crustal flow during orogenesis or large magnitude extension. The results illustrate the evolving strength of continental lower crust. Neoarchean subhorizontal flow of weak lower crust was followed by near‐isobaric cooling and strengthening. Paleoproterozoic deformation events produced steep fabrics (S2), steeply dipping shear zones, and reactivation of S1, a record of strain localization and strain hardening in an isobarically cooled anisotropic medium.

Granite genesis and mafic-felsic magma interaction in the lower crust

“…2.6 Ga (Williams and Hanmer, 2006;Dumond et al, 2010). The AGT may have remained at deep crustal levels or may have experienced minor exhumation, but a very large portion of the terrane underwent a second period of high P-T (1.0− 1.2 GPa) metamorphism and deformation at 1.9 Ga (Mahan and Williams, 2005;Williams and Hanmer, 2006;Flowers et al, 2006a).…”

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

“…The Cora Lake shear zone (CLsz) has been previously mentioned in Mahan et al (2008Mahan et al ( , 2011 and Dumond et al (2010). Mahan et al (2008) suggested that Chipman dykes are highly strained within the CLsz region, and to a lesser extent further east, suggesting that the CLsz was active after emplacement of the dyke swarm.…”

“…It has been hypothesized that the region was resident in the deep crust for ca. 650 million years (i.e., 2560-1900Mahan and Williams 2005;Williams et al 2009;Dumond et al 2010), making the East Athabasca mylonite triangle a premier location to study lower crustal processes (Williams and Hanmer 2006).…”

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

“…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. The shear zones that are the focus of this study (Cora Lake and Grease River) both belong to this Paleoproterozoic group of structures (S 2 and S 3 ).…”

“…The rocks display gneissic layering at the outcrop scale with variations in modal garnet, alumino-silicate, plagioclase and Kfeldspar defining the layers and largely explaining the measured variations in bulk compositions. The origins of the felsic gneisses are generally interpreted as the highly deformed products of late Archean partial melting of biotite-bearing protoliths (Snoeyenbos et al, 1995;Hanmer, 1997;Baldwin et al, 2006;Dumond et al, 2010).…”