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

Abstract

a b s t r a c tThe deformation behavior of crustal materials in variably hydrated metamorphic environments can significantly influence the rheological and seismic properties of continental crust. Optical observations and electron backscatter diffraction (EBSD) analyses are used to characterize sillimanite deformation behavior in felsic tectonites from two deformation settings in the Athabasca granulite terrane, western Canadian Shield. Under estimated conditions of 0.8e1.0 GPa, 725e850 C in the Cora Lake shear zone, the data suggest that sillimanite deformed by dislocation creep with slip in the [001] direction accompanied by subgrain rotation recrystallization. Where sillimanite locally remained undeformed, strain was concentrated in surrounding weaker phases. Under hydrated conditions of 0.4e0.6 GPa, 550e650 C in the Grease River shear zone, textures and cathodoluminescence imaging point to dissolutionprecipitation creep as the major deformation mechanism for sillimanite, resulting in synkinematic growth of foliation-parallel euhedral sillimanite in a preferred orientation with [001] parallel to the lineation. The results suggest that temperature, fluid content, and modal mineralogy of the surrounding phases may all have significant influence on sillimanite deformation but that preferential alignment of sillimanite [001] parallel to the lineation persists regardless of contrasts in the conditions or mechanisms of deformation.

Neoarchean arc magmatism and subsequent collisional orogenesis along the eastern Rae domain, western Churchill Province: Implications for the early growth of Laurentia

“…One major characteristic of the eastern Athabasca granulite terrane is a block style architecture, with distinct lithotectonic subdomains divided by thick ductile shear zones (briefly discussed below; Flowers et al, 2006b;Leslie et al, 2015). Detailed work has demonstrated that the steeply to moderately dipping shear zones that divide subdomains are Paleoproterozoic in age (Williams et al, 2009;Mahan et al, 2006a;Dumond et al, 2013;Regan et al 2014).…”

“…The Upper Deck (also referred to as the southern subdomain (Baldwin et al 2004;Flowers et al, 2006b)) consists of abundant migmatitic paragneiss traditionally referred to as felsic granulite, mafic granulite, and minor eclogite (Snoeyboss et al, 1995;Baldwin et al, 2003;2004). The dominant rock type is a garnetiferous quartzofeldspathic gneiss, interpreted to represent a deformed migmatite that has had variable amounts of melt loss and is referred to as the "white gneiss" (Snoeyenboes et al, 1995;Baldwin et al, 2004;Dumond et al, 2015). The white gneiss contains an assemblage of ternary feldspar, quartz, kyanite, garnet, and rutile +/-sillimanite (Snoeyenboes et al, 1995).…”

“…The white gneiss contains an assemblage of ternary feldspar, quartz, kyanite, garnet, and rutile +/-sillimanite (Snoeyenboes et al, 1995). Although production of eclogite assemblages was initially interpreted to be Paleoproterozoic and approximately 700 my after migmatization (Baldwin et al, 2006), recent work by Dumond et al (2015) suggests that eclogitization was synchronous with anatexis during the Neoarchean.…”

“…Commonly, monazite cores in the leucosome are characterized by high Y 2 O 3 , and low ThO 2 content, interpreted to have grown during prograde metamorphism. High ThO 2 innerrims, that have low Y 2 O 3 contents and vary in morphology are interpreted as syn-melting (see Dumond et al, 2015). Surrounding these domains are outer rims that are both low Y 2 O 3 and ThO 2 , which are consistently parallel to S 2 fabric (syn-S 2 ; Regan et al, 2014).…”

Introduction to Journal of Structural Geology special issue on “Deformation of the lithosphere. How small structures tell a big story”

“…Based on these results, they reveal the sensitivity of seismic anisotropy calculations of gabbroic rocks to texture symmetry, which should definitively be taken into account when interpreting geophysical data and building models of the lower crust. Leslie et al (2015) compares sillimanite microstructure and texture development in felsic tectonites from both anhydrous granulite-and hydrous amphibolite-facies shear zones (Athabasca granulite terrane, western Canadian Shield), primarily using electron backscatter diffraction. They reveal that sillimanite deformation is strongly influenced by temperature, fluid content and mineralogy of the sillimanite-bearing rock.…”