Date of Award
Master of Science (MS)
G. Lang Farmer
The deformation behavior of crustal materials in variably hydrated metamorphic environments can drastically alter the rheological and seismic properties of continental crust. The Athabasca granulite terrane in the western Canadian Shield exposes tectonized high-pressure granulite that is locally overprinted by crustal-scale amphibolite facies shear zones and thus, provides a natural laboratory for studying deep crustal deformation under variable metamorphic conditions. This study focuses on sillimanite deformation behavior in felsic tectonites from two general deformation settings. First, felsic tectonites with an anhydrous assemblage of Grt + Sil + Kfs + Pl + Qtz in the Cora Lake shear zone (CLsz) experienced sinistral shear under estimated conditions of 1.0 GPa, 800-900 °C. Optical observations and electron backscatter diffraction (EBSD) analyses indicate that dislocation creep-accommodated subgrain-rotation on both (100) and (010) was the dominant dynamic recrystallization mechanism operating in sillimanite under these conditions. Locally, strain was apparently concentrated in surrounding weaker phases (quartz and feldspar) and intracrystalline deformation did not occur in sillimanite. New monazite geochronology suggests that high-strain in the CLsz occurred at c. 1.89 Ga.
The deformation behavior of sillimanite is markedly different under hydrous conditions in the Grease River shear zone (GRsz). The GRsz is characterized by c. 1.80 Ga exhumation involving dextral synkinematic hydration and retrograde metamorphism at 0.4-0.5 GPa, 550- 650 °C. Dissolution-reprecipitation is the major dynamic recrystallization mechanism operating in sillimanite under these conditions. Synkinematic growth of foliation-parallel euhedral sillimanite is in a preferred orientation with  parallel to the shear direction and either (100) or (010) parallel to the foliation. One important conclusion of this study is that sillimanite  preferentially aligns parallel to the stretching lineation regardless of contrasts in the conditions and/or mechanisms of deformation, which has significant implications for crustal anisotropy.
Leslie, Shannon Rae, "Contrasts in sillimanite deformation in felsic tectonites from anhydrous granulite- and hydrous amphibolite-facies shear zones, western Canadian Shield" (2012). Geological Sciences Graduate Theses & Dissertations. 39.