THE ROLE OF MELT-ENHANCED SHORTENING AND SHEARING AT GRANULITE GRADE: EVIDENCE FOR LOCALLY VARIABLE STRENGTH OF THE DEEP CRUST FROM THE ATHABASCA GRANULITE TERRANE, NORTHERN SASKATCHEWAN

Deep crustal granite exposed along the western edge of the Athabasca granulite terrane in the Canadian Shield provide insight into deformation processes near the base of continental crust. The ca. 2.6 Ga Fehr granite experienced extensive anatexis at ca. 1.9 Ga during regional heating that is attributed to voluminous mafic magmatism. New granitic leucosome was produced by at least two distinct partial melting reactions: 1) eutectic disequilibrium melting, and 2) garnet-in peritectic melting. Leucosomes define two distinct styles of deformation: Type 1 leucosomes occur as veins and narrow dikes parallel to both a pre-existing, west-trending, shallow S₁ fabric and a north-trending, upright S₂ fabric. Early-formed leucosome is concentrated along S₁ and may contribute to a multi-layer character (weak-strong-weak) that promoted coarse crenulation-like buckling of S₁ and infilling of S₂ with leucosome. The Type 2 migmatite deformation style involved granitic leucosome in shear-dominated S₂ domains. Blocky K-feldspar megacrysts commonly preserve Carlsbad twinning and sigmoidal leucosome tails define an S-C fabric. Leucosome veins commonly define the C’ shear bands. Coarse deformation fabrics, defined by relatively unstrained minerals, indicate that a significant melt fraction existed during deformation. Microstructural and micro-geochemical studies of leucosome-dominated domains were used to calculate melt-solid dihedral angles as a proxy for interfacial energies. Calculated melt fractions were compared with static-melting experimental data to determine the relationship between melt pathway interconnectivity and a rheologically critical melt percentage. Instability of the crystalline framework in leucosome-bearing samples led to partitioning of strain into melt-rich domains. However, each reaction created differing volumes of partial melt, potentially at different rates. Variable melt production may have controlled the degree to which shortening was manifested as either Type 1 or Type 2 deformation. Peritectic leucosome samples preserve higher concentrations of melt-pseudomorphed grain boundaries compared with eutectic melting samples. This suggests a link between specific partial melting reactions and the relative strength of fertile zones within the deep crust.