STRENGTH VARIABILITY OF THE FERTILE DEEP CRUST DURING DEFORMATION AT GRANULITE GRADE: EVIDENCE FROM THE ATHABASCA GRANULITE TERRANE IN NORTHERN SASKATCHEWAN

The 2.6 Ga Fehr granite, exposed at the western edge of the Athabasca Granulite Terrane, experienced extensive anatexis during 1.9 Ga regional heating. Evidence of partial melting is characterized by progressive replacement of blocky K-feldspar megacrysts by granitic leucosome. Partial melting occurred via at least two distinct reactions: 1) eutectic disequilibrium melting, and 2) garnet-in peritectic melting. Where leucosome abundance is low, leucosome segregations occur within or along the pre-existing, subhorizontal S₁ fabric as: 1) tails on peritectic garnet porphyroblasts, 2) lozenge-shaped segregations of leucosome in megacryst strain-shadows and/or 3) S₁-parallel leucosome stringers. Aligned quartz and feldspar stringers are commonly S₁-parallel implying that S₁ was reactivated, at least to some degree, during the regional anatexis of the Fehr granite. With greater leucosome abundance, S₂-parallel leucosome segregations become more common as megacrysts become progressively smaller, with more irregular margins, during feldspar breakdown. This suggests a link between the dominant partial melting reactions, the volume of partial melt produced and the rate of partial melt extraction. Granitic leucosome initially migrated only locally (centimeters to meters) by exploiting the S₁ fabric until a mechanical threshold was reached. At this rheological limit, the S₁ fabric drastically decreased in strength, initially forming broad, open, upright F₂ folds. Locally, these folds tightened to form either Type 1 structures: tight to isoclinal folds associated with a relatively steeply-dipping, northeast-striking S₂ fabric or “mega-crenulation cleavage” or Type 2 structures: S₂-parallel shear domains. Microstructural studies of melt pseudomorphs in Type 1 and Type 2 structural domains were used to calculate melt-solid dihedral angles as a proxy for interfacial energies. Melt fractions, based on whole-rock geochemistry and microgeochemistry, were compared with static-melting experimental data to relate melt pathway interconnectivity and a rheologically critical melt percentage. These relationships appear to define a set of partial melting reaction-specific instability points for the crystalline framework that led to partitioning of strain and promoted progressive crustal weakening.