THE MARY REACTION: TIMING DEEP CRUSTAL DEFORMATION AND METAMORPHISM WITH IMPLICATIONS FOR STRENGTHENING AND STABILIZATION OF FLOWING LOWER CRUST

Field studies of exposed deep crustal terrains are critical to our understanding of tectonic processes. They provide information about evolving rheological conditions near the crust – mantle boundary. The Mary granite of the Athabasca Granulite Terrain, Saskatchewan, preserves two deformational fabrics with intensity that varies at all scales. The first fabric is a subhorizontal foliation with a strong lineation, attributed to crustal flow during the Archean. The second is a heterogeneously developed NE-striking, subvertical foliation interpreted to reflect 1.9 Ga shearing of the deep crust. The Mary batholith preserves a distinctive metamorphic reaction informally known as the “Mary reaction,”that is:

Opx + Pl₁ = Grt + Cpx + Pl₂

Depending on timing, the reaction could reflect one of two different crustal processes: strengthening and stabilization of flowing deep crust, or drastic alteration of the composition and rheology of the deep crust long after stabilization. A regional scale F₁ fold was identified in the field and oriented samples from the fold hinge and limbs were collected. Textures observed in oriented thin sections indicate that the reaction products are aligned along the S₁ fabric. Furthermore, electron microprobe analysis of dynamically recrystallized plagioclase feldspar shows Ca rich cores (Pl₁) and Na rich (Pl₂) recrystallized rims indicating synchronous deformation and reaction. The coreandmantle structures are aligned in the S₁ fabric, also indicating that the Mary reaction occurred during S₁ crustal flow. All data are consistent with reaction synchronous with S₁subhorizontal, deep crustal, gravity-driven flow. We suggest that the reaction may serve to strengthen the flowing ductile crust and may help to arrest flow and stabilize the deep crust after orogeny.Furthermore, the reaction represents an increase in density of the lower crustleading to decreased elevation of continental crust and possible initiation of delamination of the lower crust. This process records the evolution of the deep crust from a metastable igneous assemblage to a stable metamorphic crustal assemblage. Characterization of the Mary reaction will provide insight into seismic images and numerical models of the deep crust, and better our understanding of lower continental crust in modern orogenic settings.