ON THE ROLE OF FLUIDS IN DEEP CRUSTAL SHEAR ZONES

Fluids and deformation have long been recognized as strong influences on rock strength. Their influence is mutually accentuated due to the common positive feedback between deformation and fluid infiltration. Parameterizing the role of fluids in the lower crust faces the challenge of a sufficient number of appropriate natural laboratories. The southwestern margin of the Parry Sound domain, Grenville Province, Ontario, affords 100% exposure across dozens of meter-scale shear zones in which coupled fluid-deformation processes appear to have played a major role in strain localization. In this contribution, we describe the microstructural and petrological signatures of the shear zone strain gradients, with two goals: (1) to further test the hypothesis that hydration was a principle cause of shear zone development in this region, and (2) to document the grain-scale mechanisms by which fluids enhanced deformation occurred in the rocks.

Petrologic data and thermodynamic modeling from samples collected across the Twelve Mile Bay shear zone has shown that the influx of potassium- and water-rich fluids, initially along spaced, pegmatite-filled fractures, caused the breakdown of the stable garnet- and pyroxene-bearing granulite assemblage to sheared and strongly recrystallized amphibolites. Infrared spectroscopy of sheared samples indicates that recrystallized quartz and feldspars contain substantial amounts of hydrous species, which likely resulted in a dramatic decrease in their creep strength. Given that the reaction products and the hydrated, recrystallized matrix grains should be weaker than the existing granulite assemblage, a bulk weakening of sheared rocks is inferred.

As described in a companion paper (Gerbi et al., this volume), the decameter-scale weakening increased with more extensive shear zone development, further supporting a positive feedback relationship between fluid access and rock strength. Thus, localized hydration, in this case adjacent to pegmatite-filled fractures, exerted a significant influence on rock strength at a much larger spatial scale than that of the fractures.