Calcite textures, microstructures and rheological properties of marble mylonites in the Bancroft shear zone, Ontario, Canada

Abstract

Calcite textures in several microstructural domains from the Bancroft shear zone reveal a sequence of transitions which indicate contributions from a variety of deformation mechanisms at greenschist facies conditions. The coarse marble protolith has undergone low strains accommodated by twinning and grain boundary migration. Porphyroclastic mylonite has a well developed crystallographic preferred orientation and microstructures indicative of dislocation creep and rotation recrystallization mechanisms. The textures are consistent with high-temperature deformation experiments where r, f and c slip systems were active. Complete dynamic recrystallization produced equant, medium-sized grains (50 μm) with a texture that is nearly random. Secondary calcite growth observed under cathodoluminescence and variation in δ 18O value with microstructurc indicate large fluid fluxes during mylonitization. Fluid-assisted grain-boundary sliding and a minor component of dislocation creep are the inferred deformation mechanisms. Finer grained S-C mylonite has a well developed shape fabric and crystallographic preferred orientation with a point maximum and great-circle girdle attributed to attainment of an ‘easy slip’ orientation for most grains. Ultramylonites have homogeneous textures with finc, elongate grains (20–30 μm) mantled by disseminated secondary phases. Textures in ultramylonites are characterized by single point maxima and great circle girdles. The most evolved ultramylonite has a point maximum oblique to the shear plane with a symmetry indicating rotation of the maximum toward the extension direction. Texture/microstructure relations in these mylonites indicate that the competition between deformation mechanisms was highly sensitive to grain size, strain and secondary phases in addition to temperature, strain rate and differential stress. The varied deformation mechanisms and application of experimentally-derived constitutive equations indicate significant differences in differential stress and strain rate among microstructural domains. These differences reflect strain softening associated with dynamic recrystallization and work hardening associated with dissemination of secondary phases and the transition from grain-boundary sliding to dislocation creep.