Deformation mechanisms of phyllosilicates and amphibole and the interdependence of deformation in polymineralic rocks

Abstract:

Plate boundary shear zones commonly serve as fluid pathways and the ubiquitous presence of hydrous minerals in these shear zones is often invoked to explain their relative weakness. However, the deformation mechanisms allowing creep in these minerals as well as the interdependence of deformation mechanisms in polymineralic rocks remain understudied. We evaluate the deformation mechanisms operating within the Leech River Shear Zone on Vancouver Island, British Columbia, a 600-m wide zone of mylonites across a lithologic contact with syn-kinematic temperatures of ~550 ºC that represents a paleo-subduction interface. Chlorite, muscovite, and biotite are abundant in the schists of the accretionary complex, while the metabasalts of the oceanic plate contain predominantly amphibole and plagioclase. Within the schist, phyllosilicates exhibit kink band formation and undulose extinction, which suggest the activation of dislocation glide. However, grain sizes and shapes vary with quartz and feldspar abundance and rotations of the foliation around veins, quartz-feldspar lenses, and porphyroclasts demonstrate that phyllosilicate deformation was coupled with deformation in quartz and feldspar. The metabasalt has a strong shear fabric defined by the prograde growth of asymmetric tails on amphibole porphyroblasts, suggestive of pressure solution creep. Electron backscatter diffraction (EBSD) mapping reveals subgrain development and a crystallographic preferred orientation (CPO) within amphibole, indicating that dislocation creep was also active. Plagioclase grains define a network of shear bands between amphibole grains. Plagioclase has a weak shape preferred orientation (SPO) but no CPO, which combined with the fine grain size, indicates deformation by diffusion creep. These observations illustrate that although shear strain was likely partitioned into the weaker phase (i.e. phyllosilicates in the schist and plagioclase in the metabasalt), the degree of phase mixing and interactions between strong and weak phases ultimately controls the bulk rock rheology.