DEFORMATION HISTORY OF MANTLE XENOLITHS OF THE CANADIAN CORDILLERA

Seismic imaging of the mantle underlying the Canadian Cordillera has revealed a relatively flat Moho of 30-35 km. However, due to physical constraints, the complex tectonic setting of the Cordillera and the complicated accretionary history of western North America, the genesis and evolution of the Cordilleran mantle are not well understood. Here, to better understand the processes at work during mantle deformation history, we studied mantle xenoliths from five Tertiary to Post-Glacial volcanic suites in the Canadian Cordillera. The samples exhibit a range of textures that includes equigranular, protogranular, porphyroclastic and mylonitic, as well as transitional textures between these. This progression is marked by an increasingly defined foliation, increased elongation of olivine grains and an overall reduction in grain size conditioned by the formation of subgrains. We used Electron Back-Scattered Diffraction mapping to analyse fabric, texture, lattice-preferred orientation and grain size. Most xenoliths exhibit A-type fabrics, corresponding to the [100] (010) slip system in olivine and indicative of low water content and low to intermediate stress. Dynamic recrystallization by progressive rotation of planar subgrains at high angle from axis of elongation appears as the dominant stress-accommodating mechanism controlling the geometry of foliation. All samples are characterized by a weakly developed lattice preferred orientation, which qualitatively indicates low strain in the mantle sampled by the volcanic suites, suggesting minor spatial variations along the study area. The large grain size suggests the deformation of the mantle occurred under low stress conditions. Together, these observations suggest the mantle has not participated in significant tectonic deformation, requiring a detachment between the mantle and the crust, where significant shortening and lateral translation have occurred since the Mesozoic. These textural results will be combined with published chemical and isotopic trends to draw a more complete picture of the sub continental lithospheric mantle of the Canadian Cordillera.