FLUID-ENHANCED WEAKENING OF MESO-SCALE SHEAR ZONES IN THE INTERIOR PARRY SOUND DOMAIN, SOUTHWESTERN GRENVILLE PROVINCE, ONTARIO, CANADA

A series of ~meter-wide shear zones within layered granulites of the interior Parry Sound domain are examined to investigate the processes involved in shear zone formation. The spaced structures accommodate meters to tens of meters of top-down displacement, and overprint penetrative, thrust-sense fabrics that characterize the kilometer-wide Parry Sound Shear Zone (PSSZ). Multiple generations of K-feldspar-rich pegmatites, which are spatially related to the shear zones, pre- and post-date deformation. Adjacent to the shear zones the pyroxene and garnet bearing mafic granulite layers are locally converted to hornblende and biotite-rich “schists”, with the foliation reoriented in to the shear plane. Thus, the volatile-consuming reactions involving the production of hornblende and biotite at the expense of pyroxenes and garnet appear to have occurred synchronously with the development of the shear zone fabric. Similar changes in mineralogy and microstructure have been shown to cause a drastic weakening of the reacted volume, and promote further strain localization into these zones. The observed reactions are presumably enhanced by the introduction of pegmatite-related fluids that may have been focused along fractures or zones of increased permeability during shear zone inception. Mineralized fractures and incipient shear zones preserved in other parts of the domain provide insight into possible localization mechanisms.