Origin and incremental evolution of brittle/ductile shear zones in granitic rocks: natural examples from the southern Abitibi Belt, Canada

Abstract

Metre-scale shear zones developed in the Mooshla granitic pluton exhibit a plethora of internal mesoand microstructures revealing their mode of nucleation, growth and termination. The subvertical E-W brittle/ ductile shear zones are developed in the most isotropic part of the intrusion and consist of phyllonites characterized by mylonitic fabrics and well-developed down-dip mineral lineations. They are exposed on a single, large and flat outcrop perpendicular to the mineral lineation.These brittle/ductile shear zones are in close spatial association with a fracture system along which they have nucleated and propagated. Joints, fractures and brittle faults contain abundant evidence of ductilely deformed and recrystallized minerals suggesting that they underwent ductile shearing after their formation. The deformation was accompanied by the influx of a metasomatic fluid which altered original FeMg-bearing minerals and plagioclase into quartz, epidote, chlorite, sericite and carbonate that precipitated in dilatant fractures and narrow breccia zones. We recognize two different types of lateral terminations: (i) shear zones that terminate into joints and fractures that are parallel or oblique to the shear direction, and (ii) curved segments of shear zones that connected with neoformed splay fractures at high angle to the shear direction. In the latter case, sharp bending and severe perturbation of the shear zone orientation occur at the shear zone termination where ductile shearing is transfered into an adjacent shear zone via dilatational fractures. This demonstrates that shear zones tended to grow towards each other and coalesce to form a well-developed anastomosing network.We propose that narrow dilatational fractures and joints acted as paleo-weakness planes along which fluid-rock interaction and resulting reaction-softening occurred. The deformation scenario involves episodic fluid drawn into dilatant sites responsible for cyclic fluid pressure fluctuation in the system. Dilatancy-related fracturing and reaction-softening lead to deformation under brittle-to-plastic conditions and to shear strain localization.