Defining shear zone deformation and alteration gradients: Pocologan Kennebecasis shear zone, Canadian Appalachians

The impact of fluid-rock interaction during deformation is difficult to characterize at the regional scale because of the lack of continuous outcrops to provide textural evidence and meaningful spatial comparisons. Herein we introduce the use of bench top μXRF data as a cost-effective means to quantify chemical and deformation gradients within representative specimens from across the strain gradient associated with the Pocologan Kennebecasis shear zone (PKSZ) in the Canadian Appalachians. Within the Pocologan Harbour granitoid belt, the main foliation is defined by three distinct mineral assemblages characterized by: (A) zoisite, no muscovite; (B) zoisite and muscovite; (C) muscovite and potassium feldspar. We first test whether these mineral assemblages are indicative of primary lithological variation or hydrothermal alteration using major and trace element geochemistry. Then, we test if these mineral assemblages impact the fabric anisotropy using quantitative image analysis of thin section phase maps derived using a benchtop μXRF. We found that whole rock compositions of specimens with an assemblage characterized by potassium feldspar and muscovite show significant element mobility compared to the least altered specimens and that the thin section phase map anisotropy best defines the deformation gradient of the PKSZ regardless of the metasomatic assemblage. This deformation gradient is mimicked by the shape anisotropy of quartz, plagioclase, biotite and muscovite aggregates. Within the most altered specimens, sericite alteration of plagioclase increased the mineral aggregate anisotropy through strain localization in shear bands while the crystallization of potassium feldspar at the expense of plagioclase of muscovite, plagioclase and quartz decreased the mineral aggregate anisotropy which indicates an overall strengthening of the shear zone. Moreover, the combination of grain-size reduction mechanisms and syn-deformation metasomatism in the high strain portion of the shear zone triggered a change of deformation mechanism from plastic deformation to diffusion creep. •Regional-scale mylonite in the Canadian Appalachians•Mylonite deformation involved multiple mineral reactions related to potassic alteration.•Syn-deformation potassic alteration of granitoid lithologies produces competing rheological softening and strengthening.