Quartz porosity in amorphous SiO2 of granitic shear bands

Abstract

International audienceAbstract Important porosity is commonly observed in quartz-rich rocks that viscously deformed at depths of the metamorphic continental crust. Although the presence of such pores – often occurring with angular, pyramidal shapes – has major implications for fluid circulation, mass transfer and rock strength, whether or not they are directly produced by deformation remains unclear. Here we provide detailed observations of plastically deformed, pure quartz aggregates decorated by (sub)micrometric pores in granitic shear bands (western granite, Naxos, Greece). Using electron microscopy, we demonstrate that pores (1) decorate both grain boundaries and intra-grain substructures, (2) do not necessarily align with substructures when decorating grain boundaries, and (3) interconnect through layers of amorphous SiO 2 . Transmission electron microscopy further reveals dislocation densities one (or two) order(s) of magnitude below the predictions based on the lattice curvature gradients, suggesting the occurrence of residual stress along pore-decorated, partly amorphized substructures. Challenging long-lasting hypotheses, these features are here proposed to result from stress concentration at grain and subgrain boundaries, followed by fluid exsolution into mechanically amorphized quartz. Although involving syn-kinematic processes that remain to be fully understood, our findings corroborate recent studies that emphasise stress-induced amorphization as a general process of lithospheric rock deformation