Development of single-crystal σ-shaped quartz porphyroclasts by dissolution–precipitation creep in a calcite marble shear zone

Within a greenschist facies calcite marble shear zone, isolated quartz grains change shape across the shear zone profile. Whereas quartz grains have a spherical to elongated shape in the coarse grained marble protolith and protomylonite, they are asymmetric σ-shaped porphyroclasts with wedge shaped appendages (wings) in ultramylonites. In all cases quartz grains are single-crystals. They are sometimes twinned but never recrystallized. Stress-induced dissolution–precipitation creep is the favourable shape-controlling process during deformation providing a relatively undeformed core, truncation of pre-existing cathodoluminescence (CL) patterns and syntaxial precipitation of wings. The necessary fluids may have been released from fluid inclusions during dynamic recrystallization of the calcite matrix. The development of σ-shape is not related to the crystallographic orientation of the quartz porphyroclasts. Crystallographic orientation analysis by electron backscatter diffraction (EBSD) and CL analyses exclude crystal-plasticity as a shape-controlling mechanism of quartz grains during mylonitisation. However, in ultramylonites quartz clasts have a strong crystallographic preferred orientation (CPO), with c-axes (sub)parallel to the shear direction. This fabric is uncommon for quartz under greenschist facies conditions. It might be explained by a strengthening of a pre-existing weak CPO during mylonitisation by rigid particle rotation of elongated quartz grains.