Porosity induced by dislocation dynamics in quartz-rich shear bands of granitic rocks

Abstract

The production of micro-pores is a driving mechanism for fluids to interact with deep environment and influence rock properties. Yet, such a porosity still remains misunderstood to occur in viscous rocks and may be attributed to either grain boundary sliding (GBS), dissolution effects or sub-grain rotation. Here we focus on quartz-rich shear bands across the Naxos western granite (Aegean Sea, Greece), where we document sub-micron pores at quartz boundaries. While most of these pores are observed along grain boundaries, some of them occur at intra-grain boundaries, which excludes dissolution or GBS to produce them, but instead involves the dynamic of dislocations. We then confirm that quartz is dominated by dislocation creep with evidence of a moderate to strong lattice-preferred orientation (LPO) and numerous tilt/twist boundaries, including at the pluton margin where rocks embrittled. These features coincide with (1) randomly oriented ‘inclusion’ quartz grains along tilt/twist boundaries and (2) a partial dependency of the LPO strength on grain size. Our findings suggest that pores arise from coalescing dislocations at boundaries of rotating sub-grains, providing nucleation sites for new grains to be precipitated during plastic flow. Fluid infiltration, rock embrittlement and related implications are also expected through pores accumulation with increasing strain.

Superplastic deformation inside the knife-sharp shear bands in mid-crustal granites

“…The low-angle MXDs of the quartz grains from all the shear bands (Fig. 9) indicate a dominance of prism-áañ slip-system activity, which confirms the temperature range mentioned above (Wilson, 1975;Schmid and Casey, 1986;Précigout et al, 2022). Although rare, dislocation glide and slip system activity in K-feldspar grains can occur at 400-500°C (Sacerdoti et al, 1980;Fitz Gerald and Stünitz, 1993).…”