Development of Low-Angle Boundaries in Amphibole and Their Implications for Accommodating Grain Boundary Sliding in Naturally Deformed Amphibolite

The deformation and interaction of amphibole grains are crucial for comprehending the rheological behavior and physical properties of middle to lower crust. However, the mechanisms of strain accommodation and grain boundary processes in amphibolites are poorly studied. In this study, we analyzed a naturally deformed amphibolite from an exhumed continental strike-slip shear zone. The amphibole grains can be categorized into two distinct types: type I and type II, with the type II being embedded within type I. Type I amphibole grains exhibit typical plastic deformation behavior, distinguished by the presence of discernible dislocation arrays and formation of subgrains. In contrast, type II amphibole grains predominantly display microfractures in the middle of grains and voids occur in their elongated tails. Meanwhile, we identified three types of low-angle boundaries in amphibole grains with varying microstructural and nanoscale characteristics. Our findings indicate that low-angle boundaries in minerals are not exclusively associated with crystal-plastic deformation. Furthermore, the deformation characteristics in type II amphibole grains are related to grain boundary sliding (GBS) process. To relieve stress concentration during grain boundary sliding in type II amphibole grains, two accommodation mechanisms are proposed: (a) Grain boundary diffusion with elimination of grain boundary irregularities. (b) Intragranular deformation of adjacent grains through either a brittle or a ductile process. Our findings hold significant implications for understanding the stress concentration and accommodation during deformation process in amphibolite.