Sillimanite deformation mechanisms within a grt-sil-bt gneiss: Effect of pre-deformation grain orientations and characteristics on mechanism, slip-system activation and rheology

Sillimanite-bearing metapelites are common in the middle to lower crust and play an important role in the rheology of these crustal levels. We report deformation microstructures in a metasedimentary gneiss (Greenland) produced during simple shear at c. 700 °C and c. 4.5 kbar. Electron backscatter diffraction analysis shows activation of the (001)[010] and (001)[100] slip systems in large grains. Additionally, (100)[001] and (010)[001] are activated in needles with their c-axes parallel to lineation. Medium-grained sillimanite aggregates mixed with biotite show a weak crystallographic preferred orientation (CPO) suggesting activation of (001)[010] and (001)[100] together with grain boundary sliding (GBS). GBS is the dominant deformation mechanismin the outer parts of fine-grained clusters of sillimanite stemming from andalusite replacement. Associated features are phase mixing, clockwise dispersion of c-axis orientations and change of misorientation axes from original c-axis rotation to rotation close to the kinematic Y axis. The latter features which result in strain localization around these clusters allow their 'preservation'. Rheologically, the medium-grained aggregates together with zones around clusters take up most of the deformation. Our study shows that deformation mechanism and slip-system activation, and CPOs are strongly dependent on the orientation relationship between stress axes and pre-deformation grain orientations and characteristics.