Dynamic recrystallization of garnet and related diffusion processes

Abstract

Electron backscatter diffraction and compositional data of Permian metapegmatite garnet from the Koralpe basement (Eastern Alps, Austria) show the evolution of distinct intracrystalline shear zones in c. 10 mm sized almandine–spessartine garnet. Orientation maps reveal continuous distortion of the crystal lattice. New garnet grains (grain size 20–30 μm) within intragranular deformation zones, oriented subparallel to the main foliation of the metapegmatite, have similar crystal orientations to the host garnet, whereas recrystallized grains along fractures or along growth heterogeneities show a much weaker crystallographic preferred orientation (CPO). Needles of kyanite as well as inclusions of apatite, xenotime and rutile occur along the deformation zones. Together with the Ca variation across the deformation zones these inclusions give evidence of material diffusion related to crystal plastic deformation of garnet. The individual garnet porphyroclasts record the evolution of recrystallization microfabrics. New grains, once formed by subgrain rotation recrystallization, are able to deform partly by diffusion accommodated grain boundary sliding resulting in a weakening of the CPO. Whereas magmatic garnet growth had taken place at LP conditions in equilibrium with andalusite, the occurrence of kyanite in some deformation zones, triggered by the localized crystal plasticity of garnet, indicates that the deformation was related to the Cretaceous HP metamorphic event.