Plastic deformation and recrystallization of garnet: a mechanism to facilitate diffusion creep

Elongate and deformed garnets from Glenelg, NW Scotland, occur within a thin shear zone transecting an eclogite body that has undergone partial retrogression to amphibolite facies at circa 700°C. Optical microscopy, back-scattered electron imaging, electron probe microanalysis and electron back-scatter diffraction reveal garnet sub-structures that are developed as a function of strain. Subgrains with low-angle misorientation boundaries occur at low strain and garnet orientations are dispersed, around rational crystallographic axes, across these boundaries. Towards high-strain areas, boundary misorientations increase and there is a loss of crystallographic control on misorientations, which tend towards random. In high-strain areas, a polygonal garnet microstructure is developed. The garnet orientations are randomly dispersed around the original single-crystal orientation. Some garnet grains are elongate and Ca-rich garnet occurs on the faces of elongate grains oriented normal to the foliation. Commonly, the garnet grains are admixed with matrix minerals, and, where in contact with other phases, garnet is well faceted. We suggest that individual garnet porphyroclasts record an evolution from low-strain conditions, where dislocation creep and recovery accommodated deformation, through increasing strain, where dynamic recrystallization occurred by subgrain rotation, to highest strains, where recrystallized grains were able to deform by diffusion creep assisted grain boundary sliding with associated rotations.