Garnet plasticity in the lower continental crust: implications for deformation mechanisms based on microstructures and SEM-electron channeling pattern analysis

Elongated garnets which preserved deformational microstructures (boudinage, pinch and swell structures) occur in granulite facies quartzites in the Highland Complex of Sri Lanka. In spite of these microstructures and in contrast with previous reports on garnet plasticity, there are no or only few signs of intracrystalline deformation like subgrains or lattice distortion. This can be explained by annealing and slow static cooling from high temperatures. SEM-electron channeling pattern analysis reveals that the garnets have a significant crystallographic preferred orientation. Aspect ratio/grain-size analysis shows that the deformation mechanism is grain-size sensitive. These features indicate a diffusion assisted dislocation glide mechanism with dominant 1/2 <111>{110} slip system. A comparison of the deformation behavior between garnet, quartz and feldspar shows, that differences in flow strength are low under the high-grade conditions (850±50°C). Garnet is still the phase with the highest flow strength, but (dry) quartz is slightly stronger than feldspar.