INTRACRYSTALLINE DEFORMATION OF GARNET IN A QUARTZ MATRIX DURING HIGH-STRAIN SHEAR

Garnet is a common mineral in the lithosphere and widely used for interpreting microstructures, mineral stability, metamorphic P-T conditions, and orogenic timing. Because garnet is volumetrically minor in many crustal rocks, has an equant habit, and is a strong mineral (elastic modulus ≈ 300), it typically resists deformation even during high strain of the host rock. In a quartzite from Antarctica deformed at ~700°C under high strain rates (γ ≥ 5), rather than behaving as rigid bodies in a weak quartz-rich matrix, garnet porphyroblasts show bulk plastic strain and formation of subgrains along low-angle misorientation boundaries. The sample contains ≥95% quartz with small garnet porphyroblasts (≤1 mm) and minor biotite. It is a high-grade L-S tectonite with an annealed quartz matrix that preserves a strong lattice preferred orientation indicative of non-coaxial shear strain. The isolated garnets are elliptical to lens-shaped with long axes aligned with macroscopic foliation. Band contrast and grain boundary mapping by electron back-scattered diffraction (EBSD) shows that the garnets are divided into numerous subgrains with irregular subgrain boundaries. Misorientation profiles show that the subgrain boundaries correspond to typical lattice mismatches of 2-10°, but locally up to 40°. Local misorientation maps show that internal strain is focused along subgrain boundaries and highlight areas of incipient subgrain formation. The average crystallographic orientations for 43 individual garnets show a near-random distribution, indicating that initial orientation did not play a role in garnet deformation, nor did deformation result in a lattice preferred orientation within the sample. We interpret these observations as evidence that deformation was not concentrated on a dominant slip system. Because the elastic modulus of garnet is three times that of quartz (elastic modulus ≈ 90), it should be difficult to transfer intracrystalline strain via quartz to isolated garnet crystals. The garnets are compositionally homogeneous, so stress-induced dissolution cannot be ruled out. However, the evidence for grain-boundary misorientation and subgrain formation, as well as their lozenge shape, indicates that garnet can deform plastically in a relatively weak matrix at high shear-strain rates.