EVIDENCE FOR SUPERPLASTICITY IN GARNET AND THE ROLE OF DEFORMATION IN ENHANCING REACTION PROGRESS IN A HIGH-PRESSURE SHEAR ZONE, HARAMSØYA, NORWAY

Orientation contrast (OC) imaging, compositional mapping, and mapping of lattice preferred orientations (LPO) using electron backscattering diffraction (EBSD) in the SEM are used provide insight into the interplay between deformation and metamorphism in eclogitized shear zones from the Haram corona gabbro. The general reactions involved include: 1) Ca plag + Opx/Cpx + spinel=Omph + Gar + Na plag and 3) Ca plag + Ilm + spinel=Rut + Garnet + Na plag. Plagioclase and kyanite show a strong preferred grain shape orientation related to deformation in the shear zones. Omphacite forms polycrystalline layers in strain shadows around orthopyroxene porphryoclasts. The LPOs of plagioclase, ilmenite, and omphacite are all related to the deformation in the shear zone. The observed microstructures and textures indicate that part or all of the garnet growth occurred during deformation. Garnet occurs in layers up to a few hundred microns in width and as isolated grains down to a few microns in diameter. Garnet has an overall LPO that is random. In the foliated and mylonitic samples the most striking feature of the orientation contrast imaging of garnet is the bimodal grain size in the garnet layers with fine grains (1-15 µm) in the interior and coarse grains (50-150 µm) in the outer parts of one garnet layer adjacent to plagioclase. The characteristics observed in the fine grained parts of the garnet layers including fine grain size, grain boundary sliding and migration, a preferred grain shape orientation at low accumulated strains, curved grain boundaries, polygonal grain shape, and lack of a significant LPO are all features characteristic in materials that exhibit superplasticity. Coarse garnets on the outer part of the layer adjacent to plagioclase have grain boundaries that bulge sharply into adjacent grains. There is no obvious preferred grain shape orientation of the coarse garnet in contact with plagioclase in the outer part of the coronas. The plagioclase near the interfaces is granoblastic and typically shows no preferred grain shape orientation adjacent to garnet. These features are interpreted to have developed during post-kinematic growth of garnet and plagioclase and their restriction to narrow regions is interpreted to indicate significant shortening of diffusion distances in response to that cessation of deformation.