FORMATION OF ELONGATED GARNETS IN THE SPRING POINT AMPHIBOLITE, HARPSWELL, MAINE
Electron backscatter diffraction (EBSD) maps reveal complex patterns of crystal lattice orientations for the elongated garnets. High-angle grain boundaries are common, and suggest that many – but not all – garnets are polycrystalline. Most polycrystal domains, as well as single crystal garnets, show evidence of low-angle subgrain boundaries, and gradual lattice orientation shifts about a single axis. Field observations and backscattered electron images show brittle cracks, spiraled inclusion trails and deformed pressure shadows indicating dominant pre- to syn-kinematic growth of garnet.
The elongated garnets appear to have formed through a combination of growth and deformation mechanisms. Some polycrystals seemed to have formed early in the growth history given the observed concentric major-element zoning and high-angle boundaries. During deformation, subgrains formed (possibly by reprecipitation?) leading to low-angle boundaries and inclusion-poor terminations of the garnet. Dislocation creep may explain the observed gradual lattice misorientations about a common axis. Finally, boudinage and post-kinematic brittle deformation further elongated the garnets.