MICROSTRUCTURE AND CRYSTALLOGRAPHIC PREFERRED ORIENTATION OF POLYCRYSTALLINE MICROGARNET AGGREGATES DEVELOPED DURING FLATTENING, GRAIN BOUNDARY SLIDING, AND DIFFUSION CREEP

Optical microscopy, electron probe microanalysis, and electron backscatter diffraction methods were used to examine a broad range of garnet microstructures within the Mount Dumplin high strain zone, which marks the western margin of a major zone of transpression and extrusion in the southern New England Appalachians in south-central Massachusetts. Garnet accommodated variable states of finite strain, expressed as low strain porphyroblasts (Type 1), high strain polycrystalline aggregates (Type 2), and transitional morphologies (Type 3) that range between these endmembers. Type 1 behaved as rigid porphyroclasts and is characterized by four concentric Ca growth zones. Type 2 help define foliation and lineation, are characterized by three Ca zones, and possess a consistent bulk crystallographic preferred orientation of (100) symmetrical to the tectonic fabric. Type 3 show variable degrees of porphyroclast associated with aggregate, where porphyroclasts display complex compositional zoning that corresponds to lattice distortion, low-angle boundaries, and subgrains, and aggregate CPO mimics porphyroclast orientation. All aggregates accommodated a significant proportion of greenschist facies flattening facilitated by grain boundary sliding, grain rotation and impingement, and pressure solution, which lead to a cohesive behavior and overall strain hardening of the aggregates. The characteristic CPO could not have been developed in this manner, and was likely a result of an older phase of amphibolite facies dislocation creep, recovery, and recrystallization of porphyroclasts. This study demonstrates the significance of strain accommodation within garnet under a range of PT conditions, and emphasizes the importance of utilizing EBSD methods with studies that rely upon a sound understanding of garnet composition and behavior. The garnet microstructures, including folded aggregates of microgarnets, display sinistral asymmetry, consistent with shear sense in the sinistral Mount Dumplin high strain zone.