MICROSTRUCTURAL AND ELECTRON BACKSCATTER DIFFRACTION EVIDENCE FOR PORPHYROBLAST ROTATION DURING NON-COAXIAL FLOW

The question of whether or not metamorphic porphyroblasts rotate relative to one another and/or an externally fixed reference frame has persisted for decades. This debate is important because porphyroblasts commonly exhibit a record of coupled metamorphism and deformation in their chemical zoning and internal inclusion fabrics. The use of inclusion microstructures to interpret kinematic and structural histories requires knowledge of the rotational behavior of porphyroblasts during deformation. Attempts to better understand porphyroblast kinematics have employed theoretical, computational, and observational approaches, but microstructural studies of porphyroblastic rocks have typically led to equivocal results. This inherent ambiguity arises for a number of reasons, most notably the lack of knowledge of the original orientations of porphyroblasts and their included fabrics prior to the deformation of interest.

Acadian aged deformation in south-central Maine resulted in multiple episodes of temporally distinct deformation events. The latest event was marked by dextral non-coaxial flow representing the early development of the Norumbega shear zone system. Using multiply oriented thin sections, we evaluated porphyroblast kinematics and inclusion microstructures in three dimensions in a staurolite-andalusite schist. Optical microstructures suggest that fragments of boudinaged porphyroblasts have rotated relative to one another, and electron backscatter diffraction provides an exact measure of the relative rotation. Where unique reconstruction of fragmented staurolite porphyroblasts was possible, the data indicates relative rotations of ~30-40°. Thus, we conclude that during distributed dextral shear, asymmetric microboudinage and relative rotation of staurolite crystals occurred at syn- to post-peak amphibolite facies conditions. The amount and style of relative rotation is influenced by a number of factors including grain/twin shape and initial orientation relative to the kinematic reference frame, rigid particle interaction, and reaction-driven disaggregation. Despite these complications, the evidence shows unequivocally that porphyroblasts can rotate relative to one another and therefore also to an externally fixed reference frame during non-coaxial deformation.