Abstract

This contribution shows unequivocally that porphyroblasts rotate relative to one another during ductile deformation. The porphyroblasts described here have special significance because they are from the original “millipede” rocks that led to the nonrotation hypothesis. Thus, the debate that has lasted for more than 20 years is settled. Despite this finding, porphyroblast microstructures continue to provide important evidence for deformation and metamorphic histories. Although porphyroblasts clearly rotate relative to one another during ductile deformation, there are several factors that contribute to relatively minor rotation in many instances, including (1) low strain during and after porphyroblast growth in comparison, for example, to mylonitic shear zones; (2) small axial ratios combined with relatively low internal vorticity during growth and post-growth deformation; and (3) strain localization at the porphyroblast-matrix interface. Thus, given the right circumstances, porphyroblasts may preserve the approximate orientations of deformation fabrics present at the time of their growth, but each case must be individually assessed.

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