Porphyroblast inclusion trail geometries in the Grand Canyon: evidence fornon-rotation and rotation?

Porphyroblasts in Paleoproterozoic supracrustal rocks of the Grand Canyon g rew during a progressive deformation that involved shortening and heterogen eous development of NE-Striking subvertical foliation (S-2) that crenulates and transposes an earlier foliation (Si). Inclusion trails in syn-S-2 porp hyroblast cores show; a remarkably consistent NW strike across the regional transect, suggesting a lack of appreciable porphyroblast rotation relative to the trace of the NE-striking S-2 foliation. These apparently non-rotate d porphyroblast cores thus preserve a regional NW-striking SI orientation. Porphyroblast rims and younger porphyroblasts locally overgrow NE-striking S-2 indicating that porphyroblast growth (and continued non-rotation) spann ed S-2 Apparent non-rotation of most porphyroblasts across the transect is interpreted to reflect a dominance of coaxial shortening. In contrast, sing le thin sections show domains of these 'non-rotated' garnets with sigmoidal inclusion trails adjacent to domains wheres the characteristic sigmoids ar e rotated by about 45 degrees. This documents local rotation of garnets rel ative to St during a heterogeneous late or post-S-2 'reactivation' of the f oliation. Thus these types of studies should continue to look for both rota tional and non-rotational behaviors at all scales and continue to specify t he frame of reference. (C) 2000 Elsevier Science Ltd. All rights reserved.