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

Porphyroblasts in Paleoproterozoic supracrustal rocks of the Grand Canyon grew during a progressive deformation that involved shortening and heterogeneous development of NE-striking subvertical foliation (S2) that crenulates and transposes an earlier foliation (S1). Inclusion trails in syn-S2 porphyroblast 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 S2 foliation. These apparently non-rotated porphyroblast cores thus preserve a regional NW-striking S1 orientation. Porphyroblast rims and younger porphyroblasts locally overgrow NE-striking S2 indicating that porphyroblast growth (and continued non-rotation) spanned S2. Apparent non-rotation of most porphyroblasts across the transect is interpreted to reflect a dominance of coaxial shortening. In contrast, single thin sections show domains of these ‘non-rotated’ garnets with sigmoidal inclusion trails adjacent to domains where the characteristic sigmoids are rotated by about 45°. This documents local rotation of garnets relative to S2 during a heterogeneous late or post-S2 ‘reactivation’ of the foliation. Thus these types of studies should continue to look for both rotational and non-rotational behaviors at all scales and continue to specify the frame of reference.