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

Ilg, Bradley; Karlstrom, Karl

"The results of this study suggest either `rotation' or `non-rotation' porphyroblast behavior is possible during deformation, even in adjacent thin section-scale domains. More importantly, it highlights the importance of specifying the reference frame, (e.g. Kraus, 1998) timing, degree, and type of `rotation'. However, in spite of the various possible rotations of porphyroblasts, there remains a remarkable consistency of inclusion trail orientations across the Grand Canyon transect, with NW-striking S1 cores at high angle to NE-striking subvertical S2 matrix, or with S2 inclusion trails parallel to matrix. This suggests that many porphyroblasts, that grew early-, syn-, and late-D2, did not rotate appreciably with respect to the trace of developing S2 cleavage during ca 1.7 Ga NW/SE shortening deformation. This does not disprove rotation (especially given the puzzle of diverse dips of S1 inclusion trails and the possibility of passive rotation of geometric elements), but it seems to preclude models for shear-related rotation of porphyroblasts relative to developing S2 foliation for most samples. To the extent that the porphyroblasts did not rotate, they may be used to evaluate the pre-D2 fabric geometry. In the case of Paleoproterozoic rocks in the Grand Canyon, the inclusion trail geometries suggest that S1 was regionally NW striking. S1 may have been variably dipping, but more likely was subhorizontal across a wide region. Thus, we infer both rotation and non-rotation behavior and point out the importance of specifying timing and reference frame when discussing porphyroblast behavior during deformation." [authors' conclusion, from ScienceDirect online database] Includes geologic map of the Upper Granite Gorge, rose diagram, photomicrograph, block diagram, and more.

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

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