New Mexico Geological Society Annual Spring Meeting — Abstracts

Metamorphic porphyroblasts in supracrustal rocks of the Granite Gorge Metamorphic Suite exposed in the Upper Grainte Gorge of the Grand Canyon have responded variability to noncoaxial heterogeneous shortening deformation; some rotated others remained fixed with respect to geographic coordinates. Thin section-scale domains of roatated verses nonrotated garnets are developed adjacent to one another in garnet-staurolite demonstrating endmember behavior on a mm-scale. However, in spite of local of rotation, inclusion trails porphyroblasts in most consistently either NW or NE, in keeping with regional and that most porphyroblasts did not significantly rotate across the 70 km-long transect. Porphyroblasts with NW -striking inclusion trails are interpreted to have overgrown a regionally penetrative, NW -striking foliation (S1) pre- to early-S2 development. Porphyroblasts with NE-striking inclusion trails are interpreted to have overgrown the NE- striking S2 late during S2. Nonrotated porphyroblasts yield insights about both the early geometry of S1 throughout the transect and the timing of their growth with respect to S2-related deformation. Rotated porphyroblasts yield information about the local finite strain history. These methods provide a powerful tool for evaluating the tectonic evolution of middle crustal rocks in an orogen and for understanding the complex behavior of porphyroblasts during ductile deformation.