USING CRYSTALLOGRAPHIC PREFERRED ORIENTATION IN MYLONITIC GARNET LEUCOGNEISS TO STUDY PALEOPROTEROZOIC TECTONICS IN THE RUBY RANGE, MONTANA

The tectonic history of the northwestern margin of the Wyoming Province and its role in the assembly of Laurentia are still poorly understood. The Ruby Range of SW Montana sits at the northwestern margin of the Wyoming Province. There, mylonitic garnet leucogneiss (MGL) crosscuts fabric in adjacent Dillon Gneiss and layering in metasupracrustal units of the lower Christensen Ranch suite, and is interpreted to be a meta-intrusive rock. MGL contains evidence of metamorphism at 2.45 Ga, associated with the cryptic Beaverhead orogeny. Unlike surrounding metamorphic units, however, the MGL is not overprinted by the 1.78 Ga Big Sky orogeny, and can therefore provide rare information on the earlier tectonic event.

MGL is consistently composed of quartz, feldspar, and garnet, but has significant variation in mode and grain size across the Ruby Range and even within individual outcrops. MGL fabric ranges from barely detectible foliation to extremely mylonitized, with continuous quartz ribbons, feldspar augen, and garnet occurring both as euhedral grains and in elongate clusters of very small grains associated with quartz ribbons. In some samples, MGL has mesoscopic S-C fabric and asymmetric feldspar augen, but these were not sufficiently well sampled to form the basis for kinematic analysis. Within quartz ribbons, quartz-to-quartz grain boundaries are typically polygonal or straight, and in some cases hexagonal, evidence of annealing that suggests the MGL remained in a high heat environment after the termination of the shear strain that caused mylonitization. Consequently, crystallographic preferred orientation (CPO) in quartz is used to assess the shear that produced the mesoscopic mylonitic fabric. Electron backscatter diffraction (EBSD) analysis on a scanning electron microscope provides CPO patterns of quartz and feldspar grains in polished thin sections from nine locations of the MGL. Results of these analyses constrain the conditions of deformation associated with the 2.45 Ga Beaverhead orogeny, including direction and sense of shear strain, providing further understanding of tectonism along the northwestern margin of the Wyoming Province during the Paleoproterozoic.