PALEOPROTEROZOIC GREEN-MICA QUARTZITE IN THE RUBY RANGE OF SOUTHWEST MONTANA

The crystallographic preferred orientation (CPO) of quartz grains in green-mica quartzite in the Ruby Range, SW Montana provides new constraints on the history of tectonism on the NW flank of the Wyoming Province prior to and during the Paleoproterozoic Blue Sky orogeny. This distinctive green-mica-bearing quartzite unit is approximately 5-10 m thick, and occurs discontinuously within the Precambrian Christensen Ranch Suite of metasupracrustal rocks on the western side of the Ruby Range. The unit consistently separates two suites of rock: one that contains evidence of collision-related metamorphism and deformation at both 2.45 Ga during the Beaverhead orogeny and again at 1.78 Ga during the Big Sky orogeny, and another suite that only records effects of the 1.78 Ga orogeny. Geochemical analysis indicates that the mica is Fe- rather than Cr-bearing, in contrast to fuchsitic quartzites reported from other Precambrian metamorphic rock suites in SW Montana. Oriented samples from nine locations along strike of the quartzite unit provide the basis for CPO analysis through electron backscatter diffraction (EBSD) on the scanning electron microscope (SEM). Samples were selected to reflect the observed range of quartz grain sizes (5-15 mm), quartz grain shapes (nearly-equant to elongate), and green-mica concentrations (0-20%) throughout the unit. Polygonal quartz-to-quartz grain boundaries suggest that annealing followed any simple shear that might have occurred along the boundary between the two geochronologically distinct suites. Quartz CPO patterns rather than subgrain shapes are therefore used to investigate the strain history of the unit. Structural and compositional studies of this quartzite unit provide further insight into the stages of tectonic assembly of Paleoproterozoic North America.