KINEMATICS AND TIMING OF PROTEROZOIC MYLONITIZATION IN THE HIGHLAND MOUNTAINS OF SOUTHWESTERN MONTANA

Widespread mylonitization accompanied the 1.77 Ga Big Sky orogeny in the Highland Mountains of southwestern Montana. The kinematics of mylonitization are determined using analysis of fabric elements measured in the field and petrographic assessment of microscopic shear sense indicators from oriented samples of mylonitic biotite-garnet-sillimanite schist, migmatite, and foliated granitoids that flank the core of the structural Highland dome. The relative timing of shear, metamorphism and intrusion is constrained using field and petrographic evidence and U-Th-Pb geochronology of a mylonitic granite from the Camp Creek region of the Highland Mountains. Field evidence suggests that the schists and migmatite predate the granitoids. All of the rock types are foliated, and foliations and lineations in each rock type are consistent, suggesting that an episode of deformation post-dated metamorphism of the schist, melting of the migmatite, and emplacement of the granitoids, or was ongoing during these events. Mylonitic fabrics commonly parallel metamorphic fabrics, suggesting that mylonitization may have represented a continuation of the same episode of deformation responsible for metamorphism and migmatization, or, if the mylonitization occurred in a separate event, that the second event was kinematically similar to the first. The nearby Tobacco Root Mountains preserve evidence of a crustal-scale upper-amphibolite facies metamorphic event, the Big Sky orogeny, at 1.77 Ga. The proximity of the Highland Mountains to the locus of the Big Sky orogen makes them of great importance in our understanding of the Big Sky orogeny and the growth of Laurentia.