TIMING OF DEFORMATION ASSOCIATED WITH A MAJOR DUCTILE SHEAR ZONE, RUBY MOUNTAINS, SW MONTANA
Separating the lithologically distinct terranes is a 5-10 meter thick shear zone of mylonitic leucogranite and granite gneisses. Foliations within the shear zone strike NE and dip moderately to the NW and mineral lineations are predominantly down dip. Kinematic indicators with mylonitic gneisses reveal a two-phase history of movement, with an earlier reverse stage overprinted by later normal motion. The timing of deformation is uncertain.
Age dating in the western part of the Ruby Mountains and the adjacent ranges has identified a long and complex history of deformation and metamorphism ranging in age from 1.7 to 2.5 billion years ago. However, the age of the deformation and shear zone formation in the southern Ruby Mountains is not well constrained. We have analyzed hornblende samples from amphibolite units above, within, and below one of these shear zones in an effort to establish the timing of deformation and the thermal history associated with the development of this shear zone.