TIMING OF DEFORMATION ASSOCIATED WITH A MAJOR DUCTILE SHEAR ZONE, RUBY MOUNTAINS, SW MONTANA

The Ruby Mountains in southwest Montana are one of several Archean-aged basement cored uplifts exposed during Cretaceous-Tertiary time. A high strain shear zone exposed across the southern portion of the Ruby Mountains separates two distinct lithostratigraphic terranes. The Archean terrane to the north consists of granitic gneiss, marble, amphibolite, pelitic gneiss, and quartzite, with minor, metaconglomerate, meta-banded iron formation, and calc-silicate schist. The terrane to the south is composed dominantly of granitic gneiss, locally migmatitic pelitic gneiss, and amphibolite, with minor quartzite. In addition, the southern terrane contains a narrow (~1 km) belt of ultramafic rocks, both peridotite and megacrystic metapyroxenite.

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.