SYMPLECTIC KYANITE, GARNET, AND CORDIERITE IN ORTHOAMPHIBOLITES FROM THE RUBY RANGE, SOUTHWESTERN MONTANA

Garnet, cordierite, and kyanite-bearing orthoamphibolites from the Christensen Ranch Metasedimentary Suite (CRMS) in the Ruby Range in southwestern Montana yield insights into the pressure and temperature conditions during the 1.77 Ga Big Sky orogeny. These small (commonly less than 1 meter in thickness) discontinuous orthoamphibolite layers occur within layers of marble, calc-silicate rock, and other amphibolites. Major and trace element geochemistry applied to the Ruby Range rocks constrain the tectonic environment associated with their formation. Orthoamphibole + garnet-dominated mineral assemblages with remnant kyanite and staurolite found in these CRMS orthoamphibolites preserve evidence for high pressure (greater than 6 kbar) conditions associated with crustal thickening. Garnet and cordierite corona textures separating kyanite and staurolite from orthoamphibole reflect lower pressures (less than 5 kbar). These data are consistent with a clockwise P-T path for these rocks. Additional complexities are indicated by the rimming of kyanite by staurolite, the breakdown of kyanite to form sapphirine and spinel, and spinel-kyanite symplectites. The role of calcium bearing minerals (An-rich feldspar in some samples, zoesite in others) remains unresolved, but may be associated with the breakdown of garnet.

Similar symplectic aluminous orthoamphibolites have been used in past studies to document a clockwise P-T path in the Tobacco Root Mountains to the northeast of the Ruby Range. The mineral assemblages present in the Ruby Range, however, are characteristic of lower grade rocks formed at shallower depths than those experienced by the Tobacco Root Mountains. The Ruby Range and Tobacco Root Mountains were both involved in the Big Sky orogeny, which contributed to the formation of Laurentia. By comparing the pressure and temperature conditions previously determined in the Tobacco Root Mountains with the conditions in the Ruby Range constrained in this study, we can derive a greater understanding of the dynamics at play during the Big Sky orogeny.