ANALYSIS OF THE RELATIONSHIP BETWEEN STRAIN AND METAMORPHIC EQUILIBRIUM AS PRESERVED IN ARCHEAN AMPHIBOLITES FROM THE TOBACCO ROOT MOUNTAINS, MONTANA

This study focuses on Archean amphibolites from the Tobacco Root Mountains, MT. Hbl-grt-plag-cpx-opx-qtz amphibolites from a 1 x 2 square kilometer study area have widely differing fabrics including well developed foliation and lineation, isotropic fabric, and S-C fabrics. Several samples include small pockets and lenses (1-2 cm) of melt. Representative samples of each fabric type are analyzed using comparative hbl-plag-grt thermobarometry and petrography to determine possible causes for differences in strain, and to test whether all samples equilibrated at the same pressure and temperature.

These amphibolites are part of the Indian Creek Metamorphic Suite (ICMS), which is dominated by quartzofeldspathic gneiss but also includes amphibolite, marble, pelitic schist, quartzite and iron formation. The ICMS was metamorphosed to upper amphibolite grade at around 2450 Ma and again around 1770 Ma at pressures of 7-9 kbars and temperatures of 650-750 degrees C.