Significance of Leucogranitic Gneiss in the Archean Teton Range

Abstract

In previous years of this project, we have developed the hypothesis that the high-pressure granulites exposed in the Moose Basin area of the Teton Range represent evidence of a 2.7 billion year­old continent-continent collision. We have described gneisses in the Teton Range that show two distinct metamorphic histories. In the northwest there are high­pressure granulites suggesting metamorphism and deformation resulted from a 2685 to 2671 Ma Himalayan type orogeny (Frost et al., 2006). Gneiss exposed in the northwest is dominated by migmatites, with lesser kyanite bearing pelite, and some garnet amphibolite. Their ENd values at 2685 Ma are negative. Thermobarometric studies in the northwestern gneiss suggest burial of pelitic rocks to the base of continental crust, with pressures reaching 12 kilobars and temperatures of ~950° C during peak metamorphism (Fitz-Gerald, 2008). In modem plate tectonic environments these high pressures are associated with continent-continent collision. If the collisional hypothesis is correct, then this is the oldest documented example in the world.