PETROGRAPHY AND GEOCHEMISTRY OF ARCHEAN GARNETIFEROUS GNEISSES FROM THE SOUTH BOULDER RIVER VALLEY, TOBACCO ROOT MOUNTAINS, MONTANA

The Precambrian rocks in the Tobacco Root Mountains in southwestern Montana formed over through a series of diverse Archean and Paleoproterozoic orogenic events. Metamorphic suites in the region include metavolcanics/metasedimentary rocks (greenstone belts) and deformed crustal fragments (gneisses) from the Archean Wyoming Craton and metasedimentary gneisses and migmatites of the Paleoproterozoic Big Sky Orogeny. This study analyzes a representative collection of amphibolic gneisses, amphibolites, and garnetiferous gneisses along a 1 km transect near a nonconformity with overlying Paleozoic sedimentary rocks in the South Boulder River valley. Petrographic and geochemical analyses from this metamorphic suite provides insight into the orogenic development and geologic history of this region. General similarities in structural fabrics, mineralogy, and trace element geochemistry supports a common petrogenesis for most of the amphibolitic and tonalitic gneisses on this transect. In contrast, the fine grained, granoblastic, banded garnetiferous texture, high Fe and Mn content of the garnets, and associated foliated amphibolites are suggestive of a supracrustal suite that formed independently of the other gneisses. This study concludes that the garnetiferous gneiss and nearby amphibolites were deposited in a submarine extensional environment with evidence of intense hydrothermal activity. Preliminary geochemistry of the amphibolites indicate a tholeiitic magmatic affinity suggestive of an oceanic setting. The garnetiferous gneiss and amphibolite represent a 3.13-2.85 Ga exhalative iron formation and associated volcanic/intrusive rocks and are likely genetically related to the other banded iron formations elsewhere in the Archean Wyoming Craton. With peak metamorphic grade as upper amphibolite across the entire transect, it is assumed that peak metamorphism was during the 1.78-1.72 Ga Big Sky Orogeny.