ORIGIN OF PRECAMBRIAN GARNETIFEROUS GNEISSES FROM THE SOUTH BOULDER RIVER VALLEY, TOBACCO ROOT MOUNTAINS, MONTANA

The Precambrian geology of the Tobacco Root Mountains in Southwestern Montana formed over almost 1 billion years through a series of diverse geologic settings. Precambrian metamorphosed rocks in the region include metavolcanics/metasedimentary rocks (greenstone belts) and deformed crustal fragments (gneisses) from the Archean Wyoming Craton and Paleoproterozoic Big Sky Orogeny. The samples analyzed are a representative collection of amphibolic gneisses, amphibolites, and garnetiferous gneisses along a 1 km transect near a nonconformity with overlying Paleozoic sedimentary rocks. Petrographic and geochemical analyses from this metamorphic suite collected from the South Boulder River Valley provides insight into the orogenic development and geologic history of this region. General similarities in structural characteristics, mineralogy, and trace element geochemistry supports a common genesis for most of the gneisses on this transect. In contrast, the fine grained, granoblastic, banded garnetiferous texture and high Fe and Mn content of the garnetiferous gneisses suggests that these rocks formed as a result of an isolated process or event. This study concludes that the garnetiferous gneiss and nearby amphibolites along this transect were deposited in a submarine back-arc environment with evidence of intense hydrothermal activity. The garnetiferous gneiss and amphibolite represent a 3.13-2.85 Ga exhalative iron formation genetically related to the other banded iron formations in the Archean Wyoming Craton. With similarities in metamorphic grades, it is assumed that peak metamorphism was during the Big Sky Orogeny, at about 1.78-1.72 Ga.