Determining the Precambrian Structure and Thermotectonic Evolution of the Central Ruby Range, southwest Montana

The Ruby Range in southwestern Montana is a fundamental location within the Archean Wyoming craton for examining Precambrian crustal architecture and determining the thermotectonic evolution of the region. Monazite, zircon, and garnet geochronology from each of the three major units within the Ruby Range reveal two distinct metamorphic age populations: an older population ca. 2.55-2.45 Ga (the Tendoy orogeny) and a second, younger population ca. 1.78-1.72 Ga (the Big Sky orogeny). Phase equilibria modeling for the Big Sky orogeny within the confines of the Mine Gulch 7.5’ quadrangle reveal discrete pressure-temperature (P-T)histories for each of the three major structural units within the range. The Christensen Ranch metasedimentary suite yields peak metamorphic pressures of ~7 kbar, however peak temperatures increase from the top of the unit towards the base from ~700 °C to ~760 °C. The Dillon Gneiss yielded peak metamorphic conditions of ~8.4 kbar and ~760 °C, and the Elk Gulch Suite on average yielded peak conditions of ~8.8 kbar and ~770 °C. Additionally, new mapping interpretations from the Mine Gulch quadrangle reveal that all three units within the Ruby Range are closely structurally related, sharing an entwined Precambrian metamorphic and deformational history. Furthermore, the separation of the ca. 2.45 Ga garnet leucogneiss from the Dillon Gneiss as a distinct unit within the range ascribes wide spread crustal melting to the Tendoy orogeny. Combining these new observations with previous work aid in constraining a revised tectonic history for southwestern Montana between ~2.55-1.72 Ga. Prior to ~2.55 Ga, sediment accumulated on the margin of the Wyoming province and was followed by continental collision, the Tendoy orogeny, between ~2.55-2.45 Ga. A second cycle of sedimentation on the cratonic margin initiated immediately after the conclusion of the Tendoy orogeny and lasted between ~2.45-1.8 Ga. Between ~2.1-2.0 Ga, the Wyoming Province experienced a short period of extensional tectonism resulting in the emplacement of mafic dikes and sills. Between ~1.9-1.8 Ga, the Wyoming Province began to subduct beneath the Medicine Hat Block, ultimately culminating in the collision between the two continents resulting in the Big Sky orogeny between ~1.78-1.72 Ga