GARNET LU-HF AND ZIRCON U-PB DATING CONSTRAINS EARLIEST PALEOPROTEROZOIC (2.43 GA) THERMAL EVENT IN THE RUBY RANGE, SW MONTANA

The Wyoming province contains some of the oldest rocks in North America and preserves a long history of magmatism and metamorphism from the Mesoarchean to the Mesoproterozoic. The details of these geological events throughout the province, however, remain elusive. Determining when and how these rocks formed—and how they have been modified by later tectonic processes—is important not only for understanding the geologic history of this region, but also for understanding how North America was formed and assembled. In this study we use U-Pb zircon and Lu-Hf garnet geochronology to determine magmatic and metamorphic ages to constrain the geologic history in the Ruby Range in southwestern Montana, located in northwest Wyoming province. In detail, the focus of this study is on the Christensen Ranch Metasedimentary Sequence (CRMS), a package of metasedimentary lithologies located along the western flank of the Ruby Range. Previous work by co-author Baldwin and students (Cramer, 2014) on a garnet-sillimanite paragneiss revealed monazite dates from 1.78 to 1.71 Ga, interpreted to indicate a major tectonic event involving the collision of two continental blocks during the Big Sky orogeny (Harms et. al. 2004). In the present study, we determined garnet Lu-Hf and zircon U-Pb ages from a mylonitic garnet leucogneiss that intrudes the base of the CRMS. The garnet Lu-Hf analyses yield a well-defined date of 2428.1 ± 6.8 Ma (MSWD = 0.68) based on 3 garnet fractions and associated whole rock. Zircons separated from this leucogneiss yield U-Pb dates identical to the garnet date: a weighted-average ²⁰⁷Pb/²⁰⁶Pb date, based on the best 4 concordant LA-ICPMS analyses, yield an age of 2429±26 Ma (MSWD = 1.3). These new dates indicate a period of crustal melting (and metamorphism?) preserved in the Ruby Range prior to the 1.78-1.71 Ga metamorphism attributed to the Big Sky orogeny. The Hf isotope compositions of the zircons, calculated at their ²⁰⁷Pb/²⁰⁶Pb age, are e_Hf ~9, indicating an older crustal precursor for these rocks. Integrated U-Pb zircon and monazite geochronology—as well as Lu-Hf and Sm-Nd garnet geochronology—are needed to further constrain the complicated magmatic and metamorphic history of this important crustal section.