Tectonothermal Evolution of the NW Wyoming Province: Insights From Coupled Garnet Lu-Hf and Sm-Nd Geochronology

Understanding how Paleoproterozoic orogenic processes shaped the assembly of Laurentia remains a critical puzzle in deciphering Earth's ancient tectonic history. To address this challenge, the Montana metasedimentary terrane in the northwest Wyoming Province, which preserves a complex record of multiple metamorphic episodes, provides a unique opportunity. In this study, we integrate garnet composition with coupled Lu-Hf and Sm-Nd geochronology to unravel the polymetamorphic history of this terrane and constrain the timing and mechanisms of orogenic processes. Our new garnet Lu-Hf and Sm-Nd ages reveal three distinct age groups: 2.42, 2.19–2.06 and 1.83–1.70 Ga. Most analysed garnet samples demonstrate typical prograde zoning patterns with enriched Mn and Lu contents in their core, suggesting that the Lu-Hf ages from these samples reflect the timing of prograde metamorphism. The ~30 Myr younger Sm-Nd ages for most samples document the cooling process that followed peak metamorphic conditions during the orogenic cycle. One amphibolite sample shows reverse Lu zoning that likely resulted from mineral breakdown reactions, suggesting complex trace-element incorporation during high-grade metamorphic processes. By integrating our age data with the estimated peak metamorphic conditions reported by previous studies, we identify a spatial and temporal trend of decreasing metamorphic grade that progressed southeastward from the northwestern boundary of the terrane between 1.8 and 1.7 Ga. This pattern reveals a propagation of orogenic activity during the Big Sky orogeny, providing insights into the thermal evolution and incorporation of terranes during Laurentia assembly.