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The creation of Laurentia involved accretion of crustal fragments around a central portion of Archean crust, including the Wyoming Province. The southern boundary of the Wyoming Province, the Cheyenne belt, separates Archean rocks to the north from <1.8 Ga Paleoproterozoic rocks of the Yavapai/Mojavia blocks that were previously interpreted to have collided with the southern Wyoming Province at ~1.7 Ga. Though the location of the Cheyenne belt is well-known in southern Wyoming, its location farther west, such as in the Uinta Mountains of eastern Utah, is poorly known due to conflicting U/Pb zircon ages from basement rocks of the Red Creek Quartzite and the Owiyukuts Complex. Here, we present new U/Pb zircon ages from a quartzite and felsic orthogneiss of the Owiyukuts Complex near Beaver Creek and two quartzites and an amphibolite from the structurally overlying Red Creek Quartzite from Beaver Creek and Jesse Ewing Canyon. New maximum depositional ages of the quartzites span from ~2.67 Ga to ~2.32 Ga and agree with the relative structural positions hypothesized by prior workers on the basis of structural mapping and metamorphic grade. Results also suggest that the quartzite sediments came from sources dominated by ~2.7 Ga ages, with one quartzite sample yielding ages as young as ~2.3 Ga. Two quartzites, including from both the Owiyukuts Complex and Red Creek Quartzite, have distinctly zoned zircon rims on CL images that yield high-U/Th, ~1.75 Ga ages that we interpret to represent the timing of high-grade metamorphism. Substantial Pb loss precludes estimation of the crystallization age of the felsic orthogneiss. Finally, a coarsely crystalline amphibolite sill exposed at Beaver Creek lacks a significant foliation and yields an age of 1.68 Ga, which we interpret to post-date high-grade metamorphism and deformation. In summary, our results suggest that basement of the eastern Uinta Mountains is dominated by ~2.67-2.32 Ga metasedimentary rocks, which enjoyed high-grade metamorphism at ~1.75 Ga. Given their similarities with the southern Wyoming Province and Cheyenne belt, we interpret that basement rocks in the eastern Uintas define the southern boundary of the Wyoming Province and were metamorphosed as a consequence of collision of Paleoproterozoic blocks to the south at ~1.75 Ga. 

The Wyoming Province of Laurentia, which hosts some of the oldest known crustal material on Earth including zircon 207Pb/206Pb ages up to 3.96 Ga in the Beartooth Mountains, Montana, has been subjected to multiple periods of orogenesis and burial from Proterozoic time to present. We present new zircon U-Pb geochronology and zircon (U-Th)/He thermochronology from Archean-Proterozoic metamorphic rocks exposed in the Bridger Range, Montana, to resolve details of their origins and reconstruct their deep-time tectonothermal history. Zircon U-Pb geochronology and cathodoluminescence imaging, paired with whole rock geochemistry and petrography, was obtained from four metamorphic samples including quartzofeldspathic and garnet-biotite gneisses proximal to the “Great Unconformity” (GU), where Archean-Proterozoic metamorphic rocks are unconformably overlain by ~7.5-9 km of compacted Phanerozoic strata. Single grain 207Pb/206Pb ages range from 4099 ± 44 Ma to 1776 ± 24 Ma, extending the age of known crustal material in the northern Wyoming Province into the Hadean and recording high-grade conditions during the Paleoproterozoic Great Falls/Big Sky orogeny. Zircon (U-Th)/He thermochronology from five metamorphic samples proximal to the GU record cooling ages ranging from 705 Ma to 10.3 Ma, reflecting the variable He diffusivity of individual zircon grains with a large range of radiation damage as proxied by effective uranium (eU) concentrations, which range from ~5 to ~3000 ppm. A negative correlation between cooling age and eU is observed across the five samples suggesting the zircon (U-Th)/He system is sensitive to Proterozoic through Miocene thermal perturbations. Ongoing thermal history modeling seeks to reconstruct the temperature-time histories of these metamorphic rocks, including testing whether this dataset is sensitive to thermal effects imparted by the rifting of Rodina and erosion related to Cryogenian glaciation (i.e., hypotheses related to formation of the GU), and the onset of modern, active extension. These datasets and models provide crucial new constraints on the obscured Proterozoic tectonic history of the northern Wyoming Province and have important implications for our understanding of the formation of early crustal material on Earth. 

Unconformities, or gaps in the rock record, have typically limited our understanding of Earth’s history. However, geologic processes such as erosion and burial leave a thermal imprint on the rocks below unconformities that can be recovered using thermochronology. Thermochronology uses the temperature-sensitive diffusive loss of radiogenic daughter products within a mineral to obtain information about a sample’s temperature and time evolution, or its thermal history. In the Uinta Mountains of northeastern Utah, Paleoproterozoic metamorphic rocks are nonconformably overlain by Neoproterozoic sedimentary rocks of the Uinta Mountain Group (UMG), resulting in a nearly 1 Gyr interval of missing time. The UMG is a valuable record during early stages of rifting of Rodinia along the western margin of Laurentia where, elsewhere, this early rifting is rarely preserved. In our work, we use the unconformity as an independent thermochronologic constraint: at the time of deposition of the overlying rock, the underlying rock must have been at the surface and at cool, near-surface temperatures. As deposition ensued, we expect that the underlying rocks were heated. To evaluate whether the thermal record of this burial is preserved in the underlying rock and extract information about the pre-depositional history of the region, we sampled basement rocks in depositional contact with the overlying Uinta Mountain Group in a well-characterized structural context for thermochronologic analysis. Here, we present new zircon (U-Th)/He data to constrain the thermal history of each sample. Despite the complexity of multiple episodes of deformation within the study area, these preliminary data and thermal history models support an episode of heating ca. 800-700 Ma, corresponding with deposition of the UMG. Ongoing work will evaluate the spatial heterogeneity of this thermal record within the study area and thus the ability of deep-time thermochronology to fill in the knowledge gaps left by unconformities, particularly in other localities where the sedimentary record of Neoproterozoic rifting is not preserved.