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1. EVIDENCE OF LATEST ARCHEAN TO HADEAN CRUSTAL MATERIAL IN THE NORTHERN WYOMING PROVINCE AND THE DEEP-TIME TECTONOTHERMAL HISTORY OF THE BRIDGER RANGE, MONTANA

   Kennedy, R; Orme, D; Lageson, D; Laskowski, A; Morrow, Z; Moss, K; Murray, KE (September 2024, Geological Society of America)

   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. 

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2. Orogens of Big Sky Country: Reconstructing the Deep‐Time Tectonothermal History of the Beartooth Mountains, Montana and Wyoming, USA

   https://doi.org/10.1029/2022TC007541  

   Ronemus, Chance B.; Orme, Devon A.; Guenthner, William R.; Cox, Stephen E.; Kussmaul, Christopher A. L. (January 2023, Tectonics)

   Abstract Archean rocks exposed in the Beartooth Mountains, Montana and Wyoming, have experienced a complex >2.5 Gyr thermal history related to the long‐term geodynamic evolution of Laurentia. We constrain this history using “deep‐time” thermochronology, reporting zircon U‐Pb, biotite⁴⁰Ar/³⁹Ar, and zircon and apatite [U‐Th(‐Sm)]/He results from three transects across the basement‐core of the range. Our central transect yielded a zircon U‐Pb concordia age of 2,805.6 ± 6.4 Ma. Biotite⁴⁰Ar/³⁹Ar plateau ages from western samples are ≤1,775 ± 27 Ma, while those from samples further east are ≥2,263 ± 76 Ma. Zircon (U‐Th)/He dates span 686.4 ± 11.9 to 13.5 ± 0.3 Ma and show a negative relationship with effective uranium—a proxy for radiation damage. Apatite (U‐Th)/He dates are 109.2 ± 23.9 to 43.6 ± 1.9 Ma and correlate with sample elevation. Multi‐chronometer Bayesian time‐temperature inversions suggest: (a) Cooling between ∼1.90 and ∼1.80 Ga, likely related to Big Sky orogeny thermal effects; (b) Reheating between ∼1.80 Ga and ∼1.35 Ga consistent with Mesoproterozoic burial; (c) Cooling to ≤100°C between Mesoproterozoic and early Paleozoic time, likely reflecting continental erosion; (d) Variable Paleozoic–Jurassic cooling, possibly related to Paleozoic tectonism and/or low eustatic sea level; (e) Rapid Cretaceous–Paleocene cooling, preceding accepted proxies for flat‐slab subduction; (f) Eocene–Miocene reheating consistent with reburial by Cenozoic volcanics and/or sediments; (g) Post‐20 Ma cooling consistent with Neogene development of topographic relief. Our results emphasize the utility of multi‐chronometer thermochronology in recovering complex, non‐monotonic multi‐billion‐year thermal histories. 

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3. Proterozoic to Phanerozoic Tectonism in Southwestern Montana Basement Ranges Constrained by Low Temperature Thermochronometric Data

   https://doi.org/10.1029/2021TC006744  

   Kaempfer, Jenna M.; Guenthner, William R.; Pearson, David M. (November 2021, Tectonics)

   Abstract Crystalline basement rocks of southwestern Montana have been subjected to multiple tectonothermal events since ∼3.3 Ga: the Paleoproterozoic Big Sky/Great Falls orogeny, Mesoproterozoic extension associated with Belt‐Purcell basin formation, Neoproterozoic extension related to Rodinia rifting, and the late Phanerozoic Sevier‐Laramide orogeny. We investigated the long‐term (>1 Ga), low‐temperature (erosion/burial within 10 km of the surface) thermal histories of these tectonic events with zircon and apatite (U‐Th)/He thermochronology. Data were collected across nine sample localities (n = 55 zircon andn = 26 apatite aliquots) in the northern and southern Madison ranges, the Blacktail‐Snowcrest arch, and the Tobacco Root uplift. Our zircon (U‐Th)/He data show negative trends between single aliquot date and effective uranium (a radiation damage proxy), which we interpreted with a thermal history model that considers the damage‐He diffusivity relationship in zircon. Our model results for these basement ranges show substantial cooling from temperatures above 400°C to near surface conditions between 800 and 510 Ma. Subsequent Phanerozoic exhumation culminated by ∼75 Ma. Late Phanerozoic cooling is coincident with along‐strike Sevier belt thin‐skinned thrusting in southeastern Idaho, and older than exhumation in basement‐involved uplifts of the Wyoming Laramide province. Our long‐term, low‐temperature thermal record for these southwestern Montana basement ranges shows that: (a) these basement blocks have experienced multiple episodes of upper crustal exhumation and burial since Archean time, possibly influencing Phanerozoic thrust architecture and (b) the late Phanerozoic thick‐skinned thrusting recorded by these rocks is among the earliest thermochronologic records of Laramide basement‐involved shortening and was concomitant with Sevier belt thin‐skinned thrusting. 

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4. U/Pb zircon dating of basement rocks in the eastern Uinta Mountains, Utah suggests 2.7-2.3 Ga deposition and ca. 1.7 Ga metamorphism at the southern margin of the Wyoming Province

   https://doi.org/10.1130/abs/2024AM-404741  

   Heebner, Michael Tiago; Pearson, David M; Murray, Kendra; Miller, Anna (September 2024, Geological Society of America Abstracts with Programs)

   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. 

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5. A 3-billion-year history of magmatism, metamorphism, and metasomatism recorded by granulite-facies xenoliths from central Montana, USA

   https://doi.org/10.1007/s00410-024-02190-5  

   Ringwood, Mary_F; Ortner, Sophia_E; Seward, Gareth_G_E; Kylander-Clark, Andrew_R_C; Rudnick, Roberta_L (December 2024, Contributions to Mineralogy and Petrology)

   Abstract Lower crustal xenoliths from the Missouri Breaks diatremes and Bearpaw Mountains volcanic field in Montana record a multi-billion-year geologic history lasting from the Neoarchean to the Cenozoic. Unusual kyanite-scapolite-bearing mafic granulites equilibrated at approximately 1.8 GPa and 890 °C and 2.3 GPa and 1000 °C (67 and 85 km depth) and have compositions pointing to their origin as arc cumulates, while metapelitic granulites record peak conditions of 1.3 GPa and 775 °C (48 km depth). Rutile from both mafic granulites and metapelites have U-Pb dates that document the eruption of the host rocks at ca. 46 Ma (Big Slide in the Missouri Breaks) and ca. 51 Ma (Robinson Ranch in the Bearpaw Mountains). Detrital igneous zircon in metapelites date back to the Archean, and metamorphic zircon and monazite record a major event beginning at 1800 Ma. Both zircon and monazite from a metapelite from Robinson Ranch also document an earlier metamorphic event at 2200–2000 Ma, likely related to burial/metamorphism in a rift setting. Metapelites from Big Slide show a clear transition from detrital igneous zircon accumulation to metamorphic zircon and monazite growth around 1800 Ma, recording arc magmatism and subsequent continent-continent collision during the Great Falls orogeny, supporting suggestions that the Great Falls tectonic zone is a suture between the Wyoming craton and Medicine Hat block. U-Th-Pb and trace-element depth profiles of zircon and monazite record metasomatism of the lower crust during the Laramide orogeny at ~60 Ma, bolstering recent research pointing to Farallon slab fluid infiltration during the orogeny. 

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