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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

   https://doi.org/10.1130/abs/2024AM-404615  

   Kennedy, Rebekah; Orme, Devon; Lageson, David R; Laskowski, Andrew; Morrow, Zach; Moss, Konrad; Murray, Kendra (September 2024, Geological Society of America Abstracts with Programs)

   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. 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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3. Regional exhumation of the Laramide Province

   https://doi.org/10.1130/B37625.1  

   Jepson, Gilby; Carrapa, Barbara; Reeher, Lauren J; DeCelles, Peter G; Afonso, Walter D; Howlett, Caden J; Caylor, Emilia A; Sherpa, Tshering ZL; Wang, Jordan W; Constenius, Kurt N (February 2025, Geological Society of America Bulletin)

   Western North America is the archetypical Cordilleran orogenic system that preserves a Mesozoic to Cenozoic record of oceanic Farallon plate subduction-related processes. After prolonged Late Jurassic through mid-Cretaceous normal-angle Farallon plate subduction that produced the western North American batholith belt and retroarc fold-thrust belt, a period of low-angle, flat-slab subduction during Late Cretaceous−Paleogene time caused upper plate deformation to migrate eastward in the form of the Laramide basement-involved uplifts, which partitioned the original regional foreland basin. Major questions persist about the mechanism and timing of flat-slab subduction, the trajectory of the flat-slab, inter-plate coupling mechanism(s), and the upper-plate deformational response to such processes. Critical for testing various flat-slab hypotheses are the timing, rate, and distribution of exhumation experienced by the Laramide uplifts as recorded by low-temperature thermochronology. In this contribution, we address the timing of regional exhumation of the Laramide uplifts by combining apatite fission-track (AFT) and (U-Th-Sm)/He (AHe) data from 29 new samples with 564 previously published AFT, AHe, and zircon (U-Th)/He ages from Laramide structures in Arizona, Utah, Wyoming, Colorado, Montana, and South Dakota, USA. We integrate our results with existing geological constraints and with new regional cross sections to reconstruct the spatial and temporal history of exhumation driven by Laramide deformation from the mid-Cretaceous to Paleogene. Our analysis suggests a two-stage exhumation of the Laramide province, with an early phase of localized exhumation occurring at ca. 100−80 Ma in Wyoming and Montana, followed by a more regional period of exhumation at ca. 70−50 Ma. Generally, the onset of enhanced exhumation occurs earlier in the northern Laramide province (ca. 90 Ma) and later in the southern Laramide province (ca. 80 Ma). Thermal history models of selected samples along regional cross sections through Utah−Arizona−New Mexico and Wyoming−South Dakota show that exhumation occurred contemporaneously with deformation, implying that Laramide basement block exhumation is coupled with regional deformation. These results have implications for testing proposed migration pathway models of Farallon flat-slab and for how upper-plate deformation is expressed in flat-slab subduction zones in general. 

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4. 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 (January 2025, Contributions to Mineralogy and Petrology)

   Canil, Dante (Ed.)

   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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5. Multichronometer dating of dropstones and ice-rafted debris (latest Miocene through Pliocene) recovered from IODP drill cores offshore West Antarctica, to extend knowledge of bedrock geology and past ice sheet extent

   Siddoway, Christine S; Thomson, Stuart N; Hemming, Sidney R; Brachfeld, Stefanie A (December 2024, American Geophysical Union)

   In 2019, International Ocean Discovery Program (IODP) expeditions to offshore West Antarctica recovered deep ocean sediment cores in the outer Amundsen Sea (Exp. 379) and Dove Basin (Exp. 382). These cores are characterized by numerous ice-rafted detritus (IRD) intervals, including dropstone cobbles released by icebergs calved from past glaciers/ice streams that incised the subglacial bedrock of West Antarctica. We selected nine dropstones from latest Miocene through mid-Pliocene sediment from IODP Sites U1532C, U1533B (Exp. 379) and U1536E (Exp. 382), comprising sandstone, diorite, granitoid, basalt, and rhyolite, for petrologic characterization and multi-method geo-thermochronology. Dating methods applied include U-Pb zircon (UPbZ) geochronology, and apatite fission-track (AFT) and (U-Th)/He (AHe) low-temperature thermochronology, to reveal dates and rates of geologic events with bearing on their crustal provenance and source region bedrock thermal history. Comparison to published data reveal dropstones to be of both local and distant origin. Notable discoveries are: 1) From U1536E, a ~1200 Ma [U-PbZ] diorite cobble, with ca. 130 Ma AFT and 65-50 Ma AHe ages that most resembles cratonic crust of Queen Maud Land (East Antarctica). 2) Three granitoid rocks from U1533B with ca. 174-179 Ma (UPbZ) ages. The only known rocks of similar age and lithology in West Antarctica are described in the Whitmore Mountains (WM). AFT ages of 114 Ma, 91 Ma, and 81 Ma may thus provide the first thermochronology data from the WM. 3) A 27±1 Ma (UPbZ) diorite of from U1533B records 25.6 Ma AFT and 10.6 Ma AHe ages, suggesting origins in the western Antarctic Peninsula. 4) Two very similar distinctive green quartz arenite dropstones were recovered from latest Miocene core at U1533B and U1536E, locations separated by 3270 km. Multivariate statistical comparison of their UPbZ age populations with published data indicates a common provenance in the Ellsworth Mountains (Antarctic interior). When placed within geotectonic and paleoclimate context, discoveries from IRD-dropstones promise to advance understanding of crustal and landscape evolution of evolution of glaciated continents, variations in icesheet extent during warm periods, and ocean/atmospheric current circulation. 

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