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1. Timing of Dike and Sill Emplacement in the Inner Aureole of the Alta Stock, Utah Determined by Zircon and Monazite Petrochronology

   https://doi.org/10.1029/2024GC011946  

   Beno, C J; Stearns, M A; Bowman, J R; Bartley, J M; Fernandez, D P; Kylander‐Clark, A_R C (May 2025, Geochemistry, Geophysics, Geosystems)

   Abstract Detailed geochronology from two compositionally distinct generations of dikes and sills intruded into the Alta metamorphic aureole, north‐central Utah, complement previous geochronologic studies from the Alta stock, providing information on the timing of magmatism and the nature of emplacement. Uranium/thorium‐lead dates and chemistry were measured in zircon and monazite from these intrusions and associated reaction selvages in hornfels by split‐stream laser ablation techniques. Concordant zircon U‐Pb dates (n = 532) define a dispersed population of dates that range from ∼38 to 32 Ma. Monazite Th‐Pb dates (n = 888) from granodioritic compositions range from ∼40 to 32 Ma. Evaluation of²⁰⁸Pb/²³²Th and²⁰⁷Pb/²⁰⁶Pb‐corrected dates with respect to common Pb, U and Th/U values allows rigorous evaluation of the effects of excess²⁰⁶Pb in these young monazites, yielding concordant²⁰⁸Pb/²³²Th and²⁰⁷Pb/²⁰⁶Pb‐corrected dates in monazites from the granodiorite, consistent with zircon dates from the same thin sections. Leucogranite sills and dikes, which cross‐cut the older granodiorite, have younger monazite dates from ∼33 to 28 Ma. Elevated heavy rare earth element concentrations and trends of larger negative Eu anomalies in the youngest monazites suggest crystallization from an evolved melt. Integration of these new geochronology results and field relationships with prior results from the Alta stock indicate the granodiorite represents the oldest material emplaced in the Alta system. Leucogranite aplite/pegmatite dikes and sills in the inner Alta aureole were emplaced during the final stage of Alta stock construction by injection of evolved water‐rich magmas. 

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2. Long-term repeatability and interlaboratory reproducibility of high-precision ID-TIMS U–Pb geochronology

   https://doi.org/10.1039/D1JA00116G  

   Schaltegger, Urs; Ovtcharova, Maria; Gaynor, Sean P.; Schoene, Blair; Wotzlaw, Jörn-Frederik; Davies, Joshua F.; Farina, Federico; Greber, Nicolas David; Szymanowski, Dawid; Chelle-Michou, Cyril (January 2021, Journal of Analytical Atomic Spectrometry)

   null (Ed.)

   Age determination of minerals using the U–Pb technique is widely used to quantify time in Earth's history. A number of geochronology laboratories produce the highest precision U–Pb dates employing the EARTHTIME 202 Pb– 205 Pb– 233 U– 235 U tracer solution for isotope dilution, and the EARTHTIME ET100 and ET2000 solutions for system calibration and laboratory intercalibration. Here, we report ET100 and ET2000 solution data from the geochronology laboratory of University of Geneva obtained between 2008 and 2021 and compare the most recent data with results from the geochronology laboratories of Princeton University and ETH Zürich. This compilation demonstrates that (i) the choice of the thermal ionization mass spectrometer model has no influence on precision and accuracy of the data; (ii) the often observed excess scatter of apparent ET100 solution 206 Pb/ 238 U dates can be mitigated by more careful tracer-sample equilibration; and (iii) natural zircon reference materials are not suitable for evaluating intra-laboratory repeatability and inter-laboratory reproducibility, since they combine several phenomena of natural system complexities (especially domains of different age within the same zircon grain, and residual loss of radiogenic lead in domains of high decay damage after chemical abrasion pre-treatment). We provide our best estimates of apparent dates for the ET100 solution ( 206 Pb/ 238 U date, 100.173 ± 0.003 Ma), for ET2000 solution ( 207 Pb/ 206 Pb date, 1999.935 ± 0.063 Ma), as well as for natural reference zircon Temora-2 ( 206 Pb/ 238 U date, 417.353 ± 0.052 Ma). These data will allow U–Pb laboratories to evaluate their analytical performance and to independently calibrate non-EARTHTIME tracer solutions in use. 

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3. 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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4. 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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5. Data from: Influence of magmatism on the architecture of transpressional faults and shear zones in the deep crust of the Late Cretaceous Southern California batholith

   https://doi.org/10.5061/dryad.jh9w0vtn6  

   Klepeis, Keith; Schwartz, Joshua; Miranda, Elena; Baskin, Jillyan; Castro_Mendez, Anya; Mora-Klepeis, Gabriela (January 2026, Dryad)

   Structural analyses combined with U‐Pb zircon petrochronology show the influence of arc magmatism on the evolution of two transpressional shear zones in the deep root of the Late Cretaceous Southern California batholith.  The mid-crustal Black Belt and lower-crustal Cucamonga shear zones (eastern San Gabriel Mountains) formed at ~84 Ma shortly after a large mass of tonalite and granodiorite intruded the lower crust.  Both shear zones were active until at least ~74 Ma and probably until 72-70 Ma.  In the mid-crustal shear zone, rheological contrasts between mingling magmas localized deformation at dike margins.  The deformation began as hypersolidus flow in partially crystallized dikes and then transitioned to deformation below the solidus when alternations between viscous creep and brittle faulting produced interlayered pseudotachylyte, cataclasite, and mylonite. As the dikes solidified, strain hardening drove shear zone growth and created thin (10-30 m) high-strain zones and faults that are widely spaced across ~1 km. In contrast, the lower-crustal Cucamonga shear zone was magma-starved, lacks the variety of shear zone fabrics exhibited by its mid-crustal counterpart, and formed by the reactivation of a pre-existing fabric that records pure reverse displacements at 124-93 Ma.  The two shear zones created a partitioned style of intra-arc transpression where sinistral-reverse (mostly arc-parallel with some arc-oblique) displacements were accommodated on moderately dipping faults and shear zones and arc-normal shortening was accommodated by coeval folds.  This study shows how a magmatic surge influenced the architecture and style of Late Cretaceous transpression in the Southern California batholith, including the evolution of high-strain zones that record alternating episodes of brittle, ductile, and hypersolidus deformation.  The results illustrate how magmatism localizes strain on deep-crustal faults during orogenesis and oblique convergence. We conducted U-Pb analyses on 11 zircon-bearing samples to determine the timing of magmatism, metamorphism, and deformation in the Black Belt and Cucamonga shear zones. Some zircons exhibited interior core and rim domains that reflect a history of deformation and metamorphism at amphibolite- to granulite-facies conditions.  The methods used in this study closely follow those outlined in Kylander-Clark et al. (2013) and are described in detail by Schwartz et al. (2024b).  U-Pb ratios were collected at the University of California, Santa Barbara using a Nu Plasma multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS) with a Photon Machines 193 ArF excimer laser with HelEx cell. Spot size and frequency were 35 µm and 4 Hz, respectively.  The primary standard (91500) was accessed every 10 analyses to correct for in-run fractionation of Pb/U and Pb isotopes. A secondary standard (Temora-2) was analyzed every ~10 analyses to assess data reproducibility. Uncertainties are reported as 2SE internal calculated from Iolite and IsoplotR (Paton et al., 2010; Vermeesch, 2018).  We assigned a 2% uncertainty to all dates for interlab comparisons to account for the reproducibility of standards. Zircon ages are reported using the ²⁰⁶Pb/²³⁸U date for analyses <1100 Ma, and the ²⁰⁷Pb/²⁰⁶Pb date for those >1100 Ma. For the former, discordance is calculated as the percent difference between the ²⁰⁷Pb/²³⁵U and ²⁰⁶Pb/2³⁸U dates. Corrections for minor amounts of common Pb in zircon were made on ²⁰⁶Pb/²³⁸U dates following the methods of Tera and Wasserburg (1972) using measured ²⁰⁷Pb/²⁰⁶Pb and ²³⁸U/²⁰⁶Pb ratios and an age-appropriate Pb isotopic composition of Stacey and Kramers (1975).  Zircons with large common Pb corrections (e.g., analyses interpreted as having ~20% or greater contribution from common Pb) were discarded. No corrections were made on ²⁰⁷Pb/²⁰⁶Pb dates due to large uncertainties in measured ²⁰⁴Pb. Cathodoluminescence images were obtained using a FEI Quanta scanning electron microscope before and after ablation to evaluate analyzed areas and compare them with growth textures. Where possible, we targeted all growth domains and report ²⁰⁷Pb/²⁰⁶Pb-corrected ²⁰⁶Pb/²³⁸U ages of texturally homogeneous populations. In a few cases where a laser spots overlapped multiple domains we report the data in tables but did not considered them in weighted mean calculations. Concordia plots and error-weighted average ages are shown in the accompanying journal article. Trace elements were measured simultaneously with U-Pb isotopes by LA-SF-ICPMS using Zr as the internal standard and nominal values of 43.14 % Zr. Trace element data were reduced using Iolite (Paton et al., 2010, 2011) and concentrations calculated relative to NIST-612 as a primary standard. BHVO-2G was analyzed as a secondary standard to assess reproducibility of the data. For zircon, model Ti-in-zircon temperatures were calculated using the Ferry and Watson (Ferry and Watson, 2007) calibration. Samples of granulite-facies rocks from the Cucamonga terrane contain rutile, allowing us to estimate the activity of TiO₂ to be one.  For samples that lack rutile we assume a value of 0.6 based on the presence of ilmenite. # Data from: Influence of magmatism on the architecture of transpressional faults and shear zones in the deep crust of the Late Cretaceous Southern California batholith [https://doi.org/10.5061/dryad.jh9w0vtn6](https://doi.org/10.5061/dryad.jh9w0vtn6) ## Description of the data and file structure All isotopic, geochemical, and trace element data were collected as part of a study to determine the timing of magmatism, metamorphism, and deformation in the Black Belt and Cucamonga shear zones in the southeastern San Gabriel Mountains in Southern California. Some zircons exhibited interior core and rim domains that reflect a history of deformation and metamorphism at amphibolite- to granulite-facies conditions. Where possible, we used zircon trace-element concentrations and ratios, particularly U (ppm), U/Th, Gd/Yb, and Dy/Yb, to distinguish metamorphic age populations. For more information on methods and an interpretation of the data, please see the publication in the journal *GSA Bulletin* by Klepeis et al. (2025). ### Files and variables #### File: Supplementary_Table_1_U-Pb_zircon_FINAL.xlsx **Description:** The table in the data sheet tab titled 'Table 1. U-Pb data unknowns' presents all U-Pb isotope data for the 11 samples analyzed plus the standards used. All standards are listed at the bottom of the table. The zircon standards include: Plesovice(Ples), which is used as a zircon U-Pb age standard; 91500, which is a primary zircon reference standard used for age normalization and reproducibility; Australian zircon suite (AusZ), which is a zircon age standard that is used as a secondary check on accuracy; GJ-1 zircon (GJ), which is a widely used zircon standard, the Malinau/Malim zircon suite (Mali) which is an additional age validation standard. Trace element standards include NIST 612 (N612), which is a synthetic silicate glass standard (see more information below), and BHVO, which is a natural basalt reference standard (see more information below). In the 'Table 1. U-Pb data unknowns' tab, n/a means not applicable. This refers to the lack of best ages and absolute error calculations for the standards N612 and BVHO. This information isn't applicable because these are non-zircon trace element standards, so no age is calculated. In contrast, the zircon standards (91500, AusZ, GJ, Mali, Plesovice) all have well-known ages, so best age and absolute error calculations are included. In the Table 1 data tab, n.d. means not detected. Only 1 average value and a standard deviation for the trace element analyses are provided for each sample in columns AP and AQ, respectively (the rest of the cells are blank). The tab titled 'NIST 612' shows data that used the widely used synthetic glass standard from the U.S. National Institute of Standards and Technology. The standard is commonly analyzed to calibrate trace-element concentrations during LA-ICP-MS work. The tab titled 'BHVO' reports data from a well-characterized natural basalt reference material (from Hawaii). This is used as a secondary standard to monitor analytical accuracy and precision. In the BHVO data tab, n.d. means not detected. Isotopic ratios and Apparent ages are not corrected for common lead. For the 206Pb/238U ages, common lead was corrected by inferring the initial Pb-composition from the Stacey and Kramers (1975) two stage isotope evolution model (Vermeesch, 2018). Analyses with greater than 10% uncertainty in 207Pb/206Pb age (1-sigma) or 5% uncertainty in 206Pb/238U age (1-sigma), 20% discordance, and/or 5% reverse discordance are excluded. Accepted ages were calculated by using 206Pb/238U ages for grains younger than 1100 Ma and 207Pb/206Pb ages for grains older than 1100 Ma. ##### Variables * U, U/Th, 238U/206U, 207Pb/206Pb, 207Pb/206Pb vs 238U/206Pb isotopic ratios, apparent ages and absolute errors. * Trace-element data for P, Ti, V, Y, Zr, Nb, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta Abbreviations include: ppm is parts per million, abs is absolute, Ma is millions of years, abs err is absolute error, error corr is error correction. ## Code/software Uncertainties were calculated using Iolite and IsoplotR (Paton et al., 2010; Vermeesch, 2018) 

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