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The [sup.176]Lu-[sup.176]Hf and [sup.147]Sm-[sup.143]Nd decay systems are routinely used to determine garnet (Grt)-wholerock (WR) ages; however, the [sup.176]Lu-[sup.176]Hf age of garnet is typically older than the [sup.147]Sm-[sup.143]Nd age determined from the same aliquots. Here we present experimental data for [Lu.sup.3+] and [Hf.sup.4+] diffusion in garnet as functions of temperature, pressure and oxygen fugacity and show that the diffusivity of [Hf.sup.4+] in almandine/spessartine garnet is significantly slower than that of [Lu.sup.3+]. The diffusive closure temperature ([T.sub.C]) of [Hf.sup.4+] is significantly higher than that of [Nd.sup.3+], and although this property is partly responsible for the observed [sup.176]Lu-[sup.176]Hf and [sup.147]Sm-[sup.143]Nd Grt-WR age discrepancies, the difference between the [T.sub.C]-s of [Lu.sup.3+] and [Hf.sup.4+] could lead to apparent Grt-WR [sup.176]Lu-[sup.176]Hf ages that are skewed from the age of [Hf.sup.4+] closure in garnet. In addition, the slow diffusivity of [Hf.sup.4+] indicates that the bulk of metamorphic garnets retain a substantial fraction of prograde radiogenic [sup.176]Hf throughout peak metamorphic conditions, a phenomenon that further complicates the interpretation of [sup.176]Lu-[sup.176]Hf garnet ages and invalidates the use of analytical [T.sub.C] expressions. We argue that the diffusion of trivalent rare earth elements in garnet becomes much faster when their concentration level falls below a few hundred ppm, as in the experiments of Tirone et al. (Geochim Cosmochim Acta 69: 2385-2398, 2005), and further argue that this low-concentration mechanism is appropriate for modeling the susceptibility of [sup.147]Sm-[sup.143]Nd garnet ages to diffusive resetting.

Restudy of the Cambrian-Ordovician boundary beds, traditionally assigned to the Mila Formation Member 5 in MilaKuh, northern Iran, for the first time provides convincing evidence of the Early Ordovician (Tremadocian) age of the uppermost part of the Mila Formation. Two succeeding trilobite assemblages typifying the Asaphellus inflatus-Dactylocephalus and Psilocephalina lubrica associations have been recognized in the uppermost part of the unit. The Tremadocian trilobite fauna of Mila-Kuh shows close similarity to contemporaneous trilobite faunas of South China down to the species level, while affinity to the Tremadocian fauna of Central Iran is low. The trilobite species Dactylocephalus levificatus and brachiopod species Tritoechia tenuis from the Tremadocian of Mila-Kuh are new to science.

The Seve Nappe Complex (SNC) comprises continental rocks of Baltica that were subducted and exhumed during the Caledonian orogeny prior to collision with Laurentia. The tectonic history of the central SNC is investigated by applying in-situ zircon and monazite (Th-)U–Pb geochronology and trace element analysis to (ultra-)high pressure (UHP) paragneisses in the Avardo and Marsfjället gneisses. Zircons in the Avardo Gneiss exposed at Sippmikk creek exhibit xenocrystic cores with metamorphic rims. Cores show typical igneous REE profiles and were affected by partial Pb-loss. The rims have flat HREE profiles and are interpreted to have crystallized at 482.5 ± 3.7 Ma during biotite-dehydration melting and peritectic garnet growth. Monazites in the paragneiss are chemically homogeneous and record metamorphism at 420.6 ± 2.0 Ma. In the Marsfjället Gneiss exposed near Kittelfjäll, monazites exhibit complex zoning with cores enveloped by mantles and rims. The cores are interpreted to have crystallized at 481.6 ± 2.1 Ma, possibly during garnet resorption. The mantles and rims provide a dispersion of dates and are interpreted to have formed by melt-driven dissolution-reprecipitation of pre-existing monazites until 463.1 ± 1.8 Ma. Depletion of Y, HREE, and U in the mantles and rims compared to the cores record peritectic garnet and zircon growth. Altogether, the Avardo and Marsfjället gneisses show evidence of late Cambrian/early Ordovician partial melting (possibly in (U)HP conditions), Middle Ordovician (U)HP metamorphism, and late Silurian tectonism. These results indicate that the SNC underwent south-to-north oblique subduction in late Cambrian time, followed by progressive north-to-south exhumation to crustal levels prior to late Silurian continental collision.

This study investigates the potential of laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) U‐Pb dating for carbonate nodules in the Late Triassic Ischigualasto Formation of northwestern Argentina. We establish a fully characterized paragenetic sequence to guide the analysis of three pedogenic carbonates and compare the U‐Pb ages with published geochronology from volcanic ashes within the sedimentary succession. Our findings demonstrate the importance of interpreting U‐Pb data within a well‐defined paragenetic framework for accurate age interpretation of pedogenic carbonates. We observe variations in U‐Pb isotopic signatures across different generations of carbonate precipitates and identify syn‐pedogenic and early burial calcite cements as most suitable for precise dating. Respectively, these two calcite cements are interpreted as microcodium and crack‐lining calcite cements formed early in the paragenetic sequence during pedogenesis to early burial of the paleosols as they transitioned from the unsaturated vadose to saturated phreatic zone below the water table. The U‐Pb ages obtained from the carbonate nodules agree with the radioisotopic ages of volcanic ashes, supporting the validity of our dating strategy. These results contribute to advancing U‐Pb carbonate geochronology and highlight its increased potential for dating sedimentary records in the terrestrial realm. Future research should focus on replicating similar work on different carbonate nodules within the Ischigualasto Fm and expanding the application of LA‐ICP‐MS U‐Pb dating to other carbonate‐bearing formations, especially in successions with limited absolute ages or where volcanic ashes are sparse or absent. Plain Language Summary Carbonate minerals that formed in fossil soils can provide valuable insights into past physical, biological, and chemical processes on Earth's surface. Despite their significance in reconstructing ancient climates and environments, determining the age of these fossil soils via uranium‐to‐lead dating has proven challenging. This difficulty arises from factors associated with soil carbonate minerals, including low uranium content, high lead content, complex formation chemistry, multiple formation episodes (generations), and potential for post‐formation chemical alteration. To address these issues, we first identified the order in which carbonate minerals formed within three samples from the Late Triassic Ischigualasto Formation of Argentina. Then, we dated each generation within each sample and compared the results to identify and understand the most optimal sample locations for dating. Analysis of our data shows that carbonate minerals formed due to biological processes near the surface as well as during the burial and submersion of the fossil soils below the water table are the most suitable for dating and approximate the timing of soil formation. Importantly, the ages from these soil carbonate samples align with ages from volcanic ashes found within the Ischigualasto Formation, thus validating our results and the potential to apply our strategy in other locations. Key Points A well‐defined paragenetic framework is necessary to interpret laser ablation inductively coupled plasma mass spectrometry U‐Pb data for accurate age interpretation of pedogenic carbonates U‐Pb isotopic signatures vary across generations of carbonate precipitates, with syn‐pedogenic and early burial cements being the most suitable for precise dating Syn‐pedogenic and early burial cements approximate the timing of pedogenesis and transition from the unsaturated vadose to the saturated phreatic zone, respectively

Zircon U–Pb geochronology is a vital tool for dating geological events, but interpreting discordant zircon ages—often the result of Pb loss due to fluid‐rock interactions—remains challenging. Although frequently disregarded, discordant data can yield valuable geological insights. Here, we introduce a Python application that models the timing of Pb loss in zircon, enabling efficient analysis of discordant U–Pb data. Our application uses the Concordant–Discordant Comparison test in the Tera‐Wasserburg concordia space to classify zircon analyses based on a customizable discordance threshold. Featuring a user‐friendly interface, the application models possible Pb loss events and calculates optimal Pb loss ages by minimizing the dissimilarity between concordant and reconstructed age distributions. When applied to the Mount Isa Inlier in northeastern Australia, the tool identifies a major Pb loss event around 481 Ma, corresponding to Palaeozoic tectonic activity along Gondwana's active margin, including the Delamerian Orogeny. Older Pb loss ages cluster along major NE–SW fault systems, suggesting that long‐lived, reactivated structures facilitated fluid migration and subsequent Pb mobilization in zircon. Sensitivity analyses confirm the consistency of Pb loss age estimates across various model parameters. This study provides new insights into fluid‐rock interactions in the Mount Isa Inlier and offers a methodological framework for exploring Pb loss in zircon U–Pb data sets. The application is broadly applicable for future research in regions with complex tectonic histories. Plain Language Summary Zircon crystals are commonly used to date primary magmatic crystallization or metamorphism but may also retain information on alteration events. However, when zircons lose lead—often due to interactions with fluids—their isotopic systems become disturbed, creating “discordant” data. Such data are usually set aside as unreliable, even though they hold clues about past geological processes related to fluid rock interaction. Our study introduces a Python‐based tool that estimates when lead loss likely occurred, revealing valuable insights into processes that would otherwise go undated. The tool can analyze large data sets quickly, allowing a better understanding of fluid‐related processes affecting zircons over time. We applied the tool to the Mount Isa region in northeastern Australia and found evidence of a major lead loss event around 481 million years ago, likely related to tectonic activity along Gondwana's margin. We also observed that older lead loss events follow fault lines, suggesting that these structures repeatedly allow fluids to move through the rocks, altering associated zircon crystals. Key Points Python tool estimates Pb loss timing in zircon, turning discordant data into insights on fluid‐rock interaction Monte Carlo simulations estimate uncertainties in Pb loss timing, enhancing confidence in geological interpretations Applied to the Mount Isa Inlier, the tool reveals a Pb loss event at c. 481 Ma, coinciding with tectonic activity on Gondwana's margin

The Southern Copper Belt, Carajás Province, Brazil, hosts several iron oxide–copper–gold (IOCG) deposits, including Sossego, Cristalino, Alvo 118, Bacuri, Bacaba, Castanha, and Visconde. Mapping and U–Pb sensitive high-resolution ion microprobe (SHRIMP) IIe zircon geochronology allowed the characterization of the host rocks, situated within regional WNW–ESE shear zones. They encompass Mesoarchean (3.08–2.85 Ga) TTG orthogneiss, granites, and remains of greenstone belts, Neoarchean (ca. 2.74 Ga) granite, shallow-emplaced porphyries, and granophyric granite coeval with gabbro, and Paleoproterozoic (1.88 Ga) porphyry dykes. Extensive hydrothermal zones include albite–scapolite, biotite–scapolite–tourmaline–magnetite alteration, and proximal potassium feldspar, chlorite–epidote and chalcopyrite formation. U–Pb laser ablation multicollector inductively coupled mass spectrometry (LA-MC-ICP-MS) analysis of ore-related monazite and Re–Os NTIMS analysis of molybdenite suggest multiple Neoarchean (2.76 and 2.72–2.68 Ga) and Paleoproterozoic (2.06 Ga) hydrothermal events at the Bacaba and Bacuri deposits. These results, combined with available geochronological data from the literature, indicate recurrence of hydrothermal systems in the Southern Copper Belt, including 1.90–1.88-Ga ore formation in the Sossego–Curral ore bodies and the Alvo 118 deposit. Although early hydrothermal evolution at 2.76 Ga points to fluid migration coeval with the Carajás Basin formation, the main episode of IOCG genesis (2.72–2.68 Ga) is related to basin inversion coupled with Neoarchean (ca. 2.7 Ga) felsic magmatism. The data suggest that the IOCG deposits in the Southern Copper Belt and those in the Northern Copper Belt (2.57-Ga Salobo and Igarapé Bahia–Alemão deposits) do not share a common metallogenic evolution. Therefore, the association of all IOCG deposits of the Carajás Province with a single extensive hydrothermal system is precluded.

Zircon U‐Pb geochronological data on over 900 zircon grains for Cambrian to Silurian sandstone samples from the South China Block constrain the pre‐Devonian tectonic setting of, and the interrelationships between, the constituent Cathaysia and Yangtze blocks. Zircons range in age from 3335 to 465 Ma. Analyses from the Cathaysia sandstone samples yield major age clusters at ∼2560, ∼1850, ∼1000, and 890–760 Ma. Zircons from the eastern and central Yangtze sandstone samples show a similar age distribution with clusters at ∼2550, ∼1860, ∼1100, and ∼860–780 Ma. A minor peak at around 1450 Ma is also observed in the Cathaysia and central Yangtze age spectra, and a peak at ∼490 Ma represents magmatic zircons from Middle Ordovician sandstone in the eastern Yangtze and Cathaysia blocks. The Cambrian and Ordovician strata show a transition from a carbonate‐dominated succession in the central Yangtze Block, to an interstratified carbonate‐siliciclastic succession in the eastern Yangtze Block, to a neritic siliciclastic succession in the Cathaysia Block. Paleocurrent data across this succession consistently indicate directions toward the W‐NNW, from the Cathaysia Block to the Yangtze Block. Our data, together with other geological constraints, suggest that the Cathaysia Block constitutes a fragment on the northern margin of east Gondwana and both Cathaysia and east Gondwana constituted the source for the analyzed early Paleozoic samples. The similar age spectra for the Cambrian to Silurian sandstone samples from the Yangtze and Cathaysia blocks argue against the independent development and spatial separation of these blocks in the early Paleozoic but rather suggest that the sandstone units accumulated in an intracontinental basin that spanned both blocks. Subsequent basin inversion and Kwangsian orogenesis possibly at 400–430 Ma also occurred in an intracontinental setting probably in response to the interaction of the South China Block with the Australian‐Indian margin of east Gondwana.

Supracrustal greenstone belts of the Rae craton of north Baffin Island have been historically attributed to the Mary River Group (MRG), a key tectonostratigraphic unit of economic significance. Best‐preserved exposures of supracrustal rocks occur within the Eqe Bay Greenstone Belt. New mapping and U‐Pb zircon dating of samples from Eqe Bay, combined with previous geochronological work, reveal that greenstone belts hitherto known as MRG consist of two temporally and spatially distinct episodes of volcanism: a ca. 2863–2830 Ma Mesoarchean pulse, to which we assign the name “Tuktuliarvik Group”, and a ca. 2759–2718 Ma Neoarchean pulse, which retains the designation “MRG.” High‐purity Fe‐ores of the world‐class Mary River deposit occur exclusively within the Neoarchean‐age MRG. Detrital zircon U‐Pb dates from quartzite and psammite (e.g., Pond Inlet) constrain a transition within the MRG, from magmatism to clastic sedimentation, at ca. 2720 Ma. An extensive tract of turbiditic siliciclastic rocks of the Eqe Bay Greenstone Belt, originally thought to represent the upper succession of the MRG, yields a Paleoproterozoic maximum depositional age. This suggests that turbiditic rocks instead form part of the younger, ca. <2.19–2.16 to >1.88 Ga Piling Group, and that the MRG lacks a turbiditic sequence typical of the upper stratigraphy of many greenstone belts. An updated chronostratigraphy of the greenstone belts of north Baffin Island has implications for stratigraphic links with Greenland and mainland Canada, Archean geodynamics within the northeastern Rae craton and the footprint of Precambrian basins during the amalgamation of the Nuna supercontinent.