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Recent findings using U-Pb zircon geochronology combined with paleoenvironmental proxies and sedimentological observations have constrained the timing and extent of glacial advances and retreats across southwestern and south-central Gondwana during the Late Paleozoic Ice Age (LPIA). Sedimentary archives deposited during the transition from the latest Carboniferous to the earliest Permian reveal widespread short-term warming as reflected in the loss of subglacial deposits across Gondwana basins, some of which were fully deglaciated by the end of the Carboniferous at 299 Ma. These observations are accompanied by fluctuations in proxies for pCO2 and δ13C across the Carboniferous-Permian transition. These climate oscillations require large scale, short-lived climate forcing mechanisms. One such potential cause may be greenhouse gas emissions related to widespread magmatism across central and northwestern Europe, which has been linked to a deep-seated mantle plume. The effects of igneous activity may be further amplified by the interaction of magma with widely occurring organic-rich sedimentary rocks across this region. The products of this magmatic province predominantly show silicic compositions, but intermediate and basaltic melts also occur in outcrops and cores from the British Isles and southern Sweden (predominantly in the form of dykes and sills), the Oslo and Skagerrak Graben as well as the Northern and Southern Permian Basin, which extends from the North Sea across northern Germany to Poland (predominantly filled with rhyolitic and rhyodacitic volcanics).Available radioisotopic age constraints for these magmatic products are from different isotopic systems whose comparison requires consideration of systematic errors, and often have large uncertainties (at the several million-year level) rendering them unsuitable for studying potential causal relations to contemporaneous climate fluctuations that operate at much shorter timescales. We review these results and present new high-resolution U-Pb zircon CA-TIMS (Chemical Abrasion-Thermal Ionization Mass Spectrometry) ages from volcanic and intrusive products from the Southern Permian Basin and adjacent areas which show that the duration of magmatic activity was much shorter than previously thought. We suggest that greenhouse gas emissions from the magma, and its interaction with biogenic sedimentary rocks, may have been the cause for the observed short-term climate oscillations across the Carboniferous-Permian transition. These short-term events, and their effects, appear to be superimposed on long-term climate drivers such as continental arc volcanism, tropical silicate weathering, as well as changes in the distribution of continents in high latitudes, the opening and closing of oceanic gateways and tectonically induced variation in the equilibrium-line altitude for ice.The research presented here is supported by NSF grant 1728705

The initiation of pluton formation is rarely preserved as the rock record is typically overprinted by younger intruding pulses. An exception is the 80 km2 Kuna Crest lobe, which marks the initiation of the 95–85 Ma, 1100 km2 Tuolumne Intrusive Complex in the Sierra Nevada, California, USA. We present a detailed map of the lithologies and structure of the Kuna Crest lobe, associated sheeted complex and satellite plutons, and their host rocks, using chemical abrasion–isotope dilution–thermal ionization mass spectrometry and laser ablation–inductively coupled plasma–mass spectrometry U-Pb zircon geochronology, element and isotope geochemistry, and Al-in-hornblende thermobarometry to conclude the following: (a) The 94.91 ± 0.53 Ma to 92.75 ± 0.11 Ma Kuna Crest lobe and its marginal sheeted complex preserved the oldest intrusive pulses and most mantle-like compositions of the entire Tuolumne Intrusive Complex. (b) Emplacement began with magma wedging of low volume magma pulses resulting in a sheeted complex that is compositionally heterogeneous at outcrop scales, but isotopically homogeneous. (c) These early magmas established a pre-heated pathway within just a few hundreds of thousands of years that gave way to the formation of the ~1.5 million-year-long active, compositionally more homogeneous but isotopically more heterogeneous magma mush across the Kuna Crest lobe. The host rocks and previously intruded magma were displaced largely vertically through downward flow. (d) The melt-interconnected mush zone in the lobe allowed for magma mixing and crystal-liquid separation at the emplacement level. We interpret this lobe to represent an upper- to mid-crustal, vertical magma transfer zone that likely fed shallower plutons and potentially volcanic eruptions. We propose a filter pressing mechanism driven by vertical magma transport through the lobe resulting in margin-parallel fabrics, plagioclase-rich crystal cumulates, inward draining and upward loss (of up to 40%) of interstitial melts. Some inward drained melts hybridized with later intruding Half Dome magmas at the transition to the main Tuolumne Intrusive Complex. Some of the lobe magmas, including fractionated melts, drained laterally into the strain shadow of the lobe to form the satellite plutons, further contributing to cumulate formation in the lobe. This study documents that within only a few hundreds of thousands of years, arc magma plumbing systems are capable of establishing a focused magma pathway to build up to increasingly larger magma bodies that are capable of undergoing magma differentiation and feeding shallower plutons and volcanic eruptions.

The Early Permian Tarim large igneous province (LIP) in northwestern China comprises voluminous basaltic lava flows, as well as ultramafic and silicic intrusions. The age and duration of the Tarim LIP remains unclear, and thus the rate of magma production and models of potential environmental effects are uncertain. Here we present high-precision chemical abrasion–isotope dilution–thermal ionization mass spectrometry zircon U-Pb ages for three newly discovered tuff layers interlayered with lava flows in the Kupukuziman and Kaipaizileike formations in the Keping area (Xinjiang, northwest China). The volcanism of the Kupukuziman Formation is constrained to a short duration from 289.77 ± 0.95 to 289.41 ± 0.52 Ma. An age for the overlying Kaipaizileike Formation is 284.27 ± 0.39 Ma, bracketing the duration of the entire eruptive phase of the Tarim flood basalts at ~5.5 m.y. The low eruption rate and relatively long duration of magmatism is consistent with a plume incubation model for the Tarim LIP.

The response of sediment routing to climatic changes across icehouse-to-greenhouse turnovers is not well documented in Earth's pre-Cenozoic sedimentary record. Southwest Gondwana hosts one of the thickest and most laterally extensive records of Earth's penultimate icehouse, the late Paleozoic ice age. We present the first high-resolution U-Pb zircon chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) analysis of late Paleozoic ice age deposits in the Kalahari Basin of southern Africa, which, coupled with existing CA-ID-TIMS zircon records from the Paraná and Karoo Basins, we used to refine the late Paleozoic ice age glacial history of SW Gondwana. Key findings from this work suggest that subglacial evidence in the Kalahari region is restricted to the Carboniferous (older than 300 Ma), with glacially influenced deposits culminating in this region by the earliest Permian (296 Ma). The U-Pb detrital zircon geochronologic records from the Paraná Basin of South America, which was located downstream of the Kalahari Basin in the latest Carboniferous and Permian, indicate that large-scale changes in sediment supplied to the Paraná were contemporaneous with shifts in the SW Gondwana ice record. Gondwanan deglaciation events were associated with the delivery of far-field, African-sourced sediments into the Paraná Basin. In contrast, Gondwanan glacial periods were associated with the restriction of African-sourced sediments into the basin. We interpret the influx of far-field sediments into the Paraná Basin as an expansion of the catchment area for the Paraná Basin during the deglaciation events, which occurred in the latest Carboniferous (300–299 Ma), early Permian (296 Ma), and late early Permian (

The Triassic hothouse world experienced dynamic changes in climate and ecosystems that set the Earth's climate and biotic states for much of the Mesozoic Era. A critical time interval was the Carnian Stage (~237–227 Ma), which not only saw the first appearance of dinosaurs in the fossil record but witnessed a large igneous province eruption (Wrangellia LIP) and a sudden climate shift, the Carnian Pluvial Episode (CPE). Compelling hypotheses suggest that the Wrangellia eruptions caused the CPE, which in turn set the stage for the origin and initial diversification of dinosaurs. Unfortunately, testing these hypotheses is difficult because most relevant Carnian sedimentary archives possess few if any absolute age constraints, and there are little detailed paleoclimatic data for the CPE outside of the Tethys region. There is little existing evidence for the CPE in Gondwana, even though this region contains by far the best fossil record of early dinosaurs and their close relatives. We present new high-resolution geochronologic and paleoclimatic data from Carnian-aged strata in the Ischigualasto-Villa Union Basin of northwest Argentina, which also preserves fossils of the earliest-known dinosaurs. New CA-TIMS U Pb zircon age of 234.47 ± 0.44 Ma from the lower Los Rastros Formation demonstrates that most of the overlying lacustrine strata within this basin was deposited during or after the CPE, including large dinosauriform footprints. Multiproxy paleoenvironmental data (sedimentology, clay mineralogy, C and O stable isotopes, and fossils) from the same strata are the first detailed paleoclimate data for the CPE in Gondwana, and provide evidence that the CPE interval in western Gondwana was indeed warmer and more humid than before or after. These data are consistent with the interpretation of the CPE as a global event, but direct linkages in Gondwana and the Tethys with the origin and initial diversification of dinosaurs are less clear.

Syn-rift deposits often provide the only means to determine the chronology of rift initiation and evolution. However, the earliest syn-rift packages deposited in Jurassic – Cretaceous rift basins that formed during the breakup of SW Gondwana are poorly understood because they are deeply buried beneath overlying passive margin sequences. The exhumed remnants of several such rift basins are exposed in the southern Cape of South Africa and contain the Suurberg and Uitenhage groups, which are predominantly continental, taphrogenic, fossiliferous strata interbedded with volcaniclastics. Here we present the first robust U–Pb chronostratigraphic framework for these groups by dating zircon in nine pyroclastic and five resedimented volcaniclastic deposits using Laser Ablation – Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS). To further improve the precision and accuracy of the results, we utilize Chemical Abrasion – Thermal Ionisation Mass Spectrometry (CA-TIMS) on four selected samples minimizing the effects of Pb-loss and further constraining depositional uncertainties. We thereby show that the Suurberg Group was deposited rapidly during the emplacement of the Karoo Large Igneous Province in the Early Jurassic and likely predates the main phase of rifting, whereas the Uitenhage Group was deposited over a prolonged (>40 Ma) period beginning in the Early Jurassic and continuing into the Early Cretaceous. The Uitenhage Group records two phases of rifting: an initial Jurassic episode that roughly coincides with the separation of East and West Gondwana and is contemporaneous with widespread volcanism in SW Gondwana, and a subsequent period of renewed rifting during the Early Cretaceous opening of the South Atlantic and initiation of the Agulhas Falkland Transform. This framework illustrates the complexity of long-lived rift-basin sedimentation and highlights the importance of high-resolution chronostratigraphy when investigating and integrating the tectonic, palaeogeographic and palaeontological records from the final stages of a unified SW Gondwana.

The Upper Triassic Chinle Formation is a critical non-marine archive of low-paleolatitude biotic and environmental change in southwestern North America. The well-studied and highly fossiliferous Chinle strata at Petrified Forest National Park (PFNP), Arizona, preserve a biotic turnover event recorded by vertebrate and palynomorph fossils, which has been alternatively hypothesized to coincide with tectonically driven climate change or with the Manicouagan impact event at ca. 215.5 Ma. Previous outcrop-based geochronologic age constraints are difficult to put in an accurate stratigraphic framework because lateral facies changes and discontinuous outcrops allow for multiple interpretations. A major goal of the Colorado Plateau Coring Project (CPCP) was to retrieve a continuous record in unambiguous superposition designed to remedy this situation. We sampled the 520-m-long core 1A of the CPCP to develop an accurate age model in unquestionable superposition by combining U-Pb zircon ages and magnetostratigraphy. From 13 horizons of volcanic detritus-rich siltstone and sandstone, we screened up to ∼300 zircon crystals per sample using laser ablation–inductively coupled plasma–mass spectrometry and subsequently analyzed up to 19 crystals of the youngest age population using the chemical abrasion–isotope dilution–thermal ionization mass (CA-ID-TIMS) spectrometry method. These data provide new maximum depositional ages for the top of the Moenkopi Formation (ca. 241 Ma), the lower Blue Mesa Member (ca. 222 Ma), and the lower (ca. 218 to 217 Ma) and upper (ca. 213.5 Ma) Sonsela Member. The maximum depositional ages obtained for the upper Chinle Formation fall well within previously proposed age constraints, whereas the maximum depositional ages for the lower Chinle Formation are relatively younger than previously proposed ages from outcrop; however, core to outcrop stratigraphic correlations remain uncertain. By correlating our new ages with the magnetostratigraphy of the core, two feasible age model solutions can be proposed. Model 1 assumes that the youngest, coherent U-Pb age clusters of each sample are representative of the maximum depositional ages and are close to (227 Ma) in age, and hence the biotic turnover event cannot be correlated to the Carnian–Norian boundary but is rather a mid-Norian event. Our age models demonstrate the powers, but also the challenges, of integrating detrital CA-ID-TIMS ages with magnetostratigraphic data to properly interpret complex sedimentary sequences.

International audience; Icehouse climate systems comprise an abbreviated portion of Earth history, though they have profound impacts on biogeochemical cycling, sea water chemistry, the evolution of continental crust, and biota. On the 105-108-year timescale, increased greenhouse gas concentration, principally pCO2, is well-documented as a driver of icehouse-greenhouse turnovers. In the Phanerozoic, the turnover from icehouse to greenhouse climate systems has been correlated with increased rates of continental arc volcanism and a decrease in silicate weathering across low latitude tropical belts. These processes control atmospheric pCO2 on 107-to-108-year timescales, whereas contributions (and sinks) of pCO2 on shorter timescales (≤106 years) are largely unresolved in deep-time records as a result of limited temporal constraints. To that extent, we present a high-resolution U-Pb zircon CA-TIMS calibrated ice and volcanic record for the late Paleozoic icehouse (LPIA) to greenhouse turnover. U-Pb zircon data from interglacial and postglacial deposits from South America and southern Africa indicate a stepwise deglaciation, occurring first in the mid-latitudes of southern Brazil during the latest Carboniferous, in high-latitude Namibia in the earliest Permian and culminating in the polar regions of Gondwana (South Africa) in latest Early Permian. Our study finds that base-level records across this region are tightly coupled. Highstand sedimentation in all basins is correlative with ice-minima events, whereas lowstand sedimentation is associated with a return of glaciation in the higher-latitude regions. Fundamental shifts in the nature and scale of glaciation are congruent with new high-resolution U-Pb zircon CA-TIMS age control from at least two large volcanic provinces in the latest Carboniferous and early Permian. Our data suggest a causal relationship between the Gondwana deglaciations and large igneous volcanic events. The presented data are the first record of climatic forcing for the LPIA, calibrated at a resolution ≤106-years.

NSF National Science Foundation (NSF) [EAR 0958976, 0958723, 0958915, 0959107, 0958859, EAR-1338583]; Deutsche ForschungsgemeinschaftGerman Research Foundation (DFG); Lamont-Climate Center; Ann and Gordon Getty Foundation

The demise of the Late Paleozoic Ice Age has been hypothesized as diachronous, occurring first in western South America and progressing eastward across Africa and culminating in Australia over an ∼60 m.y. period, suggesting tectonic forcing mechanisms that operate on time scales of 106 yr or longer. We test this diachronous deglaciation hypothesis for southwestern and south-central Gondwana with new single crystal U-Pb zircon chemical abrasion thermal ionizing mass spectrometry (CA-TIMS) ages from volcaniclastic deposits in the Paraná (Brazil) and Karoo (South Africa) Basins that span the terminal deglaciation through the early postglacial period. Intrabasinal stratigraphic correlations permitted by the new high-resolution radioisotope ages indicate that deglaciation across the S to SE Paraná Basin was synchronous, with glaciation constrained to the Carboniferous. Cross-basin correlation reveals two additional glacial-deglacial cycles in the Karoo Basin after the terminal deglaciation in the Paraná Basin. South African glaciations were penecontemporaneous (within U-Pb age uncertainties) with third-order sequence boundaries (i.e., inferred base-level falls) in the Paraná Basin. Synchroneity between early Permian glacial-deglacial events in southwestern to south-central Gondwana and pCO2 fluctuations suggest a primary CO2 control on ice thresholds. The occurrence of renewed glaciation in the Karoo Basin, after terminal deglaciation in the Paraná Basin, reflects the secondary influences of regional paleogeography, topography, and moisture sources.

Carboniferous glaciogenic strata (Itarare Gp.) in the southernmost Parana Basin, Brazil exhibit soft-sediment deformation features previously interpreted as glaciotectonism. These sediments were studied in detail to confirm that they were deformed by ice and to assess the nature of the glaciation, depositional environments, and paleoclimate in this region during the Carboniferous. Five outcrops were described along a railroad transect that contains a conglomerate and diamictite facies with striated and faceted clasts, a medium sandstone facies, a fine grained silt/clay rhythmite and mudstone facies with dropstones and diamictite pellets, a sandy clinoform facies, and a folded sandstone with interbedded mudstone facies. The depositional environment for these sediments is interpreted as an outwash fan and fan delta from an ice-proximal, transitional terrestrial-to-estuarine setting. Rb/K values from the rhythmites reflect a transition from a freshwater to brackish environment and the Chemical Index of Alteration (CIA) of the rhythmites ranges from 65 to 73, reflecting a relative increase in the degree of chemical weathering through time. Deformation features include widespread folding, thrust faults, hydrofractures, decollement surfaces, and piggyback fold-thrust complexes. The deformation is interpreted as evidence of a push-moraine complex formed by at least two complete ice advance/retreat cycles. The occurrence of decollement surfaces, plastically deformed proglacial sediments, and hydrofractures indicate a dynamic, warm-based or polythermal glacier. Abundant outwash sediments, rhythmites with dropstones, and the shift to average CIA values all support a temperate paleoclimate. Deformation structures indicate a NW direction of ice shove that is in agreement with the regional-scale hypothesis that a NNW flowing lobe extended out of Uruguay during the Carboniferous and terminated in the southernmost Parana Basin. This study demonstrates that late Paleozoic glaciation in this region was more dynamic than previously understood, with high frequency fluctuations in ice marginal positions.

Low Nb Ta rhyolites from the Permian Tarim Large Igneous Province (LIP) represent the melting products of continental crust above a mantle plume. Zircon U Pb dating reveals that the Kuchehe and Dawanqi rhyolites were erupted at 292.6 ± 0.5 Ma and 274 ± 2 Ma respectively. Both rhyolite groups are characterized by pronounced negative Nb Ta anomalies, crust-like trace element ratios and negative eNd-Hf(t) values, consistent with a crustal origin. Newly acquired data, combined with available data in the literature, clearly show that there is an increasing trend of zircon saturation temperature (TZr) from 794 to 827 °C to 886–936 °C for the early to middle stage (293–287 Ma), followed by a decreasing TZr of 803–908 °C for the late stage (287–274 Ma) of the Tarim magmatism. The temporal changes in TZr for the Tarim rhyolites are interpreted to reflect the crustal melting behavior above a mantle plume, such that fusible materials in the crust melt at an earlier stage and refractory materials melt at a later stage. The gradual decrease in TZr, towards the final stage of the Tarim rhyolitic magmatism, reflects the gradual ‘decay’ of thermal input from mantle plume to the lithosphere in the region. This thermal model is also applicable to describe the crustal melting behavior in the Chon Aike silicic LIP of Patagonia and the Snake River Plateau-Yellowstone Plateau volcanic provinces.

Plesiadapiform mammals, as stem primates, are key to understanding the evolutionary and ecological origins of Pan-Primates and Euarchonta. The Purgatoriidae, as the geologically oldest and most primitive known plesiadapiforms and one of the oldest known placental groups, are also central to the evolutionary radiation of placentals and the Cretaceous-Palaeogene biotic recovery on land. Here, we report new dental fossils of Purgatorius from early Palaeocene (early Puercan) age deposits in northeastern Montana that represent the earliest dated occurrences of plesiadapiforms. We constrain the age of these earliest purgatoriids to magnetochron C29R and most likely to within 105–139 thousand years post-K/Pg boundary. Given the occurrence of at least two species, Purgatorius janisae and a new species, at the locality, we provide the strongest support to date that purgatoriids and, by extension, Pan-Primates, Euarchonta and Placentalia probably originated by the Late Cretaceous. Within 1 million years of their arrival in northeastern Montana, plesiadapiforms outstripped archaic ungulates in numerical abundance and dominated the arboreal omnivore–frugivore niche in mammalian local faunas.

International audience; The expansion of ice masses across southern Africa during the Late Palaeozoic Ice Age has been known for 150 years, including the distribution of upland areas in controlling the configuration of glaciation. In Accepted Article This article is protected by copyright. All rights reserved Namibia, increasing attention has focussed on long and deep palaeovalley networks in the Kaokoland region in the north, but comparatively little work has been attempted in the topographically subdued plains of the south, in the Aranos and Karasburg basins. The desert terrain of the Aranos area exposes diamictites of the Dwyka Group discontinuously over about 300 km, extending further south to the Karasburg area at the Namibian-South African border along the Orange River. Whilst examined at a stratigraphic level, the nature of the contact between the Dwyka glacial rocks and underlying lithologies has not been systematically investigated. This paper presents the results from fieldwork in austral winter 2019, in which a highly varying basal contact is described that records the processes of growth, flow and expansion of ice masses across this part of Gondwana. At the basin margins, subglacially-produced unconformities exhibit classic glacially striated pavements on indurated bedrock. In comparison, the basal subglacial unconformity in the more basinward regions is characterised by soft-sediment striated surfaces and deformation. In the Aranos Basin, soft-sediment shear zones originated in the subglacial environment. This type of subglacial unconformity developed over well differentiated, unconsolidated, siliciclastic materials. Where ice advanced over more poorly sorted material or cannibalised pre-existing diamictites, "boulder-pavements" recognized as single clast-thick boulder-dominated intervals formed. Importantly, these boulder-pavements are enriched in clasts, which were facetted and striated in-situ by overriding ice. By integrating measurements of striation orientations, fold vergence and palaeocurrent information, former ice flow pathways can potentially be reconstructed over a wide area.

The 238U decay constant (λU-238) is fundamental to radioisotope-based chronometry in the Earth and planetary sciences, yet only a single published λU-238 value (Jaffey et al. in Phys Rev C 4(5):1889–1906, 1971) is widely applied. We have determined λU-238 via the novel approach of measuring of 234Th ingrowth in high-purity 238U solutions, using isotope dilution mass spectrometry (ID-MS). The 234Th decay constant (λTh-234) was measured via decay counting with high-purity Ge (HPGe) γ detectors. Preliminary results for λU-238 agree with the value determined by α-counting [1] within the elevated uncertainty of 0.462% (k = 2). Ongoing efforts to reproduce λU-238 with reduced experimental uncertainties will inform future conclusions.

A precise and accurate timescale is critical for calibrating major events in Earth history and testing their spatiotemporal relationships for putative causes. The Newark-Hartford astrochronostratig...

The Parana Basin, Brazil and the Chaco-Parana Basin, Uruguay both contain sedimentary records that are critical to reconstructing late Paleozoic ice centers in central Gondwana. The orientations of subglacial landforms and glaciotectonic structures suggest that late Paleozoic glacial deposits in the eastern Chaco-Parana Basin and the southernmost Parana Basin are genetically related, as they were likely glaciated by the same ice center. However, the location and extent of the ice center responsible for depositing these sediments are unclear. Furthermore, changes in sediment dispersal patterns between glacial, inter-glacial, and post-glacial intervals are not understood for this region of Gondwana. Therefore, this study utilized U–Pb detrital zircon geochronology to assess the provenance of glacial and post-glacial sediments from the eastern Chaco-Parana Basin (San Gregorio, Cerro Pelado, Tres Islas Formations) and the southernmost Parana Basin (Itarare Group). Results show dominant age peaks at ~520–555 Ma, ~625 Ma, 750–780 Ma, and 900–1000 Ma in all samples from the eastern Chaco-Parana Basin. These zircons are interpreted to have been derived from sources in the Cuchilla Dionisio Terrane and Punta del Este Terrane in southeastern Uruguay, and possibly the Namaqua Belt in southern Namibia. Another important source was likely Devonian sedimentary rocks of the Durazno Group in central/eastern Uruguay. Meanwhile, a sample of the glaciogenic Itarare Group from the southernmost Parana Basin contains a different detrital zircon signature with peaks at ~580 Ma, ~780 Ma, ~2110 Ma, and ~2500 Ma that closely resembles underlying sedimentary and meta-sedimentary rocks of the Precambrian/Cambrian Camaqua Basin. Detrital zircon ages in the glacial and post-glacial sediments indicate that local sources were dominant. In contrast, zircon ages from relatively ice-distal glaciomarine intervals in the Chaco-Parana Basin reflect more distal sources to the east and southeast, which indicates a larger drainage catchment opened when glaciers retreated and/or the zone of maximum subglacial erosion shifted. Although most zircon ages in the Chaco-Parana Basin can be attributed to Uruguayan sources, results support the hypothesis that glaciers emanated from southern Namibia and southeast Uruguay into the Chaco-Parana Basin. From there, ice flowed northwest into the Parana Basin and then receded back towards Africa as the paleoclimate warmed. The detrital zircon inventory in our study region is distinct from the eastern Parana Basin, suggesting at least two unique African source regions for glaciers that deposited sediments in the Parana and Chaco-Parana Basins.

The Tuolumne Intrusive Complex, an upper-crustal (7–11 km emplacement depths), incrementally constructed (95–85 Ma growth history) plutonic complex (~1100 km 2 ), preserves evidence from several data sets indicating the repeated, multiscale, magmatic erosion of older units occurred and that some eroded material was recycled into younger magma batches. These include: (1) map patterns of internal contacts (hundreds of kilometers) that show local hybrid units, truncations, and evidence of removal of older units by younger; (2) the presence of widespread xenolith and cognate inclusions (thousands), including “composite” inclusions; (3) the presence of widespread enclaves (millions), including “composite” enclaves, plus local enclave swarms that include xenoliths and cognate inclusions; (4) the presence of widespread schlieren-bound magmatic structures (>9000) showing evidence of local (meter-scale) truncations and erosion; (5) antecrystic zircons (billions) and other antecrystic minerals from older units now residing in younger units; (6) whole-rock geochemistry including major element, REE, and isotopic data; and (7) single mineral petrographic and geochemical studies indicating mixing of distinct populations of the same mineral. Synthesis of the above suggests that some erosion and mixing occurred at greater crustal depths, but that thousands of “erosion events” at the emplacement site resulted in removal of ~35–55% of the original plutonic material from the presently exposed surface with some (~25%?) being recycled into younger magmas and the remainder was either erupted or displaced downward. The driving mechanisms for mixing/recycling are varied but likely include buoyancy driven intrusion of younger batches into older crystal mushes, collapse and avalanching along growing and over-steepened solidification fronts within active magma chambers (1 to >500 km 2 in size), and local convection in magma chambers driven by internal gradients (e.g., buoyancy, temperature, and composition).

Dinosaurs have been major components of ecosystems for over 200 million years. Although different macroevolutionary scenarios exist to explain the Triassic origin and subsequent rise to dominance of dinosaurs and their closest relatives (dinosauromorphs), all lack critical support from a precise biostratigraphically independent temporal framework. The absence of robust geochronologic age control for comparing alternative scenarios makes it impossible to determine if observed faunal differences vary across time, space, or a combination of both. To better constrain the origin of dinosaurs, we produced radioisotopic ages for the Argentinian Chañares Formation, which preserves a quintessential assemblage of dinosaurian precursors (early dinosauromorphs) just before the first dinosaurs. Our new high-precision chemical abrasion thermal ionization mass spectrometry (CA-TIMS) U-Pb zircon ages reveal that the assemblage is early Carnian (early Late Triassic), 5- to 10-Ma younger than previously thought. Combined with other geochronologic data from the same basin, we constrain the rate of dinosaur origins, demonstrating their relatively rapid origin in a less than 5-Ma interval, thus halving the temporal gap between assemblages containing only dinosaur precursors and those with early dinosaurs. After their origin, dinosaurs only gradually dominated mid- to high-latitude terrestrial ecosystems millions of years later, closer to the Triassic-Jurassic boundary. Fil: Marsicano, Claudia Alicia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; Argentina Fil: Irmis, Randall. University of Utah; Estados Unidos Fil: Mancuso, Adriana Cecilia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; Argentina Fil: Mundil, Roland. Berkeley Geochronology Center; Estados Unidos Fil: Chemale, Farid. Universidade do Brasília; Brasil

The upper Stormberg Group (Elliot and Clarens formations) of the main Karoo Basin is well-known for its fossil vertebrate fauna, comprising early branching members of lineages including mammals, dinosaurs, and turtles. Despite 150 years of scientific study, the upper Stormberg Group lacks radioisotopic age constraints and remains coarsely dated via imprecise faunal correlations. Here we synthesise previous litho- and magnetostratigraphic studies, and present a comprehensive biostratigraphic review of the upper Stormberg fauna. We also present the results of the first geochronological assessment of the unit across the basin, using U-Pb dates derived from detrital zircons obtained from tuffaceous sandstones and siltstones, the youngest of which are considered maximum depositional ages. Our results confirm that the Elliot Formation contains the Triassic–Jurassic boundary, making it one of the few fossiliferous continental units that records the effects of the end-Triassic Mass Extinction event. Our work suggests a mid-Norian–Rhaetian age for the lower Elliot Formation and a Hettangian–Sinemurian age for the upper Elliot Formation, although the precise stratigraphic position of the Triassic/Jurassic (Rhaetian/Hettangian) boundary remains somewhat uncertain. A mainly Pliensbachian age is obtained for the Clarens Formation. The new dates allow direct comparison with better-calibrated Triassic-Jurassic faunas of the Western Hemisphere (e.g., Chinle and Los Colorados formations). We show that sauropodomorph, but not ornithischian or theropod, dinosaurs were well-established in the main Karoo Basin ~220 million years ago, and that typical Norian faunas (e.g., aetosaurs, phytosaurs) are either rare or absent in the lower Elliot Formation, which is paucispecific compared to the upper Elliot. While this is unlikely the result of geographic sampling biases, it could be due to historical sampling intensity differences.

Dating zircon by U–Pb in the Pleistocene Bishop Tuff is challenging because the concentration of radiogenic Pb is low and the correction required for disequilibrium in the intermediate daughter products is large. The dates can be difficult to interpret, because their absolute precision is similar in magnitude to purported timescales of crystallization. Previous estimates of the duration of zircon crystallization span two orders of magnitude and appear to depend on the analytical approach. To reconcile the differing interpretations, we present new SIMS and ID-TIMS zircon Pb/U dates, including some crystals that are dated by both techniques. Both the SIMS and ID-TIMS dates have similar distributions as previous results. Normalized to a Th/Umelt = 2.81, SIMS spot dates from interior regions of sectioned grains range from 769 ± 31 ka to 845 ± 28 ka (2σ) and ID-TIMS dates have a range from 760 ± 7 ka to 793 ± 6 ka. Individual crystals dated by both methods have an average difference of 31 ka, with the SIMS dates being consistently older than the bulk crystal dates. This level of variability precludes the assignment of a geological significance to a mean (or weighted-mean) zircon date. A previous ID-TIMS investigation of single BT zircon that showed apparent statistically significant clustering of zircon ages was compromised by an incorrect treatment of the covariance structure of 230Th-corrected Pb/U data. We show approximate and exact methods for the correct treatment and demonstrate substantially more scatter in that data than previously recognized. Close scrutiny of available partition coefficient data show that they are not adequately precise for

In this study, structures in plutons and host rocks are coupled with geochronology to track paleodeformation fields from the late Paleozoic to Late Cretaceous in the central Sierra Nevada. Regional NW-striking host-rock foliation, NE- or SW-vergent thrust faults, and associated folds developed from the early Mesozoic to Early Cretaceous. Dextral transpressional shear zones developed in the Late Cretaceous. Strikes of steep-dipping magmatic foliations in Mesozoic plutons temporally vary from approximately NW (Triassic–Jurassic) to WNW (Late Cretaceous), displaying a progressive counterclockwise rotation. Joint interpretation based on combining host-rock and magmatic structures suggests that intra-arc paleodeformation fields were dominated by coaxial and arc-perpendicular contraction from the early Mesozoic to Early Cretaceous, becoming increasingly dextral transpressive in the Late Cretaceous. The switch from contraction to transpression was likely caused by oblique convergence between the Farallon and North American plates. Based on observations in the study area and other host-rock pendants in the central Sierra Nevada, we propose that the intensity of intra-arc deformation is cyclic. To some extent, it mimics the episodic pattern of arc magmatism: Stronger deformation coincides with magmatic flare-ups. Magmatism promotes intra-arc deformation, which in turn causes crustal thickening during transfer of materials downward to the magma source regions, potentially fertilizing source regions with supracrustal materials and resulting in increased magma generation. Thus, models addressing continental arc tempos should include intra-arc processes. Evolution of continental arcs may be influenced by linked cyclic processes within the arcs accompanied by noncyclic processes driven by events external to the arcs.

The Early Permian Tarim Large Igneous Province (LIP) in northwestern China contains a large area of silicic volcanics (~ 48,000 km 2 ) which are spatially and temporally associated with mafic–ultramafic rocks. In order to understand the behavior of crust above a mantle plume, selected rhyolitic samples are investigated in terms of U–Pb zircon dating, geochemical and isotopic analyses. The Tarim rhyolites have high A/CNK ratios (= molar Al 2 O 3 /CaO + Na 2 O + K 2 O), Fe # , Ga/Al ratios, concentrations of high field strength elements (HFSEs) such as Zr and Nb, and rare earth elements (REEs), along with high zircon saturation temperatures (872–940 °C), typical of aluminous A-type granitoids. Two contrasting rock types have been recognized. The low Nb–Ta type rhyolites are mainly associated with the first phase of the Tarim flood basalt magmatism at ~ 290 Ma. They are characterized by negative Nb–Ta anomalies, low e Nd (t) and e Hf (t) values, and high 87 Sr/ 86 Sr(t) and δ 18 O zircon values, consistent with a derivation from continental crustal source. The high Nb–Ta type rhyolites and their plutonic equivalents are associated with the second episode of Tarim magmatism (283–272 Ma). They are characterized by small negative to positive Nb–Ta anomalies, oceanic island basalt (OIB)-like trace element ratios, low 87 Sr/ 86 Sr(t) and high e Nd (t) and e Hf (t) values. These high Nb–Ta rhyolites are best interpreted as hybrid products of crystal fractionation of mafic magmas, coupled with crustal assimilation. The temporal and compositional evolution of the Tarim rhyolites reflects various extents of thermal and mass exchange between mantle-derived basaltic magma and crustal material above a mantle plume. When the plume head rises to the base of the Tarim craton, it first melts enriched components in the lithospheric mantle (~ 290 Ma), part of which may have ponded near the crust–mantle boundary and induced crustal anatexis leading to the formation of the low Nb–Ta type rhyolites. At ~ 280 Ma, large magma chambers and plumbing systems were formed due to increasing magma supply rate during decompression melting of the mantle plume. This led to the formation of a mafic–ultramafic and felsic association of which the high Nb–Ta type rhyolites are a part.

The age of the Emeishan lavas in SW China remains poorly constrained because the extrusive rocks are (1) thermally overprinted and so represent an open system unsuitable for 40 Ar/ 39 Ar geochronology and (2) in most cases devoid of zircon so that it is impossible for the application of U–Pb geochronology. Existing radiometric age constraints of Emeishan large igneous province are mainly from the application of SIMS and LA-ICP-MS U–Pb techniques to zircons from mafic and felsic intrusions, which represent indirect constraints for the lavas. In an attempt to directly determine the age of the Emeishan lava succession, high-resolution chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) zircon U–Pb techniques have been used on the felsic ignimbrite at the uppermost part of the Emeishan lava succession. These techniques have yielded a weighted mean 206 Pb/ 238 U age of 259.1 ± 0.5 Ma (n = 6; MSWD = 0.7). We interpret this age as the termination age of the Emeishan flood basalts. The age of the Guadalupian–Lopingian boundary is still unconstrained by high-resolution geochronology but is likely to be close to our new age for this felsic ignimbrite.

A negative δ C 13 excursion in carbonate sediments near the Guadalupian/Lopingian (Middle/Late Permian) boundary has been interpreted to have resulted from a large carbon cycle disturbance during the end-Guadalupian extinction event (ca. 260 Ma). However, the carbon isotope data alone are insufficient to uniquely determine the type and magnitude of perturbation to the global carbon cycle. Calcium isotopes can be used to further constrain the cause of a carbon isotope excursion because the carbon and calcium cycles are coupled via CaCO3 burial. In this study, we present coupled carbon and calcium isotope records from three Guadalupian–Lopingian (G/L) sections in China (Penglaitan and Chaotian) and Turkey (Koserelik Tepe). The δ C 13 and δ Ca 44 / 40 records differ among our studied sections and do not co-vary in the same manner. No section shows δ C 13 and δ Ca 44 / 40 changes consistent with massive, rapid volcanic CO2 emissions or methane clathrate destabilization. Additionally, many sections with large ( > 3 ‰ ) changes in δ C 13 exhibit δ O 18 evidence for diagenetic alteration. Only one section exhibits a large excursion in the δ Ca 44 / 40 of limestone but the absence of a similar excursion in the δ Ca 44 / 40 of conodont apatite suggests that the limestone excursion reflects a mineralogical control rather than a perturbation to the global calcium cycle. Hence, we interpret the large isotopic changes observed in some sections to have resulted from local burial conditions or diagenetic effects, rather than from a large carbon and calcium cycle disturbance. Perturbations to the global carbon and calcium cycles across the G/L transition were much less intense than the disturbances that occurred across the subsequent Permian–Triassic boundary. This finding is consistent with the much smaller magnitude of the end-Guadalupian extinction relative to the end-Permian.

The timing and geographic distribution of glaciers in high-latitude southern Gondwana during the Late Paleozoic Ice Age remain poorly constrained, ultimately precluding our ability to estimate ice volume and associated climate teleconnections and feedbacks during Earth's penultimate icehouse. Current aerial extents of glaciers, constrained by sedimentary flow directions, near exclusively infer paleo-glaciation to be highland-driven and may underestimate potential ice sources in continental regions from which ice sheets may have emanated. Here, we report new U-Pb ages and Hf isotope compositions of detrital zircons recovered from diamictites in two key mid- to high-latitude Gondwanan basins (Parana, Brazil and Tepuel, Argentine Patagonia). The results indicate regional sediment sources for both basins during the early period of late Paleozoic glaciation evolving into more distal sources during the final deglaciation along southern and western Gondwana. Similar age sediment sourced from diamictites in the Congo Basin, that require an ice center in eastern Africa suggest the possibility of a large ice sheet in this area of Africa proximal to the Carboniferous-Permian boundary, which may have sourced sediments to the Parana Basin. An inferred distal southern source of glacial sediment for the Tepuel Basin argues for the presence of an ice sheet(s) in the Ellsworth Block of Antarctica towards the end of the glaciation history in Patagonia. These findings indicate an evolution during the Late Paleozoic Ice Age from proximally to extrabasinally sourced sediment reflecting continental-scale glaciation and subsequent drainage from the Windhoek Highlands, Ellsworth Block and an east African source in west-central Gondwana. Coincidence with a long-term fall in atmospheric pCO2 during the Pennsylvanian to a minimum across the Carboniferous-Permian boundary and a subsequent rise in the early Permian suggests a primary CO2-driver for deglaciation in the Parana Basin. Additional boundary conditions including availability of moisture and paleogeography likely further contributed to the timing of nucleation, growth and demise of these Gondwanan glaciers.

The location, longevity, and geographic extent of late Paleozoic ice centers in west-central Gondwana remain ambiguous. Paleovalleys on the Rio Grande do Sul Shield of southernmost Brazil have previously been interpreted as fjords carved by outlet glaciers that originated in Africa and emptied into the Parana Basin (Brazil). In this study, the sedimentology, stratigraphy, and provenance of sediments infilling two such paleovalleys (the Mariana Pimentel and Leao paleovalleys) were examined in order to test the hypothesis that an ice center over present day Namibia drained across southernmost Brazil during the Carboniferous and Permian. Contrary to previous findings, the facies assemblage from within the paleovalleys is inconsistent with a fjord setting and no clear evidence for glaciation was observed. The facies show a transition from a non-glacial lacustrine/estuarine environment, to a fluvial-dominated setting, and finally to a restricted marine/estuarine environment. Detrital zircon results present a single population of Neoproterozoic ages (c. 800–550 Ma) from the paleovalley fill that matches the ages of underlying igneous and metamorphic basement (Dom Feliciano Belt) and is incongruent with African sources that contain abundant older (Mesoproterozoic, Paleoproterozoic, and Archean) zircons. Furthermore, results suggest that the formation of the paleovalleys and the deposition of their fill were controlled by the reactivation of Neoproterozoic basement structures during the Carboniferous and Permian. The lack of evidence for glaciation in these paleovalleys highlights the need for detailed studies of supposed late Paleozoic glacial deposits. These results are supportive of the hypothesis that well-established glacial sediments on the Rio Grande do Sul Shield (southern margin of the Parana Basin) may be the product of a separate lobe extending north across Uruguay, rather than a single, massive ice sheet draining west from Africa.

Impact Dating The large mass extinction of terrestrial and marine life—most notably, non-avian dinosaurs—occurred around 66 million years ago, at the boundary between the Cretaceous and Paleogene periods. But attributing the cause to a large asteroid impact depends on precisely dating material from the impact with indicators of ecological stress and environmental change in the rock record. Renne et al. (p. 684 ; see the Perspective by Pälike ) acquired high-precision radiometric dates of stratigraphic layers surrounding the boundary, demonstrating that the impact occurred within 33,000 years of the mass extinction. The data also constrain the length of time in which the atmospheric carbon cycle was severely disrupted to less than 5000 years. Because the climate in the late Cretaceous was becoming unstable, the large-impact event appears to have triggered a state-shift in an already stressed global ecosystem.