Radiometric Ages Research Articles

Three-phased Middle Permian–Mesozoic magmatism in the Great Xing'an Range: implications for episodic southern subduction of the Mongol–Okhotsk Ocean

The Erguna Massif is located in the southwestern portion of the Great Xing'an Range and is adjacent to the Mongol–Okhotsk suture zone. It has not been determined whether the tectonic evolutionary processes of the Erguna Massif belonged to the Mongol–Okhotsk tectonic regime during the Middle Permian–Mesozoic. In this study, a suite of rocks comprising Mesozoic S-type monzogranite (U–Pb zircon age of 248 ± 1.2 Ma), highly fractionated I-type rhyolite (204 ± 1.1 Ma), gabbro (196 ± 1.9 Ma), A2-type volcanic rocks (190 ± 0.9 Ma), A1-type trachydacite (167 ± 0.8 Ma) and Early Cretaceous A1-type alkaline rhyolite are newly identified and geochemically studied. The rhyolite, gabbro, trachydacite and alkaline rhyolite whole-rock Sr–Nd isotope analyses obtained initial values 87 Sr/ 86 Sr ratios ranging from 0.7044 to 0.7058 and ε Nd ( t ) values of −0.68 to +2.73. These samples show ε Hf ( t ) values ranging from +5.3 to +11.2 and T DM2 ranging from 0.48 to 0.90 Ga. The 248 Ma monzogranites were produced by the partial melting of greywackes. The 204 Ma rhyolites were derived from the partial melting of lower mafic crust. The 196 Ma gabbros originated from the partial melting of an enriched mantle metasomatized by subduction-slab-released fluids. The 190 Ma volcanic rocks, 167 Ma trachydacite and Early Cretaceous alkaline rhyolite were mainly formed by the partial melting of the basaltic rocks. They all show enrichment in the large-ion lithophile elements (e.g. Rb, Ba, and K) and depletion in the high-field-strength elements (e.g. Nb, Ta, and Ti), suggesting they formed in an active continental margin setting. The features of these igneous rocks indicate the southward subduction of the Mongol–Okhotsk oceanic plate. Based on compiled age data, three phases of middle Mesozoic magmatism were identified in the Erguna Massif at c. 275–225, 215–165, and 150–110 Ma. In addition, three similar magmatic phases were found in the Xing'an Massif. However, a hysteresis of c. 15–20 Ma exists between the two massifs. These magmatic rocks may record the three stages of the southward subduction of the Mongol–Okhotsk oceanic plate, and two periods of slab rollback occurred during the Middle Permian to Early Cretaceous. Supplementary material: Analytical details, radiometric ages and Tables S1–S5 are available at: https://doi.org/10.6084/m9.figshare.c.5459285

A proposed chronostratigraphic framework for the late Quaternary of southern Africa

AbstractThe principles of chronostratigraphy can inform the process of correlation between different palaeoclimate records, enabling the coherence of spatial and temporal patterns of past climates and environments to be identified based on the physical, chemical, biological and isotopic properties of individual depositional units. This study presents a chronostratigraphic framework for the late Quaternary of southern Africa, based on the integration of palaeoclimatic and palaeoenvironmental proxy data from key records across the country from the start of Marine Isotope Stage 6 (~191 ka BP) to present. The methodology adopted in this study involves, first, wiggle-matching between sufficiently long and continuous records from different regions across southern Africa, informed by radiometric age controls from individual records. Based on interpretive limitations of these records, we then integrate different geomorphic and archaeological data types in proposing successive chronostratigraphic time periods that collectively extend through the late Quaternary of southern Africa. These time periods correspond to phases in which, within them, a certain set of (relative) stable climates or environments existed in different regions of southern Africa, as recorded in different ways in different proxy records. The boundaries between successive time periods are identified where there is evidence for a significant change in the workings of the climate or environmental system as reflected in the preserved proxy record found in a certain locality. These chronostratigraphic units are interpreted as reflecting the impacts of external forcing that is of regional extent, synchronous, and are not merely an outcome of local environmental variability. These chronostratigraphic phases identified for the late Quaternary period also correspond to distinctive technological and cultural phases in the southern African archaeological record, demonstrating links between coeval climate and environmental change and phases of human evolutionary development. This chronostratigraphic approach provides both a correlative framework for understanding the varied late Quaternary records of southern Africa, and a testable hypothesis for considering the synchroneity or otherwise of different records and thus their associated forcing factors.

Geochemistry and paleogeographic implications of Permo-Triassic metasedimentary cover from the Tauern Window (Eastern Alps)

Abstract. The chemical composition of metasediments is a valuable source of paleogeographic information about the protolith's sedimentary environment. Here, we compile major- and trace-element whole-rock data, including B contents, and 10/11B-isotope ratios from the Permo-Triassic metasedimentary cover of the Pfitsch–Mörchner basin, overlying the Variscan basement in the western Tauern Window, Eastern Alps (Austria and Italy). The basement consists of orthogneiss (“Zentralgneis”, metamorphosed Variscan granitoids with intrusion ages between 305 and 280 Ma), and the roof pendant consists of granites (amphibolites, paragneiss, and minor serpentinites). The Zentralgneis is partly hydrothermally altered into pyrite quartzite with high Al–S contents, low Na–Sr–Ca–Mg contents, and very strong depletion of the light rare earth elements. Comparison with published detailed mapping of this and other time-equivalent basins in the western Tauern Window, with radiometric age data in the literature, and with unmetamorphosed basins in the South Alpine realm yields a late Permian to Early Triassic age of sedimentation. Although during Alpine metamorphism all rocks were strongly deformed, the whole-rock chemical compositions of the metasediments were not pervasively changed during deformation. We show that the sediments were deposited in a small, probably lacustrine–fluviatile, intramontane basin, under arid to semi-arid climatic conditions. The sequence starts with metaconglomerates, which can be interpreted as a mixture of the different basement rocks, based on a combination of major-element ratios Na2O / (Na2O + K2O) and MgO / (MgO + Fe2O3) with concentrations of trace elements Cr, V, and Ni. The sequence is overlain by a fining-upwards sequence of clastic sediments, in which the behavior of K, Rb, and Sr allows the reconstruction of intense diagenetic K–B metasomatism, which raised the K2O contents up to ∼ 10 wt %. The average B content of 218 µg g−1 is well above the B content of common sediments, and the B-isotope composition reaches extremely low values of down to −33 ‰ δ11B. The top of the sequence is a lazulite quartzite, interpreted as a former conglomeratic phosphatic sandstone, which marks the transition from a closed Permian basin to an open Triassic basin. Within the clastic sequence, the presence of hydrothermal tourmalinite veins documents a hydrothermal event after deposition but before the onset of Alpine metamorphism. A metamorphosed mafic dike swarm in the orthogneiss indicates a post-Variscan event of basaltic magmatism, and this event is tentatively correlated with increased heat flow in the Triassic basin and hydrothermal activity. A consistent conceptual model of this basin and its diagenetic modifications, based on a combination of geochemical data with petrographical and field information, provides the geodynamic context of the European margin at the onset of the Alpine orogeny.

Skarn fluid sources as indicators of timing of Cordilleran arc emergence and paleogeography in the southwestern United States

Abstract Oxygen isotope ratios of garnet provide well-established means to investigate crustal fluid histories. Traditionally, δ18O values from skarn garnets have been used to track the hydrothermal evolution of an individual skarn body through time. We, however, use garnet from 14 skarns from the Jurassic (ca. 175 to ca. 148 Ma) Cordilleran margin arc (southwestern United States) to provide regional tectonic context to arc magmatism and hydrothermal activity. We document arc-wide garnet δ18O variability of ~19‰ (−8.9‰ to +10.3‰, n = 159), providing a record of contrasting meteoric fluid ingress between northern (Sierra Nevada) and southern (Mojave Desert) arc segments. Strongly negative garnet δ18O values (≤−3‰) are limited to the Mojave Desert arc segment and can only form in the presence of meteoric fluid, requiring shallow formation in subaerial crust. When combined with U-Pb garnet ages, the δ18O data provide a minimum radiometric age of local subaerial arc emergence and temporal constraint on the migration of the Jurassic paleoshoreline in the Mojave Desert section of the arc.

Sequence stratigraphy of the Niobrara Formation: Implications for age-constraining tectonic events and stratigraphic complexities in the Denver-Julesburg Basin, United States

The Upper Cretaceous Niobrara Formation in the Denver-Julesburg (D-J) Basin, United States, records late Turonian–early Campanian transgressive–regressive cycles that governed the rhythmic character of pelagic and detrital sedimentation along the former carbonate ramp of the Western Interior seaway (WIS). The general stratigraphic and architectural complexity of the full Niobrara section has long been recognized; however, fine-scale subdivision of the formation from a sequence stratigraphic aspect has been lacking and is critical for basin-scale analysis. The timing and controls of significant changes in architectural and sediment-accumulation patterns have important implications for the Niobrara petroleum system. As part of this study, we present a sequence stratigraphic framework appropriate for the distal carbonate ramp of the WIS. We project published biozone mapping and radiometric ages into basin-wide well control to establish the timing of chronostratigraphic surfaces. Age-constrained isochore maps reveal a chronology of dynamic paleoseafloor morphology that is critical to understanding the stratigraphic and architectural evolution of the Niobrara. At a regional scale, thickness patterns suggest that the north-south–oriented flexural forebulge in the Sevier foreland basin had migrated eastward to the position of the future Rocky Mountains, a location coincident with the western edge of dominantly pelagic carbonate deposition in the backbulge of the WIS basin. At the D-J Basin scale, chronostratigraphic isochore maps of four third-order sequences, and their finer-scale subintervals, record two broadly distinct periods of depositional influence in the Niobrara. Between ca. 90.0 and ca. 85.7 Ma (upper Turonian–lower Santonian), relatively even deposition was dominated by a pattern of differential sediment accumulation. Systematic reversals of thicks and thins through time are indicative of compensational infilling patterns with no influence of tectonic uplifts along the seafloor. In contrast, from ca. 85.7 to ca. 81.7 Ma (lower Santonian–lower Campanian), a reorganization of earlier architecture was driven by the influence of sublinear uplifts along the trend of the emergent Transcontinental arch. As a result, sediment accumulation was dominated by patterns of draping over the long-lived paleohighs. Absolute timing of the architectural changes of the Niobrara suggests a link among Sevier thrusting events, a migrating flexural forebulge, and uplifts at reactivated basement shear zones in the distal foreland. Thus, both active tectonics and the inherited Proterozoic basement fabric of the WIS influenced the evolution of the Niobrara Formation. This example from the Cretaceous WIS shows that age-constraining sequence stratigraphy for regional isochore mapping can reveal relationships among the structural response to far-field tectonic events, oceanographic variations, and stratigraphic architecture.

Middle Pleistocene Homo behavior and culture at 140,000 to 120,000 years ago and interactions with Homo sapiens

Middle Pleistocene Homo in the Levant Our understanding of the origin, distribution, and evolution of early humans and their close relatives has been greatly refined by recent new information. Adding to this trend, Hershkovitz et al. have uncovered evidence of a previously unknown archaic Homo population, the “Nesher Ramla Homo ” (see the Perspective by Mirazon Lahr). The authors present comprehensive qualitative and quantitative analyses of fossilized remains from a site in Israel dated to 140,000 to 120,000 years ago indicating the presence of a previously unrecognized group of hominins representing the last surviving populations of Middle Pleistocene Homo in Europe, southwest Asia, and Africa. In a companion paper, Zaidner et al. present the radiometric ages, stone tool assemblages, faunal assemblages, and other behavioral and environmental data associated with these fossils. This evidence shows that these hominins had fully mastered technology that until only recently was linked to either Homo sapiens or Neanderthals. Nesher Ramla Homo was an efficient hunter of large and small game, used wood for fuel, cooked or roasted meat, and maintained fires. These findings provide archaeological support for cultural interactions between different human lineages during the Middle Paleolithic, suggesting that admixture between Middle Pleistocene Homo and H. sapiens had already occurred by this time. Science , abh3169 and abh3020, this issue p. 1424 and p. 1429 ; see also abj3077, p. 1395

A Middle Pleistocene Homo from Nesher Ramla, Israel

Middle Pleistocene Homo in the Levant Our understanding of the origin, distribution, and evolution of early humans and their close relatives has been greatly refined by recent new information. Adding to this trend, Hershkovitz et al. have uncovered evidence of a previously unknown archaic Homo population, the “Nesher Ramla Homo ” (see the Perspective by Mirazon Lahr). The authors present comprehensive qualitative and quantitative analyses of fossilized remains from a site in Israel dated to 140,000 to 120,000 years ago indicating the presence of a previously unrecognized group of hominins representing the last surviving populations of Middle Pleistocene Homo in Europe, southwest Asia, and Africa. In a companion paper, Zaidner et al. present the radiometric ages, stone tool assemblages, faunal assemblages, and other behavioral and environmental data associated with these fossils. This evidence shows that these hominins had fully mastered technology that until only recently was linked to either Homo sapiens or Neanderthals. Nesher Ramla Homo was an efficient hunter of large and small game, used wood for fuel, cooked or roasted meat, and maintained fires. These findings provide archaeological support for cultural interactions between different human lineages during the Middle Paleolithic, suggesting that admixture between Middle Pleistocene Homo and H. sapiens had already occurred by this time. Science , abh3169 and abh3020, this issue p. 1424 and p. 1429 ; see also abj3077, p. 1395

Middle Pleistocene Homo in the Levant

Paleoanthropology Our understanding of the origin, distribution, and evolution of early humans and their close relatives has been greatly refined by recent new information. Adding to this trend, Hershkovitz et al. have uncovered evidence of a previously unknown archaic Homo population, the “Nesher Ramla Homo ” (see the Perspective by Mirazon Lahr). The authors present comprehensive qualitative and quantitative analyses of fossilized remains from a site in Israel dated to 140,000 to 120,000 years ago indicating the presence of a previously unrecognized group of hominins representing the last surviving populations of Middle Pleistocene Homo in Europe, southwest Asia, and Africa. In a companion paper, Zaidner et al. present the radiometric ages, stone tool assemblages, faunal assemblages, and other behavioral and environmental data associated with these fossils. This evidence shows that these hominins had fully mastered technology that until only recently was linked to either Homo sapiens or Neanderthals. Nesher Ramla Homo was an efficient hunter of large and small game, used wood for fuel, cooked or roasted meat, and maintained fires. These findings provide archaeological support for cultural interactions between different human lineages during the Middle Paleolithic, suggesting that admixture between Middle Pleistocene Homo and H. sapiens had already occurred by this time. Science , abh3169 and abh3020, this issue p. [1424][1] and p. [1429][2]; see also abj3077, p. [1395][3] [1]: /lookup/doi/10.1126/science.abh3169 [2]: /lookup/doi/10.1126/science.abh3020 [3]: /lookup/doi/10.1126/science.abj3077

Climate and the latitudinal limits of subtropical reef development

Climate plays a central role in coral-reef development, especially in marginal environments. The high-latitude reefs of southeast Florida are currently non-accreting, relict systems with low coral cover. This region also did not support the extensive Late Pleistocene reef development observed in many other locations around the world; however, there is evidence of significant reef building in southeast Florida during the Holocene. Using 146 radiometric ages from reefs extending ~ 120 km along Florida’s southeast coast, we test the hypothesis that the latitudinal extent of Holocene reef development in this region was modulated by climatic variability. We demonstrate that although sea-level changes impacted rates of reef accretion and allowed reefs to backstep inshore as new habitats were flooded, sea level was not the ultimate cause of reef demise. Instead, we conclude that climate was the primary driver of the expansion and contraction of Florida’s reefs during the Holocene. Reefs grew to 26.7° N in southeast Florida during the relatively warm, stable climate at the beginning of the Holocene Thermal Maximum (HTM) ~ 10,000 years ago, but subsequent cooling and increased frequency of winter cold fronts were associated with the equatorward contraction of reef building. By ~ 7800 years ago, actively accreting reefs only extended to 26.1° N. Reefs further contracted to 25.8° N after 5800 years ago, and by 3000 years ago reef development had terminated throughout southern Florida (24.5–26.7° N). Modern warming is unlikely to simply reverse this trend, however, because the climate of the Anthropocene will be fundamentally different from the HTM. By increasing the frequency and intensity of both warm and cold extreme-weather events, contemporary climate change will instead amplify conditions inimical to reef development in marginal reef environments such as southern Florida, making them more likely to continue to deteriorate than to resume accretion in the future.

40Ar/39Ar age and petrology of magmatic rocks from East Balkan (Bulgaria) constrain the initiation of regional subduction in SE Europe

A prominent subduction-related magmatic arc hosting significant mineralization formed in SE Europe during the Late Cretaceous. Previous studies on major magmatic centers and ore deposits suggested that this belt formed through southwards retreat of a subducting Neotethys oceanic slab. However, the timing and the petrologic characteristics of magmatic products from the less mineralized eastern portions of this belt remain largely unknown. The complete lack of radiometric ages and limited geochemical characterization of this magmatism adds considerable uncertainty on existing large-scale geodynamic reconstructions focused on the Late Cretaceous magmatism in SE Europe. Here, we address this question by studying little known Cretaceous lavas and sub-volcanic bodies from the Eastern Balkan, Bulgaria. Major and trace element contents of these volumetrically limited rocks show a clear subduction signature. The most primitive rocks are high-Al basalts, which further differentiated into andesites and dacites via fractional crystallization in relatively small magma chambers. The mineral chemistry and assemblages constrain magmatic conditions prior to crystallization to pressures of 3–7 kb, temperatures of 900–1020 °C, water contents of ~4–7 wt% and high oxygen fugacities. Phenocryst features like reverse zonation of clinopyroxene and amphibole and sieve and patchy textures of plagioclase suggest magma mixing processes. Initial εHf values of Cretaceous zircons (+2 to −2) and inherited, mainly Variscan and older zircons (+3 to −11) provide clear evidence for assimilation of crustal lithologies by mantle-derived Late Cretaceous magmas. Amphibole phenocrysts from an andesite and a dacite give 40Ar/39Ar plateau ages of 94.67 ± 0.40 Ma and 94.56 ± 0.40 Ma, respectively. These dates are the oldest recorded in the entire Late Cretaceous magmatic belt and constrain the onset of the subduction magmatism to the later parts of the Cenomanian stage. Regional correlations based on these results reveal that processes of slab retreat were active also in the eastern part of the magmatic arc. Further, these results outline a clear temporal along-arc trend of progressively younger initiation of the arc magmatism from east to west.

Mapping solid reservoir bitumen by deconvoluting formation mechanism

Solid reservoir bitumen is a geologic risk that controls reservoir quality and fluid distribution and has a profound impact on field development and near-field exploration. Understanding the exact mechanism responsible for the development of solid bitumen is therefore a prerequisite toward its proper quantification and mapping. A sandstone reservoir of an anticlinal gas condensate field was investigated geochemically, and data integrated with reservoir diagenetic history to explain abundant solid bitumen encountered within several meters immediately below a regional unconformity in crestal wells, as opposed to deeper sections and downdip wells. The reservoir is generally characterized by a similar sedimentological and diagenetic history across the field, with diagenetic illite grain coats and quartz cement occurring in both updip and downdip wells. The influx of gas condensates into the upper part of the preexisting oil reservoir, possibly facilitated by a major fault, appears to have caused thermodynamic disequilibrium, leading to de-asphalting and subsequent deposition of solid bitumen. Textural and radiometric age dating, integrated with fluid inclusion homogenization temperatures and basin modeling, suggests the initiation of bitumen deposition 129 Ma, immediately after the arrival of gas condensates into the reservoir. As the process was restricted to the uppermost part of the reservoir, it is possible that the solid bitumen, once developed, has improved sealing efficiency along the unconformity and the trapping of subsequent hydrocarbon charges in the underlying reservoir. Better reservoir quality can be inferred farther downdip in areas inaccessible to gas incursions and hence not affected by gas de-asphalting. Other processes, such as regional uplift-induced phase separation, in-reservoir thermal cracking, water-washing, and biodegradation, were also examined and found irrelevant to the formation of solid bitumen in question.

Devonian–Carboniferous regional deformation in the northeastern Lachlan Orogen, southeastern Australia

The timing of regional deformation of the turbiditic Silurian–Devonian Hill End Trough in the northeastern Lachlan Orogen has been a contentious issue with one view ascribing the regional north–south folds and axial planar foliation to Middle Devonian basin inversion. Alternatively, it has been argued that given the low-angle discordance between Lower and Upper Devonian units on the Capertee and Molong highs, and the development of a dominant episode of folding mapped from the Hill End Trough into the adjoining highs, the major regional deformation is latest Devonian to early Carboniferous and predates intrusion of Bathurst-type granites at 358–314 Ma. We have approached the problem by analysing the gentle, upright, southeast-trending folds and deformation patterns in Devonian units of the northern Capertee High (Cudgegong area), drawing a cross-section across the Hill End Trough and reviewing the structure of two areas in the Molong High (west of Orange, and south of Wellington). Upper Devonian units on the highs are affected by fold patterns that extend into the underlying successions and have been interpreted as continuous into the central Hill End Trough, confirming the extent of the latest Devonian to early Carboniferous regional deformation in the northeastern Lachlan Orogen. Constraints from timing of deposition and radiometric ages are ambiguous and imply deformation was overlapping with sedimentation, as has also been inferred from sedimentary features of the Upper Devonian succession. Steeply dipping faults along the highs predated the Late Devonian and were probably formed in extension during formation of the Hill End Trough and were reactivated during basin inversion and potentially during Upper Devonian sedimentation. Our conclusion is that the simplest hypothesis is that the regional east–west shortening was of latest Devonian to early Carboniferous age, although poorly understood basin inversion occurred in the Middle to Late Devonian. KEY POINTS The timing of the development of regional north–south folds in the northeastern Lachlan Orogen has been a contentious issue and is difficult to resolve, given the broad stratigraphic constraints and ambiguous radiometric ages. The simplest hypothesis remains that the regional deformation was of latest Devonian to early Carboniferous timing. Middle to Late Devonian deformation included reactivation of basin-forming steeply dipping faults during basin inversion and ongoing sedimentation.

The Late Carboniferous deeply eroded Tharandt Forest caldera\u2013Niederbobritzsch granite complex: a post-Variscan long-lived magmatic system in central Europe

Samples and documentation of outcrops and drillings, facies analysis, whole rock geochemistry and radiometric ages have been employed to re-evaluate the Late Carboniferous Tharandt Forest caldera (TFC) and the co-genetic Niederbobritzsch granite (NBG) in the eastern Erzgebirge near Dresden, Germany. The c. 52 km2 TFC harbours strongly welded ignimbrites with a preserved minimum thickness of 550 m. Composition of initial fallout tephra at the base of the TFC fill, comprising lithics of rhyolitic and basic lava, and of silica-rich pyroclastic rocks, suggests a bimodal volcanic activity in the area prior to the climactic TFC eruption. The lower part of the TFC fill comprises quartz-poor ignimbrites, overlain by quartz-rich ignimbrites, apparently without a depositional break. Landslides originating from the collapse collar of the caldera plunged into the still hot TFC fill producing monolithic gneiss mesobreccia with clasts ≤ 1 m in a pyroclastic matrix. Aphanitic and porphyritic rhyolitic magma formed ring- and radial dykes, and subvolcanic bodies in the centre of TFC. Whole rock geochemical data indicate a high silica (most samples have > 73 wt% SiO2) rhyolitic composition of the TFC magma, and a similar granodiorite–granitic composition for the NBG. Based on drillings and caldera extent, a minimum volume of 22 km3 of TFC fill is preserved, the original fill is assumed at about 33 km3. This estimate translates into a denudation of at least c. 210 m during Late Paleozoic to pre-Cenomanian. Telescopic subsidence of the TFC took place in two, perhaps three stages. A possible TFC outflow facies has been completely eroded and distal TFC tuff has not been recognized in neighboring basins. New CA-ID-TIMS measurements on two TFC samples gave mean zircon ages of 313.4 ± 0.4 Ma and 311.9 ± 0.4 Ma; two samples from NBG resulted in 318.2 ± 0.5 Ma and 319.5 ± 0.4 Ma. In addition, for one sample of the ring dyke an age of ca. 314.5 ± 0.5 Ma has been obtained. These ages, together with field relations, allow for a model of a long-standing evolution of an upper crustal magmatic system (~ 5 Ma?), where pulses of magmatic injection and crustal doming alternate with magmatic quietness and erosion. Together with the Altenberg–Teplice Volcanic Complex, located some 10 km to the southeast, the TFC–NBG Complex represents an early post-Variscan magmatic activity in central Europe.

Repeated extraction of aphyric melts in a rhyolitic system revealed by zircon age and composition: The Ramadas Volcanic Centre (Puna plateau), NW Argentina

Rhyolitic volcanism can provide important information about the mechanisms by which highly-evolved crystal-poor melts can be extracted from silicic crystal-mush reservoirs. In the Altiplano-Puna plateau (Central Andes), rhyolites are volumetrically less abundant than the high-volume, crystal-rich intermediate products emitted during the ignimbritic flare-up (ca. 10–1 Ma), and their geological and temporal relation with the widespread, upper crustal, dacitic mush systems is not well constrained. We studied the isotopic (UPb ages), trace (U, Th, Hf, Y, Ti, P) and rare earth element compositions of zircon contained in the rhyolitic products of the Ramadas Volcanic Centre (late Miocene), which erupted extremely-aphyric, garnet-bearing tubular pumice during a single Plinian event (Corte Blanco Tuff; northern Puna plateau). Results reveal a complex pre-eruptive magmatic history characterized by variable crystallization conditions existing at different times within an upper-crustal crystal-mush reservoir. The unmixing model applied to magmatic zircon sharing textural and geochemical features (oscillatory texture, Th/U = 0.2–0.6; Eu/Eu* = 0.1–0.7) indicates the existence of at least two mush-related crystallization events, which are separated by a protracted hiatus (ca. 2 Ma), and are supported by independent isotopic ages. An episode of zircon crystallization (average disequilibrium-corrected 206Pb/238U age of 9.06 ± 0.19 Ma) coincides with the ages determined for accessory phases associated with garnet in pumice samples (9.163 ± 0.037 Ma, UPb age determination on zircon; 8.70 ± 0.23 Ma, U-Th-Pb age determination on monazite). A further zircon crystallization event is recorded at ca. 6.64 ± 0.12 Ma, which is concordant with published radiometric ages dating the eruption at 6.3 ± 0.3 Ma (average 40Ar/39Ar age of glass shards from distal locations) and at 6.63 ± 0.28 Ma (fission track age of proximal obsidian). The existence of a late-stage to hydrothermal crystallization event is evident from another zircon population with low Th/U ratios (< 0.1) and a 206Pb/238U age of 6.514 ± 0.058 Ma, which crystallized in cold and highly-evolved parts of the reservoir resulting in negative Eu/Eu* and Ti depletion in zircon before these crystals were recycled into the erupting magma. The extreme aphyric character of the rhyolitic products and the nearly-complete lack of phenocrysts and glomerocrysts despite their evolved composition indicate that the Plinian eruption was preceded by effective extraction of crystal-poor melts from the mush zone, during which only micrometric antecrystic minerals (≤ 200 μm) were incorporated. Gas filter-pressing, combined with a compressional local stress field, likely contributed to melt-crystal separation, which was favoured by high volatile contents (H2Omelt 3–5 wt%) and shallow emplacement levels (< 10 km). Finally, the distribution of the U-Pb ages of zircon antecrysts suggests a correlation between the evolution of the Ramadas magmatic system and the fluctuating pattern characterizing the Altiplano-Puna Volcanic Complex flare-up activity, highlighting a possible geological and temporal connection between rhyolitic volcanism and the widespread dacitic mush systems in this sector of the Puna plateau.

Use of high-resolution stratigraphy and derived lithoclasts to document structural inversion: a case study from the Paleogene, Isle of Wight, UK

The effects of structural inversion generated by the Pyrenean Orogeny on the southerly bounding faults of the Hampshire Basin (the Needles and Sandown faults) and on Eocene sedimentation in the adjacent regions were studied in outcrop by sedimentary logging, dip records and the identification of lithoclasts reworked from the crests of anticlines generated during inversion. The duration and precise age of the hiatuses associated with inversion were identified using bio- and magnetostratigraphy and compared with the Geologic Time Scale 2020. The succession on the northern limb of the Sandown Anticline (Whitecliff Bay) includes five hiatuses of varying durations, which together form a progressive unconformity developed during the Lutetian–Priabonian interval (47–35 Ma). Syn-inversion deposits thicken southwards towards the southern margin of the Hampshire Basin and are erosionally truncated by unconformities. The effects of each pulse of inversion are recorded by successively shallower dips and the age and nature of clasts reworked from the crest of the Sandown Anticline. Most individual hiatuses are interpreted as minor unconformities developed subsequent to inversion, rather than eustatically generated sequence boundaries or transgressive surfaces. By contrast, the succession north of the Needles Fault (Alum Bay) does not contain hiatuses of magnitude or internal unconformities. Subsidiary anticlinal and synclinal structures developed in the NW of the island as a response to Eocene inversion events by the reactivation of minor basement faults. The new dates of the Eocene inversion events correspond closely with radiometric ages (48–36 Ma) derived from fracture vein-fill calcites to the west in Dorset.

A New Martian Crater Chronology: Implications for Jezero Crater

Abstract Crater chronologies are a fundamental tool to assess the relative and absolute ages of planetary surfaces when direct radiometric dating is not available. Martian crater chronologies are derived from lunar crater spatial densities on terrains with known radiometric ages, and thus they critically depend on the Moon-to-Mars extrapolation. This extrapolation requires knowledge of the time evolution of the impact flux, including contributions from various impactor populations, factors that are not trivially connected to the dynamical evolution of the early Solar System. In this paper, we will present a new Martian crater chronology based on current dynamical models, and consider the main sources of uncertainties (e.g., impactor size–frequency distribution; dynamical models with late and early instabilities, etc.). The resulting “envelope” of Martian crater chronologies significantly differs from previous chronologies. The new Martian crater chronology is discussed using two interesting applications: Jezero crater’s dark terrain (relevant to the NASA Mars 2020 mission) and the southern heavily cratered highlands. Our results indicate that Jezero’s dark terrain may have formed ∼3.1 Ga, i.e., up to 0.5 Gyr older than previously thought. In addition, available crater chronologies (including our own) overestimate the number of craters larger than 150 km on the southern highlands, suggesting either that large craters have been efficiently erased over Martian history or that dynamical models need further refinement. Further, our chronology constrains the age of Isidis basin to be 4.05–4.2 Ga and that of the Borealis basin to be 4.35–4.40 Ga; these are predictions that can be tested with future sample and return missions.

China's Chang'e-5 landing site: Geology, stratigraphy, and provenance of materials

China's Chang'e-5 (CE-5) mission, the first lunar sample return mission since 1976, landed at 43.06°N, 51.92°W on Dec. 1, 2020, in Northern Oceanus Procellarum. CE-5 targeted a mare plain (Em4/P58) composed of distinctive young (∼1.6-1.7 Ga) moderate-Ti mare basalts, with elevated Th abundance (inherent or extraneous). Thus, the regolith and rock fragments sampled by CE-5 come from some of the youngest mare basalts on the Moon, near Rima Sharp, and from the center of the globally anomalous Procellarum KREEP Terrane (PKT), hypothesized to be responsible for the generation of the young volcanism. To provide context for the analysis and interpretation of the returned samples and in-situ measurements of the regolith substructure with penetrating radar, we constructed a detailed geologic map and stratigraphic assessment of the site. The stratigraphy consists of ancient highland materials (PKT crust and ejecta from Iridum and Imbrium basins), local silica-rich volcanism, overlain by a sequence of mare basalts, capped with Em4/P58. A ∼4-7 m thick regolith layer developed by post-mare bombardment overlies the Em4/P58 protolith and contains admixed impact ejecta from distant sources, mainly from Harpalus (∼6 wt.%), followed by Copernicus (∼2 wt.%) and Aristarchus (∼1 wt.%). New crater size-frequency measurements of Em4/P58 provide the necessary crater spatial density reference for calibration of the lunar cratering chronology with radiometric ages of the returned samples. The geological map and assessment of regolith provenance indicate that samples returned by CE-5 will address fundamental questions in lunar chronology, thermal evolution, basalt petrogenesis, and the nature of PKT, as well as provide key calibration for lunar and planetary chronologies and remote sensing data.

Radiometric ages and time–space distribution of volcanism in the Campo de Calatrava Volcanic Field (Iberian Peninsula)

The volcanic region of Campo de Calatrava, which is located in the centre of the Iberian Peninsula, is characterized by monogenetic, strombolian and hydromagmatic volcanic activity. It is made up of more than two hundred volcanoes distributed over an area of about 4000 km2. They are mainly comprised of sodic alkaline and ultra-alkaline rocks, few evolved rocks (basalts, basanites, olivine nephelinites and olivine melilitites) and occasionally ultrapotassic rocks (olivine leucitites) and carbonatites. New radiometric ages (K/Ar and 40Ar/39Ar) allow a better understanding of the distribution and temporal evolution of volcanic activity in this region. The oldest volcanism is ultrapotassic and restricted to a single vent. The new 40Ar/39Ar ages date this centre between 7.4 Ma and 7.1 Ma, so it was contemporary with other ultrapotassic activity in the Iberian Peninsula (the lamproites of the SE of Spain). After a break of about 3 million years, the main volcanic activity in the region took place, without apparent major pauses, from about 4 Ma to less than 0.7 Ma, with most radiometric ages concentrated between 3.5 and 1.5 Ma. Except in the case of olivine leucitites, there are no significant compositional differences related to the ages of the volcanoes, although there may be minor variations. The mildly alkaline volcanoes (basalts and basanites) tend to be concentrated in the central areas of the region, while on the peripheral areas, the volcanoes are essentially ultra-alkaline. The spatial age distribution of the volcanoes seems to show a shift of the volcanic activity over time towards the northeast. The spatial age distribution of the volcanoes seems to show a displacement of volcanic activity over time towards the northeast, accompanied by an increase in its intensity.

Late Neogene evolution of the Peruvian margin and its ecosystems: a synthesis from the Sacaco record

The highly productive waters of the Humboldt Current System (HCS) host a particular temperate ecosystem within the tropics, whose history is still largely unknown. The Pisco Formation, deposited during Mio-Pliocene times in the Peruvian continental margin has yielded an outstanding collection of coastal-marine fossils, providing an opportunity to understand the genesis of the HCS ecosystem. We present a comprehensive review, completed with new results, that integrates geological and paleontological data from the last 10 My, especially focusing on the southern East Pisco Basin (Sacaco area). We discuss the depositional settings of the Pisco Formation and integrate new U/Pb radiometric ages into the chronostratigraphic framework of the Sacaco sub-basin. The last preserved Pisco sediments at Sacaco were deposited ~ 4.5 Ma, while the overlying Caracoles Formation accumulated from ~ 2.7 Ma onwards. We identified a Pliocene angular unconformity encompassing 1.7 My between these formations, associated with a regional phase of uplift. Local and regional paleoenvironmental indicators suggest that shallow settings influenced by the offshore upwelling of ventilated and warm waters prevailed until the early Pliocene. We present an extensive synthesis of the late Miocene–Pleistocene vertebrate fossil record, which allows for an ecological characterization of the coastal-marine communities, an assessment of biodiversity trends, and changes in coastal-marine lineages in relation to modern HCS faunas. Our synthesis shows that: (i) typical endemic coastal Pisco vertebrates persisted up to ~ 4.5 Ma, (ii) first modern HCS toothed cetaceans appear at ~ 7–6 Ma, coinciding with a decline in genus diversity, and (iii) a vertebrate community closer to the current HCS was only reached after 2.7 Ma. The genesis of the Peruvian coastal ecosystem seems to be driven by a combination of stepwise transformations of the coastal geomorphology related to local tectonic pulses and by a global cooling trend leading to the modern oceanic circulation system.

40Ar/39Ar and LA-ICP-MS U–Pb geochronology for the New England portion of the Early Cretaceous New England-Quebec igneous province: Implications for the postrift evolution of the eastern North American Margin

The Early Cretaceous New England-Quebec igneous province is a classic example of postrift magmatism along the eastern North American passive margin. Although a suite of ⁴⁰Ar/³⁹Ar ages has been available for the Monteregian Hills lobe in the Quebec portion of the New England-Quebec igneous province for many years, only a single high accuracy radiometric age has been published for the Burlington lobe and none for the Taconic lobe in the New England portion of the province. As a result, the timing of and driving mechanisms behind the magmatism have remained unresolved, and a hotspot origin for the entire province persists in the literature. We have dated four dikes and one pluton in the Burlington and Taconic lobes using ⁴⁰Ar/³⁹Ar and U–Pb geochronology to improve understanding of the age of magmatism in the New England portion of the province. In the Burlington lobe, ⁴⁰Ar/³⁹Ar plateau ages include a 137.55 ± 1.80 Ma biotite age and a 136.9 ± 4.2 Ma amphibole age for a lamprophyre dike from Charlotte, Vermont, and a 133.6 ± 2.2 Ma biotite age for a lamprophyre dike from Colchester, Vermont. In the Taconic lobe, ages include an ⁴⁰Ar/³⁹Ar plateau amphibole age of 107.09 ± 1.32 Ma for a lamprophyre dike from Castleton, Vermont, a 122 Ma minimum ⁴⁰Ar/³⁹Ar biotite age for a lamprophyre dike from Poultney, Vermont, and a 103.13 ± 0.53 Ma LA-ICP-MS U–Pb zircon age from the quartz syenite of the Cuttingsville complex. These results show that magmatism spanned at least 35 Ma, from ∼138 to 103 Ma, which is broadly consistent with the span of magmatism suggested by workers in the 1970s and 1980s based on K–Ar and Rb–Sr ages. This extended span of magmatism for the Burlington and Taconic lobes is in contrast to the brief 1 to 2 Ma episode of magmatism at ∼124 Ma inferred for the Monteregian Hills lobe. The New England-Quebec igneous province has traditionally been attributed to passage of the Great Meteor hotspot. However, given the close proximity of the Burlington and Taconic lobes, the magmatism in these lobes should span only a few Ma if the product of a hotspot. The age data are also difficult to reconcile with a more complex expression of hotspot magmatism in continental lithosphere related to either plume head magmatism or long-distance migration of plume material. Instead, the extended duration of Early Cretaceous New England-Quebec igneous province magmatism in New England may represent an expression of edge-driven convection, a process known to occur along passive margins and inferred to be operating beneath the eastern North American margin today.