Pb Geochronology Research Articles

Genesis of the Devonian Habaqin ultramafic arc complex and associated low-grade Fe–Ti oxide mineralization in the northern North China Craton

In the northern North China Craton (NCC), abundant non-layered ultramafic complexes and associated low-grade Fe–Ti oxide ores (over 1000 million tons) were uncovered. However, their petrogenesis and metallogeny have remained unclear. The Habaqin complex is one of the larger intrusions of the region and hosts the largest low-grade magnetite occurrence. Here we present petrography, zircon U–Pb geochronology, mineral and whole-rock major and trace element compositions, and whole-rock Sr–Nd and zircon Lu–Hf isotopes of the complex to constrain its genesis. The complex includes early pyroxenite (∼75% clinopyroxene, ∼15% amphibole, ∼10% magnetite) and late hornblendite (∼80% amphibole, 15–20% magnetite, 0–5% apatite). Zircon grains from the hornblendite yielded a weighted mean 206Pb/238U age of 395 ± 2 Ma. The increasing whole-rock FeOtot and TiO2 contents from pyroxenite (FeOtot = 18.9 wt%, TiO2 = 1.31 wt%) to hornblendite (FeOtot = 19.1–30.4 wt%, TiO2 = 2.63–3.12 wt%) correlate with higher modal proportions of amphibole and magnetite. The pyroxenite and hornblendite display similar arc-like trace element characteristics of enrichment in LILEs and LREEs, and depletion in HFSEs. The complex formed by fractional crystallization of clinopyroxene, amphibole, magnetite, and apatite from a hydrous parent magma. Early removal of clinopyroxene caused H2O enrichment in evolved residual magmas and crystallization of magnetite and amphibole, forming low-grade mineralization with Ti-V-poor magnetite in hornblendite. The complex displays high initial 87Sr/86Sr ratios of 0.706413–0.707341, negative εNd(t) values of −16.0 to −14.2 and negative εHf(t) value of −30.6 to −12.0, suggesting a parent magma derived from an enriched lithospheric mantle, with possible existence of amphibole-bearing pyroxenite veins within the source. During the Early Devonian, the retreat of the subducted Paleo-Asian oceanic slab as well as arc-continental collision along the northern NCC induced partial melting of the veins and eventually formation of the complex.

Tectonic setting and provenance of the Triassic Gande Formation in the northern Bayanhar, NW China: constraints from whole-rock geochemistry and zircon U–Pb geochronology

Tectonic setting and provenance of the Triassic Gande Formation in the northern Bayanhar, NW China: constraints from whole-rock geochemistry and zircon U–Pb geochronology

Impact of sediment provenance and depositional setting on chlorite content in Cretaceous turbiditic sandstones, Norway

AbstractChlorite minerals, mainly in the form of clay coats, play a critical role in determining the reservoir quality of siliciclastic rocks. They can positively influence reservoir quality by preserving porosity during deep burial, but they can also play a negative role by reducing permeability through pore filling. The main aim of this research is to determine the optimal conditions for chlorite growth in sedimentary basins. This study investigates the Lower Cretaceous turbidite sandstone of the Agat Formation in the North Sea. We used a source‐to‐sink approach to investigate the impact of sediment source composition, chemical weathering and depositional environment on chlorite formation. Understanding the interplay between these processes can help refine exploration and exploitation strategies, optimise hydrocarbon recovery, and reduce exploration risks. Representative samples from two hydrocarbon fields (the Duva and Agat fields) were investigated using petrography, geochemistry, heavy mineral identification and quantification, and U–Pb geochronology of detrital zircons. Our results show a strong heterogeneity in the sediment provenance between the two turbidite systems. In the Duva field, the sandstone is derived from a mixture of mafic and felsic sources, producing Fe‐rich sediments. Intense chemical weathering generates fine fraction materials rich in kaolinite, vermiculite, and hydroxy‐interlayered clays, which are transported into shallow marine settings. Subsequent interaction with seawater results in the formation of glauconitic materials, Fe‐illite, and phosphatic concretions. These Fe‐rich materials are remobilised into deep marine settings, providing precursors for the development of authigenic Fe‐clays such as berthierine and chlorite. Conversely, in the Agat field, the sandstone is predominantly sourced from felsic rocks that underwent low chemical weathering, producing sediment rich in quartz and feldspar with a low amount of clays. With few Fe‐rich materials transported into the basin, the development of chlorite in the Agat field was less pervasive. Basin configuration and depositional environment exerted additional control on chlorite distribution. In the confined turbidite system (e.g. Duva field), chlorite is typically found as coating, whereas in less confined turbidite systems (e.g. Agat field) chlorite shows complex distribution related to depositional environment and dewatering processes. Our findings demonstrate the importance of considering the entire sediment routing system, from source to sink, when predicting chlorite occurrence and its impact on reservoir quality in deep marine settings. This integrated approach can guide exploration and development efforts in deepwater clastic reservoirs.

Redefining timing, genesis and geodynamic setting of polymetallic skarn mineralization, Gangdese belt, Tibet, from LA–ICP–MS garnet U–Pb geochronology

Iron, Cu, Pb–Zn, and W–Mo skarn deposits occur in the Gangdese metallogenic belt, Tibet. A lack of precise age constraints for mineralization hinders understanding of the processes contributing to skarn mineralization, particularly identification of the causative intrusions. In this study, new garnet U–Pb geochronological data are provided for the Qiagong (Fe) and Jiaduobule (Fe–Cu) skarns. We place particular emphasis on skarn garnet mineralogy, using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS), to characterize garnet textures, compositions, and U–Pb isotope systematics. All garnets studied belong to the grossular-andradite series and display oscillatory compositional zoning (And100Gr0 to And20Gr80). Andradite-dominated garnet features a chondrite-normalized REE fractionation pattern exhibiting enrichment in LREE relative to HREE and a positive Eu anomaly, whereas grossular-dominated garnet displays relative LREE depletion and a marked flat HREE fractionation trend. Ore-forming elements, including Sn, W, As, and U, are detected at concentrations up to several hundreds of ppm in the same garnets. Concentrations of W and As are higher in andradite and correlate positively with one another. The correlation between U, Sn and the major oxide composition of garnet is insignificant, potentially resulting from the changing valence states of these elements under different redox conditions. Together with previously published and limited new data for the Gebunongba (Fe), Ri’a (Cu), and Hahaigang (W–Mo) skarns, the new garnet U–Pb age data indicate that the major Fe, Pb–Zn, and W–Mo mineralization event in the Gangdese metallogenic belt took place between 65 Ma and 50 Ma, with marked peaks at ∼ 65 Ma and ∼ 53 Ma. Partial melting of a crustal-dominated source due to the slab rollback (∼65 Ma) and break off (∼53 Ma) of Yarlug–Zangbo Tethyan Ocean and subsequent asthenosphere upwelling induced this regional event during the Paleogene to early Eocene. A less common but regionally widespread Cu (±Au) mineralization event in the same belt formed during the Late Cretaceous (∼85 Ma) and may have resulted from the coupled influence of the northward subduction of the Yarlug–Zangbo Tethyan Ocean and the southward subduction of the Bangong–Nujiang Tethyan Ocean. This contribution highlights the potential significance of the Late Cretaceous Cu(±Au) event and hence the prospectivity of the belt for both skarn and porphyry systems. More broadly, our findings carry implications for Neo-Tethyan metallogeny in the region extending from southwestern China to eastern Europe during the Late Cretaceous.

Sources of Neogene Rare Metal–Titanium Placers of the Northeastern Wall of the Dnieper–Donets Basin: First Results of U–Pb Geochronological Studies of Detrital Zircon

Sources of Neogene Rare Metal–Titanium Placers of the Northeastern Wall of the Dnieper–Donets Basin: First Results of U–Pb Geochronological Studies of Detrital Zircon

Determination of multistage oil charge processes in the Ediacaran Dengying gas reservoirs of the southwestern Sichuan basin, SW China

The Sichuan Basin has experienced multi periods of tectonic uplift event, accompanied with multiple hydrocarbon accumulation and phase transitions processes in the deep and ultra-deep reservoirs. The formation of paleo-oil reservoir plays an important role in the distribution of present-day gas fields, yet many records of early oil reservoirs have been destroyed. A series of methods including, petrography, cathodoluminescence, fluid inclusion, in-situ U–Pb geochronology, and basin modeling, were used to determine the multistage oil charge process in the Ediacaran Dengying Formation of the Weiyuan area, SW Sichuan Basin. The lace-like dolomite lamination and three stages of dolomite cement were petrographically, and geochronologically distinguished in the Dengying Formation, dated at c. 542 Ma, 486–482 Ma, 410 Ma, and 270 Ma, respectively. Three stages of bitumen in the Dengying reservoir indicated three episodes of oil charge. The first and second oil-charging events occurred at 500–486 Ma and 410–400 Ma, by combining the modeled timing of oil generation with mineral ages of the two generations before and after the solid bitumen. The modification of the Weiyuan paleo-oil pool occurred during the Caledonian tectonic uplift, after the second oil-charging event. The timing of the third oil charge was at 270–230 Ma, according to the Th value of aqueous inclusions coeval with secondary bitumen inclusions in the stage 3 dolomite cement (CD-3), close to the timing of main oil generation for the Lower Cambrian source rocks. The first oil charge may be a contribution from the Doushantuo source rock, and the Lower Cambrian source rocks provide the major contribution for the third oil charge. This research reveals the timing of multistage oil charging events in the southwest Sichuan basin, and provides a further method for determining the timing of oil charge in multi-cyclonic composite basins.

Testing in-situ apatite Lu–Hf dating in polymetamorphic mafic rocks: a case study from Palaeoproterozoic southern Australia

In mafic systems where primary mineral assemblages have witnessed moderate- to high-temperature hydrous overprinting and deformation, little is known about the retentivity of the Lu–Hf isotopic system in apatite. This study presents apatite laser-ablation Lu–Hf and U–Pb geochronology, zircon geochronology, and detailed petrological information from polymetamorphic mafic intrusions located in the central-western Gawler Craton in southern Australia, which records an extensive tectonometamorphic history spanning the Neoarchaean to the Mesoproterozoic. Zircon records magmatic crystallisation ages of c. 2479–2467 Ma, coinciding with the onset of the c. 2475–2410 Ma granulite-facies Sleafordian Orogeny. The amphibole-dominant hydrous assemblages which extensively overprint the primary magmatic assemblages are hypothesised to post-date the Sleafordian Orogeny. The Lu–Hf and U–Pb isotopic systems in apatite are used to test this hypothesis, with both isotopic systems recording significantly younger ages correlating with the c. 1730–1690 Ma Kimban Orogeny and the c. 1590–1575 Ma Hiltaba magmatic event, respectively. While the early Mesoproterozoic apatite U–Pb ages are attributed to thermal re-equilibration, the older Lu–Hf ages are interpreted to reflect re-equilibration facilitated primarily by dissolution-reprecipitation, but also thermally activated volume diffusion. The mechanisms of Lu–Hf isotopic resetting are distinguished based on microscale textures and trace element abundances in apatite and the integration of apatite-amphibole textural relationships and temperatures determined from the Ti content in amphibole. More broadly, the results indicate that at low to moderate temperatures, apatite hosted in mafic rocks is susceptible to complete recrystallisation in rocks that have weak to moderate foliations. In contrast, at higher temperatures in the absence of strain, the Lu–Hf system in apatite is comparatively robust. Ultimately, the findings from this study advance our understanding of the complex role that both metamorphism and deformation play on the ability of mafic-hosted apatite to retain primary Lu–Hf isotopic signatures.

Geochemical characterization and zircon U–Pb geochronology of the Tirodi Gneissic Complex, Central Indian Tectonic Zone (CITZ): constraints on petrogenesis, Proterozoic crustal evolution and tectonic setting

Geochemical characterization and zircon U–Pb geochronology of the Tirodi Gneissic Complex, Central Indian Tectonic Zone (CITZ): constraints on petrogenesis, Proterozoic crustal evolution and tectonic setting

Hydrothermal alteration mechanisms of an Archaean metamorphic buried hill and the models for reservoir zonation, Bozhong depression, Bohai Bay Basin, China

Multiple metamorphic buried-hill hydrocarbon reservoirs have been recently discovered in the Bozhong Depression, Bohai Bay Basin. Among them, the buried-hill reservoirs in the BZ13, BZ19 and BZ26 regions have the largest condensate gas reserve in eastern China. The hydrocarbon-bearing reservoir has a large thickness with a maximum of ∼1 km, making the oil-equivalent reserve exceed 5 × 108 tons. This study used petrography, isotopes, gas composition, zircon U–Pb geochronology, X-ray diffraction, electronic probe and hydrothermal dissolution experiments to discuss the formation mechanisms of the metamorphic buried-hill reservoirs. The results indicate that the reservoirs 500 m below the surface of the buried hill have been significantly altered by the mantle-derived CO2. This is supported by the carbon isotope of CO2, He isotope of the helium gas, strong negative excursions in δ18OPDB, positive excursions in δ13CPDB, Co/Ni ratio greater than 1 in pyrite and secondary characteristic minerals such as dawsonite. Hydrothermal dissolution experiments using CO2 result in the precipitation of kaolinite, ankerite and quartz cements, which is consistent with petrographic observation. The mantle-derived CO2 reacts with feldspars, hornblende, biotite and carbonates, creates secondary dissolution pores and significantly improves reservoir quality. Moreover, partial carbonate cements exhibit negative excursions in δ13CPDB and the depletion of light rare earth elements, indicating the influence of organic acids-bearing low-temperature hydrothermal fluids. Therefore, three development models illustrating the formation of metamorphic buried-hill reservoirs were proposed. This study may serve as a reference for the hydrocarbon exploration in metamorphic buried-hill reservoirs.

Reassessing the timing of high-strain deformation in the Strangways Metamorphic Complex, Central Australia, by in situ mica Rb–Sr and titanite U–Pb geochronology

Dating the timing of deformation within shear zones is critical to quantifying orogenic processes and developing time-resolved regional tectonic frameworks. Such work requires the integration of detailed microstructural analysis with in situ geochronology to quantify when deformation mechanisms were active at the micro-scale. The Proterozoic, granulite-facies Strangways Metamorphic Complex was exhumed during the Devonian to Carboniferous intracontinental Alice Springs Orogeny. New microstructural observations, quartz c -axis orientation analyses, mica chemistry, Ti-in-biotite thermometry, in situ mica Rb–Sr and titanite U–Pb geochronology outline a detailed history of ductile shearing across the complex. Movement along the north boundary shear zone of Strangways Metamorphic Complex appears to have initiated c. 382 Ma, preceding peak metamorphism in the area during the Alice Springs Orogeny. Widespread reverse-sense ductile shearing occurred within the Strangways Metamorphic Complex between c. 365 and 355 Ma, and correlates with rapid cooling of the region. Late-stage ductile deformation is recorded at c. 335 Ma, likely reflecting the terminal exhumation of the Strangways Metamorphic Complex. Finally, the new in situ muscovite and biotite Rb–Sr data collected herein permit comparison with previous two-point mica Rb–Sr isochrons and 40 Ar– 39 Ar dates from the same specimens. In the rocks analysed, the biotite Rb–Sr system returned dates similar to the previous 40 Ar– 39 Ar white mica dates, perhaps indicating a similar effective closure temperature. Supplementary material: Test results and previous geochronology data from the Alice Springs Orogeny are available in tabular form at https://osf.io/yjp24/?view_only=0bf5bec9f5c94a1aa4fb413320ec24e8

Detrital zircon U–Pb geochronology and geochemistry of sandstones in the Siziwang Banner, Central Inner Mongolia: Implication for tectonic evolution

The central Inner Mongolia, located at the intersection of the northern margin of the North China Craton (NCC) and the Central Asian Orogenic Belt, is crucial for deciphering the Late Palaeozoic tectonic evolution associated with the subduction and closure of the Palaeo‐Asian Ocean (PAO). Our study focused on petrology, detrital zircon LA–ICP–MS U–Pb geochronology and whole‐rock geochemistry for the Late Carboniferous to Permian sandstones within the Shuanmazhuang, Dahongshan, Naobaogou, and Laowopu formations in Siziwang Banner, central Inner Mongolia. This comprehensive analysis shed light on the dynamic interplay between the NCC and the South Mongolia Block. Detrital zircon U–Pb ages in investigated samples mainly cluster between 250 and 2650 Ma, with significant peaks at 2.4–2.5 Ga, 1.8–2.0 Ga, 400–430 Ma, and 250–320 Ma, respectively. The geochemistry data are characterized by SiO2 contents (56.29–77.95 wt. %), Na2O / K2O ratios (0.45–1.58) and SiO2/Al2O3 ratios between 4.33 and 7.44. Moreover, they exhibit the slight enrichment in large ion lithophile elements (Rb and Ba) and the depletion in high field strength elements (Nb, Ta, Th, and U). These facts indicate that the sedimentary detritus predominantly originates from felsic sources, probably deriving from the Late Carboniferous–Permian continental island arc‐related intermediate‐acid igneous rocks, the Late Ordovician‐Silurian magmatic rocks in the Bainaimiao arc and the basements of the NCC. Furthermore, our present results also suggest that during the Early–Middle Permian, accelerating oceanic crust subduction triggered significant magmatic events in Siziwang Banner, leading to rapid uplift and the erosion of arc magmatic rocks, as well as the abundant corresponding sediments. Subsequently, the gradual convergence and eventual collision between the NCC and the Southern Mongolian Block took place at the end of the Permian, representing final closure of the PAO.

Late Paleozoic oxygenation of marine environments supported by dolomite U-Pb dating

Understanding causal relationships between evolution and ocean oxygenation hinges on reliable reconstructions of marine oxygen levels, typically from redox-sensitive geochemical proxies. Here, we develop a proxy, using dolomite U–Pb geochronology, to reconstruct seawater U/Pb ratios. Dolomite samples consistently give U–Pb dates and initial 207Pb/206Pb ratios lower than expected from their stratigraphic ages. These observations are explained by resetting of the U–Pb system long after deposition; the magnitude of deviations from expected initial 207Pb/206Pb are a function of the redox-sensitive U/Pb ratios during deposition. Reconstructed initial U/Pb ratios increased notably in the late-Paleozoic, reflecting an increase in oxygenation of marine environments at that time. This timeline is consistent with documented shifts in some other redox proxies and supports evolution-driven mechanisms for the oxygenation of late-Paleozoic marine environments, as well as suggestions that early animals thrived in oceans that on long time scales were oxygen-limited compared to today.

Magmatic, Metamorphic and Structural History of the Variscan Lizard Ophiolite and Metamorphic Sole, Cornwall, UK

AbstractThe Lizard ophiolite, Cornwall, South‐West England, is the largest and best‐preserved ophiolite within the Variscan orogenic belt. It forms part of the Rheic‐Rhenohercynian suture zone, and was obducted northwestward onto the passive continental margin of Avalonia (Laurussia) during the Middle Devonian. It comprises an almost complete thrust slice of oceanic crust with sheeted dykes, gabbros, Moho transition sequence, and upper‐mantle peridotites, underlain by a metamorphic sole. Despite the importance of the Lizard ophiolite in understanding Variscan tectonics, the origin and age of the Lizard ophiolite are debated. We present new field observations, structural maps and cross‐sections of the Lizard ophiolite from extensive re‐mapping, integrated with U–Pb geochronology, petrology, thermobarometry, and whole rock geochemistry. We report new U–Pb zircon (CA‐ID‐TIMS and LA‐ICPMS) ages of 386.80 ± 0.25/0.31/0.52 Ma (Givetian) from a plagiogranite dyke intruding the Crousa Gabbros at Porthoustock, and 395.08 ± 0.14/0.22/0.47 Ma (Emsian) from partial melts of the metamorphic sole Landewednack Amphibolites at Mullion Cove. These ages, respectively, precisely date the formation of the Lizard ophiolite oceanic crust, and the age of cooling post peak‐metamorphism of the sole. Petrological modeling on the Landewednack Amphibolites suggests peak metamorphic conditions of 10 ± 2 kbar and 600 ± 75°C. We demonstrate that the Lizard ophiolite formed as a supra‐subduction zone ophiolite overlying an inverted metamorphic sole, and we combine our observations and data into a new geodynamic model for the formation and obduction of the ophiolite. The current data supports an induced subduction initiation model.

Minimizing the effects of Pb loss in detrital and igneous U–Pb zircon geochronology by CA-LA-ICP-MS

Abstract. Detrital zircon geochronology by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) is a widely used tool for determining maximum depositional ages and sediment provenance, as well as reconstructing sediment routing pathways. Although the accuracy and precision of U–Pb geochronology measurements have improved over the past 2 decades, Pb loss continues to impact the ability to resolve zircon age populations by biasing affected zircon toward younger apparent ages. Chemical abrasion (CA) has been shown to reduce or eliminate the effects of Pb loss in zircon U–Pb geochronology but has yet to be widely applied to large-n detrital zircon analyses. Here, we assess the efficacy of the chemical abrasion treatment on zircon prior to analysis by LA-ICP-MS and discuss the advantages and limitations of this technique in relation to detrital zircon geochronology. We show that (i) CA does not systematically bias LA-ICP-MS U–Pb dates for 13 reference materials that span a wide variety of crystallization dates and U concentrations, (ii) CA-LA-ICP-MS U–Pb zircon geochronology can reduce or eliminate Pb loss in samples that have experienced significant radiation damage, and (iii) bulk CA prior to detrital zircon U–Pb geochronology by LA-ICP-MS improves the resolution of age populations defined by 206Pb/238U dates (Neoproterozoic and younger) and increases the percentage of concordant analyses in age populations defined by 207Pb/206Pb dates (Mesoproterozoic and older). The selective dissolution of zircon that has experienced high degrees of radiation damage suggests that some detrital zircon age populations could be destroyed or have their abundance significantly modified during this process. However, we did not identify this effect in either of the detrital zircon samples that were analyzed as part of this study. We conclude that pre-treatment of detrital zircon by bulk CA may be useful for applications that require increased resolution of detrital zircon populations and increased confidence that 206Pb/238U dates are unaffected by Pb loss.

Geochemistry and Zircon LA–ICP–MS U–Pb Geochronology of the Shuangwang Au Deposit, Shaanxi Province: Implications for Tectonic Evolution and Metallogenic Age

The Shuangwang Au deposit (with a gold resource of approximately 70 t Au), is located in the Fenxian-Taibai fore-arc basin in the West Qinling Orogen of central China. Igneous intrusions in the region include the Xiba granitic pluton and granite porphyry and lamprophyre dykes. The Xiba pluton is composed of granodiorite and monzonite granite. The granodiorite is typical I-type granite, and it yields a crystallization age of 221.1 ± 1.2 Ma and a two-stage Hf model age of 1432–1634 Ma. The monzonite granite shows a transitional characteristic between I-type and A-type granite, and it yields a crystallization age of 214.8 ± 1.2 Ma and a two-stage Hf model age of 1443–1549 Ma. The granitoid was derived mainly from a crust–mantle mixed source. The ages indicate that the granodiorite and monzonite granite formed during two different stages. The REE distribution patterns of the Xiba granitoid exhibit significant fractionation between LREE and HREE, showing right-dipping curves, with an enrichment of LREE and a deficit of HREE. The granodiorite displays a light negative Eu anomaly, while the monzonite granite displays an obvious negative Eu anomaly. The granite porphyry dikes are distributed in the No. I breccia and Jiupinggou granite porphyry, and they yield crystallization ages of 219.9 ± 1.5 Ma and 213.1 ± 0.89 Ma, respectively, and two-stage Hf model ages of 1382–1501 Ma and 1373–1522 Ma, respectively. The lamprophyre dikes in the deposit yield a crystallization age of 214.4 ± 2.7 Ma. After the collision event between the Yangtze and the North China Plates along the Qinling orogenic belt, at approximately 220 Ma in the Late Triassic, the detachment of the slab produced the upwelling of the asthenosphere material. Under conditions of mantle heat and tectonic stress, widespread partial melting of the subducted continental crust and the upper lithosphere mantle occurred, forming granitoids with various degrees of adakite characteristics.

U–Pb geochronology and geochemistry of volcanic ash layers in the Eastern Caucasus intermountain Neogene basin: implications for their sources

U–Pb geochronology and geochemistry of volcanic ash layers in the Eastern Caucasus intermountain Neogene basin: implications for their sources

Pleistocene deformation of the Malargüe fold–thrust belt from structural modelling and geochronology of syntectonic sedimentation

We report uplift and shortening rates from a late Neogene–Pleistocene deformation stage of the frontal fold–thrust belt and adjacent wedge-top in the Principal Cordillera of the southern Central Andes (latitude 33–39°S). A structural model is presented based on integration of surface field data and subsurface 2D seismic sections. Shortening, uplift and sedimentation rates were calculated from different steps of kinematic modelling. Our structural interpretations and modelling are integrated with new detrital zircon U–Pb geochronology to define a previously overlooked Pleistocene period of orogenic shortening and syntectonic sedimentation in the Malargüe basin. This task was possible as a result of the dating of three samples yielding ages between ∼12 and 1 Ma obtained from a 900 m deep well located in the foreland. From stratigraphic correlations, our data record an active Plio-Pleistocene wedge-top depozone coeval with retreat of the volcanism, and the emplacement of retroarc basalts. Structural modelling, together with detrital zircon U–Pb provenance data, registers shortening producing a foredeep to wedge-top Plio-Pleistocene transition, adjusting and completing the knowledge of the frontal fold–thrust belt and foreland basin in the southern Central Andes. Supplementary material: Supplementary tables are available at https://doi.org/10.6084/m9.figshare.c.7033425

Neoarchean magmatism and Rhyacian reworking in the northeast of the Amapá Block, southeastern Guiana Shield, Brazil: Geochemistry, U–Pb geochronology, and Hf–Nd isotopes

This study investigates the age, geochemical nature, sources, and Rhyacian reworking of the Neoarchean magmatism at the northeastern portion of the Amapá Block, southeastern Guiana Shield (northeast Amazonian Craton). This block consists of a large Archean continental landmass strongly reworked during the Transamazonian orogeny (2.26–1.95 Ga). We provide the very first geochemical results for Archean basement units of the Amapá Block, which are the Mesoarchean Tumucumaque and Neoarchean Guianense complexes, and Neoarchean Mungubas Granite. The first two exhibited geochemical affinity with enriched TTG-type and hybrid granitoids, respectively, while the Mungubas Granite showed affinity with biotite granites. Such affinity with C-type granitoids indicates that the genesis of these units involves partial melting of crustal sources. LA-MC-ICP-MS zircon U–Pb and Lu–Hf, and whole-rock Sm–Nd data were obtained for the Neoarchean Mungubas Granite and Guianense Complex. In both units, zircon U–Pb dating provided a distribution of dates that span ∼50 Ma forming two groups with distinct mean 207Pb/206Pb ages of ∼2.70 Ga and ∼2.65 Ga, respectively. The older age is considered as the crystallization/emplacement age of the Mungubas Granite and the Guianense orthogneiss protolith, while the younger one is ascribed to concealed Pb loss. The recognition of Mesoarchean inherited zircon core and xenocryst with ages of 2.84 Ga and 2.99 Ga suggest that these units reworked older basement rocks from the Amapá Block. Subchondritic ԐHf(2.70 Ga) of zircon and ԐNd(2.70 Ga) of whole-rock, along with their respective Hf-TDMC (3.6–3.2 Ga) and Nd–TDM (3.6 Ga) model ages, for both units, reflect the substantial crustal reworking of predominant Paleoarchean sources during the Neoarchean in the northeastern Amapá Block. In addition, U–Pb dating on titanite crystals from the Guianense orthogneiss yielded an unprecedented ∼2.05 Ga age, arguing that a Late Rhyacian metamorphic event affected the Archean orthogneisses. This 2.05 Ga event coincides with the emplacement of Rhyacian granitoids in the area, as well as with the 2.06–2.04 Ga late orogenic HT granulite facies and amphibolite facies metamorphic events identified, respectively, at the northern border and in the central-southern portion of the Amapá Block.

Microstructural and isotopic analysis of shocked monazite from the Hiawatha impact structure: development of porosity and its utility in dating impact craters

U–Pb geochronology of shocked monazite can be used to date hypervelocity impact events. Impact-induced recrystallisation and formation of mechanical twins in monazite have been shown to result in radiogenic Pb loss and thus constrain impact ages. However, little is known about the effect of porosity on the U–Pb system in shocked monazite. Here we investigate monazite in two impact melt rocks from the Hiawatha impact structure, Greenland by means of nano- and micrometre-scale techniques. Microstructural characterisation by scanning electron and transmission electron microscopy imaging and electron backscatter diffraction reveals shock recrystallisation, microtwins and the development of widespread micrometre- to nanometre-scale porosity. For the first time in shocked monazite, nanophases identified as cubic Pb, Pb3O4, and cerussite (PbCO3) were observed. We also find evidence for interaction with impact melt and fluids, with the formation of micrometre-scale melt-bearing channels, and the precipitation of the Pb-rich nanophases by dissolution–precipitation reactions involving pre-existing Pb-rich high-density clusters. To shed light on the response of monazite to shock metamorphism, high-spatial-resolution U–Pb dating by secondary ion mass spectrometry was completed. Recrystallised grains show the most advanced Pb loss, and together with porous grains yield concordia intercept ages within uncertainty of the previously established zircon U–Pb impact age attributed to the Hiawatha impact structure. Although porous grains alone yielded a less precise age, they are demonstrably useful in constraining impact ages. Observed relatively old apparent ages can be explained by significant retention of radiogenic lead in the form of widespread Pb nanophases. Lastly, we demonstrate that porous monazite is a valuable microtexture to search for when attempting to date poorly constrained impact structures, especially when shocked zircon or recrystallised monazite grains are not present.

Sedimentology, geochronology and provenance of the late Permian and Triassic Mitu Group in Peru—The evolution of continental facies along a transform margin

AbstractIn the late Permian and Triassic, the continental Mitu Group formed in extensional basins along the length of the Cordillera Oriental and Altiplano of present‐day Peru. Given the presence of coeval arc systems only in northern Chile and southern Ecuador but not in Peru the tectonic setting of the Mitu basin has been interpreted variably as orthogonal continental rift, sinistral transtensional rift, aulacogen and back‐arc basin. The Mitu Group comprises continental mass flow and alluvial fan, fluvial, aeolian and minor lacustrian facies and hosts thick piles of subalkaline and alkaline intermediate and felsic ignimbrites and mafic lavas. The age of the Mitu Group had originally been established as ranging from the Late Permian to the late Triassic on the basis of structural considerations and scarce biostratigraphic data. Recently, U–Pb zircon ages from ignimbrites and sedimentary rocks have been taken to constrain the Mitu Group to the Middle and Late Triassic. We performed a sedimentological, heavy mineral, and zircon geochronological and Lu‐Hf isotope study of the Mitu Group in 14 sections mainly in southern and central Peru, and one section in northern Peru. Ten new U–Pb concordia ages on ignimbrites intercalated in the Mitu Group successions offer a new robust stratigraphic framework and constrain the stratigraphy of the Mitu Group between 260 and 205 Ma. In combination with maximum likelihood ages of deposition derived from detrital zircon, U–Pb geochronology places the deposition of the Mitu Group between ca. 270 and 194 Ma (lower Guadalupian into the Sinemurian). Detrital zircon U–Pb age distributions and heavy mineral assemblages reflect a strongly recycled Precambrian Amazonian and Palaeozoic proto‐Andean provenance. The Palaeozoic detrital age patterns are highly variable, and temporally and spatially random. A local provenance can generally not be identified. εHf(t) values in zircon obtained from ignimbrites and sedimentary rocks indicate variable degrees of crustal recycling. In the course of the Palaeozoic, εHf(t) values become on average progressively less negative, with a large proportion particularly of Mitu age zircons' εHf(t) values encompassing less evolved and moderately juvenile compositions. Along strike of the basin stratigraphic thicknesses, and rates and times of accumulation vary strongly with larger thicknesses and rates being registered in southern Peru. This suggests that the Mitu basin had been divided into a number of subbasins with individual histories of subsidence, accumulation, and volcanism. Absent a magmatic arc, late Permian–Triassic Peru evolved in a sinistral plate tectonic and regional framework expressed particularly in the sinistral Late Gondwanide orogeny predating the Mitu Group. We interpret that a sinistral transform fault linked the subduction zones in southern Ecuador and northern Chile and that the Mitu Group basin has formed by sinistral transtensional and transpressional movements along a related transcurrent fault inboard of the transform margin. In the broader framework of the accretionary Terra Australis orogen along the western Gondwana margin, this interpretation fits its evolution in the Andean segment in an internally consistent way.