Geochronological Investigations Research Articles

Ordovician tectonic transition from passive margin into peripheral foreland in the southern Ordos: A diagnostic insight into the closure of Erlangping Ocean between the North Qinling Arc and North China Block

AbstractAchieving a reliable closure time of a back‐arc ocean is an essential aspect in studies on detailed tectonic processes of an active continental margin and arc–continent collision. This is particularly the case for the northern Qinling Orogen, which records the accretion of the North Qinling Arc (NQA) onto the North China Block (NCB) after the Erlangping back‐arc ocean closure. Sedimentological, petrological and geochronological signatures from the Ordovician successions in the southern Ordos reveal a tectonic transition from passive continental margin to peripheral foreland in the southern NCB at the beginning of Katian. Sedimentological and geochronological investigations reveal an abrupt shift of accelerating basin subsidence and deepening at the earliest Katian, separating ca. 300‐m‐thick shallow‐marine carbonate shelf assemblages from overlying ca. 2000‐m‐thick deep‐water carbonate slope and turbidite associations. Zircon age spectra of the Katian turbidites are characterized by early‐Palaeozoic and Neoproterozoic age clusters, which are different from those of the Middle Ordovician quartz arenites sourced merely from the NCB basement. Instead, these age patterns match well with those of the coeval successions in the northern NQA, indicating a spatially linked abyssal deposystem. Stratigraphic architecture deciphers a typical foreland basin geometry, involving, from south to north, northward‐propagating turbiditic wedge, northward‐backstepping carbonate slope and progressively shoaling carbonate platform, embodying foredeep, forebulge and backbulge, respectively. These characteristics of basin‐fill evolution reflect the northward migration of the flexural wave as a dynamic response to the northward expansion of the thickened NQA thrust wedge. Together with the other geological and geochronological data, our new insights indicate a southward subduction polarity of the Erlangping back‐arc oceanic crust followed by its termination at ca. 450 Ma, which was earlier than that of the main Proto‐Tethyan Shangdan Ocean between the NCB and South China Block. Our new data provide an updated view of the complex history of the Proto‐Tethys closure during the Gondwana assembly.

A Late Cambrian Continental Convergent Margin in the North Qilian Orogenic Belt, Northwestern China: Geochemical and Geochronological Evidence from Hongtugou Mafic Rocks

The tectonic setting and subduction polarity of the early Paleozoic North Qilian Orogenic Belt (NQOB) in northwestern China is poorly constrained due to complex tectonic deformation. Mafic and ultramafic rocks in the South Ophiolite Belt of the NQOB are interpreted to be middle ocean ridge ophiolite or suprasubdcution zone ophiolite. To address this, we have conducted geochemical and geochronological investigations of the mafic rock sequence (cumulate gabbros, diabases, isotropic gabbros, and basalts) in Hongtugou in the South Ophiolite Belt. Trace element characteristics of the pillow basalts and the isotropic gabbros with enrichment of Th and La relative to Nb on the N-MORB normalized multi-element diagram are consistent with a suprasubduction setting, where similarities with the Panamanian proto-arc rocks suggest they formed shortly after subduction initiation. Major element modelling for cumulate gabbros and basalts indicates the hydrous condition of crystallization which further supports a suprasubduction setting. The Proterozoic zircon crystals captured in a cumulate gabbro and a diabase suggest this suprasubduction zone is a continental convergent margin. A weighted mean zircon SHRIMP age of 507 ± 6 Ma from an isotropic gabbro is consistent with crystallization ages of other mafic rocks in this belt. This suggests the North Qilian oceanic lithosphere subducted beneath the continent in the late Cambrian. Mafic rocks in this study along with the serpentinized peridotite do not fall into the category of ophiolite, despite displaying an ophiolite sequence.

Palaeogeography and provenance transition of Precambrian–Cambrian unconformity at the southern margin of the North China Craton

The Precambrian–Cambrian unconformity boundary is ubiquitous and globally documented, which is of great significance to studying major global scientific issues such as environmental transition, positions of the supercontinent, Cambrian explosion, and global geochemical cycle. However, few studies have been on the palaeogeographic features and provenance changes of the Precambrian–Cambrian unconformity boundary at the southern margin of North China Craton (NCC). This uncertainty hampers us from further understanding the early climate change and plate evolutionary history of the NCC. To improve the knowledge of the palaeogeographic features and provenance changes of the Precambrian–Cambrian unconformity boundary on the southern margin of the NCC, the Luoquan and Xinji formations at the southern margin of the NCC were selected as the research object integrated with sedimentary petrology, trace elements, and detrital zircon geochronology investigation. The results suggest a depositional gap between the Luoquan and Xinji formations. The former is glacial to deglaciation deposits, while the latter is shallow to coastal sediments. Compared with the sedimentary stratum of the South China Craton, the Luoquan Formation lacks at least one transgression and regression cycle. The Xinji Formation has detrital zircon age peaks of 1200, 1100, and 950 Ma, unavailable in the Luoquan Formation. The provenance areas of the Luoquan and Xinji formations have changed, indicating that the provenances of the Luoquan Formation belong to the NCC and part of the provenances of the Xinji Formation belong to the North Qinling Terrane (NQT). The trace element data indicate that the tectonic setting of the Luoquan and Xinji formations has changed from a continental island arc to a passive continental margin. It is suggested that the NCC be involved in the evolution of Rodinia supercontinent, which leads to the change of the provenance and tectonic setting. This research provides an explanation for the Precambrian–Cambrian tectonic history, basin development, stratigraphic correlation, and palaeogeographic reconstruction of NCC.

ISOTOPE-GEOCHRONOLOGICAL U-Pb (SHRIMP-RG) AND GEOCHEMICAL CHARACTERISTICS OF ZIRCON FROM RARE METAL (W, Li) APLITE GRANITE OF THE MURMANSK BLOCK

The article presents the results of geochemical and isotope geochronological investigations of zircon from aplite granite with scheelite and spodumene mineralization. The mineralized granite is of scientific interest as a possible source of matter for the world’s largest spodumene pegmatite Kolmozero deposit, and for the tungsten mineralization in the Kola metallogenic province. As of now, no data are available on the age of the spodumene pegmatite Kolmozero deposit, and the estimated age of possible ‘mother’ granites is 2.7 to 1.9 Ga. Zircons from the studied granite are characterized by internal inhomogeneity, with less altered central parts and intensely altered outer parts. Uranium content in zircon is changeable as well, being 2–3 times higher in the altered parts of the grains. Zircons are of two geochemical types – magmatic and metasomatic. New isotope geochronological U-Pb data for zircon indicate the 2723 ± 11 Ma age of crystallization of the aplite granite, and 2207 ± 15 Ma age of alteration and, probably, of the formation of spodumene and scheelite mineralization. These figures may indicate the age of the rare metal spodumene pegmatite in the Kolmozero deposit.

Neoarchean–Paleoproterozoic HP-HT metamorphism in the Bhopalpatnam granulite belt, Bastar Craton (India): Insights from phase equilibria modelling and monazite geochronology

The Bhopalpatnam Granulite Belt (BGB), located within the southwestern part of the Bastar Craton (India), is composed of granite gneisses with numerous enclaves of high-grade rocks, including quartzofeldspathic gneisses, orthopyroxene-free mafic granulite, and garnetiferous two-pyroxene granulite. In addition, the marginal parts of the belt preserve patches of garnetiferous psammo-pelitic gneisses. New petrographic, metamorphic (conventional and phase equilibria modelling) and monazite geochronological investigations of these high-grade rocks constrain the conditions and timing of granulite-facies metamorphism and enable to demonstrate the tectonometamorphic history of the BGB. The garnet-bearing psammo-pelitic gneisses underwent medium pressure-high temperature (MP-HT) and retrograde metamorphism at P-T conditions of 9.4 ± 1.0 kbar/797 ± 39 °C and 5.1 ± 1.1 kbar/697 ± 69 °C, respectively. The quartzofeldspathic gneisses experienced high pressure-high temperature metamorphism (HP-HT; 12.0 ± 0.9 kbar/785 ± 48 °C), followed by retrogressive cooling at 10.6 ± 0.8 kbar/683 ± 42 °C. The orthopyroxene-free mafic granulites and two-pyroxene granulites witnessed HP-HT metamorphism at 11.0 ± 0.8 kbar/827 ± 45 °C, followed by MP-HT metamorphism at 8.4 ± 0.9 kbar/803 ± 66 °C and retrograde cooling at 5.7 ± 1.5 kbar/555 ± 80 °C. The U–Th–total Pb dating of monazites from the quartzofeldspathic gneiss yielded peak (HP-HT) metamorphic age at 2588–2461 Ma, MP-HT metamorphism at 2499–2308 Ma, and thermal events associated with granite magmatism at 2292–2111 Ma. Based on similar clockwise P–T history and Late Archean-Early Mesoproterozoic tectonothermal events, we suggest that the Bhopalpatnam and Karimnagar granulite belts evolved coherently until the Mesoproterozoic time and eventually rifted apart during the development of the Pranhita-Godavari rift basin between ∼ 1.6–1.4 Ga. The HP-HT and MP-HT granulite-facies metamorphic episodes identified in the BGB are correlated with the continent–continent collision between Bastar and eastern Dharwar cratons.

Late Neoarchean high-grade regional metamorphism in the eastern North China Craton: New constraints from monazite dating in northern Liaoning

Late Neoarchean tectono-thermal events in the eastern North China Craton have been comprehensively characterized in terms of mineralogy, petrology, geochemistry, metamorphic evolution, geophysics and geochronology. The Late Neoarchean metamorphic event is debated to correlate with vertical and/or horizontal tectonism, potentially an indicator as initiation of plate tectonics. With a growing number of zircon geochronological investigations, the precise timing of the Late Neoarchean metamorphic event should be questioned due to the complexity of post-Archean multiple tectono-thermal overprints. Here we demonstrate the capabilities of LA–ICP–MS U–Pb and LA–MC–ICP–MS Sm–Nd isotope measurements applied to monazite endowed with LREE-enriched geochemical peculiarities as well as zircon U–Pb–Hf isotope, to reevaluate the Late Neoarchean metamorphic event in northern Liaoning, eastern North China Craton. Monazites within the biotite two-feldspar gneiss from the Qingyuan greenstone belt are Ce-monazite in composition, with negligible amounts of the huttonite and brabantite components. The internally homogeneous and anhedral rounded granular monazites show textural equilibration in contact with plagioclase, quartz, and K-feldspar, indicating that monazite is a product of metamorphism. The apparent ages and Eu/Eu* values of monazite analyses show a positive correlation, which confirms the continuous formation of monazite and coeval feldspar during the metamorphic evolution. These metamorphic monazites yield a weighted mean 207Pb/206Pb age of 2493 ± 8 Ma and an upper intercept age of 2496 ± 8 Ma, consistent with the metamorphic zircon dates of 2492–2468 Ma. These results, along with monazite Sm–Nd isochron of 2459 Ma, thus provide a critical clue to understand the Late Neoarchean high-grade regional metamorphism in the eastern North China Craton. As a consequence, the Qingyuan greenstone belt was only partially influenced by the Paleoproterozoic tectono-thermal events, not largely as previously thought by some workers.

Assessing the robustness of geochronological records from the Arabian Peninsula: A new synthesis of the last 20 ka

The timing and extent of Late Quaternary climate and environmental changes in the Arabian Peninsula were critical for human demographic changes. High resolution palaeoclimate archives suggest that monsoon variability was a key driver of change, but these records obscure environmental heterogeneity at finer spatial scales. Recent decades have seen the production of new, chronometrically controlled records from a range of new sites, with a greater spatial coverage. However, records often appear contradictory vis a vis the hydro-climatic state of the environment and so synthesising them can be challenging. To enhance collation and interpretation of records we have compiled a new database of terrestrial and coastal environmental chronologies for the Arabian Peninsula over the last 20 ka, a key period of marked hydrological change. The SCAPE (Synthesising Chronologies for Arabian PalaeoEnvironments) database contains records that include >700 luminescence and radiocarbon ages. Robustness criteria are applied to identify the utility of each data source in regional hydroclimate reconstruction. The most robust data provide a unique synthesis of 20 ka of palaeoenvironmental change at sub-regional scales. We find that Arabian environmental change does not necessarily follow broad narratives of ‘wet’ or ‘dry’ states but rather the climatic signal is a mosaic of substantial landscape heterogeneity. SCAPE also highlights regions in which more high-quality geochronological investigation is warranted to refine both temporal and spatial records, signifying important avenues for future research.

Tectonic transformation from orogen-perpendicular to orogen-parallel extension in the North Himalayan Gneiss Domes: Evidence from a structural, kinematic, and geochronological investigation of the Ramba gneiss dome

The North Himalayan Gneiss Domes consist of a series of isolated domes cored by leucogranite and/or gneiss body. The Ramba dome core comprises gneiss and two-mica granite, mantled mainly by schist sequences, and the degree of deformation decreases outward from pervasive mylonitization to weak mylonitization. Pre-doming deformation is characterized by the formation of regional-scale top-to-the-north shearing (D1) in the Tethyan Himalayan Sequence, locally preserved strong folding (F1), and intrusion of 46.3 Ma-40.6 Ma leucogranite. The D2 deformation consists of map-scale ductile shearing fabrics and top-to-the-north detachment along the South Tibetan Detachment System (STDS) in the middle and lower units of the dome. Intrusion of 26.1–21.0 Ma syn-kinematic leucogranite veins in the schist units occurred during the D2 north-directed extensional episode. The D3 deformation was characterized by top-to-the-east shear during 12.0–10.2 Ma, concurrent with the initiation of activity in the N–S-trending Yadong–Gulu rift. The latest deformation (D4) involved the ascent of two-mica leucogranite through diapirism at about 8.17 Ma. Microstructural analyses, as well as quartz c-axis fabrics measured by EBSD (Electron backscatter diffraction), indicate deformation temperature ranges of D2 extensional shearing from 500 to 600 °C in the middle unit of the Ramba dome to >600 °C in the lower unit. These new data imply that this type of doming in the Ramba region represents a classic type of transformation from orogen-perpendicular to orogen-parallel extensional deformation during Miocene India–Asia collision.

Petrological and geochronological investigations on the individual nappes of the Meran-Mauls nappe stack (Austroalpine unit/South Tyrol, Italy)

Abstract The Meran-Mauls nappe stack is part of the Austroalpine unit in South Tyrol (Italy). There it holds a special position directly in front of the Southalpine Dolomites indenter and west of the Tauern Window. It is situated in the hanging wall of the Southalpine unit, above a NW dipping segment of the Periadriatic fault system, namely the Meran-Mauls fault. Also all other sides are defined by Oligocene-Miocene strike-slip and normal faults. Based on recent mapping the Meran-Mauls nappe stack consists of three nappes separated by NW to NNW dipping shear zones. The lowermost nappe in the southwest is represented by the Schenna (Scena) unit. It is overlain along the Masul shear zone by a nappe consisting of the Hirzer (Punta Cervina) unit and the Pens (Pennes) unit including Triassic (meta)sediments. Separated by the Fartleis fault the St. Leonhard (San Leonardo) unit forms the uppermost nappe. The aim of this study is to describe the individual units and the separating structural elements more properly, based on new structural, petrological, geothermobarometric and geochronological data and to compare these units to other Austroalpine elements in the vicinity. Sillimanite-bearing paragneiss, minor amphibolite and quartzite as well as a distinct marble layer close to its base characterise the Schenna unit. Further, it contains pegmatite dikes, presumably Permian in age. Amphibolite-facies P-T conditions of c. 0.55 ± 0.15 GPa and 600 ± 100°C are thus correlated with a Permian metamorphic imprint. The Masul shear zone mostly consists of mylonitic paragneiss of the Hirzer unit. It is pre-Alpine in age and probably formed during the Jurassic. For the paragneiss of the Hirzer unit upper greenschist- to amphibolite-facies metamorphic conditions of 0.4-0.50 ± 0.15 GPa and 550 ± 70°C are attributed to the Variscan tectonometamorphic imprint. The whole Pens unit represents a shear zone. Due to the occurrence of Permotriassic (meta)-sediments within this shear zone, it is an Alpine structure, as well as the bordering Fartleis fault. Rb/Sr biotite ages yield sometimes partly reset pre-Alpine age values in the whole Meran-Mauls nappe stack, indicating a pervasive anchizonal to lowermost greenschist-facies metamorphic overprint during the Eoalpine tectonometamorphic event. Tectonostratigraphically the Meran-Mauls nappe stack can be attributed to the Drauzug-Gurktal nappe system. The latter forms the uppermost structural element of the Austroalpine nappe stack and thus only shows a weak Eoalpine metamorphic overprint. With respect to its special lithologic composition the Schenna unit can be correlated with the Tonale unit in the southwest and the Strieden-Komplex in the east.

Paleomagnetic and geochronological results of the Risong Formation in the western Lhasa Terrane: Insights into the Lhasa-Qiangtang collision and stratal age

To constrain the Lhasa-Qiangtang collisional age, a combined paleomagnetic and geochronologic investigation has been performed on the Risong Formation redbeds and volcanic rocks in the Wuma area of the western Lhasa terrane (LT). A U-Pb dating result from the volcanic interlayer at the bottom of the Risong Formation redbeds yields the youngest weighted mean 206 Pb/ 238 U age of 110.6 ± 1.3 Ma, and the two detrital zircon U-Pb chronological results from the Risong Formation redbeds provide the youngest weighted mean 206 Pb/ 238 U age of 106.9 ± 3.3 Ma (the top strata of section A) and 120.5 ± 3.0 Ma (the top strata of section D), respectively. Our new geochronological results suggest that the studied Risong Formation redbeds formed during the Early Cretaceous, rather than during the Late Jurassic as assigned by 1:250,000 scale Wuma regional geological survey report. Characteristic remanent magnetization (ChRM) directions from two different limbs of folds concentrate in two different groups, and both site-mean directions pass positive fold tests. One group including 32 sites yields a tilt-corrected site-mean direction that is D = 356.1°, I = 33.1° with α 95 = 2.3°, corresponding to a paleopole at 75.2°N, 278.1°E with dp/dm = 1.5°/2.6° and a paleolatitude of 18.1° ± 1.5°N for the study area (32.41°N, 83.39°E). The other group including 24 sites provides a tilt-corrected site-mean direction that is D = 60.5°, I = 48.5° with α 9 5 = 2.0°, corresponding to a paleopole at 38.7°N, 159.6°E with dp/dm = 1.7°/2.6° and a paleolatitude of 29.5° ± 1.7°N for the study area (32.41°N, 83.39°E). After syntectonic-sedimentation-correction, the Fisherian site-mean inclination of 41.7° ± 1.6° and corresponding paleolatitude of 24.0° ± 1.2°N obtained from 56 sites of two different limbs are consistent with the inclination-only mean of 39.6 ± 3.1° and corresponding paleolatitude of 22.5° ± 2.2°N deduced from the ChRM directions of all the 56 paleomagnetic sites, suggesting that the observed inclination discrepancy of the Risong Formation redbeds may result from the syntectonic sedimentation. Comparing the Early Cretaceous paleolatitudes observed from the western LT with those from the western Qiangtang terrane shows insignificant paleolatitude difference, supporting that the Lhasa-Qiangtang collision in the western part occurred at or before the Early Cretaceous. • New dating shows the sampled Risong Formation redbeds formed during the Early Cretaceous rather than the Late Jurassic. • The Wuma sampling area was located at ~22.5° ± 2.2°N during ~120.5–106.9 Ma. • The Lhasa-Qiangtang collision occurred before ~120 Ma in the western part.

Northwest Africa 6486: Record of large impact events and fluid alteration on the L chondrite asteroid

AbstractWe report the results of petrological, geochemical, and geochronological investigations of the unusual K‐rich L chondrite melt rock Northwest Africa 6486 (NWA 6486). The rock has slightly fractionated siderophile elements and a mostly unfractionated L chondrite pattern of lithophile elements with the exceptions of enrichments in K and Rb and chondritic Sr abundance similar to the K‐rich inclusions found in the ordinary chondrites and indicating a fractionation of alkaline elements through the vapor. We suggest that NWA 6486 and related K‐rich chondritic inclusions were formed in situ on the OC parent bodies and that K and Rb enrichment of these rock most probably is a result of the selective impact evaporation of volatile alkali elements followed by the reaction of a vapor with shock melt. NWA 6486 recorded a breakup event of the L chondrite parent asteroid at 470 Ma during which it was formed. Unusual veins, depleted in K, Na, Ca, and Al relative to the host rock were found in NWA 6486. We suggest that NWA 6486 was affected by aqueous fluids that produced alteration zones depleted in a feldspar component on the walls of opened fractures. The melt veins could be formed during a subsequent impact event by in situ melting of the fracture walls or due to decomposition of an injected supercritical aqueous silicate fluid. The aqueous alteration and the second impact event had no detectable effect on Ar and oxygen isotopic systems. Cosmic ray exposure ages indicate that NWA 6486 was ejected from its parent asteroid ~3–4 Ma ago.

Review of the Geology of the Arun-Tamor Region, Eastern Nepal: Present Understndings, Controversies and Research Gaps

Systematic study of the eastern Nepal Himalaya was started after 1950 when Nepal opened up for foreigners. Thereafter, several geological studies have been carried out in the Arun-Tamor region of eastern Nepal Himalaya. The Tibetan-Tethys sedimentary sequence, the Higher Himalayan amphibolite to granulite facies metamorphic crystalline sequence, the Lesser Himalayan sedimentary and greenschist facies metasedimentary sequences, and the Siwalik foreland molassic sedimentary sequence are the four major tectonic units of this area. The individual nomenclature schemes of stratigraphic units, the correlational dispute, the positions and interpretations of regional geological structures are some examples that have created controversies regarding the lithostratigraphy and structural arrangements. The difference in age and genesis of the Main Central Thrust and its effects in the metamorphism of the eastern Nepal Himalaya are the exemplification of the contradiction in the interpretation of the tectonometamorphic history. There is a gap in research in the tectonics and episodic metamorphic evolution of the area owing to the bare approach in the microstructural and geochronological investigation. Future investigations should be focused on solving the above mentioned controversies and narrowing down the research gaps in tectonic and metamorphic evolution.

Downdip Development of the Ni-Cu-PGE-Bearing Mafic to Ultramafic Uitkomst Complex, Mpumalanga Province, South Africa

The about 2055-Ma-old mafic to ultramafic Uitkomst Complex in the Mpumalanga Province of South Africa hosts the low-grade-large-tonnage Ni-Cu-PGE deposit, Nkomati. The complex is regarded to represent a satellite to the Bushveld Complex and a feeder to an eroded magmatic reservoir in the southeast. Aeromagnetic surveys and previous drilling indicated an overall northwestern-downdip extension of the complex, but the question is to what extent and in which expression can the complete intrusion be found under cover in the northwest? Answering this, a mineralogical, geochemical and geochronological investigation of a borehole intersection of the whole complex at Little Mamre was carried out, using petrography, XRF, EPMA and LA-ICP-MS U–Pb analyses of zircons for age determination. Although the total thickness of the rock units is larger than to the southeast, emplacement, litho- and mineral chemistry trends, expression of alteration mineralogy and style of sulphide mineralisation are similar. The amount of sulphide mineralisation is on average less than in the southeast. The upper ultramafic unit contains, more frequently, pegmatoidal sections, and the Chromitiferous Harzburgite unit has less massive chromitite layers than the southeastern parts of the complex, whereas the gabbro(-norite) units contain more interstitial liquid with late-stage minerals. The findings confirm that the anvil-shaped intrusion in cross section continues with increased thickness towards northwest at a shallow dip; although approaching the deeper part of the igneous reservoir, mineral compositions are partially more evolved. The overall mineralogical consistency downdip supports a situation of multiple magma replenishment along a flat-lying, northwest–southeast trending conduit, resulting in an evolved cumulus mineral assemblage in the upper part.

Geochemical, isotopic and U-Pb geochronological investigation of the late Cretaceous Karaçayır carbonatite (Sivas, Turkey): Insights into mantle sources within a post-collisional tectonic setting

The Karaçayır carbonatite is located within the northeastern edge of the Central Anatolian Crystalline Complex, and represents one of the significant rare earth element (REE) mineralizations in Turkey. The carbonatite intrudes the associated syenite as dikes and veins, and is mainly composed of medium- to coarse-grained calcite, and lesser amounts of apatite, mica, amphibole, pyroxene, and magnetite. This study reports new and combined geochemical, radiogenic (Sr, Pb), stable (O and C), and for the first time B and Nd isotopic ratios and in-situ U-Pb age data from the Karaçayır carbonatite. The carbonate separates from the Karaçayır carbonatite are characterized by high Ba (30 to 484 ppm), Sr (613 to 9106 ppm) and total rare earth element (TREE) concentrations (172 to 2892 ppm), which are consistent with geochemical signatures for mantle-derived carbonate melts. The combined δ18OV-SMOW (+10.7 to +19.0 ‰), δ13CV-PDB (−2.9 to +1.8 ‰) and δ11B (−13.9 to −2.9 ‰) data for a majority of the carbonate separates investigated here in contrast indicate that they are the result of open system behavior involving contamination by the country rock (marble), and/or sub-solidus, low temperature alteration. Based on the combined stable and radiogenic isotopic signatures for the least altered/contaminated samples, the carbonatite magma at Karaçayır most likely represents a low-degree partial melt of an isotopically heterogeneous (metasomatized) mantle source containing EM (enriched mantle)-like components and recycled crustal carbon; the latter finding is consistent with those recorded for young (<300 Ma old) carbonatites worldwide. A weighted mean 206Pb/238U age of 69.3 ± 1.7 Ma is obtained for apatite from the Karaçayır carbonatite, and indicates its emplacement within a post-collisional tectonic setting during the late Cretaceous.

Reconstructing the middle to late Pleistocene explosive eruption histories of Popocatépetl, Iztaccíhuatl and Tláloc-Telapón volcanoes in Central México

The Sierra Nevada Volcanic Range (SNVR), which includes Popocatépetl, Iztaccíhuatl and Tláloc-Telapón volcanoes, has been the source of multiple large explosive eruptions that have dispersed tephra across central México. Several eruptions since 40 ka have previously been described, particularly from Popocatépetl, the southernmost volcano of the range. However, the longer-term eruption history of the SNVR is poorly understood, due to challenges with correlating limited exposures of older pyroclastic sequences, and in discriminating between tephras derived from different sources. Here we describe two extensive exposures located between Popocatépetl and Iztaccíhuatl volcanoes, which provide a more complete and longer-term explosive eruption record of the SNVR: the Nepopualco and Xalitzintla tephra sequences. A detailed tephrostratigraphic survey, together with new 40Ar/39Ar geochronological analyses and glass geochemistry, has permitted the characterization of identified eruption units further leading to the determination of geochemical fields for each volcano and the subsequent discernment of volcanic sources. Our results show that, since the collapse of the Los Pies Cone, which destroyed the Paleo-Iztaccíhuatl edifice at 631 ± 44 ka (2σ), Iztaccíhuatl has produced at least 6 explosive rhyolitic eruptions. After coeval activity with Popocatépetl, between ~600 and ~500 ka, Iztaccíhuatl's explosive activity ceased while Popocatépetl's continued until present day. Popocatépetl has produced at least 27 medium to large explosive eruptions (inferred VEI 4–6), commonly of andesitic to dacitic compositions. Some of these eruptions deposited pumice fallout of >1 m thick in both the Nepopualco and Xalitzintla sequences (e.g. the 339 ± 16 ka [2σ] NT-23/WRT-7 eruption), suggesting that Popocatépetl has produced several eruptions similar in magnitude to the well-known ~14 ka Tutti Frutti Pumice (a VEI 6 eruption with a ~5 km3 tephra volume). The Popocatépetl and Iztaccíhuatl tephras are interbedded with deposits from more distal volcanoes, including some mafic to intermediate products of unknown sources (possibly from nearby monogenetic cones) and tephras related to the late Pleistocene eruptions of Tláloc-Telapón (including the tephra layer produced by the San Valentin Ignimbrite, recently 40Ar/39Ar dated in this study at ~102 ka; 2σ). Our new chemical, stratigraphic and geochronologic investigations of these pyroclastic deposits, predominantly from Popocatépetl and Iztaccíhuatl, provide information on the scale and frequency of medium to large magnitude explosive eruptions over a longer-time period than currently known and that have had potential to disperse tephra across central México since the middle to late Pleistocene. This new data can be used to determine the source of further unknown tephras in the region as well as to better assess the volcanic hazard to the densely populated megalopolis of México City.

Tectono-sedimentary evolution of the Paleoproterozoic succession of the Carajás Basin, southeastern Amazonian Craton, Brazil: Insights from sedimentology, stratigraphy, and U–Pb detrital zircon geochronology

The Carajás Basin, situated in the southeastern Amazonian Craton in northern Brazil, hosts a > 6 km thick pile of volcano-sedimentary rocks deposited from the Neoarchean to the early Paleoproterozoic eras (ca. 2.75–2.06 Ga). It is claimed that this basin started as a rift configuration in which volcanic and banded iron formation strata were deposited during the Neoarchean Era. However, the tectono-sedimentary evolution of this basin through the Paleoproterozoic Era remains unexplored. Based on a sedimentological, stratigraphic, and U–Pb detrital zircon geochronological investigation of a marine to a fluvial succession of this basin, this study suggests that the Carajás Basin evolved in a foreland setting during the early Paleoproterozoic Era. Probably, the Carajás Foreland Basin was sedimented in the underfilled stage (ca. 2.5–2.3 Ga) by deep-marine strata (i.e., glaciogenic submarine fan deposits), likely from the Siderian–Rhyacian Serra Sul Formation. Subsequently, shallow-marine deposits of the Azul Formation were deposited in the filled stage (ca. 2.3–2.1 Ga), followed by deposition of the fluvial to alluvial deposits of the Águas Claras and Gorotire formations during the overfilled stage (ca. 2.1–1.9 Ga). The Azul and Águas Claras formations were supplied mainly by Meso- to Neoarchean source rocks, whereas Paleoproterozoic and Paleoarchean rocks played the role of a subordinate source of sediments. The youngest U–Pb age cluster, at ca. 2.27 Ga, is interpreted as its maximum depositional age. In terms of paleogeography, the occurrence of Rhyacian zircon grains in the Azul Formation deposits suggests, at least partially, a connection between the Carajás Protocontinent with the Bacajá Domain during that period. The integrated data corroborate the hypothesis that the foreland basin was formed in a scenario of collision between the Bacajá Domain and the Carajás Protocontinent during the Transamazonian Cycle. These dramatic upheavals are directly related to the configuration of the Columbia Supercontinent, which initially led to the emergence of the Carajás Foreland Basin, and soon after, the cratonization of proto-Amazonia

Using a ‘speedy’ unspiked K–Ar methodology to investigate age patterns in giant mafic dyke swarms

Abstract We present the results of a geochronological investigation that was conducted on mafic rocks (dolerites) that form two giant dyke swarms in NE Brazil, whose intrusion was correlated to the break-up of the West Gondwana supercontinent and the opening of the Atlantic Ocean. Despite their impressive dimensions, these swarms lack any geological information, which motivated us to develop a speedy, low-cost analytical protocol, modified from the Cassignol unspiked K–Ar technique, to define age patterns. The results were interpreted in light of basic statistical treatments and, although some limitations were mostly related to grain-size heterogeneities, they successfully matched other 40 K-based ages (conventional K–Ar and 40 Ar/ 39 Ar) reported in the literature and showed that the dyke swarms share two main age intervals of rock formation identified as the Early Cretaceous and Late Jurassic.

The Požarevac loess–paleosol sequence: a record of increased aridity in the south‐eastern margin of the Carpathian Basin during the last 350 ka

ABSTRACTThe loess sequence preserved in the Požarevac brickyard in north‐eastern Serbia comprises eight loess units separated by seven paleosols. Geochronological investigation using amino acid racemization and luminescence dating support stratigraphic correlations of loess units L3, S2LL1 and L1 at the Požarevac section with loess of glacial cycles E [Marine Isotope Stage (MIS) 10], D (MIS 9–8), C (MIS 7–6) and B (MIS 5–2) across central Europe. Correlation with the marine oxygen‐isotope stratigraphy and associated paleoclimatic inferences are further supported by magnetic susceptibility, particle size and carbonate content measured in Požarevac sediments. Malacological investigations at the Požarevac section reveal the continuous presence of the Chondrula tridens and Helicopsis striata faunal assemblages throughout the last 350 ka. The loess malacological fauna, which is characterized by the complete absence of cold‐resistant and cold‐preferring species, suggests a stable, dry and relatively warm glacial and interglacial climate, compared with other central European loess localities. Together these data suggest that the south‐eastern part of the Carpathian (Pannonian, Middle Danube) Basin was a refugium for warm‐preferring and xerophilous land‐snails during the generally unfavorable glacial climates of the late Middle and Late Pleistocene.

Bashkirian taeniate bisaccate pollen‐dominated palynological assemblage from northwestern Junggar Basin, Xinjiang Province, China

The Early Pennsylvanian land plant community is characterized by widespread forests consisting of lycopsids, ferns, and seeds fern in swampy wetlands. The contemporaneous palynological assemblages are mainly composed of spores of Lycospora spp., Densosporites spp., and Laevigatosporites spp. and monosaccate prepollen of Florinites spp. Taeniate bisaccate pollen appeared for the first time during the Early Pennsylvanian but never prevailed in the coeval palynological assemblages. Here, we report a taeniate bisaccate pollen‐dominated palynological assemblage from the Jiamuhe Formation of Well Ke 85 in the Zhongguai Uplift, northwestern Junggar Basin, Xinjiang. Nineteen miospore species belonging to 14 genera were identified from the assemblage. Based on their relative abundance, one assemblage is nominated informally as Calamospora breviradiata–Lunatisporites tersus (BT) assemblage. SIMS U–Pb geochronological analysis of zircons from two volcanic ash beds from the basal and upper parts of the formation yields weighted mean 206Pb/238U dates of 316.3 ± 2.2 Ma (MSWD = 1.2, n = 19) and 316.5 ± 2.8 Ma (MSWD = 0.85, n = 20) in an ascending order. A Bashkirian age is thus assigned to the BT assemblage. Our combined palynological and geochronological investigations reveal an early stage of the rise of conifers in the Kazakhstan Plate and offer a rare glimpse into the Early Pennsylvanian land vegetation in the subtropical area at the northern hemisphere.

Carboniferous sedimentary provenance and tectonic setting in the Darbut region of Western Junggar (NW China): evidence from mineralogy, geochemistry and detrital zircon U–Pb dating

The Carboniferous tectonic evolution of Western Junggar is crucial to understanding the subduction–accretion process of the Central Asian Orogenic Belt, but the nature of this setting is still controversial. In this work, composite mineralogical, geochemical and detrital zircon U–Pb geochronological investigations have been conducted on Carboniferous clastic rocks in the Darbut region. The chemical compositions and sedimentary features show low sediment maturity and limited recycling, suggesting short-distance transportation and rapid accumulation. The samples contain igneous rock debris, mainly andesite and small amounts of basalt and granite, and a heavy mineral assemblage of Zr + Ap + Aug + Hbl + iron-bearing minerals (Hem-Lm, Ilm, Mag and Py). The samples feature moderate ratios of Zr/Sc (average 15.47) and Th/Sc (average 0.61), and low ratios of La/Sc, Co/Th and La/Th, as well as low Hf content, suggesting intermediate to felsic arc-related igneous provenances. Detrital zircon grains from the clastic rocks show prominent age peaks in the Devonian and Carboniferous with positive ε Hf ( t ) values, indicating a consistent provenance associated with the Tiechanggou–Halaalate island arc. Combining the petrology, geochemistry and geochronology of the sedimentary and magmatic rocks, we conclude that the Darbut Carboniferous volcanic–sedimentary strata were deposited in a back-arc basin during c. 327–311 Ma. Supplementary material : Tables S1-S6 and Figures S1-S6 are available at https://doi.org/10.6084/m9.figshare.c.5357293