Pb Isotopic Ages Research Articles

The Oldest Vendian (Ediacaran) Fossils of Eurasia: U–Pb Isotope Age of the Basa Formation (Asha Group, Southern Urals)

U–Th–Pb (SHRIMP II) isotopic dating of accessory zircons from Vendian (Ediacaran) ash tuffs of the Basa formation section (Asha Group, Southern Ural) was performed for the first time. The concordant age obtained (573.0 ± 2.3 Ma) can be interpreted as the upper age limit of the onset of the Timan Orogeny in the structure of the Southern Urals. Fossil remains of Kuckaraukia multituberculata and several paleopascichnids, belonging to the genera Palaeopascichnus and Orbisiana, were previously found in the rocks of the Upper Basa Subformation, and the first occurrence of Arumberia banksi was established. The age obtained indicates a more ancient age of this Vendian assemblage compared to the previously accepted one.

Paleoproterozoic basement beneath the Eastern Cathaysia Block revealed by zircon xenocrysts from late Mesozoic volcanics

The Eastern Cathaysia Block located to the western side of the Pacific Plate was mainly covered by late Mesozoic intrusive and volcanic rocks, and the age and composition of its basement remain enigmatic. Here we firstly report U–Pb and Hf isotopes in zircon xenocrysts from late Mesozoic mafic sub-volcanic rocks in Pingtan Island, the east coastal area of the Eastern Cathaysia Block, with surprising Paleoproterozoic to Archean U–Pb isotopic ages. Neither ca. 1.8 Ga nor 1.0–0.7 Ga age peaks that are very typical in the outcropping rocks (including the Neoproterozoic sedimentary rocks in Pingtan Island) in the South China Block is identified. The two main Precambrian age peaks of 3.4–3.0 Ga and 2.6–2.2 Ga of these zircon xenocrysts are explained by contamination from the sediments in the unexposed basement. The youngest two Paleoproterozoic zircon xenocrysts may indicate the deposition of the basement close to ca. 1.8 Ga. The identification provides clues to the existence of the Paleoproterozoic basement in the Eastern Cathaysia Block. It may further suggest that the late Mesozoic (continental arc?) magmatism in the Eastern Cathaysia Block was most likely developed on and contaminated by the Paleoproterozoic basement.

Mantle Evolution of Asia Inferred from Pb Isotopic Signatures of Sources for Late Phanerozoic Volcanic Rocks

We present a systematic study of Pb isotope ages obtained from sources of the late Phanerozoic volcanic rocks from unstable Asia and also volcanic rocks and kimberlites from stable regions of the Siberian and Indian paleocontinents. In the mantle sources, we have recorded events of the Early, Middle, and Late epochs of the Earth’s evolution. Evidence on the Early epoch are preserved in sources of the protolithosphere and viscous lower protomantle likely generated from the Hadean magma ocean about 4.51 and 4.44 Ga and in sources of the viscous upper mantle that acquired low µ and elevated µ (LOMU and ELMU) signatures in the early Archean (4.0–3.7 Ga). The Middle and Late epochs are denoted by sources of the viscous upper mantle that was generated, respectively, in the late Archean-Paleoproterozoic (2.9–2.6 Ga and 2.0–1.8 Ga) and in the Neoproterozoic-late Phanerozoic (0.7–0.6 Ga and < 0.25 Ga). Our results show the specific role of the mantle beneath unstable Asia in terms of globally varied µ signatures and the same mantle epochs in sources of the late Phanerozoic volcanic rocks and kimberlites from stable regions of the Siberian and Indian paleocontinents, but with high μ (HIMU) signatures that are distributed worldwide and explained by sulfide sequestration of Pb from the mantle to the core. We refer the LOMU-ELMU mantle sources to the Asian high-velocity lower mantle domain and propose that the HIMU generating processes were focused mainly in the South Pacific and African low-velocity lower mantle domains in the Middle Mantle Epoch of the Earth’s evolution due to influence of the unbalanced solid core.

On the Age of the Buor-Kemus Flora of Northeast Asia Based on the Data on the Nonmarine Cretaceous in Eastern Chukotka

On the basis of the analysis of literature and cartographic materials, our field observations, results of U–Pb isotope dating of zircons, and examination of paleobotanical collections, it was established that the “Mesophytic” Buor-Kemus fern–gymnosperm flora existed in Eastern Chukotka in the Late Cretaceous longer than in other regions. The volcanics of the Nyrvakinot sequence are partly facially replaced by the sedimentary deposits of the Olkhovaya Formation. Both units contain plant remains of the Buor-Kemus flora. The Amgen volcanogenic sequence lies conformably and without any hiatus on the Nyrvakinot sequence and contains “Cenophytic” plant assemblages with a significant amount of angiosperms, ascribed to the Turonian–Coniacian. These three units were previously considered as the three major constituents of the Etelkuyum Formation. The U–Pb isotope ages of zircons from volcanogenic and tuffaceous sedimentary rocks of the Nyrvakinot sequence show that they accumulated in the Late Сretaceous in the age range of 93–87 Ma (Turonian–Coniacian). Taking into account all data available, it is evident that the Buor-Kemus flora (at least within the Eastern Chukotka segment of the Okhotsk–Chukotka Volcanogenic Belt) existed in the Albian, Cenomanian, Turonian, and, probably, in the early Coniacian.

A mid-Palaeozoic ocean–continent transition in the Mazongshan subduction–accretion complex, Beishan, NW China: new structural, chemical and age data constrain the petrogenesis and tectonic evolution

AbstractAccretionary orogens contain key evidence for the conversion of oceanic to continental crust. The late tectonic history and closure time of the Palaeo-Asian Ocean are recorded in the Mazongshan subduction–accretion complex in the southern Beishan margin of the Central Asian Orogenic Belt. We present new data on the structure, petrology, geochemistry and zircon U–Pb isotope ages of the Mazongshan subduction–accretion complex, which is a tectonic mélange with a block-in-matrix structure. The blocks are of serpentinized peridotite, basalt, gabbro, basaltic andesite, chert and seamount sediments within a matrix that is mainly composed of fore-arc-trench turbidites. U–Pb zircon ages of two gabbros are 454.6 ± 2.5 Ma and 434.1 ± 3.6 Ma, an andesite has a U–Pb zircon age of 451.3 ± 3.5 Ma and a tuffaceous slate has the youngest U–Pb zircon age of 353.6 ± 5.1 Ma. These new isotopic ages, combined with published data on ophiolitic mélanges from central Beishan, indicate that the subduction–accretion of Beishan in the southernmost Central Asian Orogenic Belt lasted until Late Ordovician – Early Carboniferous time. Structure and age data demonstrate that the younging direction of accretion was southwards and that the subduction zone dipped continuously to the north. Accordingly, these results record the conversion of oceanic to continental crust in the southern Beishan accretionary collage.

Sedimentary history and provenance analysis of the Sanbagawa Belt in eastern Kii Peninsula, Southwest Japan, based on detrital zircon U–Pb ages

U–Pb isotope ages of detrital zircons from psammitic rocks of the Sanbagawa Belt in eastern Kii Peninsula, Japan, were determined by laser ablation–inductively coupled plasma–mass spectrometry. The age spectra patterns allow four sedimentary types to be distinguished on the basis of dominant zircon age proportions: Permian–Jurassic, Cretaceous, Precambrian and Mixed types. Combining the age data with geological features in the area, we subdivide this belt into the Mayoidake, Hachisu, Kayumi, and Haze complexes, with their deposition–accretion ages estimated as 116–95, 103–84, 98–76, and 76–73 Ma, respectively. The Mayoidake Complex is dominated by Permian–Jurassic-type sediments, whereas the Haze Complex is dominated by Precambrian-type sediments. Cretaceous-type and Mixed-type sediments are found in the Kayumi and Hachisu complexes. Most of the Cretaceous zircons are inferred to have been derived from a Late Cretaceous volcanic arc on the eastern margin of the Eurasian continent, whereas the Precambrian zircons are interpreted to have been sourced from basement of the North China Block. By also considering results of previous studies, we conclude that the Mayoidake Complex belongs to the neighboring Shimanto Accretionary Complex rather than the Sanbagawa Metamorphic Rocks. Furthermore, detrital zircon U–Pb age spectra for the interval 100–95 Ma differ between the Mayoidake and Hachisu complexes. This difference suggests that the depositional basin of the Sanbagawa Metamorphic Rocks was separate and lay some distance from that of the Shimanto Accretionary Complex, with the two belts finally being juxtaposed by lateral displacement along a steeply dipping strike-slip.

Exploring the efficiency of stepwise dissolution in removal of stubborn non-radiogenic Pb in chondrule U-Pb dating

Chondrules in chondritic meteorites are unique witnesses of nebular and asteroidal processes that preceded large-scale planetary accretion. Together with refractory calcium-aluminium-rich inclusions (CAIs), they are the sources of our knowledge of the initial evolution of the early Solar System. We have investigated a single very large (>10 mm in longer dimension) chondrule, hereafter, the mega-chondrule A25-2, extracted from the Allende CV3 chondrite. We characterised texture, mineralogy and mineral chemistry of this chondrule, and studied its Al-Mg, U-Pb and U-isotope systematics. We also studied the distribution of U, Th and Pb, and measured Pb isotopic composition in individual minerals of A25-2 by secondary ion mass-spectrometry (SIMS). The main difficulty in absolute age determination was the presence of pervasive and resilient non-radiogenic Pb. In the search for the best way to separate radiogenic Pb from non-radiogenic Pb components of terrestrial and asteroidal origins, we used various protocols of multi-step leaching and assessed their efficiency in generating data suitable for the construction of an isochron. Testing the data filtering procedure led us to explore the behaviour of the stepwise leaching method in the presence of pervasive and resilient non-radiogenic Pb. The model age patterns observed in the final HF partial dissolution steps were probably induced by isotopic fractionation. Although step leaching did not yield fractions with highly radiogenic Pb, a Pb-Pb isochron age, corrected for measured 238U/235U was obtained by: (1) data filtering process based on strict analytical and geochemical criteria to include in the Pb-Pb isochron only leaching steps free from terrestrial contamination and (2) arithmetically recombined analyses to cancel the effects of leaching-induced isotopic fractionation.This extensive data processing yielded the age of 4568.5 ± 3.0 Ma, which we consider reliable within its uncertainty limits, although it is not as precise as, and more model dependent than, the age that could have been obtained if Pb isotopic compositions were more radiogenic. The 238U/235U ratio of the mega-chondrule is 137.764 ± 0.016, which is similar to the ratios obtained from single chondrules yet slightly different from small pooled Allende chondrules. The initial 27Al/26Al ratio inferred from internal isochron obtained from SIMS Al-Mg isotope measurements is (5.4 ± 6.5) × 10–6, which corresponds to 4565.0 + 0.8/−∞ Ma, assuming homogeneous distribution of 26Al throughout the protoplanetary disk at the canonical level (∼5.2 × 10−5). This age is 3.5 ± 3.1 Ma younger than the Pb-isotopic age. Calculation of 26Al-26Mg age assuming initial (27Al/26Al)0 of (1.36 ± 0.72) × 10–5 in the CV chondrule-forming region yields the age of 4566.4 + 0.8/−∞, which is consistent with the Pb-isotopic age.

Late Carboniferous mafic to felsic intrusive rocks in the central Great Xing’an Range, NE China: petrogenesis and tectonic implications

The timing of the amalgamation of the Xing’an and Songliao–Xilinhot blocks in the eastern Central Asian Orogenic Belt has long been the subject of debate. To explore this question, we studied a suite of late Carboniferous mafic to felsic intrusive rocks in the central Great Xing’an Range, NE China. These rocks yielded late Carboniferous zircon U–Pb isotopic ages of 320–300 Ma. The felsic monzogranites and granodiorites are strongly peraluminous with high A/CNK ratios (≥ 1.1), and they have signatures transitional between I- and S-type granites. Their geochemical characteristics suggest a magmatic source in a heterogeneous juvenile crust that included both clay-rich pelitic material and metabasalt. On the other hand, the intermediate biotite granodiorites have characteristics that suggest a dual magmatic source involving both mantle material and crustal contributions. The mafic gabbros were derived from the partial melting of mantle materials including both spinel and garnet lherzolite. The upwelling of the asthenospheric mantle contributed greatly to the formation of these magmas, with the upwelling as a result of the delamination of dense lithosphere and/or previously trapped mafic slabs. The petrology, geochemical signatures, field characteristics, and rock associations of the late Carboniferous mafic to felsic intrusive rocks imply that they formed in a post-collisional extensional tectonic setting that developed after the amalgamation of the Xing’an and Songliao–Xilinhot blocks and closure of the Nenjiang Ocean between them. Considering that the tectonic setting during the early Carboniferous was one of regional subduction, our data constrain the amalgamation of the Xing’an and Songliao–Xilinhot blocks and the closure of the Nenjiang Ocean to the early late Carboniferous.

Petrogenesis and tectonic implications of the early Carboniferous volcanic rocks in West Junggar, NW China

Carboniferous magmatism in the Darbut region is critical for understanding the evolutionary history of the accretionary orogenesis of the West Junggar area in the Central Asian Orogenic Belt. In this paper, zircon U–Pb geochronological, whole‐rock geochemical and Sr–Nd–Pb–Hf isotopic analyses of the basalts, basaltic andesite, and dacite from the Carboniferous Tailegula Formation, Darbut region, have been performed. The magmatic zircons from basalt, basaltic andesite, and dacite yield concordia U–Pb isotope ages of 344 ± 4, 347 ± 3, and 340 ± 5 Ma, respectively, which are interpreted as the crystallization ages of these rocks. The basalts and basaltic andesite are characterized by an alkali composition; high Nb/Yb, Th/Yb, Ta/Yb, La/Sm, and Sm/Yb trace element ratios and Pb isotopic ratios; moderate (87Sr/86Sr)i ratios; low (143Nb/144Nb)i ratios; and positive εNd(t) (+0.66 − +4.73) and εHf(t) (+1.7 − +15.2) values. These characteristics indicate that the magmas were derived from mixed mantle sources, that is, enriched mantle sources and high U/Pb ratio mantle (HIMU) component sources with ≤10% partial melting of garnet and spinel lherzolite. The samples display pronounced light rare earth elements (LREEs) and incompatible element enrichment patterns with positive Nb–Ta anomalies and a high Pb content and Nb/La ratios but low (Th/Nb)N ratios and a lower degree of partial melting, implying that these rocks were formed in a continental extensional setting. Based on our results and that of previous studies, we conclude that the early Carboniferous volcanic rocks in the Darbut region formed by mantle plume or regional‐scale mantle upwelling in a continental extensional setting after closure of the Junggar Ocean.

The Upper Riphean of the Yenisei Range: Sr Chemostratigraphy and Pb–Pb Age of Limestones of the Tungusik and Shirokaya Groups

The Pb–Pb isotope age and the Sr isotope signature of the Upper Riphean sedimentary limestones from the south of the Yenisei Range on the western margin of the Siberian Craton were obtained. The Pb–Pb age of carbonate deposits of the Dadykta Formation of the Tungusik Group in the Kamenka facial zone in the east of the Yenisei Range is 1020 ± 20 Ma. The Pb–Pb age of limestone of the Gorevka Formation of the Shirokaya Group in the Glushikha facies zone in the western part of the region is estimated at 1020 ± 70 Ma. The 87Sr/86Sr ratio in limestones of the Dadykta Formation lies within 0.70536–0.70590 and that of the Gorevka Formation lies within 0.70552–0.70578. This coincides with a decrease in the 87Sr/86Sr ratio in a paleo-ocean immediately after the culmination of the Grenville orogeny. The correlation of the Shirokaya Group with the Oslyanka Group of the Kamenka Zone in combination with other data has shown that the accumulation of the Tungusik, Shirokaya, and Oslyanka groups occurred within an interval of 1030–950 Ma. The isotope age of the Middle and Upper Riphean boundary at the base of the Tungusik Group of the Yenisei Range is estimated at 1030 Ma, which is consistent with the age of this boundary in the sections of the Turukhansk Uplift and the Uchur-Maya Region of Siberia. It was revealed that the greater part of the Upper Riphean (Neoproterozoic) in the sedimentary record of the Yenisei Range is missing. The observed facial zoning of the Riphean deposits of the Yenisei Range represents the configuration of the ancient sedimentary basin that existed at the western margin of the Siberian Craton long before the collisional events accompanied by granite intrusions and superimposed metamorphism in the Neoproterozoic.

Element behaviour during interaction of magma and fluid: A case study of Chamuhan Granite, and implications on the genesis of W – Mo mineralisation

The Chamuhan Granite is located in the eastern part of the Central Asian Orogen where it crops out as an elongated, NE-trending composite monzogranite. The pluton consists of two phases with a fine – grained monzogranite (G2) at depth in sharp contact with a medium – grained monzogranite (G1) above. The medium-grained monzogranite can be subdivided into G1a and G1b based on the degree of metasomatism affecting it, with the G1b being more altered. Tungsten–molybdenum mineralisation is hosted by quartz (−fluorite) veins or forms disseminated sulfides in G1a, indicating a genetic association between the monzogranite and ore. Magmatic zircons from G1a yield a weighted mean age of 144 ± 2 Ma, and hydrothermal zircons from G1a yield a weighted mean age of 143 ± 1 Ma, indicating that the hydrothermal zircons are very close in age to the medium-grained monzogranite. Monazite grains yield U – Pb isotope ages of 138 ± 3 Ma for G1a, 137 ± 1 Ma for G1b, and 139 ± 2 Ma for the fine-grained monzogranite (G2) phase. These dates are the same within analytical error, and are coeval with the molybdenite Re – Os weighted mean age of 140 ± 2 Ma, indicating that the monazite and molybdenite were deposited during hydrothermal alteration. The Chamuhan Granite is geochemically a metaluminous to slightly peraluminous high – K calc-alkaline granite, and is especially characterised by its REE tetrad pattern and a huge Eu depletion that are accompanied by a non – “Charge-and Radius-Controlled” (CHARAC) trace element behaviour. This and the O-isotopic data from the granite phases are indicative of a highly differentiated magma that interacted strongly with a co–existing fluid emanating from the magma during the late stage of its crystallisation. The interaction between the magma and related hydrothermal fluid resulted in the enrichment of W–Mo(–Nb–Ta–Th–U–Hf–Rb–Y). All the studied granitic rocks have homogeneous initial isotope values with positive εNd(t) values of 0.1–3, and TDM2 dates between 924 and 691 Ma. This is consistent with granites derived from a pre-existing juvenile crust derived from a depleted mantle during the Neoproterozoic. The molybdenite in the Chamuhan W – Mo deposit has a narrow δ34SVCDT range of 0.1–1.6‰ characteristic of a magmatic origin, and the Pb isotopic compositions with 206Pb/204Pb ratios between 18.298 and 18.882, 207Pb/204Pb ratios between 15.494 and 15.538 and 208Pb/204Pb ratios between 38.046 and 38.206, are similar to the calculated initial Pb isotopic ratios of the Chamuhan Granite. We thus suggest that there is an intimate relationship between the Chamuhan Granite and mineralisation, and that the mineralising process is related to a post-collisional extensional event following the formation of an over-thickened continental crust and during closure of the Mongol-Okhotsk and Paleo-Pacific oceanic plates.

Details Geological Mapping and Petrological Characteristics of EL Shereik Study Area, River Nile State, North Sudan

The present study focused on details geology of the lithostratigraphy, phenomena and petrogenesis, with full classification of rock units and correlate by the regional geology in the study area. The previous studies, focused on regional geology, which can be divided in Bayuda Terrane, represented by Dem El Tor Shear Zone (DTSZ), its Precambrian to Tertiary ages (isotopic signatures of later cooling 590 to 550 Ma), with dominated by metamorphic and intrusive rocks belonging to the Pre-Nubian basement complex. The study area lies at El Shereik, River Nile State, Sudan. It is characterized by low-lying bed plains, covered with superficial deposits, with an arid climate. The techniques of this work, represented in an official, field works and laboratory test, with used Land Sat Images ETM. And Nubian Arabian Shield, represented by the Keraf Shear Zone (KSZ), its Precambrian to Phanerozoic ages (Neoproterozoic, Pb isotope ages dating, of forms is -730 -710Ma and ended in -565Ma), with dominated by metamorphic, ophiolitic mélange, volcano-sedimentary sequences, and Phanerozoic sediments. But through this detailed surveyed, we discovered different types of rock units; they were found as accumulated and highly deformed, affected by various thrusting faults. collected more than 40 samples, through six traverses, for classification and petrographic studies and classified more than 9 types of rooks units didn’t mentioned before in previous study, all of them well exposed as following: metamorphic rocks, included, migmatites, calc-silicates, wollastonite, talc-schist, carbonaceous (dolomitic marble), amphibolites, graphitic schist, mica schist, quartzo-feldspathic schist, and grey gneiss, While the Igneous rocks, consisted of dykes, as dolerite, trachy-basaltic, Rhyolite porphyry, pegmatites, diorite, and quartz veins, whereas the superficial deposits, included, of aeolian, fluvial, and collovial. These rocks extend and spread to the outside of the limits of the study area and most of them oriented parallel with KSZ SE - NW region and a few of them are oriented E-W, This is maybe due to the collusion of the contact boundary between KSZ and DTSZ Bayuda Tehran. Talc-schist, Wollastonite and graphitic schist, represent a strategic stockpile besides gold mining. Studies conducted with DTSZ, its old age, occurred before Neoproterozoic compared to KSZ. Because it has the first deformation of the folding of pre- KSZ proportion to the presence of folding in the west and east of the Nile zone, according to the border between Bayuda Terrane and Nubian Arabian Shield, as the suggest result. And the previous studies of the ages dating confirm it.

Geochemistry and Isotopic Age of Zircons from Rocks of Ultramafic Massifs in the Southern Folded Framing of the São Francisco Craton (Southeastern Brazil)

Abstract —We present the first data on the geochemistry and isotopic age of zircons from rocks of the Roseta (lherzolite and olivine orthopyroxenite) and Córrego da Areia (olivine orthopyroxenite) ultramafic massifs localized among Archean–Proterozoic metamorphic strata in the southern folded framing of the São Francisco Craton. The examined zircons differ in crystal morphology, the intensity and type of cathodoluminescence, oscillatory zoning, the distribution of trace elements, and U–Pb isotopic age. Zircon crystals from the Roseta lherzolite have an age of 800-728 Ma and show negligible cathodoluminescence. Some of them have reaction rims with an age of 663–619 Ma. The crystal cores are enriched in rare earth elements (REE) but are often depleted in U as compared with the reaction rims. The age of zircons from the Roseta olivine orthopyroxenite showing moderate cathodoluminescence is within 622.6–596.9 Ma. Zircons from the Córrego da Areia olivine orthopyroxenite show moderate to intense cathodoluminescence and often have a rhythmic oscillatory zoning. They are characterized by an uneven distribution of REE and other trace elements and have high contents of U and Th. Most of these zircons have an ancient isotopic age (2558–2100 Ma). The examined zircons are of several types: relict, xenogenic, syngenetic, and epigenetic. The Roseta and Córrego da Areia massifs are considered to be protrusive outliers of a deep-seated Precambrian mafic–ultramafic complex, which were tectonically transferred along faults into the intensely metamorphosed Archean–Proterozoic strata in the southern folded framing of the São Francisco Craton.

Composition and U–Pb (LA-ICP-MS) Isotopic Age of Zircons from Composite Dikes of Western Sangilen (Tuva-Mongolian Orogen)

Abstract —Composite dikes (mingling dikes) of Western Sangilen (southeastern Tuva) are believed to indicate a shift from tectonic contraction, related to the mountain fold system formation, to postcollisional extension that ended with the disintegration of the collisional orogen. However, the position of composite dikes in the general sequence of magmatic events was based only on their geologic location and has not been confirmed by U/Pb isotope-geochronological data. The proposed study is concerned with the geologic structure, composition, and isotopic age (485–490 Ma) of West Sangilen composite dikes. The isotopic age of the host granites from the Matut pluton is 509 Ma; thermal events of Riphean (679–934 Ma) and early Paleozoic (484 Ma) age are reflected in zircons from gneiss-granites of the Erzin metamorphic complex. The obtained data are consistent with the earlier determined sequences of West Sangilen tectonometamorphic events. Basites from the examined composite dikes are strongly different in the contents of indicator elements (Rb, Nb, Ta, Zr, and Hf) and the Nb/Y, Zr/Y, Th/Ta, Zr/Nb, and Nb/Th ratios, which is indicative of different (probably, different-depth) mantle sources. The protolith of the salic component of the composite dikes might have been rocks similar in composition to the autochthonous/parautochthonous gneiss-granites of the Erzin metamorphic complex.

U-Pb, Rb-Sr and Ar-Ar systematics of the ungrouped achondrites Northwest Africa 6704 and Northwest Africa 6693

We report U-Pb, 87Rb-87Sr, 40Ar-39Ar, and 238U/235U isotopic data for paired ungrouped achondrites NWA 6704 and NWA 6693 that were derived from a highly oxidised parent body with broadly chondritic composition (Warren et al., 2013; Hibiya et al., 2019). Pb-isotopic ages derived from isochrons for multiple acid-leached pyroxene fractions are 4562.76 + 0.22/−0.30 Ma for NWA 6704 and 4562.63 + 0.29/−0.21 Ma for NWA 6693, calculated using 238U/235U ratio of 137.7784 ± 0.0097 measured in NWA 6704. The Rb-Sr mineral isochron age of 4543 ± 46 Ma (initial 87Sr/86Sr = 0.699013 ± 0.000055) is consistent with the Pb-isotopic age. Together with 187Re-187Os isochron age of 4576 ± 250 Ma for NWA 6704 (Hibiya et al., 2019), and 26Al-26Mg and 53Mn-53Cr ages calculated using the rapidly crystallized angrite D’Orbigny as a time anchor are also consistent with the Pb-isotopic age (Sanborn et al., 2019), these data indicate that the parent rocks of NWA 6693 and NWA 6704 remained closed to migration of both lithophile and siderophile elements since crystallisation and initial cooling. The whole rock 40Ar-39Ar age of 4199 ± 32 Ma suggests a complete resetting of the K-Ar system approximately 360 Ma after crystallisation. A later event at ≤2.12 Ga partially reset the K-Ar system as shown by the low temperature heating steps. Both meteorites have high 87Rb/86Sr ratios (up to 7.0 in NWA 6693 pyroxene) and very radiogenic 87Sr/86Sr up to 1.15. Together with the absence of secondary disturbance in the Rb-Sr and U-Pb systems, this makes them suitable for cross-calibration of the isotopic chronometers. These meteorites are also promising candidates to serve as age reference samples for the early Solar System chronology, as an alternative or complement to angrites of the early generation (D’Orbigny, Sahara 99555) that are currently used for this purpose. Plagioclase in NWA 6704 has a sufficiently low Rb/Sr ratio to define precise initial 87Sr/86Sr of 0.698997 ± 0.000027, which corresponds to the time of separation of the parent body precursor material from the solar nebula of 1.5 ± 2.1 Ma. This value suggests that the parent asteroid accreted within 3.6 Ma after CAI formation, or before 4563.7 Ma using the CAI age of 4567.3 Ma (Connelly et al., 2012).

Reconstruction of Source-Rock Composition of the Middle and Upper Riphean Rocks of the Isherim and Bashkir Anticlinoria, Urals

It is shown that the Riphean sediments of the Bashkir and Isherim anticlinoria of the Ural mobile belt were formed in different settings. Sediments of the Bashkir Anticlinorium are autochthonous and were formed in the eastern (in present-day coordinates) Baltica from proximal siliciclastics, whereas the complexes of the Isherim Anticlinorium did not belong to it. Geochemical features of the Middle and lower Upper Riphean metapelites of the Isherim Anticlinorium and U–Pb isotope ages of detrital zircons from sandstones suggest that their provenances were the northern and northwestern parts of Baltica. In the Late Riphean (?)–Vendian, the Isherim block migrated along the Timan margin of Baltica into the present-day position.

Carboniferous continental arc in the Hegenshan accretionary belt: Constrains from plutonic complex in central Inner Mongolia

The architecture and tectonic evolution of the Hegenshan accretionary belt in the Central Asian Orogenic Belt (CAOB) remains debated. Here we present an integrated study of zircon U–Pb isotopic ages, whole rock major-trace elements, and Sr–Nd–Pb isotopic data from the Hegenshan volcanic-plutonic belt in central Inner Mongolia. Field observations and zircon U–Pb ages allow us to divide the intrusive complex into an early phase at 329–306 Ma and a late phase at 304 to 299 Ma. The intrusive bodies belong to two magma series: calc-alkaline rocks with I-type affinity and A-type granites. The early intrusions are composed of granodiorite, monzogranite and porphyritic granite, and the late calc-alkaline intrusions include gabbro though diorite to granodiorite. The calc-alkaline intrusive rocks exhibit a well-defined compositional trend from gabbro to granite, reflecting continuous fractional crystallization. These rocks show obvious enrichment in LILEs and LREEs and relative depletion of HFSEs, typical of subduction-related magma. They also exhibit isotopic characteristics of mantle-derived magmas such as low initial 87Sr/86Sr (0.7029–0.7053), positive ɛNd(t) values (0.06–4.76) and low radiogenic Pb isotopic compositions ((206Pb/204Pb)I = 17.907–19.198, (207Pb/204Pb)I = 15.474–15.555, (208Pb/204Pb)I = 37.408–38.893). The marked consistency in geochemical and isotopic compositions between the intrusive rocks and the coeval Baoligaomiao volcanic rocks define a Carboniferous continental arc. Together with available regional data, we infer that this east-west trending continental arc was generated by northward subduction of the Hegenshan ocean during Carboniferous. The late alkali-feldspar granites and the high-Si rhyolites of the Baoligaomiao volcanic succession show similar geochemical compositions with high SiO2 and variable total alkali contents, and low TiO2, MgO and CaO. These rocks are characterized by unusually low Sr and Ba, and high abundances of Zr, Th, Nb, HREEs and Y, comparable to the features of typical A2-type granites including their high ratios of FeOT/MgO, Ga/Al and Y/Nb. Our study suggests that the A-type granite was derived from a distinct magma source rather than through fractional crystallization of the coeval calc-alkaline magmas. Their Nd–Pb isotopic compositions are similar to those of calc-alkaline arc rocks and are compatible with partial melting of pre-existing juvenile basaltic crust in the continental arc. Notably, the widespread eruptions of A2-type rhyolitic magmas (305.3 Ma–303.4 Ma) following a short period of magmatic quiescence was temporally and spatially associated with bimodal magmatism with mantle-derived gabbro-diorites and A-type granites (304.3 Ma–299.03 Ma) in the pre-existing arc volcanic-plutonic belt (329 Ma–306 Ma). Such a marked change in the magma affinity likely indicates subducted slab break-off resulting in a change of the regional stress field to an extensional setting within the Carboniferous continental arc that runs E-W for few thousands of kilometers. Thus, the onset of the late magmatism (305–299 Ma) likely represents the maximum age for the cessation of the northward subduction in the Hegenshan ophiolite-arc-accretion belt.

Early Stages of the Evolution of Uralides as Evidenced from the U–Pb Systematics of Detrital Zircons from Rift Complexes

The U–Pb isotope data and corresponding ages of detrital zircons from rocks of the basal complexes of the Uralides of different segments of the Ural Fold Belt are considered. It was established that complexes of ancient domains of the East European Platform (Volga-Uralia, Sarmatia, Kola, etc.) seem to have been the main provenance areas of the clastic material for the Southern, Middle, and Northern Urals. This means that there were relatively remote and local (igneous formations of the pre-Uralides) provenance areas. Rift rock associations of the Uralides of the Subpolar and Polar Urals were formed mainly through erosion of local provenance areas (predominantly, Late Riphean–Vendian island-arc and orogenic magmatic complexes of the Proto-Uralides–Timanides). Detrital zircons of Riphean age dominate in rocks of the basal complexes of the Uralides. A source for them could have been rock complexes of Svecofennian-Norwegian Orogen and Cadomides of the Scythian-Turan Plate, intraplate magmatic formations, and metamorphic complexes, as well as blocks accreted to the margin of the East European Platform in the Late Precambrian–Cambrian and later detached and displaced during the Ordovician rifting and spreading. In general, the basal complexes of Uralides were formed owing to supply of clastic material from both remote and local sources. Despite the appearance of information of a totally new level (U–Pb isotope ages of detrital zircons, their Lu–Hf systematics, and the distribution features of rare earth and trace elements), the contribution of these sources to the formation of the Late Cambrian–Early Ordovician clastic strata is hardly possible at present to evaluate.

Spatial and temporal distributions of the late Mesozoic volcanic successions and their controlling effects on the Changling fault depression of the Songliao Basin, northeast China

Volcanic successions, including their spatial, temporal, tectonic, and geochemical attributes, were identified based on well data, 2D and 3D seismic data, U–Pb isotopic ages, and major and trace element data from the Changling fault depression of the Songliao Basin in northeastern China. Three eruption cycles developed in the Changling fault depression: K1h (Huoshiling Formation) (124–118 Ma), K1yc1 (first member of the Yingcheng Formation) (115–106 Ma), and K1yc3 (third member of the Yingcheng Formation) (106–102 Ma). These three eruption cycles comprised seven eruption stages. The volcanic successions in every eruption cycle were bimodal and evolved from basic to acidic. The magma of the basic and intermediate rocks in these cycles was derived from partial melting of the asthenosphere. In K1yc1 and K1yc3, partial melting of the newly formed basic rocks in the lower crust formed the high-SiO2 acidic magma. In addition, fractional crystallization of basic magma formed the small-scale intermediate–acidic magma. In K1h, the most likely origin of the acidic magma was fractional crystallization of basic magma. During the early stage of the fault depression, the volcanic rocks of K1h were controlled primarily by the boundary fault activity. In K1yc1 and K1yc3, basic–intermediate rocks were distributed widely along syngenetic faults, and acidic rocks were concentrated in areas where the syngenetic faults had their largest amplitudes. Multiple cycles and stages of the volcanic successions were controlled by magmatic evolution, and the spatial distribution was controlled by basin tectonics.

Early formation of planetary building blocks inferred from Pb isotopic ages of chondrules.

The most abundant components of primitive meteorites (chondrites) are millimeter-sized glassy spherical chondrules formed by transient melting events in the solar protoplanetary disk. Using Pb-Pb dates of 22 individual chondrules, we show that primary production of chondrules in the early solar system was restricted to the first million years after the formation of the Sun and that these existing chondrules were recycled for the remaining lifetime of the protoplanetary disk. This finding is consistent with a primary chondrule formation episode during the early high-mass accretion phase of the protoplanetary disk that transitions into a longer period of chondrule reworking. An abundance of chondrules at early times provides the precursor material required to drive the efficient and rapid formation of planetary objects via chondrule accretion.