Isotopic Ages Research Articles

First Data on the U–Pb Isotopic Age for Zircon (LA-ICP-MS Method) from Leucocratic Granites of the Tyrnyauz Mo–W Deposit (North Caucasus, Russia)

First Data on the U–Pb Isotopic Age for Zircon (LA-ICP-MS Method) from Leucocratic Granites of the Tyrnyauz Mo–W Deposit (North Caucasus, Russia)

Glacial Ripping in Sedimentary Rocks: Loch Eriboll, NW Scotland

Glacial ripping is a newly recognized process sequence in which subglacial erosion is triggered by groundwater overpressure. Investigations in gneiss terrain in lowland Sweden indicate that ripping involves three stages of (i) hydraulic jacking, (ii) rock disruption under subglacial traction, and (iii) glacial transport of rock blocks. Evidence for each stage includes, respectively, dilated fractures with sediment fills, disintegrated roches moutonnées, and boulder spreads. Here, we ask: can glacial ripping also occur in sedimentary rocks, and, if so, what are its effects? The case study area is in hard, thinly bedded, gently dipping Cambrian quartz-arenites at Loch Eriboll, NW Scotland. Field surveys reveal dilated, sediment filled, bedding-parallel fractures, open joints, and brecciated zones, interpreted as markers for pervasive, shallow penetration of the quartz-arenite by water at overpressure. Other features, including disintegrated rock surfaces, boulder spreads, and monomict rubble tills, indicate glacial disruption and short distance subglacial transport. The field results together with cosmogenic isotope ages indicate that glacial ripping operated with high impact close to the former ice margin at Loch Eriboll at 17.6–16.5 ka. Glacial ripping thus can operate effectively in bedded, hard sedimentary rocks, and the accompanying brecciation is significant—if not dominant—in till formation. Candidate markers for glacial ripping are identified in other sedimentary terrains in former glaciated areas of the Northern Hemisphere.

Locating the Indo-Antarctica suture – Correlating the Rengali, Rauer and Ruker terranes in Gondwana

The Rayner Complex of East Antarctica and the Eastern Ghats Province (EGP) of India are thought to have been contiguous in past supercontinents like Rodinia and Gondwana. These terranes have been correlated on the basis of similar granulite facies metamorphic imprints and isotopic age data that testify to Grenvillian (1100-900 Ma) and Pan-African (650-450 Ma) thermal signatures. The Grenvillian granulite facies metamorphic event is generally thought to represent collision between Antarctica and cratonic India, but the precise location of the Indo-Antarctic suture is disputed. The intensity of Pan-African age geological imprints is also variable in both continents, and their significance remains unclear. In this review, we correlate structural, metamorphic and geochronological data in both terranes and parts of their bounding cratons, and suggest that the Ruker Terrane and Rauer Group in Antarctica were continuous with the Rengali Province in India. Together with the established correlation between the EGP and the Rayner Complex, this implies that cratonic India along with the EGP-Rayner amalgam collided with the Archaean Ruker Terrane (part of the Crohn craton) at ~520 Ma along the southern Prince Charles Mountain in East Antarctica. This suture is distinct from the Grenvillian suture between EGP-Rayner and cratonic India.

Unravelling metamorphic ages of suture zone rocks from the Sabzevar and Makran areas (Iran): Robust age constraints for the larger Arabia–Eurasian collision zone

AbstractIn this study we establish a time frame for metamorphic processes affecting Neotethyan suture zones at the northern (Sabzevar) and southern (Makran) margin of the Central Iranian Microcontinent. For this purpose, we have applied Rb–Sr and U–Pb geochronology to blueschist‐ and amphibolite facies rocks from these occurrences. So far, no isotopic ages are available for high‐P/low‐T rocks from the Sabzevar area which are exposed in the Govain and Chili mountains near Soltanabad. Three newly dated blueschists from the Govain mountain yielded Rb–Sr ages of 56.52 ± 0.32, 57.20 ± 0.32, and 55.12 ± 0.33 Ma respectively. These ages provide the first unambiguous evidence for late Palaeocene to early Eocene high‐P metamorphism in the Sabzevar zone and confirm the accuracy of earlier age assignments that were derived indirectly. The clearly confirmed age difference between late Palaeocene–early Eocene blueschists of the Govain mountain and kilometre‐scale dismembered HP granulites with Early Cretaceous (Albian) metamorphic ages from outcrops c. 60 km to the west of Soltanabad further corroborates interpretations that these rocks are not related to the same subduction zone complex. The blueschists record closure of an Albian to Late Cretaceous back‐arc basin, whereas the granulites formed due to subduction of older oceanic lithosphere. Uranium–Pb zircon dating of an amphibolite from the Iranshahr region in the Makran Zone yielded an intercept age of 87.8 ± 1.8 Ma and weighted mean age of 88.1 ± 1.7 Ma. Rb–Sr mineral isochrons for seven blueschist‐ and amphibolite facies samples from the same area generated very consistent ages of c. 84 Ma (± <1–3 Ma), regardless of metamorphic grade. The apparent contradiction that rocks recording different P–T conditions have identical metamorphic ages either indicates tectonic mixing of rocks during exhumation that originally had formed at the same time at different depths of the subduction system, or very rapid exhumation so that differences in closure temperatures became indistinguishable. The new data are similar, but much more precise, than previously reported K–Ar ages and suggest that most of the existing data had not been significantly compromised by contamination with extraneous Ar. Potassium–Ar indications for older blueschist‐ and/or amphibolite facies metamorphism (>85 Ma) have not yet been confirmed but can be reconciled with the larger geological frame implying that northward subduction of the Arabian plate beneath the Bajgan–Durkan continental block started shortly after c. 120 Ma.

Construction of an intraplate island volcano: The volcanic history of Heard Island

Heard Island is a subantarctic volcanic island in the southern Indian Ocean and includes Australia’s only active volcano, Big Ben (2813 m high, 18 km diameter). The most recent eruption of lava at Big Ben occurred in November 2020. Heard Island is the southernmost subaerial exposure of the Kerguelen Plateau, a mainly submerged large igneous province formed as a result of the activity of the Kerguelen plume over the last 130 Myrs. Using published and unpublished data and reports, modern satellite images, and private and public rock collections, we produced a new geological map of Heard Island supplemented by new 40Ar/39Ar isotopic ages. The volcanoes on Heard Island have been constructed on a ~ 300-m-thick platform of 10–5-Myr-old submarine volcaniclastic rocks known as the Drygalski Formation, now exposed above sea level. The Drygalski Formation is underlain by the Oligocene Laurens Peninsula Limestones. Exposed Big Ben lavas are dominated by basanite, basalt, and trachybasalt compositions. Two Big Ben lavas were dated at 11.1 ± 1.1 ka and 23.9 ± 2.1 ka, expanding the range of ages previously reported for nonhistorical lavas from Big Ben (356 ± 51–33 ± 52 ka). Sector collapse occurred on Big Ben prior to the formation of the current eruptive centre, Mawson Peak (likely after 23 ± 16 ka). Laurens Peninsula lies to the west of Big Ben and consists of trachyte and basanite lavas and domes. Laurens Peninsula lavas have ages in the range 5.1 ± 3.5 to 76.6 ± 5.5 ka. Young basaltic cones are located around the coast of Heard Island and range in age from 1.9 ± 3.8 to 13.3 ± 4.1 ka. Heard Island is a relatively small volcanic edifice with a low average magma discharge rate (~0.0002 km3/yr) when compared with other intraplate basaltic volcanic islands.

Protogenetic sulfide inclusions in diamonds date the diamond formation event using Re-Os isotopes

Sulfides are the most abundant inclusions in diamonds and a key tool for dating diamond formation via Re-Os isotopic analyses. The manner in which fluids invade the continental lithospheric mantle and the time scale at which they equilibrate with preexisting (protogenetic) sulfides are poorly understood yet essential factors to understanding diamond formation and the validity of isotopic ages. We investigated a suite of sulfide-bearing diamonds from two Canadian cratons to test the robustness of Re-Os in sulfide for dating diamond formation. Single-crystal X-ray diffraction (XRD) allowed determination of the original monosulfide solid-solution (Mss) composition stable in the mantle, indicating subsolidus conditions of encapsulation, and providing crystallographic evidence supporting a protogenetic origin of the inclusions. The results, coupled with a diffusion model, indicate Re-Os isotope equilibration is sufficiently fast in sulfide inclusions with typical grain size, at mantle temperatures, for the system to be reset by the diamond-forming event. This confirms that even if protogenetic, the Re-Os isochrons defined by these minerals likely reflect the ages of diamond formation, and this result highlights the power of this system to date the timing of fluid migration in mantle lithosphere.

Siderian to Rhyacian evolution of the Juiz de Fora Complex: Arc fingerprints and correlations within the Minas-Bahia Orogen and the Western Central Africa Belt

The Juiz de Fora Complex (JFC) is one of the basement units of the Occidental Terrane, Central Ribeira belt, in the Neoproterozoic framework of Western Gondwana, regarded as part of the reworked border of the São Francisco Paleocontinent. The JFC comprises Paleoproterozoic granitoid rocks with TTG, calc-alkaline, sanukitoid, and tholeiite-type geochemical compositions. They exhibit complex deformational fabrics associated with two episodes of granulite-facies metamorphism, occuring in the early Orosirian and the Ediacaran. The new U-Pb (SHRIMP) isotopic ages and Lu-Hf (LA-MC-ICPMS) data combined with available lithogeochemical, whole-rock Sm-Nd and Rb-Sr isotopic data constrain the tectonic evolution of the JFC to a Siderian-Rhyacian evolved arc system. Five samples were selected for U-Pb geochronology and Lu-Hf analysis on zircon grains: a low-K tholeiite, one TTG, two sanukitoids, and a mafic tholeiite. Combined whole-rock Sm-Nd, Rb-Sr isotopes, and zircon Lu-Hf isotopic data suggest the following evolution: an initial moderately juvenile (2445 Ma) low-K tholeiitic magmatism (εNd(t) of − 0.1; εHf(t) from − 8 to + 3), followed by a moderately juvenile to evolved (2200–2182 Ma) coeval TTG-sanukitoid pulse (εNd(t) from − 0.6 to − 4.8; εHf(t) from − 11.7 to + 2.5), and eventually by a moderately juvenile (2130 Ma) tholeiitic magmatism (εNd(t) = − 0.05). The integrated information provides preliminary petrogenetic insights for these magmatic episodes, such as interaction between mantle sources and supracrustal subducted material, melting of the oceanic slab, and crustal assimilation. A ca. 2035 Ma collisional granulite event recorded in metamorphic rims and neoformed zircon grains in leucosomes, possibly marks the amalgamation of the JFC onto the Mantiqueira Complex. Geochronological and isotopic correlations with coeval terranes of the Minas-Bahia Orogen in the Brazilian and African counterparts are addressed for tectonic implications at a broader scale.

Detrital zircon U-Pb-Hf isotopes from the Yanliao intracontinental rift sediments: Implications for multiple phases of Neoarchean-Paleoproterozoic juvenile crustal growth in the North China Craton

Late Neoarchean-Paleoproterozoic marks a major period of continental growth and recycling, the culmination of which witnessed the assembly of the Columbia supercontinent and its subsequent breakup. The North China Craton (NCC) preserves important records of tectonic activities and crust building and recycling processes during this time. In this study, we carried out U-Pb geochronology and Lu-Hf isotopic analyses of detrital zircon grains from four sedimentary formations of the Changcheng Group in the Yanliao intracontinental rift in NE NCC. Our isotopic ages constrain the timing of initiation of the Yanliao rift as ca. 1680 Ma. The zircon U-Pb data show two major age populations at 2.7–2.5 Ga and 2.2–1.9 Ga, with high and positive εHf(t) values of + 0.1 to + 34.6. Both the Neoarchean and Paleoproterozoic detrital zircon populations were derived from the basement rocks of the NCC. The results from our study indicate major juvenile crust accretion during Neoarchean and Paleoproterozoic associated with the craton building process of the NCC.

Maria Basalts Chronology of the Chang’E-5 Sampling Site

Chang’E-5 is the first lunar sample return mission of China. The spacecraft was landed in the northwest of the Procellarum KREEP Terrane (43.0576°N, 308.0839°E) on 1 December 2020 and returned 1731 g samples from a previously unvisited region. The landing area has been proposed as one of the youngest mare basalt units of the Moon and holds important information of lunar thermal evolution and chronology. However, the absolute model ages estimated in previous studies are quite different, ranging from 2.07 Ga to 1.21 Ga. Such significant difference may be caused by (1) different crater counting areas, (2) different crater diameter ranges, (3) effects of secondary craters, and (4) biases in crater identification. Moreover, the accurate landing site was unknown and the ages were estimated over the Eratosthenian-aged mare unit (Em4) instead. In light of the above unsatisfactory conditions, this study seeks to establish a standard crater size-frequency distribution of the CE-5 landing site. We derived the concentrations of FeO and TiO2 to map out the pure basaltic areas where external ejecta deposits are negligible and thus secondary craters are rare. Based on the geochemistry of basaltic ejecta excavated by fresh craters in the mare unit, the FeO concentration threshold for mapping pure basaltic areas is 17.2 wt.%. The morphologically flat subunits in the pure basaltic areas were selected for crater identification and age dating to exclude the contamination of external ejecta to the best as we could. In the Chang’E-5 sampling site subunit, we detected 313 craters with a diameter greater than 100 m and derived the absolute model age as 1.49−0.084+0.084 Ga. The craters identified in all pure basaltic subunits of Em4 gave the model age of 1.41−0.028+0.027 Ga. As least affected by secondary craters, the crater size-frequency distribution of the sample-collected pure basaltic subunit can provide important constraints for lunar cratering chronology function in combination with isotopic age of returned samples.

Environmental isotopes and noble gas ages of the deep groundwater with coupled flow modelling in the Baltic artesian basin

In this study, modelled groundwater actual flow times in intermediate and deep aquifers, covered by regional scale impermeable aquitards, were compared with 4He and 81Kr age dating results. To improve the reliability of the steady state 3D groundwater flow model, the isotopic ages of deep groundwater were compared to the MODPATH modelled travel times. The highest helium values in groundwater reservoirs coincide with fault zones in the crystalline basement and sedimentary cover near Rapakivi granite massifs. Insights into isotope-geochemical anomalies of the Baltic Artesian Basin intermediate and deep groundwater support their main distribution peculiarities in the flow path towards the Baltic Sea coast lowland and seabed depression as the regional groundwater discharge area.

SOME PROBLEMS GEOCHEMISTRY OF VOLCANITES OF THE PECHENGA STRUCTURE (KOLA PENINSULA)

Some problems of dating the most important stratigraphic units of the Early Proterozoic Pechenga Structure are considered. The structure is composed of rocks of the Pechenga Complex, which in turn is divided into well-studied formations of the North-Pechenga subcomplex and poorly studied formations – the South-Pechenga subcomplex. This structure is one of the most studied Early Precambrian structures in the world. Volcanites of the Pechenga Structure have been studied for nearly half a century. However, many determinations of the isotopic ages of reference levels of geological crossection were carried out by insufficiently accurate Rb-Sr, Sm-Nd, and other methods. Nevertheless, in recent years, using the modern U-Pb method, age characteristics of the most important stratigraphic units of the North-Pechenga subcomplex have been obtained.

Petrogenesis and Tectonic Significance of Carboniferous Granites on the North Side of the Solonker Suture, Central South Mongolia

AbstractThe central part of South Mongolia, located to the north of the Solonker Suture, is a key region for studying the late Paleozoic tectonic evolution of the Central Asian Orogenic Belt (CAOB). Voluminous late Paleozoic granitic rocks, especially of Carboniferous age, were intruded in this area. However, these granitoids have not been well studied and there is a lack of precise ages and isotopic data. This has hampered our understanding of the tectonic evolution of southeastern Mongolia, and even the entire CAOB. In this paper, we provide new U‐Pb isotopic ages and geochemical analyses for these Carboniferous granites. One granite from the Ulaanbadrakh pluton yielded a zircon U‐Pb age of 326 Ma, which indicates emplacement in the Early Carboniferous, and three other granites from the Khatanbulag region gave zircon U‐Pb ages of 316 Ma, 315 Ma, and 311 Ma, which indicate emplacement in the Late Carboniferous. The Early Carboniferous granite has SiO2 contents of 70.04–70.39 wt% and K2O + Na2O contents of 6.48–6.63 wt%, whereas the Late Carboniferous granites have more variable compositions (SiO2 = 65.29–77.91 wt% and K2O + Na2O = 5.30–7.27 wt%). All the granites are weakly‐peraluminous I‐types that are relatively enriched in U, Th, K, Zr, Hf, and LREEs. The whole rock Sr‐Nd and zircon in situ Lu‐Hf isotope analyses for the Early Carboniferous granite gave positive values of ∊Nd(t) (2.87) and ∊Hf(t) (4.31–12.37) with young Nd (TDM = 860 Ma) and Hf (TDMc = 1367–637 Ma) two‐stage model ages, indicating derivation from juvenile crustal material. In contrast, the Late Carboniferous granites had more diverse values of ∊Nd(t) (–4.03 to 2.18) and ∊Hf(t) (–12.69 to 5.04) with old Nd (TDM = 1358–1225 Ma) and Hf (TDMc = 2881–1294 Ma) depleted mantle two‐stage model ages, suggesting derivation from remelting of Precambrian basement. Based on the existing results, the tectonic setting of the Late Carboniferous granites in the central part of South Mongolia is known for its diversity, and this paper believes that the tectonic background of the carboniferous granite records the tectonic transition from a continental‐margin‐arc to a post‐collisional extensional setting during the Late Carboniferous–Permian.

Age of the Archean Strata with Banded Iron Formation in the Kostomuksha Greenstone Belt, Karelian Craton, Fennoscandian Shield: Constraints on the Geochemistry and Geochronology of Zircons

The paper presents results of the first geochronologic study (LA-ICP-MS zircon method) of metasedimentary rocks that host the banded iron formation (BIF) and the metarhyolites (halleflinta) of the Gimoly Group in the Kostimuksha greenstone belt,Karelian Craton. Felsic magmatism that produced the rhyolite sills and dikes was dated at 2759 ± 19–2743 ± 15 Ma. The clearly predominant group of zircons in the schists (metagraywacke) has an isotope age of 2753 ±19 Ma, and scarce zircon grains of these rocks vary in age from 3.1 to 2.8 Ga. Geochemically, zircons from metarhyolites fall into three types (1) Eu-depleted and Ce-enriched, (2) Eu- and Ce-enriched, and (3) LREE- and MREE-enriched. The zircons of geochemical types (1) and (2) are also predominant in the sediments. The morphology and structure of zircon crystals in the rhyolites and sediments are also identical. The rhyolites were thus a major source of zircons for the metagraywacke, which alternates with BIF, while the Mesoarchean rocks played a subordinate role. The BIF of the Gimoly Group in the Kostomuksha Greenstone Belt was formed simultaneously with felsic volcanism at 2760–2740 Ma.

Application of Strontium Isotopic Stratigraphy to Dating Marine Sedimentary Units: A Case Study from the Permian Stratotype Section in Southern China

AbstractThe calibration of sedimentary rock absolute dates is one of the difficulties in sedimentological and stratigraphic research. Since strontium (Sr) resides in seawater much longer (≈106 a) than the seawater intermixing time (≈103 a), the Sr isotopic composition of global seawater is uniform at any time and results in a stable system throughout geological history, based on which a global Sr isotope composition dating database has been established for age‐calibration of marine strata. The Permian stratigraphic sections in the northern part of the Upper Yangtze block, southern China, record continuous marine sediments with clear stratigraphic boundaries and is suitable for stratigraphic dating of Sr isotopes. Based on sampling and Sr isotopic compositions of Permian carbonate strata in the northern part of the Upper Yangtze, a Permian Sr isotope evolution curve was established. According to the basic principles of Sr isotope stratigraphy, the global Strontium isotope age database can be used to calibrate the Permian stratigraphic dates in the northern Upper Yangtze. The results show that the Sr isotope evolution curves for the marine carbonate rocks in the Permian stratigraphic section of the Upper Yangtze present a decreasing trend from the mid‐Qixia stage (P2) to the mid‐Wujiaping stage (P3), and then rise from the middle Wujiaping stage to the end of Changxing stage (P3). When the Permian Sr‐isotope evolution curve is compared with the global Sr isotope evolution curve in the northern Upper Yangtze, the two are consistent in their long‐term evolutionary trend, indicating that Permian global geological events are important controlling factors for the composition and evolution of Sr isotopes. The 87Sr/86Sr value decreased gradually in the background of large‐scale regressions at the turn of middle to late Permian period, revealing that the Emeishan basalt eruption occurred near the Maokou/Wujiaping boundary (GLB). Sr‐isotope stratigraphy dating was performed on the boundaries of the Qixia Formation/Maokou Formation, Maokou Formation/Wujiaping Formation (GLB), Wujiaping Formation/Changxing Formation (WCB) and the Permian/Triassic (PTB) using the Global Strontium Isotope Age Database. The results are 270.4 Ma, 261.2 Ma, 254.5 Ma and 249.7 Ma, respectively. Based on this, the eruption age of the Emeishan basalts is defined at about 261.2 Ma., which is more coincident with that acquired from other previous dating methods on the eruption age of the Emeishan basalts, and therefore proves that the application of Sr isotopic stratigraphy to dating marine sedimentary units is an effective method.

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

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

Eo‐Variscan metamorphism in the Bohemian Massif: Thermodynamic modelling and monazite geochronology of gneisses and granulites of the Góry Sowie Massif, SW Poland

AbstractIn the Variscides of central Europe, the tectonothermal history of a few units jointly assigned to the Teplá‐Barrandian/Bohemian microplate had already terminated in the Devonian. One of these is the Góry Sowie Massif (GSM; Sudetes, SW Poland), mainly consisting of variably migmatitic gneisses with subordinate granulite and peridotite inserts. The eo‐Variscan evolution of the GSM was completed in five tectonothermal stages; however, the timing and pressure–temperature (P–T) conditions of the individual stages remain unclear because isotopic ages overlap within error, and previous P–T estimates have been determined without reference to these ages. Our thermodynamic modelling and LA–ICP–MS U–Pb and electron probe microanalysis (EPMA) U–Th–total Pb monazite geochronology provide new insights into the early Variscan multistage evolution of the GSM. The new P–T–time (t) data show that granulites were metamorphosed concurrently with gneisses but at different depths and temperatures. During subduction of Saxothuringia beneath Teplá‐Barrandia at 395–390 Ma, the granulites of crustal provenance experienced high‐P–high‐T conditions (740–750°C, 17–18 kbar) and came in contact with the overlying mantle. Slab break‐off gave rise to decompression from conditions of ~17 kbar/840°C to 11–12 kbar/850–860°C, which continued at 380–370 Ma when the granulites were sheared off and extruded as tectonic slices to lower–middle crustal depths, where gneisses were undergoing metamorphism and migmatization under amphibolite facies conditions. In the gneisses, the oldest monazite date of c. 390 Ma constrains the age of the early tectonometamorphic event D1. Two subsequent events (D2 and D3, sillimanite grade) occurred under conditions of 660–680°C and 5–9 kbar at 380–375 Ma, when, in various parts of the future massif, the gneisses were (re)folded and migmatized due to heat arising from underplated mantle. Isostatic exhumation was accompanied by pegmatite intrusions (D4) at 370 Ma and subsequent orogenic collapse (D5). Concurrently, high temperature conditions (~700°C) promoted a disturbance of the U–Pb system in monazite in some granulites.

О геологическом и изотопном возрасте золоторудных месторождений на примере золото-серебряного месторождения Кубака (Северо-Восток России)

Information on the geological and isotopic age of the Kubaka gold-silver deposit in the Omolon middle massif in the North-East of Russia is presented. It has been established that the Kubaka deposit geological age lies in between the Late Devonian age of the Kedon series volcanites, containing the gold-silver mineralization, and the Early Carboniferous age of the Korbinsky suite terrigenous rocks, overlapping the volcanites and the mineralization. The post-ore nature of the Omolon complex dykes, which produce no significant impact on the distribution of gold mineralization in ore bodies, is shown. According to isotope dating, the following stages of the Kubaka deposit formation are distinguished: the accumulation of the Kubaka suite tuffs (369 Ma); the introduction of subvolcanic intrusions (344 and 337 Ma); the formation of ore metasomatites (335±5 Ma); the formation of gold-silver mineralization (330 and 334 - 324 Ma); the introduction of post-ore dikes (179±8 - 176±10 Ma).

Mid-Ediacaran bimodal magmatism and peri-Baltic affinity of the Brunovistulian terrane documented by the U-Pb isotope and palaeomagnetic data from the Brno Massif (Central Europe)

Palaeomagnetic and isotope (U-Pb, 40Ar-Ar39) studies were conducted in the area of the Brno Massif (Central Europe). The basaltic and granitic dykes, so far regarded as Silurian, were sampled in four localities. New U-Pb isotope ages and palaeomagnetic data indicate that the studied magmatic intrusions, most probably of bimodal type, penetrated the Brno Massif in the mid-Ediacaran (ca 575-565 Ma ago). Palaeomagnetic data support the thesis that the Brunovistulian terrane, a larger tectonostratigraphic unit containing the Brno Massif, was incorporated into the peri-Baltic part of the Cadomian orogenic belt and, at least since the mid-Ediacaran, had a common drift history with Baltica. It crossed the polar region of the southern hemisphere between 580 and 560 Ma. The Brno Massif rotated clockwise by 50 – 60o with respect to stable Europe in the Carboniferous, most probably during the oroclinal bending of the north-eastern segment of the Variscan orogenic belt.

Two key switches in regional stress field during multi-stage deformation in the Carboniferous–Triassic southernmost Altaids (Beishan, NW China): Response to orocline-related roll-back processes

AbstractThe orogenic architecture of the Altaids of Central Asia was created by multiple large-scale slab roll-back and oroclinal bending. However, no regional structural deformation related to roll-back processes has been described. In this paper, we report a structural study of the Beishan orogenic collage in the southernmost Altaids, which is located in the southern wing of the Tuva-Mongol Orocline. Our new field mapping and structural analysis integrated with an electron backscatter diffraction study, paleontology, U-Pb dating, 39Ar-40Ar dating, together with published isotopic ages enables us to construct a detailed deformation-time sequence: During D1 times many thrusts were propagated northwards. In D2 there was ductile sinistral shearing at 336–326 Ma. In D3 times there was top-to-W/WNW ductile thrusting at 303–289 Ma. Two phases of folding were defined as D4 and D5. Three stages of extensional events (E1–E3) separately occurred during D1–D5. Two switches of the regional stress field were identified in the Carboniferous to Early Permian (D1-E1-D2-D3-E2) and Late Permian to Early Triassic (D4-E3-D5). These two switches in the stress field were associated with formation of bimodal volcanic rocks, and an extensional interarc basin with deposition of Permian-Triassic sediments, which can be related to two stages of roll-back of the subduction zone on the Paleo-Asian oceanic margin. We demonstrate for the first time that two key stress field switches were responses to the formation of the Tuva-Mongol Orocline.

Geological features and the first isotopic data of volcanic rocks in the Iset river basin, East-Urals megazone

Relevance of the work. The Iset river basin contains the most extensive outcrops of volcanogenic formations of the Beklenishchevsky complex of the East Ural megazone, the age of which is determined as Early Carboniferous by the ratio of volcanic rocks with faunistically characterized sedimentary deposits. Volcanics here compose flows of andesite-basaltic and andesitic lavas and lava breccias. There are no geochronological dates specifying the age of the rocks, which makes it difficult to assess their role in the formation of the megazone. Therefore, isotopic dating of these formations is very important. Methods. The U – Pb age and data on the geochemistry of zircons were obtained by laser ablation (LA – ICP – MS). Purpose of the research is to study the features of the geological structure, the material composition of volcanic rocks in the Iset river basin, the geochemistry of zircons from andesites and the determination of their isotopic age. Results of the work and the scope of their application. Lava flows of andesites and basaltic andesites with minor amounts of basalts and dacites have tectonic contact with sedimentary rocks of the Early Carboniferous age. The distribution of rare elements in volcanics is typical of supra-subduction formations. Zircons in andesites are represented by prismatic and isometric crystals. Prismatic differences in the nature of the distribution of REE and the content of Li, Ti, Sr, Th, U refer to zircons of magmatic genesis, isometric – to “hydrothermal”. According to the U / Yb – Y ratios, the former correspond to the zircons of the ocean floor, while the latter are related to the continental ones. Isotopic dating of zircons from andesites was carried out for the first time. Their age was 311 million years. The data can be used in geological mapping, as well as in the compilation of large-scale geodynamic maps and diagrams. Conclusions. Volcanic rocks in the Iset river basin were formed in supra-subduction continental-marginal geodynamic conditions that took place in the Urals in the Carboniferous. The obtained value of the age of zircons from andesites, possibly, fixes the stage of their transformation. Keywords: East-Ural megazone, volcanic rocks, zircon, isotopic age.