Isotopic Ages Research Articles

ВОЗРАСТ И СОСТАВ МАГМАТИЧЕСКИХ ОБРАЗОВАНИЙ КАЙНОЗОЙСКОГО ОСАДОЧНОГО ЧЕХЛА ШЕЛЬФА О. САХАЛИН

For the first time, data on the K-Ar isotopic age and petrological-geochemical composition were obtained for igneous rocks using the core samples recovered from the previously drilled holes on the Sakhalin shelf. Based on these data, the igneous rocks were compared to the rocks of the Anivsky, Orlovsky, Lamanonsky, and Chekhovsky volcanic complexes. Isotopic dating on rocks from the Lesogorsky complex of hypabyssal small intrusions exposed on the west coast of Sakhalin Island allowed tighter constrains to be placed on the timing of its formation (8.9–16.2 Ma).

Uranium-lead isotopic age of monazites of plagiogneyses of the Ingulo-Inguletse series (Ingul megablock of the Ukrainian Shield)

Among the supercrustal formations of the Ingul megablock, Paleoproterozoic rocks, represented by the Ingul-Ingul series, are significantly predominant. In general, the composition of the Ingul-Ingulets series includes (from bottom to top): Zelenorichen, Artemiv, Rodioniv, Kamyano-Kostovatsky (age analogue of Spasivsk) and Roschakhiv (age analogue of Chechelivsk) worlds. The Chechelivsk suite is represented by biotite and garnet-biotite gneisses, places with sillimanite and cordierite, as well as underlying layers of pyroxene gneisses and schists. Suite rocks are widespread within the Ingul Synclinorium, in the framework of the Korsun-Novomyrhorod pluton and in the northern part of the Bratsky Synclinorium. Currently, the age of monazites from plagiogneisses distributed in the Novgorodkiv quarry and south of the village Lavrovka has been determined using the uranium-lead isotope method. The obtained monazite isotopic age values of 2022.9 ± 6.7 Ma and 2031.5 ± 1.5 Ma, respectively, can only serve as the upper age limit for the formation of the Ingul-Ingul series. Analysis of the internal structure of zircon crystals from plagiogneiss distributed south of Lavrivka, showed that the majority of zircons are complex. In almost all crystals there are mainly thin shells of light pink zircon. Light pink zircon cores are found in the middle of the light pink crystals. Crystals with a brown color have a more complex structure. Along with isometric cores of light pink zircon, cores with fine «magmatic» zoning occur, as well as highly fractured uniform brown cores and complex cores.On this basis, it is assumed that this plagiogneiss was formed by clastogenic material from several sources.

U-Pb isotopic geochronology of the monazite gneisses of the «zelenolevadivska suite» of the Middle Bug region

On the right bank of the South Bug river valley, opposite the village of Zelena Levada crystalline rocks are observed in separate scattered rock outcrops, more than 1 km long along the river bed. A bundle of hypersthene-biotite and biotite plagiogneisses is exposed here, which in some areas has undergone calishpatization, manifested in the form of unevenly placed porphyroblasts of potassium feldspar, thin veins of quartz-feldspar composition and migmatization. Gneisses are dissected by single veined and lenticular bodies of granites. The gneiss pack is assigned to the Zelenolevada stratum of the Dniester- Bug series. The section is considered to be a stratotype for the Zelenolevada Formation. So far, the following values of isotopic ages have been obtained for monazites from microclinized plagiogneiss and from a lenticular body of granite exposed in this area — 2031.5 ± 6.7 million years and 2030.3 ± 1.7 million years, respectively. This publication presents the dating results of monazites from calishpatized plagiogneiss. microclination is manifested in the formation of thin veins composed of quartz+microcline+plagioclase+ biotite and porphyroblasts of microcline. Optical studies of polished sections of zircon crystals revealed that the crystals of this mineral have a complex structure, they are formed by heterogeneous, often with rounded contours, light pink (brownish pink) cores and shells of milky white zircon. The presence of heterogeneous, including cores with rounded (rolled) cores indicates the primary-sedimentary (volcanogenic-sedimentary) nature of plagiogneiss. According to the results of uranium-lead isotope dating of multigrain samples of monazite, the age of monazite (weighted aver- age value based on the ²⁰⁷Pb/²⁰⁶Pb ratio) is 2031.5 ± 2.7 million years.

ВОЗРАСТ ЗОЛОТОГО ОРУДЕНЕНИЯ КЕТКАПСКО-ЮНСКОЙ МАГМАТИЧЕСКОЙ ПРОВИНЦИИ, ФОРМАЦИОННАЯ ПРИНАДЛЕЖНОСТЬ ЗОЛОТОНОСНЫХ КОМПЛЕКСОВ И РАЗНОВРЕМЕННОСТЬ ПОЗДНЕМЕЗОЗОЙСКОГО МАГМАТИЗМА В РАЗНЫХ ЧАСТЯХ АЛДАНСКОГО ЩИТА

The first 40Ar/39Ar isotopic age data for hydrothermal vein gold mineralization in the late Mesozoic Ketkap-Yuna igneous province of the Aldan shield confirm the correlation between this style of mineralization and the Early Cretaceous sub-alkali magmatism, which was established by geological observations. The combination of geological characteristics and U-Pb data on magmatites enabled us to indirectly determine the age of the highly productive bimetasomatic «massif-skarn» type of mineralization associated with sub-alkali magmatogenic formations of the province. The isotopic dating of magmatites and gold mineralization in the Ketkap-Yuna igneous province and other late Mesozoic igneous provinces of the Aldan shield shows consistency in the ages of ore-bearing magmatites and associated ores. The delay in time of the Late Mesozoic magmatism manifestations in the Ketkap-Yuna igneous province and the associated gold mineralization relative to the intensification of tectonomagmatic processes in the Western and Central Aldan, as well as differences in the correlations of different types of igneous formations in the provinces, are explained by the characteristics of the regional tectonic structure and, as a consequence, the specific nature of the Late Mesozoic magmatism development in different parts of the Aldan shield. Two large areas of the late Mesozoic intensification of tectonomagmatic processes are identified in the Aldan shield that differ both in the time of the onset of polyformational magmatism and the accompanying different-type mineralization, and in the dominant formational type of magmatites: West-Central Aldan, on the one hand, and East Aldan, on the other. The former is characterized by continuous magmatic activity from the Berriasian to the early Albian (≈ 30 Ma) and predominance of the leucitite–alkali-(foid)-syenite formation, and the latter by manifestations of magmatism over a timeframe half of the above period (≈ 15 Ma) and predominance of the subalkaline diorite-granodiorite-granite formation. The termination of the late Mesozoic magmatism in both areas was subsynchronous. A «set» of magmatogenic formations in them is also similar: leucite-alkaline-(foid)-syenite with alkali granites, monzonite (shonkinite)-syenite and subalkaline diorite-granodiorite-granite. The Coniacian–Santonian outbreak of alkali volcano-plutonism during the intensification of tectonomagmatic processes is characteristic of the East Aldan region, which manifested itself in the Ketkap-Yuna igneous province after a long (about 30 Ma) period of amagmatism.

ВОЗРАСТ (U-Pb, LA-ICP-MS) ДЕТРИТОВЫХ ЦИРКОНОВ ИЗ ОСАДОЧНЫХ ПОРОД КОМСОМОЛЬСКОЙ СЕРИИ (CЕВЕРНЫЙ CИХОТЭ-АЛИНЬ)

A geochronological study of detrital zircons from sedimentary rocks of the Gorinskaya (91 zircons with concordant ages), Pionerskaya (117 zircons with concordant ages), and Pivanskaya (115 zircons with concordant ages) formations of the Komsomolskaya Group was carried out. Comparison of the 206Pb/238U isotopic age of the youngest zircon populations from these formations with their stratigraphic age revealed that they are consistent only for one of them (Pivanskaya Formation). For the other two formations, the youngest zircons turned out to be significantly younger than the upper limit of their stratigraphic age. New evidence shows that: 1) the age of the «Berriassian-Valanginian» deposits of the Pionerskaya Formation is not older than the Barremian; and 2) the age of the «Titonian-Berriasian» deposits of the Gorinskaya Formation is not older than the Hauterivian. The division of deposits in the Komsomolskaya Group into formations is based primarily on lithological criteria, and since the composition of the formations is very similar, similar deposits in other Lower Cretaceous sections of the region may have been assigned to other stratigraphic units, so the above conclusion is valid only for the sections we studied.

ВРЕМЕННЫЕ ИМПУЛЬСЫ КАЙНОЗОЙСКОГО ЭКСПЛОЗИВНО-ФРЕАТИЧЕСКОГО ВУЛКАНИЗМА В ЮГО-ЗАПАДНОМ ПРИМОРЬЕ. КОРРЕЛЯЦИЯ ИЗОТОПНЫХ И ФИТОСТРАТИГРАФИЧЕСКИХ ДАТИРОВОК

The Cenozoic tephra deposits - products of explosive phreatic eruptions of maar volcanoes in the southwest of Primorye are studied. The deposits represent rhyolitic ash and pumice pyroclastic beds with a high terrigenous component, including tephroid pseudo-conglomerates. Isotopic dating of tephra beds established two time pulses of explosive volcanism: 30–34 Mya and 23–24 Mya. The first time pulse coincided with the beginning of the formation of marginal seas and continental coal basins. It corresponds to the most productive stage of coal accumulation, the burial of wood wastes and their coalification at a faster rate, and the development of a high-temperature geothermal field and can be compared with the well-known catastrophic eruption of Mount St. Helens in the U.S.A. The second time pulse of explosive volcanism had a regional character of manifestation. It is characterized by the formation of green tuff complexes on submarine elevations of the Sea of Japan as well as along the western and eastern coasts of Japan. Synchronously with the volcanic activity started the acceleration of the sinking rate of the Sea of Japan bottom in response to the active rising of asthenospheric diapirs. The established isotopic ages do not conform to the ages determined for fossil leaves and pollen from the deposits, which may reflect the climate-forming type of such an explosive process.

Origin and the U-Pb isotopic age of detrital zircons from the Yaregsk oil-titanium and Pizhemsk titanium deposits (Timan)

Origin and the U-Pb isotopic age of detrital zircons from the Yaregsk oil-titanium and Pizhemsk titanium deposits (Timan)

URANIUM-LEAD GEOCHRONOLOGY BY TITANITE, ADVANTAGES AND LIMITATIONS

The uranium-lead isotope method is used to date titanites from granitoids of the Ros'-Tikych megablock of the Ukrainian Shield (middle reaches of the Ros' river, below Bila Tserkva). Their isotopic behavior is analyzed along with published results, of other titanites found in crystalline rocks of the Ukrainian Shield. Titanites of granitoids from the Ros'-Tikych megablock contain impurity lead and this leads to an abnormal isotopic composition. The overestimation in age values calculated by methods that do not require correction for ordinary lead (method normalized, agreed differences and the slope of the regression line on the graph in the coordinates 207Pb/204Pb — 206Pb/204Pb) is up to several tens of millions of years, compared to, calculated on the model of Ahrens-Wetherill. The influence of impurity lead of anomalous isotopic composition on the distortion of the calculated values of the isotopic age, in the ratio 207Pb/206Pb is modeled. It was found that the entry into the isotope system, 2.0 billion years old, of ordinary lead with a modern isotopic composition, even with a ratio of impurity lead to radiogenic lead 1:1, will reduce the age by 207Pb/206Pb by only 4 million years. Significantly greater distortions (overestimations) of the isotopic ages is will be caused by impurity lead, in the isotopic composition of which there is an ancient (for example, 500 million older than the age of crystallization of titanite) radiogenic lead. Thus, when the ratio share of impurity lead is 20%, the overestimation of age is more than 50 million years, and it reaches 235 million years forat a ratio of 1:1 (50%). The numerical values of the isotopic age will be even more inflated at the ratio of 207Pb/206Pb in cases when impurity lead contains radiogenic lead of older age, even with a smaller share of it in the composition of impurity lead.

Interpretation of geological significance of Re-Os isotopic age of bitumen

Re、Os亲有机质的特性使得Re-Os同位素在富有机质地质样品中得到了广泛应用；沥青由大量重组分有机质组成，对Re、Os具有较强的富集能力，并且沥青样品中Re-Os同位素封闭性较好，因此沥青Re-Os同位素已经成为当今国际上研究的一个热点。然而，沥青Re-Os同位素年龄究竟是代表烃源岩的沉积时代、油气成藏时代、还是油气藏破坏后油气运移时代，至今还没有明确的定论。本文对新疆乌尔禾沥青矿、四川矿山梁沥青矿样品进行了Re-Os同位素分析，结果显示，乌尔禾沥青矿Re-Os同位素年龄为269±9Ma，老于周围白垩系砂岩地层沉积时代，Os同位素初始比值¹⁸⁷Os/¹⁸⁸Os （Os_i）为0.367±0.028，小于二叠纪海水的Os_i同位素比值，表明得到的乌尔禾沥青Re-Os同位素年龄反映的是乌尔禾油气藏的成藏时代。矿山梁沥青矿Re-Os同位素年龄为537.3±5.2Ma，老于围岩（寒武系、奥陶系、志留系地层）沉积时代，Os同位素初始比值为0.716±0.028，与早寒武世海水的Os_i同位素比值一致，表明得到的矿山梁沥青Re-Os同位素年龄代表烃源岩的沉积时代。结合前人发表的沥青Re-Os同位素数据，沥青Re-Os同位素等时线年龄可能反映烃源岩的沉积时代，也可能代表油气藏的形成时代，这主要取决于油气成藏过程中，具有不同Re-Os同位素特征的原油是否能够混合均匀。沥青Re-Os同位素年龄能否反映油气藏遭受破坏而发生运移的时代，主要取决于沥青的热演化程度以及油气藏破坏过程中原油的混合程度。沥青Re-Os同位素研究不仅能够为油气成藏时代的厘定提供了一种有力的工具，而且对于油气烃源岩的判别以及油气成因研究都具有十分重要的意义。

Geochemical heterogeneity of polychronous relict zircons from restitogenic ultramafic rocks of the Shaman massif (Eastern Transbaikal region)

Research subject. The distribution patterns of rare earth elements (REE), as well as Y and Th, in the grains of polychromous zircons from the restitogenic ultramafic rocks of the Shaman massif (Eastern Transbaikalia). This massif is a steeply inclined protrusion that is part of the eastern branch of the Baikal-Muya ophiolite belt.Materials and methods. 31 zircon grains 100–150 μm in size were isolated from a composite sample of harzburgites and dunites with a total weight of 4 kg for their subsequent U-Pb isotope dating. These analyzes were performed by the LA-ICP-MS method by scanning along straight profiles on the plane of sections of representative zircon grains.Results. All zircon grains from the general collection are characterized by a rounded shape, a rough surface, microfracturing, a weak cathodoluminescent glow to a complete absence, and an irregular oscillatory zoning. In some grains, microinclusions of epigenetic minerals, such as quartz, mica, etc. were found. It was previously determined that, within the entire collection of zircon grains, the values of their age, as well as U and Th contents, vary across rather wide intervals (3049–502 Ma), the reasons for which are the subject of discussions. The LA-ICP-MS scanning over the profiles of representative zircon grains from the general collection showed that REE, Th, and Y are distributed highly unevenly, occasionally showing signs of zoning. It is assumed that the zircons found in the ultramafic rocks of this massif are a relict phase and appeared as a result of the transformation of very ancient (more than 3 billion years old) juvenile crystals of this mineral, which had been originally located in the upper mantle protolith.Conclusions. Transformations of juvenile zircons and their transformation into a relict phase occurred in the process of partial melting of the protolith, during which they underwent thermal action (annealing), chemical resorption, as well as disturbances in their U-Pb systems, which caused uneven “rejuvenation” of their isotopic age. It is also assumed that the revealed geochemical heterogeneity of relict zircons was mainly due to the later redistribution of trace elements with the simultaneous formation of microinclusions of epigenetic minerals in the process of infiltration along microcracks into ultramafic rocks, precipitated by acidic melts.

Ophiolites and ocean plate stratigraphy (OPS) preserved across the Central Mongolian Microcontinent: A new mega-archive of data for the tectonic evolution of the Paleo-Asian Ocean

Following the breakup of Rodinia and the opening of the Paleo-Asian ocean the relative positions of geological units (now in Mongolia) remain uncertain, and in consequence relevant paleogeographic reconstructions are poorly construed. In this paper, we apply an integrated approach to the reconstruction of the Paleo-Asian ocean using preserved ophiolites and ocean plate stratigraphy (OPS) in the Mongolian Lake and Altay zones in western Mongolia, which surround the Precambrian Central Mongolian Microcontinent (CMM) in the center of Central Asia.The Lake Zone of Mongolia contains many sediments of Ocean Plate Stratigraphy (OPS) and their underlying basalt-dominated ophiolites, which occur in three main zones along the western margin of the Precambrian crystalline basement of the CMM. The ophiolites in all zones have similar isotopic ages of ca. 570 Ma. The sections of OPS, together with serpentinite mélanges and other volcanogenic-sedimentary assemblages, have been thrust in nappe sheets over the Zavkhan block of the CMM.In the Neoproterozoic microcontinents derived from Rodinia drifted towards Siberia, and a passive margin developed on the Zavkhan block on the eastern margin of the Paleo-Asian ocean. In mid-upper Neoproterozoic times the eastern side of the Paleo-Asian oceanic plate was thrust over the CMM, and in the early Cambrian the Khasagt and Tas Khairkhan ophiolites were obducted and a mid-ocean ridge (?) reached the CMM, preventing further obduction. In the Neoproterozoic the western margin of the Paleo-Asian ocean was subducted under the Rodinia-derived microcontinent and evolved into an active continental margin. In mid-Cambrian–early Ordovician times thick turbidites were deposited in the Altay. Marine sediments, which had accumulated in the Devonian to Carboniferous in the Paleo-Asian ocean are preserved in a remnant ocean in the Lake Zone. Locally in the Devonian, terrigenous – volcanogenic sequences were deposited in the Ulgey extensional superimposed basin, and later in Meso-Cenozoic time (?) some ophiolitic and OPS fragments (Tsagaan Nuur, Ulaankhus and Bodonch) were exposed along fault escarpments that were possibly re-activated during transpression related to the India-Eurasia collision.Our analysis of the Neoproterozoic–Middle Paleozoic tectonic development of western Mongolia leads to a new holistic model for the evolution of the western Altaids, according to which the ophiolites and OPS formed in the Mongolian Lake and Altay zones between two microcontinents within a new tectonic framework of the western Altaids.

Serpentinite as a tracer for tectonic setting and mantle metasomatism of ophiolites: A case study of the Aoyougou ophiolite in the Qilian Orogenic Belt, NW China

Serpentinites, particularly those transformed from mantle peridotites, are potential tracers for the petrogenesis and tectonic geodynamics of peridotitic protoliths and certain mantle processes (e.g., fluid/melt metasomatism). This study presents new petrographic observations, mineral chemistry, geochemistry, and Re-Os isotopic compositions of the serpentinites from the Aoyougou ophiolite, the most problematic ophiolite in the Qilian Orogenic Belt, NW China. Our data indicates that the protoliths are melting residues of oceanic lithospheric mantle in a forearc setting. The high Mg# values (90.49–93.06) of serpentinites confirm their refractory nature, which reflects multiple episodes of partial melting and melt extraction (13.86–18.90%). Moreover, the geochemical fingerprints (e.g., U-shaped rare earth element chondrite-normalized patterns, enrichments of fluid-mobile elements, positive Eu anomalies, extremely high 187Re/188Os ratios, high Re abundances) and the “future” model ages (TMA) of most of the serpentinites show that they had undergone intensive melt/fluid metasomatism. Metasomatism was probably triggered by the input of slab-derived melts/fluids in the forearc setting, which resulted in the introduction of a high abundance of Re. Moreover, the Re-Os isotopic ages record a two-stage evolutionary history of mantle residues that included Precambrian melt extraction and late-Cambrian Re input by mantle metasomatism. Combined with the geological context, an intra-oceanic subduction model was proposed to interpret the origin and evolution of the ophiolite. As a result of the intra-oceanic subduction of the North Qilian Ocean, the oceanic crust of the ophiolite was generated by decompressive melting of the previously depleted forearc mantle.

LA-ICP-MS U-Pb dating and geochemical characterization of oil inclusion-bearing calcite cements: Constraints on primary oil migration in lacustrine mudstone source rocks

Abstract To date, few isotope age constraints on primary oil migration have been reported. Here we present U-Pb dating and characterization of two fracture-filling, oil inclusion-bearing calcite veins hosted in the Paleocene siliciclastic mudstone source rocks in Subei Basin, China. Deposition age of the mudstone formation was estimated to be ca. 60.2–58.0 Ma. The first vein consists of two major phases: a microcrystalline-granular (MG) calcite phase, and a blocky calcite phase, each showing distinctive petrographic features, rare earth element patterns, and carbon and oxygen isotope compositions. The early MG phase resulted from local mobilization of host carbonates, likely associated with disequilibrium compaction over-pressuring or tectonic extension, whereas the late-filling blocky calcite phase was derived from overpressured oil-bearing fluids with enhanced fluid-rock interactions. Vein texture and fluorescence characteristics reveal at least two oil expulsion events, the former represented by multiple bitumen veinlets postdating the MG calcite generation, and the latter marked by blue-fluorescing primary oil inclusions synchronous with the blocky calcite cementation. The MG calcite yields a laser ablation–inductively coupled plasma–mass spectrometry U-Pb age of 55.6 ± 1.4 Ma, constraining the earliest timing of the early oil migration event. The blocky calcite gives a younger U-Pb age of 47.8 ± 2.3 Ma, analytically indistinguishable from the U-Pb age of 46.5 ± 1.7 Ma yielded by the second calcite vein. These two ages define the time of the late oil migration event, agreeing well with the age estimate of 49.7–45.2 Ma inferred from fluid-inclusion homogenization temperature and published burial models. Thermodynamic modeling shows that the oil inclusions were trapped at ~27.0–40.9 MPa, exceeding corresponding hydrostatic pressures (23.1–26.7 MPa), confirming mild-moderate overpressure created by oil generation-expulsion. This integrated study combining carbonate U-Pb dating and fluid-inclusion characterization provides a new approach for reconstructing pressure-temperature-composition-time points in petroleum systems.

Silver isotope and volatile trace element systematics in galena samples from the Iberian Peninsula and the quest for silver sources of Roman coinage

Abstract Silver played a key role in the progressive monetization of early Mediterranean civilizations. We combine Pb and Ag isotopes with volatile trace elements (Bi, Sb, and As) to assess whether, during the Roman occupation of Iberia, galena constituted a significant source of silver. We find that the Pb and Ag isotopic compositions of 47 samples of galena from eight different Iberian mining provinces, many of them exploited during Roman times, are uncorrelated. This indicates that their respective isotopic variabilities depend on different petrogenetic processes. Moreover, the range of Ag isotopic abundances is approximately six times wider than that displayed worldwide by silver coins in general and Roman silver coins in particular. Although galena from the Betics provides the best fit for Pb isotopes with Roman coins, their fit with Ag isotopic compositions is at best sporadic. We suggest that, together with Sb, Bi, and As, silver is primarily derived from fluids boiled off from differentiated mantle-derived magmas. These fluids, in turn, reacted with preexisting galena and functioned as a silver trap. Lead sulfides with ε109Ag of ~0 and unusually rich in Ag, Sb, Bi, and As were the most probable sources of ancient silver, whereas samples with ε109Ag departing significantly from ~0 reflect low-temperature isotopic fractionation processes in the upper crust.

When did the North Anatolian fault reach southern Marmara, Turkey?

AbstractWe dated syntectonic calcites on fault planes from the southern branch of the western North Anatolian fault (NAF) in northern Turkey using U-Th geochronology. We selected strike-slip faults that are kinematically related to the current regional strain field. The isotopic ages cluster around different periods during the past ~700 k.y. The most prominent cluster peak of 510.5 ± 9.5 ka (1σ) is consistent with the maximum cumulative strike-slip offset data and tectonic plate motions measured by GPS data, highlighting the fact that the present configuration of the NAF in the southern Marmara region started at ca. 500 ka or earlier. These new isotopic ages, combined with previous considerations of regional tectonics, reveal that faulting along the western NAF initiated primarily in the southern Marmara region at least a few hundred thousand years earlier than the timing suggested for the northern branch of the western NAF. This study presents an innovative approach to constrain the timing of initiation of currently active fault segments along the NAF in southern Marmara. U-Th geochronology of fault-hosted calcite thus has a wide application in determining absolute ages of fault episodes in wider shear zones along plate boundaries.

Reassessing the chronostratigraphy and tempo of climate change in the Lower-Middle Permian of the southern Sydney Basin, Australia: Integrating evidence from U–Pb zircon geochronology and biostratigraphy

The Permian of the southern Sydney Basin (Australia) comprises both marine and non-marine sedimentary sequences with evidence of glacial deposition and syndepositional magmatism represented by tuffs, lava flows and hypabyssal intrusions. This succession has been well-studied in lithostratigraphy and sedimentology, but the Lower-Middle Permian chronostratigraphy remains poorly constrained due to a lack of radiometric isotope ages and strong faunal endemism. In this study, we report the first suite of reconnaissance U–Pb zircon ages from this succession, comprising two Wandrawandian Siltstone (WS) tuffs (Green Point tuff 272.6 ± 6.5 Ma population, Callala Bay tuff ≥290 Ma Permian populations) and magmatic zircon ages from three discrete syndepositional intrusions: the Termeil Complex intruding the Snapper Point Formation dated 278.9 ± 3.7 Ma (95% confidence), the Milton Complex intruding the Snapper Point Formation and likely the WS dated 275.5 ± 4.3 Ma, and a dyke/sill at Kinghorn Point intruding the WS dated 275.6 ± 4.3 Ma. In light of the new preliminary U–Pb ages, combined with stratigraphic and biostratigraphic evidence, the age of the Wasp Head Formation is reassessed to be late Asselian-Sakmarian, Pebbley Beach Formation Artinskian, Snapper Point Formation early Kungurian, Wandrawandian Siltstone late Kungurian-Roadian, Nowra Sandstone and Berry Siltstone both confined within early Wordian, and Broughton Formation from mid-Wordian to early Capitanian. The ages and duration of the Permian glacial events are also revised, with the first episode (P1) lasting from basal Permian to early Sakmarian (~299–293 Ma), P2 spanning Artinskian (~290–283 Ma), P3, possibly the longest of all Permian glacial intervals, extending from mid-Kungurian to early Capitanian (~278–263 Ma), and P4 from mid-Capitanian to late Wuchiapingian (~260–254.5 Ma). The ≥290 Ma Permian zircon populations in the Callala Bay tuff reflect either an early Permian volcanism or older zircons recycled from elsewhere.

The Changshagou gold deposit, Eastern Tianshan, NW China: orogenic gold mineralization overprinting a porphyry gold occurrence

Abstract The temporal–spatial relationships of porphyry and orogenic gold mineralization in the Eastern Tianshan Orogenic Belt are ambiguous. The newly discovered Changshagou deposit in this belt contains both porphyry and orogenic gold mineralization, which are characterized by polymetallic–sulfide veinlets and quartz–pyrite veins, respectively. Fluid inclusions in the porphyry mineralization episode were trapped at 290–340°C with salinities of 3.0–8.0 wt% NaCl equiv . The homogenization temperatures and salinities in the orogenic mineralization episode range from 240 to 300°C and from 1.0 to 5.0 wt% NaCl equiv . Coexisting V- and L-type fluid inclusions with similar homogenization temperatures are indicative of fluid immiscibility. The δ 18 O w and δ D w values are in the ranges 7.6–9.1 and −70.9 to −84.0‰ in the porphyry mineralization episode, and 6.4–7.1 and −65.7 to −72.1‰ in the orogenic mineralization episode, overlapping magmatic and metamorphic ranges, respectively. The pyrite δ 34 S values range from 3.5 to 4.9‰, falling into the magmatic range. Pyrite in porphyry and orogenic mineralization episodes yield Re–Os isotope ages of 269.1 ± 2.9 and 257.4 ± 2.4 Ma. The porphyry and orogenic gold mineralizations are genetically associated with the quartz syenite porphyry and Kanggur strike-slip shear activity, respectively.

Whether short-lived or prolonged duration of multistage combined magmatic and hydrothermal events in the giant Chalukou porphyry Mo deposit, China

The duration (or time scales) of multistage combined magmatic and hydrothermal events provides key information constraining the metallogenic history of porphyry deposits, but the duration (time scale) of the mineralising hydrothermal process remains uncertain. Here, we report zircon U–Pb isotopic age, fluorite Sm–Nd isochron ages, and Ti-in-quartz diffusion chronometry for the giant Chalukou porphyry Mo deposit with reserves of 2.46 Mt @ 0.087 wt% Mo in NE China. The deposit is genetically related to hydrothermal fluids associated with a Late Jurassic (148–147 Ma) porphyritic magmatic event coincident with our new fluorite Sm–Nd isochron age of 148 Ma. Porphyry Mo mineralisation is characterised by the superposition of multiple generations of crosscutting quartz-bearing veins including: quartz veins with minor magnetite and hematite associated with K-silicate alteration (A-veins), quartz-molybdenite veins with K-feldspar alteration halos that host the bulk of the Mo mineralisation (B-veins), and late base-metal mineralisation with phyllic alteration (D-veins), as well as post-mineralisation veins with argillic alteration. Hydrothermal veins from the Chalukou porphyry Mo deposit contain multiple generations of quartz that are distinct in their Ti concentrations and scanning electron microscope-cathodoluminescence images. The A-veins preserve subhedral to euhedral characteristics with alternating CL bright and dark growth zoning. The A-veins probably formed at a near-lithostatic pressure of ∼ 1.35 kbar and contain CL-bright quartz assaying 31–71 ppm Ti with Ti-in-quartz temperatures averaging 603 ± 30 °C, and CL-dark quartz assaying 6–25 ppm Ti with Ti-in-quartz temperatures averaging 496 ± 36 °C. The mineralised B-veins also CL-dark quartz, although they display less growth zoning with lower Ti assays of 0.4–27.9 ppm. These veins have near 0.7 kbar pressure and Titanium-in-quartz temperature averaging 390 ± 79 °C. The D-veins are dominated by CL-dark quartz and systematically lower Ti concentrations of 0.4–8 ppm, being lower than the relatively latter vein generations, formed at a temperature of 348 ± 58 °C and pressure of 0.4 kbars. The Ti diffusion in quartz has become a successful method constraining time scales and duration of magmatic and hydrothermal processes. Ti diffusion modelling indicates that the A-veins were deposited and cooled in a period between 2500 and 50,000 years. This shows that the earliest magma-related hydrothermal fluids activities formed within tens of thousands of years.

Petrology of Granitoids of the Selennyakh Ridge (Verkhoyansk-Kolyma Orogenic Belt)

The Verkhoyansk-Kolyma orogenic belt is characterized by intense Late Mesozoic granitoid magmatism. Numerous granitoids plutons form longitudinal belts, elongated parallel to the boundaries of major tectonic structures (Main and Northern), and transverse belts, oriented across or at an angle to them. The Main belt is dominated by massifs of granodiorite-granite composition, accompanied by tin-tungsten, boron-tin, and gold mineralization of various scale. Therefore, understanding their petrological and genetic characteristics and crystallization conditions leading to the generation of mineralization is of not only theoretical but also practical interest. The aim of the research was a detailed study of petrography, geochemical features and crystallization conditions of granodiorite-granite massifs of the Selennyakh block of the Omulevka terrane of the Kolyma-Omolon microcontinent that forms part of the Verkhoyansk-Kolyma orogenic belt. It was found that the formation of granitoids took place in an active continental margin setting and was long-term and complex. During the evolution of magmatism, the homodrome character of development (granodiorites → granites → leucogranites and aplites) was replaced by the antidrome one (granite-porphyries and granodiorite-porphyries). The Rb-Sr isotopic age of the rocks varies from 136 to 122 Ma. The generation of the parent melts for the granitoid massifs occurred within the lower crust at the boundary between amphibolite and dacite-tonalite substrates at temperatures of 1070–990° C and a pressure of 1.1–0.9 GPa. These parameters are comparable to those of the melt that formed the granodiorite-porphyry dikes: 990° C and 0.94 GPa. Maintaining high temperatures of the melt formation from initial to final derivatives at deeper levels of the magma chamber with a simultaneous increase in their fluid saturation requires the supply of juvenile heat and fluids. The main mineral in the territory is tin. The formation of mineralization is associated with late fluid-saturated derivatives of the granitoid melt. During the crystallization of leucogranites and pegmatites, fluorine was the main Sn-extracting agent. With depth, in the course of crystallization of granite and granodiorite porphyries, boron and then sulfur became the major extractants of tin.

Digital geological map and geochronological database of the Cenozoic cover of the southern Mesa Central province, Mexico

The digital geological map of the southern sector of the Mesa Central province is presented, covering an area of ~41 000 km2 in central Mexico. This first interactive map is a compilation of the geological maps available in the literature and the unpublished maps done by our work team. The map focuses on the Cenozoic stratigraphy, supported by a geochronological database of 261 isotopic ages derived from K-Ar, U-Pb, Ar-Ar, and fission tracks methods. The interactive map displays the lithostratigraphic and chronostratigraphic units and the major and second-order Cenozoic faults. Map construction considered lithostratigraphy and geochronological correlation criteria and the main unconformities. Integrating these data in a single digital map allows a regional vision of the southern Mesa Central, making the map a valuable work tool to better understand the Cenozoic geologic evolution of central Mexico.