Isotope Geochronology Research Articles

Measurement, decomposition and level-switching in historical science: Geochronology and the ontology of scientific methods.

Philosophers of the historical sciences have focused to a significant extent on the problem of epistemic access facing these sciences: how do historical scientists overcome the relative scarcity of data about the past, compared to the present? Solving this problem usually requires solving another one, which I call the 'problem of ontic access:' how do historical scientists get access to entities and processes with properties that are potentially informative about the past? The case of geochronology illustrates one solution to this problem: historical scientists can get access to entities and processes with properties that are potentially informative about the past by exploiting the metaphysical structure of their domain. Geochronology experienced a spectacular explosion of its research boundaries in the 20th century. I explain this productivity by analyzing the ontology implicit in geochronological techniques. The productivity of isotope geochronology was based on (a) mereological decomposition in order to (b) exploit differences of properties obtaining between the parts and the whole, and (c) an exceptional complementarity between mass spectrometry and the lower-level properties, allowing application to a wide range of geological contexts. The technologically mediated ability of the scientists to exploit the metaphysical structure of their domain was crucial to their success.

Problems of dating the Au-Pd mineralization of the Chudnoye deposit (the Subpolar Urals)

The ages values of the minerals that make up the ores of the Chudnoe deposit, established using isotope geochronology methods, do not allow us to correctly estimate the time of formation of Au-Pd mineralization due to the lack of reliable signs of their syngeneticity with gold. Based on linking isotope-geochronological data with the history of the geological development of the Subpolar Urals and the entire complex of endogenous and exogenous processes which determined the features of its geological structure and minerageny, the Early Paleozoic age of mineralization is substantiated. The similar age of the metarhyolites hosting the mineralization suggests not only a spatial, but also a paragenetic relations between ore mineralization and felsic volcanites. When studying the mineral composition and structural and textural features of ores, it is necessary to take into account the imposition of post-ore processes, especially the most powerful metamorphism manifested in the Subpolar Urals, the peak of which occurred in the Upper Paleozoic (~250 Ma).

Diffusion-induced stress in crystals: Implications for timescales of mountain building

Intracrystalline chemical diffusion offers valuable insights into the durations of metamorphic and igneous processes. However, it can yield timescale estimates for orogenic and subduction zone events that are considerably shorter than those obtained via isotopic geochronology. One potential explanation that has been offered for the discrepancy is that the interdiffusion of species with different atomic or ionic radii may generate intracrystalline, compositional stresses that alter or limit diffusional relaxation. In this study we test this idea by developing and applying the compositional stress theory of materials scientists F. Larché and J. Cahn to garnet from the Barrovian sillimanite zone, Scotland. Relaxed contacts from the garnet, independent diffusion chronometers, and thermal modeling all indicate a >100 kyr duration for peak temperature metamorphism. Nonetheless, the garnet records sharp, μm-scale variations in calcium and iron contents that standard diffusion treatments predict should relax in 1–10 kyr at peak temperature conditions. Our results show that the development of compositional stress during diffusional relaxation can explain the preservation of the observed short wavelength compositional oscillations at a >100 kyr timescale. Thus, it may be necessary to account for compositional stress when modeling diffusion in solid solutions with appreciable differences in their endmember molar volumes. This will be particularly relevant when considering sharp, μm-scale chemical gradients involving grossular, the garnet endmember with the largest molar volume relative to pyrope, almandine, and spessartine. Neglecting compositional stress in such cases could result in the underestimation of the timescales of lithospheric processes by potentially orders of magnitude. The effects of compositional stress in garnet are predicted to be the most pronounced under amphibolite and blueschist–eclogite facies conditions. At lower temperatures diffusion is limited, and at higher temperatures both plastic deformation and more ideal solid solution behavior will act to diminish the impact of stress.

Pressure–temperature evolution, protolith characteristics and tectonic setting of granulite facies rocks from Laojinchang area in the Longgang Block, China

The Archean supracrustal rock series in the Laojinchang area of Jilin Province, China provide valuable insights into the Precambrian geological evolution of the North China Craton. This study presents new results on petrography, geochemistry, mineralogy, isotope geochronology and phase equilibrium modeling of the Laojinchang granulite facies metamorphic rocks. The protolith ages of garnet two-pyroxene granulite and amphibolite from Sidaolazihe Formation, and two-pyroxene granulite from Laoniugou Formation are 2565 ± 5.9 Ma, 2512 ± 17 Ma, and 2509.2 ± 8.6 Ma, respectively, with the metamorphic age recorded by amphibolite being 2475.6 ± 9 Ma. New geochemical data indicates that TTGs (tonalite-trondhjemite-granodiorite) weathering in an island arc environment formed the protolith of argillaceous gneiss of Sidaolazihe Formation. Basaltic andesite, formed under an island arc environment, was the protolith of garnet two-pyroxene granulite of Sidaolazihe Formation; while calc-alkaline basalt was the protolith of amphibolite from the same formation and two-pyroxene granulite from Laoniugou Formation. All these Archean supracrustal rocks exposed in the Laojinchang area have recorded nearly isobaric cooling (IBC) anticlockwise P-T paths with distinct prograde, peak and post-peak stages. The peak P-T conditions reached granulite facies, suggesting a heat source from underplated magmas. A tectonic model involving an island-arc setting is proposed for the evolution of these Neoarchean supracrustal rocks in Laojinchang area.

Solid bitumen Rhenium-Osmium (Re–Os) isotope geochronology and existing problems: Sampled of Sinian-Cambrian gas reservoir in Sichuan Basin, China

Understanding the key timings related to petroleum evolution is crucial for optimizing exploration targets and assessing oil/gas resources, attract petroleum geologists’ attention worldwide. Recently, hydrocarbon (oil and bitumen) Re–Os isotope dating has been innovatively applied to constrain the timing related to oil/gas generation, however, the resulting Re–Os isochron ages can be complex and challenging to interpret. This study utilizes various geochemical and geochronological data from Sinian to Cambrian natural gas reservoirs in the Sichuan Basin to reconstruct the hydrocarbon evolution process and discuss the significance of different bitumen Re–Os dating results. The gas accumulation in the Sinian-Cambrian reservoirs experienced four stages of evolution: (1) initial oil generation during the Ordovician to Silurian periods, (2) secondary oil generation during the Triassic period, (3) gas generation through thermal cracking of liquid oil from the Jurassic to Cretaceous periods, and (4) gas reservoir redistribution since the late Cretaceous. The Re–Os dates (ca. 485 Ma) of low maturity and biodegraded bitumen from the western Sichuan Basin record the oil generation during the Ordovician before the Caledonian tectonic event. The Re–Os dates (ca.184–128 Ma) of highly mature bitumen associated with MVT Pb–Zn deposits in northern Sichuan Basin provide insights into both liquid oil-cracking and thermochemical sulfate reduction (TSR) processes. The complex Re–Os dates (ca.414 Ma, ca.154 Ma) of highly mature bitumen from the central Sichuan Basin may represent different periods related to either oil or gas generation. Future studies should explore the genetic type, maturity, thermal cracking, or TSR degrees of bitumen to better understand the significance of Re–Os dates.

In Situ Lu–Hf Garnet Dating of Archean Deep Crust Granulites from the Polymetamorphic Grenville Front Tectonic Zone

Abstract Recent advances in geochronological techniques now make it possible to efficiently decipher the timing and duration of geological processes in complex high-grade polymetamorphosed orogenic terranes. This is the case of the Grenville Front Tectonic Zone, which truncates the Superior Craton to the southeast. The zone exposes parautochthonous Archean rocks that underwent mid- to high-pressure granulite facies metamorphism of uncertain age. The metamorphic assemblages have been either interpreted as Archean and associated with the final stages of the Superior Craton assembly or as the result of Mesoproterozoic Grenvillian metamorphism, based on cross-cutting relationships and traditional geochronology methods such as U–Pb zircon and 40Ar–39Ar mica dating. Herein, we revisit the extent of the Grenvillian metamorphic overprint in the parautochthonous domain and provide new age constraints for granulite-facies metamorphic assemblages through in situ garnet dating within migmatitic paragneiss, migmatitic orthogneiss, and mafic granulites, combined with in situ trace element mapping. Six samples, which show bell-shaped and occasionally sharp and oscillatory lutetium growth zoning in garnet, yield garnet Lu–Hf isochrons with identical Archean dates of c. 2.6 Ga. Sparse analyses of the material trend towards Grenvillian ages (c. 1 Ga) in one sample from which garnet shows lutetium zoning consistent with post-growth fluid-assisted disturbance. Overall, our results indicate that the widespread granulite-facies metamorphism within the Grenville Front Tectonic Zone is dominantly late Neoarchean in age, unveiling a rare exposure of Archean lower crust in the southeastern Superior Craton. Our results also point towards a limited Grenvillian metamorphic overprint, though the spatial extent and precise thermal conditions of this metamorphism are still unknown. The results presented herein demonstrate the potential of in situ isotopic geochronology on rock-forming minerals like garnet in polymetamorphic terranes.

Moisture availability and groundwater recharge paced by orbital forcing over the past 750,000 years in the southwestern USA

Quaternary climate changes are driven in part by variations in the distribution and strength of insolation due to orbital parameters. Continental climate variability is well documented for the most recent glacial-interglacial cycles, yet few records extend further back in time. Such records are critically needed to comprehensively assess the entire spectrum of natural climate variability against the backdrop of anthropogenic warming. Here, we apply uranium isotope geochronology to calcite deposits to date groundwater-table changes in Devils Hole cave, Nevada. The deposits record multi-meter groundwater-table fluctuations over the last 750,000 years, reflecting the long-term evolution of hydroclimate in this presently arid region. During periods between glacial or interglacial extremes, the water table responded sensitively to variations in 65°N summer insolation, likely caused by the increasing extent of North American ice sheets during cold period, which steered moisture-laden trajectories towards the southwestern USA. These orbitally-driven hydroclimatic changes are superimposed on a tectonically-driven long-term decline in the regional groundwater table observed prior to 438,000 ± 14,000 years ago.

The mafic–ultramafic roots of a Mesoarchean microblock in southern India: Insights from petrochemistry and isotope geochronology of the Coorg Block

The Coorg Block in southern India is an archive of Paleo-Mesoarchean arc magmatism and continental building in the early Earth. Here we investigate a suite of mafic and ultramafic rocks and associated lithologies to characterize the roots of this microcontinent. We present petrologic, geochemical, as well as U-Pb and Lu-Hf data on zircon and U-Pb data on monazite from these rocks which provide insights into the petrogenesis of the rocks as well as the timings of magmatism and subsequent metamorphism. The geochemical features of the mafic granulites indicate that the rock suite was derived from the fractionation of a sub-alkaline tholeiite magma. Most of the rocks correspond to arc setting in a subduction-related tectonic regime. The rocks exhibit relative enrichment in LILE, Pb, and LREE with depletion in HFSE such as Nb and Ta suggesting partial melting of a metasomatized mantle wedge. Their high LILE/HFSE and LREE/HFSE ratios indicate slab dehydration and mantle wedge melting in typical island arc settings. The overall trace element and REE patterns of the rocks indicate that the source magma of the ultramafic and mafic rocks might have been derived from a refractory mantle source followed by the metasomatic enrichment of lithospheric mantle wedge by incompatible elements through the influx of subduction-derived fluids/melts. We present comprehensive zircon U-Pb data and Lu-Hf data on a suite of mafic granulite, garnet-bearing granulite, tremolite actinolite schist, charnockite, and anorthositic gabbro as well as monazite U-Pb data from a charnockite sample. Most zircon grains show cores with magmatic crystallization features and many grains are mantled by metamorphic rims. The age data show a prominent peak at 3.1 Ga for most of the rocks suggesting that the major crustal building in the Coorg Block occurred during the Mesoarchean. Rocks on the margins of the block were affected by the younger (Neoarchean) tectonothermal event in the surrounding blocks. The Lu-Hf isotopic data on zircon grains with ca. 3.1 Ga ages show εHf(t) values ranging from −4.4 to + 3.7 (average + 0.3). In the εHf(t) versus 207Pb/206Pb age diagram, the plots are distributed close to the CHUR line suggesting Paleo-Eoarchean juvenile magma sources. The voluminous mafic lower crust at the root of the Coorg Block is consistent with addition of mafic magmas through slab melting induced by high mantle potential temperatures in the Archean, as well as the interaction of fluids/melts with the mantle wedge.

Isotopic Geochronological Constraints on the Formation and Evolution of the Moon

One major task of studying the formation and evolution of the Moon is to construct a timeline of the important events with precise isotopic ages. Here, we review recent major isotopic geochronological progress in the past decade and the unsolved problems in isotopic geochronology. The Moon probably formed between 4.52 and 4.42 Ga. Recent high-precision whole-rock and mineral Sm-Nd isotopic dating results suggested that ferroan anorthosite and highlands magnesian suite rocks formed contemporarily around 4.37 to 4.33 Ga. Although the major mare basaltic volcanism took place from 3.85 to 2.93 Ga, new geochronological data from lunar meteorites and Chang’e-5 basalts suggested that lunar basaltic volcanism took place as old as up to 4.37 Ga and at least as young as 2.0 Ga, respectively. Impact events older than 3.9 Ga have also been revealed based on U-bearing minerals Pb/Pb ages and Ar-Ar ages and can provide important clues to understand the late heavy bombardment hypothesis. However, the reliable isotopic ages for the important events on the Moon are still far from conclusive, due to lack of pristine samples that directly crystallized from Lunar Magma Ocean and samples from impact melt sheets in large impact basins (e.g., the South Pole-Aitken basin). In the future, collection and return of pristine samples of ferroan anorthosite and highlands magnesian suite rocks from the farside, cryptomare basalts and late-stage basalts, quartz monzogabbros, granites/felsites, and rocks from impact melt sheets in large impact basins are required for better understanding the formation and evolution of the Moon.

Metamorphic P-T evolution and tectonic implications of UHT metamorphism from the Shillong Meghalaya Gneissic Complex, India: evidence from phase equilibria modelling, monazite U-Pb-Th geochronology, K-Ar dating and geochemistry

Abstract The study area Sonapahar is an integral part of Shillong Meghalaya Gneissic Complex (SMGC), which is located in the Northeastern part of India. This complex mainly comprises metamorphic formations spanning from Upper Amphibolite to Ultra-high temperature granulite, interspersed with various igneous intrusions. In this study, particular attention is directed towards unravelling the metamorphic history of Mg-Al granulite. For the very first time, we establish the pressure–temperature (P-T) trajectory of the Mg-Al granulite from Sonapahar, SMGC. Employing conventional thermobarometry along with winTWQ analysis, the inferred metamorphic conditions for this granulite reveal temperatures exceeding 900°C and pressures of approximately >8 kbar. These conditions firmly indicate the presence of ultra-high-temperature metamorphism. By utilizing the Perple_X software in the MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 compositional system, we construct a P-T pseudosection. This gives a clockwise P-T path, signifying an episode of cooling (+ minor decompression). Such a pattern also suggests rapid cooling of the tectonically-thickened crust. Concurrently, a geochemical exploration of trace and rare earth elements in the rocks offers further insights. These investigations give an idea about the protolith, having a clay-to-sandstone in nature. Additionally, chemical data from monazite within the studied rock provide a weighted mean age of 682 Ma for the peak metamorphic stage. This age aligns with the global Pan-African orogenic events. The biotite K-Ar isotopic geochronology from the symplectite position provides decompression history or cooling age of 442 Ma. This age corresponds to a period after the last peak metamorphic phase that occurred during the Pan-African thermal event.

FIRST RESULTS OF COMPLEX DATING AND GROWTH RATE ESTIMATION OF SPELEOTHEM FROM VORONTSOVSKAYA CAVE (KRASNODAR REGION, RUSSIA)

Speleothems are unique archives of information about climate, geomagnetism, and ecological conditions of past epochs of the Earth, and over the past two decades have been successfully used for paleoclimatic and paleomagnetic studies. The uniqueness of these geological objects lies in the peculiarities of their formation and preservation in them of a wide range of geochemical, geological and geophysical proxies, and, most importantly, in the ability to obtain time series of these proxies in an unprecedentedly accurate resolution using 230Th/U, 14C and some other geochronological methods, as well as incremental chronology. This paper presents the first results of dating the Vor speleothem from the Vorontsovskaya Cave (Krasnodar region), which preserved a record of the geomagnetic excursion, obtained by 230Th/U α-spectrometry, 14C dating, and incremental chronology. Such studies have been carried out in Russia for the first time. Despite the limitations of using the methods of isotope geochronology, it was possible to obtain a limit on the age of the excursion, which probably occurred no earlier than 5500–6000 years ago. At the same time, the incremental chronology made it possible to determine with great accuracy the duration of the main phase of the excursion, which equals 871 ± 16 years, during which the virtual geomagnetic pole was in the southern hemisphere, and also to obtain the upper limit on the age of the excursion, which probably occurred no later than 5.5–6 thousand years ago.

Lower Cretaceous (Aptian-Albian) Bisbee Group, Arizpe area, northern Sonora, Mexico: Integrated biostratigraphy, age and provenance from U-Pb and Hf isotopes

New data on the stratigraphy, biostratigraphy, detrital zircon U-Pb, and Hf isotope geochronology of a Lower Cretaceous succession of the Bisbee Group in northern Sonora, Mexico that accumulated in the Altar-Cucurpe sub-basin help to constrain ages of their formations and documents the detrital provenance. These tectonically shortened strata, studied in the Arizpe area, crop out in the Cerro La Ceja block (CCB) composed of the upper Morita Formation, the complete Mural Limestone, and Cintura Formations, and the lower part of La Juana Formation. The Sierra Los Azulitos block (SAB), composed of the Mural Limestone, is thrust eastward over the CCB. A measured section of the Mural Limestone in the SAB block includes the upper Tuape Shale, Los Coyotes, Cerro La Puerta, and part of the Cerro La Espina members whose age is constrained as upper Aptian to lower Albian based on benthic and planktonic foraminifera, bivalves and echinoderms. The age of the boundary between the Morita and Tuape Shale Member is constrained by detrital zircon maximum depositional ages (MDA) of 115.8 ± 1.1 Ma and 113.9 ± 0.8 Ma, respectively. The MDA of the upper Mural is 111.6 ± 1.6 Ma and the MDA of basal Cintura Formation is 108.8 ± 1.1 Ma. The La Juana Formation has upper Albian bivalves and echinoderms.
 About one-quarter of the detrital zircon grains dated from the five sandstone samples have Proterozoic ages with age peaks that clearly indicate provenance from basement rocks of southwestern Laurentia. More than two-thirds of the grains are of Jurassic and Early Cretaceous age in about the same proportions, and have main peaks at 166, 150, and 118 Ma. The Jurassic grains may have sources in rocks of the Jurassic Cordilleran continental magmatic arc of southwestern North America, and the most probable sources for the Cretaceous grains are arcs of the Peninsular Ranges batholith that by that time were located adjacent to coastal Sonora. Jurassic and the Early Cretaceous zircons dated from 195.3 to 106.8 Ma both have mixed εHf(t) values that range from 9.53 to -21.28, and TDM model ages between 0.4 to 0.8 Ga for the primitive grains, and 0.8 to 1.3 Ga for the more evolved zircons. The probable source for the Jurassic grains with negative εHf values might be local granites of that age in northern Sonora that have Neoproterozoic TDM model ages. In contrast, sources for the Jurassic zircon with positive Hf values may be Jurassic igneous and metaigneous rocks of the Peninsular Ranges Batholith of the Californias, although data reported are scarce. Similarly, the Early Cretaceous detrital zircon grains with mixed positive and negative εHf values may have provenance from igneous rocks of the Peninsular Ranges Batholith-Sierra Nevada magmatic arc.

The evolution of the majiang paleoreservoir, west margin of xuefeng uplift: Insights from in situ calcite U–Pb dating and bitumen and shale trace element analysis

The Majiang Reservoir, situated within the Yangtze Block, is the largest paleoreservoir in the region. Employing a comprehensive methodology involving the analysis of trace and rare earth elements in bitumen and shale, in situ U–Pb dating of hydrocarbon-associated calcite, and incorporating previously published data on oil-source correlation and significant stages of reservoir development, we successfully reconstructed the complete evolution of the Majiang Reservoir quantitatively. The bitumen trace elements data show the ratio of V/(V + Ni) ranging from 0.64 to 0.82, Ni/Co exceeding 7.0 and Zr/Cr below 1.0. PAAS-normalized rare earth element distribution patterns of both bitumen and Cambrian shales present zig-zags distributions. In addition, biomarker distribution acquired by catalytic hydropyrolysis and bulk δ13C values of both bitumen and Cambrian shales (−29.1‰ to −34.7‰ vs −30.2‰ to −32.3‰) also show similarity. All the geochemical parameters together indicate that the primary source of bitumen stems from Early Cambrian shales. Integrating fluid inclusion analysis, specifically examining two Th groups with temperatures of 97.2 °C and 142.6 °C, and U–Pb dating on hydrocarbon-bearing calcite (∼300 Ma), referencing prior Re–Os dates on bitumen (∼430 Ma) and pyrobitumen (∼80 Ma) and quartz fluid inclusion Ar–Ar dating (∼225 Ma), we successfully reconstructed the entire hydrocarbon evolution. The findings reveal that initial oil generation occurred during the Caledonian event, followed by secondary continuous oil and gas generation and migration, influenced by the Indosinian event. The combination of fluid inclusion analysis and isotopic geochronology data exemplifies immense potential in precisely determining the progression of petroleum evolution.

NEW DATA ON THE GEOLOGICAL STRUCTURE AND PRECAMBRIAN EVOLUTION OF THE DENMAN GLACIER WEST SIDE MOUNTAIN FRAME (EAST ANTARCTICA): FIRST PALEOARCHEAN AGE FOR PLAGIOGNEISES

Mountain frame of the Denman Glacier is a little explored and at the same time an most essential region of East Antarctica in the context of studying the Precambrian geological history and geodynamic evolution of the Archean protocratons of East Antarctica. Present paper provides original U–Pb isotope geochronology data from zircons of metamorphic and intrusive rocks sampled from the Denman Glacier western flank outcrops. Geodynamic interpretation of geochronology data are also presented. For the first time, for such East Antarctic location, crystallization time (3355 ± 5.4 Ma) for plagiogneisses magmatic protolith was obtained. The Davis Paleoarchean protocraton in the Archean time interval of evolution was subject to multistage polymetamorphism in the intervals ~3100–3000, 2900–2800 Ma ago. The late stage is associated with crustal extension, which is marked by syntectonic intrusion of ultramafic dikes and pyroxenite sills (2827 ± 6 Ma). The formation time of granite veins and subalkaline granitoid pluthons corresponds to the time of tectono-thermal activisation in the interval of 550–510 Ma traced also along significant part of East Antarctic (the Pan-African tectono-thermal event). The Davis terrane shows a significant similarity in the time of formation and evolution of geodynamic processes with the Paleo-Mesoarchean protocratons of East Antarctica, as well as India and Australia.

Precambrian tectono-magmatic evolution of the western margin of the Yangtze block, South China: evidence from zircon U-Pb-Lu-Hf isotopes, REE and trace elements of Yuanmou-Miyi complexes

ABSTRACT Here new evidence of the isotope geochronology and the geochemistry from the Yuanmou-Miyi complexes reveal the Precambrian tectono-magmatism geodynamic process in the study area. The Yangtze Block basement with 1.81- ~1.77 Ga and negative ε Hf(t) values of −7.8 to −0.7 corresponded to the assembly of the Columbia-supercontinent. However, the rifting magmatic event of ~ 1.70 Ga, which caused the continental crust extension-thinning in the study area, may exert influence on the subduction-magmatism in the Grenvillian period. The Yuanmou-Miyi complexes of 1.19- ~1.00 Ga are characterized by (La/Sm)N＞1, depletion in HFSEs, negative anomalies of Nb-Ta, and enrichment of LILEs, which show a genetic link with the subduction-magmatism. The temporal tendency that the magmatism from Yuanmou to Miyi gradually became younger, may be responsible for the subducting direction. The subduction caused the reactivation of the Anninghe-Lvzhijiag fault zone, and created the rupture windows of the subduction slab, leading to upwelling of the deep magma. These granitoids were derived from the partial melting of the mantle wedge, while the small-scale gabbro dikes intruded into the granitoids were likely spillover products from the asthenosphere mantle melts through the rupture windows.

New Data on the Geological Structure and Precambrian Evolution of the Mountain Framework of the Denman Glacier Western Side (East Antarctica): First Paleoarchean Age for Plagiogneisses

The mountain framework of the Denman Glacier is a poorly explored area. However, it is a key region of East Antarctica in the context of studying the Precambrian geological history and geodynamic evolution of the Archean protocratons of the Antarctic Shield and in their comparison with the Archean crust-forming events of other regions of the Earth. Original U–Pb isotope geochronology data from zircons of metamorphic and intrusive rocks sampled from the western side of the Denman Glacier are reported. A geotectonic interpretation of the data was carried out. The Paleoarchean age of crystallization (3355 ± 5.4 Ma) of the magmatic protolith of tonalitic orthogneisses of the Davis granite–gneiss complex was obtained for the first time in this sector of Antarctica. In the Archean time interval, the Davis Paleoarchean protocraton was affected by multistage polymetamorphism in the intervals ~3100–3000 and 2900–2800 Ma ago. A crustal extension associated with the late stage is indicated by syntectonic intrusion of ultramafic dikes and pyroxenite sills (2827 ± 6 Ma). The time when granite veins and subalkaline granitoid plutons formed corresponds to the time of tectono-thermal Pan-African activization in the interval of 550–510 Ma typical for the Rayner Province of East Antarctica. The terrane of Davis craton is similar to the Paleo-Mesoarchean protocratons of East Antarctica, India, and Australia for the time of formation and the evolution of geodynamic processes.

Late Neoarchean charnockite in Daqingshan terrane, North China Craton: petrogenesis and implications for continental crustal evolution

ABSTRACT Modern analogues of Archaean charnockite are important because they facilitate investigations into processes of early crustal formation and tectonic transition. Herein, we present the results of the petrography, geochemistry, and isotope geochronology of Late Neoarchean charnockites in the Daqingshan terrane at the northern margin of the North China Craton. The sources, petrogenesis, and tectonic implications of the samples obtained are determined based on their geochemical characteristics. Charnockites mainly exhibit magnesian and calc–alkaline characteristics; furthermore, they contain high amounts of light rare-earth elements, with slightly positive Eu anomalies. Analysis of zircon U–Pb dating using a sensitive high-resolution ion microprobe reveals that Hademengou–Xiwulanbulang (HDMG–XWLBL) charnockites were emplaced during the (i) Late Neoarchean (ca. 2500 Ma), with zircon εHf(t) values ranging from 1.57 to 7.81; (ii) single-stage model ages (TDM1) from 2529 to 2637 Ma; and (iii) two-stage model ages (TDM2) from 2710 to 2820 Ma. A comparative study has performed on the petrography, geochemistry, and geochronology of charnockite and granulite shown that the parent magma of the charnockites is the product of the partial melting of granulite. Combining our results with geological data pertaining to the Archaean structural evolution of the Daqingshan terrane, we speculate that the Late Neoarchean HDMG–XWLBL charnockite was formed during a subduction-related thermo-tectonic event in the northern margin of the North China Craton.

Two episodes of Mesozoic granitic magmatism in East Qinling, central China: Constraints from geochemistry and isotope geochronology of the Huangbeiling pluton

The East Qinling Orogen as an important segment of central China that records a complex tectonic evolution history. The Huangbeiling pluton is a rare S-type granite in this orogen and has vital significance in understanding the tectonic evolution of the Qinling Orogen during the Late Mesozoic. The biotite monzogranite of this pluton is the major ore-hosting rocks of the Yumugou Mo-W deposit. In this study, we newly reported a muscovite monzogranite and conducted detailed petrological, geochemical (whole-rock, biotite, muscovite major and trace elements), and zircon U-Pb dating investigations on both the biotite and muscovite monzogranites from the Huangbeiling pluton, with a view to reveal their physico-chemical conditions of formation, petrogenesis and tectonic evolution. Zircon U-Pb dating yielded a weighted mean 206Pb/238U age of 126.8 ± 0.6 Ma for muscovite monzogranite, integrated with published zircon U-Pb age (156–132 Ma) of biotite monzogranite, indicating that the Huangbeiling pluton was produced by two episodes of magmatism during the Late Mesozoic. The pluton shows formation temperatures in the range of 670–772°C, and is classified as cold granites that formed in a subduction-related setting. The emplacement depth of Huangbeiling pluton is constrained at ca. 0.3–4.6 km, with an average depth of 2.0 km. The biotite monzogranite has higher oxygen and fluorine fugacities than the muscovite monzogranite, suggesting high potential for Mo mineralization. Geochemically, the muscovite monzogranite is identified as a S-type granite sourced from crust-derived clay-rich mudstone, and underwent muscovite dehydration melting and subsequent fractional crystallization of K-feldspar and plagioclase during magma ascend. In combination with the syn- to post-collision geochemical affinities of the Huangbeiling pluton and the Late Mesozoic tectonic evolution of the Qinling Orogen, we propose two episodes of magmatism in the Huangbeiling pluton during the tectonic transition (150–125 Ma) from compression to extension in the Qinling Orogen. The first episode (156–132 Ma) of magmatism is correlated with the delamination of lithospheric mantle and intra-continental subduction of the North China Craton under compressional tectonics, whereas the second episode (∼126 Ma) of magmatism occurred through muscovite dehydration melting during pressure decrease.

Concerning the discussion about the Bug series of the Middle Bug (Ukrainian shield)

The materials of several articles and reports published in 2022 by the authors of this work and V.P. Kirilyuk (Kirilyuk, 2022 a,b), who discussed the structure and stratigraphy of the Вug area granulite complex of the Ukrainian Shield. The authors, relying on their geological, geophysical and geochemical studies, isotopic geochronology data, as well as on the world experience in studying the Early Precambrian complexes of the Canadian and Australian shields, argue that during 3.8—1.7 Ga, the Bug area granulite complex underwent at least a threefold structural metamorphic processing. Its modern structural plan, as well as, to a large extent, the composition and structural and textural features of metamorphic rocks, are of Early Proterozoic age. Therefore, its division into suites of primary sedimentary-volcanogenic rocks and their vertical distribution by age does not make sense. Only a division into two series is possible — the Dniester-Bug and the Bug, between which there is a tectonic contact. Contrary to this, V.P. Kirilyuk, relying on the experience of his stratigenic-metamorphogenic studies and the authority of the well-known school of L.I. Salopa, E.M. Lazko et al., defends the point of view on the stratigenic nature of the Bug area granulite complex, in which the primacy and stability of the position of strata-formations relative to each other were preserved throughout the entire time. According to V.P. Kirilyuk, there are no breaks and disagreements between the suites, so he proposes to abandon the identification of the early Precambrian USh Bug Group. And the seven suites identified by him, the youngest of which is 3.8 billion years old, are proposed to be attributed to a single Dniester-Bug series. The authors of both alternative points of view apply to the National Stratigraphic Committee of Ukraine with a proposal to revise the stratigraphic scheme of the Early Precambrian of the USh.

Siderophile element and Hf-W isotope characteristics of the metal-rich chondrite NWA 12273 – Implication for its origin and chondrite metal formation

Metal phases in primitive chondrites contain important information for understanding early Solar System processes. Northwest Africa (NWA) 12273 is an ungrouped chondrite and, due to its preternatural high modal abundance of metal grains (>60%), provides a unique perspective on the formation and evolution of primitive planetesimals. To investigate the formation of metal phases in early solar system materials we report detailed petrography, siderophile geochemistry and Hf-W isotope geochronology for NWA 12273. The primitive textures and heterogeneous mineral chemistry of chondrules, as well as siderophile element compositions of Ni-rich metal, all indicate an affinity of NWA 12273 to unequilibrated (type 3.8) ordinary chondrites that experienced limited thermal metamorphism. Modeling shows that crucial inter-element fractionations among refractory siderophile elements for kamacite in NWA 12273, were most likely caused by solid metal-liquid metal partitioning, for example during partial melting of Fe-Ni metal containing ∼3.9 wt.% of S. Analysis of the metal fractions within NWA 12273 gives a Hf-W model age of ∼2.4 Ma after CAI formation (ε182W6/4=−3.29±0.11), older than metal formation in most of H chondrites and IIE irons. In comparison with published data for chondrites and ‘non-magmatic’ iron meteorites, siderophile element data suggest that the metal phases in NWA 12273 may have originated from similar precursor materials and/or accreted in adjacent nebular locations to IIE irons and H chondrites. These primary planetesimals experienced impact-related evaporation and mixing followed by rebuilding of the secondary body. This scenario is consistent with that proposed for IIE and other silicate-bearing iron meteorites, indicating that the building blocks of some chondrite parent bodies were not entirely primitive (undifferentiated) materials.