The results of Sr-chemostratigraphic study of carbonates of the Staraya Rechka and Nemakit-Daldyn formations which make up the upper part of the Precambrian cover of the Anabar Uplift in Northern Siberia are presented. A Pb–Pb age of the Staraya Rechka Formation dolostone has been obtained (549 ± 25 Ma, MSWD = 1.4) for the first time. An improved stepwise dissolution procedure was used to determine the 87Sr/86Sr, 206Pb/204Pb and 207Pb/204Pb isotopic ratios in carbonates rocks. The methodology for studying the Rb–Sr systematics included the chemical removal of about a third of the crushed sample [fraction L(Rb–Sr)1] by preliminary acid leaching in 0.2N CH3COOH and subsequent partial dissolution [fraction L(Rb–Sr)2] of the remaining part of the sample in CH3COOH of the same concentration. The Pb–Pb isotopic systematics of dolostones was studied by six to nine-step dissolution in 0.5N HBr. Chemical procedures resulted in the removal of secondary epigenetic carbonate material, which improved the quality of Sr-chemostratigraphic and U–Pb geochronological information. The initial 87Sr/86Sr ratios in the least altered carbonate material [fraction L(Rb–Sr)2] of the Staraya Rechka Formation dolostone are 0.70822‒0.70836, and in the Nemakit-Daldyn Formation limestone, 0.70854–0.70856. The Pb–Pb age of early diagenesis of dolostones of the Staraya Rechka Formation (549 ± 25 Ma) was calculated from fractions [L(U–Pb)2–L(U–Pb)n], where n for different samples varied in the range from 6 to 9. Epigenetic carbonate fractions L(U–Pb)1 are characterized by a Pb–Pb age of 360 ± 190 Ma (MSWD = 0.8). The obtained results prove that the Staraya Rechka Formation of the Anabar Uplift belong to the Late Vendian (Late Ediacaran), allowing them to be confidently correlated with the carbonate rocks of the upper part of the Yudoma Group of the Uchur-Maya region and to include the named strata into a single Yudoma Complex of Siberia.

The geochemical characteristics (REE, trace elements) and Sr and Nd isotopic composition of apatite from corundum-bearing metasomatites of the Khitoostrov occurrence (Belomorian Mobile Belt), associated plagioclasites, and host rocks represented by garnet amphibolites and kyanite–garnet–biotite gneisses of the Chupa sequence have been studied. Apatites from the corundum-bearing metasomatites and kyanite–garnet–biotite gneisses are enriched in medium REE and have a negative Eu anomaly (Eu/Eu* 0.20–0.35). Apatite from the corundum-bearing rocks differs from apatite from the gneisses of the Chupa sequence in the increased content of Sr, LREE, decreased content of HREE, as well as a lower 87Sr/86Sr(t) ratio and an increased ɛNd(T) value: 0.70 865–0.70 896 and –9.3 ± 0.2 compared to 0.72 533 and –8.1, respectively. Apatite from the garnet amphibolites is enriched in MREE, lacks Eu-anomaly (Eu/Eu* 0.98), and has a low ɛNd(T) = –9.3 and the lowest 87Sr/86Sr(t) ratio of 0.70 560. The Sm-Nd age estimate of apatite is 1.80 ± 0.15 Ga, which coincides with the Svecofennian metamorphism in the Belomorian Mobile Belt. The geochemical features of the apatite and 87Sr/86Sr(t) ratios indicate that the metasomatic alteration of the gneisses was caused by the lower crustal fluid and was accompanied by the influx of LREE and the removal of HREE. The slightly lower Eu anomaly and higher Ce vs Th and REE vs La/Sm relations reflect the fact that apatite from the corundum-bearing metasomatic rocks was formed in a more oxidizing environment than apatite from host rocks. Neither the corundum-bearing metasomatites and plagioclasites, nor the host rocks revealed any Sr-isotopic and REE-geochemical traces of interaction with surface (meteoric) waters.

An Erratum to this paper has been published: https://doi.org/10.1134/S0016702923200016

This paper presents new age estimates and results obtained by the chemostratigraphic study of dolostones of the Billyakh Group, which consists of the Kotuikan and the Yusmastakh formations. The Billyakh Group forms the upper part of the Riphean section of the Anabar uplift in northern Siberia. The stepwise dissolution technique was used for the first time to determine the 87Sr/86Sr, 206Pb/204Pb, and 207Pb/204Pb isotopic ratios in dolostones. The Rb‒Sr systematics was studied by the chemical removal of about a third of the crushed sample (fraction L1) by the preliminary acid leaching in 0.2 N CH3COOH and the subsequent partial dissolution (fraction L2) of the remaining part of the sample in CH3COOH with the same concentration. The Pb‒Pb isotope systematics of dolostones was studied by the six-step dissolution of crushed samples in 0.5 N HBr. These procedures led to the removal of secondary carbonate material and greatly improved the quality of Sr chemostratigraphic and geochronological information. The initial 87Sr/86Sr ratios of the least altered carbonate material (fraction L2) of the Billyakh Group dolostones are 0.70502 ± 0.00029 in the Kotuikan Formation, 0.70519 ± 0.00026 in the lower subformation of the Yusmastakh Formation, and 0.70511 ± 0.00018 in the upper subformation of the Yusmastakh Formation. The Pb‒Pb age of early diagenesis of Kotuikan and Yusmastakh dolostones (1519 ± 18 Ma at MSWD = 1.8) was calculated from the results obtained for carbonate fractions L3–L6. Secondary carbonate fractions L1‒L2 are characterized by a Pb‒Pb age of 1466 ± 54 Ma at MSWD = 0.6. The δ13C values vary from ‒1 to ‒0.4‰ in dolostones of the Kotuikan Formation and from ‒0.4 to +0.8‰ in those of the Yusmastakh Formation (from ‒0.1 to +0.4‰ in the lower subformation and from ‒0.4 to +0.8‰ in the upper subformation). Comparison of these variations, as well as variations in the initial 87Sr/86Sr ratios in dolostones of the Kotuikan Formation and the Lower and Upper Yusmastakh subformations (0.70460‒0.70499, 0.70450‒0.70525, and 0.70462‒0.70523, respectively), does not make it possible to distinguish these units on the basis of chemostratigraphic characteristics.

Представлены результаты Pb-Pb геохронологического и Sr-, Cхемостратиграфического изучения известняков и доломитов араошейской свиты Тункинских гольцов Восточного Саяна. Значение δ13C в известняках варьирует от -0.6 до 2.3‰ PDB, отношение 87Sr/86Sr лежит в пределах 0.70851-0.70864, Pb-Pb возраст известняков равен 521±21 млн лет (СКВО = 0.9). Значение δ13C в доломитах меняется от -3.9 до 0.5‰ PDB, отношение 87Sr/86Sr 0.70844-0.70882, Pb-Pb возраст доломитов равен 417±8 млн лет (СКВО = 2.5). U-Pb изотопная система доломитов была нарушена в ходе геологической эволюции региона, что привело к омоложению их Pb-Pb возраста в раннем девоне. Полученные Sr-хемостратиграфические и Pb -Pb данные показывают, что известняки араошейской свиты накапливались в раннем кембрии. Это позволяет надежно доказать принадлежность известняков араошейской свиты к венд-кембрийскому осадочному чехлу ТувиноМонгольского массива.

The mineralogical, structural, crystal-chemical, and isotope-geochronological data were obtained for the first time in two samples of globular phyllosilicates (GPS) of the glauconite–illite series collected from terrigenous rocks in two sections of the lower part of the Lower Vendian Maastakh Formation (Khorbusuonka and Ulakhan-Sololi rivers, northwestern slope of the Olenek Uplift). Simulation of experimental X-ray diffraction patterns for both samples in the air-dried and ethylene glycol-solvated states was used to determine the expandable layer contents (9, 10%), types of expandable layers (smectite- and vermiculite-type), their ratios in a three-component mixed-layer structure, parameter csinβ (9.98 Å) of mica layers, and the short-range order factor describing the alternation of different layer types (R = 2, 3). Values of the unit-cell parameter b (9.027, 9.039 Å) correspond to the dioctahedral Al, Fe-bearing micas. The mica components in the studied mixed-layer minerals are shown to be represented by Fe-illites (KAl = VIAl/(VIFe3+ + VIAl) = 0.71, 0.82) with the K2O content of 7.77 and 8.40%. The Rb–Sr dating of the Maastakh GPS was carried out for the first time in combination with the calculation of theoretical patterns in the cation distribution in the mineral structure and comparison of the calculation results with the Mössbauer and IR spectroscopy data using the Optima software package supplemented with the new Irmes software. The Rb–Sr data obtained for two Fe-illites (Khorbusuonka and Ulakhan-Sololi rivers: 1033, 913 ± 12 Ma, respectively) are “older” relative to the Vendian Maastakh Formation (~640 ± 5), indicating a terrigenous origin of the studied grains. Globular phyllosilicates from the Maastakh Formation are similar in age and composition to the previously studied Lower Khaipakh (Middle Riphean) GPS (1172 ± 18 and 1112 ± 24 Ma, respectively). The age of GPS samples from the Maastakh Formation is younger (1033–913 Ma), probably, due to the initial cationic disordering in the GPS structure and partial loss of radiogenic elements during secondary alterations (rewashing and redeposition of Fe-illite grains in sediments of the Maastakh Formation) and, probably, during their catagenetic changes as well.

The first direct Pb–Pb dating of carbonate rocks of the Kamo Group has been carried out. The Pb–Pb age of carbonates of the lower units (the Madra, Jurubchen, Kuyumba, and Vingold formations) of this sedimentary sequence is 1510 ± 50 Ma, while the age of the upper unit (the Iremeken Formation) is 1490 ± 50 Ma. The ages obtained show that all the carbonate formations of the Kamo Group are Lower Riphean (Early Mesoproterozoic), and their deposition continued for a period of no longer than several tens of millions of years. The new geochronological data allow us to correlate the Riphean deposits of the Baikit Anteclise with the rocks of the Riphean cover of the Anabar Uplift.

— Based on Sm–Nd data, a crustal source of iron-ore fluid was substantiated and the probability of age estimation for hydrothermal–metasomatic siderite of the Bakal Group, Southern Urals, was shown for the first time. The εNd (Т) values of siderite (from –13.4 to –17.6) plot in the field of Riphean shale and not the Precambrian rift gabbro and granite of this region. The obtained Sm–Nd age of the Bakal siderite is 970 ± 40 Ma, which is consistent with the Pb–Pb age of siderite from the major ore phase (~1000 Ma). The established age boundary coincides with tectonic restructuring, including the formation of a number of barite–polymetallic deposits, as well as ferruginous magnesite and fluorite in the Riphean deposits on the western slope of the Southern Urals.

The mineralogical, crystallochemical and geochronological study of globular phyllosilicates (GPS) of glauconite–illite series from the Bakeevo Formation was performed. This formation is the basal member of the Vendian Asha Group in the South Urals. Monomineral GPS-fractions representing the globules of varied size and density are composed of Al-glauconite and illite. Rb–Sr dating of the GPS of the Bakeevo Formation in combination with modeling of cation distribution in their structure and comparison of the model data with Mossbauer and IR spectroscopy data was carried out for the first time. Rb–Sr age of the Bakeevo glauconite is 642 ± 9 Ma. Mineralogical and geochemical characteristics in combination with theoretical calculations show that the stratigraphic significance of this age is quite reasonable. The initial ratio 87 Sr/ 86 Sr in the glauconitization environment is 0.7080 ± 0.0009 and within the experimental error is consistent with this ratio in Vendian sea water. Hence the lower boundary of the Vendian in the South Urals has an age of about 640 Ma.

— Clay subfractions (SFs) with particles size of 2–5, 0.6–2.0, 0.3–0.6, 0.2–0.3, and 0.1–0.2 μm of two shale samples (Upper Riphean Inzer Formation, South Urals) are studied using transmission electron microscopy, X-ray diffractometry (XRD), and U–Pb, Sm–Nd, Rb–Sr, and K–Ar methods. The SFs are composed of low-temperature 1M d illite; quartz, chlorite, and 2M 1 illite are observed only in some coarse SFs. Irrespective of the size, the clay particles are equant. The standardized illite crystallinity indices (CIS) of all SFs are typical of the dia(cata)genesis zone. The CIS value increases, the I 002 /I 001 ratio of XRD patterns decreases, and the K content and K/Rb ratio increase with decreasing particle size of the SFs from 2–5 to 0.1–0.2 μm. Leaching with 1N HCl and 1N NH 4 OAc and U–Pb, Sm–Nd, and Rb–Sr analysis of untreated SFs, leachate, and residue allowed us to study the isotope systematic of mixing in mobile and silicate material of shales. The 208 U/ 204 Pb and 87 Rb/ 86 Sr ratios of leachates are lower and the 147 Sm/ 144 Nd ratio is higher than those of residues. The leachates are also characterized by less radiogenic Pb and Sr and more radiogenic Nd relative to residues. With decreasing size of SF particles, the U, Pb, Sm, Nd, and Sr contents of leachates gently decrease and the Rb content increases. The 87 Rb/ 86 Sr and 87 Sr/ 86 Sr ratios of leachates of fine SFs are significantly higher, whereas their 238 U/ 204 Pb ratio is lower in comparison with coarse SFs. What is more, the data points of residues of various SFs occur along the mixing lines in the 87 Rb/ 86 Sr‒ 87 Sr/ 86 Sr and 1/ 86 Sr‒ 87 Sr/ 86 Sr plots. The data points of corresponding leachates also form linear trends in 238 U/ 204 Pb‒ 206 Pb/ 204 Pb, 206 Pb/ 204 Pb‒ 207 Pb/ 204 Pb, 147 Sm/ 144 Nd‒ 143 Nd/ 144 Nd, and 87 Rb/ 86 Sr‒ 87 Sr/ 86 Sr coordinates. Apparent Rb–Sr ages calculated from slopes of “inner isochrons” (“leachochrons”) as well as K-Ar ages gradually decrease from 835–836 and 721–773 Ma, respectively, for SFs of 2–5 μm and to 572‒580 and 555‒580 Ma, respectively, for SFs of 0.1–0.2 μm. Thus, the XRD and isotopic data indicate that both clay and mobile shale constituents are mixtures at least of two components, and the silicate phase contains authigenic illites of various ages. First-generation illite abundant in the coarse SFs of 2–5 μm and 0.6–2.0 μm was formed immediately after the deposition of the Inzer sediments and its age of 803–836 Ma is consistent with stratigraphic age of the Inzer Formation. The formation of this illite was facilitated either by lithostatic burial or intense horizontal fluid flow caused by tectonic inversion in the eastern regions of the Uralian paleobasin. The age of the second-generation illite is 572–580 Ma; it was formed as a result of vertical movements or renovation of the composition of the pore fluids during deformations and metamorphism of the South Urals related to the evolution of the Beloretsk metamorphic complex.

The 2–5, 0.6–2, 0.3–0.6, 0.2–0.3 and 0.1–0.2 µm clay subfractions (SFs) separated from two shale samples of the Upper Riphean Inzer Formation, the southern Urals, were studied by the TEM, XRD, and U–Pb, Sm–Nd, Rb–Sr and K–Ar isotopic methods. All the SFs consist of the low-temperature 1Md illite; admixtures of quartz, chlorite and 2M1 illite occur only in the coarsest SFs. The clay particles are isometric, regardless of their size. The CIS (Crystallinity Index Standard) illite values for the all SFs are typical for the dia(kata)genetic zone. As the size of particles in the SF decreases from 2–5 to 0.1–0.2 µm, the CIS rises, the I002/I001 ratio on the XRD diagrams decreases, and the K content and the K/Rb ratio increase. Leaching with 1N HCl and 1N ammonium acetate (NH4OAc) and subsequent U–Pb, Sm–Nd and Rb–Sr analyses of the untreated SF, acid (acetate) leachate and residue made possible to study the mixing systematics in mobile and silicate materials of the shales. The 238U/204Pb and 87Rb/86Sr ratios in the acid and acetate leachates are below, and the 147Sm/144Nd ratio is above those in the residues. Less radiogenic Pb and Sr and more radiogenic Nd are also common for the leachates compared to the residues. As the size of particles in the SFs decreases, the U, Pb, Sm, Nd and Sr contents in the residues are smoothly reduced, whereas the Rb content shows an increase. The 87Rb/86Sr and 87Sr/86Sr values in the residues for fine-grained SFs are well above, and the 238U/204Pb value is well below those for coarse-grained SFs. What is more, in the 87Rb/86Sr–87Sr/86Sr and 1/86Sr–87Sr/86Sr diagrams, data points for the residues of variable size are arranged on the mixing lines. The data points of respective acid and acetate leachates also form linear trends in the 238U/204Pb–206Pb/204Pb, 206Pb/204Pb–207Pb/204Pb, 147Sm/144Nd–143Nd/144Nd, and 87Rb/86Sr–87Sr/86Sr coordinates. The apparent Rb–Sr age values, calculated from the slopes of “inner isochrons” (“leachochrons”), along with the K–Ar ages are smoothly lowered from 835–836 and 721–773 m.y. for the 2–5 µm SF to 572–580 and 555–580 m.y. for the 0.1–0.2 µm SF. Hence the XRD and isotopic data testify that the clay as well as the mobile material of the shale represent the mixtures of at least two components, the silicate phase containing authigenic illites of different ages. The first illite generation enriched in coarse-grained 2–5 and 0.6–2 µm SFs was formed shortly after deposition of the Inzer sediments, and its age of 803–836 m.y. is in agreement with the stratigraphic age of the formation. Simple lithostatic burial or intensive lateral fluid flow induced by tectonic inversion in the eastern regions of the Urals paleobasin may be considered as the geological processes responsible for the forming of this illite. The second illite generation was formed 572–580 m.y. ago. As the starting points for its formation, alternatively, may be concerned either vertical tectonics or renewal of pore fluid compositions during deformations and metamorphism on the southern Urals area related to evolution of the Beloretsk metamorphic complex.

Представлена Sr-изотопная характеристика карбонатных пород центральноприазовской серии (0,70322–0,70352), вычислен Nd-модельный возраст силикатных осадков этой серии (2,34–2,31 млрд лет) и определён U-Pb-возраст трондьемитов (2052 ± 5 млн лет), прорывающих карбонатные породы. Полученные данные доказывают, что морской осадочный чехол Приазовского блока, являющегося фрагментом раннедокембрийского континента Сарматия, формировался в раннем палеопротерозое 2,23–2,34 млрд лет назад.

The Sr-isotope composition of the Central Azov Group carbonates (0.70322-0.70352) and Nd model age of silicate sediments (2.34-2.31 Ga) has been reported. The U-Pb age of trondhjemite (2052+5 Ma) cutting the carbonates has been determined. According to the obtained data, the marine sedimentary cover of the Azov block making up the Early Precambrian Sarmatia Continent emplaced in the Early Paleoproterozoic at 2.23-2.34 Ga.

— This work presents the results of the mineralogical, crystallochemical, and geochronological study of globular phyllosilicates (GPS) of glauconite–illite series from the Bakeevo Formation. This formation is the basal member of the Vendian Asha Group in the Southern Urals. The monomineral GPS fractions representing the globules varying in size and density are composed of Al-glauconite and illite. The Rb–Sr dating of the GPS of the Bakeevo Formation in combination with the modeling of cation distribution in their structure and comparison of the model data with Mössbauer and IR spectroscopy data was performed for the first time. The Rb–Sr age of the Bakeevo glauconite is 642 ± 9 Ma. The mineralogical and geochemical characteristics in combination with model calculations show that the stratigraphic significance of this age is quite reasonable. The initial 87 Sr/ 86 Sr ratio in the glauconitization environment is 0.7080 ± 0.0009 and is consistent within the experimental error with this ratio in the Vendian seawater. Thus, the lower boundary of the Vendian in the Southern Urals has an age of about 640 Ma.

Выполнено минералогическое, кристаллохимическое и изотопно-геохронологическое изучение глобулярных слоистых силикатов (ГСС) глауконит-иллитового ряда из бакеевской свиты, базального члена ашинской серии венда Южного Урала. Мономинеральные размерные и плотностные фракции глобул представлены Al-глауконитом и иллитом. Впервые Rb–Sr датирование ГСС бакеевской свиты проведено в сочетании с моделированием распределения катионов в их структуре и сопоставлением с данными мессбауэровской и ИК-спектроскопии. Rb–Sr возраст глауконитов бакеевской свиты равен 642 ± 9 млн лет. Минералого-геохимические характеристики и теоретические расчеты показывают, что стратиграфическая значимость этого возраста представляется достаточно обоснованной. Первичное отношение 87 Sr/ 86 Sr в среде глауконитизации равно 0.7080 ± 0.0009 и в пределах погрешности согласуется с этим отношением в морской воде венда. Таким образом, нижняя граница венда на Южном Урале имеет возраст около 640 млн лет.

The Sr isotope composition in glendonites from two sections of the Middle Jurassic of Northern Siberia has been determined for the first time. Glendonite was formed by the replacement of icite by calcite during diagenesis. The 87 Sr/ 86 Sr in most of the samples of glendonites (0.70687–0.70715) is insignificantly lower than this ratio in the Late Bathonian–Early Bajocian ocean. This may indicate a contribution from the Callovian seawater in the formation of individual calcite generations or the influence of methanogenic fluid of deep origin.