Siderite microconcretions in the glauconite-bearing clayey-silty rocks of the Khaipakh Formation (Middle Riphean,Olenek Uplift)

Siderite microconcretions in the glauconite-bearing clayey-silty rock member of the lower sub-formation of the Khaipakh Formation (Middle Riphean, Olenek Uplift) are scrutinized for the first time. In two Khorbusuonka River sections located with a spacing of 12 km, the microconcretions occur as lenses and interlayers. Together with glauconitites, they serve as a distinct marker horizon of this stratigraphic interval. Their structures, morphologies, diffraction characteristics, chemical compositions, and isotope data are considered. They were examined comprehensively with modern investigation methods (X-ray diffraction, scanning electron microscopy with the application of local microprobe analysis, and others). Structural-morphological types of microconcretions are identified. Calculation of the chemical composition of carbonates based on numerous microprobe analyses made it possible to reveal different degrees of mineral heterogeneity in each type in terms of the distribution of macro- and microlevel isomorphous trace-elements and to refine the character of their secondary alterations at different stages of lithogenesis. The results of oxygen and hydrogen isotopic studies are presented for algal dolomites and limestones from the Middle and Lower Riphean sections in the Olenek Uplift (Debengda, Arymass, and Kyutingda formations). They demonstrated that siderites are similar to limestones and dolomites in terms of the oxygen isotopic composition (δ¹⁸O = 17.6–24.8, δ¹⁸O_av = 20.0 ± 2.4‰), but are marked by low δ¹³C values (from ?6.3 to ?12.0‰ (δ¹³C_av = ?8.6 ± 2.1‰), suggesting the formation of microconcretions during early diagenesis. Siderite microconcretions were formed in the clayey-silty sediment slightly after glauconite, whose grains could serve as crystallization centers and (or) be entrapped during the growth of separate microcrystals. The role of catalyzers during the formation of both glauconite and siderite was played by bacterial communities, whose poorly preserved remnants have been detected not only in the studied member of the lower Khaipakh subformation, but also above and below the section. Separate types of microconcretion could be formed during the replacement of oncolites by siderite.     

Glauconitites in terrigenous rocks of the Khaipakh Formation (Middle Riphean,Olenek Uplift)

Structure of the lower subformation of the Khaipakh Formation from the upper portion of the Middle Riphean Olenek Uplift (northern Siberia) is considered. It has been noted for the first time that the glauconite-containing sandy-aleurolitic rocks (hereafter, sandstones and siltstones) in sections of the Khorbusuonka River include glauconitite laminas and siderite lenses that are distinct marker horizons of this stratigraphic interval. It has been shown that the glauconitites are weakly cemented and almost completely composed of glauconite grains and glauconite cement. The paper presents detailed mineralogical and structural-crystallochemical characteristics of Al-glauconite in specimens with different degrees of cementation (solid and loose rocks). The paper discusses genetic features of glauconite grains and their secondary alterations. Comparative characteristics of glauconite from solid and loose rocks from both Khaipakh sections and previously studied terrigenous rocks of the Arymas and Totta formations (Middle Riphean Olenek Uplift and Uchur-Maya region) are given. Suitability of glauconite extracted from the glauconitites for isotope-geochronological investigations is estimated. Literature and original data on glauconitites formed in situ in Precambrian and Phanerozoic sections are compared. It is concluded that their primary macroscopic and microscopic features are very similar.     

Isotopic geochronology and biostratigraphy of Riphean deposits of the Anabar Massif,North Siberia

The structure of Riphean deposits developed on the western slope of the Anabar Massif is described with analysis of their depositional environments, distribution of stromatolite assemblages and organic-walled and silicified microfossils through sections, and evolution of views on stratigraphic significance of some of these assemblages. The investigation included complex mineralogical, geochemical, structural, and isotopic?geochronological study of globular phyllosilicates (GPS) of the glauconite?illite series from paleontologically well substantiated Riphean sequences (Ust’-Il’ya and Yusmastakh formations of the Billyakh Group) of the Anabar Massif in the Kotuikan River basin. Isotopic dating of monomineral size and density fractions of GPS from the Billyakh Group was performed in combination with simulation of the distribution of octahedral cations and comparison of the results obtained with Mössbauer spectrometry data. The applied approach is based on an assumption that the formation and transformation of Rb?Sr and K?Ar systems in GPS are synchronous with stages in their structural evolution, which are determined by the geological and geochemical processes during depositional history. Such an approach combined with the mineralogical and structural analysis contributes to correct interpretation of stratigraphic significance of isotopic data. The results obtained provide grounds for the conclusion that isotopic dates of GPS from the Ust’-Il’ya (Rb?Sr, 1485 ± 13 Ma; K?Ar, 1459 ± 20 Ma) and Yusmastakh (Rb?Sr, 1401 ± 10 Ma; K?Ar, 1417 ± 44 Ma) formations mark the stage of early diagenesis of sediments and are suitable for estimating the age of formations in question.     

Microfossils from the Arymas and Debengda formations, the Riphean of the Olenek Uplift: Age and presumable nature

Studied assemblages of diverse organic-walled microfossils separated from the Arymas and Debengda formations of the Olenek Uplift include several paleobiological groups of microorganisms. Sufficiently large morphotypes of the first group are identified with remains of cyanobacteria. Morphotypes of variable spiral structure, which dwelt in association or in symbiosis with cyanobionts, are attributed to the same bacterial community. The other group includes a series of different acritarch genera whose characters suggest their affinity with green algae of the order Desmidiales. It is very likely that this group coexisted on siliciclastic shoals with large ancestral forms of the present-day brown algae. Several microfossil taxa have been known before from the Neoproterozoic deposits only. With due regard for the relatively gradual accumulation of sedimentary succession lacking large hiatuses and for the regular series of K-Ar dates characterizing three Riphean formations of the Olenek Uplift, it is possible to suggest that there was the Arymas-Debengda-Khaipakh cycle of long-lasted, almost uninterrupted sedimentation within the time span of 1250–900 Ma. It is also admissible that age ranges of some Late Precambrian microfossils are much larger than their distribution intervals postulated formerly.     

RbSr and KAr ages of paleozoic glauconites from Ohio—Indiana and Missouri,U.S.A.

Glauconite pellets from the Lower Silurian Brassfield Formation on both limbs of the Cincinnati Arch in Ohio and Indiana give a RbSr age of 370 ± 11 Ma, which is substantially younger than the stratigraphic age of the formation. The age is concordantcwith conventional KAr ages of 355 ± 6 and 368 ± 5 Ma for two of the same glauconites. Concordant ages were also obtained from the Viburnum Trend in Missouri, where glauconite pellets from the Davis Formation in an ore-zone collapse structure into the underlying Bonneterre Formation give a RbSr age of 387 ± 21 Ma and conventional KAr ages of 368 ± 5 and 369 ± 5 and 369 ± 5 Ma. A third suite of glauconite from the Bonneterre Formation in the Old Lead Belt ～ 10 km from the nearest ore body has given a RbSr age of 423 ± 7 Ma and slightly older conventional KAr ages of 434 ± 6, 445 ± 6 and 441 ± 11 Ma.Because these glauconite-bearing rocks have been buried to depths of less than 1 km, thermal resetting of the RbSr and KAr systematics appears unlikely. The initial ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of the RbSr isochrons are similar to the ratios for vein- and vug-filling dolomite and calcite. This is consistent with resetting of the RbSr and KAr systems during diagenetic changes which included the isotopic equilibration (perhaps by cation exchange) of the Sr in the glauconite with that in the diagenetic and Mississippi Valley-type ore fluids. This interpretation implies that the age of the Mississippi Valley-type mineralization in the Viburnum Trend is Devonian rather than Carboniferous—Permian as has been inferred from paleomagnetic measurements.Cation-exchange experiments with a dilute Sr-bearing solution and an artificial oilfield brine indicate that glauconite adsorbs large amounts of Sr, some of which is sufficiently strongly attached to the glauconite lattice as to resist leaching with ammonium acetate. The introduction of this strongly attached Sr may be the first step in the resetting of the RbSr systematics of glauconite by cation exchange.     

Mössbauer characteristics,mineralogy and isotopic age (Rb-Sr,K-Ar) of Upper Riphean glauconites from the UK Formation,the southern Urals

Comprehensive mineralogical analysis, Mössbauer spectroscopy and isotopic-geochronological study have been carried out for globular phyllosilicates (GPS) of glauconite group from the Uk Formation, the second one below the top of the Upper Riphean stratotype in the southern Urals. Glauconites have been sampled in the Kurtaza and Kulmas sections remote from each other in the Alatau anticlinorium that corresponds to western facies zone of the Bashkirian meganticlinorium. As is shown, size and density monomineral fractions of globules are represented by Al-glauconite according to established structural formula. The isotopic (Rb-Sr and K-Ar) dating of glauconites from the Uk Formation is performed for the first time along with computer simulation of cation arrangements in their crystal lattice and comparison of the results obtained with data of Mössbauer spectroscopy. It has been assumed by simulation that origin and transformation of the Rb-Sr and K-Ar systems in glauconite are concurrent to stages in structural evolution of this mineral, which have been controlled by geological and geochemical events in the history of sedimentary successions. The approach has been aimed at recognition of stratigraphically meaningful isotopic dates corresponding to the glauconite formation at the stage of the early diagenesis close to sedimentation time and the “rejuvenated” dates characterizing ages of subsequent geological events. The comparison of simulated cation arrangements with data of Mössbauer spectroscopy shows that the Rb-Sr (663 ± 9 Ma) and K-Ar (669 ± 16 Ma) dates established for glauconites correspond to the time of early diagenesis in their host sediments, being suitable for age assessment of the Uk Formation. The dates obtained are of interregional and wider significance, as they must be taken into consideration when constructing the general curve of Sr isotope variations in the Late Riphean oceans.     

Isotopic constraints on age and duration of fluid-assisted high-pressure eclogite-facies recrystallization during exhumation of deeply subducted continental crust in the Sulu orogen

Fluid availability during high‐grade metamorphism is a critical factor in dictating petrological, geochemical and isotopic reequilibration between metamorphic minerals, with fluid‐absent metamorphism commonly resulting in neither zircon growth/recrystallization for U‐Pb dating nor Sm‐Nd isotopic resetting for isochron dating. While peak ultra‐high pressure (UHP) metamorphism is characterized by fluid immobility, high‐pressure (HP) eclogite‐facies recrystallization during exhumation is expected to take place in the presence of fluid. A multichronological study of UHP eclogite from the Sulu orogen of China indicates zircon growth at 216 ± 3 Ma as well as mineral Sm‐Nd and Rb‐Sr reequilibration at 216 ± 5 Ma, which are uniformly younger than UHP metamorphic ages of 231 ± 4 to 227 ± 2 Ma as dated by the SHRIMP U‐Pb method for coesite‐bearing domains of zircon. O isotope reequilibration was achieved between the Sm‐Nd and Rb‐Sr isochron minerals, but Hf isotopes were not homogenized between different grains of zircon. The HP eclogite‐facies recrystallization is also evident from petrography. Thus this process occurred during exhumation with fluid availability from decompression dehydration of hydrous minerals and the exsolution of hydroxyl from nominally anhydrous minerals. This provides significant amounts of internally derived fluid for extensive retrogression within the UHP metamorphosed slabs. Based on available experimental diffusion data, the consistent reequilibration of U‐Pb, Sm‐Nd, Rb‐Sr and O isotope systems in the eclogite minerals demonstrates that time‐scale for the HP eclogite‐facies recrystallization is c. 1.9–9.3 Myr or less. This provides a maximum estimate for duration of the fluid‐facilitated process in the HP eclogite‐facies regime during the exhumation of deeply subducted continental crust.     

Microfossils in cherts from the Mesoproterozoic (Middle Riphean) Debengda Formation, the Olenek Uplift, northeastern Siberia.

Silicified shallow-water marine carbonate deposits of the Proterozoic Debengda Formation (the Olenek Uplift, northeastern Siberia) contain well preserved microfossils. One or two distinct assemblages consists only of filamentous Siphonophycus microfossils, which are presumably the extracellular sheaths of hormogonium cyanobacteria. The other is dominated by coccoidal microfossils, first by the entophysalidacean cyanobacterium Eoentophysalis. The coccoidal assemblage was recognized in the layered carbonate precipitate structures of a superficially stromatolite appearance. Despite its simple composition, the microfossil assemblage supports the generally accepted Mesoproterozoic (middle Riphean) age of the Debengda Formation. This conclusion corresponds to the available data on isotopic geochronology, and to the composition of columnar stromatolites from the Dehengda Formation. Both the structural features and carbon isotopic composition of its rocks are comparable to those of rocks of known Mesoproterozoic age, but differ from the characteristics of definitely Neoproterozoic deposits.     

Isotopic disequilibrium in ultrahigh-pressure and retrograde metamorphism of eclogite and gneiss from the Chinese Continental Scientific Drilling in the Sulu orogen, China: evidence from mineral Nd–Sr–O isotopic composition

Diffusion rates of Sr and O in minerals are often comparable while Nd has a lower diffusion rate during thermal overprint(s); thus, the O isotope systems between metamorphic minerals can serve as an indicator to evaluate whether equilibrium of Rb–Sr and Sm–Nd systems has been preserved in the metamorphic minerals that experienced retrograde metamorphism. This study presents a combination of investigation on Sm–Nd, Rb–Sr, and O isotopic compositions of minerals separated from ultrahigh-pressure eclogite and gneiss that were collected from the main hole of the Chinese Continental Scientific Drilling project located in the Sulu orogen, eastern China. Oxygen isotopic compositions of minerals from gneiss and eclogite yield two temperature groups of 620–740 and 460–590°C, representing diffusion cessation of isotopic exchange during the eclogite-facies recrystallization and later amphibolite-facies retrograde overprint. Rb–Sr mineral regressions of two eclogite samples give consistent Triassic ages of 244 Ma, corresponding to eclogite-facies metamorphism, while the same minerals do not yield meaningful Sm–Nd isochron ages. This phenomenon likely suggests that Rb–Sr isotopic equilibrium was achieved during eclogite-facies metamorphism and preserved during late amphibolite-facies retrogression. In contrast, Sm–Nd isotopic equilibrium between the minerals of eclogite was not achieved under UHP metamorphic conditions. Regressions of epidote and biotite of one gneiss sample give a Triassic Sm–Nd age of 243 ± 34 Ma, corresponding to the time of the eclogite-facies metamorphism, and a Jurassic mineral Rb–Sr age of 187.5 ± 1.8 Ma. These results imply that fluids have played an important role to achievement of the Sm–Nd isotopic equilibrium during eclogite-facies metamorphism and re-equilibration of the Rb–Sr isotopic system during later retrograde overprint.     

Rb-Sr, K-Ar, H-and O-Isotope systematics of the Middle Riphean shales from the Debengda Formation, the Olenek Uplift, North Sibera

Clay subfractions (SFs) of <0.1, 0.1–0.2, 0.2–0.3, 0.3–0.6, 0.6–2 and 2–5 μm separated from Middle Riphean shales of the Debengda Formation are studied using the TEM, XRD, K-Ar and Rb-Sr isotopic methods. The oxygen and hydrogen isotope compositions in the SFs are studied as well. The low-temperature illite-smectite is dominant mineral in all the SFs except for the coarsest ones. The XRD, chemical and isotopic data imply that two generations of authigenic illite-smectite different in age are mixed in the SFs. The illite crystallinity index decreases in parallel with size diminishing of clay particles. As compared to coarser SFs, illite of fine-grained subfractions is enriched in Al relative to Fe and Mg, contains more K, and reveals higher K/Rb and Rb/Sr ratios. The Rb-Sr age calculated by means of the leachochron (“inner isochron”) method declines gradually from 1254-1272 Ma in the coarsest SFs to 1038-1044 Ma in finest ones, while the K-Ar age decreases simultaneously from 1225–1240 to 1080 Ma. The established positive correlation of δ¹⁸O and δD values with dimensions of clay particles in the SFs seems to be also consistent with the mixing systematics. The isotopic systematics along with data on mineral composition and morphology lead to the conclusion that mixedlayer illite-smectite was formed in the Debengda shales during two periods 1211–1272 and 1038–1080 Ma ago. The first period is likely close to the deposition time of sediments and corresponds to events of burial catagenesis, whereas the second one is correlative with the regional uplift and changes in hydrological regime during the pre-Khaipakh break in sedimentation.     

Microfossiles (Acritarches) du Proterozoique superieur dans les shales de la formation d'Atar (Mauritanie)

Earlier studies of Stromatolites had permitted the dating of the Atar Formation (Mauritania) as late Riphean by comparing these structures with those of the U.S.S.R. Geochronological data (Rb/Sr) obtained subsequently on illite had given ages ranging between 890 ± 36 and 595 ± 43 Ma for the whole Atar Group, the Atar Formation being dated at 890 ± 36 Ma (late Riphean). The microfossils (Acritarchs) studied here come from black shales cored while drilling for water near Atar. These shales are equivalent to the intercalations between the stromatolitic limestones of the Atar Formation. They have yielded 20 species of Acritarchs, six of which are most important as they provide a diagnostic assemblage of the late Riphean (1000 ± 50–650 ± 10 Ma) of the U.S.S.R. and northern Europe. These are: Chuaria circularis, Kildinosphaera chagrinata, K. verrucata, Leiosphaeridia asperata, Stictosphaeridium cf. sinapticuliferum, Trematosphaeridium holtedahli. The late Riphean age of the Atar Formation is thus confirmed by this microfossil assemblage.     

Stages in the formation of uranium mineralization in the Salla-Koulajarvinskaya zone (Northern Karelia): Geological and isotope geochronological data

On the basis of U–Pb, Rb–Sr and Sm–Nd isotopic data, it is shown that formation of uranium mineralization in the Paleoproterozoic Salla-Koulajarvinsky belt (Northern Karelia) was a long-lasting mult-stage process that developed over more than 1 Ga: from the Paleoproterozoic to the Paleozoic. The first stage, 1.75 Ga ago, corresponds to the Svekofennian metamorphic event—regional albitization. The process was dated by the Rb–Sr (isochronic age of albitites is 1754 ± 39 Ma) and U–Pb methods (the age of rutile is 1756 ± 8 Ma). At this stage, with a lower temperature limit of 400–450°C, conditions were favorable for the mobilization and migration of uranium, but not for its deposition in minerals. The second stage, 1.62 Ga ago, was a time of alteration of rocks at the regressive stage of the Svekofennian metamorphic event, when carbonate and chlorite rocks formed after albitites. The age of this stage was estimated as 1627 ± 42 Ma according to ThO₂, UO₂, and PbO contents in uraninite. Probably, the deposition of uraninite took place at this stage at temperature not higher than 300–350°C. The final, third stage, 385 Ma ago, corresponds to the Paleozoic tectonic activation and formation of Caledonian alkaline intrusions. Uranium minerals were probably redeposited at this stage; the U–Pb age of brannerite is 385 ± 2 Ma.     

Carbonate Rocks from the Riphean–Vendian Boundary Deposits of the Anabar Uplift: Isotope (<Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr, δ<Superscript>13</Superscript>C, δ<Superscript>18</Superscript>O) Systematics and Chemostratigraphic Consequences

New ⁸⁷Sr/⁸⁶Sr, δ¹³C, and δ¹⁸О chemostratigraphic data were obtained for carbonate rocks of the Lower Riphean Yusmastakh and the Vendian Starorechenskaya formations. The δ¹³С values in dolomites of the Yusmastakh Formation varies from–0.6 to–0.1‰ and in dolomites and dolomitic limestones of the Starorechenskaya Formation, from–1.2 to–0.4‰ PDB, and δ¹⁸О values, from 24.4 to 26.4‰ and from 25.3 to 27.6‰ SMOW, respectively. The Rb–Sr systematics of carbonate rocks was studied using the refined method of stepwise dissolution of samples in acetic acid, including chemical removal of up to one-third of the ground sample by preliminary acid leaching and subsequent partial dissolution of the rest of the sample. Owing to this procedure, secondary carbonate material is removed, which enables one to improve the quality of the Sr-chemostratigraphic data obtained. The initial ⁸⁷Sr/⁸⁶Sr ratios in carbonate rocks of the Yusmastakh (0.70468–0.70519) and Starorechenskaya (0.70832–0.70883) formations evidence the Riphean–Vendian boundary in the Precambrian sequence of the Anabar Uplift.     

Sr isotopic characteristic of neoproterozoic carbonate sediments from the southern Yenisei Ridge

This paper presents the first Sr isotopic data for the Late Precambrian carbonate rocks of the southern Yenisei Ridge. Their geochemical study allowed estimation of the degree of secondary alterations and gave the possibility to reveal rocks with a less disturbed Rb-Sr isotopic system. The Sr isotopic data indicated Neoproterozoic sedimentation of the rocks about 1070–750 Ma ago. Sr and C isotopic data showed that carbonate rocks of the Sukhoi Pit, Tungusik, and Shirokino groups are Late Riphean and could be comparable with sedimentary sequences of three Precambrian key sections of the Northern Eurasia: the subsequent Derevnino, Burovaya, and Shorikha formations from the Turukhansk Uplift, the Lakhanda Group from the Uchur-Maya region, and the Karatav Group from the South Urals. All studied carbonate rocks are older than 750 Ma and, according to the International Stratigraphic Chart, accumulated prior to global glaciations in the Cryogenian. This is evident from sedimentological study indicating the absence of tillite horizons in the studied sections. δ¹³C values in the sections vary from +0.4 up to +5.3‰, which testifies to the absence of periods of great cold.     

Age and sources of matter for the Kedrovskoe gold deposit,Northern Transbaikal tegion,Republic of Buryatia: Geochronological and isotopic geochemical constraints

The paper presents new geochronological and isotopic geochemical data on gold mineralization of the Kedrovskoe deposit. The deposit is located in the northeastern part of the Transbaikal metallogenic province, Russia’s largest. The Early Permian age (273 ± 4 Ma) of mineralization based on the results of Rb–Sr study of metasomatic rocks is correlated with the age of the final phases of Hercynian magmatism in the Baikal–Muya Foldbelt. The Sr, Nd, and Pb isotopic geochemical characteristics of mineralization show that the host rocks are involved in the formation of the latter. It has been established that ore lead was supplied to the hydrothermal system of the deposit mainly from a geochemical reservoir represented by the Neoproterozoic juvenile continental crust of the Baikal–Muya Foldbelt.     

Isotopic constraints on the genesis and age of autochthonous glaucony in the Oligo-Miocene Torquay Group, south-eastern Australia

The Oligo‐Miocene Torquay Group at Bird Rock in south‐eastern Australia comprises a sequence of fine‐grained skeletal carbonates and argillaceous and glauconitic sandstones, deposited in a cool‐water, mid‐shelf environment. The Bird Rock glaucony is autochthonous and consists predominantly of randomly interstratified glauconitic smectite, which constitutes bioclast infills and faecal pellet replacements. The results of Rb–Sr and oxygen isotopic analysis of samples taken from a single glauconitic horizon (the BW horizon) indicate that the glaucony developed through a series of simultaneous dissolution–crystallization reactions, which occurred during very early diagenesis in a closed or isochemical system, isolated from the ambient marine environment. The constituent ions of the glaucony were derived primarily from terrigenous clay minerals, but considerable potassium may have been sourced indirectly from sea water, through potassium enrichment of clay precursors. The pore fluids associated with glauconitization were marine derived, but progressively modified by the dissolution–crystallization of detrital clay minerals and autochthonous glaucony. Rb–Sr data for the BW horizon indicate that dating glauconies may be somewhat problematic, as co‐genetic glauconitic minerals can show a range of initial strontium compositions, which reflect the incorporation of strontium derived from mineralogical precursors and/or contemporaneous sea water. Rb–Sr isochrons indicate that the glaucony of the BW horizon formed at 23 ± 3 Ma. This age is in good agreement with both the established biostratigraphy and a ⁸⁷Sr/⁸⁶Sr age for the horizon (23 ± 1 Ma), but could only be determined using the independent age constraint and the estimate of the ⁸⁷Sr/⁸⁶Sr ratio of contemporaneous sea water provided by analysis of associated biogenic carbonate.     

The 1501 Ma Kuonamka Large Igneous Province of northern Siberia: U-Pb geochronology,geochemistry, and links with coeval magmatism on other crustal blocks

A new large igneous province (LIP), the 1501 ± 3 Ma Kuonamka LIP, extends across 700 km of northern Siberia and is linked with coeval dikes and sills in the formerly attached Sao Francisco craton (SFC)-Congo craton to yield a short-duration event 2000 km across. The age of the Kuonamka LIP can be summarized as 1501 ± 3 Ma (95% confidence), based on 7 U-Pb ID-TIMS ages (6 new herein) from dolerite dikes and sills across the Anabar shield and within western Riphean cover rocks for a distance of 270 km. An additional sill yielded a SIMS (CAMECA) age of 1483 ± 17 Ma and sill in the Olenek uplift several hundred kilometers farther east, a previous SIMS (SHRIMP) age of ca. 1473 Ma was obtained on a sill; both SIMS ages are within the age uncertainty of the ID-TIMS ages. Geochemical data indicate a tholeiitic basalt composition with low MgO (4–7 wt%) within-plate character based on trace element classification diagrams and source between E-MORB and OIB with only minor contamination from crust or metasomatized lithospheric mantle. Two subgroups are distinguished: Group 1 has gently sloping LREE ((La/Sm)_PM = 1.9) and HREE ((Gd/Yb)_PM = 1.8) patterns, slightly negative Sr and moderate TiO₂ (2.2 wt%), and Group 2 has steeper LREE ((La/Sm)_PM = 2.3) and HREE ((Gd/Yb)_PM = 2.3), strong negative Sr anomaly, is higher in TiO₂ (2.7 wt%), and is transitional from tholeiitic to weakly alkaline in composition. The slight differences in REE slopes are consistent with Group 2 on average melting at deeper levels. Proposed reconstructions of the Kuonamka LIP with 1500 Ma magmatism of the SFC-Congo craton are supported by a geochemical comparison. Specifically, the chemistry of the Chapada Diamantina and Curaga dikes of the SFC can be linked to that of Groups 1 and 2, respectively, of the Kuonamka LIP and are consistent with a common mantle source between EMORB and OIB and subsequent differentiation history. However, the coeval Humpata sills and dikes of the Angola block of the Congo craton represent a different magma batch.     

Mineralogical,Mössbauer,and Isotopic–Geochronological Study of Upper Riphean Al-glauconites,Kil'din Group,Srednii Peninsula

The paper presents mineralogical, crystallochemical, and Rb–Sr age data on globular layer silicates of the glauconitic series from lower horizons of the Upper Riphean Kil'din Group, Srednii Peninsula. Chloritization significantly changed the globular Al-glauconite in sandstones of the Päräjarvinskaya Formation. However, it almost did not affect glauconite globules in sandstones of the Palvinskaya Formation and altered only clayey component of the rock. Mössbauer spectroscopy showed that Al-glauconite in the Palvinskaya Formation was affected by secondary transformations, which caused the Fe²⁺ oxidation in octahedral sheets and loss of radiogenic ⁸⁷Sr. The transformations were presumably related to exhumation of rocks of the Päräjarvinskaya and Palvinskaya formations to the circulation level of the oxygen-rich meteoric waters. Thus, Rb–Sr datings obtained for Al-glauconites of the Palvinskaya (781–786 Ma) and Päräjarvinskaya(744–751 Ma) formations correspond to the tectonic uplift of Kil'din rocks and have no stratigraphic significance.     

Permian copper–polymetallic mineralization in the southern Great Xing’an Range,Northeast China: the Daolundaba copper–polymetallic deposit example

The Daolundaba Cu–polymetallic deposit is a newly discovered Cu–W–Sn deposit on the western slopes of the southern Great Xing’an Range, and its mineralization was related to an early Permian coarse-grained biotite granite. However, there is little information on the age of formation of the deposit. In this article, we present the results of our investigation into the age of the Daolundaba Cu–polymetallic deposit, which involved the selection of chalcopyrite and pyrrhotite samples for Rb–Sr isochron dating. A Rb–Sr isochron defined by the chalcopyrite samples yielded a Rb–Sr isochron age of 290.0 ± 11 Ma (MSWD = 1.2) with an initial Sr isotopic composition (I_Sr) of 0.71446. The pyrrhotite samples yielded a Rb–Sr isochron age of 283.0 ± 2.6 Ma (MSWD = 1.16) with an initial Sr isotopic composition (I_Sr) of 0.71447. The Rb–Sr isochron age determined from the chalcopyrite and pyrrhotite is 282.7 ± 1.7 Ma (MSWD = 1.13). These results indicate that the Daolundaba Cu–polymetallic deposit formed during the early Permian (282.7–290.0 Ma). The Rb and Sr contents of the chalcopyrite and pyrrhotite range from ~0.1325 to ~3.6810 ppm and from ~0.1219 to ~9.5740 ppm, respectively, and the initial Sr isotope ratios (I_Sr) range from 0.71047 to 0.71869, with an average of 0.714723. These isotopic characteristics indicate the ore-forming minerals of the Daolundaba Cu–polymetallic deposit originated mainly from the crust, but with small amounts of mantle material involved. The copper was derived from the associated magma whereas the W and Sn was derived from the surrounding strata. The Permian mineralization of the Xing’an–Mongolia region occurred in an active continental margin setting during subduction of the Palaeo-Asian oceanic plate beneath the Siberian Plate.     

Geochemistry, U-Pb Geochronology and Sr-Nd-Hf Isotopes of the Early Cretaceous Volcanic Rocks in the Northern Da Hinggan Mountains

Whole-rock geochemical, zircon U-Pb geochronological and Sr-Nd-Hf isotopic data are presented for the Early Cretaceous volcanic rocks from the northern Da Hinggan Mountains. The volcanic rocks generally display high SiO2(73.19–77.68 wt%) and Na2O+K2O(6.53–8.98 wt%) contents, with enrichment in Rb, Th, U, Pb and LREE, and depletion in Nb, Ta, P and Ti. Three rhyolite samples, one rhyolite porphyry sample, and one volcanic breccia sample yield weighted mean 206Pb/238 U ages of 135.1±1.2 Ma, 116.5±1.1 Ma, 121.9±1.0 Ma, 118.1±0.9 Ma and 116.9±1.4 Ma, respectively. All these rocks have moderate(87Sr/86Sr)i values of 0.704912 to 0.705896, slightly negative εNd(t) values of –1.4 to –0.1, and positive εHf(t) values of 3.7 to 8. Their zircon Hf and whole-rock Nd isotopic model ages range from 594 to 1024 Ma. These results suggest that the Early Cretaceous volcanic rocks were originated from melting of subducted oceanic crust and associated sediments during the closure of the Mongol-Okhotsk Ocean.