Антон Кузнецов - Academia.edu (original) (raw)

Papers by Антон Кузнецов

Stratigraphy and Geological Correlation, 2008

Fe-Mg карбонатные метасоматиты в известняках суранской свиты нижнего рифея в Авзянском рудном рай... more Fe-Mg карбонатные метасоматиты в известняках суранской свиты нижнего рифея в Авзянском рудном районе Башкирского мегантиклинория представлены крупными залежами Fe-магнезита (Исмакаевское месторождение) и штоками брейнерита (проявление Богряшка). Метасоматическая зональность выражена изменениями в ряду известняк-доломит-Fe-магнезит (брейнерит). Железистый магнезит содержит до 8 мол. % FeСO 3 , в брейнерите насыщенность железом варьирует от 10 до 45 мол. % FeСO 3. Метасоматический флюид представлял собой рассол хлоридов Ca, Na, Mg с примесью Fe, и был связан с процессами ремобилизации захоронённых в отложениях нижнего рифея эвапоритовых рассолов. Соленость и температура гомогенизации флюидных включений в магнезитах-20-26 мас. % экв. NаCl и 200-240°C, а в брейнеритах-10-15 мас. % экв. NаCl и 140-190°C, соответственно. Взаимодействие флюида с терригенными породами в тектонически активной зоне машакского рифтогенного грабена привело к обогащению рассола различными лигандами и накоплению в метасоматических продуктах лантаноидов средней и тяжелой группы. При миграции флюида через известняковую толщу и его остывании, во фронтальной зоне происходило образование Fe-магнезита (Исмакаевское месторождение), а в тыловой зоне, примыкающей к терригенным сланцам-брейнерита (проявление Богряшка).

Stratigraphy and Geological Correlation, 2008

The Rb-Sr and U-Pb systematics are studied in carbonate deposits of the Satka and Suran formation... more The Rb-Sr and U-Pb systematics are studied in carbonate deposits of the Satka and Suran formations corresponding to middle horizons of the Lower Riphean Burzyan Group in the Taratash and Yamantau anticlinoria, respectively, the southern Urals. The least altered rock samples retaining the 87 Sr/ 86 Sr ratio of sedimentation basin have been selected for analysis using the original method of leaching the secondary carbonate phases and based on strict geochemical criteria of the retentivity (Mn/Sr < 0.2, Fe/Sr < 5 and Mg/Ca < 0.024). The stepwise dissolution in 0.5 N HBr has been used to enrich samples in the primary carbonate phase before the Pb-Pb dating. Three (L-4 to L-6) of seven consecutive carbonate fractions obtained by the step-wise leaching are most enriched in the primary carbonate (in terms of the U-Pb systematics). In the 206 Pb/ 204 Pb-207 Pb/ 204 Pb diagram, data points of these fractions plot along an isochron determining age of 1550 ± 30 Ma (MSWD = 0.7) for the upper member of the Satka Formation. The initial 87 Sr/ 86 Sr ratio in the least altered limestones of this formation is within the range of 0.70460-0.70480. Generalization of the Sr isotopic data published for the Riphean carbonates from different continents showed that 1650-1350 Ma ago the 87 Sr/ 86 Sr ratio in the world ocean was low, slightly ranging from 0.70456 to 0.70494 and suggesting the prevalent impact of mantle flux.

Stratigraphy and Geological Correlation, 2012

Sr/ 86 Sr ratio of global continental runoff is estimated at either 0.7119 (Palmer and Edmond, 19... more Sr/ 86 Sr ratio of global continental runoff is estimated at either 0.7119 (Palmer and Edmond, 1989) or 0.7136 (Allègre et al., 2010). The present day annual flux of strontium into the oceans is determined within the range of 2.21 × 10 12 g (Chaudhuri and Clauer, 1986) to 2.73 ± 0.09 × 10 12 g (Veizer, 1989). The seawater oceanic crust interaction in hydro thermal vents of mid ocean ridges and erosion of oce anic islands produce the other main flux of strontium into the World Ocean (Brass, 1976; Spooner, 1976; Rad et al., 2007; Allègre et al., 2010). In this flux, the 87 Sr/ 86 Sr ratio is significantly lower than in the conti nental runoff. In the Icelandic hydrothermal brines, this ratio ranges from 0.7032 to 0.7044 (Clauer and

Stratigraphy and Geological Correlation, 2006

New data on Sr-and C-isotopic systematics of carbonate rocks from the Upper Riphean stratotype (K... more New data on Sr-and C-isotopic systematics of carbonate rocks from the Upper Riphean stratotype (Karatau Group of the southern Urals) are obtained for several southwestern sections of the Bashkirian meganticlinorium, which have not been studied before. The results obtained supplement the Sr-and C-isotopic information for the group upper horizons thus detailing chemostratigraphic characterization of the entire succession. Limestone and dolostone samples used to analyze the Sr isotope composition satisfy strict geochemical criteria of the isotopic system retentivity and have been subjected to preliminary treatment in ammonium acetate to remove secondary carbonate phases. Data on 255 samples of carbonate rocks (171 studied for the first time) show that δ 13 ë value varies in the Karatau Group succession from-2.8 to +5.9 ‰ V-PDB with several in-phase excursions from the general trend in all the sections studied in the area 90 × 130 km. The δ 13 ë variation trend demarcates several levels in the carbonate succession of the Karatau Group suitable for objectives of regional stratigraphy and for C-isotope chemostratigraphic subdivision of the Upper Riphean. The results of Sr isotopic analysis of 121 samples (51 unstudied before) from the Karatau Group imply that rocks in its lower part (the Katav Formation and basal horizon of the Inzer Formation) experienced considerable secondary alterations, while limestones and dolostones of the overlying interval of the group are frequently unaltered. In the "best" samples satisfying geochemical criteria of the isotopic system retentivity, the 87 Sr/ 86 Sr initial ratio increases from 0.70521-0.70532 in the lower Inzer deposits to 0.70611 in the upper Min'yar carbonates, decreasing to <0.70600 near the top of the latter. Above the regional hiatus separating the Min'yar and Uk formation, this ratio grows from 0.70533 to 0.70605-0.70609 in the limestone succession of the last formation.

Precambrian Research, 2002

Depositional ages of high-grade marble sequences in the Ofoten district of the North-Central Norw... more Depositional ages of high-grade marble sequences in the Ofoten district of the North-Central Norwegian Caledonides have been constrained by the application of carbon and strontium isotope stratigraphy. Several marble units intercalated with various schists of the Evenes Group, previously correlated over long distances farther to the north with a late Ordovician-early Silurian, low-grade, fossiliferous succession (Balsfjord Group), have been studied for carbon, oxygen and strontium isotopes. The least altered 87 Sr/ 86 Sr ratios (0.7066 and 0.7077), and the best preserved d 13 C values (+ 5.0 and +8.0‰) obtained from the lowermost and uppermost marble formations of the Evenes Group are consistent with a seawater composition in the Neoproterozoic (650-600 and 620-610 Ma, respectively). The isotopic data indicate that only one formation (87 Sr/ 86 Sr= 0.7083, d 13 C = +5.1‰), forming the middle Evenes Group, is consistent with the previously suggested early Silurian correlation. This is supported by new isotopic data obtained from coral and brachiopod-bearing, Llandovery (443-428 Ma) metalimestones (87 Sr/ 86 Sr= 0.7083, d 13 C= +4.3‰) of the Balsfjord Group. The remaining, fourth formation of the Evenes Group shows isotope data (87 Sr/ 86 Sr=0.7088, d 13 C= +2.1‰) which are consistent with a Cambrian seawater composition. The results obtained have several implications: (i) the late Ordovician-early Silurian Elvenes Conglomerate/ophiolite assemblage has a tectonic contact with the structurally overlying marble formation dated to 650-600 Ma; (ii) the isotopic and geological data do not support the previously proposed correlation of the entire Evenes Group with a late Ordovician-early Silurian, lithostratigraphic succession farther north in the Balsford area; (ii) the Evenes Group is not a lithostratigraphic unit and should be abandoned, as it is composed of a number of marble formations of different age, tectonically emplaced in a non-chronostratigraphic order. The Neoproterozoic depositional ages combined with the palaeogeographic position of Baltica suggest that a large part of the carbonate rock succession forming the Evenes Group was initially accumulated in warm seas on a continental shelf, probably Laurentia, and

Lithology and Mineral Resources, 2012

Lithology and Mineral Resources, 2011

The Rb-Sr and U-Pb systematics have been studied in the metasedimentary carbonate rocks from the ... more The Rb-Sr and U-Pb systematics have been studied in the metasedimentary carbonate rocks from the Paleoproterozoic Kuetsjarvi Formation. Samples were taken from the borehole drilled in the northern zone of the Pechenga Greenstone Belt in the northwestern Kola Peninsula. The carbonate section of the formation is made up of three units (from the bottom to top): (I) dolomite (68 m), (II) calcareous-dolomite (9 m), and (III) clayey calcareous (1 m) ones. Dolomites (Mg/Ca = 0.55–0.61) from the lowermost unit I contain 70.3–111 ppm Sr. Initial 87Sr/86Sr ratio in them varies within 0.70560–0.70623 and characterizes the primary continental-lacustrine carbonate sediments. Calcareous dolomites (Mg/Ca = 0.39–0.59) and dolomitic limestones of units II and III (Mg/Ca = 0.02–0.36) are enriched in Sr (285–745 and 550–1750 ppm, respectively). Initial 87Sr/86Sr ratios in these rocks (0.70406–0.70486 and 0.70407–0.70431, respectively) fall within the range typical of the Jatulian seawater, which indicates that the carbonate sediments of two upper units were formed in an open marine basin. Study of dolomites from unit I showed that the Svecofennian metamorphism more significantly affected the U-Pb systems of carbonate rocks as compared to their Rb-Sr systems. In the 207Pb/204Pb-206Pb/204Pb diagram, most data points corresponding to the carbonate constituent of dolomites define isochron with an age of 1900 ± 25 Ma (MSWD = 0.5). The same samples define a positive correlation in the 208Pb/204Pb-206Pb/204Pb plot. Since sedimentary carbonates usually do not contain Th, this correlation points to secondary enrichment of the studied dolomites in Th or thorogenic 208Pb. Hence, the obtained Pb-Pb dating can be regarded as the age of the Svecofennian metamorphic event. Three samples from dolomites of unit I lack any disturbance of the initial U-Th-Pb systematics, but their trend in the 207Pb/204Pb-206Pb/204Pb diagram deviates from the 1900 Ma isochron. Based on these samples, the model U-Pb premetamorphic age of the Kuetsjarvi carbonate sediments is 2075–2100 Ma. This interval is consistent with the age range of the Lomagundi-Jatulian event, which was responsible for the formation of carbonate sediments with high positive δ13C values.

Lithology and Mineral Resources, 2007

Fine-grained clay subfractions, SFs (particle size <0.1, 0.1-0.4, 0.4-0.6, and 0.6-2.0 µ m) separ... more Fine-grained clay subfractions, SFs (particle size <0.1, 0.1-0.4, 0.4-0.6, and 0.6-2.0 µ m) separated from a sample of the Lower Cambrian blue clay of the Lontova Formation were studied with XRD and Sm-Nd methods. The relatively coarse-grained SFs include illite with a small admixture of chlorite, while the finegrained SF (<0.1 µ m) consists of mixed-layer illite-smectite. The illite crystallinity index (I c) increases with decreasing particle size. The leaching of SFs with 1N HCl, analysis of Sm-Nd systematics of leachates and residues, as well as XRD data and results of chemical analysis show that the studied rocks contain at least two generations of minerals. The first (detrital) generation is related to the transformation of provenance material, whereas the second (authigenic) generation was formed at the postsedimentary stage of the evolution of the Lontova sediments. The Sm-Nd date of the first generation (790 ± 90 Ma) is considered a minimal age of rocks in the northeastern and southwestern regions of the East European Platform that served as a source of sedimentary material of the Lower Cambrian blue clay. The date of the second generation of minerals reflects the timing of authigenic mineral formation in the course of burial and diagenetic and catagenetic reworking of clay sediments.

Journal of Asian Earth Sciences, 2013

Depositional ages for the middle part of the Neoproterozoic sedimentary succession exposed along ... more Depositional ages for the middle part of the Neoproterozoic sedimentary succession exposed along Lake Baikal in the southern margin of the Siberian Platform have been constrained by the application of Sr isotope chemostratigraphy and Pb-Pb dating of carbonate rocks. The thick siliciclastic-carbonate Baikal Group is the type section of the regional Siberian succession. Although many of the Baikal Group carbonate rocks have undergone pervasive dolomitization by mixed meteoric-marine fluids, oolitic limestones in the 800-m-thick carbonate member of the Uluntui Formation preserve primary textures and high Sr contents (up to 3200 ppm), suggesting that aragonite might have been the original precursor. The 87 Sr/ 86 Sr in the least altered high-Sr limestones (Mg/Ca < 0.08 and d 18 O > À8‰ PDB) range within 0.70842-0.70876, values we interpret as representing the best proxy for coeval seawater. Stepwise dissolution using 0.5 N HBr was applied to enrich samples in primary carbonate from the Uluntui Formation and nine data points from the five least altered, high-Sr limestones yielded a Pb-Pb isochron age of 560 ± 30 Ma with MSWD = 0.6 and probability = 0.8. This age matches that inferred from Sr isotope stratigraphy (580-550 Ma) and further supports the inference that the Baikal paleobasin was connected to the contemporary oceans. Our Sr chemostratigraphy and Pb-Pb isochron age of the Uluntui limestones cast doubt on previously proposed correlations of the Baikal Group as late Mesoproterozoic to Cryogenian. Instead, the data indicate that the entire siliciclastic-carbonate succession of the Baikal Group was most likely deposited during the Ediacaran (or Vendian, of the Russian Stratigraphic Scale).

Geological Magazine, 2002

Carbon and strontium isotope stratigraphy has been applied to constrain the depositional ages of ... more Carbon and strontium isotope stratigraphy has been applied to constrain the depositional ages of high-grade marble sequences in the Ofoten district of the North-Central Norwegian Caledonides. Two marble formations hosted by diverse schists from the Bogen Group, all previously correlated over long distances with a Late Ordovician–Early Silurian, low-grade, fossiliferous succession, have been studied for carbon, oxygen and strontium isotopes. The least altered 87Sr/86Sr ratios ranging between 0.7062 and 0.7068, and the best preserved δ13C values falling between +5.0 and +6.5‰ obtained from two marble formations, are consistent with a seawater composition in the time interval 700–600 Ma. The results obtained do not support the previously proposed correlation of the Bogen Group with an Ordovician–Silurian lithostratigraphic succession further north. The apparent depositional ages suggest that the tectonostratigraphic succession studied is inverted and that the tectonostratigraphy of the...

Doklady Earth Sciences, 2014

341 The Onot greenstone belt (OGB) forms a trough (paleorift) structure within the Prisayan margi... more 341 The Onot greenstone belt (OGB) forms a trough (paleorift) structure within the Prisayan marginal block of the Siberian Platform basement (Fig. 1). The OGP occurs on ancient tonalite–trondhjemite–gran odiorite gneiss containing zircons with a U–Pb age of ≈3.34 Ga [1] and is intruded by Early Proterozoic granite of the Shumikhin Complex (the U–Pb age of zircons is 1.87 Ga [2]). The belt is composed of metavolcanic rocks of the tholeiite–rhyolite series, metaterrigenous and biotite–garnet gneisses, amphib olite, quartzite, and magnesite and dolomite marbles [3, 4]. Rocks are metamorphosed under the condi tions of the amphibolite and epidote–amphibolite facies and have undergone intense ultrametamorphic and metasomatic transformations. The age of the pro tolith of metarhyolite from the OGB is 2.89 Ga (U– Pb age by zircon) [5]. The Rb–Sr age of ultrameta morphic granitoids from the Onot Complex is 2.64 Ga [6]. Thus, the supracrustal complex of the OGB is one of the oldest in Asia, and its age is consistent with that of typical Archean greenstone belts of the Earth.

Doklady Earth Sciences, 2007

Doklady Earth Sciences, 2011

ABSTRACT The U-Pb(Pb-Pb) age was determined for limestone from member III of the Sorna Formation ... more ABSTRACT The U-Pb(Pb-Pb) age was determined for limestone from member III of the Sorna Formation out- cropped on the Azyr-Tal Range, Kuznetsk Alatau. The weighted average value from three calculated values is 523 ± 5 Ma (MSWD = 0.02, 2sigma). This age coincides with the Early Cambrian age interval determined with Sr isotopic chemostratigraphy (87Sr/86Sr ratio is 0.70850-0.70852). The low value of mu2 = 238U/204Pb for limestone is due to the evolution of diagenetic fluid containing lead from the mantle reservoir into the Sorna sediments.

Doklady Earth Sciences, 2013

ABSTRACT The Pb-Pb age of phosphorite concretions of the Zigaza-Komarovo Formation, which compose... more ABSTRACT The Pb-Pb age of phosphorite concretions of the Zigaza-Komarovo Formation, which composes the intermediate horizons of the Riphean stratotype of the South Urals, was determined in fractions resulting from the stepwise dissolution of concretions in 0.1 N, 0.5 N, and 1 N HCl. The determination of the Sr isotopic composition in phosphate fractions was favorable for rejection the fractions polluted with extraneous material. On the 207Pb/204Pb-206Pb/204Pb diagram, the isochron based on 31 points corresponds to 1330 ± 20 Ma (MSWD = 1.12), which is in agreement with the stratigraphic position of the Zigaza-Komarovo Formation. The decreased μ2 value of 9.57 for the phosphorite concretions relative to that of the average earthly lead based on the Stacey-Kramers model (9.74) is related to the rocks with an admixture of mantle lead, which occur in the run-off area of the Zigaza-Komarovo sediments.

Stratigraphy and Geological Correlation, 2008

Fe-Mg карбонатные метасоматиты в известняках суранской свиты нижнего рифея в Авзянском рудном рай... more Fe-Mg карбонатные метасоматиты в известняках суранской свиты нижнего рифея в Авзянском рудном районе Башкирского мегантиклинория представлены крупными залежами Fe-магнезита (Исмакаевское месторождение) и штоками брейнерита (проявление Богряшка). Метасоматическая зональность выражена изменениями в ряду известняк-доломит-Fe-магнезит (брейнерит). Железистый магнезит содержит до 8 мол. % FeСO 3 , в брейнерите насыщенность железом варьирует от 10 до 45 мол. % FeСO 3. Метасоматический флюид представлял собой рассол хлоридов Ca, Na, Mg с примесью Fe, и был связан с процессами ремобилизации захоронённых в отложениях нижнего рифея эвапоритовых рассолов. Соленость и температура гомогенизации флюидных включений в магнезитах-20-26 мас. % экв. NаCl и 200-240°C, а в брейнеритах-10-15 мас. % экв. NаCl и 140-190°C, соответственно. Взаимодействие флюида с терригенными породами в тектонически активной зоне машакского рифтогенного грабена привело к обогащению рассола различными лигандами и накоплению в метасоматических продуктах лантаноидов средней и тяжелой группы. При миграции флюида через известняковую толщу и его остывании, во фронтальной зоне происходило образование Fe-магнезита (Исмакаевское месторождение), а в тыловой зоне, примыкающей к терригенным сланцам-брейнерита (проявление Богряшка).

Stratigraphy and Geological Correlation, 2008

The Rb-Sr and U-Pb systematics are studied in carbonate deposits of the Satka and Suran formation... more The Rb-Sr and U-Pb systematics are studied in carbonate deposits of the Satka and Suran formations corresponding to middle horizons of the Lower Riphean Burzyan Group in the Taratash and Yamantau anticlinoria, respectively, the southern Urals. The least altered rock samples retaining the 87 Sr/ 86 Sr ratio of sedimentation basin have been selected for analysis using the original method of leaching the secondary carbonate phases and based on strict geochemical criteria of the retentivity (Mn/Sr < 0.2, Fe/Sr < 5 and Mg/Ca < 0.024). The stepwise dissolution in 0.5 N HBr has been used to enrich samples in the primary carbonate phase before the Pb-Pb dating. Three (L-4 to L-6) of seven consecutive carbonate fractions obtained by the step-wise leaching are most enriched in the primary carbonate (in terms of the U-Pb systematics). In the 206 Pb/ 204 Pb-207 Pb/ 204 Pb diagram, data points of these fractions plot along an isochron determining age of 1550 ± 30 Ma (MSWD = 0.7) for the upper member of the Satka Formation. The initial 87 Sr/ 86 Sr ratio in the least altered limestones of this formation is within the range of 0.70460-0.70480. Generalization of the Sr isotopic data published for the Riphean carbonates from different continents showed that 1650-1350 Ma ago the 87 Sr/ 86 Sr ratio in the world ocean was low, slightly ranging from 0.70456 to 0.70494 and suggesting the prevalent impact of mantle flux.

Stratigraphy and Geological Correlation, 2012

Sr/ 86 Sr ratio of global continental runoff is estimated at either 0.7119 (Palmer and Edmond, 19... more Sr/ 86 Sr ratio of global continental runoff is estimated at either 0.7119 (Palmer and Edmond, 1989) or 0.7136 (Allègre et al., 2010). The present day annual flux of strontium into the oceans is determined within the range of 2.21 × 10 12 g (Chaudhuri and Clauer, 1986) to 2.73 ± 0.09 × 10 12 g (Veizer, 1989). The seawater oceanic crust interaction in hydro thermal vents of mid ocean ridges and erosion of oce anic islands produce the other main flux of strontium into the World Ocean (Brass, 1976; Spooner, 1976; Rad et al., 2007; Allègre et al., 2010). In this flux, the 87 Sr/ 86 Sr ratio is significantly lower than in the conti nental runoff. In the Icelandic hydrothermal brines, this ratio ranges from 0.7032 to 0.7044 (Clauer and

Stratigraphy and Geological Correlation, 2006

New data on Sr-and C-isotopic systematics of carbonate rocks from the Upper Riphean stratotype (K... more New data on Sr-and C-isotopic systematics of carbonate rocks from the Upper Riphean stratotype (Karatau Group of the southern Urals) are obtained for several southwestern sections of the Bashkirian meganticlinorium, which have not been studied before. The results obtained supplement the Sr-and C-isotopic information for the group upper horizons thus detailing chemostratigraphic characterization of the entire succession. Limestone and dolostone samples used to analyze the Sr isotope composition satisfy strict geochemical criteria of the isotopic system retentivity and have been subjected to preliminary treatment in ammonium acetate to remove secondary carbonate phases. Data on 255 samples of carbonate rocks (171 studied for the first time) show that δ 13 ë value varies in the Karatau Group succession from-2.8 to +5.9 ‰ V-PDB with several in-phase excursions from the general trend in all the sections studied in the area 90 × 130 km. The δ 13 ë variation trend demarcates several levels in the carbonate succession of the Karatau Group suitable for objectives of regional stratigraphy and for C-isotope chemostratigraphic subdivision of the Upper Riphean. The results of Sr isotopic analysis of 121 samples (51 unstudied before) from the Karatau Group imply that rocks in its lower part (the Katav Formation and basal horizon of the Inzer Formation) experienced considerable secondary alterations, while limestones and dolostones of the overlying interval of the group are frequently unaltered. In the "best" samples satisfying geochemical criteria of the isotopic system retentivity, the 87 Sr/ 86 Sr initial ratio increases from 0.70521-0.70532 in the lower Inzer deposits to 0.70611 in the upper Min'yar carbonates, decreasing to <0.70600 near the top of the latter. Above the regional hiatus separating the Min'yar and Uk formation, this ratio grows from 0.70533 to 0.70605-0.70609 in the limestone succession of the last formation.

Precambrian Research, 2002

Depositional ages of high-grade marble sequences in the Ofoten district of the North-Central Norw... more Depositional ages of high-grade marble sequences in the Ofoten district of the North-Central Norwegian Caledonides have been constrained by the application of carbon and strontium isotope stratigraphy. Several marble units intercalated with various schists of the Evenes Group, previously correlated over long distances farther to the north with a late Ordovician-early Silurian, low-grade, fossiliferous succession (Balsfjord Group), have been studied for carbon, oxygen and strontium isotopes. The least altered 87 Sr/ 86 Sr ratios (0.7066 and 0.7077), and the best preserved d 13 C values (+ 5.0 and +8.0‰) obtained from the lowermost and uppermost marble formations of the Evenes Group are consistent with a seawater composition in the Neoproterozoic (650-600 and 620-610 Ma, respectively). The isotopic data indicate that only one formation (87 Sr/ 86 Sr= 0.7083, d 13 C = +5.1‰), forming the middle Evenes Group, is consistent with the previously suggested early Silurian correlation. This is supported by new isotopic data obtained from coral and brachiopod-bearing, Llandovery (443-428 Ma) metalimestones (87 Sr/ 86 Sr= 0.7083, d 13 C= +4.3‰) of the Balsfjord Group. The remaining, fourth formation of the Evenes Group shows isotope data (87 Sr/ 86 Sr=0.7088, d 13 C= +2.1‰) which are consistent with a Cambrian seawater composition. The results obtained have several implications: (i) the late Ordovician-early Silurian Elvenes Conglomerate/ophiolite assemblage has a tectonic contact with the structurally overlying marble formation dated to 650-600 Ma; (ii) the isotopic and geological data do not support the previously proposed correlation of the entire Evenes Group with a late Ordovician-early Silurian, lithostratigraphic succession farther north in the Balsford area; (ii) the Evenes Group is not a lithostratigraphic unit and should be abandoned, as it is composed of a number of marble formations of different age, tectonically emplaced in a non-chronostratigraphic order. The Neoproterozoic depositional ages combined with the palaeogeographic position of Baltica suggest that a large part of the carbonate rock succession forming the Evenes Group was initially accumulated in warm seas on a continental shelf, probably Laurentia, and

Lithology and Mineral Resources, 2012

Lithology and Mineral Resources, 2011

The Rb-Sr and U-Pb systematics have been studied in the metasedimentary carbonate rocks from the ... more The Rb-Sr and U-Pb systematics have been studied in the metasedimentary carbonate rocks from the Paleoproterozoic Kuetsjarvi Formation. Samples were taken from the borehole drilled in the northern zone of the Pechenga Greenstone Belt in the northwestern Kola Peninsula. The carbonate section of the formation is made up of three units (from the bottom to top): (I) dolomite (68 m), (II) calcareous-dolomite (9 m), and (III) clayey calcareous (1 m) ones. Dolomites (Mg/Ca = 0.55–0.61) from the lowermost unit I contain 70.3–111 ppm Sr. Initial 87Sr/86Sr ratio in them varies within 0.70560–0.70623 and characterizes the primary continental-lacustrine carbonate sediments. Calcareous dolomites (Mg/Ca = 0.39–0.59) and dolomitic limestones of units II and III (Mg/Ca = 0.02–0.36) are enriched in Sr (285–745 and 550–1750 ppm, respectively). Initial 87Sr/86Sr ratios in these rocks (0.70406–0.70486 and 0.70407–0.70431, respectively) fall within the range typical of the Jatulian seawater, which indicates that the carbonate sediments of two upper units were formed in an open marine basin. Study of dolomites from unit I showed that the Svecofennian metamorphism more significantly affected the U-Pb systems of carbonate rocks as compared to their Rb-Sr systems. In the 207Pb/204Pb-206Pb/204Pb diagram, most data points corresponding to the carbonate constituent of dolomites define isochron with an age of 1900 ± 25 Ma (MSWD = 0.5). The same samples define a positive correlation in the 208Pb/204Pb-206Pb/204Pb plot. Since sedimentary carbonates usually do not contain Th, this correlation points to secondary enrichment of the studied dolomites in Th or thorogenic 208Pb. Hence, the obtained Pb-Pb dating can be regarded as the age of the Svecofennian metamorphic event. Three samples from dolomites of unit I lack any disturbance of the initial U-Th-Pb systematics, but their trend in the 207Pb/204Pb-206Pb/204Pb diagram deviates from the 1900 Ma isochron. Based on these samples, the model U-Pb premetamorphic age of the Kuetsjarvi carbonate sediments is 2075–2100 Ma. This interval is consistent with the age range of the Lomagundi-Jatulian event, which was responsible for the formation of carbonate sediments with high positive δ13C values.

Lithology and Mineral Resources, 2007

Fine-grained clay subfractions, SFs (particle size <0.1, 0.1-0.4, 0.4-0.6, and 0.6-2.0 µ m) separ... more Fine-grained clay subfractions, SFs (particle size <0.1, 0.1-0.4, 0.4-0.6, and 0.6-2.0 µ m) separated from a sample of the Lower Cambrian blue clay of the Lontova Formation were studied with XRD and Sm-Nd methods. The relatively coarse-grained SFs include illite with a small admixture of chlorite, while the finegrained SF (<0.1 µ m) consists of mixed-layer illite-smectite. The illite crystallinity index (I c) increases with decreasing particle size. The leaching of SFs with 1N HCl, analysis of Sm-Nd systematics of leachates and residues, as well as XRD data and results of chemical analysis show that the studied rocks contain at least two generations of minerals. The first (detrital) generation is related to the transformation of provenance material, whereas the second (authigenic) generation was formed at the postsedimentary stage of the evolution of the Lontova sediments. The Sm-Nd date of the first generation (790 ± 90 Ma) is considered a minimal age of rocks in the northeastern and southwestern regions of the East European Platform that served as a source of sedimentary material of the Lower Cambrian blue clay. The date of the second generation of minerals reflects the timing of authigenic mineral formation in the course of burial and diagenetic and catagenetic reworking of clay sediments.

Journal of Asian Earth Sciences, 2013

Depositional ages for the middle part of the Neoproterozoic sedimentary succession exposed along ... more Depositional ages for the middle part of the Neoproterozoic sedimentary succession exposed along Lake Baikal in the southern margin of the Siberian Platform have been constrained by the application of Sr isotope chemostratigraphy and Pb-Pb dating of carbonate rocks. The thick siliciclastic-carbonate Baikal Group is the type section of the regional Siberian succession. Although many of the Baikal Group carbonate rocks have undergone pervasive dolomitization by mixed meteoric-marine fluids, oolitic limestones in the 800-m-thick carbonate member of the Uluntui Formation preserve primary textures and high Sr contents (up to 3200 ppm), suggesting that aragonite might have been the original precursor. The 87 Sr/ 86 Sr in the least altered high-Sr limestones (Mg/Ca < 0.08 and d 18 O > À8‰ PDB) range within 0.70842-0.70876, values we interpret as representing the best proxy for coeval seawater. Stepwise dissolution using 0.5 N HBr was applied to enrich samples in primary carbonate from the Uluntui Formation and nine data points from the five least altered, high-Sr limestones yielded a Pb-Pb isochron age of 560 ± 30 Ma with MSWD = 0.6 and probability = 0.8. This age matches that inferred from Sr isotope stratigraphy (580-550 Ma) and further supports the inference that the Baikal paleobasin was connected to the contemporary oceans. Our Sr chemostratigraphy and Pb-Pb isochron age of the Uluntui limestones cast doubt on previously proposed correlations of the Baikal Group as late Mesoproterozoic to Cryogenian. Instead, the data indicate that the entire siliciclastic-carbonate succession of the Baikal Group was most likely deposited during the Ediacaran (or Vendian, of the Russian Stratigraphic Scale).

Geological Magazine, 2002

Carbon and strontium isotope stratigraphy has been applied to constrain the depositional ages of ... more Carbon and strontium isotope stratigraphy has been applied to constrain the depositional ages of high-grade marble sequences in the Ofoten district of the North-Central Norwegian Caledonides. Two marble formations hosted by diverse schists from the Bogen Group, all previously correlated over long distances with a Late Ordovician–Early Silurian, low-grade, fossiliferous succession, have been studied for carbon, oxygen and strontium isotopes. The least altered 87Sr/86Sr ratios ranging between 0.7062 and 0.7068, and the best preserved δ13C values falling between +5.0 and +6.5‰ obtained from two marble formations, are consistent with a seawater composition in the time interval 700–600 Ma. The results obtained do not support the previously proposed correlation of the Bogen Group with an Ordovician–Silurian lithostratigraphic succession further north. The apparent depositional ages suggest that the tectonostratigraphic succession studied is inverted and that the tectonostratigraphy of the...

Doklady Earth Sciences, 2014

341 The Onot greenstone belt (OGB) forms a trough (paleorift) structure within the Prisayan margi... more 341 The Onot greenstone belt (OGB) forms a trough (paleorift) structure within the Prisayan marginal block of the Siberian Platform basement (Fig. 1). The OGP occurs on ancient tonalite–trondhjemite–gran odiorite gneiss containing zircons with a U–Pb age of ≈3.34 Ga [1] and is intruded by Early Proterozoic granite of the Shumikhin Complex (the U–Pb age of zircons is 1.87 Ga [2]). The belt is composed of metavolcanic rocks of the tholeiite–rhyolite series, metaterrigenous and biotite–garnet gneisses, amphib olite, quartzite, and magnesite and dolomite marbles [3, 4]. Rocks are metamorphosed under the condi tions of the amphibolite and epidote–amphibolite facies and have undergone intense ultrametamorphic and metasomatic transformations. The age of the pro tolith of metarhyolite from the OGB is 2.89 Ga (U– Pb age by zircon) [5]. The Rb–Sr age of ultrameta morphic granitoids from the Onot Complex is 2.64 Ga [6]. Thus, the supracrustal complex of the OGB is one of the oldest in Asia, and its age is consistent with that of typical Archean greenstone belts of the Earth.

Doklady Earth Sciences, 2007

Doklady Earth Sciences, 2011

ABSTRACT The U-Pb(Pb-Pb) age was determined for limestone from member III of the Sorna Formation ... more ABSTRACT The U-Pb(Pb-Pb) age was determined for limestone from member III of the Sorna Formation out- cropped on the Azyr-Tal Range, Kuznetsk Alatau. The weighted average value from three calculated values is 523 ± 5 Ma (MSWD = 0.02, 2sigma). This age coincides with the Early Cambrian age interval determined with Sr isotopic chemostratigraphy (87Sr/86Sr ratio is 0.70850-0.70852). The low value of mu2 = 238U/204Pb for limestone is due to the evolution of diagenetic fluid containing lead from the mantle reservoir into the Sorna sediments.

Doklady Earth Sciences, 2013

ABSTRACT The Pb-Pb age of phosphorite concretions of the Zigaza-Komarovo Formation, which compose... more ABSTRACT The Pb-Pb age of phosphorite concretions of the Zigaza-Komarovo Formation, which composes the intermediate horizons of the Riphean stratotype of the South Urals, was determined in fractions resulting from the stepwise dissolution of concretions in 0.1 N, 0.5 N, and 1 N HCl. The determination of the Sr isotopic composition in phosphate fractions was favorable for rejection the fractions polluted with extraneous material. On the 207Pb/204Pb-206Pb/204Pb diagram, the isochron based on 31 points corresponds to 1330 ± 20 Ma (MSWD = 1.12), which is in agreement with the stratigraphic position of the Zigaza-Komarovo Formation. The decreased μ2 value of 9.57 for the phosphorite concretions relative to that of the average earthly lead based on the Stacey-Kramers model (9.74) is related to the rocks with an admixture of mantle lead, which occur in the run-off area of the Zigaza-Komarovo sediments.