INTRODUCTION

Currently, in the global practice, there are no isotope data for early Neoproterozoic carbonate rocks with the geochronological ages of these deposits determined [1]. Data about the Sr- and C-isotopic composition of carbonate rocks have been obtained for the time interval of 920–980 Ma from a single section, where the deposition time limitation is based on the detrital zircon age data, which is the carbonate sequence of the Huainan Formation in the North China Platform [2, 3]. For Northern Eurasia, one of the typical Late Mesoproterozoic and Early Neoproterozoic sections is the terrigenous–carbonate sequences of the Turukhansk uplift in the northwestern part of the Siberian Platform (Fig. 1) [4]. They were accumulated in the shallow-water settings of the carbonate platform in the tidal zone [5]. The section is dominated by carbonate deposits, with terrigenous rocks including quartz sandstones, quartz–feldspar sandstones, polymictic sandstones, and argillites. The sources of detrital material for the terrigenous rocks were Archean and Paleoproterozoic rocks in the basement of the Siberian Platform [6]. The age of the Turukhansk uplift deposits was determined based on Pb–Pb dating of carbonate rocks from the Sukhaya Tunguska Formation, which is 1035 ± 60 Ma [7]. Manifestations of tectono-magmatic activity had not been detected before in this part of the platform at the Meso- and Neoproterozoic boundary.

Fig. 1.
figure 1

Geological scheme of the Late Precambrian Turukhansk uplift and a lithological column fragment of the upper part of the Nizhny Tunguska Formation and the lower part of the Shorikha Formation. (1) Limestone, (2) clayey limestone, (3) trachyte, (4) poor bauxite ores, (5) chamosites, (6) stromatolitic limestones, (7) interruption in observations, (8) sampling site for U–Pb dating of zircon. Formations of the Turukhansk uplift: Pr3ln, Linok; Pr3sh, Sukhaya Tunguska; Pr3dr, Derevninskaya; Pr3nt, Nizhny Tunguska; Pr3ṧr, Shorikha; Pr3mr, Miroedikha; Pr3tr, Turukhansk; Pr3dm, Durnomys; and QIIIsr, Quaternary deposits.

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During the field studies, we recorded the presence of volcanic rocks, poor bauxite, and chamosite ores in the upper part of the carbonate rock section in the Nizhny Tunguska (Burovaya) Formation of the Turukhansk uplift (Fig. 1). Previously, bauxites were recognized as argillites, and chamosites, as glauconitic sandstones [6]. Volcanic rocks were identified within the formation for the first time.

The aim of our study was to solve the inverse problem of Sr-chemostratigraphy and to obtain data on the Sr-isotopic composition of paleoocean water based on study of carbonate rocks with an age substantiated geochronologically by U–Pb data of zircon from synchronous volcanic rocks with carbonates. This allowed us for the first time in the global practice to obtain the Sr-isotope data for carbonate rocks, the age of which is confirmed by the geochronological studies of synchronous volcanic rocks at the boundary of 960 Ma.

STRUCTURE OF THE UPPER PART OF THE NIZHNY TUNGUSKA FORMATION SECTION

In the coastal outcrop of the right bank of the Nizhnyaya Tunguska River, near Voronikhin Cape, we studied the upper 200-m sequence of the Nizhny Tunguska Formation and the basal 10-m portion of the overlying Shorikha Formation in a single section.

In the upper part of the Nizhny Tunguska Formation and the basal part of the Shorikha Formation, the carbonate rocks are represented by limestones (Mg/Ca is from 0.003 to 0.039) with the proportion of alumosiliciclastic rocks ranging from 1 to 30%. The carbonate rocks in the upper part of the formation, adjacent to the bauxites, are most enriched in alumosilisiclastic rocks. The limestone layers are thinly bedded and thinly laminated and are less frequently massive bituminous. Closer to the surface, thin layers of grayish green clayey limestone appear. A lens of dark-gray coarse-grained rocks (10-m long and 0.2–0.7 m wide) was discovered among the limestones approximately 60 m from the surface, within the Nizhnyaya Tunguska River incision (65°50′08.3″ N, 88°12′30″ E). The petrographic studies, including with the use of a MIRA3 TESCAN scanning electron microscope (SEM) conducted at the Center for Collective Use of Scientific Equipment of Multi-Element and Isotopic Research, Siberian Branch, Russian Academy of Sciences, revealed that these rocks are altered volcanic rocks. The impregnation consists of plagioclase, potassium feldspar, and quartz. The matrix exhibits a fluidal texture and is replaced by iron-rich chlorite. Accessory minerals are dominated by zircon, rutile with high V concentrations, and monazite, florencite, and fluorapatite. This rock is characterized by increased contents of alkalis, MgO, TiO2, Zr, Hf, Th, and rare earth elements (REE) (Table 1). Despite the low contents of silica, the composition of impregnations and fluidal texture of the matrix, as well as the geochemical characteristics, suggest that these rocks are volcanic, that is, quartz trachytes.

Table 1. Chemical composition of volcanic rocks, bauxites, and chamosite sandstones from the Nizhny Tunguska Formation and sandstones from the Derevninskaya Formation
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Oval-shaped brown-yellow segregations up to 30 cm along the axis and up to 15 cm in cross section are distributed upward through the section throughout brown argillites. They look like concretions, but both the segregations and the host argillites have identical composition and together represent poor low-grade bauxites (Al2O3 > 28%) with high concentrations of TiO2, Cr, Zn, Zr, Hf, W, Th, and REE (Table 1).

At the top of the section of the Nizhny Tunguska Formation is a horizon of black fine-grained сhamosite sandstones up to 15-m thick. These rocks are characterized by a high iron content (Table 1). The petrographic studies, including the SEM analysis, established that the matrix is represented by green round concretions of chamosite, siderite, and sharply angular unrounded quartz grains; iron–chlorite ore is encountered less frequently. Abundant accessory minerals include zircon represented by unrounded fine-to-coarse grains, chromospinelides with a well-preserved crystallographic habitus, rutile with high V concentrations, and florencite. Glauconite is absent in the terrigenous rocks of the upper part of the Nizhny Tunguska Formation.

The geochemical characteristics of trachytes, chamosites, and bauxites in the Nizhny Tunguska Formation display a similar pattern (Fig. 2). This suggests that these sedimentary rocks represent redeposited products of a widespread weathering crust, including the quartz trachytes studied. The presence of bauxites and chamosite deposits in the section implies a wide manifestation of trachytic volcanism within the paleo-depositional area of the sedimentary basin, where the deposits of the Nizhny Tunguska Formation were accumulating.

Fig. 2.
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Distribution of rare and rare-earth element contents in (1) trachytes, (2) bauxites, and (3) chamosites of the Nizhny Tunguska Formation. Normalization of elements relative to the primitive mantle and chondrite according to [9].

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Thus, the study of silicate and alumosilicate rocks from the upper part of the carbonate sequence of the Nizhny Tunguska Formation revealed an episode of volcanism synchronous with sedimentation and widespread chemical weathering processes near the landmass until the formation of bauxites and chamosite deposits.

RESULTS OF U–Pb ZIRCON DATING

To determine the time when these events occurred, U–Pb dating of zircon from trachytes was conducted using the LA-ICP-MS method at the Institute of Geology, Siberian Branch, Russian Academy of Sciences (Ulan-Ude). The results show that the volcanic rocks of the Nizhny Tunguska Formation contain only zircon grains of the same time interval, with concordant values ranging from 1.05 to 0.96 Ga. The concordant age of the youngest zircon population is 963 ± 6 Ma, which we consider as the age of the volcanic event (Fig. 3).

Fig. 3.
figure 3

Concordia diagram for the youngest zircon population from trachyte of the Nizhny Tunguska Formation.

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In this respect, U–Pb dating of zircon grains using the ICP-MS method at the Center for Collective Use of Scientific Equipment of Multi-Element and Isotopic Research, Siberian Branch, Russian Academy of Sciences, for the underlying quartz sandstones of the Derevninskaya Formation and the chamosite sandstones at the top of the Nizhny Tunguska Formation revealed Paleoproterozoic, Neo-, and Mesoarchean ages for the rocks of the provenance area (Fig. 4). We note that the main provenance area in the sandstones of the Derevninskaya Formation was Neoarchean rocks (2.5 Ga) without any contribution from the Paleoproterozoic sources of detrital material, although they are the dominant contributors of detrital material to the Precambrian sedimentary basins of the Siberian platform ([6] etc.). Thus, the provenance area for the Turukhansk sedimentary basin changed after the volcanic events 960 Ma ago, and Mesoarchean (2.87 Ga) and Paleoproterozoic (1.86 Ga) rocks became the dominant contributors. There are also isolated zircon grains about 1 Ga in age, which are likely remnants of the volcanic event we identified.

Fig. 4.
figure 4

Distribution of ages of zircon grains in histograms and relative probability curves for sandstones of the Derevninskaya Formation (sample K75/16), as well as trachyte (sample K73/16) and chamosite sandstones (samples K74/16 and K201/19) of the Nizhny Tunguska Formation.

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Based on the data presented, we infer that the carbonate deposits accumulated in the upper part of the Nizhny Tunguska Formation around 960 Ma ago. Therefore, the rocks studied in this formation are currently the only geochronologically substantiated early Neoproterozoic section in the world for which the age of the carbonate deposition has been determined. This gives us a chance to obtain reliable information about the Sr-isotopic composition of the paleoocean water in the early Neoproterozoic.

Sr-STRATIGRAPHY OF CARBONATE ROCKS

To obtain data on the Sr-isotopic composition in the paleoocean water around 970–960 Ma ago, we collected samples of carbonate rocks from the upper 200-m part of the Nizhny Tunguska Formation and the 10-m base of the Shorikha Formation along the Nizhny Tunguska River at 1 m. As a result of the petrographic and geochemical studies, we determined that the lower 60-m section of this carbonate sequence consists of dolomites, which are less preferable for the purposes of Sr-chemostratigraphy than the limestones composing the upper part of the section (Table 2). The topmost 40-m part of the section of the Nizhny Tunguska Formation located at the contact with bauxites consists of clayey limestones. The dolomites and clayey limestones were excluded from this study, and the Sr-isotopic composition data were obtained for a 100-m section of limestones, including the studied trachytes of the Nizhny Tunguska Formation (95 samples), and an 8-m section of the lower part of the Shorikha Formation (eight samples). Based on the petrographic and geochemical studies, we selected 13  samples of limestones among them, having a micritic texture, without secondary veins, nonferruginous, with minimal values of the reference element ratios, indicating the least post-sedimentary alterations in the studied rocks (Table 2).

Table 2. Chemical and isotopic composition of carbonate extracts from the studied carbonate rocks of the Nizhny Tunguska (sample K100/19–K187/19) and Shorikha (sample K202/19–K210/19) formations
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Chemical decomposition of the samples and determination of element contents in the carbonate extracts were carried out by the atomic absorption method at the Center for Collective Use of Scientific Equipment of Multi-Element and Isotopic Research, Siberian Branch, Russian Academy of Sciences. The isotopic ratio of 87Sr/86Sr was measured by a Triton Plus Thermo Fisher mass spectrometer (Yekaterinburg, Zavaritsky Institute of Geology and Geochemistry, Ural Branch, Russian Academy of Sciences) using the SRM987 isotopic standard. As a result, we found that the values of 87Sr/86Sr in the carbonate rocks of the upper part of the Nizhny Tunguska Formation vary within the range of 0.70532–0.70573 and correspond to the isotopic composition of the water in the paleoocean 970–960 Ma ago. Variations in this ratio were observed within the section of the Nizhny Tunguska Formation. The values of this ratio were recorded to increase from 0.70532 to 0.70573 upward through the section and to decrease to 0.70559 in the horizon at the contact with trachytes, and then again to rise to 0.70573 in the overlying limestones of this formation (Fig. 5). In the basal limestones of the Shorikha Formation, the 87Sr/86Sr ratio increases to 0.70581–0.70582 (Table 2, Fig. 5).

Fig. 5.
figure 5

Variations in the 87Sr/86Sr isotopic ratio in carbonate rocks of the upper part of the Nizhny Tunguska Formation. See legend in Fig. 1. Black dots on the right side of the lithological column denote the sampling sites for geochemical and isotopic studies of carbonate rocks.

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The obtained isotopic characteristics coincide with such of the carbonate rocks from the Huainan Formation of the North China Platform [3], the age of which was determined based on U–Pb data of detrital zircon grains from the underlying and overlying sandstones. Our research refines the data obtained previously by Chinese colleagues, showing an older shift by ten million years of the curve of Sr-isotopic composition variations. Thus, we have obtained comparable data on the isotopic ratio of 87Sr/86Sr in the paleoocean water at the very beginning of the Early Neoproterozoic at the present time for two distant sections in the world (Fig. 6).

Fig. 6.
figure 6

Comparison of Sr-chemostratigraphy data for early Neoproterozoic carbonate rocks. (1) Curve of Sr-isotopic composition variations in the water of the early Neoproterozoic ocean [2, 3], (2) fragment of the refined curve of Sr composition variations in the water of the early Neoproterozoic ocean based on the data obtained in this work, (3) 87Sr/86Sr values in carbonate rocks of the upper part of the Nizhny Tunguska Formation, (4) age of volcanic rocks synchronous with the accumulation of carbonate deposits in the Nizhny Tunguska Formation, and (5) 87Sr/86Sr values in carbonate deposits at the base of the Shorikha Formation.

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CONCLUSIONS

The study of the upper part of the Nizhny Tunguska Formation in the Turukhansk uplift of the northwestern Siberian Platform allowed us not only to reveal and date an episode of alkaline volcanism as synchronous with the accumulation of carbonate deposits in this formation but also to identify the widespread occurrence of chemical weathering processes (up to the formation of poor bauxite and chamosite ores) on the paleocatchment of this sedimentary basin around 960 Ma ago. Alterations in the composition of the provenance area were detected, Neoarchean rocks contributing to the underlying sandstones of the Derevninskaya Formation and Mesoarchean, and Paleoproterozoic rocks to the chamosite sandstones at the boundary with the overlying Shorikha Formation.

The geochronological evidence for the time of sedimentation of the upper part of the Nizhny Tunguska formation allowed us for the first time to determine the Sr-isotopic composition of the paleoocean water 960 Ma ago. For the least altered limestones in the upper part of the Nizhny Tunguska Formation, we obtained values of 87Sr/86Sr ranging from 0.70532 to 0.70573, which are close to those established in carbonate rocks of the Huainan Formation of the North China Platform, the time interval of formation of which is limited only by the U–Pb isotope age of detrital zircons. The studied section of the Nizhny Tunguska Formation compares favorably with the Huaian formation, which is the only geochronologically substantiated (by sub-synchronous volcanic rocks) carbonate sequence from the early Neoproterozoic in the world.