Geology and Mineralogical-Geochemical Characteristics of the Volcanic Rocks in the Tunguska Syneclise, Permo-Triassic Siberian Traps Province

Abstract

The Siberian Traps have been a reference object for geologists for many years in understanding the origin of Large Igneous Provinces on Earth. Data from high-magnesium rocks in its northern region have traditionally been used to create genetic models. However, the central and southern parts, consisting of the tholeiitic basaltic rocks (platform basalts, or typical traps), remain significantly unexplored, despite their important role in volcanic evolution within the province. This article presents new structural, mineralogical, and geochemical data from the tuff-lava sequences in the central regions of the Tunguska syneclise, i.e., along the Lower Tunguska River Valley and near Vanavara settlement in the Podkamennaya Tunguska Valley. The studied sections include pyroclastic rocks of the Korvunchansky Formation and overlaying basalts and tuffs of the Nidymsky and Kochechumsky Formations (Early Triassic, 251 Ma). Major and trace element analyses of 64 samples show that basalts from these formations are similar in composition, with MgO contents ranging from 4.2% to 7.8%. Trace element patterns exhibit negative Ta-Nb anomalies and positive Pb anomalies, which are typical of the Siberian platform basalts. Tuffs, on the other hand, differ from the basalts by having higher SiO₂ and K₂O contents and steeper rare earth element (REE) patterns. Isotopic analysis (Sr-Nd) confirms the difference between basalts and tuffs and suggests their origin from two distinct magma sources. The Nidymsky basalts have positive Ɛ_Nd values (+2.21 to +3.10), similar to those of the Morongovsky and Mokulaevsky basalts in the Norilsk area. Tuffs, on the other hand, exhibit negative Ɛ_Nd (−1.33 to −4.5) and elevated ⁸⁷Sr/⁸⁶Sr (>0.706), in comparison with the basalts (0.704–0.706). Geological observations reveal that pyroclastic rocks play a significant role at the initial stage of volcanic activity. The main result of this study is that tuffs from the Korvunchansky and Khakanchansky Formations (in the south and north of the province, respectively) are identical in their geochemical characteristics. This similarity suggests the existence of a single pyroclastic cover, closed in composition to Lower crust, over the Siberian platform. These pyroclastic rocks mark the beginning of trap magmatism in the Siberian province, rather than picrites, as suggested by the plume model.

1. Introduction

The Siberian Large Igneous Province [1,2,3,4], which includes the rocks from Western and Eastern Siberia, is the largest continental province on Earth (251 Ma; [5]). It attracts the attention of many researchers as an etalon area for LIP modeling. Two types of basalts are distinguished in this province, i.e., rift-related and platform-related [6,7,8] (Figure 1a). Rift-related rocks are found in Western Siberia and northern Eastern Siberia as narrow zones along deep faults. Their composition varies widely from picrite to dacite. Platform-related rocks, also known as typical traps or flood basalts, cover vast areas of Eastern Siberia and have a very constant tholeiitic composition. A combination of these rock types forms thick volcanic sequences in the Maymecha–Kotuy and Norilsk regions, reaching up to 3.5 km (Figure 1b).

The stratigraphy of tuffs and lavas in Eastern Siberia was established during the geological mapping in the 1960s and has been used in subsequent publications [2,9,10,11,12,13,14,15,16,17]. Rift basalts consist of subalkaline rocks, such as trachybasalts of the Ivakinsky and Syverminsky Formations, picritic and tholeiitic basalts of the Gudchikhinsky and Nadezhdinsky Formations in the Norilsk region [18,19]; and Arydzhangsky and Delkansky alkaline Formations, Maymechinsky Formation in the Maymecha–Kotuy region, as well as Ayansky picrites and Ankaramites in Putorana [20,21]. Platform rocks (typical traps) include tholeiitic basalts from the Tuklonsky, Morongovsky, Mokulaevsky, Kharaelahsky, Kumginsky, and Samoedsky Formations in the Norilsk region; Aynasky, Honnamkitsky in the Putorana plateau and Korvunchansky, Nidymsky, Kochechumsky and Yambukansky in the Tunguska syneclise [22,23]. Because it is difficult to show the area of each Formation on the map in Figure 1a, we give the contour of their distribution. The locations of separate Formations were shown in the articles [24,25,26,27] and in the book ([15], p. 93).

The geochemistry of basalts, including their isotopic characteristics, in the Norilsk area has been studied in detail by many researchers [28,29,30]. This interest is largely due to the occurrence of unique PGE-Cu-Ni deposits here [31,32,33,34,35]. Furthermore, high-magnesium rift rocks have been studied in detail in the Norilsk and Maymecha–Kotuy areas [26,27,36,37,38,39,40,41,42]. A.V.Sobolev et co-authors [38] estimated the depth of substrate melting for the Gudchikhinsky magma as 130–180 km (4.5 to 6 GPa) with a mantle temperature of 1500–1580 °C. According to I.D.Ryabchikov et co-authors [40], the temperature of the primary magma for the Ayansky picrites was 1650 °C, and P varied from 6 to 7 GPa. These rocks are considered to be primary derivatives for traps that are regarded as contaminants of the lithospheric mantle by crustal material (25 to 30 vol.%) [11,28]. However, typical traps do not have any mantle characteristics, so their relationship with rift rocks is uncertain.

The features mentioned above, i.e., the high thickness of volcanic rocks and the occurrence of high-magnesium rocks and PGE-Cu-Ni deposits, led to a better study of the northern part of the province in comparison with its other areas [13,23,25,28,43,44,45]. All existing models for the Siberian province are based on these data. Different hypotheses have been proposed to explain the origin of large volumes of magma within a supposed relatively short time period (less than 1 million years [4,46]) and the genesis of ore deposits [16,17,32,47] as well as mass extinctions at the Permian–Triassic boundary [48,49,50,51].

The most widely accepted theory suggests that all Permian–Triassic igneous rocks in Siberia and the related deposits are the result of a plume, or superplume activity [52,53,54]. It is proposed that the Siberian Traps represent the head of this plume, which contains high-magnesium rocks formed at a significant depth (up to 120–150 km) and at a high temperature. According to this classic model [55], the initial platform magmatism should have been replaced by rifting, leading to the splitting of the continent. However, this hypothesis has several significant drawbacks. Many researchers have pointed out the lack of evidence for the uplift of the region [54,55], which is necessary for deep matter to reach the surface. Furthermore, Siberian rift zones appeared at the start of rock formation [6], and the rift magmatism existed simultaneously with the platform one [8,15,56]. High-Mg rocks were found only in rift zones, such as the Yenisei–Khatanga trough and its satellites, and not among platform basalts. While there are other hypotheses on trap formation [57,58,59,60], the plume theory remains the most widely accepted.

Figure 1. Distribution of rift and platform basalts (a), cross-section A-B (b), and stratigraphic column (c) in Eastern Siberia, compiled by the authors of [18,19]. Volcanic formations are named in the column as follows: P₃iv—Ivakinsky, T₁sv—Syverminsky, T₁gd—Gudchikhinsky, T₁nd—Nadezhdinsky, T₁hk—Khakanchansky, T₁tk—Tuklonsky, T₁mr—Morongovsky, T₁mk—Mokulaevsky, T₁hr—Kharaelahsky, T₁km—Kumginsky, T₁sm—Samoedsky, T₁ar—Arydzhangsky, T₁del—Delkansky, T₁on—Onkuchaksly, T₁m—Maymechinsky, T₁ayn—Aynasky picrites, T₁ank—Ankaramites, T₁nid—Nidymsky, T₁kč—Kochechumsky, T₁ym—Yambukansky. T₁tt+kv—Tutonchansky + Korvunchansky. Sedimentary rocks: S—Silurian, D—Devonian, C₂-P₂—Tunguska series. Rectangles, areas: 1—Norilsk, 2—Maymecha–Kotuy.

Figure 1. Distribution of rift and platform basalts (a), cross-section A-B (b), and stratigraphic column (c) in Eastern Siberia, compiled by the authors of [18,19]. Volcanic formations are named in the column as follows: P₃iv—Ivakinsky, T₁sv—Syverminsky, T₁gd—Gudchikhinsky, T₁nd—Nadezhdinsky, T₁hk—Khakanchansky, T₁tk—Tuklonsky, T₁mr—Morongovsky, T₁mk—Mokulaevsky, T₁hr—Kharaelahsky, T₁km—Kumginsky, T₁sm—Samoedsky, T₁ar—Arydzhangsky, T₁del—Delkansky, T₁on—Onkuchaksly, T₁m—Maymechinsky, T₁ayn—Aynasky picrites, T₁ank—Ankaramites, T₁nid—Nidymsky, T₁kč—Kochechumsky, T₁ym—Yambukansky. T₁tt+kv—Tutonchansky + Korvunchansky. Sedimentary rocks: S—Silurian, D—Devonian, C₂-P₂—Tunguska series. Rectangles, areas: 1—Norilsk, 2—Maymecha–Kotuy.

In order to understand the history of magmatic evolution and create a comprehensive theory of the Siberian Province origin, it is essential to have information about its overall structure, especially about typical traps, which compose the central and southern regions of the province and which have received less attention than rift basalts despite their vast exposure and large volume (they make up 90% of the province territory). The widespread models of the Siberian Traps are based on data on rift rocks (Gudchikhinaky, Maymechinsky Formations, Ayansky picrites [25,39,40]). Thus, it turns out that the origin of traps is considered without taking into account their structure and composition. The gap in data on the Tunguska syncline volcanic rocks is detrimental to our understanding of the province origin. This article is dedicated to the structure and geochemical characteristics of the tuff-lava sequence in the central part of the Tunguska syneclise (Figure 1). The results allow us to compare these rocks with those from the northern part of Eastern Siberia and, based on this, draw conclusions about their formation.

2. Geological Background

The Siberian platform, the largest part of the Siberian Large Igneous Province, is composed of a crystalline basement (AR-PR) and an overlying sedimentary cover (R-J) [61,62,63]. Ancient rocks are exposed on the Anabar and Aldan Shields (Figure 1, Figure 2 and Figure 3). Cambrian–Permian sedimentary deposits of the cover are exposed along the edges of the Tunguska syneclise, while in the central part, they are overlain by volcanic rocks. We use the tuff-lava term for a sequence of interlayers of tuff and lavas, while tuffolava means a rock of intermediate composition.

Geological and geophysical data reveal the complex structure of both the basement (Figure 2) and the sedimentary deposits (Figure 3). The crystalline basement, according to the geophysical data [64], is composed of Archean and Proterozoic gneisses and granulites. It comprises several large depressions—Kharaelakhsky, Khantaisky, Tunguska and Taseevsky (Figure 3), as well as several smaller ones. Before the trap magmatism at the end of the Permian period, the depths of the Khantaisky and Tunguska depressions ranged from 100 to 400 m. Therefore, the thickness of volcanic, mainly pyroclastic, rocks varies significantly within the Tunguska syneclise [65]. The cover of the Siberian platform consists of the Cambrian–Devonian carbonate-terrigenous rocks and Middle Carboniferous–Middle Permian coal-bearing terrigenous deposits forming a thick section around 10,000 m [19].

The sedimentary deposits in the Tunguska River Valley were penetrated by numerous but shallow boreholes. Only one of them, located near the village of Babkino (600 m deep, b1 in Figure 2), revealed almost the entire Paleozoic rocks that underlie the volcanic rocks, but it did not reach the basement. This borehole also found numerous dolerite sills. The structure of the Late Permian–Early Triassic igneous rock in Eastern Siberia has been zoned, largely determined by the subsequent erosion of elevated areas of the Tunguska syneclise. This erosion led to the appearance of tuffs and intrusions on the periphery of the syneclise (Figure 1). The studied area, dominated by exposed basalts, is located in the center of the Tunguska syneclise [66]. According to V.S. Staroseltsev [62], this syneclise formed in the Early Carboniferous, and its outer boundaries are defined by the outer ranges of trap sills (Figure 1 and Figure 2). At the base of the syneclise, there are numerous shafts and depressions formed during the first half of the Paleozoic era (Figure 3).

The crystalline basement, according to the geophysical data [64], is composed of Archean and Proterozoic gneisses and granulites. It comprises several large depressions—Kharaelakhsky, Khantaisky, Tungusky and Taseevsky (Figure 3), as well as several smaller ones. The long and deep faults restrict these structures. As a rule, in the north of the syneclise they represent a continuation of the borders of the Yenisey–Khatanga trough and its branches (paleorift zones). The southern part of one of them, the Norilsk–Igarka structure, occurs in the west–northern part of the Tunguska syneclise, to the east of the Norilsk–Turukhan ridge (IV in Figure 3). Their faults control the mantle magma intrusion in this region. In the central part of the Tunguska syneclise, an extensive Putoransky depression was formed (III in Figure 3). Its axial zone has a north–westerly direction. Apparently, this trough is a paleorift having an end joint with the Yenisei–Khatanga one.

Figure 2. The scheme of distribution of trap facies and the basement relief of the Siberian platform. The position of the scheme is shown in the inset here and in Figure 3, compiled by the author on the basis of [63,67]. Depressions: I—Kharaelakhsky, II—Khantaysky, III—Tungusky, IV—Taseevsky.

Figure 2. The scheme of distribution of trap facies and the basement relief of the Siberian platform. The position of the scheme is shown in the inset here and in Figure 3, compiled by the author on the basis of [63,67]. Depressions: I—Kharaelakhsky, II—Khantaysky, III—Tungusky, IV—Taseevsky.

The southern part of the Tunguska syneclise is isolated into an independent Vanavarsky trough, separated from the Putoransky by the Katanga Saddle (KS in Figure 3) formed at the beginning of the Mesozoic period. P.E. Offman [68] concluded that the superimposed syncline junctions with residual anticlines were the places of the most intense volcanism. But their bounding faults were not as deep as those which restricted paleorifts. They were channels only for crustal magmas.

Figure 3. Tectonic map of the sedimentary cover in the western part of the Siberian platform at the end of the Carboniferous period. Structures: IV—Turukhan–Norilsk ridge; V—Yenisei anticline (eastern slope). Synclines: III—Putoransky; VI—Angarsk; VII—Tunguska syneclise. Superimposed depressions: A—Aganyl depression, Mk—Muruktin depression; AV—Angaro–Vilyui depression; M—Murskaya depression; KS—Katanga Saddle. From [65], with the author’s corrections.

Figure 3. Tectonic map of the sedimentary cover in the western part of the Siberian platform at the end of the Carboniferous period. Structures: IV—Turukhan–Norilsk ridge; V—Yenisei anticline (eastern slope). Synclines: III—Putoransky; VI—Angarsk; VII—Tunguska syneclise. Superimposed depressions: A—Aganyl depression, Mk—Muruktin depression; AV—Angaro–Vilyui depression; M—Murskaya depression; KS—Katanga Saddle. From [65], with the author’s corrections.

At the end of the Paleozoic era, the Siberian platform, almost entirely, together with the Southern Taimyr, represented an extensive area of accumulation of terrigenous coal-bearing sediments formed under continental conditions. The world’s largest coal basin, the Tunguska coal basin, was formed here [69]. Already at the end of the Permian period, during the existence of this huge basin, weak volcanic activity began to occur in a number of areas.

The rocks containing volcanic material were combined [9] into the following Formations (the summary section is shown in Figure 4), i.e., the Pelyatkinsky and Degalinsky Formation (P₃), with a whole thickness of 0–240 m; the overlapping Tutonchansky Formation (T₁), ranging from 0 to 100 m; and the Korvunchansky Formation with thicknesses from 200 up to 900 m [19,70]. The Degalinsky Formation is characterized by the abundance of Cordaites, which indicate its Late Paleozoic (Permian) age [71]. The first two Formations consist of sandstones with a few admixtures of ash, while the third and fourth ones comprise only pyroclastic rocks. These Formations overlap sequentially with basalts and tuffs of the Nidymsky, Kochechumsky, and Yambukansky Formations, described below. The most complete sections, up to 600 m in length, were studied from boreholes drilled in the Lower Tunguska Valley (Figure 5). They have penetrated underlying sedimentary rocks as well.

Figure 4. Stratigraphic column for the rocks in the Tungska Syneclise and symbols for the maps in Figure 5 (from [72], with the authors’ corrections).

3. Materials and Methods

The subject of this study includes volcanic rocks exposed at three locations: (a) near the Tura village on the right bank of the Lower Tunguska River (see Table S1 in ESM S1); (b) near the Babkino village located 70 km south of Tura on the left bank of the river, where a number of flows and tuff horizons of the Korvunchansky and Nidymsky Formations were exposed, and the Krutoe deposit of Icelandic spar is located; and (c) near the Vanavara settlement in the Podkamennaya Tunguska River Valley (Figure 3). Thin sections and materials for analyses were prepared from 64 samples taken from the less altered rocks.

To study the textural characteristics and chemical composition of minerals, standard methods of optical and scanning electron microscopy were used. An Olympus microscope and a Field Emission Gun (FEG) scanning electron microscope (SEM) Tescan Mira 3 FEG SEM were employed at the Vernadsky Institute of Geochemistry and Analytical Chemistry of the Russian Academy of Sciences in Moscow, Russia (GEOKHI RAS). This method is described in ESM S5.

Major elements in minerals were also determined in the polished thin sections using a Jeol JXA 8200 SuperProbe electron microprobe at the Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry (IGEM RAS, Moscow, Russia) with the assistance of analyst V.I. Taskaev. These data are in ESM S3 (55 analyses). Well-characterized homogeneous natural and synthetic mineral standards were used for calibration of element sensitivities, i.e., almandine, apatite, jadeite, celestite, olivine, diopside, augite, plagioclase, rutile, orthoclase. All analyses used 20 kV accelerating voltage, 20 nA focused beam, 20 s on-peak counting time and 10 s for each background. For matrix correction, the PAP method was applied. Detection limits for silicate minerals were 0.01–0.02 (wt.%) for all oxides.

The contents of the major and trace elements in the rocks were determined using X-ray fluorescence analysis (XRF) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) (ESM S4). XRF analyses were performed at the IGEM RAS by A.I. Yakushev using an AXIOS mAX vacuum spectrometer from PANalytical (Almelo, The Netherlands), equipped with a 4 kW X-ray tube with a Rh anode. The maximum voltage on the tube was 60 kV, and the maximum current on the anode was 160 mA. For calibration of the device, standard industry samples of the chemical composition of rocks were used. The quality control of the results was carried out through the analysis of USGS rock standards. The relative standard deviations (RSDs) for the main oxides (wt.%) are as follows: SiO₂: 0.12, TiO₂: 0.05, Al₂O₃: 0.12, Fe₂O₃ total: 0.13, MnO: 0.005, MgO: 0.07, CaO: 0.08, Na₂O: 0.08, K₂O: 0.03, P₂O₅: 0.01.

Trace element analyses in the rocks were carried out using ICP-MS at the Institute of Microelectronics and High Purity Materials, Russian Academy of Sciences, Chernogolovka, by V. K. Karandashev, according to the methodology [73]. The detection limits for isotopes can be found in the ESM S4 document. Andesite (AGV-2) and basalt (BHVO-2) were used as standards (ESM S4). A detailed description of the method is given in ESM S5.

The isotope composition of the samples was analyzed using a Triton solid-phase multi-collector mass spectrometer (Thermo, Waltham, MA, USA) at the Karpinsky Russian Geological Institute in St. Petersburg (analyst B.V. Belyatsky), it is characterized in ESM S5. The values of ⁸⁸Sr/⁸⁶Sr = 8.375209 and ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219 were used for normalization. To recalculate the isotopic composition of lead at the time of its formation 251 million years ago, we used the contents of lead, thorium, and uranium, which are given in Table 1.

4. Results

Only volcanic and pyroclastic rocks of the Upper Permian to Lower Triassic period are exposed in the studied areas (Figure 5; ESM S6, Figures S1 and S2) in the center of the Siberian Trap province. Boreholes have penetrated underlying sedimentary deposits (Figure 5e). We have studied three Formations from the six shown in Figure 4, i.e., the Korvunchansky, Nidymsky, and Kochechumsky Formations. Below is a description of the rock Formations that we studied in the outcrops (ESM S6, Figure S1) according to their geological setting on the maps (lines I and II in Figure 5).

Figure 5. Geological maps (a,c) and cross-sections (b,d,e) for the Lower Tunguska River Valley from [73], with authors’ corrections. Blue lines in (a,c) (I and II) correspond to the sections in Figure 6. Blue triangles in (c) are boreholes Al13 and Al14.

4.1. Structure of the Tuff-Lava Sequence in the Center of the Tunguska Syneclise

We described in detail two sections (I and II in Figure 5a,c). We demonstrate field observations and the petrographic, mineralogical and geochemical characteristics of the rocks. These sections include the most exposed rocks in the stratigraphic sequence in this area.

The section of igneous rocks begins from pyroclastic varieties, subdivided into two Formations, i.e., the Tutonchansky and Korvunchansky ones (Figure 5e and Figure 6). Only a small portion of the total volume of tuffogenic strata is available for research, and it is primarily composed of products of explosive activity. Ashy pisolitic tuffs and tuffites dominate in the Tutonchansky Formation, while tuff sandstones and tufofaleurolites are less common. These rocks are characterized by a pronounced, thin layering, fine-grained structure, and good sorting. The Tutonchansky Formation has a very local distribution (its thickness changes from 0 to 200 m, Figure 6) and is not exposed on the surface in the studied area. Therefore, we do not include it in the article. We have studied the pyroclastic rocks of the Korvunchansky Formation and overlapping rocks of the Nidymsky and Kochechumsky Formations (Figure 4) (ESM S6, Figure S2). Below, we give a description of these rocks according to their geological setting (lines I and II in Figure 5). Despite the smaller total thickness of section II (325 m in comparison with 550 m), it comprises more basaltic flows and tuff horizons than section I because the thickness of its individual units is less (Figure 6).

Figure 6. Sections I and II of the tuff-lava sequences in the Lower Tunguska River Valley. Positions of these sections are indicated in Figure 5a,c. 1—tuff, 2—tuff breccia, 3—pillow lava, 4–6—basalts: 4—aphyric, 5—poikilophitic, 6—amygdale; 7—not exposed; 8—position and number of sample. Formations: T₁kv₂—Upper part of Korvunchansky, T₁nid₁—Lower part of Nidymsky, T₁nid₂—Upper part of Nidymsky, T₁kč—Kochechumsky.

4.1.1. The Korvunchansky Formation

The lowest pyroclastic Formation, Tutonchansky (Figure 5e), has a very local distribution and is not exposed on the surface in the studied areas. Although the overlapping Korvunchansky Formation also consists of pyroclastic rocks, it covers almost the entire Tunguska syneclise. The thicknesses of this Formation vary between 100 and 900 m. Rare interbeds of volcanogenic-sedimentary rocks within these formations only contain lake fauna and a continental spore–pollen complex of Early Triassic age [19,74].

Currently, the Korvunchansky Formation is divided into two parts. The lower one belongs to the Ucham Formation, while the upper one belongs to either the Bugariktinsky or Gagaryeostrovsky Formation, depending on the location in the area [73]. We have retained the original name of the Korvunchansky Formation with its subdivision into the Lower and Upper parts because it is accurately described in many publications [75,76].

Volcanoclastic and pyroclastic-sedimentary rocks of the Lower part are a clear example of ancient volcanic stratigraphy [70]. Within their distribution, two structural and facies zones can be identified: (1) a zone with intense explosive activity, the main source of pyroclastic and erupted material (vent and near-vent facies); and (2) a zone dominated by accumulative processes in low paleorelief places (facies of covered sedimentary pyroclastic rocks). The thicknesses of the Korvunchansky Formation change between 100 and 900 m, mostly due to variations in its Lower part. This is evident in the boreholes drilled along the Lower Tunguska River Valley. The boreholes 6-P and 3-P, for instance, show a significant increase in tuff thicknesses in this cross-section, reaching 500 and 400 m, respectively (Figure 7). This is due to the rocks’ origin in rough terrain conditions, which is a result of post-Tutonchansky erosion. The same powerful transformations of tuff thickness are evident on a regional level within the Tunguska syneclise, extending from the study areas in the Lower Tunguska Valley to the Vanavara settlement and then to its southern border (Figure 8).

Figure 7. Cross-section along the Lower Tunguska River to the west of the Chunchan River (borehole 3-P is located 10 km to the west of the Chunchan delta). hβT₂?—Gabbro-dolerites, supposed Middle Triassic age.

The predominant lithological types of rocks in the near-vents areas are agglomerate tuff-breccia and lapilli tuffs (Figure 8). These rocks are found in distinct areas throughout the syneclise and occur as tubular structures. These rocks form outcrops near ancient craters with tower-shaped weathering patterns (Figure 9a) along the rivers. They are distinguished by a rough and often uneven texture. The lapilli tuffs and breccias can be found up to 200 m from the vent. The agglomerate tuffs in the northwestern part of the Tunguska show no signs of transporting the blocky material after the initial deposition, indicating that they were formed by explosive volcanic activity. The depth of the volcanic structures in this part of the Tunguska syneclise did not reach the base of the Devonian formations, as there are no fragments of older rocks present in the vast deposits of tuff, according to the authors [19,77].

The volcanic rocks of the surface facies were created in areas that were far from syneclise the epicenters of volcanic activity. The presence of tuffites and pisolite tuffs, as well as local erosion, indicates that these areas were close to low-lying regions and that redeposition processes played a significant role in their formation.

Figure 8. Structure of pyroclastic rocks in the Tunguska syneclise. Sites, River Valleys (in Figure 2): Sections: 1—Lower Tunguska, 2—Podkamennaya Tunguska, 3—Tetere. Composed by the authors using their own data and [78].

Figure 8. Structure of pyroclastic rocks in the Tunguska syneclise. Sites, River Valleys (in Figure 2): Sections: 1—Lower Tunguska, 2—Podkamennaya Tunguska, 3—Tetere. Composed by the authors using their own data and [78].

The Upper part of the Korvunchansky formation consists of tuffs that can be related to the covered facies in terms of their volume and areas of development (ESM S6, Figure S2). It includes diverse, mostly small-block tuffs, such as psephitic and psammitic varieties. It has a more sustained thickness throughout the territory.

The period of its accumulation and the flora that developed during this time are referred to as the “Korvunchansky” or conifer-fern post-cordaitic [19,79]. As noted by Kutygin and colleagues [75], “the change of the cordaitic flora to the Korvunchansky flora is a significant event in the development of the Angara plant kingdom”. It is suggested that it was formed in the Triassic period [19,74].

The upper boundary of the Formation is drawn along the base of the basalts of the Nidymsky Formation (Figure 9b). The total thickness of its upper part ranges from 145 to 250 m. These rocks were redeposited and are characterized by distinct layering (2–5 cm) of gray and green silty lithocrystalline andesite–basalt tuffs, with relatively good sorting. Sometimes, there is a shell-like structure in the tuffs (ESM S6, Figure S2). Upon close examination, the rocks exhibit a yellowish-brown color (Figure 10). Debris, which accounts for 60%–80% of the volume, consists of angular-shaped or oval glass fragments of basalt or andesite–basalt composition, with an average size of 300 microns. Additionally, there are rare (5 vol. %) oval quartz–sericite fragments that are cemented together with the smallest glass particles and calcite, accounting for about 30% of the cement volume in some places.

Figure 9. Outcrops of the tuffs: (a)—outcrops of the agglomerate tuffs near Vanavara settlement; (b)—contact of the psammitic tuffs of the Korvunchansky Formation (T₁kv₂) and basalts of the Nidymsky Formations (T₁nid₁) (see geological settings in Figures 2 and 3). Here and in Figures 13 and 15, photos by Krivolutskaya.

Figure 9. Outcrops of the tuffs: (a)—outcrops of the agglomerate tuffs near Vanavara settlement; (b)—contact of the psammitic tuffs of the Korvunchansky Formation (T₁kv₂) and basalts of the Nidymsky Formations (T₁nid₁) (see geological settings in Figures 2 and 3). Here and in Figures 13 and 15, photos by Krivolutskaya.

Figure 10. Texture and structure of the lithoclastic tuffs of the Korvunchansky Formation. Samples: (a,b)—50; (c,d)—HT-60.

Figure 10. Texture and structure of the lithoclastic tuffs of the Korvunchansky Formation. Samples: (a,b)—50; (c,d)—HT-60.

The size of the crystal fragments is slightly larger than that of the glassy fragments, averaging 50 microns in diameter (Figure 11). They represent separate crystals (indicated by points in Figure 11—Ab(1), Ab(2), Ab(16), and Ab(17)—and substitute for plagioclase in the main basalt matrix. Most of the material consists of plagioclase, primarily Ab(10, 11), as well as larger crystals (Ab(33) and Ab(44)). Potassium feldspar is also present, such as Fsp(12) and Fsp(45), occasionally with a high concentration of barium (Ba 1.8 wt.%). Natrolite occurs in interstitial spaces, including Ntr(4), Ntr(15), Ntr(23), and Ntr(29). Iron hydroxide forms a thin rim around these crystals. Coarse quartz grains (>100 nm) are also present in the rocks, as is titanite (Ttn(14) in Figure 11c), along with Ttn(41) in Figure 11f,h. A particularly interesting finding is the presence of chromium spinel with an 8.1 wt.% Zn content. In Figure 11h, this spinel is located within a small, irregularly shaped fragment that is enclosed by a basalt matrix. This high-chromium spinel has not been found in any nearby rocks.

There are two types of glasses in the sample (e.g., sample 50). The first type has a tholeiitic composition with 6.6 wt.% FeO, and the second type is “ferruginous” basalt, with a high content of magnetite (FeO content is 13.7 wt.%). The cementing material also contains increased concentrations of ore minerals (Figure 11e), although not as much as in the “ferruginous” glass. The composition of the samples varies slightly (Figure 11a) due to variations in SiO₂ and FeO, and corresponds mostly to an andesite composition due to the quartz occurrence in the tuffs. However, it can partially reflect fractionation of primary magma because the MgO content changes as well.

Smectite group of minerals, especially ferrosaponite [Ca_0.3(Fe²⁺,Mg,Fe³⁺) (Si,Al)₄ O₁₀ (OH)₂·4H₂O] or ferrisaponite where Fe³⁺ dominates, often occur in the tuffs. These minerals have a constant admixture of sodium, at 4–5 wt.% Na₂O. This mineral has been described in basaltic lavas and tuffs of the Siberian Traps, on the left bank of the Lower Tunguska River (section C-D in Figure 5) [80]. It forms spherulites, veined secretions, and radiant structures, with the central parts being more magnesian and the margins being more ferruginous. Radially radiant aggregates of chlorite from the iron-magnesia group, such as chamosite (in Figure 11h), are also present in the rocks. These minerals indicate green-facies metamorphism.

The discovery of lithoclastic psammitic tuffs, consisting of fragments of pyroxenophyric and plagioclase–pyroxenophyric basalts (Figure 12) in the studied section, is unusual, as they are not found in the tuff-lava stratum of the central part of the Tunguska syneclise. These fragments may be remnants of deeper volcanic rocks from the vent facies of volcanoes formed during the Korvunchansky period. The pieces of basalt are rounded and altered along the edges, which can indicate that they were carried by water streams, in contrast to the other pyroclastic rocks of this area. The detailed pictures of clinopyroxene from these rocks are shown in ESM S7, Figure S3, where zoned crystal textures are demonstrated.

Figure 11. Backscattered electron images of the tuffs from the Korvunchansky Formation. (a–c,f)—sample 318; (d,e,g,h)—sample 50. Here and in Figures 11 and 13: Mx—matrix, Chr—chromite, Ab—albite, Chl—chlorite, Fsp—feldspar, Ttn—titanite, Ntr—natrolite, Sap—saponite.

Figure 11. Backscattered electron images of the tuffs from the Korvunchansky Formation. (a–c,f)—sample 318; (d,e,g,h)—sample 50. Here and in Figures 11 and 13: Mx—matrix, Chr—chromite, Ab—albite, Chl—chlorite, Fsp—feldspar, Ttn—titanite, Ntr—natrolite, Sap—saponite.

4.1.2. The Nidymsky and Kochechumsky Formations

The Nidymsky Formation (500 m thick) is divided into two parts, the Lower and Upper, which differ in the proportions of pyroclastic rocks they contain, i.e., tuffs and tuffites predominate in the Lower part and are rare in the Upper one. Traditionally, the lower boundary of the Nidymsky Formation is drawn along the top of volcanogenic clastic rocks of the Korvunchansky Formation [19]. Usually, boundaries between Formations are drawn along marking horizons. Although the literature data indicate that the Nadayansky (35–40 m), Delochinsky (25–30 m) and Yagtaliysky (25–60 m) marking covers composed of plagiophyre varieties of basalts take part in the structure of the Nidymsky Formation in the described area, they were not found in the bedrock outcrops. Maybe they are covered by scree or vegetation.

The Lower part of this Formation is characterized by a combination of dense, light gray tuff, whose thickness can reach up to 10 m. This tuff overlaps with pillow lava composed of basalt. The contact between these two rock types is very smooth and clear, as shown in Figure 13a. In contrast, the relationship between similar rocks in the Upper Nidymsky Formation is somewhat different. There is a very uneven contact surface between them (Figure 13b). This tuff has a mottled color that consists of a combination of different shades.

The tuffs of the Nidymsky Formation (Figure 13a) are characterized by good sorting and the presence of weakly crystallized glass (Figure 14). The fragments (0.5 mm on average) of this glass have undergone varying degrees of secondary alteration. There are two types of glass in the rocks: sideromelane and tachylyte. Sideromelane is a glass that forms when a weakly crystallized or non-crystallized basalt or andesite–basalt melt solidifies. In thin sections, it has a greenish or greenish-yellow color, and is sometimes almost colorless. Plagioclase inclusions are rare, and there may be pores in it. Iron Fe²⁺ dominates this glass (up to 10 wt.%), and it corresponds in composition to andesites (SiO₂ > 56 wt.%) or, less often, to basalts (for example, Mx = 62 in Figure 14d). The amygdales in glasses are filled with radially radiant aggregates of secondary minerals, chlorite, epidote (Mx 51, 55, 58 in Figure 14a–c).

Figure 12. Photomicrographs of the lithoclastic psammitic tuffs from the Korvumchansky Formation: (a)—oval fragments of the aphyric and rarely plagioclase porphyric basalts cemented by basaltic matrix; (b)—sharp-angled fragment of plagioclase–pyroxenophyric basalt with a distinct Fe-rich rim around the boundaries; (c)—altered sharp-angled fragment of plagioclase–pyroxenophyric basalt; (d)—pyroxene crystals in the interstitial space; (e)—large Px-Pl crystals in matrix; (f)—fragment of pyroxene-plagioclase basalt; (g)—fragment of plagioclase basalt with a distinct Fe-rich rim around the boundaries; (h)—oval fragments of aphyric and plagioclase porphyric basalts with Fe-rich rims cemented by crashed basalt. Scale is 50 µm.

Figure 12. Photomicrographs of the lithoclastic psammitic tuffs from the Korvumchansky Formation: (a)—oval fragments of the aphyric and rarely plagioclase porphyric basalts cemented by basaltic matrix; (b)—sharp-angled fragment of plagioclase–pyroxenophyric basalt with a distinct Fe-rich rim around the boundaries; (c)—altered sharp-angled fragment of plagioclase–pyroxenophyric basalt; (d)—pyroxene crystals in the interstitial space; (e)—large Px-Pl crystals in matrix; (f)—fragment of pyroxene-plagioclase basalt; (g)—fragment of plagioclase basalt with a distinct Fe-rich rim around the boundaries; (h)—oval fragments of aphyric and plagioclase porphyric basalts with Fe-rich rims cemented by crashed basalt. Scale is 50 µm.

Figure 13. Contacts of tuffs with basalts of the Nidymsky Formation, parts: (a)—Lower, (b)—Upper (circles show the location of samples HT-6.43, HT-6.44, and HT-7, demonstrated in Figures 5a and 6).

Figure 13. Contacts of tuffs with basalts of the Nidymsky Formation, parts: (a)—Lower, (b)—Upper (circles show the location of samples HT-6.43, HT-6.44, and HT-7, demonstrated in Figures 5a and 6).

The structure of sideromelane glasses ranges from hyaline, which is 95%–100% glass with rare dendritic crystals of magnetite and titanomagnetite, to hyalopilitic, which has 70%–95% glass and plagioclase and Fe-Ti oxides, and microlith, which has 40%–70% glass. The lowest percentage of glass is in the intersertal structure, where it makes up only 40% of the total composition. The tuffs also contain numerous irregularly shaped fragments of basalt, often with a high iron content, up to 19% in anhydrous terms (FeO). These fragments are called tachilites and they are dominated by Fe³⁺ iron in the form of magnetite crystals. Tachylites are black when viewed under a microscope, but they have a bright color in back-scattered electrons (BSE images, Figure 14). These rocks contain natrolite (Ntr 59 and 67, 73 in Figure 14c–e), feldspar Fsp(53), (5), (6) in Figure 14b, plagioclase Pl(70) in Figure 14e, and albite Ab(68), in Figure 14d. Quartz is also present. Clinopyroxene is rare, Cpx(71) in Figure 14e. Small fragments of coal sometimes occur in these rocks in the upper part of the Nidymsky Formation (C, Figure 14f).

The basalts of the Nidymsky and Kochechumsky Formations have similar structural and textural features. They are fine- to medium-crystalline aphyritic rocks with ophitic or poikilophitic structures (ESM S7, Figure S4).

Figure 14. BSE images of the tuffs from the Nidymsky Formation, samples: (a,c,d)—HT-6.43; (b,e)—HT-6.44; (f)—HT-51. Abbreviations are in Figure 9; Q—quartz, C—coal.

Figure 14. BSE images of the tuffs from the Nidymsky Formation, samples: (a,c,d)—HT-6.43; (b,e)—HT-6.44; (f)—HT-51. Abbreviations are in Figure 9; Q—quartz, C—coal.

The basalts of the Lower part of the Nidymsky Formation are often characterized by pillow separation and thick amygdaloidal lower zones (Figure 15a). According to some authors, this indicates the outpouring of the melt into the aquatic environment, such as swamps or lakes. The average flow thickness ranges from 2 to 10 m; in a rocky cliff, they form outcrops up to 50 m high. These basalts represent a combination of oval and round massive fine-grained dense bodies (from 0.2 up to 8.0 m in diameter) enclosed in a vitreous hardening tube with a thickness of 1–3 cm. The distribution of balls within the amygdaloid basalts is highly irregular (ESM S8, Figure S5). Within these spaces, there is also evidence of Iceland spar, quartz, anthracite, apophyllite, prehnite, and datolite, among other minerals. The distribution of nests in the genus is uneven. They vary in size from 2–4 cm to 30 cm across these rocks. These extremely rich accumulations were discovered by S.V. Obruchev [69] and formed the basis for the extraction of Iceland spar in Russia for several decades.

Figure 15. Basalts of the Lower part of the Nidymsky Formation. (a)—pillow lava from the lowest horizon; (b)—columnar separation in the basalts from the Upper part of the Nidymsky Formation.

Figure 15. Basalts of the Lower part of the Nidymsky Formation. (a)—pillow lava from the lowest horizon; (b)—columnar separation in the basalts from the Upper part of the Nidymsky Formation.

The basalts of the Upper part of the Nidymsky Formation differ from those of the Lower one. As we move higher along the section, from the bottom to the top of the Formation, the thickness of basalt flows increases up to 20 m, and they acquire a columnar structure (Figure 15b; ESM S8, Figure S6). In addition to vertical fractures, horizontal fractures can also be observed in the basalts, which divide the flow into separate parts eroded unevenly (ESM S8, Figure S6c–f). The base of the Kochechumsky Formation lies along the lower boundary of the Agitkansky marking cover (30–35 m), consisting of dark gray, fine-grained basalts with thin, fan-shaped columns. The top of the Formation extends along the upper boundary of a rock unit (8–50 m), represented by tuffites, tuff-sandstones, tuff-aleurites, and tuff-argillites of brown, yellow and ash-gray colors. Compared to the Nidymsky Formation, the Kochechumsky Formation has a thicker strike length and fewer almond-shaped zones in its upper layers. The total thickness of the Kochechumsky Formation is 150 m.

The main minerals include plagioclase, clinopyroxene, and magnetite, while olivine and orthopyroxene are less common (ESM S3). These rocks differ from those of the Norilsk region and Putorana plateau in their texture of individual minerals, which indicates their very fast cooling. Magnetite skeletal and dendritic crystals in recrystallized glass are a clear example of this process (Figure 16). Zoned pyroxene and plagioclase crystals are also present (magnetite dendrites contain up to 20 wt.% TiO₂, Figure 16a, ESM S2). Additionally, these rocks underwent a higher degree of alteration compared to the rocks in the northern part of the province. Secondary minerals first appear in the interstitial spaces and replace glass. Then, substitution of clinopyroxene begins at the margins and eventually covers the entire grain, leaving plagioclase unaffected by secondary changes (Figure 16b). The main secondary minerals in the rock are chlorite, epidote, and calcite (and possibly siderite). The calcination losses reach 5–6 wt.% (ESM S4), while those in basalts of the Norilsk area reach 1–2 wt.%. Olivine is always replaced by serpentine.

Figure 16. Backscattered electron images of the basalts from the Nidymsky Formation (sample HT-46): calcite-saponite amygdala (a), magnetite dendrites (b). Abbreviation of minerals in Figure 11. Point numbers correspond to ESM S2.

The Yambukansky Formation (T₁jm) consists of the upper part of the volcanic section that begins with a layer of volcanic sedimentary rocks, followed by the Yambukansky porphyritic basalt layer. This typical section of the Formation can be found in the middle river flow in the Yambukan River basin. The thickness of the Yambukansky Formation varies between 75 and 150–180 m (in the central part of the Q-47 map, on the left bank of the Tembenchi River).

4.2. Geochemical Features of the Rocks from the Tuff-Lava Sequence

4.2.1. Major Elements in the Rocks of the Tuff-Lava Sequence

A plot of the entire set of studied rock compositions in the TAS diagram (Figure 17) indicates that most of the rocks are basalts. Only a small number of compositions correspond to andesites and trachybasalts [81]. The latter primarily include the tuffs from both the Nidymsky and Korvunchansky Formations. The tuff compositions also correspond to typical basalts.

Figure 17. Diagram SiO₂—Na₂O + K₂O for the studied rocks. Here and in Figures 18, 19 and 23 formations are T₁kč—Kochechumsky; T₁nid₂—Nidymsky. Upper part: T₁nid₁—Nidymsky. Lower part; T₁Kv—Korvunchansky.

The series of diagrams in Figure 18 illustrates the behavior of the major oxides in the rocks from the Korvunchansky, Nidymsky, and Kochechumsky Formations. Additionally, basalts and tuffs from the same Formations are shown separately. In general, the basalts of all Formations are very similar in chemical composition, as can be seen in Figure 18a–f. Their composition fields overlap on the diagrams, especially on the plots of MgO versus SiO₂ (Figure 18a) and CaO (Figure 18c). The concentrations of most elements do not depend significantly on the MgO content in the rocks, which varies between 4 and 8 wt.%. There are no correlations between MgO and FeO and TiO₂. The most notable exception is the Kochechumsky Formation, where the TiO₂, Fe₂O₃, and Al₂O₃ contents increase with the MgO concentration decline (R² = 0.72, 0.89, and 0.76, respectively). This correlation is less pronounced for the basalts of the Lower and Upper Nidymsky Formation (R² = 0.2–0.5) (Figure 18b–d).

Figure 18. Diagrams MgO–SiO₂ (a), TiO₂ (b), Al₂O₃ (c), Fe₂O₃ (d), CaO (e), K₂O (f), Na₂O (g), and MnO (h) for the rocks in the Tunguska syneclise.

The tuffs from the studied Formations differ from the basalts in high SiO₂ and K₂O contents, and are depleted in TiO₂, Fe₂O₃, CaO, and MnO (Figure 18b,d,e,h). The tuffs from the Nidymsky and Kochechumsky Formations have the same Na₂O concentrations as basalts (Figure 18e), although they contain natrolite. In general, the tuffs of the last Formation differ in chemistry from the first ones. They have an andesitic composition due to the occurrence of quartz in the rocks (Figure 14) and magma fractionation (the trendline for tuffs demonstrates a correlation between MgO and SiO₂ (R² = 0.80)). The diagrams in ESM S4 demonstrate the absence of correlations between SiO₂ and K₂O and a low correlation between SiO₂ and Na₂O (ESM S4). These data support the diagrams in Figure 18, where it is clear that only Na is mobile.

Almost all studied rocks belong to the low- and moderate-potassium types. Low-potassium rocks (with less than 0.5 wt.% K₂O), in particular, are mainly found in the upper Formations of the basalt section, i.e., Nidymsky, Kochechumsky, and partially in the lowest one, Korvunchansky Formations. This low potassium content is a distinguishing feature of the Trap Formations in the Siberian province, including all basalts of the Norilsk area.

4.2.2. Metals in the Rocks

The studied basalts have copper concentrations ranging from 100 to 200 parts per million (ppm). We did not find any rocks depleted in Cu in our samples (Figure 19), as observed in the Norilsk region. However, concentrations below 50 ppm in the Nidymsky Formation have been previously reported [19]. Nickel concentrations are lower than copper and vary between 80 and 150 ppm (Figure 19d). The range of cobalt values is very narrow; Co content changes from 35 to 57 ppm (Figure 19e). Similarly, the range of zinc concentrations in rocks is also relatively small, varying from 95 to 115 ppm (R² = 0.03–0.19), except for two samples where the concentration reaches 150 ppm (Figure 19f). The vanadium and chromium contents are typical of the platform basalts of the Siberian province and range from 231 to 322 and 116 to 232 ppm, respectively (Figure 19a,b). The correlation between metal contents and magnesium in the basalts from the three studied Formations is very low, with R² values ranging from 0.03 to 0.19, and the highest correlation being for nickel, with an R² value of 0.76.

Figure 19. Diagrams of MgO versus V (a), Cr (b), Co (c), Ni (d), Cu (e), and Zn (f) for the rocks from the Tunguska syneclise. Blue stars mean average concentrations of metals in the Kharaelakhsky Formation [82].

A comparison of the obtained metal contents in the studied rocks indicates their close concentrations to those in the Morongovsky, Mokulaevsky, and Kharayelakhsky Formations (typical platform basalts), i.e., (ppm): Cu—93, 141, 139; Ni—136, 132, 124; Zn—100, 83, 87; Co—47, 50, 49; Cr—139, 108, 109 (georock.com). The average data for these metals (except V) from the basalts of the Kharaelakhsky Formation are indicated by blue stars in Figure 19 (from [82]).

Tuffs contain low concentrations of non-ferrous and ferrous metals compared to basalts. Only rare samples (ESM S4) contain non-ferrous metals at concentrations approaching those found in basalts. In contrast, the tuffs from the Korvunchansky Formation show high positive correlations between copper and zinc and MgO (R² up to 0.93), unlike all basalts.

4.2.3. Trace Elements in the Rocks of the Tuff-Lava Sequence

The distribution of trace elements in the rocks is also typical of the platform basalts of the Siberian igneous province, with negative Ta-Nb and positive Pb anomalies (Figure 20). The examination of the basalt spectra from both the Nidymsky Formation (the Lower and Upper parts) and the Kochechumsky Formation shows their surprising similarity. We combined the effusive rocks into individual groups according to their localities (Figure 20a–d). They demonstrate a range of spectra, especially the rocks from the central part of the B-C section (Figure 20b; number HT-40th). The other rock groups show only minor variations in composition. The concentrations of all elements are approximately 10 times higher than their contents in the primitive mantle, and their patterns are almost parallel to the X-axis with a slight inclination on the right side of the spectrum, which characterizes the behavior of HREE in magmas. These patterns significantly differ from those of E-MORB and N-MORB due to the presence of the Ta-Nb negative anomaly and a Pb positive anomaly, despite being similar in trace element contents to E-MORB (Figure 20).

Figure 20. Spider diagrams for the rocks of the Nidymsky Formation. Sample numbers: (a)—HT-13–HT-24, (b)—HT-28–HT-36, (c)—HT-38–HT-50, (d)—HT-51–HT-68. Here and in Figures 21–24 and 27–31, rock concentrations normalized to primitive mantle from [83].

Figure 21 and Figure 22 show that the trace element chemistry of the basalts differs significantly from that of tuffs, as indicated by the ratio of reference elements, which characterizes the topology of their spectra and the magnitude of anomalies. Tuffs have a steeper slope for their spectra relative to the X-axis in the left part (Figure 22a), indicating an enrichment in large-ion elements such as Th, U, and Ba (Figure 22b) and a general slope that reflects higher (La/Sm)n and (La/Yb)n ratios (Figure 23a,b and Figure 24). This trend is also seen to a lesser extent on the right side of the spectra, indicating depletion of HREEs (Figure 25a). Therefore, the (Gd/Yb)n ratio is slightly higher in the tuffs (1.65–1.82) compared to the basalts (1.50–1.67).

Figure 21. Spider diagrams for the tuffs from the Korvunchansky Formation: (a) Vanavara; (b) L. Tunguska; and the Nidymsky Formation: Lower part, samples (c) HT-7, (d) HT-6; Upper part, (e) HT-43, HT-36.1.

Figure 22. Spider diagrams for the tuffs (brown lines) and basalts (green lines) from the Nidymsky Formation (a); diagram (Ba/Lu)n-(Ta/Lu)n for the tuffs and basalts from different Formations in the Tunguska syneclise (b). Square marks are tuffs; diamonds are basalts.

Figure 23. Diagrams (La/Yb)n-(U/Nb)n (a) and (La/Sm)n-(Gd/Yb)n (b) for the tuffs and basalts from different Formations in the Tunguska syneclise.

Figure 24. Diagrams (La/Sm)n-(Gd/Yb)n (a) and (La/Yb)n-(U/Nb)n (b) for the tuffs of the Korvunchansky (samples VNR) and Nidymsky (samples HT) Formations.

These changes in the Gd/Yb ratio (Figure 23b) cannot be explained by the presence of quartz and potassium silicate minerals in the rocks. As the main component of these rocks is glass, it is likely that they have slightly different origins of primary magma. The tantalum-niobium and lead anomalies are more pronounced in tuffs, as well as a negative titanium anomaly, which is not present in basalts. The tuffs, which have different positions in the section, also slightly differ from each other: those located in the Upper part of the Nidymsky Formation are close to basalts (Figure 23), while the tuffs of the Korvunchansky Formation and those lying in the lower part of the Nidymsky Formation are very different (Figure 24).

4.2.4. Isotope Compositions of the Rocks

We have analyzed nine samples (Table 1) (Figure 25), which can be divided into three groups based on their isotopic composition. These groups have more or less similar isotopic compositions within each group, but differ in their average elemental composition. The first group includes the basalts HT-13, HT-14, HT-16, NT-27, and NT-28, with an average content (ppm): Ru 8.08, Sr 185, Sm 3.59, Nd 12.6, Pb 2.22, U 0.52, and Th 1.24 ppm. The second group combines the two basalt samples (HT-33 and HT-36), with the following contents: Rb is 3.42, Sr is 258, Sm is 3.42, Nd is 11.8, Pb is 1.92, U is 0.44, and Th is 1.14 ppm. The third group includes a single tuff sample, HT-6.44. Its isotopic composition differs significantly from the other samples. The Rb concentration in the HT-6.44 sample is 109 ppm, Sr is 233 ppm, Sm is 4.17 ppm, Nd is 18.0 ppm, Pb is 4.89 ppm, U is 1.02 ppm, and Th is 3.90 ppm. The isotopic composition of the studied samples perfectly corresponds to the differences in elemental content. For the first group, the average Rb/Sr isotopic ratio is 0.12761, while for the second group it is 0.03798, and for the tuff, it is 1.35174. The Sm/Nd ratio for the first group is 0.17324, while for the second it is 0.17587, and for the tuff, it is 0.14007.

The main question concerns the influence of secondary mineral phases on the measured isotopic composition in certain varieties of tuff and basalt. We have not subjected ground samples to acid leaching to avoid introducing uncertainty into the true isotopic composition of the samples. Therefore, we cannot completely rule out the influence of these secondary minerals. However, the presence of consistent elemental and isotopic compositions in these groups supports the idea that the influence is minimal. Moreover, the samples from the first group on the Rb-Sr isochron diagram (Figure 25a) form a range with ages between 260 and 290 million years ago and an initial isotopic composition of 0.7048. This indicates that they are close to their actual age and their isotopic systems have not undergone significant changes due to secondary alterations.

Figure 25. Diagrams ⁸⁷Rb/⁸⁶Rb–⁸⁷Sr/⁸⁶Sr (a), ¹⁴³Nd/¹⁴⁴Nd–¹⁴⁷Sm/¹⁴⁴Nd (b), and Ma–²⁰⁶Pb/²⁰⁴Pb (c) for the rocks in the Lower Tunguska River Valley.

Figure 25. Diagrams ⁸⁷Rb/⁸⁶Rb–⁸⁷Sr/⁸⁶Sr (a), ¹⁴³Nd/¹⁴⁴Nd–¹⁴⁷Sm/¹⁴⁴Nd (b), and Ma–²⁰⁶Pb/²⁰⁴Pb (c) for the rocks in the Lower Tunguska River Valley.

The only candidate for a sample with altered isotopic characteristics is the HT-6.01 sample. It obviously has an increased strontium isotopic ratio (0.707659) compared to other basalts with similar Rb content and lower Sr content, which means that its Sr isotopic ratio is not due to radiogenic accumulation. It can be explained by the presence of secondary minerals, or by the occurrence of more ancient material in the composition of this tuff breccia than in the basalts (251 million years ago). The latter suggestion is clearly supported by the two-point model trend conducted through the tuff composition and the initial composition of the first group of basalts (0.7048), indicating a model age of 380 million years. As a result, when calculating the initial isotopic composition of the samples for an estimated age of formation of 251 million years, variations in the composition of the selected groups may reflect both the initial composition of the melt (for the first group, the average is 0.704773, and for the second group, it is 0.705467) and the presence of older substances that were trapped within the host rock, such as tuff breccia, which has an initial isotopic composition of 0.707236.

At the same time, the isotopic composition of the Sm-Nd system (Figure 25b) fully confirms the correlations established in the Rb-Sr system. For example, a negative ℇ_Nd (−1.33) for the tuff sample indicates the presence of ancient material (possibly host rocks of the crust) and reflects less influence from secondary minerals. In general, the variations in the Sm-Nd system are less pronounced than in the Rb-Sr system, which reflects a relatively young age of formation for the long-lived isotopic system of rare earth elements. The isotopic composition variations within the U-Th-Pb system are relatively small (Figure 25c), although there are differences in the elemental composition between the indicated groups of samples. The µ (mu) values range from 13.3 for tuff samples to 14.9 for the basalts in the first group. The κ (kappa) value (²³²Th/²⁰⁴Pb) ranges from 37 for the first group to 53 for the tuff, with the same ratio of thorium to uranium (2.7–3.4). This results in a difference of no more than 0.5% in the initial calculated isotopic composition. In addition, as shown in the chapter on petrography (Figure 11 and Figure 16), in all samples (basalts and tuffs), the changes correspond to the green shale stage of metamorphism. In this case, only sodium and potassium, as well as water and CO₂, are mobile. Other elements, especially rare earths, do not change. The clear separation of the samples according to their patterns confirms this conclusion (Figure 24).

Table 1. Isotope composition of the rocks in the Lower Tunguska River Valley.

Sample	Rock	Weight, g	Age, Ma	Rb, ppm	Sr, ppm	87Rb/86Sr	±2σ, %	87Sr/86Sr		±2σ
HT-6.44	Tuff	0.10178	251	109	233	1.35174	0.194	0.712043		0.000006
HT-6.01	Tuff-breccia	0.10493	251	10.5	320	0.09517	0.121	0.707659		0.000014
HT-13	Basalt	0.10651	251	4.90	191	0.07435	0.115	0.705168		0.000005
HT-14	Basalt	0.10659	251	5.67	192	0.08540	0.115	0.705084		0.000003
HT-16	Basalt	0.11930	251	10.3	186	0.16009	0.120	0.705194		0.000004
HT-27	Basalt	0.10171	251	9.84	167	0.17005	0.118	0.705477		0.000004
HT-28	Basalt	0.10161	251	9.66	189	0.14819	0.118	0.705213		0.000005
HT-33	Basalt	0.11074	251	4.06	285	0.04121	0.116	0.705566		0.000005
HT-36	Basalt	0.10228	251	2.78	232	0.03475	0.114	0.705639		0.000005
Sm, ppm	Nd, ppm	147Sm/144Nd	±2σ, %	143Nd/144Nd	±2σ
4.17	18.0	0.14007	0.240	0.512477	0.000002
3.71	13.8	0.16221	0.192	0.512695	0.000002
3.71	13.1	0.17104	0.188	0.512729	0.000004
3.65	12.7	0.17347	0.182	0.512747	0.000004
3.84	13.5	0.17263	0.456	0.512747	0.000002
3.06	10.4	0.17796	0.144	0.512744	0.000003
3.71	13.1	0.17110	0.179	0.512748	0.000004
3.67	12.9	0.17268	0.224	0.512757	0.000003
3.16	10.7	0.17906	0.151	0.512758	0.000003
206Pb/204Pb	±2σ	207Pb/204Pb	±2σ	208Pb/204Pb	±2σ	Pb, ppm	Th, ppm	U, ppm	ꜪNd (T)	(87Sr/86Sr)i
18.6141	0.0007	15.5552	0.0006	38.6110	0.0021	4.89	3.90	1.02	−1.33	0.707236
18.7707	0.0018	15.4637	0.0016	38.1350	0.0045	2.76	1.63	0.74	2.21	0.707320
18.6353	0.0024	15.5325	0.0028	38.5701	0.0043	2.31	1.31	0.58	2.60	0.704903
18.7875	0.0010	15.5076	0.0010	38.2128	0.0035	2.21	1.27	0.55	2.88	0.704780
18.7153	0.0010	15.5028	0.0008	38.1990	0.0021	2.35	1.34	0.59	2.90	0.704624
18.5353	0.0012	15.5082	0.0011	38.2643	0.0030	1.89	0.98	0.33	2.67	0.704872
18.8009	0.0004	15.5557	0.0003	38.3350	0.0008	2.32	1.29	0.57	2.98	0.704686
18.5706	0.0024	15.4196	0.0019	38.5943	0.0050	2.41	1.29	0.54	3.10	0.705419
18.6644	0.0016	15.4967	0.0021	38.3548	0.0045	1.43	0.98	0.35	2.91	0.705515

Table 1. Isotope composition of the rocks in the Lower Tunguska River Valley.

Sample	Rock	Weight, g	Age, Ma	Rb, ppm	Sr, ppm	87Rb/86Sr	±2σ, %	87Sr/86Sr		±2σ
HT-6.44	Tuff	0.10178	251	109	233	1.35174	0.194	0.712043		0.000006
HT-6.01	Tuff-breccia	0.10493	251	10.5	320	0.09517	0.121	0.707659		0.000014
HT-13	Basalt	0.10651	251	4.90	191	0.07435	0.115	0.705168		0.000005
HT-14	Basalt	0.10659	251	5.67	192	0.08540	0.115	0.705084		0.000003
HT-16	Basalt	0.11930	251	10.3	186	0.16009	0.120	0.705194		0.000004
HT-27	Basalt	0.10171	251	9.84	167	0.17005	0.118	0.705477		0.000004
HT-28	Basalt	0.10161	251	9.66	189	0.14819	0.118	0.705213		0.000005
HT-33	Basalt	0.11074	251	4.06	285	0.04121	0.116	0.705566		0.000005
HT-36	Basalt	0.10228	251	2.78	232	0.03475	0.114	0.705639		0.000005
Sm, ppm	Nd, ppm	147Sm/144Nd	±2σ, %	143Nd/144Nd	±2σ
4.17	18.0	0.14007	0.240	0.512477	0.000002
3.71	13.8	0.16221	0.192	0.512695	0.000002
3.71	13.1	0.17104	0.188	0.512729	0.000004
3.65	12.7	0.17347	0.182	0.512747	0.000004
3.84	13.5	0.17263	0.456	0.512747	0.000002
3.06	10.4	0.17796	0.144	0.512744	0.000003
3.71	13.1	0.17110	0.179	0.512748	0.000004
3.67	12.9	0.17268	0.224	0.512757	0.000003
3.16	10.7	0.17906	0.151	0.512758	0.000003
206Pb/204Pb	±2σ	207Pb/204Pb	±2σ	208Pb/204Pb	±2σ	Pb, ppm	Th, ppm	U, ppm	ꜪNd (T)	(87Sr/86Sr)i
18.6141	0.0007	15.5552	0.0006	38.6110	0.0021	4.89	3.90	1.02	−1.33	0.707236
18.7707	0.0018	15.4637	0.0016	38.1350	0.0045	2.76	1.63	0.74	2.21	0.707320
18.6353	0.0024	15.5325	0.0028	38.5701	0.0043	2.31	1.31	0.58	2.60	0.704903
18.7875	0.0010	15.5076	0.0010	38.2128	0.0035	2.21	1.27	0.55	2.88	0.704780
18.7153	0.0010	15.5028	0.0008	38.1990	0.0021	2.35	1.34	0.59	2.90	0.704624
18.5353	0.0012	15.5082	0.0011	38.2643	0.0030	1.89	0.98	0.33	2.67	0.704872
18.8009	0.0004	15.5557	0.0003	38.3350	0.0008	2.32	1.29	0.57	2.98	0.704686
18.5706	0.0024	15.4196	0.0019	38.5943	0.0050	2.41	1.29	0.54	3.10	0.705419
18.6644	0.0016	15.4967	0.0021	38.3548	0.0045	1.43	0.98	0.35	2.91	0.705515

5. Discussion

The results of this research on the structure of the tuff-lava sequence and its mineral and chemical composition can be used to clarify several aspects of the formation of the Siberian magmatic province.

5.1. The Structure of the Volcanic Rocks

The studied sections (Figure 6) show that the sequence of rocks in the Lower Tunguska River Valley represents the alternation of pyroclastic rocks and lavas. The tuffs dominate in the lower part of the volcanic pile, reaching up to 700–900 m in thickness. The two lowest Formations, i.e., Tutonchansky and Korvunchansky, consist only of tuffs and tuffits, while the overlapping Nidymsky and Kochechumsky Formations comprise tuffs and lavas; moreover, tuffs predominate in their lower parts. Therefore, pyroclastic rocks lying on the sedimentary rocks of the Tunguska series within the Tunguska syneclise mark the beginning of trap magmatism on the Siberian platform.

5.2. Rock Compositions

5.2.1. Tuff Compositions

The tuffs of the Korvunchansky Formation and the tuffs located within the Nidymsky one differ from the basalts of the Nidymsky and Kochechumsky Formations in terms of trace elements (Figure 22 and Figure 23).

Furthermore, tuffs differ significantly from basalts in composition in the contents of radioactive isotopes (Table 1; Figure 26). This may be partly due to the admixture of the sedimentary component in the latter, but most likely from different magma sources. Since the glass is the main component in pyroclastic rocks, it mainly determines the isotopic characteristics of tuffs. First of all, tuffs have lower ℇ_Nd, which varies from −1.33 to −4.5. This range of compositions comprises tuffs of the Nidymsky Formation (sample HT-6.44, this study), as well as tuffs of the Khakanchansky Formation located in the eastern part of the Norilsk area, in the South Iken River Valley (samples 76/1, 76/2), in the Kuluymber River Valley [30]), and on Mount Sunduk.

Figure 26. Diagram ⁸⁷Sr/⁸⁶Sr-Ɛ_Nd for tuffs and basalts of the Siberian Traps Province. Data from Table 1 and [11,28,30]. Formations, basalts (squares): Khr—Kharaelakhsky, Mk—Mokulaevsky, Nid—Nidymsky, Mr—Morongovsky. Formations, tuffs (circles): Khakanchansky—tuff-Sunduk, Tuff-Khakancha, Khakancha-SG-22; HT-6.44—Nidymsky.

The tuff compositions form a field below the line with a value of 0. These rocks are characterized by elevated ⁸⁷Sr/⁸⁶Sr ratios that are higher than 0.706. Due to these characteristics, tuffs are separated from basalts in the diagram ⁸⁷Sr/⁸⁶Sr-ℇ_Nd. Basalts of the Nidymsky Formation have ℇ_Nd close to that of the main formations of the Siberian Traps, i.e., Morongovsky, Mokulaevsky, and Kharaelakhsky [11,28]. It changes slightly from +2.21 to +3.10. The ⁸⁷Sr/⁸⁶Sr ratio in basalts varies more significantly than in tuffs (0.704620–0.707320; ESM S4). The highest values of ℇ_Nd for Nidymsky basalts lie near those from the Kharaelakhsky Formation. According to our data, there is no geographic variation in the isotopic characteristics of the samples, which means no evolution of the source.

Thus, there are two principally different fields for basalts and tuffs in the Siberian province, which evidence two different magma sources for these rocks. The different composition (Figure 22) of these tuffs and interlayered basalts (samples HT-60, HT-2, HT-6.43, 6.44 and HT-56, NT-57, HT-58, HT-53 in Figure 6) means that they formed simultaneously from different magmatic sources.

The question arises as to how tuffs, especially those from areas of ancient volcanic activity, reflect the composition of parental magma, as they almost always contain a mixture of foreign material and, therefore, may differ from basalts. In fact, typical tuffs with small amounts of sedimentary material, such as quartz, are often very similar in terms of trace element contents to lavas. As proof of this position, we demonstrate the compositions of rocks from the areas of the Siberian platform. The compositions of tuffs and tholeiitic basalts from the Kharayelakhsky Formation are identical in the Norilsk area, as well as for the alkaline Delkansky Formation in the Maymecha–Kotuy region (Figure 27a,b). These diagrams confirm the origin of tuffs and basalts from single sources for both Formations.

Figure 27. Spider diagrams for basalts (blue lines) and tuffs (red lines), Formations: (a)—Kharaekakhsky, (b)—Delcansky. Data in (ESM S3, [15,84]).

For the first time, we show that tuffs of the Korvunchansky Formation, which are widespread in the south of the platform, are similar to the Khakanchansky one in the Norilsk area (Figure 28). The occurrence of tuffs around the Siberian platform is reliably established by extensive geological survey works conducted in 1960–2020. However, they were related to different formations due to geological mapping being carried out by different performers. Unfortunately, their compositions were not analyzed practically. The similarity between the Korvunchansky and Khakanchansky tuffs suggests that the entire Siberian platform was completely covered with tuffs at the initial stage of magmatism, before the flood basalts erupted. Tuffs have slightly higher HREE concentrations than those in Lower and Bulk Crust, and intermediate LREE values between them, while Rb, Ba, U and Nb contents are similar to Lower crust (Figure 28).

Figure 28. Spider diagrams for the tuffs of the Khakanchansky (samples 76/1, 76/2) and Korvunchansky (samples vnr20 and vnr20_4) Formations. Data in ESM S3 and [30]. Lower and Bulk crust from [85].

5.2.2. Basalt Compositions

In many works [19,86], there are doubts about the possibility of subdivision of volcanic rocks into Formations according to their geochemical characteristics. This opinion is based on the idea that the rock compositions indicate events that occurred in small individual structures. However, we have shown that the composition of platform basalts remains almost unchanged over vast areas. Therefore, the mapped Nadayansky cover with a distinct glomeroporphyric structure (at the base of the Khonnamakitsky Formation on Putorana Plateau, or equivalently, the Mokulaevsky Formation in the Norilsk area) is widespread over an area of 48,000 square kilometers [62]. We have studied this cover on half of this area [87], and found that it has a remarkable consistency of composition (Figure 29), with the contents of both major oxides and trace elements varying only within the range of method errors. This homogeneity of composition is typical of all trap Formations. In contrast, rift Formations vary significantly in chemical composition [88], distinguishing them from platform basalts. The idea that rocks that are completely identical in geochemistry may appear at different times [86] seems untenable, since differences in their composition always occur.

Figure 29. Spider diagrams for the basalts of the Nadayansky flow. 83-33 is sample number. Data in [87].

The Nadayansky cover provides evidence of the homogeneity of its parental melt composition and the extent of the magmatic chamber. Therefore, a comparison of the geochemical features of the platform basalts from various sections located in different parts of the Siberian province is quite legitimate. Platform magmatism in the northwest of the Tunguska syneclise started from the Tuklonsky Formation, which occurs very locally in the east of the Norilsk area (Lake Glubokoe).

The most complete section of platform basalts is preserved in the Kharayelakh syncline [2,8,29] and comprises basalts from the Morongovsky, Mokulaevsky, Kharayelakhsky, Kumginsky, and Samoyedsky Formations. On the Putorana Plateau, only rocks from the Ayansky and Khonnamakitsky Formations, which correspond to the Morongovsky and Mokulaevsky Formations in the Norilsk area, have been preserved [20].

We have compared the studied basalts in the central part of the Tunguska syneclise with these Formations. The rocks from the central part of the Tunguska syneclise exhibit significant variations in magnesium compared to the basalts of the other Formations. Specifically, their magnesium oxide (MgO) varies from 4.2 to 7.8 wt.%, while other types of basalts fall within a narrower range between 6.2 wt.% and 8.0 wt.% MgO. With respect to other oxides, the Nidymsky and Kochechumsky rocks differ from the Morongovsky basalts but are similar to the Kharayelakhsky rocks for most elements. However, there is an exception for K₂O, which may be caused by secondary alteration of the rocks. The ratios of reference elements (La/Sm)n-(Gd/Yb)n and (La/Yb)n-(U/Nb)n, however, demonstrate the opposite trend—the rocks from the Nidymsky and Korvunchansky Formations are more homogeneous than the basalts from the other Formations (Figure 30). Nevertheless, they still show similarities to the rocks from the Kharaelakhsky Formation overall.

Figure 30. Diagrams (La/Yb)n-(U/Nb)n (a) and (La/Sm)n-(Gd/Yb)n (b) for the volcanic rocks of the Siberian Traps province. Abbreviations in Figure 26.

5.3. Reconstruction of the Volcanic Evolution in the Siberian Traps Province

The basalts of the central part of the Tunguska syncline, namely the Nidymsky and Kochechumsky Formations, correspond to the Kharayelakhsky Formation and were formed at a later stage of trap evolution. This conclusion differs from the findings of previous researchers. For instance, Almukhamedov et al. [66] correlated the Nidymsky Formation with the Morongovsky and Mokulaevsky Formations, while comparing the Kochechumsky Formation with the Kharayelakhsky one. However, Figure 30 and Figure 31 show little difference between the Nidymsky and Kochechumsky formations, and they both resemble the Kharayelakhsky formation very well. A.V.Latyshev and his colleagues [86] compared the Nidymsky–Kochechumsky formations to the part of the Norilsk section, which includes three formations: Morongovsky, Mokulaevsky, and Kharayelakhsky. Despite their widespread distribution on the Putorana plateau and in the north of the Tunguska syneclise, the Morongovsky and Mokulaevsky basalts do not extend to the Lower Tunguska River (Figure 32). Furthermore, the authors of the last article concluded that the basalts of the central Tunguska syneclise were formed at the initial stage of trap magmatism. This conclusion is based on the similarity of the paleomagnetic poles for the volcanic and intrusive rocks.

Figure 31. Spider diagrams for the rocks of the Karaelakhsky (a) and Kochechumsky (b) Formations. Data in ESM S3 and [11].

Figure 32. Correlation of the volcanic cross-sections in the Norilsk area and Tunguska syneclise. Solid lines demonstrate correlation of the Formations; dotted lines show schematically wedging out the Formation (abbreviations are in Figure 1). ? means no data.

Indeed, the Korvunchansky tuffs, on the one hand, correlate with the Khakaychansky rocks, which are close to the magmatic start, and, on the other hand, with the Kharaelakhsky Formation, which belongs to the end of volcanism. Maybe this fact could be explained by the long period of pyroclastic volcanism in the Tunguska syneclise compared to the Norilsk area. While the tuffs in the Tunguska River valleys accumulated, the basalts of the Morongovsky–Mokulaevsky Formations erupted in the Norilsk area. The different thickness of pyroclastic rocks in these regions (Figure 32) proves this suggestion, although more research is needed to confirm. We do not take into account numerous data on U-Pb age of zircons from the intrusive bodies in this region [77,89], as the problem is not in the accuracy of the analysis, but in the number of zircons with a large spread in each sample [34,35,90].

Based on our research and previously published data, we have made a preliminary reconstruction of the spatial and temporal evolution of volcanic activity in the Siberian Traps Province. This reconstruction is not complete, as it does not include several formations located on the Taimyr Peninsula. Before the Khakanchansky tuffs in the north of Eastern Siberia, the rift rocks of three Formations—Ivakinsky, Syverminsky and Gudchikhinsky—formed in a narrow zone along the Yenisei–Khatangsky strip. Later, during the formation of the Khakanchansky tuffs in the north of the Tunguska syneclise, Tuklonsky basalts were simultaneously erupted on the surface [91]. This situation is similar to that observed in the area of the Lower and Podkamennaya Tunguska Rivers. Thus, three successive stages are indicated, i.e., I, II, and III. Figure 33 presents three paleogeographic maps (a, b, and c) with the boundary of the traps’ province.

Figure 33. Scheme of volcanic evolution in the Siberian Traps Province. Stages, Formations: I—Ivakinsky + Syradasaysky (P₃iv + P₃sr), Syverminsky T₁sv, and Gudchikhinsky T₁gd (a); II—Tuklonsky + Khakanchansky + Korvunchansky (T₁tk + T₁hk + T₁kv) (+Tutonchansky T₁tt) (b); III—Morongovsky T₁mr (Ayansky T₁ay), Mokulaevsky T₁mk (Honnomakitsky T₁hn), Kharaelakhsky T₁khr (Nidymsky T₁nid + Kochechumsky T₁kč, Betlingsky T₁bt) (c). Dashed line is the boundary of the Siberian platform; solid lines are boundaries of the Yenisei-Khatanga rift.

Stage I (Figure 33a) represents the earliest phase of magmatism, corresponding to the Late Permian–Early Triassic period. This stage includes the Ivakinsky + Syradasaysky (P₃iv + P₃sr), Syverminsky (T₁sv), Gudchikhinsky (T₁gd) and Nadezhdinsky (T1nd) Formations. These Formations show that volcanic activity during this stage was concentrated almost exclusively in the northern part of the province, along the Yenisei–Khatangsky Trough and in the Norilsk area. Their origin is linked to the activity of this rift, and their eruptions are controlled by Yenisei–Khatangsky and other deep faults. Subalkaline and high-Mg magmas were involved in the formation of these rocks. The latter formed at a depth of 120 km and represent a single example of mantle products in the Norilsk area [38]. The Tunguska syneclise and the vast platform interior to the south remained unaffected by magmatism.

Stage II (Figure 33b) marks the beginning of the platform, or trap, magmatism. During this stage, the Tuklonsky, Khakanchansky, and Korvunchansky Formations (T₁tk + T₁hk + T₁kv) and the Tutonchansky (T₁tt) Formations appeared. Compared to Stage I, the area of volcanic activity has expanded dramatically southward, covering a significant portion of the Tunguska syneclise. Importantly, this stage is dominated by pyroclastic volcanism—the Khakanchansky Formation in the north and the Korvunchansky Formation in the center and south are both tuff-dominated. Thus, the tuffs of the same composition covered the entire Siberian platform, suggesting far-traveled pyroclastic material from fissure-type eruptions. The Tuklonsky Formation, present only locally near Norilsk, marks the very beginning of the basalt volcanism, which appeared with the last tuff eruption.

Stage III (Figure 33c) represents the main phase of basalt volcanic activity on the platform. The Formations include the Morongovsky (T₁mr), equivalent to the Ayansky (T₁ay), the Mokulaevsky (T₁mk), equivalent to the Khonnamakitsky (T₁hk), the Kharaelakhsky (T₁khr), with equivalents the Nidymsky (T₁nid) + Kochechumsky (T₁kč), and the Betlingsky (T₁bt) in Taimyr. The basalts cover a large area, encompassing the entire Tunguska syneclise, the Norilsk area, the Putorana Plateau, and extending through the Yenisei–Khatangsky Trough to the Taimyr peninsula. This stage records the main period of trap magmatism—the eruption of voluminous tholeiitic basalts, also known as flood basalts, or the Siberian Traps. The geochemical uniformity of these basalts across the entire province is demonstrated by the data presented in this article. Rift magmatism also operated during this period (not indicated in Figure 33), forming small volcanic areas such as Aynsky picrites, Ankaramites, etc. [8].

The causes of intraplate magmatism in different regions on the continents are being actively discussed [92,93,94,95,96]. Many researchers accept the plume origin for the Siberian Igneous Province. IH Campbell and GF Davis [55] describe plumes as follows: “Plumes consist of a large head followed by a narrower tail. Picrites are expected to dominate the first eruptive products of a new plume and should be concentrated near the center of the volcanic province”. Later, the constant influx of mantle magmas through a channel leads to the reappearance of picrites, mantle derivatives, at the final stage, followed by rift formation and the breakup of the continent. The authors conclude: “All of these predictions are confirmed by observations, and so provide strong support for the validity of the mantle plume hypothesis”.

Based on the previous data on the composition and distribution of P-T volcanic rocks in Eastern Siberia, which are described in this article, we want to show that the geological structure and magmatism evolution do not follow the proposed model. There are several points that are inconsistent with the plume hypothesis: (1) the absence of uplift prior to magmatic activity; (2) the lack of mantle characteristics of the rocks, such as high-Mg rocks and high-Mg olivine in them, trace element distribution, the Sr and Sm-Nd isotopic data, which are closer to those of Lower crust; and the absence of mantle xenoliths; and (3) the development of the rift before the flood basalts and its continuation with platform magmatism.

These points are regarded in many publications [97,98,99,100]. First of all, S.V.Sobolev and A.V.Sobolev [54] explained the absence of uplift by the presence of ocean crust with low density in the source of the Siberian Traps. This conclusion is based on data from picrites of the Gudchikhinsky Formation in the Norilsk area, i.e., on high Ni and Fe/Mn ratios in olivine. But these picrites occur only in the local rift zone; they do not connect with traps. The volume of this Formation is less than 0.1 vol.% of igneous rocks of the province. It cannot have an influence on the density and uplift of territories. Another geological contradiction is the sequence of volcanic rocks: the Gudchikhinsky Formation is not the first one (as it is suggested in the article), but it is the third Formation after the Ivakinsky and Syverminsky, which do not demonstrate any mantle features. Second, the article suggests the occurrence of mantle rocks at a depth in the province.

Nevertheless, the plume model supposes that picrites are the first rock. Because they are effusive rocks, they must be on the surface. Furthermore, thousands of intrusions are located under basalts, i.e., from the Riphean to Triassic deposits. They are exposed on the surface around the Tuguska Syncline and penetrated by deep boreholes. None of them have mantle signatures either. High-Mg rocks are located only within rift zones. Finally, isotopic and REE data, regarded as evidence of contamination of continental lithosphere by a mantle plume [11], can be interpreted in a different way.

L.T.Elkins Tanton and B.H.Hage [57] proposed as a triggering mechanism for the origin of Siberian Traps delamination, i.e., replacement of the mantle lithosphere by deep magma material that increases its density as the melt freezes into eclogite. However, modern seismic data do not support this idea. L.P.Vinnik and co-authors write a [100]: “If as a result of delamination the lithospheric upper mantle of the Siberian craton within the trap province has been replaced by material with a significantly higher (by 700–800 °C) temperature and enriched chemical composition, this is equivalent to a rejuvenation which could manifest itself by a reduction in the S-wave velocities to 4.4–4.5 km/s and shallowing (up to −90 km) of the seismic discontinuity at LAB. The absence of such observations means that the question of the formation mechanism of the Siberian Traps needs to be studied further (our italics)”.

Therefore, we conclude, after L.P. Vinnik, that further research is necessary to create a new model for the Siberian igneous province origin.

6. Conclusions

The study of the tuff-lava sequence in the central part of the Tunguska syneclise has yielded the following principal results:

• The basalts of the Nidymsky and Kochechumsky Formations are low- to moderate-potassium tholeiites with MgO concentrations ranging from 4.2 to 7.8 wt.%. They have remarkably uniform compositions throughout the study area, and their trace element patterns exhibit negative Ta–Nb and positive Pb anomalies, which are typical of the Siberian platform basalts.
• Geochemical comparison confirms that the Nidymsky and Kochechumsky basalts are mostly correlated with the Kharaelakhsky Formation in the Norilsk area.
• The tuffs of the Korvunchansky Formation differ fundamentally from the basalts in terms of chemical and isotopic compositions. They are enriched in SiO₂, K₂O, and large-ion lithophile elements. Their rare earth element (REE) patterns are steeper, and they have negative Ɛ_Nd values (−1.33 to −4.5), with elevated ⁸⁷Sr/⁸⁶Sr ratios (>0.706). In contrast, basalts have positive Ɛ_Nd values (+2.21 to +3.10); thus, they are derived from a slightly different magma sources
• The tuffs from the Korvunchansky Formation in the south of the province are similar to those from the Khakanchansky Formation in the north of the same province based on their trace element and isotopic characteristics (close to the Lower crust). Taking into account the widespread distribution of tuffs in eastern Siberia, it is possible to suggest that they covered the entire Tunguska syneclise, the Norilsk area, and the Putorana plateau, i.e., the entire Siberian platform.
• The absence of picrites and the occurrence of tuffs at the bottom of the traps sequence do not support the plume model, which suggests that mantle picrites were the primary rocks.
• The volcanic evolution in the Siberian Traps Province occurred in three stages:

  -

  Initial rift-localized magmatism in the north of the province,

  -

  An explosive pyroclastic phase that spread across the platform,

  -

  Eruption of geochemically homogeneous basalts over the entire province.