Siberian Trap Province Research Articles

Composition, structure and formation conditions of the Arydzhangsky Formation of the Maimecha-Kotuy District of the Siberian Trap Province

We studied 6 sections of the Arydzhangsky lava formation (P2-T1) in the Kotuy river valley. The results of petrographic and geochemical studies of the composition of rocks of the Arydzhangsky, Khardaksky and Pravoboyarsky formations are presented. The stratigraphic columns of the sections were built and the composition of the rocks was additionally determined using a scanning electron microscope. In this regard, the relative position of these formations was established, the mantle and crust sources of magmas were confirmed. A geochemical identity of the rocks of the Khardaksky formation with the rocks of the Arydzhangsky formation was established, which suggests a similar age of their formation.

The Influence of Trap Magmatism on the Geochemical Composition of Brines of Petroliferous Deposits in the Western Areas of the Kureika Syneclise (Siberian Platform)

Abstract —We present the results of study of the influence of trap magmatism on the geochemical composition of brines and on the geothermal regime of the Earth’s interior in the western areas of the Kureika syneclise. The Siberian trap province, which unites all cutting and layered tholeiite–basic magmatic intrusions and erupted basaltic lava, is the world’s largest Phanerozoic continental basalt province. Brines, hydrocarbon deposits, and organic matter of the sedimentary cover were subjected to a significant thermal impact as a result of the Permo-Triassic trap magmatism. During the trap intrusion, the maximum paleotemperatures in major Silurian (D’yavolskii), Ordovician (Baikit), and Cambrian (Deltula–Tanachi, Abakun, and Moktakon) productive horizons reached 650 °C. The Paleozoic and Proterozoic deposits of the study area contain brines with TDS = 50–470 g/dm3. By chemical composition, they are of Na, Na–Ca, Ca–Na, Ca–Mg, and Ca chloride types (according to the classification by S.A. Shchukarev), with mixed Ca–Na and Na–Ca chloride brines dominating. The studied brines can be divided into three groups according to the degree of metamorphism: low (S1), medium (S2), and high (S3). The first group includes mainly sodium chloride brines with TDS = 50–370 g/dm3 (rNa/rCl = 0.60–0.95; S ≤ 100). The second (dominating) group comprises Na–Ca, Ca–Na, Ca, and Ca–Mg chloride brines with TDS = 150–470 g/dm3 (rNa/rCl = 0.10–0.87; 100 ≤ S ≤ 300). The third group is Ca–Na and Ca chloride brines with TDS = 223–381 g/dm3 (rNa/rCl = 0.12–0.45; S ≥ 300). We have first established changes in the hydrogeochemical field (major- and trace-component and gas compositions) with distance from the contacts of intruded dolerite sills and dikes. Hydrocarbons (CH4, C2H6, C3H8, i-C4H10, n-C4H10, i-C5H12, n-C5H12, and C6H14) and water-soluble components I, B, and NH4 were most actively subjected to destruction. For example, at a distance of 100 m from the intrusion zone, the water-dissolved gases are dominated by CO2 (>90 vol.%), and CH4 amounts to 5 vol.%, whereas at a distance of 250 m, the concentration of CO2 decreases to 30 vol.%, and that of CH4 increases to 60–70 vol.%. In addition to the negative effect on the hydrocarbon preservation in the contact zone (≤400 m), the intrusive trap magmatism favored the formation of hydrocarbons in remote horizons. The reaction of intruding traps with brines of the sedimentary cover led to the saturation of the latter with iron, aluminum, and silica, which suggests extraction of metals in the form of salts from magmatic melts into an ore-bearing fluid.

Petrographical and Geochemical Characteristics of Magmatic Rocks in the Northwestern Siberian Traps Province, Kulyumber River Valley. Part II: Rocks of the Kulyumber Site

The origin of the Siberian trap province is under discussion even though numerous models of its formation have been created over the last three decades. This situation is mainly due to lack of modern geochemical data on magmatic rocks around the province. These data are a very important tool to reconstruct of magmatic evolution within the province in time and space and to understand a mechanism of province formation. Geochemical study has only been carried out so far for the Norilsk and Meimecha–Kotuy areas. For the first time, we have studied the geochemical and mineralogical characteristics of magmatic rocks at the Kulyumber river valley located 150 km to south from the Norilsk ore district, in the junction of the Tunguska syneclise and Norilsk–Igarka zone. It comprises three sites, i.e., Khalil, Kaya, and Kulyumber. The geochemical data on the magmatic rocks of the Khalil and Kaya sites were published earlier (Part I). This article (Part II) regards geochemical and mineralogical data on igneous rocks at the Kulyumber site. Seventeen intrusive bodies (41 samples) and six samples of sedimentary rocks were studied by X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS). Isotopes analyses (Sr, Nd, Pb) were conducted for 12 samples. These data were compared with data for intrusions of the Norilsk area, the Dzhaltulsky massif, Kureyka river, and intrusions in Angara river valley published earlier. The whole list of analyses includes 102 items. Three groups of intrusive rocks were recognized: (1) Mafic rocks with elevated K2O without negative Ta-Nb and Pb-positive anomalies, with (Gd/Yb)n = 2.0 and εNd = −1.0; attributed to a new Kulyumbinsky complex; (2) subalkaline rocks with elevated SiO2,TiO2, P2O5, and K2O with small negative Ta-Nb and positive Pb anomalies and (Gd/Yb)n = 1.8, εNd = −3.8; Ergalakhsky complex; and (3) mafic rocks with strong Ta-Nb and Pb anomalies and (Gd/Yb)n = 1.2–1.4, εNd = +0.4–+2.2. The third group is rather nonhomogeneous and includes intrusions of the Norilsk, Kuryesky, Katangsky, Ogonersky, and Daldykansky complexes differing in MgO content and trace element distribution (values of Ta-Nb, Pb, and Sr anomalies). Three groups of intrusive bodies had different magma sources and different condition of crystallization reflecting their origin in rift and platform regimes.

Structure and Composition of the Nadayansky Lava Flow: an Example of the Homogeneity of Lava Flows of the Siberian Trap Province

The Nadayansky Flow is one of the main markers of the volcanic pile of the Siberian Trap Province and covers an area of approximately 48 000 km2 at an average thickness of 30 to 50 m. The paper is the first to present data demonstrating the constancy of the major- and trace-element composition of the Nadayansky Flow, typifying basalt lava flows in the province. The flow makes up the bottom part of the Mokulaevsky Formation in the Norilsk area and the bottom of the Khonnamansky Formation at the Putorana Plateau and, correspondingly, overlies the Morongovsky and Ayansky formations (the names of these formations differ because they are used in different schemes for the stratigraphic subdivision of the volcanic rock sequence). The rocks show an obvious glomerophyric texture, which makes this flow clearly distinguishable from the under- and overlying rock units. The composition of the Nadayansky Flow was studied throughout its length of a few hundred kilometers and shows very little varying concentrations of major components (48.31 SiO2, 1.26 TiO2, 15.8 Al2O3, 12.71 Fe2O3, 0.19 MnO, 6.89 MgO, 11.1 CaO, 2.25 Na2O, 0.37 K2O, 0.14 P2O5, 0.02 Cr2O3) and trace elements (2.44 La/Sm and 1.56 Gd/Yb), whose variations are within the analytical uncertainties (XRF and ICP-MS analyses). The basalt of the flow crystallized from tholeiitic melt, whose composition was analogous to those of melts that produced all other flows of the Mokulaevsky Formation. For comparison, the paper displays the inner structures of the underlying Morongovsky and Mokulaevsky formations in the basin of the Mikchangda River in the eastern part of the Norilsk area. The composition of the basalts of these formations also varies insignificantly. The main difference is an increase in TiO2 concentration from 1.19 to 1.3 wt % with the transition from the lower formation to the upper one. In spite of the insignificant difference between the concentrations, it is of principal importance and makes it possible to distinguish between the basalts of these formations. The detected constancy of the compositions of the formations as a whole and the Nadayansky Flow in particular are principally important for studying continental flood basalt provinces and demonstrates the compositional homogeneity of the erupted magmas and their sources. These results are important as an example how geochemical data can be used to correlate widely spaced sequences of volcanic rocks.

First Data on the Age of Metamorphic Schists from the Taz Peninsula (Arctic, Western Siberia)

The 40Ar/39Ar dating of metamorphic schists from the basement of the West Siberian Platform exposed by Lenzitskaya well no. 77 on the Taz Peninsula was performed. The samples collected at depths of 3503 and 3515 m yielded age dates of 251.6 ± 3.5 and 271.2 ± 3.5 Ma. The age of ~271 Ma reflects the age of metamorphism of schists, which was induced by a strong tectonomagmatic event, associated with the onset of emplacement of rift zones and outflow of basalts in the Permian–Triassic. The age of ~252 Ma coincides with the period of the most intense emplacement of submeridional rifts in the basement of the West Siberian Platform. This process was accompanied by extension and thinning of the Earth’s crust in this region, which resulted in the exhumation of rocks from the mid-crystal level. The age boundary of ~250 Ma was established in the metamorphic and intrusive complexes of western Siberia and the Urals. This age is fixed almost everywhere and is correlated with the age of formation of the Siberian Trap Province.

Structure and Geochemical Features of the Volcanic Rocks in the Tunguska Syneclise (Siberian Trap Province)

To understand the origin of the Siberian trap province, it is necessary to know its internal structure in detail. Only two areas were described, the Noril’sk and Maimecha-Kotuy, while the most important part of the province, the Tunguska syneclise, is still poorly characterized. The main goal of our study is an investigation of the structure and geochemistry of volcanic rocks located in the central part of the province. We have studied basalts and tuffs and compared them with the rocks of the Noril’sk area. All studied rocks belong to three formations: Korvunchansky, Nidymsky and Kochechumsky. The whole thickness of this cross-section is around 1,000 m. Our geochemical data demonstrate that the lower part of the cross-section comprises basalts with TiO2=1 wt% while the upper part consists of the basalts with the average TiO2 content 1.4-1.5wt%. Based on this data, these rocks can be correlated with Morongovsky and Mokulaevsky formations located in the Noril’sk area. Despite many similarities between the volcanic rocks of these two areas, the basalts and tuffs of the Tunguska syneclise have their specific features, i.e. many tuff horizons, small thickness of flows and their poikiloophitic structure that reflect the rock formation near the boundary of the Tunguska syneclise.

Could the Norilsk Region Dolerite Sills Have Recorded Geomagnetic Field Reversals? Results of Mathematical Modeling

The possibility of reconstructing geologic events by identifying patterns in variations of the geomagnetic field related to reversals is one of the fundamental applications of paleomagnetism. The most detailed records of reversal events, whose duration averages 1–10 thous. years, are known from studies of flood basalts of large igneous provinces. At the same time, there have been recent publications presenting facts interpreted as records of geomagnetic reversals in intrusive bodies. Specifically, such data were obtained for relatively thin dolerite sills of the Ergalakh complex in the Norilsk region of the Siberian trap province that supposedly recorded the Permian-Triassic “Ivakin-Syvermin” reversal. This interpretation is based on the hypothesis of a slowly cooling intrusion, in which its apical parts magnetized in the Ivakin epoch of reversed polarity and the central parts—after the reversal in the Syvermin epoch of normal polarity. In this paper, using results of mathematical modeling, we discuss the validity of such assumptions and the potential attractiveness of subvolcanic intrusions for studies of geomagnetic reversals. We demonstrate that the duration of their cooling, including the most probable interval of magnetization is several orders of magnitude less than the duration of reversal transitions, and that the most probable cause of the occurrence of both polarities is the self-reversal effect.

Unique PGE–Cu–Ni Noril’sk Deposits, Siberian Trap Province: Magmatic and Tectonic Factors in Their Origin

The unique and very large PGE–Cu–Ni Noril’sk deposits are located within the Siberian trap province, posing a number of questions about the relationship between the ore-forming process and the magmatism that produced the traps. A successful answer to these questions could greatly increase the possibility of discovering new deposits in flood basalt provinces elsewhere. In this contribution, we present new data on volcanic stratigraphy and geochemistry of the magmatic rocks in the key regions of the Siberian trap province (Noril’sk, Taimyr, Maymecha-Kotuy, Kulyumber, Lower Tunguska and Angara) and analyze the structure of the north part of the province. The magmatic rocks of the Arctic zone are characterized by variable MgO (3.6–37.2 wt %) and TiO2 (0.8–3.9 wt %) contents, Gd/Yb (1.4–6.3) and La/Sm (2.0–10.4) ratios, and a large range of isotopic compositions. The intrusions in the center of the Tunguska syneclise and Angara syncline have much less variable compositions and correspond to a “typical trap” with MgO of 5.6–7.2 wt %, TiO2 of 1.0–1.6 wt %, Gd/Yb ratio of 1.4–1.6 and La/Sm ratio of 2.0–3.5. This compositional diversity of magmas in the Arctic zone is consistent with their emplacement within the paleo-rift zones. Ore-bearing intrusions (the Noril’sk 1, Talnakh, Kharaelakh) are deep-situated in the Igarka-Noril’sk rift zone, which has three branches, namely the Bolsheavamsky, Dyupkunsky, and Lower Tunguska, that are prospected for discovering new deposits. One possible explanation for the specific position of the PGE–Cu–Ni deposits is accumulation of sulfides in these long-lived zones from the Neoproterozoic to the Mesozoic era during magmatic and metamorphic processes. Thus, trap magmatism, itself, does not produce large deposits, but mobilizes earlier formed sulfide segregations in addition carrying metals in the original magmas. These deposits are the results of several successive magmatic events, in which emplacement of the traps was the final event.

Late Permian Intraplate Magmatism of the Baikal–Muya Belt: U–Pb Geochronology and Nd-Isotope Data

This paper presents the results of U–Pb isotope–geochronological study on zircons from subalkali gabbros and granodiorites of the Dogaldyn intrusive massif located in the Karalon–Mamakan zone of the Baikal–Muya belt. The ages of 254 ± 2 and 256 ± 1 Ma yielded for rocks of the two main intrusion phases do not differ significantly from each other and correspond to the Late Permian period. The Nd-isotope and geochemical data indicate the relationship between the primary melts of intraplate rocks from the Dogaldyn Massif and various sources, including plume–mantle and ancient crustal ones. The manifestations of Late Permian intraplate magmatic activity in the Baikal–Muya belt could have been induced by the Iberian superplume framed by the Siberian trap province.

Prospects of search for diamondiferous kimberlites in the northeastern Siberian Platform

The objects of study are Triassic hypabyssal diamondiferous kimberlites with an age of 220-245 Ma, containing macrocrysts of unaltered olivine. The latter are close in the time of formation to the main stage of intrusion of the Siberian Trap Province (252 Ma), which lasted less than 1 Myr. A comparative high-precision analytical study of the Ti, Ca, Cr, and Al impurity patterns in about 1000 olivine macrocryst samples with a forsterite content Fo = (100Mg/(Mg + Fe)) of 78 to 93 has demonstrated the effect of traps on the lithospheric composition. A comprehensive comparative study of diamonds from northern placers and Triassic kimberlites, including determination of their carbon isotope composition, was performed. Chromatography-mass-spectroscopic analysis of submicron fluid inclusions in diamonds from northern placers and kimberlites has shown predominant hydrocarbons of a wide range of compositions and subordinate contents of N2, H2O, and CO2. These findings, together with the results of previous studies of subcalcic Cr-pyropes and diamonds found in the Lower Carboniferous gritstones of the Kyutyungde graben, lead to the conclusion that the Toluopka kimberlite field is promising for Paleozoic kimberlites. The results of comprehensive studies of diamonds and indicator minerals and U/Pb isotope dating of numerous detrital zircon samples from the basal horizon of the Carnian Stage (Upper Triassic) of the Bulkur site in the lower reaches of the Lena River suggest the presence of diamondiferous kimberlites within the northeastern Siberian Platform. The age of the probable primary diamond sources in the study area can be evaluated by an integrated U/Pb isotope dating of zircons, perovskites, and rutiles from the developed diamond placers and the basal horizon of the Carnian Stage.

World-Class PGE-Cu-Ni Talnakh Deposit: New Data on the Structure and Unique Mineralization of the South-Western Branch

The Talnakh deposit is one of the largest PGE-Cu-Ni deposits in the world. It is located inside the North-Western part of the Siberian Trap province and consists of three branches. The problem of massive ore origins has been discussed for several decades. The structure of the South-Western branch and the mineralogy of related its Cu-rich ore are discussed in this article. The Southern-2 orebody has a deep inclination (45°) inside the intrusive body, close to its wall, in contrary to the horizontal orebodies of the North-Eastern branch. Mineral composition of the Southern-2 orebody differs from the composition the other orebodies of the Talnakh intrusion as well. It consists ofchalcopyrite, pentlandite with subordinate cubanite, and pyrrhotite. Its specific feature is a large amount of bornite and chalcocite. These minerals occur in disseminated and massive ores. The ores of the other branches of the Talnakh intrusion have chalcopyrite-pyrrhotite compositions whilethe Southern-2 massive ore is enriched in Cu (19.03–25.8 wt %; Cu/Ni = 3.8–8.6) and PGE: ΣPGE changes from 39.1 to 279 ppm, Pd/Pt = 1.3–32. Twelve minerals of the systems Pd-Sn-Cu, Pd-Pb-Bi, Pd-Ni-As, Au-Ag-Pd-Cu, and intermetallics of the Pt-Fe-Cu-Ni types were discovered in ores, but are also widespread in other orebodies. Isoferroplatinum, sperrilite and cooperitewere not found. The unusual structure and composition of the Southern-2 orebody suggest its origin from a separate magma impulse. The correlation between disseminated and massive ores of the Southern 2 orebody in term of chemical and mineralogical composition is evidence of the formation of massive oresin situ, without displacement along the bottom of the massif.

Verifying the mesozoic dipole low hypothesis by the Siberian trap data

A complex study is carried out for the collection of trap rocks sampled from two sequences in the geographically distant regions of the Siberian trap province: the Ergalakh section (Norilsk region) and Tyvankitskii and Delkanskii formations (Maymecha-Kotuy region). The magnetic and thermomagnetic properties are studied, petrographic and microscopic investigations are carried out, the paleointensities are determined, and the domain structure of ferromagnetics is assessed using the Day diagram and the thermomagnetic criterion. It is shown that small single-domain and/or pseudo-single-domain grains are the carriers of remanent magnetization. The absolute paleointensities H anc are determined by the Thellier-Coe method with test heating of the samples to lower temperatures (check-point procedure). The paleointensity estimates that meet the modern reliability criteria are obtained in more than 130 samples. The mean H anc values in the lava flows from the Ergalakh section and the TyvankittskiiFormation, as well as in samples from the Delkanskii Formation, vary within 2.1–24.6 mkT, which is considerably lower than the present magnetic field at the sampling site (∼ 50 mkT). The corresponding mean VDM values in these sites vary within (0.54–3.2) × 1022 Am2 (with a dispersion of ∼0.9 × 1022 Am2), which is far lower than the mean VDM value (∼8 × 1022 Am2) in the Late Cenozoic. The agreement of low H anc and VDM determined in the Ergalakh section, Tyvankitskii, and Delkanskii formations with the previous similar data on the traps (Solodovnikov, 1994; Heunemann et al., 2004; Shcherbakova et al., 2005; 2013) supports the Mesozoic Dipole Low (MDL) hypothesis. In the Ergalakh section, low and extremely low H anc (11.2 and 2.7 mkT) are determined in two pulses (five flows) of the Ivakinskii Formation, which precedes the geomagnetic reversal. This probably indicates that these flows recorded a sharp decrease in H anc prior to the geomagnetic reversal (or at its beginning), which occurred at the earliest stage of the Norilsk tuff-lava sequence formation. The abnormal values of paleodirections and paleointensities in the group of the flows of the Tyvankitskii Formation suggest that they could have been formed during the anomalous nonstationary state of the geomagnetic field such as the excursion.

Paleomagnetism of trap intrusions, East Siberia: Implications to flood basalt emplacement and the Permo–Triassic crisis of biosphere

Despite decades of concentrated studies, large igneous provinces (LIPs) are still among the most enigmatic phenomena of the Earth evolution. Among the myriad questions facing researchers, the duration of eruptive magmatic activity and its distribution through time are probably the most mysterious ones. Although the accuracy of modern dating techniques offers relatively high resolution, it is impossible to discriminate whether magmatic activity was more or less uniform or concentrated in few powerful outbursts. Consequently, many “trap-related” questions, from their origin to possible impact on the biosphere, still cannot be answered. Some of the above questions can be addressed with paleomagnetic data as have been effectively done in the case of the end-Cretaceous Deccan traps. The Siberian traps are one of the largest LIPs in the Phanerozoic and are thought, by some, to be the triggering event for the end-Permian mass extinction. We conducted a paleomagnetic study of trap intrusions along two long profiles in the eastern part of the province and found that most site-mean directions are grouped several times tighter than is common for the data on thick lava series and dyke swarms; similar over-grouping is found in other parts of the Siberian trap province too. We argue that this phenomenon is the most likely related to nearly simultaneous emplacement of trap intrusions over areas of several ten to few hundred kilometers in dimension for few millenniums or even faster. Moreover, such brief events account for all, or nearly all, magmatism in each area; how such events correlate over the entire Siberian LIP, however, remains an open question. Still, our data together with paleomagnetic results on thick lava series in Siberia strongly indicate that magmatic activity consisted of a number of brief pulses. In comparison to the other LIPs, the Siberian one is underlain by the thickest succession of carbonates, evaporites and coal-bearing clastics that are capable of producing huge amount of green-house and poisonous gases. We hypothesize that these very fast injections of magma into thick underlying sediments and the unique geological setting of the Siberian LIP are the major reasons for the Permo–Triassic mass extinction being the strongest in Earth history.

FeO activity and oxygen potential in magnesian magmas

An equation for the excess free energy of melts containing all major components is derived from published experimental data on equilibria between silicate liquids with either metallic iron or spinel at controlled oxygen partial pressure. The FeO activities in mafic and ultramafic magmas calculated with this equation were used to develop an oxygen barometer (geoxometer) for the association spinel + melt. Data derived with the application of the geoxometer confirm that meymechite magmas in the Siberian trap province had oxygen fugacities higher than in other magmatic systems. The origin of lower lithospheric domains with elevated redox potential can be explained by the intense diffusion of hydrogen from zones in which water-rich near-solidus melts from asthenospheric sources were emplaced into harzburgites, which had lost water and other incompatible elements in the course of earlier large-scale melting.

Provenance and characteristics of rocks from the Yermak Plateau, Arctic Ocean: Petrographic, geochemical and geochronological constraints

The Yermak Plateau is a prominent bathymetric feature of the Arctic Ocean. To the west it is bordered by the Fram Strait, which forms the only deep-water connection between the Arctic and the other global oceans. Origin, crustal nature and age of the Yermak Plateau are largely unknown. For this study, we investigated dredged rocks of two sites from the Yermak Plateau. Based on petrography, geochemistry, and geochronology, we distinguished between ice-transported and in-situ rocks. Ice-transported material was most likely derived from outcrops of the High Arctic Large Igneous Province (HALIP) on Franz Josef Land, the Siberian trap province, and presumably from northern Svalbard. Our data from the in-situ rocks, in conjunction with previously published geophysical data, show that the investigated parts of the Yermak Plateau are composed of stretched continental crust strongly affected by alkaline magmatism. The continental rocks represent a direct continuation of the exposures on northern Svalbard. Alkaline magmatism took place at ~ 51 Ma and was related to continental rifting in an extensional setting. The melts were formed by low degrees of partial melting of the sub-continental lithospheric mantle and are probably associated with the high-amplitude magnetic anomalies described for the northeastern Yermak Plateau. Extension of the Yermak Plateau was contemporaneous with spreading of the adjacent young Eurasian Basin, and occurred during the peak of compressional deformation affecting North Greenland, Svalbard, and Ellesmere Island. These contrasting regimes were probably compensated by transpression and strike-slip movements along the DeGeer and Wegener Faults. The date of ~ 51 Ma for extension-related magmatism also provides age constraints for the extension-related formation of the Sophia Basin (and thus for water exchange between the Eurasian Basin, the area of the DeGeer Fault and the young Norwegian-Greenland Sea), and for the sediments covering the horst-and-graben structures of the Yermak Plateau.

Parental melt of the Nadezhdinsky Formation: Geochemistry, petrology and connection with Cu-Ni deposits (Noril'sk area, Russia)

The paper addresses genetic relationship between volcanic rocks and ore-bearing intrusions of the Noril'sk area, Siberian Trap Province. Mg-rich rocks were found inside layered the Mikchangdinsky and Ikensky flows at the highest level of the vertical section of a tuff-lava pile in the Nadezhdinsky Formation in the eastern part of the Noril'sk area. The high La/Sm, Th/U ratios, depletion in Cu, highly radiogenic Sr and unradiogenic Nd isotope compositions suggest their affiliation with the Lower Nadezhdinsky Formation and distinguish them from the Sunduksky layered flow, which has similar internal structure but belongs to the Tuklonsky Formation. The composition of layered Mg-rich basalt flow (the Mikchangdinsky flow) indicates that these rocks likely represent previously unknown parental melt of Nadezhdinsky lavas which are thought to play a crucial role in the genesis of ore mineralisation. Compositionally this melt was significantly different from the parental melt, which formed lavas of the Tuklonsky Formation and this precludes their direct genetic relationship. Olivine composition of the Mikchangdinsky flow indicates rapid depletion of melt by Ni and Cu suggesting early precipitation and fractionation of sulphide melt. Chemical composition and Sr and Nd isotope compositions of rocks from the Mikchangdinsky flow are similar to the Low Talnakh intrusion, while composition of olivine and coexisting spinels manifest significant differences. The compositional differences between the olivine (contents Mn and V) and coexisting spinel (Fe2+/Fe3+ ratios) from the picrite gabbro-dolerites of the Low Talnakh intrusion and picrite of the Mikchangdinsky flow suggest that they may have crystallised from magma of similar composition but at different oxygen fugacity (intrusions being unusually oxidised). The substantial differences in contents of Ca, HREE and Y in olivines from these objects, however, leave open the question of genetic relationship between Nadezhdinsky parental melt and the Low Talnakh intrusion. Although some rocks of the Nadezhdinsky Formation contain unusually low concentrations of chalcophile elements, their role in the ore-forming processes of the Noril'sk district still remains largely uncertain.

Unique zoned olivines from an ultrabasic—Basic massif in the Noril’sk disitrict

The specifics of the morphology, internal structure, and composition of olivine, one of the major rock� forming minerals of ultrabasic and basic rocks, contains valuable information on the conditions of its formation and are widely used in petrology [11, 8, 3, 7, 12]. One of its important characteristics is zoning, which is most pronounced in lunar [8] and Martian [2] meteorites. Olivine exhibits both normal and reverse zoning, with the core–rim difference reaching 25 mol % forsterite. However, this phenomenon is uncommon in the ter� restrial intrusive rocks, because subsolidus diffusion usually “smoothes out” the initial compositional het� erogeneities of the mineral. At present, the available data are reduced to scarce olivine grains with zoned distribution of major (Fe and Mg with Fo gradient of 8–10 mol % [1, 4, 5]) and trace elements Ca, Ni, as well as Al, Cr, and P [4, 9]. Therefore, the find of zoned olivines with the core–rim difference more than 20 mol % Fo in the magmatic rocks of the Noril’sk district is a unique phenomenon not only for this region but also for ultrabasite–basite complexes around the world. The Noril’sk district is located in the NW Siberian trap province. It consists of the rocks of the crystalline basement and platform cover represented by the Early Cambrian–Late Permian terrigenous–carbonate rocks and Early Triassic basalts. Numerous ultrabasite– basite hypabyssal massifs in the forms of chonoliths and bodies of irregular shape are localized among the sedimentary and volcanic rocks of the area. Distinctly differentiated intrusions are combined into the pro� ductive Noril’sk complex, which is characterized by an elevated average weighted MgO content (10–12 wt %) relative to tholeiitic basalts predominant in the section (6–7 wt %). Some of them contain superlarge Pt– Cu–Ni deposits (Talnakh, Oktyabr’skoe, Noril’sk�1), whereas others are either weakly mineralized or bar� ren. Their age relations with volcanic rocks were not established exactly, because in most cases the intru� sions are localized in the underlying sedimentary rocks. One of the representatives of this type of magmatic complexes is the massif studied by us in the eastern part of the area, in the Mikchangda River basin. It is located in the Devonian carbonate–terrigenous rocks

Siberian meimechites: origin and relation to flood basalts and kimberlites

Here we combine petrological-geochemical and thermomechanical modeling techniques to explain origin of primary magmas of both Maimecha–Kotui meimechites and the Gudchikhinskaya basalts of Norilsk region, which represent, respectively, the end and the beginning of flood magmatism in the Siberian Trap Province. We have analyzed the least altered samples of meimechites, their olivine phenocrysts, and melt inclusions in olivines, as well as samples of dunites and their olivines, from boreholes G-1 and G-3 within the Guli volcanoplutonic complex in the Maimecha–Kotui igneous province of the northern Siberian platform. The Mn/Fe and Ni/MgO ratios in olivines indicate a mantle peridotite source of meimechites. Meimechite parental magma that rose to shallow depths was rich in alkalis and highly magnesian (24 wt.% MgO), largely degassed, undersaturated by sulfide liquid and oxidized. At greater depths, it was, likely, high in CO 2 (6 wt.%) and H 2O (2 wt.%) and resulted from partial melting of initially highly depleted and later metasomatized harzburgite some 200 km below the surface. Trace-element abundances in primary meimechite magma suggest presence of garnet and K-clinopyroxene, in the mantle source and imply for genetic link to the sources of the early Siberian flood basalts (Gudchikhinskaya suite) and kimberlites. The analyzed dunite samples from the Guli complex have chemistry and mineralogy indicating their close relation to meimechites. We have also computed thermomechanical model of interaction of a hot mantle plume with the shield lithosphere of variable thickness, using realistic temperature- and stress-dependent visco-elasto-plastic rocks rheology and advanced finite element solution technique. Based on our experimental and modeling results we propose that a Permian–Triassic plume, with potential temperature of about 1650 °C transported a large amount of recycled ancient oceanic crust (up to 15%) as SiO 2-supersaturated carbonated eclogite. Low-degree partial melting of eclogite at depths of 250–300 km produced carbonate-silicate melt that metasomatized the lithospheric roots of the Siberian shield. Further rise of the plume under relatively attenuated lithosphere (Norilsk area) led to progressive melting of eclogite and formation of reaction pyroxenite, which then melted at depths of 130–180 km. Consequantly, a large volume of melt (Gudchikhinskaya suite) penetrated into the lithosphere and caused its destabilization and delamination. Delaminated lithosphere that included fragments of locally metasomatized depleted harzburgite subsided into the plume and was heated to the temperatures of the plume interior with subsequent generation of meimechite magma. Meimechites showed up at the surface only under thicker part of the lithosphere aside from major melting zone above because otherwise they were mixed up in more voluminous flood basalts. We further suggest that meimechites, uncontaminated Siberian flood basalts and kimberlites all shear the same source of strongly incompatible elements, the carbonated recycled oceanic crust carried up by hot mantle plume.

Petrology of the parental melts and mantle sources of Siberian trap magmatism

Based on the investigation of olivine phenocrysts and melt and spinel inclusions in them from the picrites of the Gudchikhinsky Formation and olivine phenocrysts and the whole-rock geochemistry from the Tuklonsky and Nadezhdinsky formations of the Noril’sk region, the compositions and conditions of formation and evolution of the parental melts and mantle sources of Siberian trap magmatism were evaluated. Olivine phenocrysts from the samples studied are enriched in Ni and depleted in Mn compared with olivines equilibrated with the products of peridotite melting, which suggests a considerable role of a nonperidotitic component (olivine-free pyroxenite) in their mantle source. The onset of Siberian trap magmatism (Gudchikhinsky Formation) was related to the melting of pyroxenite produced by the interaction of ancient recycled oceanic crust with mantle peridotite. During the subsequent evolution of the magmatic system (development of the Tuklonsky and Nadezhdinsky formations), the fraction of the pyroxenite component in the source region decreased rapidly (to 40 and 60%, respectively) owing to the entrainment of peridotite material into the melting zone. The formation of magmas was significantly affected by the contamination by continental crustal material. The primitive magmas of the Gudchikhinsky Formation crystallized under near-surface conditions at temperatures of 1250–1170°C and oxygen fugacities 2.5–3.0 orders of magnitude below the Ni-NiO buffer. Simultaneously, the magmas were contaminated by continental silicic rocks and evaporites. The parental magmas of the Gudchikhinsky rocks corresponded to tholeiitic picrites with 11–14 wt % MgO. They were strongly undersaturated in sulfur, contained less than 0.25 wt % water and carbon dioxide, and were chemically similar to the Hawaiian tholeiites. They were produced by melting of a pyroxenite source at depths of 130–180 km in a mantle plume with a potential temperature of 1500–1580°C. The presence of low melting temperature pyroxenite material in the source of Siberian trap magmas promoted the formation of considerable volumes of melt under the thick continental lithosphere, which could trigger its catastrophic collapse. The contribution of pyroxenite-derived melt to the magmas of the Siberian trap province was no less than 40–50%. This component, whose solid residue was free of sulfides and olivine, played a key role in the origin of high contents of Ni, Cu, and Pt-group elements and low sulfur contents in the parental trap magmas and prevented the early dispersion of these elements at the expense of sulfide melt fractionation. The high contents of Cl in the magmas resulted in considerable HCl emission into the atmosphere and could be responsible for the mass extinction at the Paleozoic-Mesozoic boundary.

Physicochemical conditions of magma formation at the base of the Siberian plume: Insight from the investigation of melt inclusions in the meymechites and alkali picrites of the Maimecha-Kotui province

Based on the compositions of melt inclusions and coexisting minerals from meymechites and alkali picrites, the temperatures and pressures of the ascending material of the Siberian plume were estimated at the level of the lithosphere-asthenosphere boundary. The melts trapped in olivine show high contents of titanium and other incompatible elements. The rocks crystallized under high oxygen fugacity conditions. The calculated compositions of primary magmas are similar to the compositions of near-solidus melts derived from a dry fertile lherzolite at 7 GPa. The estimated potential temperature is close to 1650°C, which is much higher than the potential temperature of plumes that generate the primary basaltic magmas of mid-ocean ridges. The obtained data show that, during the activity of the giant magma-generating system of the Siberian trap province, hot peridotite masses ascended probably from the core-mantle boundary up to the base of the continental lithosphere. Our results are at odds with the suggestion that the basalt flows of the Siberian and other large igneous provinces are not related to mantle plumes.