Valeri Fedorenko

Rapid eruption of Siberian flood-volcanic rocks and evidence for coincidence with the Permian–Triassic boundary and mass extinction at 251 Ma

The Siberian flood-volcanic event is the most voluminous and explosive, continental, volcanic event known in the Phanerozoic record. U^Pb perovskite and zircon ages were obtained for lavas of the lowermost unit (251.7 9 0.4 Ma) and near-uppermost unit (251.1 9 0.3 Ma), respectively, of the volcanic sequence in the Maymecha^Kotuy area, Russia. Along with stratigraphic correlations and paleomagnetic evidence, these ages suggest that rapid extrusion of the entire V6500 m thick composite sequence occurred in less than 1million years. The time of extrusion coincides precisely with an age of 251.4 9 0.3 Ma previously obtained for the Permian^Triassic mass-extinction event, the most devastating biotic crisis known. Emplacement of the Noril’sk^Talnakh ore-bearing intrusions, notable for their prodigious Cu^Ni^PGE deposits, was synchronous with these two major geologic events at 251.2 9 0.3 Ma. The Guli volcanic-intrusive complex in the Maymecha^Kotuy area appears to represent the final mafic magmatism of the entire Siberian flood-volcanic event. Baddeleyite from a carbonatite that intrudes the complex gives an age of 250.2 9 0.3 Ma, and shows possible 231 Pa excess. The Bolgokhtokh granodiorite stock has a zircon age of 229.0 9 0.4 Ma, and represents the youngest known magmatism in the region.

Remobilisation of the continental lithosphere by a mantle plume: major-, trace-element, and Sr-, Nd-, and Pb-isotope evidence from picritic and tholeiitic lavas of the Noril'sk District, Siberian Trap, Russia

The Late Permian to Early Triassic Siberian Traps have been sampled by drill core (core SG-9) and from surface exposure (section 1F) in the Norilsk region of the Siberian Platform, Russia. Combined major, trace element, and Nd-, Sr-, and Pb-isotope data on selected samples through the Siberia Trap, offer new chemostratigraphic criteria for the identification and characterisation of two fundamentally different magma types and 9 of the 11 formations of lava developed near Norilsk. A Lower Sequence of sub-alkalic basalts, tholeiites, and picritic basalts (upwards these are the Ivakinsky, Syverminsky, and Gudchichinsky formations) are overlain by an Upper Sequence of picritic basalts and tholeiites interbedded with tuffs (upwards, these are the Khakanchansky, Tuklonsky, Nadezhdinsky, Morongovsky, Mokulaevsky and Kharayelakhsky formations).The Gudchichinsky and Tuklonsky formations contain both picritic and tholeiitic lavas. The Tuklonsky formation tholeiites and picrites have moderate Gd/Yb (1.6–1.8), low TiO2 (0.45–0.95 wt%), a significant negative Ta and Nb anomaly (Nb/La =0.42–0.57) and unradiogenic Nd (ɛNdCHUR = to -4.6). In contrast, both the Gudchichinsky formation tholeiites and picrites have high Gd/Yb (2.3–3.1), and TiO2 (1.2–2.3 wt%), no significant Nb or Ta anomaly (Nb/La =0.8–1.1), and radiogenic Nd (ɛNdCHUR = to 7.3). The low-Ti and Nb/La, high La/Sm, and unradiogenic Nd-isotope signatures of the picritic Tuklonsky formation lavas and the tholeiitic lavas of the Upper Sequence are characteristic of magmas strongly influenced by material from the continental lithosphere, whereas the high-Ti and Nb/La, low La/Sm and radiogenic Nd-isotope signatures of the Lower Sequence are more comparable to deeper asthenospheric mantle-plume generated lavas similar to oceanic island basalts. The lavas overlying the Tuklonsky formation have mg-numbers of 0.63 to 0.68, and are more evolved than the Tuklonsky (Mg-number < 0.62) and have more radiogenic ɛNdCHUR (Tuklonsky:-0.03 to-4.66; Mokulaevsky: + 0.60 to + 1.61), but have many of the incompatible trace element features of the Tuklonsky sky type magma. These lavas show a progressive upwards decline in SiO2 (55–49 wt%), La/Sm (4.6–2.0), and ɛURSr ( + 67 to + 13) which has previously been attributed to a decrease in the proportion of crustal material contributed to the magma. This paper explores and alternative model where a component of the crustal contribution might be derived from within an ancient region of the mantle lithosphere as recycled sediment rather than from the overlying continental crust.

Isotopic and trace-element constraints on mantle and crustal contributions to Siberian continental flood basalts, Noril'sk area, Siberia

Abstract We present a tightly controlled and comprehensive set of analytical data for the 250-Ma Siberian flood-basalt province. Consideration of major- and trace-element compositions, along with strontium, lead and neodymium isotopic compositions, strongly supports earlier Russian subdivision of this magmatism into three magmatic cycles, giving rise to three assemblages of eleven basalt suites in the ascending order Ivakinsky-Gudchikhinsky, Khakanchansky-Nadezhdinsky and Morongovsky-Samoedsky. Geochemical and isotopic discontinuities of varying magnitude characterize most of the boundaries between the eleven recognized basalt suites in the Norilsk area. Although we conclude that the dominant volume of erupted magma originated from an asthenospheric mantle plume, none of the lavas is interpreted to directly represent asthenospheric melts, which would have been far more magnesian. On the basis of thermal considerations, we consider it unlikely that vast volumes of basaltic melt were produced directly from the continental lithospheric mantle beneath the Siberian craton. Moreover, there is little evidence from mantle xenoliths that the geochemical signatures of such melts would correspond to those of the Siberian flood basalts. Studies of melt migration lead us to conclude that transport of asthenospheric melt through the lithospheric mantle would be rapid, by fracture propagation. Lavas from the Gudchikhinsky suite have negligible Ta-Nb anomalies and positive ϵ Nd values and their parental magmas presumably interacted little with the continental lithospheric mantle or crust. All other lavas have negative Ta-Nb anomalies and lower ϵ Nd values that we attribute to interaction with continental crust. The model that we have developed requires discrete contributions from the plume and complex processing of all erupted magmas in the continental crust. The earliest magmas represent small percentages of melt formed in equilibrium with garnet. Over time, the percentage of melting in the source region and the volume of magma produced increased, and garnet was no longer stable in the plume source. All of the plume-derived melts initially contained more than 20 wt% MgO and became less Mg rich by fractionation of olivine as they traversed the lithospheric mantle. We conclude, however, that the most significant control on the geochemical and isotopic compositions of all the erupted lavas was processing of mantle-derived magma in crustal reservoirs during periodic replenishment, periodic tapping, continuous crystal fractionation and wallrock assimilation. Rapid eruption of an extremely large volume of processed magma that varied little in chemical and isotopic composition produced the sequence of relatively monotonous tholeiitic basalts that constitute the 2,300-m-thick third assemblage of the Siberian flood-basalt province near Norilsk.

Mantle and crustal contributions to continental flood volcanism

Arndt, N.T., Czamanske, G.K., Wooden, J.L. and Fedorenko, V.A., 1993. Mantle and crustal contributions to continental flood volcanism. In: M.J.R. Wortel, U. Hansen and R. Sabadini (Editors), Relationships between Mantle Processes and Geological Processes at or near the Earths Surface. Tectonophysics, 223: 39–52. Most continental flood basalts are enriched in incompatible elements and have high initial 87Sr/86Sr ratios and low ϵNd values. Many are depleted in Nb and Ta. The commonly-held view that these characteristics are inherited directly from a source in metasomatized lithospheric mantle is inconsistent with the following arguments: (1) thermomechanical modelling demonstrates that flood basalt magmas come mainly from an asthenospheric or plume source, with minimal direct melting of the continental lithospheric mantle. The low water contents of most flood basalts argue against proposals that hydrous lithosphere was the source. (2) Lithospheric mantle normally has low concentrations of incompatible elements, and chondrite-normalized Nb and Ta contents similar to those of other incompatible elements. Such material cannot be the unmodified source of Nb-Ta-depleted basalts such as those from the Karoo, Ferrar, or Columbia River provinces. We suggest there are two main controls on the compositions of continental flood basalts. The first is lithospheric thickness, which strongly influences the depth and degree of mantle melting of a plume or asthenospheric source, and thus has an important influence on the composition of primary magmas. All liquids formed by partial melting of peridotite at sub-lithosphere depths are highly magnesian (20–25 wt.% MgO) but have variable trace-element contents. Where the lithosphere is thick, the source melts at high pressure, garnet is present, the degree of melting is low, and trace-element concentrations are high. This type of magma evolves to produce the high-Ti type of continental flood basalt. Where the lithosphere is thinner, the source ascends to shallower levels, the degree of melting is greater, garnet may be exhausted, and the magmas have lower trace-element contents; these magmas yield low-Ti basalts. The second control is processing of magmas in chambers that were periodically replenished and tapped, while continuously fractionating and assimilating their wall rocks. The uniform compositions of basalts that evolve in such chambers are far removed from those of their picritic parental magmas. Major elements in continental flood basalts reflect control by olivine, pyroxene, and plagioclase crystallization, and this assemblage places the magma chambers at crustal depth. We believe that trace-element and isotopic compositions are also buffered, and that the erupted basalts represent steady-state liquids tapped from these magma chambers. These processes impose a crustal signature on the magmas, as expressed most strongly in the concentrations of incompatible elements (e.g., Nb-Ta anomalies) and their isotopic characteristics.

Geochemistry of the Siberian Trap of the Noril'sk area, USSR, with implications for the relative contributions of crust and mantle to flood basalt magmatism

The sequence investigated of the Siberian Trap at Norilsk, USSR, consists of at least 45 flows that have been divided into six lava suites. The lower three suites consist of alkalic to subalkalic basalts (the Ivakinsky suite), overlain by nonporphyritic basalts (the Syverminsky suite), and porphyritic and picritic basalts (the Gudchikhinsky suite). The upper three suites are tholeiitic. The uppermost 750 m of dominantly non-porphyritic basalt belong to the Mokulaevsky suite and are characterized by a nearly constant Mg number (0.54–0.56), SiO2 (48.2–49.1 wt%), Ce (12–18 ppm), and Ce/Yb (5–8). The underlying 1100 m of dominantly porphyritic basalt belong to the Morongovsky and Nadezhdinsky suites. There is a continuous increase in SiO2 (48.1–55.2 wt%), Ce (12–41 ppm), and Ce/Yb (5–18) from the top of the Mokulaevsky to the base of the Nadezhdinsky with little change in the Mg number (0.53–0.59). Mokulaevsky magmas have trace element signatures similar to slightly contaminated transitional type mid-ocean ridge basalts. The change in major and trace element geochemistry in the upper three suites is consistent with a decline in the degree of anatexis and assimilation of tonalitic upper crust by Mokulaevsky magma. The Nadezhdinsky and underlaying lavas thicken within and thus appear to be related to an elongate basin centred on the Norilsk-Talnakh mining camp. The Mokulaevsky and Morongovsky lavas thicken to the east and appear to be related to a basin centred more than 100 km to the east of the Norilsk region; these magmas may have risen up out of a different conduit system.

RE-OS ISOTOPIC EVIDENCE FOR AN ENRICHED-MANTLE SOURCE FOR THE NORIL'SK-TYPE, ORE-BEARING INTRUSIONS, SIBERIA

Magmatic Cu-Ni sulfide ores and spatially associated ultramafic and mafic rocks from the Norilsk I, Talnakh, and Kharaelakh intrusions are examined for Re-Os isotopic systematics. Neodymium and lead isotopic data also are reported for the ultramafic and mafic rocks. The Re-Os data for most samples indicate closed-system behavior since the ca. 250 Ma igneous crystallization age of the intrusions. There are small but significant differences in the initial osmium isotopic compositions of samples from the three intrusions. Ores from the Norilsk I intrusion have γOs values that vary from +0.4 to +8.8, but average +5.8. Ores from the Talnakh intrusion have γOs values that range from +6.7 to +8.2, averaging +7.7. Ores from the Kharaelakh intrusion have γOs values that range from +7.8 to +12.9, with an average value of +10.4. The osmium isotopic compositions of the ore samples from the Main Kharaelakh orebody exhibit minimal overlap with those for the Norilsk I and Talnakh intrusions, indicating that these Kharaelakh ores were derived from a more radiogenic source of osmium than the other ores. Combined osmium and lead data for major orebodies in the three intrusions plot in three distinct fields, indicating derivation of osmium and lead from at least three isotopically distinct sources. Some of the variation in lead isotopic compositions may be the result of minor lower-crustal contamination. However, in contrast to most other isotopic and trace element data, Os-Pb variations are generally inconsistent with significant crustal contamination or interaction with the subcontinental lithosphere. Thus, the osmium and lead isotopic compositions of these intrusions probably reflect quite closely the compositions of their mantle source, and suggest that these two isotope systems were insensitive to lithospheric interaction. Ultramafic and mafic rocks have osmium and lead isotopic compositions that range only slightly beyond the compositions of the ores. These rocks also have relatively uniform ϵNd values that range only from −0.8 to + 1.1. This limited variation in neodymium isotopic composition may reflect the characteristics of the mantle sources of the rocks, or it may indicate that somehow similar proportions of crust contaminated the parental melts. The osmium, lead, and neodymium isotopic data for these rocks most closely resemble the mantle sources of certain ocean island basalts (OIB), such as some Hawaiian basalts. Hence, these data are consistent with derivation of primary melts from a mantle source similar to that of some types of hotspot activity. The long-term Re/Os enrichment of this and similar mantle sources, relative to chondritic upper mantle, may reflect 1. (1) incorporation of recycled oceanic crust into the source more than 1 Ga ago, 2. (2) derivation from a mantle plume that originated at the outer core-lower mantle interface, or 3. (3) persistence of primordial stratification of rhenium and osmium in the mantle.

Demise of the Siberian Plume: Paleogeographic and Paleotectonic Reconstruction from the Prevolcanic and Volcanic Record, North-Central Siberia

The Siberian flood basalts are underlain almost everywhere by terrigenous, coal-bearing sedimentary rocks of the Tungusskaya Series, which is Middle Carboniferous to Late Permian in age and commonly ranges in thickness from 100-150 m to 1400 m. Systematic studies of paleogeographic and paleotectonic conditions during Tungusskaya Series accumulation indicate that its deposition was accompanied by well-balanced subsidence throughout the area occupied by well-developed flood-basalt sequences. The surrounding territories, which experienced denudation and fed this accumulation, subsequently experienced little or no flood-basalt activity. Pronounced inheritance is observed in the evolution of the areas of accumulation and denudation, with no reorganization in the Late Permian that can be ascribed to the influence of a mantle plume. Moderate erosion (from tens to several hundreds of meters), with some local uplifts and local, complex folding, have been observed at the sedimentary/volcanic interface, but these up...

Petrogenesis of the flood-basalt sequence at Noril'sk, North Central Siberia

The 3500-m-thick sequence of volcanic rocks at Norilsk, formed during a brief interval (∼1 m.y.) at the Permian/Triassic time boundary (∼251 Ma), represents the earliest part of the ∼6500-m-thick sequence presently ascribed to the Siberian flood-basalt province. It is composed of picritic and basaltic lavas of both low-Ti and high-Ti parentage. Extensive geological, geochemical, and isotopic study of the lava sequence and related intrusions allows detailed reconstruction of its petrogenesis. Various crustal-related processes-fractionation, crustal contamination, sulfide separation, and magma mixing-participated in the formation of the lavas. The geochemical and isotopic characteristics indicative of these processes, as well as mantle-related signatures of lava compositions, are discussed. Based on these characteristics, detailed interpretations of lava genesis and evolution throughout the Norilsk sequence are presented. Eight varieties of lavas are recognized to be primitive, similar in composition to p...

Osmium and neodymium isotopic constraints on the temporal and spatial evolution of Siberian flood basalt sources

High-Mg volcanic rocks from the ca. 250 Ma old Siberian Flood Basalt Province (SFBP) were analyzed for their osmium and neodymium isotopic compositions in order to help to constrain source characteristics as the system evolved. Picrites from the Gudchikhinsky suite, the oldest rocks examined, have γOs of +5.3 to +6.1 and ϵNd of +3.7 to +4.0. The osmium and neodymium isotopic compositions of these rocks are similar to some modern ocean-island basalts (OIB), consistent with their derivation from a mantle plume, and show little evidence for interaction with either subcontinental lithospheric mantle (SCLM), or the Precambrian Siberian craton through which the parental melts passed. Picrites from the stratigraphically higher Tuklonsky suite have similar γOs of +3.4 to +6.5, but ϵNd Of −0.9 to −2.6. The similar γOs but lower ϵNd for the Tuklonsky picrites as compared with the Gudchikhinsky picrites suggest that some magmas from the same OIB-type, mantle source were contaminated by lithospheric components. The osmium isotopic composition of the Tuklonsky picrites was not significantly affected by this interaction, possibly because Os concentrations in the magmas were substantially greater than those in the contaminant. A differentiated ankaramite flow, associated with the top of the stratigraphically higher Morongovsky suite, has γOs of +9.8 to +10.2 and ϵNd of +1.3 to +1.4. The higher γOs may indicate that the plume source was heterogeneous with respect to osmium isotopic composition, consistent with osmium isotopic measurements in rocks from other plume sources. In contrast to these rocks, Mg-rich, alkaline rocks (meymechites) from the Guli area that erupted much nearer the end of the flood-basalt event have γOs of −1.2 to −2.6 and ϵNd of +3.7 to +4.9. These rocks were probably produced by low degrees of partial melting of mantle after the main stages of flood-basalt production. The relatively low γOs and high ϵNd for the meymechites, together with a variety of trace-element characteristics, are most consistent with derivation from a mixed source—one that included both the 0113-type source that fed the majority of the flood-basalt system and a major component from the SCLM underlying the Siberian craton. These results, taken together with earlier investigations of the Norilsk-type ore-bearing intrusions, suggest that much of the SFBP consists dominantly of plume-derived material, until relatively late in the magmatic event, when the SCLM became a significant source of material.

Results of New Field and Geochemical Studies of the Volcanic and Intrusive Rocks of the Maymecha-Kotuy Area, Siberian Flood-Basalt Province, Russia

The Maymecha-Kotuy area, comprising ∼70,000 km2 of the northern part of the Siberian flood-basalt province, is of unusual interest because it appears to be the only such province in the world where high-Ti, alkaline-ultramafic rocks with associated carbonatites predominate over basaltic extrusive and intrusive rocks. New field and geochemical studies of the igneous rocks of this area were initiated with the goals of (1) correlating them with the magmatic formations of the well-studied Norilsk area and (2) reconstructing the entire magmatic evolution of Siberian flood-basalt volcanism. This report presents the first complete stratigraphic section for the volcanic sequence of the Maymecha River Basin (the most extensive in the Maymecha-Kotuy area), based on flow-by-flow mapping and sample collection. The geochemical and lithologic characteristics of the volcanic and intrusive rocks are thoroughly documented and show an unusually broad range in composition–e.g., SiO2 and MgO contents range from 40 to 70 and...