Siberian Flood Basalt Province Research Articles

Late Triassic (Carnian) lamproites from Noril'sk, polar Siberia: Evidence for melting of the recycled Archean crust and the question of lamproite source for some placer diamond deposits of the Siberian Craton

Two typical lamproitic dykes were found in Noril'sk region of the north-western Siberian Craton, which according to mineralogical, geochemical and isotopic criteria belong to anorogenic, non-diamondiferous type of lamproites. According to the geologic relationships, they cut through the Noril'sk-1 intrusion of the Siberian flood basalt province and thus are younger than ~251Ma. 40Ar/39Ar dating of the two dykes yielded ages of 235.24±0.19Ma and 233.96±0.19Ma, showing that they were emplaced in Carnian of the Late Triassic, about 16Ma after the flood basalt event. There are some indications that there were multiple lamproitic dyke emplacements, including probably emplacement of diamondiferous lamproites, which produced Carnian-age diamond-rich placer deposits in other parts of the Siberian Craton and in adjacent regions. Lead isotope modelling shows that the source of the studied lamproites was formed with participation of recycled crust, which underwent modification of its U/Pb ratio as early as 2.5Ga. However, the exact mechanism of the recycling cannot be deciphered now. It could be either through delamination of the cratonic crust or subduction of a mix of ancient terrigenous sediments into the mantle transition zone.

The deep water cycle and flood basalt volcanism

Experimental data combined with numerical simulations suggest that fast-subducting oceanic slabs are cold enough to carry a significant amount of H2O into the deep mantle in antigorite; with increasing depth, this mineral undergoes solid–solid transitions to phase A and then to phase E and/or wadsleyite. Ice VII and clathrate hydrates can also be stable under pressure and temperature (PT) conditions of cold slabs and represent other potential phases for water transport into the deep mantle. Cold slabs are expected to stagnate in the mantle transition zone. With time, they are heated to the temperature of the ambient transition zone and release excess water as a H2O-bearing fluid. This may cause voluminous melting of overlying upper mantle rocks. If such a process operates in nature, magmas geochemically similar to island-arc basalts are expected to appear in places relatively remote from active arcs at the time of their emplacement. Dolerites of the southeastern margin of the Siberian flood basalt province, located about 700 km from suggested trench, were probably associated with the fast (cold) subduction of the Mongolia–Okhotsk oceanic slab, and originated by the dehydration of the stagnant slab in the transition zone. We show that the influence of the subduction-related deep water cycle on Siberian flood basalt magmatism gradually diminished with increasing distance from the subduction zone. Thus, the unique size and volume of the Siberian flood basalt province could have originated due to long-term underflow beneath Siberia with or without the existence of a lower mantle plume.

Phanerozoic within-plate magmatism of North Asia: Absolute paleogeographic reconstructions of the African large low-shear-velocity province

The phanerozoic within-plate magmatism of Siberia is reviewed. The large igneous provinces (LIPs) consecutively arising in the Siberian Craton are outlined: the Altai-Sayan LIP, which operated most actively 400–375 Ma ago, the Vilyui LIP, which was formed from the Middle Devonian to the Early Carboniferous, included; the Barguzin-Vitim LIP (305–275 Ma); the Late Paleozoic Rift System of Central Asia (318–250 Ma); the Siberian flood basalt (trap) province and the West Siberian rift system (250–247 Ma); and the East Mongolian-West Transbaikal LIP (230–195 Ma), as well as a number of Late-Mesozoic and Cenozoic rift zones and autonomous volcanic fields formed over the last 160 Ma. The trace-element and isotopic characteristics of the igneous rocks of the above provinces are reviewed; their mantle origin is substantiated and the prevalence of PREMA, EM2, and EM1 mantle magma sources are shown. The paleogeographic reconstructions based on paleomagnetic data assume that the Iceland hot spot was situated beneath the Siberian flood basalts 250 Ma ago and that the mantle plumes retained a relatively stable position irrespective of the movements of the lithospheric plates. At present, the Iceland hot spot occurs near the northern boundary of the African large low shear velocity province (LLSVP). It is suggested that the within-plate Phanerozoic magmatism of Siberia was related to the drift of the continent above the hot spots of the African LLSVP.

Petrology, sedimentology, geochemistry, and absolute age of Triassic volcanosedimentary rocks from the southwest of the West Siberian geosyneclise ( Kurgan Region)

In this study we report new Ar-Ar ages for basalt (261.0 ± 15, 249.4 ± 3.0, 247.0 ± 3.1 Ma) and rhyolite (246.9 ± 2.6 Ma) samples collected from the Triassic volcanosedimentary sequence, which is one of the remarkably complete and structurally complex sections in the southwest of the West Siberian geosyneclise (WSG) (East Kurganskaya-42 (EK-42) and East Kurganskaya-43 (EK-43) wells), largely comprising basalts and conformable layers of rhyolites, acid tuffs, limestones, and silty mudstones. It was shown that basalts, which constitute most of the section, are geochemically similar to medium-K tholeiitic and calc-alkaline basalts in the southern extension of the Koltogory–Urengoi rift (Nikol’skaya-1 well). Rhyodacites and rhyolites from the section of interest belong to the potassic to potassic-sodic series of normal alkalinity. The trace element compositions of the rhyolites and host basalts suggest fractionation from primary magmas during formation of felsic rocks. No low-K sheet-flow basalts have been encountered in the sections penetrated by the three wells (EK-42, EK-43, and Nikol’skaya-1), as opposed to sections in the northern part of the Koltogory–Urengoi rift (TSD-6 well) and the Siberian flood basalt province. Despite their small thickness, the sedimentary units provide evidence for different depositional settings during the evolution of a volcanosedimentary basin. Similar to the Koltogory–Urengoi rift, deposition of the Presnogor’kovsk graben fill indicates intermittent marine conditions with accumulation of hyaloclastites, and fine, calcareous, silt- and clay-rich carbonaceous oozes associated with a background sedimentation, which contain abundant marine fauna, pyrite and rare distal turbidites with glauconites. The geochemical features of these sedimentary rocks were largely defined by a bimodal syndepositional volcanism. During the Early Triassic, a system of rifts and grabens that was developed within the continental plateau over much of the WSG allowed the intermittent ingress of sea-water from the north (in present coordinates) far southwards and southwestwards up to the latitude of Omsk and Kurgan. As a result of extensive erosion of the continental plateau, the packages of low-K sheet-flow basalts were removed from the upper part of the sections of deep wells, which became dominated by the eroded roots of the West Siberian traps (subalkaline and alkaline rift basalts).

Siberian flood basalt magmatism and Mongolia-Okhotsk slab dehydration

AbstractExperimental data combined with numerical calculations suggest that fast subducting slabs are cold enough to carry into the deep mantle a significant portion of the water in antigorite, which transforms with increasing depth to phase A and then to phase E and/or wadsleyite by solid-solid phase transition. Clathrate hydrates and ice VII are also stable at PT conditions of cold slabs and represent other potential phases for water transport into the deep mantle. Some cold slabs are expected to deflect while crossing the 410 km and stagnate in transition zone being unable to penetrate through 660 km discontinuity. In this way slabs can move a long way beneath continents after long-lived subduction. With time, the stagnant slabs are heated to the temperature of the ambient transition zone and release free H~2~O-bearing fluid. Combining with transition zone water filter model this may cause voluminous melting of overlying upper mantle rocks. If such process operates in nature, magmas geochemically similar to island-arc magmas are expected to appear in places relatively remote from active arcs at the time of their emplacement. Dolerites of the south-eastern margin of the Siberian flood basalt province, located about 700 km from suggested trench, were probably associated with fast subduction of the Mongolia-Okhotsk slab and originated by dehydration of the stagnant slab in the transition zone. We show that influence of the subduction-related deep water cycle on Siberian flood basalt magmatism gradually reduced with increasing distance from the subduction zone.

A Pb isotope investigation of the Guli massif, Maymecha-Kotuy alkaline-ultramafic complex, Siberian flood basalt province, Polar Siberia

Covering a vast area of the northern Siberian platform are the Siberian flood basalts (SFB), which make up one of the world’s largest magmatic provinces. Along the northeastern margin of the SFB province lies the Maymecha-Kotuy alkaline-ultramafic complex, consisting of a large volume of alkaline lavas, numerous dykes, and the Guli massif together with numerous other, smaller alkaline plutons. The genetic link between the SFB and the Maymecha-Kotuy complex continues to be a subject of active debate. Although the rocks in both units have essentially the same age close to the Permian-Triassic boundary, questions remain as to the relative order of emplacement and the contributing source materials of each lithology. This study builds upon earlier petrologic, geochemical, and isotopic work to further an understanding of the relationship between SFB and alkaline rocks.

Sediment source strength, transport pathways and accumulation patterns on the Siberian–Arctic's Chukchi and Laptev shelves

In this study, we estimate sediment source strength and determine sediment transport/accumulation patterns on the Chukchi and Laptev shelves for lithogenic material from several key source regions. In the western Laptev Sea, sediments from the Siberian flood basalt province account for <20% of surface sediments, while detritus from the eastern Laptev's Lena and Yana Rivers constitutes as much as 40% of seafloor sediments. Eastern Laptev Sea sediments similarly reflect inputs from multiple sources; here, however, local inputs from the Lena and Yana Rivers are most prevalent. Chukchi Sea sediments are more homogeneous in composition, with sediments originating from the Okhotsk-Chukotsk volcanic belt and Bering Strait inflow comprising over 60% of surface sediments except near Wrangel Island and along sections of the Siberian coast. In the Chukchi portion of our study area, the dominant sediment dispersal pathways are from the Bering Strait region to the north and from Long Strait to the east, into the central Chukchi Sea. There appears to be comparatively minimal sediment transport parallel to the Siberian coast at our sample sites (>40 m water depth). In contrast, at our Laptev Sea sites (typically >20 m water depth) sediments supplied by the Lena and Yana Rivers move primarily eastward/northeastward parallel to the coast, with relatively minimal transport to the west/northwest or north to the central shelf. Good correspondence between sediment accumulation patterns and currents on the Chukchi shelf indicates water circulation is an important sediment transport mechanism in this marginal sea. In the Laptev Sea, a combination of river outflow, cyclonic water circulation and the Siberian Coastal Current controls sediment distribution. We speculate that sediment ice rafting may also affect shelf sedimentation patterns through a combination of factors. Our findings have implications for the fate of particle reactive contaminants released to the Siberian continental margin.

A temporal link between the Emeishan large igneous province (SW China) and the end-Guadalupian mass extinction

Previous studies have suggested that there were two mass extinction events in the Late Permian: one that occurred at the Permo-Triassic (P/T) boundary (251 Ma) and a second, smaller mass extinction that occurred 5–8 Myr earlier at the end of the Guadalupian. Many workers have argued that there is a causal relationship between large-scale volcanic activity and mass extinctions. The major mass extinction event at the P/T boundary coincides with the outpouring of huge quantities of lava that formed the Siberian flood basalt province in Russia. Courtillot et al. [Earth Planet. Sci. Lett. 166 (1999) 177–195] and Wignall [Earth Sci. Rev. 53 (2001) 1–33] suggested that the earlier Late Permian mass extinction coincided with the eruption of the lavas that formed the Emeishan flood basalt (EFB) province in SW China. However, the age of eruption of the EFB lavas is poorly constrained. Using the Sensitive High-Resolution Ion Microprobe to analyze zircons, we have established the age of the Xinjie intrusion, believed to be a feeder to the main phase of EFB volcanism, to be 259±3 Ma. Hence, the formation of the EFB is coincident with a proposed extinction event at 256–259 Ma. This result supports a temporal link between the Emeishan large igneous province and the end-Guadalupian mass extinction.

Spinels and Mg Ilmenites from the Noril'sk 1 and Talnakh Intrusions and Other Mafic Rocks of the Siberian Flood Basalt Province

Spinels and Mg Ilmenites from the Noril'sk 1 and Talnakh Intrusions and Other Mafic Rocks of the Siberian Flood Basalt Province

Spinels and Mg Ilmenites from the Noril'sk 1 and TalnakhIntrusions and Other Mafic Rocks of the Siberian Flood Basalt Province

Spinels from the mineralized Noril’sk 1 and Talnakh intrusions show a broad and distinctive compositional spectrum, from normal chromite to Ti-enriched aluminous magnetite. In the Talnakh intrusion, in particular, the range of spinel compositions extends across a compositional space where spinels are rare in other layered intrusions. Spinel compositions vary widely on a scale of individual thin sections and are influenced by the nature of the enclosing silicate phase. Spinels enclosed in olivines are less Ti enriched than those outside olivine grains. Compositional variance in the spinels is attributed to variable but generally advanced reaction between cumulus chromite grains and solidifying trapped intercumulus liquid. The advanced nature of the process is attributed to the open nature of the intrusions. Continuous, prolonged flow-through of hot magma enabled cumulates within the intrusions to remain above their solidus temperature for longer than would be normal for sills of comparable thickness. The barren Chibacheka River intrusion shows some of the same features but differs from Talnakh in that spinel-olivine pairs record equilibration temperatures higher than those at Talnakh. Temperatures recorded by spinel-olivine pairs at Talnakh are anomalously low for such a small intrusion, again indicating prolonged slow cooling consistent with a flow-through model for the sill. Spinel compositions support a hypothesized comagmatic relationship between the Chibacheka River intrusion and the Ti-, Cr-rich Gudchichinsky picritic basalts. Spinels in the Noril’sk and Talnakh intrusions imply a parent magma with a lower Cr/Al ratio than the Gudchichinsky picritic basalts. Spinels from the Siberian intrusions show a similar range in compositions to the feeder intrusions of the Karoo flood basalt province. Jinchuan spinels share the features of high Ti contents and anomalously low equilibration temperatures with the Noril’sk-Talnakh spinels. Mg ilmenites are a feature of the Siberian and Jinchuan intrusions. They are attributed to crystallization of ilmenite from evolved trapped intercumulus liquid buffered by surrounding Mg-rich cumulus phases. Their existence implies moderately but not unusually Ti-rich parent magmas, consistent with compositions of the Siberian Traps lavas.

Grain size separation and sediment mixing in Arctic Ocean sediments: evidence from the strontium isotope systematic

The 87 Rb/ 86 Sr and 87 Sr/ 86 Sr ratios of Laptev Sea sediments, of Arctic Ocean sediments and of suspended particulate matter (SPM) from Siberian rivers (Lena and Khatanga) form `pseudo-isochrons' due to grain-size separation processes which are referred to as `Lena Mixing Envelope' (LME) and as `Flood Basalt Envelope' (FBE). At the land–ocean transition the reduction of the particle velocity causes a deposition of coarser grained material and the contact with saline water enhances a precipitation of finer-grained material. The coarse-grained material is enriched in Sr showing less radiogenic 87 Sr/ 86 Sr ratios whereas fine grained material is depleted in Sr relative to Rb showing more radiogenic 87 Sr/ 86 Sr ratios. The experimentally determined spread of the 87 Rb/ 86 Sr and 87 Sr/ 86 Sr ratios as a function of grain size in one sediment sample is on the same order as the natural spread of the 87 Sr/ 86 Sr ratios observed in all samples from the Arctic Ocean. Chemical Index of Alteration (CIA) for the Lena river SPM tend to confirm previous observations that chemical alteration is negligible in the Arctic environment. Thus, these `pseudo-isochrons' reflect an average age and the average isotope composition in the river drainage area. Calculated apparent ages from the FBE reflect the age of the Siberian flood basalt of about 220 Ma and the initial ratio of 0.707(1) reflects their mantle origin. The age calculated from the LME of about 125 Ma reflects accidentally the Jurassic and Cretaceous age of the sediments drained by the Lena river and the initial ratio of 0.714(1) reflects the crustal origin of their source rocks. Comparison of geographical locations reveals that all samples from the eastern Laptev Sea (east of 120°E) fall along the LME whereas all samples from the western Laptev Sea (west of 120°E) fall between LME and FBE. Mixing calculations based on 143 Nd/ 144 Nd measurements, not influenced by grain size, show that about 75% of the western Laptev Sea sediments originate from the Lena drainage area whereas about 25% of the sediments are delivered from the Siberian flood basalt province. Sediments from the central Arctic Ocean are isotopically related to the Lena drainage area and the Siberian flood basalt province. However, sediments from the Arctic Ocean margins close to Novaya Semlya, Greenland, the Fram Strait and Svalbard originate from sources not yet identified.

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 Noril'sk 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 from <1 to 38 wt%, respectively. New geochemical data, considered together with earlier paleomagnetic data, indicate that as much as 3000 m of the Maymecha-Kotuy sequence consists of lavas younger than those preserved near Noril'sk. Thus, the combined thickness of the lavas and tuffs that constitute the Siberian flood-basalt province is estimated as ∼6500 m. As at Noril'sk, the volcanic sequence lies with unconformity on sedimentary rocks of the Tungusskaya Series. At Maymecha, the sequence begins with a major, basaltic-tuff unit and an overlying suite of low-Ti, tholeiitic basalts, but the magmatism then became more diverse, with several alternations of low-Ti and high-Ti lavas, and evolved to eruption of high-Ti trachybasalts, melanephelinites, trachyandesites, trachytes, trachydacites, and trachyrhyodacites. The uppermost ∼1400 m of the sequence consists of high-Ti, limburgitic and picritic lavas, as well as Mg-rich (23 to 38 wt%), high-Ti, meymechitic lavas, apparently unique in the world. Graphic evidence of the volcanic nature of the meymechites is presented here for the first time. The numerous dikes and several sills of the Maymecha River Basin are subdivided into seven types, which can be geochemically related to the volcanic sequence and in some cases can be reliably correlated with magmatic rocks of the Noril'sk area. Preliminary indications are that the complex body commonly referred to as the Guli intrusion is, in fact, an intrusive-volcanic complex occupying ∼2000 km2, in which a large, laccolithic intrusive mass of dunitic to peridotitic rocks was breached by a central volcanic edifice from which flowed ankaratrite and picritic ankaratrite lavas. Cutting the ankaratrites and picritic ankaratrites of the complex, but intimately related to it, is a variety of intrusive, alkaline rock types and two carbonatite bodies that may be related to a central feeder zone. We recognize four magma types as products of Siberian flood-basalt volcanism and refer to them herein as low-Ti-I, low-Ti-2, moderate-Ti, and high-Ti. In contrast to the Noril'sk area, where high-Ti magmas constitute <1 vol% of the igneous rocks, their proportion is ∼50 vol% in the Maymecha River Basin. These high-Ti rocks can be subdivided into trachybasaltic, melanephelinitic, and meymechitic rock series; clearly related to these series, judging from trace-element characteristics, are associated trachyandesitic, trachytic, and felsic lavas. In general, the trachyandesitic, trachytic, and felsic lavas appear to be related to the trachybasaltic series through fractionation, but some of the felsic tuffs display characteristics that suggest their relation to the melanephelinitie series. Previous investigators have concluded that meymechitic magma formed by low degrees of mantle-peridotite melting (<7%) at pressures corresponding to depths of 250 to 300 km. The distinctions between the trachybasaltic, melanephelinitic, and meymechitic rock series must relate to different sources and conditions of melting. It is remarkable that no geochemical signature of crustal contamination can be recognized for any of the high-Ti rocks, including the felsic lavas and tuffs. We conclude that the alternation of low-Ti and high-Ti magma types observed in the lower half of the volcanic sequence in the Maymecha River Basin resulted from repetitive expulsion from discrete mantle sources through independent plumbing systems.

40Ar– 39Ar age of Siberian basaltic volcanism

Five of six samples representing the entire stratigraphic sequence (∼3700 m thick) of basaltic flows in the Noril'sk section in the northwestern part of Siberian flood basalt province, Russia yield 40Ar– 39Ar plateau ages between 248.3±1.7 Ma and 246.9±2.5 Ma relative to an age of 520.4 Ma for the hornblende standard, MMhb-1. These results clarify some discrepancies in existing data and confirm that eruption of the bulk of Siberian basalts was close to 248 Ma and possibly lasted less than 2 Ma. Although the violence of this volcanism may have caused the massive extinctions at the Permo–Triassic boundary, the onset of changes across this boundary seems to have been distinctly earlier.

Petrogenesis of the Flood-Basalt Sequence at Noril'sk, North Central Siberia

The 3500-m-thick sequence of volcanic rocks at Noril'sk, 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 Noril'sk sequence are presented. Eight varieties of lavas are recognized to be primitive, similar in composition to primary mantle melts; they varied from low-Mg basalts to olivine tholeiites or picrites, with normal tholeiites predominating. The primitive lavas are subdivided into four groups (magma types) on the basis of trace-element ratios (principally, Gd/Yb, Th/U, La/Yb, Ta/La, Ti/Sc, and V/Yb) and isotopic data. Three of the groups include both basaltic and picritic primitive lavas (with low-Mg basalts present in one of them), whereas the fourth group is represented exclusively by tholeiites. Distinctions among the groups cannot be related to degree of melting, and isotopic data indicate that none of the magma types could have formed by mixing or contamination of other types. Apparently, only differences in source composition and/or depth of melting can explain the magmatic diversity. This multitude of primitive magma types may be explained by melting in different layers of the upper mantle, which is complexly layered beneath Siberia to depths of 270 km. Moreover, no clear boundary between lithosphere and asthenosphere is evident in the deep seismic profile. A large-scale event is necessary to account for melting in different parts of the upper mantle and formation of the great volume of the Siberian flood basalts in ∼1 m.y. Extension, caused by ascent of a mantle plume, would provide a reasonable explanation, but no plume-related uplift is documented in north-central Siberia prior to, or during, eruption of the volcanic sequence.

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 γ Os of +5.3 to +6.1 and ϵ Nd 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 γ Os of +3.4 to +6.5, but ϵ Nd Of −0.9 to −2.6. The similar γ Os but lower ϵ Nd 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 γ Os of +9.8 to +10.2 and ϵ Nd of +1.3 to +1.4. The higher γ Os 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 γ Os of −1.2 to −2.6 and ϵ Nd 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 γ Os and high ϵ Nd 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 Noril'sk-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.

High- 3 He Plume Origin and Temporal-Spatial Evolution of the Siberian Flood Basalts

An olivine nephelinite from the lower part of a thick alkalic ultrabasic and mafic sequence of volcanic rocks of the northeastern part of the Siberian flood basalt province (SFBP) yielded a (40)Ar/(39)Ar plateau age of 253.3 +/- 2.6 million years, distinctly older than the main tholeiitic pulse of the SFBP at 250.0 million years. Olivine phenocrysts of this rock showed (3)He/(4)He ratios up to 12.7 times the atmospheric ratio; these values suggest a lower mantle plume origin. The neodymium and strontium isotopes, rare earth element concentration patterns, and cerium/lead ratios of the associated rocks were also consistent with their derivation from a near-chondritic, primitive plume. Geochemical data from the 250-million-year-old volcanic rocks higher up in the sequence indicate interaction of this high-(3)He SFBP plume with a suboceanic-type upper mantle beneath Siberia.

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

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 Noril'sk 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 Noril'sk.

Temporal Sr-, Nd- and Pb-isotopic variations in the Siberian flood basalts: Implications for the plume-source characteristics

The Sr-, Nd- and Pb-isotopic compositions of lavas from the 248-Ma-old Siberian Flood Basalt Province (SFBP) become increasingly uniform with height as the main phase of volcanic activity is approached. The late-stage lava sequences from the Putorana subprovince, which constitute approximately 90 vol% of the SFBP, require a homogeneous mantle as their source. Volcanostratigraphic and volume arguments imply that extreme isotopic compositions, displayed by the early lavas from Norilsk subprovince, are not the end-member compositions for the bulk of the SFBP. In 147/Sm 144Nd-initial ε Nd space, most of the Putorana basalts define a horizontal array with a majority clustering at (0.17, +1.8). In contrast, the Norilsk lavas show a positively correlated trend indicative of continental lithospheric contributions to the primary plume. Among the three isotopic systems considered, Nd shows the least effect of crustal contamination. We estimate an initial ε Nd value of +1.8 ± 0.7 (1 σ) for the uncontaminated Siberian plume, which points to a near-chondritic, slightly depleted lower mantle source. The Siberian basalts, which show an ε Nd of ⩾ 0, display a remarkably uniform Sr-isotopic composition with an average 87/Sr 86Sr of 0.7050 ± 3 (1 σ). However, in a ( 87/Sr 86Sr) i-ε Nd diagram these lavas are significantly displaced to the right-hand side of a mixing line between bulk earth and depleted mantle. We suggest that these basalts suffered selective contamination of Sr during ascent through the continental crust. This element-specific contamination, which may also be concluded from Nd—Pb and Sr—Pb isotopic variations in the basalts, is at a maximum for Pb-isotopes and at a minimum for Nd. The Pb-isotopic ratios of the contaminated basalts indicate significant contributions from the lower crust with little or no input from the upper crust. The early Norilsk lavas define the extreme compositions and help constrain the initial Pb-isotopic composition of the plume source as 206/Pb 204Pb= 18.3, 207/Pb 204Pb= 15.5 and 208/Pb 204Pb= 38.0, which falls slightly to the right of the geochron at 248 Ma B.P.. We propose a two-stage growth history of Pb for the plume source: An early core-mantle differentiation stage, with a μ value of 0.21 for the first 100–150 m.y. after the Earth formed, was followed in the second stage by an increase in the μ value to about 9.2 for the rest of Earth's evolution.

NdSr isotopes, petrochemistry, and origin of the Siberian flood basalts, USSR

The Siberian Flood Basalt Province (SFBP) of Permo-Triassic age is one of the largest flood basalt provinces with an estimated area of exposure of 337,000 km 2, average thickness of 1 km, and a magma volume of 337,000 km 3. Forty-seven basaltic rocks from two main subprovinces, Norilsk (5–10% of area, thickness up to 3 km) and Putorana (90–95% of area, thickness of more than 2 km), were selected, on the basis of petrography and volcano-stratigraphic relation, for major-element analysis. Twentysix of these basalts, twelve from Norilsk and fourteen from Putorana, were analyzed for Nd- and Srisotopic compositions. The Norilsk and Putorana basalts show some contrasting behavior in terms of the ratios of the highly incompatible elements of Ti, P, and K as a function of their Mg ∗. The Norilsk basalts are more variable, suggesting the role of fractional crystallization-assimilation in their evolution. In contrast, the Putorana basalts show remarkable uniformity in their bulk chemical compositions. There is no overall apparent correlation between the Mg ∗ and the initial ϵ Nd for the basalts. Most of the Putorana and some Norilsk lavas show a narrow range of initial ϵ Nd between 0 and +2.5 at Mg ∗ = 0.53 , suggesting that the bulk of the SFBP was derived from a mantle-source with near-chondritic Ndisotopic signature. Our data and six previously published analyses define a hyperbolic array near the mantle correlation line in 87 Sr 86 Sr(t) -ϵ Nd ( t) space. However, the majority (70%) of the volumetrically significant Putorana basalts fall in the ranges of 0 to +2.5 and 0.7046 to 0.7052 for the initial ϵ Nd and 87 Sr 86 Sr , respectively. In Nd- and Sr-isotopic space, most of the Siberian basalts also fall within the field defined by the ocean island basalts data, implying common mantle sources. It is concluded that the SFBP originated by hotspot volcanism due to the rise of a large and relatively primitive lower mantle-derived plume beneath the Siberian continent.