Home – Home – Jornals – Russian Geology and Geophysics 2005 number 9

Studies of the Emeishan and Siberian trap provinces have demonstrated abundance of basalt series and giant volumes of lavas and sills (~16 · 10⁶ km³) formed for a very short period of the major step of volcanism. The obtained data permit a model for superplumes with three steps of their formation: early (picrites and alkali basalts), major (tholeiite plateau basalts), and final (ultrabasic and alkaline intrusions). These steps reflect the evolution of a superplume from several independent plumes until the formation of thick lenses of mantle melts at the bottom of the lithosphere and, finally, plumes of differentiated mantle melts. Synchronous syenite-granite intrusions and bimodal volcanic series abundant in the framing of the Siberian traps are the result of melting of the lower crust at depths of 65-70 km under the effect of plume melts. Superplumes promoted the synchronization of events of magmatism and geologic processes repeated at intervals of 30 and 120 Myr. A similar synchronization of global geological events is observed during the activity of superplumes 120, 250 and, possibly, 360 and 480 Ma. During the effusion and intrusion of (8-16) · 10⁶ km³ of volcanic material for a short interval of 0.6-2 Myr, huge volumes of CO₂, SO₂, and HF might have been released, capable to destroy the atmospheric system, to disturb the budget of oxygen-free oceanic water, and to lead to the mass extinction of living organisms at the Permian-Triassic boundary.

We present results of laboratory modeling of thermochemical plumes rising from the core-mantle boundary. Laboratory modeling of plume rise based on the laboratory experiments shows that a plume keeps the same diameter till its eruption to the surface if the melting temperature of the ambient material remains invariable but develops a mushroom-shaped head if impinges on a high-melting layer.
Using theoretical modeling, we obtained pressure distribution in a rising plume and estimated an excess of pressure in the melt beneath the plume roof over the lithostatic pressure. The critical thickness of above-plume massif at which eruption conduit forms is compared with the available estimates from geological data.
A high-melting layer can occur in the upper mantle between 400 and 650 km at a certain relation between the melting curve of natural peridotite KLB-1 and melt temperature at the plume roof. The structure of a plume impinging on a high-melting layer is predicted in the reported model.

Based on new data on melt inclusions in minerals, the physicochemical and geochemical parameters of plateau-basalt magmatic systems of the Siberian Platform and Ontong Java, Pacific, have been established. The studied melts are enriched in Fe, which differs them from the magmatic melts of mid-ocean ridges (MOR). A comparative analysis of data on inclusions showed a similarity of continental and oceanic plateau-basalt magmatic systems, differing considerably from those of MOR and within-plate oceanic islands. Crystallization of oceanic plateau basalts took place at lower temperatures and pressures as compared with such rocks of the Siberian Platform. The data on inclusions evidence that the melts of the Siberian Platform and Malaita Island underwent a serious evolution in contrast to magmas of the Nauru Basin with more stable geochemical parameters. The most fractionated low-temperature high-Fe magmas with elevated contents of trace and rare-earth elements have been revealed for Malaita Island (Ontong Java Plateau).

The Emeishan flood basalts (EFB), which occur in southwestern China, are tightly associated with the Paleo-Tethyan volcanism. In order to find out relationships between the EFB and Paleo-Tethyan volcanism, a comprehensive geochemical study of EFB, MORB and IAB was carried out. Based on geochemical features, EFB can be divided into five groups, in which HM (high-Mg basalts and picrites), HT1 (high-Ti and high-HFSE basalts) and LT1 (low-Ti and high-HFSE basalts) occur in the eastern district, and LT2 (low-Ti and low-HFSE basalts) and HT2 (high-Ti and low-HFSE) occur in the western and central districts. Geochemically, the western EFB show transition features between the eastern EFB and the Paleo-Tethyan MORB-IAB in the Jinshajian-Alaoshan zones. HFSE and REE tracing indicates that HT1 flooded from an aggregate magma layer derived from fraction melting of a primitive mantle, and LT1 magmas had a major component derived from partial melting of subducted slabs. HT1, LT1 and HM with mg# varying from 82 to 32 covered an area of about 150,000 km² yield consistent Nd isotopic compositions from primitive to slight depleted mantles (ε _Nd ( T ) = 0...+3) and a mg#- ε _Nd ( T ) inverse correlation. Their (²⁰⁶Pb/²⁰⁴Pb)-(²⁰⁷ Pb/²⁰⁴Pb) data can fit NHLR. These isotopic features cannot be explained by mixing between a strong depleted upper mantle and subducted material, which implies that there existed plume magma components from the primitive lower mantle. Whereas Nd and Pb isotopic compositions for LT2, HT2 and MORB-IAB related to the Tethyan opening and subduction indicate that the magmas were derived from random mixing between strong depleted upper mantle, primitive mantle and recycled continental crust components. Sr isotopic compositions for all rocks demonstrate mainly sea-water influence from subducted oceanic crusts. Therefore, the geochemical data from the Permian EFB and MORB-IAB recorded the whole process from the Tethyan plate subduction through asthenosphere upwelling to development of the Emeishan mantle plume.

This paper is aimed at characterizing the Permian-Triassic mafic-ultramafic magmatism of the southeastern segment of Eurasia. Given a sinistral strike-slip fault in the Song Hong zone (ASRR) that occurred at about 40-25 Ma, the ultramafic-mafic complexes of the Song Da zone and intrusive bodies of the Cao Bang complex are considered the southwestern flank and southeastern continuation of the Emeishan traps, respectively. It has been shown that petrochemical and geochemical characteristics of the products of the Permian-Triassic ultramafic-mafic magmatism are controlled by effects of the interaction of a mantle-derived plume with blocks characterized by different structure of the lithosphere. It is supposed for the Song Da rift zone that heavily depleted mantle is in places beneath the continental lithosphere. Geochemical features of the picrites from northeastern Vietnam (Cao Bang complex) indicate that the subduction-related lithosphere mantle is present there. High contents of PGE in picritoids and basaltoids of the Permian-Triassic step in southeastern Asia are bearing on the plume originated in the lower mantle.

Comprehensive studies with the use of geological, petrologo-geochemical, and thermobarogeochemical methods have revealed specific features of the formation of Paleoasian ocean structures. Because of intense secondary alteration of rocks, it is especially important to analyze melt inclusions, which have retained the direct signature of ancient magmatic systems. Data on inclusions testify to the formation of boninites by crystallization of high-temperature (1330-1150

The Early Paleoproterozoic (2.5-2.3 Ga) Baltic large igneous province (BLIP) of siliceous high-magnesium series (SHMS) is localized in the eastern Baltic Shield. Within the Kola and Karelian cratons, the BLIP is made up of volcanics (from low-Ti picrites and basalts through andesites to dacites and rhyolites) in rift structures, gabbronorite dike swarms, and large layered mafic-ultramafic intrusions. Small synkinematic mafic-ultramafic intrusions are abundant in the Belomorian mobile belt. All this suggests the existence of mantle superplume beneath the region at that time. The major difference of the BLIP from Phanerozoic large igneous provinces is the composition of magmatic melts: In the BLIP, they were close in geochemistry to subduction-related magmas but were generated in within-plate environment. This suggests that the SHMS magmas were generated as a result of the melting of highly depleted mantle source and the subsequent large-scale assimilation of the lower-crust matter by primary mantle-derived high-temperature magmas during their ascent to the surface. Magmatic systems of the BLIP include four activity levels: (1) head of local plumes, where primary mantle-derived melts were generated; (2) lower crust, where mantle-derived magmas were contaminated by crustal matter and transformed into SHMS magmas; (3) upper crust, where transitional magma chambers (preserved as large layered intrusions) were localized; and (4) volcanic plateaus at the surface and subvolcanic sills beneath them.

Several Late Mesoproterozoic (0.9-1.1 Ga) tholeiitic dyke swarms crop out along the eastern border of the Sao Francisco Craton (SFC, Brazil), inside the Neoproterozoic Aracuai mobile belt (Diamantina and Conceicao do Mato Dentro) and in Archean-Paleoproterozoic terrains (Belo Horizonte, Olivenca-Itabuna and Salvador). These last dykes were virtually unaffected by the Brasilian metamorphic events and show well-preserved magmatic textures and phases.
All the dykes are characterized by high (>2 wt%) and low (<2 wt%) TiO₂ contents and show similar major element compositions, while important differences concern incompatible element contents and ratios. The southern SFC dykes (Diamantina, Conceicao do Mato Dentro and Belo Horizonte) show Nb/Y and Zr/Y ratios up to ~6 times higher than the northern ones (Olivenca-Itabuna and Salvador) implying variable residual source garnet and then heterogeneous mantle sources, due to the tholeiitic nature of these dykes. Furthermore, geochemical features as La/Nb in the ~0.7-1.9 range and Sr-Nd isotopic compositions recall both OIB (

Assessments of generalized mass-extinction causality scenarios should be made on a statistical - as opposed to an anecdotal or speculative - basis that takes explicit account of the principle that multiple occurrences of valid causal mechanisms should produce demonstrably similar effects. Previous Monte Carlo analyses of single-cause scenarios have suggested that only the time series of subareal LIP volcanic events exhibit sufficient similarity with the time series of local extinction-intensity peaks to support a causal link. Here, this Monte Carlo approach is extended to support consideration of multiple-cause extinction scenarios and applied to the consideration of links between the multiple effects of sea-level regression, bolide impact, and LIP volcanism and the extinction-intensity time series. Results indicate that all multiple-cause scenarios-including those that do not take account of LIP volcanism-exhibit sufficient numbers of positive matches with the extinction-intensity time series to be regarded as potentially valid causes for the overall pattern. Although these results do not alter the previous single-cause results, or the preference for LIP volcanism as the single most likely cause of the extinction-intensity peak distribution, they do underscore the important roles these mechanisms likely played in the formation of these biotic events (e.g., by accentuating or mitigating their magnitudes).