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41.
Main part of the Siberian Traps Large Igneous Province was formed in a short time-span at the Permo-Triassic boundary c. 250 Ma. New 40Ar/39Ar dating results for the Usol'skii dolerite sill in south-eastern part of the province indicate its probable emplacement c. 6 Myr after the main Permo-Triassic magmatic phase. Compilation of the published 40Ar/39Ar and U-Pb ages implies that basaltic and related magmatism lasted in total as long as 22–26 Myr. Therefore, similar to other large igneous provinces, magmatism of the Siberian Traps combined voluminous short-lived and less prominent long-lived events.  相似文献   
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43.
Siberian Traps Large Igneous Province (STLIP) is one of the most voluminous volcanic provinces on Earth. The dominant erupted rocks are low-Ti basalts, which make up 80% by volume of the classical Noril’sk lava sequence. In the west Siberian basin and Maymecha-Kotuy area, the low-Ti basalts make up about 99% and 50% by volume, respectively. Dolerite sills in the Angara-Taseevskaya Syncline at the southeastern STLIP exhibit trace element patterns and Sr isotope ratios typical of the low-Ti basalts of the Noril’sk sequence. The most Mg-rich (MgO 9.5–11 wt%) and hence least differentiated dolerites are characterized by trace element patterns with Ta-Nb depletion, low Ce/Pb and high Sr/Pr. These trace element features are similar to water-saturated, mantle wedge-derived island arc basalts. These imply an important role of subduction fluid-derived trace elements in the source of melting beneath the Angara-Taseevskaya Syncline and other regions of the STLIP. Less magnesium rocks (MgO 3.8–6.1 wt%) with less prominent Ta-Nb depletion, higher Ce/Pb and lower Sr/Pr could be produced via olivine-plagioclase fractionation of primary high-magnesium melts.  相似文献   
44.
Classical models of lithosphere thinning predict deep synrift basins covered by wider and thinner post‐rift deposits. However, synextensional uplift and/or erosion of the crust are widely documented in nature (e.g. the Base Cretaceous unconformity of the NE Atlantic), and generally the post‐rift deposits dominate basins fills. Accordingly, several basin models focus on this discrepancy between observations and the classical approach. These models either involve differential thinning, where the mantle thins more than the crust thereby increasing average temperature of the lithosphere, or focus on the effect of metamorphic reactions, showing that such reactions decrease the density of lithospheric rocks. Both approaches result in less synrift subsidence and increased post‐rift subsidence. The synextensional uplift in these two approaches happens only for special cases, that is for a case of initially thin crust, specific mineral assemblage of the lithospheric mantle or extensive differential thinning of the lithosphere. Here, we analyse the effects of shear heating and tectonic underpressure on the evolution of sedimentary basins. In simple 1D models, we test the implications of various mechanisms in regard to uplift, subsidence, density variations and thermal history. Our numerical experiments show that tectonic underpressure during lithospheric thinning combined with pressure‐dependent density is a widely applicable mechanism for synextensional uplift. Mineral phase transitions in the subcrustal lithosphere amplify the effect of underpressure and may result in more than 1 km of synextensional erosion. Additional heat from shear heating, especially combined with mineral phase transitions and differential thinning of the lithosphere, greatly decreases the amount of synrift deposits.  相似文献   
45.
A nearly complete frozen mummy of a woolly rhinoceros (Coelodonta antiquitatis Blum., 1799) was discovered in a gold mine on the lower reaches of the Kolyma River, north–eastern Siberia. This is the first find of the whole body of woolly rhino in permafrost. A large part of the mummified body was preserved, including the left part of the body, covered by skin, including skin of the head and ear, fore and hind legs. The skull with 2 horns and the lower jaw were also preserved. Most of the internal organs were lost, except the intestines, stomach, and their contents. A rib fragment from this individual was dated by AMS-radiocarbon method to 39,140 ± 390 years BP (OxA-18755). Spore and pollen analyses of the stomach contents indicate that grasses and sagebrushes formed the main part of the diet of C. antiquitatis in this region of Arctic Siberia.  相似文献   
46.
We determined the concentrations of platinum‐group elements (PGE) and rhenium in granular spinel lherzolites entrained as xenoliths in the Late Cenozoic volcanic rocks of the East Sayan within the Sarkhoi palaeo‐arc block of the Tuva‐Mongolian massif. Major element, PGE and rhenium variations in the East Sayan xenoliths can be explained by impregnation of up to 15% of arc‐type melt into initially depleted mantle harzburgite. Such a refertilization process probably took place in the Middle Neoproterozoic, when the Tuva‐Mongolian massif was in a subduction environment. East Sayan xenoliths show close similarities to Vitim xenoliths, whose host basalts erupted within an off‐cratonic crustal block. Both East Sayan and Vitim xenoliths are different from cratonic and circum‐cratonic peridotite xenoliths of worldwide localities.  相似文献   
47.
Palaeoenvironmental reconstructions from peat are strongly focused on ombrotrophic mires, but this study demonstrates that eutrophic mires can also be used. A multi-proxy approach was applied to a eutrophic mire on a floodplain terrace in the southern taiga of West Siberia. The results of the reconstruction were considered in the wide geographic context of the surrounding regions, including Siberia and Central Asia. Different palaeoecological proxies (analysis of plant macrofossils, testate amoebae, oribatid mites, molluscs, peat humification, ash content and spectral characteristics of humic acids) were used in this study. The results of different proxies showed a high level of consistency among themselves, which allowed for a robust interpretation of Holocene mire development. Throughout the ~7800 years history of the mire, there was a high level of surface wetness. The presence of mineral matter in the peat between 7800 and 5100 cal. a BP indicates regular flooding caused by the intensive fluvial activity, apparently resulting from increased precipitation. This was followed by a trend towards a gradual decrease in surface wetness from conditions of high surface moisture (stagnant water) between 5100 and 3000 cal. a BP to present day conditions of moderate surface moisture with a water table slightly below the mire surface. This pattern is consistent with the well-documented long-term trend from palaeoecological records throughout the taiga and arctic zones in West Siberia and central arid Asia. Our data further support the idea that the westerlies were the dominant driver of climate for the southern taiga of West Siberia during the Middle to Late Holocene.  相似文献   
48.
We present a summary of late Paleoproterozoic to Neoproterozoic mafic magmatism in the Siberian craton, including recently published U–Pb and 40Ar–39Ar dates. These new precise ages suggest that at least some of the previously published K–Ar ages of Siberian mafic bodies should be ignored. The time–space geochronological chart, or the ‘barcode’ of mafic magmatic events shows significant differences between northern and southern Siberia. Both are characterized by ∼1900–1700 Ma magmatic events, but then there was an almost 1 Ga mafic magmatic ‘pause’ in south Siberia until ∼800 Ma. Meanwhile there are indications of multiple mafic magmatic events in North Siberia (Anabar shield and Olenek uplift) between ∼1600 and 1000 Ma. A series of magmatic events probably related to the breakup of Rodinia occurred in southern Siberia after ∼800 Ma. So far, there are no indications of late Neoproterozoic mafic magmatism in North Siberia. Ca. 1000–950 Ma mafic sills were reported from Meso- to Neo-Proterozoic sedimentary successions in the Sette-Daban area on the east side of the Siberian craton, but their tectonic setting is debated. Recent Ar–Ar dates of ∼1750 Ma for NW-trending dykes in the Aldan and Anabar shields, together with similar-age NNE-trending Baikal uplift dykes in south-eastern Siberia suggest the existence of a giant radial dyke swarm possibly related to a mantle plume centred in the Vilyui River area.  相似文献   
49.
The Phanerozoic history of mafic magmatism in the southern Siberian craton included three major events. The earliest event (~500 Ma) recorded in dolerite dikes occurred during accretion and collision at the early stage of the Central Asian orogen. Injection of mafic melts into the upper crust was possible in zones of diffuse extension within the southern Siberian craton which acted as an indenter. The Late Paleozoic event (~275 Ma) produced dikes that intruded in a setting of subduction-related extension at the back of the active continental margin of Siberia during closure of the Mongolia–Okhotsk ocean, as well as slightly older volcanics (290 Ma) in the Transbaikalian segment of the Central Asian orogen. Early Mesozoic magmatism in the southern Siberian craton resulted in numerous 240–250 Ma mafic intrusions in the Angara–Taseeva basin. The intrusions (Siberian traps) appeared as the subducting slab of the Mongolia–Okhotsk ocean interacted with a lower mantle plume. The post-Late Paleozoic ages of flood basalts (290–275 Ma) correspond to progressive northwestward (in present coordinates) motion of the slab beneath the southern craton margin which likely ceased after the slab had reached the zone of the Siberian superplume. Since its consolidation after the Early Mesozoic activity, the crust in the area has no longer experienced extension favorable for intrusion of basaltic magma.  相似文献   
50.
Due to dynamical friction stellar mass black holes and neutron stars are expected to form high-density cusps in the inner parsec of our Galaxy. These compact remnants, expected to number around 20 000, may be accreting cold dense gas present there, and give rise to potentially observable X-ray emission. Here we build a simple but detailed time-dependent model of such emission. The possibility that these accretion flows are radiatively inefficient is taken into account and brings in some uncertainty in the conclusions. Despite this uncertainty, we find that at least several X-ray sources of this nature should be detectable with Chandra at any one time. Turning this issue around, we also ask a question of what current observational constraints might be telling us about the total number of compact remnants. In our 'best guess' model, a cusp of ∼40 000 remnants overpredicts the number of discrete sources and the total X-ray luminosity of the inner parsec, and is hence ruled out. In the most radiatively inefficient scenario that we consider, the radiative efficiency is set to be as small as  ɛ= 10−5  . In this rather unlikely scenario, a cusp of ∼40 000 black holes would be allowed by the data, but several individual sources should still be visible. Future observations of the distribution and orbits of the cold ionized gas in the inner parsec of our Galaxy will put tighter constraints on the cusp of compact remnants.  相似文献   
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