Oxidation of the Kaapvaal lithospheric mantle driven by metasomatism

The oxidation state, reflected in the oxygen fugacity (fO₂), of the subcratonic lithospheric mantle is laterally and vertically heterogeneous. In the garnet stability field, the Kaapvaal lithospheric mantle becomes progressively more reducing with increasing depth from Δlog fO₂ FMQ-2 at 110 km to FMQ-4 at 210 km. Oxidation accompanying metasomatism has obscured this crystal-chemical controlled depth-fO₂ trend in the mantle beneath Kimberley, South Africa. Chondrite normalized REE patterns for garnets, preserve evidence of a range in metasomatic enrichment from mild metasomatism in harzburgites to extensive metasomatism by LREE-enriched fluids and melts with fairly unfractionated LREE/HREE ratios in phlogopite-bearing lherzolites. The metasomatized xenoliths record redox conditions extending up to Δlog fO₂ = FMQ, sufficiently oxidized that magnesite would be the stable host of carbon in the most metasomatized samples. The most oxidized lherzolites, those in or near the carbonate stability field, have the greatest modal abundance of phlogopite and clinopyroxene. Clinopyroxene is modally less abundant or absent in the most reduced peridotite samples. The infiltration of metasomatic fluids/melts into diamondiferous lithospheric mantle beneath the Kaapvaal craton converted reduced, anhydrous harzburgite into variably oxidized phlogopite-bearing lherzolite. Locally, portions of the lithospheric mantle were metasomatized and oxidized to an extent that conversion of diamond into carbonate should have occurred. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Nature and(in-)coherent metamorphic evolution of subducted continental crust in the Neoproterozoic accretionary collage of SW Mongolia

In continental subduction complexes minor volumes of high-pressure mafic rocks(eclogites) often co-exist with much more abundant felsic(granitic) and metasedimentary rocks, which are vital for resolving the origin and metamorphic evolution of subducted continental crust.In SW Mongolia, the Alag Khadny eclogite-bearing accretionary complex(AKC) is assumed to represent either a remnant of oceanic slab, or a continental margin,subducted in the Early Cambrian.Here we present geochronological, geochemical and petrological evidence of subduction records for the three major types of lithologies that host mafic eclogites, including Mesoproterozoic and Neoproterozoic granitic basement and overlying Neoproterozoic continental-margin sediments.Variably deformed, ferroan and peraluminous metagranitoids compose a major part of AKC and are interlayered with eclogites in its southern and eastern margins.They have geochemical features of post-collisional/intraplate high-K calc-alkaline granites.LA-ICP-MS U-Pb zircon geochronology of three distinct metagranite samples show uniform protolith crystallization ages of ca.0.96 Ga and uncertain re-crystallization in the Late Neoproterozoic or Early Paleozoic metamorphic event, whereas abundant zircon inheritance indicates older,Mesoproterozoic to Paleoproterozoic crustal substrate during granite generation.The existence of Mesoproterozoic crust is highlighted by finding of distinct metagranitoids with the U-Pb zircon crystallization age of ca.1.6 Ga.Hafnium isotope signatures(T_(DM)~C 2.88–1.85 Ga) of zircons from all lithologies preserved the evidence of reworked Neoarchean to Paleoproterozoic crust, similar to that of the Baidrag block(southern Mongolia), for both Mesoproterozoic and Neoproterozoic rocks.Regardless of the specific lithology, the rocks display indicators of high-pressure metamorphic re-equilibration, including garnet(X_(Ca)up to 0.65) + epidote +phengite(Si p.f.u.up to 3.56) ± rutile assemblage in metagranitoids, garnet + phengite(Si p.f.u.up to 3.42)in quartz-rich semi-pelites and garnet + phengite(Si p.f.u.up to 3.39) + medium-Mg chloritoid(X_(Mg)up to 0.25) + kyanite + rutile in metapelites.Corresponding P-T conditions recovered from different lithologies reveal incoherent subduction of rocks, which could be shallow for granitic basement(1.1–1.4 GPa and 600–670 ℃) and clastic metasediments(1.4–1.6 GPa, 570–620 ℃), but deeper for metapelites(2.1–2.3 GPa, 500–570 ℃).consistent with that of eclogites, The combined data show that the Alag Khadny complex represents a remnant of a rifted Mesoproterozoic to Neoproterozoic(ca.1.6–0.96 Ga) continental margin consequently metamorphosed under HP conditions during Late Neoproterozoic–Early Cambrian evolution of the southern Central Asian Orogenic Belt.Acquired P-T estimates imply that high-pressure metagranitoids and metasedimentary rocks equilibrated at different depths, but most likely shared a common subduction-related metamorphic evolution.     

40Ar/39Ar dating of Usol'skii sill in the south-eastern Siberian Traps Large Igneous Province: evidence for long-lived magmatism

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.     

Low-Ti melts from the southeastern Siberian Traps Large Igneous Province: Evidence for a water-rich mantle source?

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.     

Development of sedimentary basins: differential stretching,phase transitions,shear heating and tectonic pressure

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.     

Woolly rhino discovery in the lower Kolyma River

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.     

Platinum‐group elements and rhenium in mantle xenoliths from the East Sayan volcanic field (Siberia,Russia): evaluation of melt extraction and refertilization processes in lithospheric mantle of the Tuva‐Mongolian massif

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.     

Multi-proxy climate and environmental records from a Holocene eutrophic mire,southern taiga subzone,West Siberia

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.     

Proterozoic mafic magmatism in Siberian craton: An overview and implications for paleocontinental reconstruction

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.