Devonian volcanics in the Voronezh Crystalline Massif,East European Platform: Evolution of the melts and characteristics of crustal contamination

The rift system of the Dnieper–Donets trough (DDT) is the largest magmatic area in the East European Platform. Basalts of the Voronezh Crystalline Massif (VCM) are spatially constrained to the eastern shoulder of DDT and occur far away (at a distance of 150–200 km) from the rift axis. The rocks are hosted in the Paleoproterozoic Vorontsovskii terrane and are grouped in a few fields within an area of 200 × 100 km. Basalts at most of the fields were erupted at the boundary between mid- and late Frasnian time in the Late Devonian and can be studied exclusively in core material recovered by boreholes. Newly obtained mineralogical, geochemical, and isotopic-geochemical data show that the Devonian volcanic rocks in VCM are tholeiites (Bas) and basaltic andesites/andesites (ABas). The geological section was examined most exhaustively in the Novokhopersk area (Borehole 175). The bottom of the vertical section is made up of basaltic andesites and andesites (ABas) (thickness 34 m), which rest on an eroded surface of late Frasnian sandstones. The rocks are overlain by a thin (8 m thick) tholeiite sheet (Bas2), which gives way to ABas (13 m) upsection. The top portion of the vertical section is composed of tholeiites with petrography and geochemical evidence of crustal contamination (Bas1) (apparent thickness 5 m). Geochemical parameters of Bas (mg# 42–52 at SiO₂ 47–51 wt %) are typical of continental tholeiites. The rocks have (⁸⁷Sr/⁸⁶Sr)₀ = 0.7043–0.7048 and ε_Nd(372) = 2.1–3.5. ABas (mg# 28–31 at SiO₂ 52–60 wt %) are enriched in Y (48 ppm), and possess Nb/Nb* = 0.7–0.8 and high Zn/Cu = 1.9–2.3. The rocks have (⁸⁷Sr/⁸⁶Sr)₀ = 0.7034–0.7048 and Nd–ε_Nd(372) = 0.1. Some portions of Bas melts assimilated the upper crustal material, which was similar to Paleoproterozoic granites, and ABas are contaminated in the lower crust with derivatives of Early Cambrian alkaline mafic melts. Petrographic data and simulations of fractional crystallization show that olivine and high-Mg clinopyroxene were the first to crystallize from the melt. After this, clinopyroxene and plagioclase simultaneously crystallized at temperatures from 1070 to 1020°C in Bas and at 1040–900°C at f _O2 below QFM + 1 in ABas. The source of ABas was likely a network of hornblendite or amphibole pyroxenite veins in peridotite in the lithospheric mantle or amphibolized peridotite cumulate in an underplating zone; and Bas were derived from spinel peridotites of an asthenospheric diapir. The setting of the basalts relative to the DDT axis and the asymmetric zoning of magmatism in DDT (with kimberlites and other deep rocks constrained to the western shoulder and tholeiites occurring in the axial part of the rift and its eastern shoulder) can be explained by the model of an asymmetric rift structure with a translithospheric detachment gently dipping beneath VCM.     

Sheared peridotite xenolith from the V. Grib kimberlite pipe,Arkhangelsk Diamond Province,Russia: Texture,composition, and origin

The petrography and mineral composition of a mantle-derived garnet peridotite xenolith from the V. Grib kimberlite pipe (Arkhangelsk Diamond Province, Russia) was studied. Based on petrographic characteristics, the peridotite xenolith reflects a sheared peridotite. The sheared peridotite experienced a complex evolution with formation of three main mineral assemblages: (1) a relict harzburgite assemblage consist of olivine and orthopyroxene porphyroclasts and cores of garnet grains (Gar1) with sinusoidal rare earth elements (REE) chondrite C1 normalized patterns; (2) a neoblastic olivine and orthopyroxene assemblage; (3) the last assemblage associated with the formation of clinopyroxene and garnet marginal zones (Gar2). Major and trace element compositions of olivine, orthopyroxene, clinopyroxene and garnet indicate that both the neoblast and clinopyroxene-Gar2 mineral assemblages were in equilibrium with a high Fe-Ti carbonate-silicate metasomatic agent. The nature of the metasomatic agent was estimated based on high field strength elements (HFSE) composition of olivine neoblasts, the garnet-clinopyroxene equilibrium condition and calculated by REE-composition of Gar2 and clinopyroxene. All these evidences indicate that the agent was a high temperature carbonate-silicate melt that is geochemically linked to the formation of the protokimberlite melt.     

Genesis of ringwoodite during metamorphism induced by impact waves: Experimental data

Ringwoodite, a high-density olivine modification, was first synthesized by loading plagioclase-biotite-quartz schist containing garnet and staurolite by impact waves. Ringwoodite was identified in the impact-thermal aggregates that replaced biotite (together with a mineral corresponding to spinel in chemical composition and with a biotite residue). The physical parameters under which ringwoodite was synthesized in this experiment (P _imp ～ 20–30 GPa and T ～ 1060–1500°C) include a pressure approximately 1.5 times higher than that in static analogous experiments. The ringwoodite was formed via the regrouping of and the associated removal and addition of material, as follows from the development of ringwoodite after biotite, a mineral of principally different composition. Component migration was reliably confirmed by the microprobe mapping of the chemistries of the original and newly formed minerals, which makes the origin of the ringwoodite similar to the origin of diamond (togorite) in the Kara astrobleme, where the impact loading was up to ten times higher than the static pressure.     

Late Pleistocene to Holocene vegetation and climate changes in northwestern Chukotka (Far East Russia) deduced from lakes Ilirney and Rauchuagytgyn pollen records

This paper presents two new pollen records and quantitative climate reconstructions from northern Chukotka documenting environmental changes over the last 27.9 ka. Open tundra- and steppe-like habitats dominated between 27.9 and 18.7 cal. ka BP. Betula and Alnus shrubs might have grown in sheltered microhabitats but disappeared after 18.7 cal. ka BP. Although the climate was rather harsh, local herb-dominated communities supported herbivores as is evident by the presence of coprophilous spores in the sediments. The increase in Salix and Cyperaceae ~16.1 cal. ka BP suggests climate amelioration. Shrub Betula appeared ~15.9 cal. ka BP, and became dominant after ~15.52 cal. ka BP, whilst typical steppe communities drastically reduced. Very high presence of Botryococcus in the Lateglacial sediments reflects widespread shallow habitats, probably due to lake level increase. Shrub Alnus became common after ~13 cal. ka BP reflecting further climate amelioration. Simultaneously, herb communities gradually decreased in the vegetation reaching a minimum ~11.8 cal. ka BP. A gradual decrease of algae remains suggests a reduction of shallow-water habitats. Shrubby and graminoid tundra was dominant ~11.8–11.1 cal. ka BP, later Salix stands significantly decreased. The forest-tundra ecotone established in the Early Holocene, shortly after 11.1 cal. ka BP. Low contents of green algae in the Early Holocene sediments likely reflect deeper aquatic conditions. The most favourable climate conditions were between ~10.6 and 7 cal. ka BP. Vegetation became similar to the modern after ~7 cal. ka BP but Pinus pumila came to the Ilirney area at about 1.2 cal. ka BP. It is important to emphasize that the study area provided refugia for Betula and Alnus during MIS 2. It is also notable that our records do not reflect evidence of Younger Dryas cooling, which is inconsistent with some regional environmental records but in good accordance with some others.     

Postglacial history of East European boreal forests in the mid-Kama region,pre-Urals,Russia

The Ural Mountains are an important climatic and biogeographical barrier between European and Siberian forests. In order to shed light on the postglacial formation and evolution of the boreal forests in the European pre-Urals, we obtained a peat sediment core, Chernaya, from the Paltinskoe bog located between the southern taiga and hemiboreal forest zone in the mid-Kama region. We carried out pollen analysis, non-pollen palynomorph analysis, loss-on-ignition tests and radiocarbon dating. Radiocarbon dated records provide centennial to decennial resolution of the vegetation and environmental history of the European pre-Urals for the last 8.8 ka. The postglacial formation of the pre-Uralian hemiboreal forests reveals four important phases: (i) the dominance of Siberian taiga and forest-steppe in the Early Holocene and beginning of the Middle Holocene (8.8–6.9 ka), indicating a dry climate; (ii) the spread of spruce and European broadleaved trees in the Middle Holocene (6.9–4 ka) under wetter climate conditions; (iii) the maximum extent of broadleaved trees coinciding with the arrival and spread of Siberian fir in the Late Holocene (4–2.3 ka); and (iv) the decline of broadleaved trees since the Early Iron Age (2.3 ka – present) possibly due to general climate cooling and logging. While temperate broadleaved trees possibly spread from local refugia in the Urals, fir arrived from Siberia and spread further west. The carbon accumulation rate of Paltinskoe bog (18.9±10.16 g C m⁻² a⁻¹) is close to the average value of carbon accumulation of northern peatlands. Local development of peat is characterized by non-gradual growth with a phase of intensive carbon accumulation between 3.5 and 2.3 ka. The vegetation was strongly influenced by fire in the Early Holocene and by humans since the Early Iron Age practicing deforestation, agriculture and pasture. Phases of increased anthropogenic activity correlate well with the local archaeological data.     

Environmental conditions in northwestern Russia during MIS 5 inferred from the pollen stratigraphy in a sediment core from Lake Ladoga

Lake Ladoga hosts preglacial sediments, although the Eurasian ice sheet overrode the area during the LGM. These sediments were first discovered by a seismic survey and are investigated using a 22.75‐m‐long core. Its upper 13.30 m comprise Holocene and Lateglacial sediments separated from the lower 11.45 m of preglacial sediments by a hiatus. They consist of highly terrigenous lacustrine sediments, which according to OSL dating, were deposited during an early stage of the last ice age (MIS 5). The palynological data allow a first reconstruction of the Early Weichselian environmental history for northwestern Russia. Birch and alder forests with broad‐leaved taxa dominated during MIS 5d (c. 118–113 ka), suggesting a climate more favourable than in the Holocene. A high content of well‐sorted sands and poorly preserved palynomorphs indicates a shallow‐water environment at least temporarily. More fine‐grained sediments and better preserved organic remains suggest deeper water environments at the core location during MIS 5c (c. 113–88 ka). Pine and spruce became dominant, while broad‐leaved taxa started to disappear, especially after c. 90 ka, pointing to a gradual climate cooling. An increase in open herb‐dominated habitats at the beginning of MIS 5b (c. 88–86 ka) reflects a colder and dryer climate. However, later (c. 86–82 ka) pine and spruce again became more common. Birch and alder forests dominated in the area c. 82–80 ka (beginning of MIS 5a). Although open treeless habitats also became more common at this time, a slight increase in hazel may point to somewhat warmer climate conditions coinciding with the beginning of MIS 5a. The studied sediments also contain numerous remains of freshwater algae and cysts of marine and brackish‐water dinoflagellates and acritarchs documenting that the present lake basin was part of a brackish‐water basin during the Early Weichselian, probably as a gulf of the Pre‐Baltic Sea.     

MHD shock interactions in coronal structures

We consider the magnetohydrodynamic (MHD) interactions of solar coronal fast shock waves of flare and/or nonflare origin with the boundaries of coronal streamers and coronal holes. Boundaries are treated as MHD tangential discontinuities (TD). Different parameters of the observed corona are used in the investigation. The general case of the oblique interaction is studied.It is shown that a solar fast shock wave must be refracted usually as a fast shock wave inside the coronal streamer. For the special case of the velocity shear across TD, a slow shock wave is generated. On the contrary, the shock wave refracted inside the coronal hole is indeed a slow shock wave.The significance of different effects due to the interaction of fast and slow shock waves on the coronal magnetic field is noticed, especially at the time of a coronal mass ejection (CME). It is also shown, that an oblique fast MHD coronal shock wave may trigger an instability at the boundary of a streamer considered as a TD. It might have a relation with the observed process of abrupt disappearance of the streamer's boundary in the solar corona.On leave from the Academy of Sciences, Central Astronomical Observatory Pulkovo, 196140, St. Petersburg, Russia.     

Past and recent changes in air and permafrost temperatures in eastern Siberia

Air and ground temperatures measured in Eastern Siberia has been compiled and analyzed. The analysis of mean annual air temperatures measured at 52 meteorological stations within and near the East-Siberian transect during the period from 1956 through 1990 demonstrates a significant and statistically significant (at 0.05 level) positive trend ranging from 0.065 to 0.59 °C/10 yr. A statistically significant (at 0.05 level) positive trend was also observed in mean annual ground temperatures for the same period. The permafrost temperature reflects changes in air temperature on a decadal time scale much better than on an interannual time scale. Generally, positive trends in mean annual ground temperatures are slightly smaller in comparison with trends in mean annual air temperatures, except for several sites where the discordance between the air and ground temperatures can be explained by the winter snow dynamics. The average trend for the entire region was 0.26 °C/10 yr for ground temperatures at 1.6 m depth and 0.29 °C/10 yr for the air temperatures. The most significant trends in mean annual air and ground temperatures were in the southern part of the transect, between 55° and 65° N. Numerical modeling of ground temperatures has been performed for Yakutsk and Tiksi for the last 70 yr. Comparing the results of these calculations with a similar time series obtained for Fairbanks and Barrow in Alaska shows that similar variations of ground temperatures took place at the same time periods in Yakutsk and Fairbanks, and in Tiksi and Barrow. The decadal and longer time scale fluctuations in permafrost temperatures were pronounced in both regions. The magnitudes of these fluctuations were on the order of a few degrees centigrade. The fluctuations of mean annual ground temperatures were coordinated in Fairbanks and Yakutsk, and in Barrow and Tiksi. However, the magnitude and timing of these fluctuations were slightly different for each of the sites.     

Mesoproterozoic within-plate igneous province of the western urals: Main petrogenetic rock types and their origin

The Mesoproterozoic (1.38?C1.30 Ga) Kama-Belsk igneous province (KBP) was formed at the eastern margin of the East European Platform (EEP), in the Volga-Ural area and Bashkirian anticlinorium. It is made up of plutonic, volcanic, and subvolcanic (numerous dike and sill swarms) rocks of bimodal composition. KBP, as most of large igneous provinces, contains two geochemical types of basites: high-titanium (HTi) rocks with TiO₂ > 1.5 wt % and low-titanium (LTi) rocks with TiO₂ < 1.5?C2.0 wt %. They demonstrate zoned distribution, were derived from different mantle sources at different regimes of their partial melting. The high and low-titanium basites significantly differ in geochemical and isotopic (Sr, Nd, O) parameters. The HTi rocks are characterized by Ti/Y > 400, (Gd/Yb) _n = 1.62?C4.08, (Dy/Yb) _n = 1.31?C2.43; Nb/Nb* from 0.5 to 1.3, while the LTi rocks have Ti/Y < 400, (Gd/Yb) _n = 1.23?C1.51, (Dy/Yb)n = 1.01?C1.26, and Nb/Nb* from 0.3 to 0.9. The HTi rocks have ?_Nd(T) from + 1.3 to ?2.4, while the LTi rocks are characterized by ?_Nd(T) from + 0.5 to ?6.1. The oxygen isotopic composition ??18O is 5.0?C5.9?? in the LTi rocks and 7.0?? in the HTi picrobasalts. According to obtained estimates, the parental melts for the LTi type (Mg# = 0.76) are comparable with high-Mg primary melts inferred for within-plate picrites. The parental melts for the HTi type (Mg# = 0.69) had higher Fe contents, which in combination with lowered Al₂O₃ and elevated TiO₂, Na₂O, and P₂O₅ make these rocks similar to ferropicrites. The HTi melts were presumably derived by partial melting of a pyroxenite in equilibrium with garnet-bearing residue, whereas the LTi melts were generated from peridotite protolith and left spinel-bearing residuum. Both the varieties of the basites were contaminated mainly by Paleoproterozoic crustal material.     

Neoproterozoic riftogenic subalkali basites of the Central Urals: Geochemical specifics of clinopyroxene

Trace-element distribution in clinopyroxenes of different generations was used to decipher the intricate melt fractionation history of the trachybasalts, trachyandesites, and gabbroids from different rift-related magmatic complexes, which were formed during evolution of the Neoproterozoic passive margin of the East European craton (western slope of the Central Urals). It was established that chromian and high-magnesian cores of the early Cpx phenocrysts in the trachybasalts are not xenogenic, but represent relict minerals that were formed at the early stage of fractionation of a high-Mg melt. The fact that the trachybasalts and trachybasaltic andesites contain high-Mg Cpx with trace-element patterns similar in shape and element abundance indicates their formation via differentiation of a common melt. However, their subsequent evolution was different: trachybasaltic andesites were subjected to significant crustal contamination, which was recorded in composition of late Cpx. All subalkali basites were presumably formed from comparatively deepseated melts, because, according to thermodynamic calculations, early cores in the gabbroids crystallized at temperature more than 1200°C and pressure between 10 and 14–15 kbar, i.e., at depths of approximately 35–50 km. Some differences observed between trace-element composition of Cpx from the studied subalkali basatoids and gabbroids are inconsistent with their derivation from a common source, but similarity of their Cpx in many characteristics undoubtedly indicates close depths and compositions of their sources. Geochemical peculiarities of Cpx in the Neoproterozoic rift rocks from the western slope of the Central Urals testify that they were derived from melts formed by relatively low degree partial melting with garnet in residue. The geochemical specifics of clinopyroxenes from the Neoproterozoic riftogenic subalkali basites of the Central Urals and basaltoids from the Paleozoic Tagil structure of the same region showed that this island-arc system reflects the composition of melt source and its reworking by mantle fluids, which were different in the course of plume-lithosphere interaction and suprasubduction processes.