Comparative analysis of marine paleogene sections and biota from West Siberia and the Arctic Region

The analysis of the main biospheric events that took place in West Siberia and the Arctic region during the Early Paleogene revealed the paleogeographic and paleobiogeographic unity of marine sedimentation basins and close biogeographic relations between their separate parts. Most biotic and abiotic events of the first half of the Paleogene in the Arctic region and West Siberia were synchronous, unidirectional, and interrelated. Shelf settings, sedimentation breaks, and microfaunal assemblages characteristic of these basins during the Paleogene are compared. The comparative analysis primarily concerned events of the Paleocene-Eocene thermal maximum (PETM) and beds with Azolla (aquatic fern). The formation of the Eocene Azolla Beds in the Arctic region and West Siberia was asynchronous, although it proceeded in line with a common scenario related to the development of a system of estuarine-type currents in a sea basin partly isolated from the World Ocean.     

Models of the REE distribution in the black shale Bazhenov Formation of the West Siberian marine basin,Russia

Rare-earth elements abundance in black shales of the Upper Jurassic (Tithonian Stage)–Lower Cretaceous (Berriassian Stage) Bazhenov Formation is discussed. This formation is the principal oil source rocks of West Siberia. The deposits within the formation can be subdivided into two main marine groups: (a) moderately hemipelagic deposits (clayey-siliceous, including phosphatic and carbonate rocks) and low-density distal clayey turbidites (argillites), both are considered as normal and (b) silty argillites and clayey-silt rocks, which are channel deposits and considered as anomalous. The hemipelagic rocks of normal sections, which are enriched in the rare-earth elements (REE), accumulated under both slow rates of sedimentation (clayey-siliceous rocks) and faster rates of sedimentation (argillites). The channel deposits of anomalous sections, which are impoverished in the REE, accumulated exclusively under fast rates of sedimentation.Within the hemipelagic group the rate of sedimentation of the argillites was faster than of the clayey-siliceous rocks, but the REE concentration in the former rocks (140.4 ppm) is higher than in the latter group (97.4 ppm). The argillites are more than twice enriched in clayey material than clayey-siliceous rocks. It is likely that the clay fraction was the main carrier of REE in these rocks. In the channel group of rocks, the REE abundance in clayey-silt rocks (21.2 ppm) is lower than in the silty argillite (84.6 ppm), in which the clay content is elevated.With respect to redox potential the Bazhenov Formation rocks can be subdivided further into three groups, based on the degree of pyritization index (DOP): (1) the highly reducing clayey-siliceous rocks of normal sections, with high DOP; (2) the substantially reducing argillites and carbonate rocks of normal sections, with intermediate DOP; (3) the moderately reducing rocks of anomalous sections with low DOP. The rocks with the high DOP (group 1) are characterized by ΣLREE/Σ(M+H)REE ratios between 7.37 and 7.5, whereas the rocks with the lower DOP (group 2 and 3) are characterized by ΣLREE/Σ(M+H)REE ratios between 12.8 and 13.5. Negative Ce anomalies are either small or absent in all deposits, which is typical for reducing conditions.Thus, the Bazhenov Formation exemplifies the complex depositional conditions that influence the REE concentration in black shale. However, it is this very complexity that has contributed to the development of six separate depositional models (REE contents in ppm are given in brackets). (1) Phosphatic clayey-siliceous rocks of normal sections (367.95); (2) argillites of normal sections (130.73); (3) clayey-siliceous rocks of normal sections (85.97); (4) carbonate rocks, largely dolomites of normal sections (23.23); (5) silty argillites of anomalous sections (78.7) and (6) clayey-silt rocks of anomalous sections (19.66).     

U–Pb and Hf-isotope analysis of zircons in mafic xenoliths from Fuxian kimberlites: evolution of the lower crust beneath the North China craton

Mafic xenoliths from the Paleozoic Fuxian kimberlites in the North China craton include garnet granulite, and minor pyroxene amphibolite, metagabbro, anorthosite and pyroxenite. The formation conditions of the amphibolites are estimated at 745–820 °C and 7.6–8.8 Kb (25–30 km); the granulites probably are derived from greater depths in the lower crust. LAM-ICPMS U–Pb dating of zircons from four granulites reveals multiple age populations, recording episodes of magmatic intrusion and metamorphic recrystallisation. Concordant ages and upper intercept ages, interpreted as minimum estimates for the time of magmatic crystallisation, range from 2,620 to 2,430 Ma in three granulites, two amphibolites and two metagabbros. Lower intercept ages, represented by near-concordant zircons, are interpreted as reflecting metamorphic recrystallisation, and range from 1,927 to 1,852 Ma. One granulite contains two metamorphic zircon populations, dated at 1,927±55 Ma and 600–700 Ma. Separated minerals from one granulite and one amphibolite yield Sm–Nd isochron ages of 1,619±48 Ma (¹⁴³Nd/¹⁴⁴Nd)i=0.51078), and 1,716±120 Ma (¹⁴³Nd/¹⁴⁴Nd)i=0.51006), respectively. These ages are interpreted as recording cooling following metamorphic resetting; model ages for both samples are in the range 2.40–2.66 Ga. LAM-MC-ICPMS analyses of zircon show a range in ¹⁷⁶Hf/¹⁷⁷Hf from 0.28116 to 0.28214, corresponding to a range of _Hf from –34 to +12. The relationships between ²⁰⁷Pb/²⁰⁶Pb age and _Hf show that: (1) the granulites, amphibolites and metagabbro were derived from a depleted mantle source at 2.6–2.75 Ga; (2) zircons in most samples underwent recrystallisation and Pb loss for 100–200 Ma after magmatic crystallisation, consistent with a residence in the lower crust; (3) metamorphic zircons in several samples represent new zircon growth, incorporating Hf liberated from breakdown of silicates with high Lu/Hf; (4) in other samples metamorphic and magmatic zircons have identical ¹⁷⁶Hf/¹⁷⁷Hf, and the younger ages reflect complete resetting of U–Pb systems in older zircons. The Fuxian mafic xenoliths are interpreted as the products of basaltic underplating, derived from a depleted mantle source in Neoarchean time, an important period of continental growth in the North China craton. Paleoproterozoic metamorphic ages indicate an important tectonic thermal event in the lower crust at 1.8–1.9 Ga, corresponding to the timing of collision between the Eastern and Western Blocks that led to the final assembly of the North China craton. The growth of metamorphic zircon at 600–700 Ma may record an asthenospheric upwelling in Neoproterozoic time, related to uplift and a regional disconformity in the North China craton.     

A statistical analysis of methanol maser groups

An analysis of the flux densities of the 5₁-6₀ A ⁺ (6.7 GHz) and 2₀-3_?1 E (12.2 GHz) class II methanol maser lines in a large and homogeneous sample of maser sources has been carried out. For convenience, the maser lines were divided into three groups: group I contains spectral features for the lines most prominent in the 5₁-6₀ A ⁺ (6.7 GHz) transition, group II contains spectral features for the lines strongest in the 2₀-3_?1 E (12.2 GHz) transition, group III contains spectral features for which the velocities of the emission maxima of the two lines coincide. The same dependence was found for group II and group III: log S _6.7=(0.79±0.05)×log S _12.2+(0.79±0.05). The spectral features in group I do not obey this relation, and deviations from a linear dependence are considerably greater. It is suggested that methanol class II masers be divided into a subclass IIa, which has special conditions favoring 6.7 GHz masers, and a subclass IIb, which is comprised of the 12.2 GHz masers and those 6.7 GHz masers that necessarily accompany them under the same conditions.     

The magnetic fields and density of relativistic electrons in the nucleus of NGC 1052

We analyze observations of the compact GHZ-peaked-spectrum radio source in the nucleus of the weakly active galaxy NGC 1052, assuming that the low-frequency turnover in its spectrum is due to synchrotron self-absorption. The analysis is based on a model for an inhomogeneous source of synchrotron radiation. It is shown that the magnetic field is not uniform, but the change in the field strength from the center to the edge of the compact radio source does not exceed an order of magnitude. The maximum magnetic-field strength in the nucleus of NGC 1052 is 20 G < H _⊥ < 200 G, and the density of relativistic electrons is 0.018 cm⁻³ < n _e < 0.18 cm⁻³ on scales of 0.1 pc; everywhere in the radio source, the energy density of the magnetic field exceeds the energy density of the relativistic electrons. The physical conditions are similar to those in the nuclei of the nearby radio galaxies 3C 111 and 3C 465, and differ strongly from those in the nucleus of the radio galaxy 0108+388, which is a compact GHz-peaked-spectrum source (these three galaxies were studied by the authors earlier using the same method).     

The Taihua group on the southern margin of the North China craton: further insights from U–Pb ages and Hf isotope compositions of zircons

The "Taihua Group" is a collective term for a series of old terranes scattered along the southern margin of the North China Craton. The timing of formation and thermal overprinting of the Taihua Group have long been contentious, and its relationship with the Qinling orogenic belt has been unclear. In this study, new data from integrated in-situ U–Pb dating and Hf isotope analysis of zircons from an amphibolite (from the Xiong’ershan terrane) and a biotite gneiss (from the Lantian-Xiaoqinling terrane) indicate that the Upper Taihua Group formed during the Paleoproterozoic (2.3–2.5 Ga) and thus was originally part of the southern edge of North China Craton, detached during the Mesozoic Qinling orogeny and displaced about 100 km north from its original location. This suggests that the Taihua Group became part of the tectonic terrane associated with the Qinling orogeny and now forms part of the overthrust basement section of the Qinling belt. Before the Qinling orogeny, the Taihua Group was metamorphosed at 2.1 Ga. The initial Hf-isotope compositions of zircons, together with positive εNd(t) values for the whole-rocks, imply that the original magmas were derived from a juvenile source with some assimilation of an Archean crustal component.     

The Berezitovoe gold-polymetallic deposit (East Siberia): mineralogy,age, and relation with magmatism

The Berezitovoe gold-polymetallic deposit is localized in the west of the Selenga–Stanovoi superterrane composed of a wide spectrum of Early and Late Precambrian igneous rocks and abundant Paleozoic and Mesozoic intrusive and volcanoplutonic associations. The ⁴⁰Ar/³⁹Ar ages determined for metasomatites bearing gold-polymetallic mineralization are as follows: garnet-quartz-muscovite-sericite-K-feldspathic metasomatites (129.7±3.2–127.3±4.4 Ma); muscovite-quartz-sericite metasomatites (132.0±2.9–131.3±2.3 Ma). According to the age and general scheme of evolution of the Early Cretaceous magmatism in the Selenga–Stanovoi superterrane, the metasomatites of the Berezitovoe deposit are nearly coeval to the intrusive rocks of the Amudzhikan complex (132–128 Ma). The revealed platinum potential of gold-polymetallic ores and metasomatites permits ranking the Berezitovoe deposit as a specific complex gold-polymetallic-platinum deposit, which considerably increases its commercial value.     

Seismites in Quaternary sediments of southeastern Altai

We study earthquake-induced soft-sediment deformation (seismites) in reference Quaternary sections of southeastern Altai. Sediments in the sections bear signature of liquefaction and fluidization and deformation is localized in thin (few centimeters to 0.5–1.0 m) continuously striking and frequently repeated layers sandwiched between undeformed sediments. The soft-sediment deformation records coseismic motion of different slip geometries. Seismic origin is also inferred for layers and lenses of coarse colluvium slid into the lake bottom from the slopes, which intrude plane-bedded silt and sand and vary in thickness from a few centimeters to one meter. The occurrence of seismic soft-sediment deformation at different stratigraphic levels of the Quaternary and in the Upper Pliocene Beken Formation confirms the high seismicity of southeastern Altai in Quaternary time.