Home – Home – Jornals – Russian Geology and Geophysics 2015 number 11

Analysis and summary of publications on southern East Sayan, eastern Tuva, and northern Mongolia have shown that the late Pleistocene glaciation covered a large area and had a complicated dynamics of glacier advance and retreat. Starting with MIS 5, the Todza Basin and, partly, the Oka Plateau, Azas Volcanic Plateau, Mondy Basin, and river valleys in southern East Sayan were periodically covered with ice. The thickness of ice in the eastern Todza basin was 700 m, on the Azas Volcanic Plateau it reached 300–600 m, and in the val-leys of southern East Sayan it is estimated as 700–800 m. The thickness of ice in the Mondy Basin was 300–350 m. Geological and geomorphological studies and isotope surface exposure dating (10Be method) of boul-ders from terminal moraine complexes have provided evidence for extensive MIS 2 glacier advance in the Mondy Basin and in the Sentsa, Jombolok, and Sailag river valleys (southern East Sayan). The average age of exposure for three groups of samples is 14, 16, and 22 ka.

Isolated lenses of diamictites laying discordantly over the Late Riphean (Cryogenian) Kirgitei Forma-tion were found in the immediate vicinity of the Vendian Taseeva Group stratotype in the Taseeva River val-ley and assigned to the Shishina Member a few meters in visible thickness. The Shishina diamictites are, likely, of glacial origin as they (i) lie at the base of the Vendian section, (ii) consist of unsorted dolomitic clasts from fine gravel to more than 0.5 m boulders suspended in a mud matrix, and (iii) show glacial striation on clasts. The glacial origin is further supported by the morphology of stones, which resemble a smoothing iron or a bullet, with a swelly top, a flat bottom, and a steeply cut rear and form clusters produced by disinte-gration of larger boulders. The stones bear signatures of cleavage, cracks, grooves, and striation on the faces, while the matrix looks undeformed. The Shishina Member has no genetic relation with the underlying Kirgitei Formation but rather correlates sedimentologically with the Ulyakha Member tillites at the base of the Ven-dian Marnya Formation (Oselok Group) in the Sayan foothills. The Shishina Member stones may derive from the Late Riphean (Cryogenian) Dzhura Formation exposed 4 km downstream of the site along the Taseeva. They occur near the base of the Aleshinsky Formation (lowermost unit of the Taseeva Group) of cross-bedded glaciofluvial sandstone, gravelstone, conglomerate, and sandy gravel mixtite transported from east to west (from the Siberian Craton interior to its margins) and deposited in channel bars or as gravel lags. The lower member of the Aleshinsky Formation comprises two associations of clasts: (i) coarse quartzose sand and gravel and (ii) fine and medium quartz and lithoclastic sand. Rocks in the former are well rounded, with traces of wind erosion, while the latter association is composed of mechanically eroded angular material transported to short distances from a metamorphic and metasedimentary source on the Craton margin. The Aleshinsky clastics have their composition and grain size patterns similar to those of the glaciofluvial Plity, Nersya, and Kedrovy members of the Marnya Formation in the Sayan area. According to sedimentological evidence, the Shishina diamictites are tillites identical to the Ulyakha moraine at the base of the Sayan Oselok Group and may be a missing link in the Taseeva Group stratigraphy.

The composition of stable carbon and oxygen isotopes in the section of an isolated carbonate platform is analyzed. Traces of several global and subglobal transgressions and regressions have been found in the limestone strata. Some of these phenomena were associated with the Upper Kellwasser (at the Frasnian–Famennian boundary) and multiphase Hangenberg (at the Devonian–Carboniferous boundary) Events. Never-theless, there were no considerable sea level fluctuations in the platform water area. There is no evidence for subaerial erosion. The bottom water during sedimentation was mostly in oxic conditions.

We present results of study of mineral assemblages and PT-conditions of metamorphism of mafic gar-net–two-pyroxene and two-pyroxene granulites in the Early Precambrian metamorphic complex of the An-gara–Kan terrane as well as the U–Pb age and trace-element and Lu–Hf isotope compositions of zircon from these rocks and the zircon/garnet REE distribution coefficients. The temperatures of metamorphism of two-pyroxene granulites are estimated as 800–870 to ~900 ºC. Mafic garnet–two-pyroxene granulites contain gar-net coronas formed at 750–860 ºC and 8–9.5 kbar. The formation of the garnet coronas proceeded probably at the retrograde stage during cooling and was controlled by the rock composition. The age (1.92–1.94 Ga) of zircon cores, which retain the REE pattern typical of magmatic zircon, can be taken as the minimum age of protolith for the mafic granulites. The metamorphic zircon generation in the mafic granulites is represented by multifaceted or soccerball crystals and rims depleted in Y, MREE, and HREE compared to the cores. The age of metamorphic zircon in the garnet–two-pyroxene (~1.77 Ga) and two-pyroxene granulites (~1.85 and 1.78 Ga) indicates two episodes of high-temperature metamorphism. The presence of one generation (1.77 Ga) of metamorphic zircon in the garnet–two-pyroxene granulites and, on the contrary, the predominance of 1.85 Ga zircon in the two-pyroxene granulites with single garnet grains suggest that the formation of the garnet coro-nas proceeded at the second stage of metamorphism. The agreement between the zircon/garnet HREE distribu-tion coefficients and the experimentally determined values at 800 ºC suggests the simultaneous formation of ~1.77 Ga metamorphic zircon and garnet. Zircon formation by dissolution/reprecipitation or recrystallization in a closed system without exchange with the rock matrix is confirmed by the close ranges of 176Hf/177Hf val-ues for the core and rims. The positive εHf values (up to +6.2) for the zircon cores suggest that the protoliths of mafic granulites were derived from depleted-mantle source. The first stage of metamorphism of the mafic granulites and paragneisses of the Kan complex (1.85–1.89 Ga) ended with the formation of collisional grani-toids (1.84 Ga). The second stage (~1.77 Ga) corresponds to the intrusion of the second phase of subalkalic leucogranites of the Taraka pluton and charnockites (1.73–1.75 Ga).

Omphacite is a typomorphic mineral of eclogites, which is inappropriate to mineral assemblages of per-idotites. Nevertheless, findings of this mineral in inclusions in peridotitic diamonds can be considered as indi-rect evidence for the existence of this paradoxical mineral assemblage. In this paper we present experimental results on the interaction between carbonate-bearing amphibolite and olivine that model processes operated at the crust–mantle boundary in subduction zones. The experiments demonstrate growth of omphacite at the in-terface between acid melt and peridotite media at 2.9 GPa and 850–900 ºC; the omphacite coexists either with garnet and orthopyroxene or with phlogopite. The synthetic omphacite is exclusively of reactive-magmatic origin and does not form in metasomatic way. Findings of omphacite inclusions in peridotitic diamonds and in some pyroxenites from kimberlites are discussed in scope of the obtained experimental data.

Upper-mantle xenoliths in Cenozoic basalts of northwestern Spitsbergen are rocks of peridotite (spinel lherzolites) and pyroxenite (amphibole-containing garnet and garnet-free clinopyroxenites, garnet clinopyroxenites, and garnet and garnet-free websterites) series. The upper mantle section in the depth range 50-100 km is composed of spinel peridotites; at depths of 80-100 km pyroxenites (probably, dikes or sills) appear. The equilibrium conditions of parageneses are as follows: in the peridotites - 730-1180 ºC, 13-27 kbar, and oxygen fugacity of -1.5 to +0.3 log.un.; in the pyroxenites - 1100-1310 ºC, 22-33 kbar. The pyroxenite minerals have been found to contain exsolved structures, such as orthopyroxene lamellae in clinopyroxene and, vice versa, clinopyroxene lamella in orthopyroxene. The formation temperatures of unexsolved phases in orthopyroxene and clinopyroxene are nearly 100-150 ºC higher than the temperatures of the lamellae-matrix equilibrium and the parageneses in the rock. The normal distribution of cations in the spinel structure and the equilibrium distribution of Fe2+ among the M1 and M2 sublattices in the orthopyroxenes point to the high rate of xenolith ascent from the rock crystallization zone to the surface. All studied Spitsbergen rock-forming minerals from mantle xenoliths contain volatiles in their structure: OH-, crystal hydrate water H2Ocryst, and molecules with characteristic CH and CO groups. The first two components are predominant, and the total content of water (OH- + H2Ocryst) increases in the series olivine → garnet → orthopyroxene → clinopyroxene. The presence of these volatiles in the nominally anhydrous minerals (NAM) crystallized at high temperatures and pressures in the peridotites and pyroxenites testifies to the high strength of the volatile-mineral bond. The possibility of preservation of volatiles is confirmed by the results of comprehensive thermal and mass-spectral analyses of olivines and clinopyroxene, whose structures retain these components up to 1300 ºC. The composition of hypothetic C-O-H fluid in equilibrium (in the presence of free carbon) with the underlying mantle rocks varies from aqueous (>80 % H2O) to aqueous-carbonic (~60 % H2O). The fluid becomes essentially aqueous when the oxygen activity in the system decreases. However, there is no strict dependence of the redox conditions on the depth of formation of xenoliths.

Three minerals of the mayenite supergroup have been found in fluorellestadite-bearing metacarbonate rock (former fragment of petrified wood of ankerite composition) from the burned dump of the Baturinskaya-Vostochnaya-1-2 mine. These are eltyubyuite Ca ₁₂Fe ³⁺ ₁₀Si ₄O ₃₂Cl ₆, its fluorine analog Ca ₁₂Fe ³⁺ ₁₀Si ₄O ₃₀F ₁₀, and chlormayenite-wadalite Ca ₁₂(Al,Fe) ₁₄O ₃₂Cl ₂-Ca ₁₂(Al,Fe) ₁₀Si ₄O ₃₂Cl ₆. The first two phases occur in the reaction mantle around hematite, magnesioferrite, and Ca-ferrite aggregates (“calciohexaferrite” CaFe ₁₂O ₁₉, “grandiferrite” CaFe ₄O ₇, and “dorrite phase” Ca ₂(Fe ³⁺ ₅Mn ³⁺ _0.5Mg _0.5)(Si _0.5Fe ³⁺ _5.5)O ₂₀) and, rarely, as individuals in the fluorellestadite-cuspidine (± larnite ± rusinovite Ca ₁₀(Si ₂O ₇) ₃Cl ₂) granular aggregate. Assemblages of zoned chlormayenite-wadalite crystals are found in the fluorellestadite-cuspidine granular aggregate, which lacks Ca-ferrite aggregates. Also, harmunite CaFe ₂O ₄, chlorellestadite, fluorapatite, anhydrite, rondorfite Ca ₈Mg(SiO ₄) ₄Cl ₂, fluorine analog of rondorfite Ca ₈Mg(SiO ₄) ₄F ₂, “Mg-cuspidine” Ca _3.5(Mg,Fe) _0.5(Si ₂O ₇)F ₂, fluorite, barioferrite BaFe ₁₂O ₁₉, zhangpeishanite BaFCl, and other rare phases are identified in this rock. Data on the chemical composition and Raman spectroscopy of the mayenite supergroup minerals are given. The genesis of metacarbonate rock is considered in detail: “oxidizing calcination” of Ca-Fe-carbonates with the formation of hematite and lime; reaction between hematite and lime with the formation of different Ca-ferrites; formation of larnite as a result of reaction between SiO ₂ and lime or CaCO ₃; and reactionary impact of hot Cl-F-S-bearing gases on early assemblages. Eltyubyuite and its fluorine analog crystallized at the stages of gas impact. It is presumed that the maximum temperature during the formation of rock reached 1200-1230 ºC.

The 1370 km long 4-AR reference profile crosses the North Barents Basin, the northern end of the No-vaya Zemlya Rise, and the North Kara Basin. Integrated geophysical studies including common deep point (CDP) survey and deep seismic sounding (DSS) were carried out along the profiles. The DSS was performed using autonomous bottom seismic stations (ABSS) spaced 10–20 km apart and a powerful air gun producing seismic signals with a step size of 250 m. As a result, detailed P- and S-wave velocity structures of the crust and upper mantle were studied. The basic method was ray-tracing modeling. The Earth’s crust along the entire profile is typically continental with compressional wave velocities of 5.8–7.2 km/s in the consolidated part. Crustal thickness increases from 30 km near the islands of Franz Josef Land to 35 km beneath the North Barents Basin, 50 km beneath the Novaya Zemlya Rise, and 40 km beneath the North Kara Basin. The North Barents Basin 15 km deep is characterized by unusually low velocities in the consolidated crust: The upper crust layer with velocities of 5.8–6.4 km/s has a thickness of about 15 km beneath the basin (usually, this layer wedges beneath deep sedimentary basins). Another special property of the crust in the North Barents Basin is the destroyed structure of the Moho.

We studied changes in the properties of the geologic environment in the vicinity of the stationary 40-ton vibration source at time intervals between the vibration sessions. Experiments have shown that the quantity of the energy released in the environment with time depends on the frequency of an amplitude spectrum. We introduced parameter α characterizing this dependence and have established a linear regular increase in its module between the series of switch-on of the vibration source during the field observations. A hypothesis is put forward that there are zones that can rapidly change their stressed state, both accumulating and giving the accumulated energy. Comparative analysis of the stressed state of the experimental zone showed significant differences in the spatial distribution of the gradient of a new parameter (β) before and after active low-frequency impacts on the geologic environment.

The magnetic viscosity of geologic media exerts a significant, often high and sometimes predominant influence on the pulse induction characteristics measured in the laboratory and in the field. Compared with the frequency methods, measurement of pulse magnetization parameters has advantage, namely, magnetic viscosity is observed in the absence of the primary field, and the transitive pulse parameter is measured over a wide time range. This reduces the error of measurement of parameters characterizing magnetic viscosity. In contrast to the transitive parameter, its derivative, i.e., a pulse parameter, is not influenced by a constant (slowly decreasing) component of the total residual magnetization. This eliminates the problem of the uncertainty on the separation of low-viscosity component from the total magnetization. The temporal decrease in the pulse parameters of magnetization is described by the power function a · t-b , where a is the initial value (varies over a wide range) and b is the exponent close to unity. As shown by the measurements made with the use of induction coil systems, the parameter a shows a strong linear correlation with the frequency-dependent magnetic susceptibility Δ k , which is commonly used to evaluate the content of superparamagnetic particles. This suggests that the pulse induction systems can be used for an express study of a large number of samples in order to identify SP-particles and estimate their contents. Although the exponent b differs negligibly from unity, this difference is much higher than the error of determination of this parameter from the experimental data. Mathematical modeling of the pulse parameters of magnetization has shown that both are influenced by the distribution of the particle volume, which makes prerequisites for solving the inverse problem, i.e., finding the distribution that provides the best explanation of the experimental pulse parameters.

In this paper the results of magnetostratigraphic studies of the Upper Cretaceous penetrated by two wells (S-124 and S-114) drilled in the Tomsk structural-facies zone (Bakchar iron ore deposit) are presented. The obtained biostratigraphic data show that the sediments formed in the Campanian–Maastrichtian time in-terval. The high-temperature component of the remanent magnetization identified in the rocks allowed us to compile paleomagnetic columns for each well and correlate the columns, using paleontological data, with each other and with the general magnetostratigraphic and magnetochronological scales. In magnetostratigraphic sections of two wells, the Campanian reverse-polarity Slavgorod Formation (R(km)) with a normal-polarity horizon is correlated with Chron C33(r) (top) and C33(n) (bottom) of the Gradstein scale, and the Maas-trichtian normal-polarity Gan’kino Formation with a thin reverse-polarity horizon (N(mt)) is correlated with Chron C30 of this scale.