Home – Home – Jornals – Russian Geology and Geophysics 2014 number 3

Two metamorphic complexes of the Yenisei Ridge with contrasting composition are analyzed to unravel their tectonothermal evolution and geodynamic processes during the Riphean geologic history of the area. The structural, mineralogical, petrological, geochemical and geochronological data are used to distinguish two stages of the evolution with different ages, thermodynamic regimes, and metamorphic field gradients. Reaction textures, chemical zoning in minerals, shapes of the P–T paths, and isotope dates provide convincing evidence for a polymetamorphic history of the region. The first stage is marked by the formation of the ~ 970 Ma low-pressure zoned And–Sil rocks (P = 3.9–5.1 kbar, T = 510–640 ºC) of the Teya aureole and a high metamorphic field gradient with dT/dH = 25–35 ºC/km typical of many orogenic belts. At the second stage, these rocks experienced Late Riphean (853–849 Ma) collisional medium-pressure metamorphism of the kyanite-sillimanite type (P = 5.7–7.2 kbar, T = 660–700 ºC) and a low metamorphic field gradient with dT/dH < 12 ºC/km. This metamorphic event was almost coeval with the Late Riphean (862 Ma) contact metamorphism in the vicinity of the granitic plutons, which was accompanied by a high metamorphic field gradient with dT/dH > 100 ºC/km. At the first stage, the deepest blocks of the Garevka complex in the vicinity of the Yenisei regional shear zone underwent high-pressure amphibolite-facies metamorphism within a narrow range of P = 7.1–8.7 kbar and T = 580–630 ºC, suggesting the burial of rocks to mid-crustal depths at a metamorphic field gradient with dT/dH ~ 20–25 ºC/km. At the second stage, these rocks experienced the Late Riphean (900–850 Ma) syn-exhumation dynamometamorphism under epidote-amphibolte facies conditions (P = 3.9–4.9 kbar, T = 460–550 ºC) and a low gradient with dT/dH < 10 ºC/km accompanied by the formation of blastomylonitic complexes in shear zones. All these deformation and metamorphic events identified on the western margin of the Siberian craton are correlated with the final episodes of the Late Grenville orogeny and provide supporting evidence for a close spatial connection between Siberia and Laurentia during early Neoproterozoic time, which is in good agreement with recent paleomagnetic reconstuctions.

The geologic position, development stages, age, and geochemical features of metasomatic and felsic igneous rocks along the southern edge of the Siberian craton are compared. The comparison shows that all the studied metasomatic rocks are confined to the faults feathering the main suture zone of the craton. From Biryusa zone in the southwest and farther northeast, from Primor’e zone to Davan shear zone and Katugino–Ayan zone in the Aldan area, the metasomatic rocks are of similar composition but show higher mineralization. The process begins with blastocataclasis (barren stage). During the second stage, ore-bearing (Nb, Zr, Hf, and REE) potassic solutions circulate along the blastocataclastic zones. They form metasomatic potassic rocks of the early alkaline stage, expressed subalkalic granitization. The next (acid) stage is marked by the formation of greisens with Sn, Be, Th, U, and W mineralization. The igneous stage might precede or follow the metasomatism. At the time of ongoing tectonic movements, it produces rapakivi–like granites rich in the same elements. Also, a huge volcanoplutonic belt develops along the craton edge during this time. The geochemical features of its felsic volcanics are close to those of the metasomatic rocks and granites. The age of all these rocks is within 2.1–1.6 Ga.

The age of the major igneous complexes in the western part of the Selenga–Stanovoy superterrane has been estimated by ⁴⁰Ar/³⁹Ar dating: trachyandesite-basalts of the Kuitun (Chichatka) complex — 259.4 ± 6.2 Ma; gabbro of the Tukuringra complex — 156.3 ± 4.8 Ma; granites and pegmatites of the Tukuringra complex — 153.1 ± 3.8, 154.0 ± 4.4, 156.8 ± 4.0, and 151.2 ± 3.2 Ma; granodiorites, granites, and leucogranites of the Amudzhikan complex — 131.7 ± 2.4, 134.5 ± 2.8, and 131.6 ± 4.2 Ma; and lamprophyre dikes — 125.2 ± 2.4 and 125.2 ± 3.4 Ma. Two stages of hydrothermal ore formation process have been recognized: 132–131 and ~125 Ma. It is shown that the deposit formation and superposed processes follow the general regularities of the Late Mesozoic evolution of the Pacific Asia margin.

Data on the Cu content in native gold and silver and the Ag and Au contents in native copper are summarized. The standard thermodynamic functions of solid solutions the Au–Cu and Ag–Cu binary systems and the Ag–Au–Cu ternary system have been estimated. The corresponding calculation module is prepared for the Selektor software.

The Late Neoproterozoic sediments of the Yenisei Ridge formed in several isolated basins. These sediments are correlated, and the composite section of this age in the region is described. Two age limits are of extreme importance: pre-Chapa (650 Ma) and pre–Vendian (600 Ma). The former, observed in a much larger area, predominates. The great importance of the pre–Chapa transformation becomes evident owing to its coevality with the Marino–Nantuo global glaciation and the preceding tectonic events. This glaciation was immediately followed by a significant biotic event that gave rise to the Doushantuo–Pertatataka microfossil assemblage and, afterward, Ediacaran fauna. The Chapa Group is proposed as a prototype of the Angarian–a unit of the General Late Precambrian scale, which is of the same rank as the Vendian. According to these data, the main Late Neoproterozoic units are the Baikalian, Angarian, and Vendian.

Chemical, radioisotopic, and palynological studies of the bottom sediments of mesotrophic Lake Arakhlei have been carried out for paleoclimatic reconstructions and estimation of the sedimentation rate.

The paper is focused on recent displacement rates in the Altai-Sayan region, obtained by hydroleveling, leveling, and satellite geodesy. Effective elastic moduli and viscosity parameters of the crust are used in the modeling of coseismic and tectonic processes. The elastic moduli are determined from measurements of periodic vertical displacements during seasonal loadings of the Sayano-Shushenskaya hydropower plant. We present the results of the modeling of coseismic displacements during the earthquakes of 10 February 2011 (M = 6.1) and 27 December 2011 (M = 6.7) in Tuva and West Sayan. The results of GPS determinations for postseismic displacements in the Chuya earthquake zone (Gorny Altai, 27 September 2003, M = 7.5) are analyzed; models for the geologic medium are selected; and its effective viscosity is estimated. The tectonic component of the recent crustal displacements in the Altai-Sayan region is defined.

A 450 km long composite crustal section of the Urals folded system at 67º N and 60–90° E has been compiled from seismic data on the Polar Urals transect and the results of targeted studies. The main elements of the subsurface structure at more southerly latitudes are preserved for this cross section. The general deep–seated structure is divided into the western and eastern branches. The western one has a northwestern trend and corresponds to the West Urals megazone. The eastern one, with a northeastern trend, includes the Central Urals and East Urals megazones.

A thermomagnetic analysis has been performed for 737 picroilmenite samples from the cores of eight boreholes in the N–S–striking drilling profile of the Zarnitsa kimberlite pipe in the Daldyn kimberlite field, Yakutia. Based on the shapes of thermomagnetic curves and the Curie points, 29 samples were chosen for detailed microprobe studies of chemical composition and elucidation of the dependence of their thermomagnetic parameters on the content of the hematite end-member in the picroilmenites. The thermomagnetic curves of most of the studied picroilmenite samples are approximated by a two-component model for the hematite end–member distribution: the main and supplementary distribution. The average hematite end-member content in the main distribution coincides with the probe microanalysis data and is always lower than the average content in the supplementary distribution. The relative hematite end–member contents in the main and supplementary distributions within the picroilmenite grains are indicators of the dynamics of the mineral formation in different parts of the Zarnitsa pipe. The data obtained testify to the multistage formation of the pipe under unstable thermodynamic conditions, which explains the intricate distribution pattern of picroilmenite.

The paper describes the effects of the passage of the Chelyabinsk meteoroid (exploded on 15 February 2013 over the Chelyabinsk Region), which were established from geophysical data from West Siberian stations. The trajectory and speed of the meteoric body from the start of the glow to the breakup were recorded by surveillance cameras and dashcams. Records from broadband seismic stations were used to determine the exact time of the explosion (03:20:34 UTC) from the arrival times of the surface wave produced by this event. The explosion energy was estimated from the surface-wave amplitudes at ~100 ktons on the assumption that the wave originated from a point source similar to a high–altitude thermonuclear explosion. A database of records from seismic stations obtained during the meteoroid passage has been compiled.