Home – Home – Jornals – Russian Geology and Geophysics 2016 number 1

The onset of the modern tectonic style that combines plate-related and plume-related mechanisms has been discussed. Such a tectonic style could have started on the Earth when all layers of our planet had formed. Analysis of available geological data shows that the solid inner core crystallized by 2.7 Ga. Typical geologic complexes appeared on the continents as a result of plate tectonics processes at that time. The layer D ″ that accumulates lithospheric slabs, which do not remain at the upper-lower mantle interface but can go down to the core-mantle boundary, was finally produced by 2 Ga. At pressures and temperatures of the D ″ layer perovskite, a principal mineral of the lower mantle transforms into postperovskite phase. The isotope data suggest the existence of mantle (EM-I and EM-II) domains, being the sources for oceanic island basalts (OIB) and the depleted upper mantle (asthenosphere), the source for mid-ocean ridge basalts (MORB), from at least 2 Ga. It is accepted that the recent tectonic style started on the Earth at that period. Deep-seated processes suggest the involvement of all Earth’s layers. The asthenosphere-lithosphere interaction is responsible for different types of large surface structures that develop as fold mountains, oceanic spaces, and subduction zones. The descending lithospheric slabs (cold mantle material) and ascending mantle material in hot mantle provinces or the so-called low shear velocity provinces are responsible for the lower mantle convection. The plume is produced in the layer D ″, which accumulates the descending lithosphere slabs as well as light elements released from the outer liquid core, which are oxidized in this layer, thus resulting in thermochemical plume formation. At the same time the molten ferric iron penetrates the core. So, we see the interaction of all Earth’s layers. The article also considers the evolutionary history of the Siberian continent over 700 Myr as early as the breakdown of Rodinia and the formation of mountain folds and rifting structures and the associated metallogeny.

This paper provides the state-of-the-art discussion of major aspects of the composition and evolution of the Earth’s core. A comparison of experimentally derived density of Fe with seismological data shows that the outer liquid core has a homogeneous structure and a ~10% density deficit, whereas the solid inner core has a complex heterogeneous anisotropic structure and a ~5% density deficit. Recent estimates of the core-mantle boundary (CMB) and inner-core boundary temperatures are equal to 3800-4200 K and 5200-5700 K, respectively. Silicon and oxygen (up to 5-7 wt.%) are considered to be the most likely light element candidates in the liquid core. Cosmochemical estimates show that the core must contain about 2 wt.% S, and new experimental data indicate that the inner-core structure yields the best match to the properties of Fe carbides. Our best estimate of the Earth’s core calls for 5-6 wt.% Si, 0.5-1.0 wt.% O, 1.8-1.9 wt.% S, and 2.0 wt.% C, with the Fe ₇C ₃ carbide being the dominant phase in the inner core. The study of short-lived isotope systems shows that the core could have formed early in the Earth’s history within about 30-50 Myr after the formation of the Solar System, t ₀ = 4567.2 ± 0.5 Ma. Studies on the partitioning of siderophile elements between liquid iron and silicate melt suggest that the core material would form in a magma ocean at ~1000-1500 km depths and 3000-4000 K. The oxygen fugacity for the magma ocean is estimated to vary from 4-5 to 1-2 log units below the Iron-Wustite oxygen buffer. However, the data for Mo, W, and S suggest addition of a late veneer of 10-15% of oxidized chondritic material as a result of the Moon-forming giant impact. Thermal and energetics core models agree with the estimate of a mean CMB heat flow of 7-17 TW. The excess heat is transported out of the core via two large low shear velocity zones at the base of superplumes. These zones may not be stable in their positions over geologic time and could move according to cycles of mantle plume and plate tectonics. The CMB heat fluxes are controlled either by high heat production from the core or subduction of cold slabs but in both cases are closely linked with surface geodynamic processes and plate tectonic motions. Considerable amounts of exchange may have occurred between the core and mantle early in the Earth’s history even up to the formation of a basal magma ocean. However, the extent of material exchange across the CMB upon cooling of the mantle was no greater than 1-2% of the core mass, which, however, was sufficient to supply thermochemical plumes with volatiles H, C, and S.

There are continuing issues concerning the formation and reconstruction of the geographic position of the Neoproterozoic Yenisei Ridge-a key element of the western framing of the Siberian craton and the Central Asian orogenic belt. This study focuses on the inner structure, composition, and boundaries of the Central Angara terrane, which is the largest in the Transangarian segment of the Yenisei Ridge. We propose a scheme of fault deformation of the region and demonstrate that the fault tectonics of the Central Angara terrane is distinct from that of adjacent terranes. We study in detail the Yeruda pluton granitoids of the Teya complex, which indicate accretionary-collisional magmatic events in this terrane prior to its collision with Siberia. New geochemistry and SHRIMP U-Th-Pb zircon geochronology of the granites indicate that they formed in a collisional setting at 880-860 Ma. Integrated petromagnetic and paleomagnetic investigations yield a paleomagnetic pole significantly different from the corresponding Neoproterozoic interval of the apparent polar wander path (APWP) for Siberia. The difference in paleolatitudes between the Central Angara terrane and the Siberian craton at the time of the Teya granites formation was at least 8.6 degrees, which equals a latitudinal separation of at least 1000 km. We consider various possible positions for the terrane relative to the Siberian craton. These results demonstrate that the 880-860 Ma magmatic events in the Central Angara terrane are not related to events in the western margin of the Siberian craton. Therefore, they do not indicate the existence of a Grenville-age orogenic belt in this location, as proposed by some authors.

The geodynamic reconstruction using new data on the composition, age, and paleomagnetism of Neoproterozoic and Vendian-Early Paleozoic island-arc complexes has provided new insights into the evolution of the subduction zone magmatism over extensive areas of the Central Asian Orogenic Belt, including eastern Altai-Sayan, Transbaikalia, and Northern Mongolia. Comparison of the igneous complexes of modern and ancient ensimatic and ensialic island arcs in the subduction zone forms a basis for possible geodynamic scenarios of the subduction zone magmatism in Neoproterozoic and Vendian-Early Paleozoic island arcs in the zone of interaction between the Siberian paleocontinent and the Paleoasian Ocean, which take into account the composition of crustal and mantle (including mantle plume) components.

This study presents a 3D model of the P and S seismic velocities above the Kamchatkan slab obtained as a result of tomographic inversion of arrival times of body waves from deep seismicity in the subduction zone. Various tests performed have shown limitations of the spatial resolution of the model and provided arguments for the reliability of the major structures used in the interpretation. In the uppermost layer down to 20 km depth, the model reveals strong low-velocity anomalies coinciding with Holocene volcanoes of the Klyuchevskoy group and Kizimen. In the seismogenic zone at depths from 80 to 150 km, we observe a low-velocity anomaly, which probably reflects the presence of the relatively thick oceanic crust sinking together with the subducting slab. This anomaly may also represent a zone of phase transitions, melting, and release of fluids from the slab. In the cross sections, we observe vertical and inclined low-velocity anomalies connecting the slab with the volcanic groups that probably represent the paths of ascending fluids and melts, which feed the volcanoes. In the case of Kizimen, we observe a single conduit connecting the volcano with the slab transformation area at 100 km depth. Beneath the Klyuchevskoy group, we identify several linear inclined patterns having different dipping angles. This may show that the volcanoes of the group are fed from different segments of the slab and might be one of the reasons for the diversity of lava compositions in the volcanoes of the Klyuchevskoy group.

Results of apatite fission track dating have been summarized and correlated with stratigraphic, geoelectrical, tectonic, and geomorphological data. The average regional rate of rock denudation in southeastern Gorny Altai is reflected in three thermotectonic events: (1) Late Cretaceous-Early Paleogene tectonic activity with a denudation rate of ~200 m/Myr, related to the Mongol-Okhotsk orogeny; (2) Middle Paleogene-Early Neogene stabilization with peneplanation; and (3) Neogene-Quaternary «stepwise» tectonic activity with a denudation rate of ≤270 m/Myr, related to the distant impact of the Indo-Eurasian collision. We present results of study of the evolution of regional tectonic processes and topography over the last 100 Myr by analysis of digital and shaded elevation models and apatite fission track dating.

Evidence for the involvement of a subduction component in diamond generation is analyzed based on literature data and our studies. Examination of xenoliths of diamond iferous eclogites, including X-ray tomography analysis, testifies to the superposed character of most diamonds. Diamond generation is accompanied by the serious modification of eclogite substratum. Isotope-geochemical data show that the eclogites originated from oceanic-crust rocks. The oxygen isotope compositions of garnets and clinopyroxenes from websterite xenoliths are similar to the mantle average (5.3-5.6‰). The eclogite minerals vary considerably in oxygen isotope composition (δ ¹⁸O of 5.3 to 12.4‰). Diamonds of eclogitic paragenesis predominate dramatically in the placers of the northeastern Yakutsk diamond-bearing province. In placer eclogitic diamonds, δ ¹³C varies from -27.2 to -3‰ ( n = 28). In diamonds of ultrabasic paragenesis, the range of δ ¹³C values is much narrower (from -7.1 to -0.5‰). All diamonds of variety V have a lighter carbon isotope composition (from -24.1 to -17.4‰). In a wide range of crystals with a contrasting carbon isotope composition, the isotope composition of the rim tends toward the average mantle value. This suggests that the eclogitic diamonds grew first with the participation of carbon from subducted continental crust and finally with the involvement of mantle carbon.

There are four main types of boninites in ophiolite suites, which either spatially coexist with ophiolites, though belong to other tectonic units (1), or are present as later constituents of ophiolite sequences (crosscutting dikes or lavas on top) (2), or build ophiolite sequences together with island-arc tholeiites and basaltic andesites, followed by younger volcanics of MORB or BABB affinites (3), or occupy the whole mafic portion of ophiolite sequences, together with island-arc tholeiites and basaltic andesites (4). The latter type, considered in more detail for the case of ophiolites from the southeastern Sayan Mountains (Siberia, Russia), presents an example of inconsistency between the model of ophiolite formation in mid-ocean ridge settings and subduction-related island-arc fingerprints in ophiolitic mafic rocks. The patterns of boninites record several evolution models of oceanic systems, with melting and intrusion of boninites in forearc, arc, and back-arc settings. The existing models are controversial, possibly, because there is no single mechanism to account for all types of boninites.

Based on the new geological, mineralogical, and geochemical data on the crystalline rocks of the Paleozoic ophiolite associations in the Sikhote-Alin (Russian southern Far East), we have reconstructed the tectonic setting of their formation. Two ophiolite associations have been recognized: peridotite-troctolite and peridotite-gabbronorite, differing mainly in the structure of the cumulative part of their sections. In the peridotite-troctolite association, the base of the cumulative unit is formed mostly by olivine-plagioclase rocks (troctolites and olivine gabbro), and in the peridotite-gabbronorite association, by essentially pyroxene rocks (pyroxenites and wehrlites). We have established that the ophiolite rocks crystallized at different pressures: troctolites - <5 kbar (judging from the stability of the olivine-plagioclase paragenesis), hercynite gabbronorites - 5-12 kbar, and garnet gabbro - >12 kbar. The crystalline rocks form a single geochemical series, with the accumulation of lithophile elements and LREE in more differentiated varieties. The REE patterns of rocks are in good correlation with their mineral composition. We assume that the Sikhote-Alin ophiolites formed at the basement of an oceanic plateau growing as a result of the mantle plume intrusion.

We present new data on geochemistry, isotopic geochemistry, and geochronology of the Early Cambrian igneous rocks of the northeastern Siberian Craton (Kharaulakh anticlinorium, contact between the Siberian Platform and the West Verkhoyansk sector of the Verkhoyansk fold-and-thrust belt) united into an Early Cambrian bimodal complex. This complex comprises trachyrhyolites forming pebbles in conglomerates near the base of the Cambrian sugcession, overlying trachybasalts, and mafic sills and dikes cutting Neoproterozoic strata. According to chemical composition, the felsic rocks are high-alkali rhyolites and correspond to A-type granites. The high contents of Ta, Nb, Hf, Tb, and Zr in these rocks suggest the presence of enriched mantle material in their magmatic sources. The mafic volcanics are high-Ti trachybasalts and trachydolerites with similar geochemical characteristics corresponding to alkali basalts or OIB. The high (Tb/Yb)PM ratios in these volcanics evidence that their magmatic source was the garnet peridotite mantle located at depths more than 90 km and characterized by a low degree of melting. However, the rhyolites, trachybasalts, and trachydolerites show high positive εNd(T) values (4.2-4.7, 7.5-8.9, and 7.2-8.2, respectively) indicating a depleted mantle source and no crustal contamination. The high (Nb/Yb)PM ratio points to the mixing of magmas from enriched and depleted mantle sources. Mafic magmas might have been generated from a heterogeneous source or interacted with depleted mantle before intrusion. Both the felsic and the mafic rocks formed in within-plate environments. U-Pb zircon dating yielded concordant ages of 525.6 ± 3.9 and 537.0 ± 4.2 Ma, corresponding to the Early Cambrian age of the rhyolites. The date of 546.0 ± 7.7 Ma obtained for one sample points (with regard to the error) to the Late Vendian-Early Cambrian age. Thus, at the Vendian-Early Cambrian boundary, the northeastern Siberian Platform was subjected to continental rifting accompanied by bimodal magmatism. According to paleotectonic reconstructions, this part of the Siberian Craton might have been connected with the eastern margin of Laurentia in the Late Neoproterozoic (Late Riphean-Late Vendian), and continental rifting that started at the Vendian-Cambrian boundary led to their separation. The obtained isotope-geochronological data suggest that the studied bimodal complex began to form at the Vendian-Cambrian boundary and this process terminated no earlier than the end of the Terreneuvian (Tommotian), i.e., the complex formed during rifting for about 20 Myr.

To estimate conditions for the stability of iron carbide under oxidation and to assess the possibility of formation of elemental carbon by interaction between iron carbide and oxides, experimental modeling of redox interaction in the systems Fe ₃C-Fe ₂O ₃ and Fe ₃C-Fe ₂O ₃-MgO-SiO ₂ was carried out on a “split-sphere” high-pressure multianvil apparatus at 6.3 GPa and 900-1600 °C for 18-20 h. During carbide-oxide interaction in the system Fe ₃C-Fe ₂O ₃, graphite crystallizes in assemblage with Fe ³⁺-containing wüstite. Graphite forms from carbide carbon mainly by cohenite oxidation: Fe ₃C + 3Fe ₂O ₃ → 9FeO + C ⁰ and FeO + Fe ₃C → (Fe ²⁺, Fe ³⁺)O + + C ⁰. At above-solidus temperatures (≥1400 °C), when metal-carbon melt is oxidized by wüstite, graphite and diamond crystallize by the redox mechanism and form the Fe ³⁺-containing wüstite + graphite/diamond assemblage. Interaction in the system Fe ₃C-Fe ₂O ₃-MgO-SiO ₂ results in the Fe ³⁺-containing magnesiowüstite-olivine-graphite assemblage. At ≥1500 °C, two melts with contrasting f _O2 values are generated: metal-carbon and silicate-oxide; their redox interaction leads to graphite crystallization and diamond growth. Under oxidation conditions, iron carbide is unstable in the presence of iron, silicon, and magnesium oxides, even at low temperatures. Iron carbide-oxide interaction at the mantle temperature and pressure leads to the formation of elemental carbon; graphite is produced from carbide carbon mainly by redox reactions of cohenite (or metal-carbon melt) with Fe ₂O ₃ and FeO as well as by interaction between metal-carbon and silicate-oxide melts. The results obtained suggest that cohenite is a potential source of carbon during graphite (diamond) formation in the lithospheric mantle and the interaction of iron carbide with iron, silicon, and magnesium oxides, during which carbon is extracted, is a process of the global carbon cycle.

U-Pb zircon dating by laser ablation and sector-field mass spectrometry with inductively coupled plasma (LA-SF-ICP-MS) is an accessible local method with easy sample preparation. At the Geological Institute, Ulan-Ude, this method was applied using a Thermo Scientific Element XR single-collector SF ICP mass spectrometer and a UP-213 (New Wave Research) laser ablation system. Measurements for standard zircons showed the error of dating less than 2%. The results of LA-SF-ICP-MS U-Pb dating of zircons from Late Paleozoic granitoids of western Transbaikalia confirm the overlapping of the time intervals of formation of the Barguzin (330-290 Ma), Chivyrkui (305-285 Ma), and Zaza (305-285 Ma) intrusive complexes.