Home – Home – Jornals – Russian Geology and Geophysics 2026 number 1

This special issue of the journal, dedicated to the memory of Academician Nikolai Leontievich Dobretsov, features papers reflecting the development of his research and ideas in the fields of his scientific interests. The diversity of Dobretsov’s scientific interests determined the broad range of topics covered in the presented papers: tectonics, deep geodynamics, the interaction of plate tectonics and mantle plumes, metamorphism, including ultrahigh-pressure metamorphism in subduction zones, structural patterns of geomagnetic and gravitational fields and their relationship with plume magmatism, and unique mineral deposits.

The late Mesozoic-Cenozoic intraplate volcanic province of Central Asia unites a number of spatially isolated volcanic regions. Based on the examples of the South-Khangai, West-Transbaikal, and East-Mongolian regions, we demonstrate that three periods of activity can be distinguished in the province’s geological history. The initial period (between ~145 and 100 Ma) was characterized by a regional extension and riftogenic magmatism. The middle period (between 100 and 30 Ma) was distinguished by a quasi-platform tectonic regime and an areal type of volcanism. The late period (the last 30 million years) is distinguished as a period of lava plateau volcanism. The magmatism of the province is determined primarily by mafic rocks with elevated alkalinity. During the riftogenic period, trachybasalts and trachybasaltic andesites were formed, the geochemical signature of which was high REE contents with depleted concentrations of Nb and Ta. During the period of areal volcanism, trachybasalts and alkaline basaltoids with OIB characteristics became predominant. This rock type remains dominant in the volcanic associations of the final period of the province’s formation. The trends in variations of trace elements and the isotopic composition of Sr, Nd, Pb in mafic rocks of different ages in the province were determined and the compositions of their magmatic sources were estimated. We show that at all stages of the province development, one of the magma sourcecomponents remained constant and was close to the asthenospheric mantle of the EMORB type. During the rift period, subduction-metasomatized mantle also participated in magmatism. During the period of areal volcanism, the metasomatized mantle was gradually removed from the composition of melt sources. Since that time, the magmatism of the province has been determined by the interaction of asthenospheric, plume (OIB-type) and depleted lithospheric mantles. The formation of the province is associated with the emergence of a hot mantle field at the base of the East Asian lithosphere. Its origin is consistent with the activation of deep geodynamic processes in the early late Mesozoic, primarily with the activity of the Pacific superplume.

We provide a brief description of the main structures in the Eastern Arctic, in the evolution of which two major stages have been distinguished and considered: the late Paleozoic-early Mesozoic and the Late Jurassic-Early Cretaceous. We have established the synchronicity of tectonic events on the Arctic margins of Northeast Asia and Arctic Alaska and within the structures of the Amerasia Basin, indicating the existence of a cause-and-effect relation between the compression (fold-and-thrust structures) and extension (rifting and spreading in the Canada Basin). We have proposed the tectonic models of the formation of fold-and-thrust structures in Chukotka and Arctic Alaska and have determined their similarities and differences. Paleotectonic reconstructions have been performed for 160 and 120 Ma. We present a critical review of the concepts about the formation of the structures in the Amerasia Basin and provide a subduction-convection geodynamic model according to the analysis of seismic tomography of the mantle and regional geology and tectonics data. This model was previously used to describe the Cretaceous and Cenozoic evolution of the Arctic lithosphere at a qualitative level. The model is based on the idea of the existence of a two-tier subduction system: a horizontally extended convection cell in the upper mantle, coupled with a conveyor mechanism of subduction of the Pacific lithosphere. As a result, there is a convergence of the “outer” Pacific subduction zone and the “inner” subduction zone located inside the South Anyui and Angayucham oceanic basins, which provides their closure and subsequent collision. Under the influence of the reverse upper mantle flow, scattered deformations of the Amerasia lithosphere occur, caused by viscous dragging with flows beneath the lithosphere, which is the reason for the diversity of the structures in the Amerasia Basin and the Canada Basin in particular. In addition, the developed geodynamic model is supplemented by a tectonic and magmatic mechanism of crustal subsidence and the formation of sedimentary basins.

Using 3D numerical modeling, we consider the formation of the postcollisional granitoids of the Kara orogen in Northern Taimyr 280-250 Ma under conditions of elevated heat flow due to the orogen’s breakup, prior to the magmatic activity of the Siberian mantle plume. The initial geometry of the model area, the boundary conditions and physical properties of the crust and mantle were set to be close to the structure of the Earth crust in the junction zone of the Kara, Central Taimyr and Siberian blocks. The model shows widespread melting in the granite-gneiss-andesite-basalt layer of middle crust, and a 1-2 km thick melting zone forming at the base of the crust in the granulite layer with the possible participation a mantle component. The magma ascent rate and the formation of groups of adjacent granitoid plutons depend on the value of the elevated mantle heat flow and on the rheology of the melting protolith rocks. The model shows the conditions for intrusion of magma and formation of plutons 10-20 km in diameter at depths 5-8 km in unmetamorphosed rocks. 3D modeling shows the mechanism of periodic magma intrusion pulses at the postcollisional stage during 30-40 Myr. The proposed formation mechanism for the plutons allows to reproduce such shapes and emplacement periodicity that are comparable to their actual geological position and age of the postcollisional granitoids of the Kara orogen. We compare the results of modeling in 2D and 3D configuration with identical model parameters and physical properties of the rocks. We conclude that 3D modeling is a more realistic and accurate means of description of the respective magmatic processes compared to the 2D one.

Based on the results of spectral analysis of the anomalous magnetic field, the depths of the roof and the base of the magnetoactive layer of the Amur Plate and adjacent territories were calculated. The causes of variations in the depth of the magnetoactive layer base (CPD) from 14 to 38 km (average 24 km) were determined. Maximum CPD depths are observed within sedimentary basins (Erlian, Songliao, and Middle Amur) in the southwest and in the central part of the plate. The areas of minimum depths in the continental part are located in the northwest within the giant granitoid batholiths (Angara-Vitim, Khentei) and in the northeast within the Bureinsky province. The third area of minimum CPD values is located within the waters of the Sea of Japan. The relatively high elevation of the magnetoactive layer base in the waters of the Sea of Japan is associated with rifting processes in the back-arc basin, which began at the end of the Oligocene, and the generation of fluids and magma chambers above the Pacific slab that is sinking under the Amur Plate. Two high CPD areas standing in the continental part of the plate are associated with the presence of two thermal anomalies. The north-western one is explained by the presence of a thermal crustal anomaly due to the process of radioactive heat generation by granitoids of the giant Angara-Vitim, Khangai and Khentei batholiths. The Northeastern Bureinskaya area is explained by the presence of an anomalous temperature of the mantle here. A comparison of the newly constructed CPD map with the boundaries of the Amur Plate, previously determined mainly from seismic data, shows that the surface boundaries of the plate coincide mainly with the zones of the greatest CPD gradients. All of them are associated with areas of increased seismic energy generation, with the exception of one small area on the southern border of the Amur Plate at its junction with the Yangtze Plate. In our interpretation, plate boundaries are not just lines on the surface, they are fairly wide zones from tens to the first hundreds of kilometers that encircle the plate.

We present an analysis of modern paleomagnetic data from large igneous provinces and paleorift structures in Siberia and the High Arctic that are potentially related to mantle plumes. The interrelationship between plume magmatism, geomagnetic reversal frequency, and field intensity over the last 600 Myr exhibits a periodicity of 70-100 Myr. Periods of mantle plume activity were preceded by an increase in geomagnetic reversal frequency, accompanied by a decrease in geomagnetic field intensity. Our proposed hypothesis explains this effect by changes in the thermal convection in the Earth’s outer core while mantle plumes are regarded as regulators of the state of the hydromagnetic dynamo. “Overheating” of the core increased the turbulence of convective currents, and therefore, the amount of reversals. During reversals, the value of the main component of the geomagnetic field - the geocentric axial dipole - first fell to zero and returned to high values only after a full reversal of the poles. Reduction of relaxation time in periods of frequent reversals led to prolonged low values of the absolute intensity of the geomagnetic field. Mantle plumes forming during such periods could remove the excess heat and stabilize the state of the geodynamo, even almost completely stopping reversals. We link the Ediacaran (Vendian) and Devonian geomagnetic phenomena to periods of ultra-frequent reversals. During these extended periods of low value of the axial dipole, the configuration of the geomagnetic field was determined by low-order non-zonal harmonics and by the global magnetic anomalies. We observe a qualitative coincidence of the position of paleopoles with centers of lower mantle gravitational and magnetic anomalies and postulate that the anomalies were stationary. This is the basis for substantiating a new reference framework for paleotectonic reconstructions in absolute coordinates. Examples of reconstructions made using this system also agree with the hypothesis of stationary hotspots. From the terminal Precambrian to the Mesozoic inclusively, the Siberian paleocontinent was located in the area of effect of the African mantle hot field, migrating northwards along the 0° meridian from the latitude of Tristan da Cunha to that of Iceland.

New evidence for the involvement of Fe-C-O melts in diamond formation from placers of the northeastern Siberian craton - combined with our data on iron carbide inclusions and previous research - provides a fresh perspective on diamond formation in subduction zones. Inclusions of iron carbides and oxides, along with moissanite and carbonates in polyphase inclusions, attest to a heterogeneous diamond-forming environment. Extreme variations in oxygen fugacity during diamond growth likely result from hydrogen and hydrocarbon generation via interaction between carbonated rocks of the subducting oceanic lithospheric slab and aqueous fluids. Separated hydrocarbon fluids can create localized ultra-reduced mantle regions where silicon carbide forms under conditions of nonequilibrium with the environment. A key characteristic of the studied diamonds is brittle fracture followed by crack healing, which is associated with the formation of polyphase iron carbide and oxide inclusions interpreted as trapped melts. We attribute brittle diamond fracture in the lower lithosphere to high strain rates localized in hypocenters of deep-focus earthquakes within the subducting lithospheric slab, triggered by dehydration or carbonatite melt formation.

Small bodies of restitic ultramafic rocks composed of dunite and harzburgite and enclosed in gneisses of the Olkhon composite terrane (West Baikal Area) are described. The estimates of the P - T conditions of metamorphism of the ultramafic rocks generally correspond to the metamorphic parameters of the host gneiss and amphibolite. The restitic rocks include unique aluminous ultramafic rocks composed of forsterite, enstatite, and chromium-free spinel. The latter are distinguished from restitic rocks by high contents of Al₂O₃ (up to 23 wt.%) at “peridotite” concentrations of magnesium (25-37 wt.% MgO) and silicon (30-42 wt.% SiO₂). It is assumed that these rocks are the products of high-temperature (Max T = 730-790 °C) metasomatism of dunite and harzburgite. As shown by comparing the compositions of restitic and aluminous ultramafic rocks, the metasomatic process involves many elements, including those considered to be weakly mobile. The input elements are Al, Ti, V, Zr, and rare earth elements; the output elements are Mg, Si, Cr, and Ni. At the same time, there are no possible magmatic sources of metasomatizing fluids. It is assumed that host felsic gneiss served as a source of fluid at high-temperature metamorphism. The impact of metasomatizing fluids on aluminosilicate rocks, occurring as small fragments in restitic ultramafic rocks, strongly enriched them with alumina (up to 50 wt.% Al₂O₃) and formed specific mineral associations with corundum and sapphirine.

The late Paleozoic granitoid province of Transbaikalia (Angara-Vitim batholith (AVB), Russia), located in the northeast of the Central Asian Orogenic Belt (CAOB), occupies an area of about 200,000 km². It is composed of rocks varying in composition from monzonites and quartz syenites to leucocratic granites. This research work is aimed at: 1) determination of the total duration and dynamics of formation of the Angara-Vitim batholith granitoids; 2) clarification of the reasons that determined the spatiotemporal heterogeneity of granitoids; 3) reconstruction of the sources of salic (granitoid) magmas and assessment of the contribution of mantle-crust interaction to the petrogenesis of granitoids. The paper is based on new petrogeochemical, isotope (Lu-Hf), and isotope-geochronological (U-Pb) data on the northern part of the AVB. Combined with the results of previous studies, it has been established that the AVB, one of the Earth’s largest granitoid provinces, formed over about 45 million years (from 320 to 275 Ma). During this time, about 90% of the batholith rocks formed. Crustal metagraywacke protoliths were the main source of salic magmas. The formation of monzonitoids, quartz syenites, and granodiorites is associated with the melting of mixed protoliths, in which the proportion of juvenile mafic material could reach 40-50%. The late Paleozoic granitoid magmatism in Transbaikalia began with the areal intrusion of calc-alkaline granites, granodiorites, and quartz syenites, which were predominant at the first stage of magmatism. At the second stage, magmatism took place in a narrow (200-250 km) permeable zone of NE strike. This zone drained crustal chambers of salic magmas and favored the penetration of mafic mantle melts into the upper crustal horizons. The AVB granitoids formed at the postcollisional stage of evolution of the eastern segment of the CAOB under the impact of mantle plume on the crust of the young orogen.

Carbonatite breccias were discovered by borehole G2 in the north of the Burannyi site in the Tomtor massif (complex) (TC) hosting the largest Sc-Y-Nb-REE ore deposit. The breccias are petrographically composite rocks consisting of fragments of dolomite, dolomite-ankerite, ankerite, and calcite carbonatites with significant amounts of F-REE carbonates, pyrite, and fluorite. They show signs of fragmentation of carbonatites and calcite rocks, transition of these fragments by younger melt-brine enriched in CO₂, F, S, and REE, and its interaction with breccia fragments. The studied rocks are classified as magmatic-fluidogenic and fluidogenic breccias, which are a unique source of information about the rock composition and deep-level processes within the TC. The trace element spidergrams of the carbonatite breccias are similar to those of carbonatites and unique high-grade Sc-Y-Nb-REE ores of the upper ore horizon of the TC. The spidergrams show enrichment in Th, Nb, La, Ce, and Nd and depletion in U, K, Sr, Zr, Hf, and Ti, as in other carbonatite complexes of the world (including those with breccias). The REE pattern shows enrichment in MREE and HREE. The formation of carbonatite breccias is accompanied by the crystallization of unique REE minerals: synchysite-(Ce), parisite-(Ce), and/or bastnaesite-(Ce), cebaite-(Ce), and burbankite. Сrystallization of REE-fluorocarbonates is associated with dolomite replacement by ankerite. The maximum amount of these minerals, as well as cebaite-(Ce) and burbankite, is found in calcite rocks in association with fluorite and pyrite. It is shown that the δ¹⁸O and δ¹³C (‰) composition points in carbonatite breccia fragments and in the interfragmentary space form a trend with high correlation, which is similar to the trend of mixing of C and O isotopes of carbonatites and sedimentary carbonates. However, it contradicts the data on the increasing contents of REE, Nb, P, and other elements typomorphic for carbonatites with increasing δ¹⁸O and δ¹³C (‰) values and is the result of the transformation of carbonates by a low-temperature deuteric fluid. The first obtained comprehensive data on carbonatite breccias give grounds to consider them a new type of mineralization in the TC.