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The special issue is focused on the problems of tectonics, paleogeography, geodynamic evolution, and mineral resources of the continental margins of the Russian Arctic. This topic is relevant, since the knowledge of the geologic structure of the Arctic Ocean and its formation and evolution can solve many global problems of geology and important regional problems, including the formation of oil- and gas-bearing sedimentary basins as well as prospecting for and development of diamonds and deposits of nonferrous, noble, rare-earth, and other minerals. In previous issues of Russian Geology and Geophysics, considerable attention was paid to the geology and oil and gas potential of the Arctic. In this special issue, emphasis is placed on the tectonics, stratigraphy, paleogeography, and petrology of the Arctic continental margins of Russia, the development of tectonic and geodynamic models for key structures, and diamond content and metallogeny of Arctic zones of the Siberian Platform, Chukotka, and the Kola Peninsula.

Tectonic and geodynamic models of the formation of the Amerasian Basin are discussed. The Arctic margins of the Chukchi region and Northern Alaska have much in common in their Late Jurassic-Early Cretaceous tectonic evolution: (1) Both have a Neoproterozoic basement and a complexly deformed sedimentary cover, with the stage of Elsmere deformations recorded in their tectonic history; (2) the South Anyui and Angayucham ocean basins have a common geologic history from the beginning of formation in the late Paleozoic to the closure at the end of the Early Cretaceous, which allows us to consider them branches of the single Proto-Arctic Ocean, the northern margin of which was passive and the southern margin was active; (3) the dipping of the oceanic and, then, continental lithosphere took place in subduction zones southerly; (4) the collision of the passive and active margins of both basins occurred at the end of the Early Cretaceous and ended in Hauterivian-Barremian time; (5) the collision resulted in thrust-fold structures of northern vergence in the Chukchi fold belt and in the orogen of the Brooks Ridge. A subduction-convective geodynamic model of the formation of the Amerasian Basin is proposed, which is based on seismic-tomography data on the existence of a circulation of matter in the upper mantle beneath the Arctic and East Asia in a horizontally elongated convective cell with a length of several thousand kilometers. This circulation involves the subducted Pacific lithosphere, the material of which moves along the bottom of the upper mantle from the subduction zone toward the continent, forming the lower branch of the cell, and the closing upper branch of the cell forms a reverse flow of matter beneath the lithosphere toward the subduction zone, which is the driving force determining the surface kinematics of crustal blocks and the deformation of the lithosphere. The viscous dragging of the Amerasian lithosphere by the horizontal flow of the upper mantle matter toward the Pacific leads to the separation of the system of blocks of Alaska and the Chukchi region from the Canadian Arctic margin. The resulting scattered deformations can cause a different-scale thinning of the continental crust with the formation of a region of Central Arctic elevation and troughs or with a breakup of the continental crust with subsequent rifting and spreading in the Canadian Basin.

We report new paleomagnetic and geochronological data for rocks of the Franz Josef Land archipelago and generalize available information about the paleomagnetism of the Barents Sea continental margin as applied to the issues of the Mesozoic Arctic tectonics. Specifically, the obtained age estimates are indicative of a brief episode of mantle plume magmatism at the Barremian-Aptian boundary (Early Cretaceous). The paleomagnetic data show that intraplate magmatism formations in the High Arctic, including the Franz Josef Land traps, are nothing more than a trace of the Iceland plume on the migrating tectonic plates of the region. Thus, the Iceland plume was geographically stationary for at least the last 125 Myr. Our paleotectonic reconstructions suggest a direct connection of the intraplate strike-slip systems of the Eurasian continent with the configuration and subsequent evolution mode of Mesozoic marginal basins and spreading axes during the initial opening stage of the Arctic Ocean.

We present a tectonothermal model for the late Paleozoic syncollisional formation stage of the Kara orogen in northern Taimyr in the Central Arctic. The model is based on new and published structural, petrological, geochemical, and geochronological data, as well as thermophysical properties obtained for the Kara orogen. The latter hosts a significant volume of granites formed as a result of the collision between the Kara microcontinent and the Siberian craton. Based on geological, geochemical, and U-Th-Pb isotope data, the granites were differentiated into syncollisional and postcollisional intrusions that were emplaced in the intervals 315-282 Ma and 264-248 Ma, respectively. The presented tectonothermal model covers only the syncollisional formation stage of the Kara orogen, during which anatectic granites formed. The 2D models help to reconstruct the main tectonothermal processes of the syncollisional stage of formation of this structure, taking into account the local peculiarities of the thermal state of the Earth’s crust in the region. The model shows the mechanisms of increase in the lower crust temperature necessary for the formation of syncollisional anatectic granites. The estimates obtained from the model constrain the time interval between the collision/tectonic stacking and the granite formation. The modeling also showed the general regularities typical of orogens at syncollisional stages.

The study of Triassic paleontology and stratigraphy of various regions of northeastern Russia and adjacent Arctic shelf is essential not only for improving and refining zonal biostratigraphic schemes, interregional and global correlation of Triassic deposits, and resolving problems of stratigraphic boundaries but also for developing and substantiating a new generation of Triassic stratigraphic schemes, which could serve as the stratigraphic basis for different regional and detailed geological investigations of the Arctic. The results of the study were used to improve existing zonal scales based on various groups of fauna and palynomorphs, develop a more detailed biostratigraphic subdivision of the Triassic, and characterize individual horizons using both terrestrial and marine palynomorphs. The zonal scales are calibrated to each other and to the regional zonal scale of the Triassic of Siberia and northeastern Russia, which provides the subsequent correlation with the International Chronostratigraphic Chart of the Triassic System. The set of coeval zonal scales for the Triassic of Kotelny Island sections based on ammonoids, nautiloids, coleoids, bivalves, brachiopods, and foraminifers and the analysis of microphytoplankton and terrestrial palynomorph assemblages are a useful tool for detailed subdivision and correlation of the eastern part of the Laptev Sea shelf and adjacent regions of northeastern Russia.

We report results of geological, mineralogical-petrographic, geochemical, isotope-geochemical (Sm-Nd, Rb-Sr), and geochronological (U-Pb, ⁴⁰Ar/³⁹Ar) studies of acid and intermediate intrusive rocks (granodiorites, leucocratic granites, subalkaline granites, and subalkaline leucocratic granites, diorites, and quartz diorites) of the Bukeschen and Samyr small plutons in the western part of the Yana-Kolyma gold belt (northeast Asia). These rocks are combined with Late Jurassic (151-145 Ma) dikes of basic, intermediate, and acid compositions into a single complex of small intrusions. They intrude the Upper Triassic-Middle Jurassic terrigenous deposits of continental margin blocks in the eastern part of the Verkhoyansk-Kolyma folded area. Our new U-Pb data for zircon (SHRIMP-II) indicate that the Bukeschen and Samyr pluton granitoids formed in the Berriasian and at 144.5 and 143 Ma, respectively. The small-intrusion granitoids have geochemical and isotope (Sm-Nd and Rb-Sr) characteristics similar to those of Late Jurassic dikes of varying composition. Therefore, they can be united into a single complex of small intrusions generated from a mixed source with the participation of mantle (OIB- and E-MORB type), lower crust, and subduction components and with Paleoproterozoic-Mesoproterozoic Sm-Nd model age estimates for the magma sources. Late Jurassic-Early Cretaceous magmatic and postmagmatic events and cooling of the intrusions played an important role in the processes of gold localization in the western part of the Yana-Kolyma gold belt. This is reflected in two tectonothermal stages (accounting for closing temperatures of the U-Pb, ⁴⁰Ar/³⁹Ar, and Re-Os isotope systems for different minerals) estimated at 151-141 and 138-137 Ma. These results for the small-intrusion complex agree with the tectonic model of the evolution of an active continental margin (northeastern Siberia) in the Mesozoic era, whose final development stage in the Berriasian age saw the formation of mostly small granitoid plutons.

We study the P - T conditions and age of metamorphic evolution of the rocks that make up the Korvatundra structure in the northeast of the Fennoscandian Shield. The rocks underwent progressive metamorphism of the amphibolite facies at 625-660 ºC and 8.7-8.8 kbar 1945 ± 34 Ma (Sm-Nd data). The pegmatite cutting the metamorphic paragenesis that formed at this stage has an age of 1917 ± 6 Ma (zircon U-Pb data). Metamorphic transformations after 1917 Ma are manifested locally as discrete zones of blastomylonites in the rocks of the northern part and some inner sites of the Korvatundra structure. Both local increases and decreases in temperature and pressure are possible in these zones. The formation of light titanite with an age of 1863 ± 44 Ma marks the next stage of shear strain. Low-temperature alterations (chloritization and silicification) took place in the zones of final deformations 1722 ± 5 Ma (Rb-Sr data). Beginning from 1.94 Ga, the general deformational and metamorphic history of the Korvatundra structure, Lapland Granulite Belt, and Tana Belt confirms the assumption of the formation of a single inverted metamorphic zoning within the Korvatundra structure and the overlying Lapland-Kolvitsa Collision Belt in the Paleoproterozoic. The obtained data supplement the idea of the Paleoproterozoic geodynamic evolution of the Lapland-Kola orogen.

During the recent exploration of the Monchegorsk ore district (MOD) in the Arctic western sector, the platinum potential of known Cu-Ni deposits (Nittis-Kumuzhya-Travyanaya (NKT), Nyud, Ore Horizon 330 (OH330), and Terrasa) has been assessed, and new sulfide PGE-Cu-Ni deposits (Western Nittis) and manifestations (Moroshkovoe Ozero, Poaz, and Arvarench), and low-sulfide Pt-Pd deposits (Loipishnyun, Southern Sopcha, and Vuruchuaivench) have been discovered. All of them are confined to Paleoproterozoic (ca. 2.5 Ga) layered intrusions (the Monchegorsk pluton (Monchepluton) and the Monchetundra massif) and are divided into two types according to their structural position: basal, located in the marginal parts of intrusions, and reef-type (stratiform). All types of ores show Pd specialization. Platinum group minerals (PGM) have a limited composition in sulfide PGE-Cu-Ni ores and are represented by predominant Pt and Pd compounds with Bi and Te and subordinate PGE arsenides and sulfides. Low-sulfide Pt-Pd ores are characterized by a significant variety of PGM, with a predominance of PGE sulfides, bismuthotellurides, and arsenides. Sulfide PGE-Cu-Ni deposits and manifestations (Western Nittis, NKT, Nyud, Moroshkovoe Ozero, Poaz, and Arvarench) formed through the accumulation of base metal sulfides and PGE in immiscible sulfides and their subsequent segregation in commercial contents. The reef-type OH330 deposit and Terrasa manifestation resulted from the injection of additional portions of sulfur-saturated magma. The basal-type low-sulfide Pt-Pd deposits (Loipishnyun and Southern Sopcha) formed from residual melts enriched in ore components and fluids separated and crystallized during long-term ore-forming processes. The reef-type Vuruchuaivench deposit is the result of deep fractionation of the parental magma with the formation of a sulfide liquid enriched in Cu and PGE. Significant reserves and large predicted resources of sulfide PGE-Cu-Ni and low-sulfide Pt-Pd ores are a reliable mineral resource base for the development of the mining industry in the Kola region of the Arctic western sector.

The research team studied the petrology, whole-rock geochemistry, zircon U-Pb age, and stable isotopic characteristics of the Rongguo Longba and Garongcuo granites of the Nuocang area to understand better the impact of Neo-Tethys ocean subduction and India-Eurasia continental collision on Paleocene tectonomagmatic processes along the southern margin of the Gangdese Belt. The Rongguo Longba granite and Garongcuo granite porphyry formed at 61.86 and 62.17 Ma, respectively. The Nuocang granitoids are characterized by (1) high SiO₂, NaO₂, and Al₂O₃ contents and low FeO_tot, MgO, and TiO₂ contents; (2) LREE and LILE enrichment and HREE and HFSE (Nb, P, and Ti) depletion; and (3) obvious negative Eu anomalies. These features indicate that the Nuocang granites are of the high-K calc-alkaline and peraluminous granite types. Furthermore, their zircon Hf isotope characteristics suggest that the magma source region has an ancient crystalline basement. The basaltic andesitic crystal tuff is the product of garnet-peridotite partial melting and crust contamination from rising magma emplacement.

The Lahroud Ophiolite in northwestern Iran contains extensive zones of Paleozoic ophiolite as remnantsof the Paleo-Tethys oceanic crust. The principal rock units are gabbro overlain by pillowbasalt, which is intruded by granites and interbedded with pelagic sedimentary units including radiolariancherts. Geochemistry and radioisotope studies, supported by Nd, Sm, Sr, and Pb isotope data, indicatethat the Lahroud Ophiolite originates from a within-plate basaltic mantle source. The isotope studiesshow that the basalts are derived from Indian-type oceanic mantle sources. The radiogenic data indicatethe involvement of subduction-related terrigenous materials in the source magma. All the rocks are geochemically cogenetic and were generated by fractionation of a melt with a composition of average E-MORB with a calc-alkaline signature. Two ⁴⁰Ar/³⁹Ar ages, 343 ± 3 Ma for muscovite minerals and 187.7 ± 7.7 Ma for glasses, suggest that metamorphic and basaltic rocks formed during the Late Paleozoic to Early Jurassic, respectively. Microfossil studies show the presence of Paleozoic biostratigraphy. The crystallization process and rifting into the oceanic crust in the Lahroud Ophiolite probably began in the Carboniferous, with volcanic activity continuing during the late Triassic.