A petrogeochemical study of basalts (lithophile elements and Sr-Nd-Pb isotopes, compositions of liquidus olivine and spinel) from the transition zone of the Southwest Indian Ridge in the area of the Du Toit and Andrew Bain faults revealed significant differences in their composition. Within the rift valley adjacent to the faults, tholeiites enriched in Na and depleted in Fe (Na-TOR genetic type) are typical. Deep-type basalts (TOR-1) are present in the western side of the Andrew Bain Fault. The outpouring of these types of magmas reflects a possible change in geodynamic regime during this zone formation: from deeper and higher temperature melting to shallower ones (Sushchevskaya et al., 2022). Differences in the primary melts of tholeiites from the rift valley and the Andrew Bain Transform Fault are also traced in the liquidus olivine compositions. The rift valley olivines are similar to typical Na-TOR olivines with a Mg content of Fo88–87, low Ni and elevated Mn. On the contrary, tholeiite olivines of the Andrew Bain Fault are enriched in Ni and depleted in Mn, which may indicate pyroxenite included in the primary melt formation. This component is either oceanic lithosphere recycled through the deep mantle or fragments of previously formed oceanic crust, which are subsequently involved in melting during the spreading axes jumping. A similar process is typical for the region of the Bouvet Triple Junction, where a significant heterogeneity of the olivine composition in terms of trace-element contents was revealed. The isotope characteristics of the Andrew Bain Fault tholeiites differ in Pb and Sr radiogenic composition and are similar to those of enriched magmas from such Indian Ocean rises as Crozet, Marion and Bouvet, but not from the Konrad and Af. Nikitin Rises. The source of such tholeiite melts is close in composition to the model HIMU type (with high U/Pb), possibly with an admixture of mantle material with EMII characteristics (with elevated Rb/Sr).

Приведены сведения о комплексе геолого-геофизических, литолого-палеоокеанологических, гидрофизических и биологических исследований в Центральной Атлантике в 45-м рейсе НИС “Академик Николай Страхов” в октябре–ноябре 2019 г. Обсуждаются предварительные результаты экспедиции.

Приводятся оригинальные данные по составам вулканитов поднятия Атлантис в Центральной Атлантике. По соотношениям некогерентных элементов вулканиты поднятия образовались при низких степенях частичного плавления в пределах гранатовой фации глубинности. Согласно изотопным отношениям Pb и Nd, их составы не могут безоговорочно быть отнесены к какой-либо из близлежащих магматических провинций (Мадейра, Канарская или Зеленомысская). Их отличает большая радиогенность изотопов Pb, свидетельствуя о существенной примеси компонента HIMU. Не наблюдается также значимой примеси EM 1. Можно считать, что вулканиты поднятия Атлантис образовались из самостоятельного мантийного источника.

New data on the composition of volcanic rocks of the Atlantis Rise (Central Atlantic) have been obtained. Incoherent element ratios suggest that those volcanics were formed at low degrees of partial melting within a garnet facies at depth. The isotopic ratios of Pb and Nd imply that their compositions cannot be unequivocally attributed to any of the nearby magmatic provinces (Madeira, Canary, or Cabo Verde). They are distinguished by high radiogenic Pb isotopes, indicating a significant addition of a HIMU component. No significant EM1 addition has been observed. It may be assumed that the Atlantis Rise volcanics were formed from an independent mantle source.

The paper analyzes original data obtained on the Atlantis‒Meteor seamount system during Cruise 33 of the R/V Akademik Nikolai Strakhov in the eastern North Atlantic. This system is a volcanic rise formed on the Canary abyssal plate and represents one of the key objects for understanding the geological history of opening of the central segment of the Atlantic Ocean. Basalts, tephrites, and organogenic terrigenous lagoonal marine sediments dredged from the Atlantis, Plato, and Cruiser seamounts are considered. Petrography and compositions of the Atlantis and Cruiser basalts reflect significant differences in settings of their eruptions. Well-crystallized vesicle-free olivine basalts from the Atlantis Seamount were ejected under deep-water conditions. Glassy vesicular basalts of the Cruiser Seamount are typical of shallow subaerial eruptions. Evidence for the accumulation of tuff breccias and tuff gravelstones of the Plato Seamount in subaerial settings are obtained. Tendencies were revealed in the lithogenetic transformations of organogenic‒terrigenous sediments of the Cruiser Seamount, which were subjected to the high-temperature impact of subaerial lava flows. During the volcanosedimentary lithogenesis, the plant lignite-like matter lost its primary structure and was transformed into anisotropic coke with the wide development of fusinite and pyrofusinite. The studied volcanic occurrences are thought to be related to the final (Late Miocene‒Pliocene) volcanic stage in the seamount system, which predated the destruction of the system, its prograde subsidence, and transformation of islands into guyots.

Приведены оригинальные данные по составу вулканитов западной части разлома Эндрю Бейн Юго-Западно-Индийского хребта, полученные в 23 рейсе нис “Академик Николай Страхов”. Можно говорить о присутствии здесь мелких мантийных неоднородностей. Рассмотрены два возможных варианта их происхождения. Это сохранение в деплетированной мантии фрагментов обогащённого материала в ходе раскола Гондваны либо “заражение” мантии плюмовым материалом с образованием жильных неоднородностей, которое произошло до начала раскрытия океана в этой области.

The paper considers the conditions and mechanisms of the formation of linear volcanic structures in the Brazil Basin, South Atlantic. Among these objects, those related to the ascent of deep mantle plumes predominate. It is shown that the ascent of melts from plume sources leads to the formation of (a) hot spot tracks in the form of linear volcanic ridges and (b) active hot lines in the form of submarine mountain chains with trends differing from those of hot spot tracks and with a more variable character of the age distribution of volcanic rocks. Fault tectonics affects the character of plume activity. In addition, plume material from a hot spot area is dragged by a moving plate as a flow or a sublithospheric lens, which leads to the long-term existence of particular independent segments of linear structures and sometimes to late volcanism reactivation within their limits. Decompression melting of the asthenospheric mantle in zones where thin lithosphere undergoes tension causes the formation of passive hot lines. The main mantle source for the considered volcanic rocks was a mixture of DMM and HIMU mantle components, with the latter abruptly dominating. In marginal oceanic regions, the EM1 component is also present (the EM2 component is found more rarely) within fragments of tectonically delaminated continental mantle that was trapped by the oceanic mantle during the breakup of Gondwana.

Рассмотрен состав пород линейных тектоно-магматических поднятий Бразильской котловины. Показано, что вулканиты цепи Витория-Триндади образовались в мантийном резервуаре, неоднородно обогащенном фосфором из расплавов, близких субщелочным пикробазальтам. В их составе значительную роль играет мантийный компонент EM I в результате “заражения” плюмового материала веществом континентальной литосферы. Для всей структуры, включая острова Триндади, Мартин Вас и плато Аброльос, характерна существующая длительное время изотопная гомогенность источника расплавов. Аналогичные изотопные соотношения пород поднятия Фернанду-ди-Норонья связываются со схожим механизмом формирования и эволюции глубинного мантийного плюмового материала, изначально располагавшегося под южно-американским континентом. По сравнению с расплавами вулканитов других рассматриваемых в работе гор, первичные расплавы вулканитов хребта Витория-Триндади формировались при меньших степенях частичного плавления обогащенного мантийного субстрата и на большей глубине. Подавляющая часть вулканитов Северной цепи гор Байя образовалась из расплавов, генерированных с участием материала типа HIMU. В то же время один из изученных образцов образовался из источника близкого по составу к DM с некоторой примесью обогащенного вещества типа EM I. Мантийный источник вулканитов гор Пернамбуку представлял собой смесь материала DM и HIMU с некоторой примесью материала EM I (EM II?). Горы 10°–11° ю.ш. сформировались вблизи осевой части САХ при декомпрессионном плавлении химически гетерогенного мантийного источника, состоявшего из вещества DM с примесью компонента типа EM I (EM II?).

During the 28th trip of the R/V Akademik Sergei Vavilov undertaken in 2009, exploration was carried out using a Parasound acoustic profilograph that permitted examining the upper portion of the sedimentary cover and getting transverse bathymetric profiles through a number of seamounts within the northern range of the Bahia group of seamounts, which were also sampled by dredging. The studies performed indicated that this linear structure consists of paleovolcanoes and formed under impulse conditions of volcanic activity. A number of seamount types have been distinguished, the morphostructural settings and the positions of which reflect the intensity of volcanic impulses and the association with deep permeable zones. The strongest impulse led to the formation of a northwest striking arched uplift, on which there are several central type paleovolcanoes of different heights. The largest of these paleovolcanoes, including the Stocks and Groll Seamounts, have flattened tops formed as a result of marine abrasion. Isolated seamounts have formed during shorter and lower power impulses of volcanic activity. The northern range of the Bahia seamounts, which have a major northwesterly strike, is an ensemble of structures with northwesterly, nearly east-west, and nearly north-south strikes, which indicates the presence of relevant deep permeable zones for the mantle matter; that mantle matter is the source of the melts, eruptions of which led to the formation of the volcanic edifices. The prevailing zone of weakness has a northwesterly strike. Volcanic activity in the Bahia group of seamounts ended in the Paleocene, but tectonic movements continued there up until the Holocene. Neotectonic structures are represented by horsts, diapirlike piercement structures, and folded zones. The paleovolcanoes are areas where slope processes of transport of sedimentary material occurred.

During the 28th cruise of R/V Akademik Sergei Vavilov in 2009, five different mountains belonging to the northern chain of the Baia Seamounts, located in the Brazil Basin, were dredged. Igneous rocks, limestones, and Fe-Mn crusts were collected. Igneous rocks are greatly altered resulting from halmyrolithic and/or low-temperature hydrothermal processes; the main secondary minerals are smectite, iron hydroxide, and phillipsite. Igneous rocks are subdivided into two groups, namely, trachybasalts and trachyandesites. Trachybasalts are aphyric rocks, consisting of basal plagioclase microlites, Fe-Ti ore mineral, olivine, and clinopyroxene. Trachyandesites are rarely porphyre rocks. Inclusions in trachyandesite are represented by acidic plagioclase, olivine, biotite, and zircon. The main mass is formed by acidic plagioclase and a small quantity of clinopyroxene.

The multidisciplinary approach is used to analyze the structure of the sedimentary cover in the northern Knipovich Rift valley, Molloy Fracture Zone and synonymous basin, Svyatogor and Hovgard rises, Gorynych Hills, Litvin and Pogrebitskii seamounts, and western slope of the Spitsbergen Archipelago studied in Cruise 24 of the R/V Akademik Nikolaj Strakhov. Materials of the bathymetric survey with multibeam echo sounder, as well as continuous seismic and vertical acoustic profiling, revealed two main (NNW- and NNE-trending) systems of fractures in the neotectonic structure of the region. It was established that a system of NNE-oriented fractures, linear zones of the dominant development of keyboard deformations included, is consistent with the strike of magnetic anomalies reconstructed for this region. Tectonic aspects of the Knipovich Rift and prospects of its further development are considered. Based on the wave field pattern of continuous seismic profiling (CSP) records, four seismocomplexes indicating contrasting sedimentation settings and intense tectonic processes at different formation stages of the northern Norwegian-Greenland Sea are conditionally defined in the sedimentary cover of the study region. It was established the Molloy Fracture Zone is responsible for a system of horizontal reflectors of acoustically transparent structureless light spots (“blankings”) in the upper well-stratified part of the sedimentary section, which are characteristic of areas with ascending pore fluids. The micropaleontological study (palynomorphs of higher plants, dinocysts, planktonic foraminifers, and diatoms) revealed the presence of Miocene assemblages in sediments. Benthic foraminifers include late Paleocene-middle Eocene assemblages. The composition of rock-forming components demonstrates a directed succession of mineral-terrigenous associations from the feldspar-quartz type to mesomictic quartz-graywacke type.