This paper provides information on the geological and geophysical studies of the structure of the Charlie Gibbs megatransform system in the North Atlantic on cruise 50 of R/V Akademik Nikolaj Strakhov in September–October 2020. The preliminary results of the expedition are discussed.

The plutonic rocks studied in the Ashadze Complex contain four groups of mineral assemblages: (I) coarse-grained gabbronorite, which is dominant; (IIa) micrograined differentiated gabbro (oxide microgabbro) enriched in ilmenite and apatite and sporadically containing zircon grains smaller than 70 µm; (IIb) local oxide microgabbro aggregates with zircon grains as large as 200 µm and quartz; and (III) biotite- and orthopyroxene-bearing plagiogranite veinlets. As an interpretation, a model of sequential differentiation of MORB-type melt is proposed. The compositional variations of the plagioclase and orthopyroxene are consistent with the general sequence of crystal fractionation and show partial overlap of assemblages (II) and (III). In the sequence of zircon-bearing assemblages (IIa), (IIb), and (III), zircon shows a systematic enrichment in Hf. Zircon of assemblage (IIb) hosts melt inclusions. The inclusions were completely remelted at 910°C and quenched into homogeneous glass. The analyzed granitoid (75–76 wt % SiO2) water-bearing (H2O ~ 3 wt %) composition of the inclusions was interpreted as a residual melt after crystallization of the evolved vein gabbro rich in ilmenite. The results offer an example of an evolved granitoid product of MORB differentiation and are consistent with high-degree magmatic differentiation in the oceanic crust, a model according to which the crystallization of evolved oxide gabbro results in a residual silicic melt.

Despite the local occurrence of silicic magmatism during the formation of the oceanic crust, the nature of felsic granitoid veins (“oceanic plagiogranite”) remains an important debatable problem of chemical differentiation in the global system of mid-oceanic ridges. To date, the actual compositions of granitoid melts in the modern oceanic crust are still poorly constrained, and this complicates petrogenetic interpretations. The structural patterns and compositional variations of minerals from the oceanic core complex at 13° N on the Mid-Atlantic Ridge show the sequence of magmatic differentiation: coarse-grained gabbronorite → vein microgabbro enriched in Fe–Ti oxides and apatite → biotite–plagiogranite veins. The transition to silicic magmatism corresponds to the local crystallization of quartz and zircon in the vein microgabbro with the further Zr/Hf fractionation by zircon. Experimental study of melt inclusions in zircon demonstrated homogenization at 850–910°C and the granitoid composition of the melt with K2O >1.8 wt % and SiO2 of 75–76 wt %. Such a K/Si ratio of the silicic melt, along with the genetic relation to highly differentiated gabbroids, allows us to prefer the model of a high degree differentiation of MORB-type magma in the oceanic core complex for the studied plagiogranite–gabbro association.

The geological and geophysical data obtained during the 50th cruise of R/V Akademik Nikolaj Strakhov on the Charlie Gibbs megatransform system structure in the North Atlantic are presented. The structure of the Charlie Gibbs Fracture Zone was examined in detail, considering previously published data. It has been shown that the northern and southern segments of the Mid-Atlantic Ridge, separated by the Charlie Gibbs transform fault, are entirely different in morphology, and hence in terms of formation. The dredged rocks are represented by an entire spectrum from the mantle to upper crustal varieties and allow us to determine the origin of the main structures of the Charlie Gibbs megatransform system considering detailed data on the morphology.

Herein we provide information on the integrated geological, geophysical, sedimentological, paleoceanographic, hydrophysical and biological investigations in the Central Atlantic during the cruise 45 of the R/V "Akademik Nikolaj Strakhov" in October-November 2019. The preliminary scientific results are discussed.

The geological and geophysical data acquired during cruise 45 of R/V Akademik Nikolaj Strakhov regarding the structure of the Doldrums megatransform system in the Central Atlantic are presented. Taking into account the data of the previous expeditions, the structure of this region is examined in detail. The bathymetric data confirm the complex morphology, consisting of five dextral transform faults, separated by four active intra-transform rift segments and indicates the variability of tectonic conditions in this region. The dredged rocks are represented by the entire spectrum from the mantle to the upper crustal varieties and, together with the detailed topographic data, make it possible to identify the origin of the key structures in the Doldrums megatransform system.

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.