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No 5 (2023)
Articles
Oceanic Crust Formation in the Mid-Atlantic Ridge Segment between Azores and Icelandic Plumes: Results of Geological and Petrogeochemical Studies
Abstract
The structure of the ocean floor and the composition of basalts and dolerites of the MAR segment between the Maxwell and Charlie Gibbs FZs (North Atlantic) were studied based on the data of the 53rd cruise of the R/V “Akademik Nikolaj Strakhov”. It was found that in this segment, along the spreading axis, areas of greater and lesser magmatic productivity alternate, which correspond to higher and lower bottom relief. In areas of high relief, spreading cells form in the axial zone, and rises of various nature dominate in the ridge zone: from tectonic to volcanic. In areas of low relief, the rift valley consists of deep rift basins, and low ridges are developed on the flanks, separated by wide depressions. Oceanic tholeites N, T and E-MORB are distinguished among the studied volcanites. The first of them are ubiquitous and were melted from mainly depleted mantle (source DM). Basalts and dolerites of E-MORB are found in areas of high relief. Their mantle substrate is formed by a mixture of DM and EM-2 material with the subordinate role of HIMU. T-MORB volcanites are mainly localized on large volcanic rises in the southern part of the studied MAR segment and were melted from a substrate formed by a mixture of DM and HIMU material with the subordinate role of EM-2. Two types of mantle inhomogeneities involved in melting are reconstructed: passive and active. The former are represented by blocks of the transformed continental lithosphere that are similar in composition to the EM-2 mantle source. Active inhomogeneities associate with the uplift near Maxwell FZ of the microplume of the enriched mantle with a composition close to HIMU and with its subaxial flow in the north direction up to the Charlie Gibbs FZ.
3-36
Ediacaran and Cambrian Volcanogenic and Sedimentary Complexes of Southern Ulutau (Central Kazakhstan): Structure, Substantiation of Age and Setting of Formation
Abstract
The article presents the results of studying and substantiating the age of the Ediacaran volcanogenic-sedimentary and Cambrian sedimentary strata isolated for the first time within the southern part of the Ulutau terrane (Southern Ulutau) in the west of Central Kazakhstan. Age Estimates (SHRIMP II) obtained 594 ± 3, 594 ± 5, 600 ± 2 Ma for effusive and tufogenic rocks, as well as their isotope-geochemical characteristics, are the first evidence of the manifestation of Ediacaran suprasubduction magmatism in the paleozoics of Kazakhstan and the Northern Tien Shan. The data obtained indicate the participation of the Ulutau terrane at the end of the Precambrian in the structure of the volcanic-plutonic belt, fragments of which are also Neoproterozoic blocks within Southwestern Kazakhstan (the Zeltava and Chui‒Kendyktas terranes) of the Southern Tien Shan and the Karakum‒Tajik terrane. The formation of the Ediacaran suprasubduction belt may be a continuation of the evolution of the Neoproterozoic active continental margin that arose in the Tonian period on the northwestern margin of the supercontinent Rodinia.
37-69
The Nature of the Puchezh-Katunki Impact Structure (the Central Part of the East European Platform): Results of the U‒Th‒Pb Isotope System Study of Detrital Zircons from Explosive Breccias
Abstract
The Puchezh-Katunki crater is located in the central part of the East European Platform in the area of the Gorky Reservoir, has a diameter of ~80 km and is morphologically expressed by the central uplift of the basement (Vorotilov knoll) and the ring depression surrounding it, on the periphery of which there is a ring terrace. The crater is filled with various coptogenic (explosive (?)) formations – breccias of various types, bodies of suvites and tagamites. The results of studying the U‒Th‒Pb isotopic system of detrital zircon grains from variegated explosive Puchezh breccias in the northwestern part of the ring terrace (three samples) are presented. The weighted average of the three youngest U‒Pb datings of detrital zircon from all studied samples is 258 ± 7 Ma, which corresponds to the Late Permian. We took this dating as the lower age limit of the Puchezh breccias. A comparison of the age sets of detrital zircon grains from the samples studied by us and from (i) crystalline rocks of the Vorotilov knoll and ring depression suvites and (ii) Upper Permian–Lower Triassic sandstones of the Zhukov ravine reference section (Moscow syneclise) was carried out. The absence among the detrital zircon from the Puchezh breccia grains, whose U‒Th‒Pb isotopic system is comparable with the parameters of zircon from the rocks of the Vorotilov knoll and suvites, indicates the local nature of the Puchezh-Katunki explosion, in which the impact-thermal impact did not affect the detrital zircons in rocks of the marginal part of the annular terrace of the crater. The high similarity of the sets of ages of detrital zircon grains from the lens of redeposited sandstones of the Puchezh breccias and Upper Permian rocks of the Zhukov ravine section indicates that the Puchezh breccias were formed mainly due to the recycling of the Upper Permian–Lower Triassic sequences underlying the explosive formations. We consider the Uralides paleoorogen as the main source for the deposits of the central regions of the East European Platform in the stratigraphic interval close to the Permian–Triassic boundary. The deposits were formed as a result of a high degree of mixing and averaging of clastic material of sedimentary flows containing the Uralian and Asha provenance signals.
70-95
Geodynamics and Early Cretaceous Magmatism of the Northern Volcanic-Plutonic Belt of Verkhoyan-Kolyma Fold Area (Northeastern Russia)
Abstract
New data on the geological position, U‒Pb SIMS zircon ages, petro-geochemical features, Sr‒Nd isotopic composition and geodynamic setting of the granitoids and volcanites of the Northern volcanic-plutonic belt, Verkhoyan-Kolyma fold area, are presented. Magmatites of the belt include granitoids of Elikchan, Kuranakh, Bakyn plutons, composed of elikchansky granite-granodiorite complex, and volcanites of predominantly intermediate-felsic Tumusskaya sequence with subvolcanic bodies of the same composition. They form single Early Cretaceous (127–121 Ma) volcanic-plutonic assemblage. Granitoid plutons are elongated in sub-latitudinal-northwestern direction and are discordant to main fold and thrust structures. Granitoids intrude and metamorphose Jurassic terrigenous and Early Cretaceous volcanites of Tumusskaya sequence and are cut by younger Late Cretaceous subvolcanic bodies. Granitoids of Bakyn, Elikchan and Kuranakh plutons combine petro-geochemical features of I-, S- and A-type granites. Such diversity of petro-geochemical granitoid types as well as interrelations of major (\({\text{F}}{{{\text{e}}}_{{\text{2}}}}{\text{O}}_{3}^{{{\text{tot}}}}\)–TiO2–MgO) and rare (Ba/La–Nb × 5–Yb × 10) elements in granitoids and the same age volcanites of Tumusskaya sequence allow to refer them to magmatites of transform margin or plates translation boundaries. Collision between Chukotka microcontinent and Siberian continent with earlier accreted Kolyma-Omolon microcontinent in Barremian-Aptian time changed to post-collisional extension and formation of volcanic-plutonic assemblage of the Northern volcanic-plutonic belt. Post-collisional extension took place in the regime of plates translation boundaries. Sr–Nd isotopic characteristics of granitoids of all plutons indicate the interrelation of mantle and crustal sources of granitoid melts in this process.
96-120