This work discusses the structural and compositional correlation of an unusual group of rocks that comprise post-folding massifs and dikes in the eastern Baltic Shield, which formed between 1.85 and ...1.7 Ga. Occurring from the Barents Sea in the north to the Gulf of Finland in the south, these structures are associated with areas of granulite facies rocks that formed under conditions corresponding to the deep continental crust. Large-scale extension of continental crust, which led to the exhumation of the granulite complex, simultaneous with the formation of metasomatic formations and manifestations of post-folding magmatism, was confined to the periphery of a large circular structure, which is interpreted by us as the Baltic nucleus, or a tectonic portion of the continental crust at the end of early Precambrian. The formation of enriched mantle, a source of the Paleozoic alkaline melts, can also be associated with these processes of extension.
The first data on the multi-isotope composition of sulfur (
32
S,
33
S,
34
S) in samples from the Fennoscandian Shield were obtained by the laser local method. An anomalous concentration of the ...stable isotope
33
S was registered in some samples. Δ
33
S ranges from–0.45 to +0.24‰, which indicates the mass-independent fractionation of S isotopes and provides evidence for the processes of primarily sedimentary accumulation of sulfides in the Archean oxygen-free atmosphere.
The paper reports the results of lithogeochemical studies of the Upper Devonian rocks from the Andoma Hill zone of fold-and-fault dislocations (SE Onega region). The rocks are characterized by the ...negative Eu anomaly (from 0.4 to 0.65) that maks them different from modern sediments of the White Sea. The latter can be regarded as the average composition of mainly Archean (Karelian–Kola) part of the Baltic Shield. In terms of the contents of some trace elements, they also differ from the Vendian rocks of the Zimnii Bereg area. Since the considered rocks are geochemically similar to the Svecofennian metamorphic rocks and Paleoproterozoic granite rapakivi, they could be formed by the erosion of these complexes. The clastic material was transported via a channel confined to the Baltic Shield and Russian Platform junction known as the Polkanov geoflexure.
Research subject.
The Riphean paleorift system of the White Sea, most of which is overlain by the waters of the White and Barents Seas and the platform cover of the East European Platform. This ...allowed numerous researchers to classify it as an aulacogen. The system was revealed by geophysical methods in the relief of the crystalline basement of the platform in the form of a frame of deep extended trenches of northwestern strike, subparallel to the edge of the East European platform.
Materials and methods
. Personal observations of the authors within the Onega-Kandalakcha paleorift, Baikal rift zone; a detailed study of seismostratigraphic sections of these zones; extensive literature data on the structure of modern rift zones. A comparative analysis of the structure of the most studied and currently active Baikal and East African rift systems, as well as the Karoo rift system of the Late Paleozoic origin with the paleorift system of the White Sea.
Results.
The following types of structural parageneses, which are characteristic of both modern rift systems and ancient paleorift systems, were identified. 1. Genetic relationship (inheritance?) of riftogenic structures with more ancient basement structures. 2. Structural paragenesis of concentric complexes in rift propagation zones. 3. Comparability of the area of horizontal extension of the lithosphere of the White Sea paleorift system with extension zones of modern continental rifts. 4. The fundamental similarity of the structure: the complex of paleorifts of the White Sea with modern continental rift systems: the presence of long deep trough segmentation of grabens and semi-grabens separated by bridges, which were accommodation zones with polarity reversal along the strike of the rift zone, displacement of the rift relative to the mantle ledge, the existence of a gently dipping normal fault (detachment), etc.
Conclusion
. The riftogenic nature of the aulacogens in the northeastern segment of the East European Platform has been confirmed.
Results of an experimental study of thermal efficiency of near-wall gas-droplet screens are reported. A description of the experimental setup and procedure used to measure characteristics of interest ...is given. Deposition of droplet from near-wall gas-droplet jets under isothermal conditions and the effects due to the temperature of the co-current flow, the velocity ratio
m, and the thermophysical properties of the vapor–liquid component of the two-phase jet, are examined. A comparison with modeling data is given.
New Data on the Early Riphean Age Terekhov, E. N; Skublov, S. G; Makeyev, A. B ...
Doklady earth sciences,
09/2022, Volume:
506, Issue:
1
Journal Article
Peer reviewed
This work presents data on the age of the volcanites of Sommers Island located to the south of the submarine extension of the Vyborg massif, as potential comagmates of rapakivi granites. The U-Pb ...system of zircon from Early Riphean volcanics is studied to determine their geochronological age using the SHRIMP-II ion microprobe (VSEGEI). For quartz porphyries composing the southern part of Sommers Island, the oldest age determinations of volcanics (1663 ± 8 Ma) were obtained for the Gulf of Finland area for the first time, which can be assigned to the lower parts of the Hogland series of the Early Riphean. In the northern part of the island, there are various rocks: quartz-feldspar porphyries (rhyodacites), andesibasalts, trachybasalts, and granodiorites. All of them underwent modifications under conditions of greenstone metamorphism in contrast to the relatively fresh rocks in the southern part of the island. The trachybasalts (1591 ± 5 Ma) are older than the quartz-feldspar porphyries (rhyodacite, 1578 ± 14 Ma), and a similar modification from older basic rocks to younger acidic ones is typical of all magmatic occurrences of the anorthosite-rapakivi granite formation in the Gulf of Finland region. The "young" ages of the rocks from the northern part of Sommers Island are likely to represent the presence of a younger massif of rapakivi granites than the Vyborg massif in the center of the Gulf of Finland. In this case, the Riphean trough structure traced eastward from Sommers Island is composed not only of rocks of the Hogland series, but part of it correlates with the bottoms of the Pasha graben section and the Priozerskaya Formation.
The structural setting and geochemistry of the Devonian dikes in the Kola Peninsula are considered. The alkaline dikes are controlled by rift- and drift-related structural elements. The first type of ...structures is exemplified by the Khibiny-Kontozero Fault Zone, which is regarded as a propagation zone of the East Barents Rift. The second type comprises Early Precambrian structural elements reactivated during Devonian drift of the East European Plate. Alkaline dikes occur in the ring structures at the ends of rifts and in the accommodation zones where the polarity of the main fault plane changes. These are the sites of accumulation and abrupt relaxation of tectonic stresses. Despite the significant differences in the petrography and chemistry of alkaline dikes, all of them are enriched in REE, apparently due to the elevated concentrations of these elements in the fluids. The dolerite dikes at the Murmansk Coast are located in the outer zone of the vast, concentrically zoned plateau basalt province with the East Barents Trough as its center. According to geophysical data, basaltic flows are suggested within this trough. The dolerite dikes increase in number in the inland propagation zone of the East Barents Trough extending in the Baltic Shield. The alkaline igneous rocks of the Kola Peninsula were formed under local compression at the periphery of the plateau basalt province.
A structural-geochemical study has been conducted on the dikes of presumably Devonian mafic rocks confined to a small graben filled in with Riphean sedimentary rocks hosted by Early Precambrian ...granite-gneiss of the Murmansk block. It has been demonstrated that the dolerite dikes of this region can be considered as manifestations of trap magmatism whose products fill in the foundation of the East Barents riftogenic downfold. In turn, manifestations of alkaline and kimberlite rocks of the White Sea region are confined to the peripheral portion of the trap magnetism area. Zircons from dolerite transecting Late Riphean sediments examined in two laboratories have a concordant age of 2.74–2.72 Ba, while zircons from a similar dike located in granite-gneiss of the basement are characterized by an age range of 2700–155 Ma, and the concordant age based on 4 points is 790 Ma. All these factors indicate that the age determinations of the mafic rocks are ambiguous, particularly in the zone of transition from the center of the trap province to its periphery, where alkaline magmatism is observed.
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