Stages of the lower crust formation of the Belomorian mobile belt,Kola Peninsula

Doklady Earth Sciences -     

Geochemical features of the Riphean fine-grained terrigenous rocks of the Southern Urals,Uchur-Maya region,and Yenisei Range: Estimation of the maturity of the pre-Riphean continental crust and its evolution within 1.65–0.6 Ga

The geochemical features of basal fine-grained terrigenous rocks from the Riphean sedimentary megasequences of the Southern Urals, Uchur-Maya region, and Yenisei Range were compared in order to estimate the maturity of the continental crust that was formed by the beginning of the Riphean. It was shown that initial shales from the base of the Riphean sequence of the Yenisei Range and fine-grained aluminosiliciclastic rocks from the base of the Riphean sections of the Southern Urals were formed by the erosion of a rather mature continental crust. In contrast, fine-grained terrigenous rocks from the base of the Riphean of the Uchur-Maya region were derived from immature Late Archean protoliths or their Early Proterozoic analogs. The fine-grained terrigenous rocks of the three sedimentary megasequences show different variations in the (La/Yb)_N ratio. In the Southern Urals, this ratio is high (12–15) in the Burzyan Group and decreases upsection to 6–10. In the shales of the Uchur-Maya region, the (La/Yb)_N ratio decreases upsection, and the La/Sc ratio shows a sympathetic behavior. This is due to a decrease in the proportion of “primitive” tonalite-trondhjemite associations of the Archean granite-greenstone terranes in the provenance area with time and the appearance of intra-plate (riftogenic?) granitoids and significant amounts of basic and ultrabasic rocks. The latter marks the onset of large rift-forming events in the Uchur-Maya region at the beginning of the Late Riphean. The (La/Yb)_N of the studied rocks from the Yenisei Range are mostly similar to the PAAS ratio, but higher values were found in the Upper Vorogovka and Chingasan groups, which was related to the contribution of strongly LREE-enriched granitoids and rift felsic and alkali basaltic volcanic associations to the formation of the terrigenous material. A comparison of Rb, Sr, Y, Zr, Ba, Hf, Th, U, Cr, and Ni contents and Zr/Y, (La/Yb)_N, Ni/Co, Cr/Th, Cr/Sc, and La/Th ratios in the fine-grained terrigenous rocks of the Riphean megasequences of the Southern Urals, Uchur-Maya region, and the Yenisei Range with those in the model geochemical objects (PAAS, UC_PR1, UC_AR2, and others) showed that, in terms of most of the parameters, the Riphean fine-grained terrigenous rocks from the three regions are similar to each other, PAAS, and Proterozoic cratonic shales. This indicates a fairly high general maturity of the protoliths that were eroded during the Riphean in the eastern East European craton and in the southeastern and southwestern parts of the Siberian craton.     

Magmatism evolution and carbonatite-granite association in the neoproterozoic active continental margin of the Siberian craton: Thermochronological reconstructions

Neoproterozoic carbonatites and related igneous rocks, including A-type granites in the Tatarka-Ishimba suture zone of the Yenisey Ridge are confined to a horst-anticlinal structure that was formed in a transpression setting during the oblique collision between the Central Angara terrane and the Siberian craton. The carbonatites, associating mafic (including alkaline) dikes as well as the Srednetatarka nepheline syenites are the oldest igneous formations of the Tatarka active continental margin complex. Geochronological data indicate that magmatic evolution continued in the studied anticline for nearly 100 m.y. On the earliest stage carbonatites were formed and on the last stage — the emplacement of mantle-crustal A-type Tatarka granites took place. According to new U/Pb zircon studies, the earliest rocks in the Tatarka pluton are A-type leucogranites aged 646 ± 8 Ma. The younger ⁴⁰Ar/³⁹Ar ages of carbonatites obtained for phlogopites (647 ± 7 and 629 ± 6 Ma) are related to the last tectonic events in the studied region of the Tatarka-Ishimba suture zone, which are coeval with the formation of the A-type granitoids (646–629 Ma).     

The Age of Nb Rare-Metal Mineralization of the Ilmeny–Vishnevogorsky Alkaline Complex (South Urals)

Doklady Earth Sciences - The results of local U–Th–Pb-SIMS-SHRIMP-II analysis of the collection of pyrochlores from Nb rare-metal deposits of the Ilmeny–Vishnevogorsky alkaline...     

The Late Riphean–Paleozoic history of the Uda–Vitim island arc system in the Transbaikalian sector of the Paleoasian ocean

New structural, petrological, chemical, isotope, and paleomagnetic data have provided clues to the Late Riphean–Paleozoic history of the Uda–Vitim island arc system (UVIAS) in the Transbaikalian sector of the Paleoasian ocean, as part of the Transbaikalian zone of Paleozoids. The island arc system consists of three units corresponding to main evolution stages: (i) Upper Riphean (Late Baikalian), (ii) Vendian–Lower Paleozoic (Caledonian), and (iii) Middle–Upper Paleozoic (Hercynian). The earliest stage produced the base of the system composed of Late Riphean ophiolite (971–892 Ma, U-Pb) and volcanic (837–789 Ma, U-Pb) and sedimentary rocks (hemipelagic siliceous sediments and dolerite sills) which represent the Barguzin–Vitim oceanic basin and the Kelyana island arc. The main event of the second stage was the formation of the large UVIAS structure (over 150,000 km2) which comprised the Transbaikalian oceanic basin, the forearc and backarc basins, and the volcanic arc itself, and consisted of many volcanic-tectonic units exceeding 100 km² in area (Eravna, Oldynda, Abaga, etc.). Lithology, stratigraphy, major–element compositions, and isotope ages of Vendian–Cambrian volcanic rocks and associated sediments indicate strong differentiation of calc-alkaline series and the origin of the island arc system upon oceanic crust, in a setting similar to that of the today’s Kuriles–Kamchatka island arc system. The Middle–Upper Paleozoic stage completed the long UVIAS history and left its imprint in sedimentary and volcanic rocks in superposed trough basins. The rocks were studied in terms of their biostratigraphic and isotope age constraints, as well as major- and trace-element compositions, and were interpreted as products of weathering and tectonic-magmatic rework of the UVIAS units.