S.V. Saraev
T.P. Baturina
V.A. Ponomarchuk A.V. Travin

: Russian Geology and Geophysics

The lithologic composition of basalts from the most complex sections of the Koltogory-Urengoi Rift is considered, and their 40Ar/390Ar ages are given: basalts from the lower section of Nikol'skaya BH-1 (Bergamak Formation) - 272.9 ± 10.5 and 249.5 ± 3.0 Ma, basalts from the uppermost section (Voinov Formation) - 247.5 ± 2.9 Ma, and basalts from the lower section of Tyumenskaya SDB-6 (Korotchaevo Formation) - 251.2 ± 2.0 Ma. According to these and the paleomagnetic data, trap magmatism began in the Koltogory-Urengoi Rift in the Permian and ended in the Early Triassic, whereas the paleontological data suggest its beginning in the Early Triassic and its termination in the late Middle Triassic. In Tyumenskaya SDB-6 we have recognized both low-K tholeiitic and medium- to high-K subalkalic and calc-alkalic basalts, which form large alternating packets up to 480 m thick. This section is juxtaposed to the anomalous sections of the Siberian Platform trap formation near the margins of the Tunguska syneclise. The section of Nikol'skaya BH-1 is composed mainly of medium- and high-K subalkaline basalts with scarce flows of low-K ones. The low-K basalts differ from the medium- and high-K varieties in having lower contents of TiO2, P2O5, Ba, Rb, Th, and ΣLREE. The Tyumenskaya SDB-6 basalts are characterized by paleohypergene alterations, up to the development of more than five levels of alumina-ferruginous cuirasses, as well as superposed chloritization, sericitization (hydromication), carbonatization, zeolitization, silicification, and epidotization, whereas the Nikol'skaya BH-1 basalts are subjected mainly to carbonatization, chloritization, and hydromication. We have concluded that the existence of a vast mega-arch above the superplume within the West Siberian geosyneclise in the Permo-Triassic was complicated by the Koltogory-Urengoi Rift. The Triassic rift deposits are both continental and marine sediments (sandstones and siltstones with glauconite, coccolith oozes, spherulitic lavas, and hyaloclastites). The basic volcanism within the rift showed the features of both rift and flood basalts as well as E-MORB.