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Russian Geology and Geophysics

2012 year, number 10

THE FIRST DATA ON THE EARLY PROTEROZOIC SIALIC BASEMENT IN THE EASTERN WEST SIBERIAN PLATFORM ( studies of the Tyn'yar rhyolite-granite pluton )

K.S. Ivanova, Yu.V. Erokhina, Yu.L. Ronkina, V.V. Khillera, N.V. Rodionovb, and O.P. Lepikhinaa
aA.N. Zavaritskii Institute of Geology and Geochemistry, Ural Branch of the Russian Academy of Sciences, Pochtovyi per. 7, Yekaterinburg, 620075, Russia
bA.P. Karpinsky Russian Geological Research Institute (VSEGEI), Srednii pr. 74, St. Petersburg, 199106, Russia
Keywords: Basement, geochronology, SHRIMP II dating (K-Ar, Rb-Sr, Sm-Nd, U-Pb), Paleozoic, granites, West Siberian megabasin
Pages: 997-1011

Abstract

Despite the long history of research, the presence of Precambrian complexes in the West Siberian basement has not been proven. The Tyn'yarskaya 100 and Tyn'yarskaya 101 wells were drilled in the Vakh-Elogui interfluve, in the eastern West Siberian Plate (eastern Khanty-Mansi Autonomous District). At a depth of 1790 m, they stripped a rhyolite extrusion, which graded into A-type alkali granitoids with rare-metal and REE mineralization (thorite, thorogummite, pitchblende, REE-carbonates, chevkinite, and others) downsection. This volcanoplutonic complex is Early Permian (K-Ar age, ~270 Ma; Rb-Sr age, 275.7 Ma; Sm-Nd age, 276 Ma; U-Pb age, 277 Ma). Some zircons from granites are much older (2049 ± 23 Ma, SHRIMP II), suggesting a relationship between the Early Permian granitic magma and the ancient matter. This might have been a granite-metamorphic basement, the partial melting of which produced the Tyn'yar rhyolite-granite body. The Sm-Nd model ages also suggest the participation of a Precambrian substratum in the formation of the rocks under study. Thus, it is quite possible that the Tyn'yar area is underlain by a Proterozoic (~2 Ga) sialic basement, which is an edge of the Siberian Platform thinned by Late Proterozoic-Early Paleozoic rifting and extension.