GEOCHEMISTRY AND ISOTOPE COMPOSITION OF PALEOPROTEROZOIC GRANITES AND FELSIC VOLCANICS OF THE ELASH GRABEN: EVIDENCE OF THE HETEROGENEITY OF THE EARLY PRECAMBRIAN CRUST
A.D. Nozhkin1, I. O.M. Turkina1,2, K.A. Savko3
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2Novosibirsk State University, ul. Pirogova 1, Novosibirsk, 630090, Russia
3Voronezh State University, Universitetskaya pl. 1, 394018, Russia
Keywords: Granitoids, felsic volcanics, Nd isotope composition, Paleoproterozoic juvenile crust, southwestern Siberian craton
Abstract
The paper presents results of a petrogeochemical and isotope-geochronological study of the granite-leucogranite association of the Pavlov massif and felsic volcanics from the Elash graben (Biryusa block, southwest of the Siberian craton). A characteristic feature of the granite-leucogranites is their spatial and temporal association with vein aplites and pegmatites of the East Sayan rare-metal province. The U-Pb age of zircon from granites of the Pavlov massif (1852 ± 5 Ma) is close to age of the pegmatites of the Vishnyakovskoe rare-metal deposit (1838 ± 3 Ma). The predominant biotite porphyritic granites and leucogranites of the Pavlov massif show variable alkali ratios (K2O/Na2O = 1.1-2.3) and ferroan (Fe*) index and peraluminous composition; they are comparable with S -granites. The studied rhyolites of the Tagul River (SiO2 = 71-76%) have a low ferroan index, an increased K2O/Na2O ratio (1.6-4.0), low (La/Yb) n = 4.3-10.5, and a clear Eu minimum (Eu/Eu* = 0.3-0.5); they are similar to highly fractionated I -granites. All the coeval Later Paleoproterozoic (1.88-1.85 Ga) granites and felsic volcanics of the Elash graben have distinct differences in composition, especially in the ferroan index and heavy REE content, owing to variations in the source composition and melting conditions during their formation at postcollision extension. A wide range of isotopic parameters of granites and felsic volcanic rocks (ɛNd from +2.0 to -3.7) and zircons (ɛHf from +3.0 to +0.8, granites of the Toporok massif) indicates the heterogeneity of the crustal basement of the Elash graben, which formed both in the Archean and Paleoproterozoic.
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