FORMATION AND MESOARCHEAN METAMORPHISM OF HYPERSTHENE GNEISSES FROM THE IRKUT GRANULITE-GNEISS BLOCK ( Sharyzhalgai uplift in the southwestern Siberian craton )
O.M. Turkinaa,b, L.N. Urmantsevaa, N.G. Berezhnayac, and S.G. Skublovd
aV.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
bNovosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia
cA.P. Karpinsky Russian Geological Research Institute (VSEGEI), Srednii pr. 74, St. Petersburg, 199026, Russia
dInstitute of Precambrian Geology and Geochronology, nab. Makarova 2, St. Petersburg, 199034, Russia
Keywords: Paleoarchean, granulite-gneiss complex, zircon composition, U-Pb age, Sharyzhalgai uplift, Siberian craton
Pages: 97-108
Abstract
U-Pb dating (SHRIMP-II) and study of the internal structure and composition were carried out for zircon from hypersthene gneiss from the Irkut granulite-gneiss block (Sharyzhalgai uplift in the southwestern Siberian craton). Three generations of zircon have been revealed in the hypersthene gneiss, which differ in zoning pattern, U and Th concentrations, and REE distribution. Zircon cores with growth zoning relics show a REE pattern typical of magmatic zircon: with a high (Lu/Gd)n value (11-36) and a distinct Ce anomaly (Ce/Ce* = 15-81). They belong to early magmatic generation with an age of ≥3.16 Ga. Multifaced soccerball crystals, rims, and unzoned cores of zircon belong to metamorphogene generation; they are depleted in REE and show a lower (Lu/Gd)n value (1.1-9.2) than the magmatic cores. This zircon generation formed as a result of the Mesoarchean high-temperature metamorphism at ~3.04 Ga. The latest zircon generation includes thin outer rims with low (Lu/Gd)n (11-12.4) and Th/U (0.02-0.05) values and long-prismatic crystals with an oscillatory zoning, which resulted from the Paleoproterozoic (~1.85 Ga) granulite metamorphism and partial melting. The different ages of high-temperature metamorphism in the granulite-gneiss (~3.04 and 2.55-2.6 Ga) and granite-greenstone (~3.2 Ga) blocks of the Sharyzhalgai uplift reflect the independent tectonothermal and geodynamic evolution of crust in these structures, up to the final amalgamation in the Paleoproterozoic (1.88-1.85 Ga).
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