Transactions of the Karelian Research Centre of the Russian Academy of Sciences (Oct 2016)
THE ALUMINOUS GNEISSES OF KOLA SERIES, BALTIC SHIELD (GEOCHEMISTRY, NATURE AND AGE OF PROTOLITH)
Abstract
The geochemical and isotopic data were obtained for gneisses of the Kola Series, Baltic Shield. They are metamorphosed in granulite facies and interpreted according to the igneous nature of zircons and gneisses. The biotitic and garnet-biotitic gneisses are paralleled to the rocks of TTG associations in terms of the structure of correlation between the main elements. Sil ± Crd-Grt-Bt gneisses are formed on Grt-Bt gneisses as a result of blastomylonitization and partial melting. U-Pb zircon ages (SHRIMP-II) were determined for both gneiss species. All analytical points of magmatic zircons in the concordia diagram for Grt-Bt gneisses form a continuous trend extended in concordia in the range of 2909 ± 8 – 2732 ± 13 Ma. It is interpreted as the result of lead loss from magmatic zircons during the slow cooling of tonalite in the lower crust. Minimal age of magmatism and early granulitic metamorphism estimated by most ancient zircon is 2.9 Ga. The time of manifestation of younger granulite metamorphism determined by newly-formed zircon shells from Crd-Sil-Grt-Bt gneiss corresponds to the interval 2707 ± 9 – 2656 ± 8 Ma. Part of the zircon from Crd-Sil-Grt-Bt gneiss is profoundly altered crystals with a discordant age of 2740 ± 6 – 2639 ± 9 Ma, which is similar to that of metamorphic shells, suggesting an event happened at that time which disrupted the crystalline grid and caused the loss of radiogenic lead. Three ancient zircons with a maximum age of 3461 ± 5 MA are interpreted as captured zircons, which indicate the presence of continental crust at this point in the northern part of the Baltic Shield.
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