Isotopic evidence for Palaeoproterozoic accretion in the basement of the East European Craton

The Precambrian basement in the western part of the East European Craton (EEC) consists of a series of arcuate tectonic belts metamorphosed in amphibolite and granulite facies. These are separated from each other by crustal discontinuities generally trending NNE-SSW. In conjunction with previously published Sm-Nd and U-Pb isotopic data, the results reported now show that the basement between the Baltic and Ukrainian Shields is Palaeoproterozoic in age. It was formed between 2.1 and 1.8 Ga ago. Igneous rocks from the various arcuate belts generally have positive initial epsilon (Nd) values between +1 and +3. Thus they are essentially juvenile and contain little material derived from older continental crust. Most metasedimentary rocks, in contrast, contain a distinct component of older crustal materials. This is demonstrated by initial epsilon (Nd) values ranging from -1 to -3 and by the results of U-Pb multigrain and single grain (SIMS) analyses of detrital zircons. The oldest magmatic activity in the region occurred 2.1-2.0 Ga ago in the southeast, along the margin of the Archaean protocontinent of Sarmatia. Two subsequent episodes of volcanism in the tectonic belts farther west have been dated at ca. 2.0 and 1.87 Ga using the U-Pb zircon approach. The Proterozoic volcanic rocks are thus younging towards the northwest, away from Sarmatia. The high-grade metamorphism, that has been dated employing U-Pb in monazite and zircon, is also younging from the southeast to the northwest. Its ages range from 1.95 to less than 1.79 Ga. The available data are explained best by accretionary plate tectonics. These involved repeated episodes of generation of juvenile continental crust along and outboard of the margin of the dominantly Archaean Craton in Sarmatia. An Andean-type of geodynamic setting between 2.1 and 2.0 Ga ago was succeeded by the formation of several island arcs and accretion of these onto the older continent between 2.0 and 1.8 Ga. Subsequent compression of the newly formed crust lasted until ca. 1.7 Ga. (C) 2001 Elsevier Science B.V. All rights reserved.