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 (E EC) consists of a series of arcuate tectonic belts metamorphosed in amphibo lite and granulite facies. These are separated from each other by crustal d iscontinuities generally trending NNE-SSW. In conjunction with previously p ublished Sm-Nd and U-Pb isotopic data, the results reported now show that t he basement between the Baltic and Ukrainian Shields is Palaeoproterozoic i n age. It was formed between 2.1 and 1.8 Ga ago. Igneous rocks from the various arcuate belts generally have positive initia l epsilon (Nd) values between +1 and +3. Thus they are essentially juvenile and contain little material derived from older continental crust. Most met asedimentary rocks, in contrast, contain a distinct component of older crus tal 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 s outheast, along the margin of the Archaean protocontinent of Sarmatia. Two subsequent episodes of volcanism in the tectonic belts farther west have be en dated at ca. 2.0 and 1.87 Ga using the U-Pb zircon approach. The Protero zoic volcanic rocks are thus younging towards the northwest, away from Sarm atia. The high-grade metamorphism, that has been dated employing U-Pb in mo nazite and zircon, is also younging from the southeast to the northwest. It s ages range from 1.95 to less than 1.79 Ga. The available data are explained best by accretionary plate tectonics. Thes e involved repeated episodes of generation of juvenile continental crust al ong and outboard of the margin of the dominantly Archaean Craton in Sarmati a. An Andean-type of geodynamic setting between 2.1 and 2.0 Ga ago was succ eeded by the formation of several island arcs and accretion of these onto t he older continent between 2.0 and 1.8 Ga. Subsequent compression of the ne wly formed crust lasted until ca. 1.7 Ga. (C) 2001 Elsevier Science B.V. Al l rights reserved.