PALAEOPROTEROZOIC U-PB ZIRCON AGES FROM BELORUSSIA - NEW GEODYNAMIC IMPLICATIONS FOR THE EAST EUROPEAN CRATON

The crust in the northwest of the East European Craton has traditionally been regarded as part of an Archaean nucleus in eastern Europe. Recent correlation with the Baltic Shield in Scandinavia, however, suggests the need of reappraisal. To study this problem, zircons from three boreholes in the basement of the Russian Platform in Belorussia (Belarus) have been dated by the U-Pb and Pb-Pb methods. The ages indicated by two U-Pb discordias are 1982 +/- 26 Ma for a metadacite from the Central Belorussian Belt of volcanic-arc igneous rocks, and 1800 +/- 7 Ma for mafic granulite from the Baltic-Belorussian Granulite Belt largely built up of mafic igneous rocks. For another Central Belorussian metadacite, a Pb-Pb age of 1975 +/- 10 Ma was obtained. In previous ''stratigraphies'' of Belorussia, all three dated rocks have been considered to be important components of an Archaean, even Early Archaean crust. Analyses of abraded crystal cores of complex zircons in one sample and separated core and rim zircons in another do not suggest substantially older ages of the core materials. Together with previous ages of granitoids and charnockites, which had, however, been explained as a result of late magmatism and metamorphism, and therefore not characterizing the age of the crust, our data indicate the dominance of Palaeoproterozoic rocks in the basement of the northwestern part of the East European Craton that adjoins the Tornquist-Line boundary between central and northeastern Europe. In conjunction with the results of a Finnish-Estonian study of similarly allegedly Archaean, but in actual fact Proterozoic rocks in southern Estonia, our results suggest the existence of large Palaeoproterozoic crustal belts extending all the way from the Finnish Gulf to eastern Poland. This implies a substantial revision of established concepts on the Precambrian geology in the northeastern half of the European continent.