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 traditional ly been regarded as part of an Archaean nucleus in eastern Europe. Rec ent correlation with the Baltic Shield in Scandinavia, however, sugges ts the need of reappraisal. To study this problem, zircons from three boreholes in the basement of the Russian Platform in Belorussia (Belar us) 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 Ce ntral Belorussian Belt of volcanic-arc igneous rocks, and 1800 +/- 7 M a for mafic granulite from the Baltic-Belorussian Granulite Belt large ly built up of mafic igneous rocks. For another Central Belorussian me tadacite, a Pb-Pb age of 1975 +/- 10 Ma was obtained. In previous ''st ratigraphies'' of Belorussia, all three dated rocks have been consider ed to be important components of an Archaean, even Early Archaean crus t. Analyses of abraded crystal cores of complex zircons in one sample and separated core and rim zircons in another do not suggest substanti ally older ages of the core materials. Together with previous ages of granitoids and charnockites, which had, however, been explained as a r esult of late magmatism and metamorphism, and therefore not characteri zing the age of the crust, our data indicate the dominance of Palaeopr oterozoic rocks in the basement of the northwestern part of the East E uropean Craton that adjoins the Tornquist-Line boundary between centra l and northeastern Europe. In conjunction with the results of a Finnis h-Estonian study of similarly allegedly Archaean, but in actual fact P roterozoic rocks in southern Estonia, our results suggest the existenc e of large Palaeoproterozoic crustal belts extending all the way from the Finnish Gulf to eastern Poland. This implies a substantial revisio n of established concepts on the Precambrian geology in the northeaste rn half of the European continent.