ISOTOPIC AGE, NATURE, AND STRUCTURE OF THE PRECAMBRIAN CRUST OF BYELARUS

The age and primary nature of the Precambrian crust of the Belarus bas ement were established by using the U-Pb isotope datings of accessory zircons and by means of Sm-Nd isotope studies of whole rock samples fr om the main structural complexes of the basement. Archean crustal mate rial is absent in the studied structures of the larger part of Belarus . The crust of the Belarussian-Baltic granulite belt and of the Centra l Belarussian belt was formed during a period of 2 - 1.8 Ga in the Pal eoproterozoic, thus making these formations concurrent with the Svecof ennids of the Baltic shield. Archean crustal material was found only a mong the metapelites of the Bragin granulite block correlative with th e formations of the Ukrainian shield. The new data allow us to extend the Svecofennian province of the Baltic shield to the south up to its boundary with the Osnitsk-Mikashevichi igneous belt. The crustal struc ture in this part of the East European platform, however, differs esse ntially from that of Svecofennids in the Baltic shield because of a un ique alternation of the metamorphic belts. This type of crustal struct ure is indicative of specific geodynamic settings during the formation of the Paleoproterozoic crust in the Baltic area and in Belarus.