RADIOACTIVE ELEMENTS IN THE EVOLUTION OF THE EARTHS CRUST ON THE SOUTHWESTERN PART OF THE SIBERIAN PLATFORM

Epochs of the formation of Na-K-granitoids and acid volcanites are ass ociated with discrete supplies of U, Th, and K to the upper layer of t he Earth's crust. Eight epochs of essentially potassium granite format ion and subalkaline magmatism have been recognized in the evolution of the Earth's crust on the southwest of the Siberian Platform; a radioc hemical description is given to corresponding rocks and complexes. It is emphasized that the Early Archean, Early Proterozoic, and Late Prot erozoic events of acid magmatism play a special role in the formation of the radiogeochemical background; they are responsible for the furth er growth, sialization of continental crust and repeated supply of rad ioactive elements to its upper layer. The main stages and related proc esses responsible for redistribution and concentration of U and Th hav e been shown. Post-Proterozoic epochs of tectonomagmatic activity are very important for the formation of uranium, uranium-thorium, and rela ted rare-metal mineralization within the rifting structures of the Pre cambrian platform basement.