EARLY STAGES IN THE EVOLUTION OF THE ANABAR SHIELD CRUST

Abstract

The predominant rocks of the Anabar shield are basic pyroxene-plagioclase schists, metaultramafics, and various gneisses and granites with a minor number of partings of amphibolite, quartz, marble, calciphyre and other rocks. The internal structure of the Anabar Shield is very complex and was produced over a long period. It is the result of a succession of many tectonic cycles, which superimposed on each other deformations, of different kinds and scales, that occurred under different geodynamic conditions. The authors collected two series of samples for geochronological tests, namely two-pyroxene plagioclase-containing schists of the Daldyn and Upper Anabar Series from the central part of the shield, and massive or slightly gneissic pyroxene-containing plagiogranite-gneiss from the basin of the Knyazeva River on its eastern periphery. They determined K, Rb and Sr in these samples by flame photometry. The radiometric ages for the Early Precambrian rocks of the Anabar Shield indicate rather reliably that they contain relicts of the primary mafic protocrust and suggest that the two-pyroxene plagioclase-containing schists of the basement of the Siberian craton are among the oldest terrestrial rocks.