RUBIDIUM-STRONTIUM AGE OF GRANITOIDS OF THE AMANANSKIY COMPLEX, EASTERN TRANSBAIKALIA

Abstract

For the eastern part of the region, the view that amphibole-biotite granitoids should be assigned to the Amananskiy complex is not supported clearly enough by petrologic and geochemical criteria or by geochronologic isotope data. Studying the chemistry of granitoids of large plutons in the eastern part of the Olekminskiy Stanovik Range and dating them would partly help to solve this problem. To determine the age of the granitoids, we used the Rb-Sr method, which differs from the customarily used K-Ar method in the possibility of self-controlled dating and obtaining genetic information. A suite of five to seven unaltered granitoid samples was used. As further support for the resulting values of isotopic parameters such as the time of crystallization of the granitoids and the strontium isotopic composition of their primary melt, we plotted control mineral isochrons that included one sample and mineral separates from it. there are differences in the time of crystallization of the granitoids of the Itaka and Malokovali plutons, which earlier were assigned to the Amananskiy complex. The fact that these differences correlate with those in the geochemical characteristics of granitoids gives us grounds for identifying an Early Jurassic (late Amananskiy) intrusive granitoid complex in the eastern part of the Olekminskiy Stanovik Range and distinguishing it from the Early Triassic Amananskiy complex in the western part.