Duration of the young (Pliocene) plutonic magmatism in Tyrnyauz ore field, Northern Caucasus: New K-Ar and Rb-Sr data

Isotope-geochronologic investigation of Tyrnyauz rhyolites and vitrophyres was conducted on all available for isolation and further analysis monomineral fractions of these rocks. The dating results show that magmatism in the considered region was completed 1.90-1.95 mln years ago with vitrophyre dike intrusion. The dikes aren't associated with the Elbrus Center volcanism, as was suggested before, since volcanic activity in Elbrus region began near 900 thousand years ago, and the volcano appeared nor earlier than 250 thousand years ago. Formation of granite Eldjurtinskii massif as well as breaking through it subvolcanic rhyolite substances and, in its turn, crossing them vitrophyre dikes proceeded without considerable pauses in the time range 2.5-1.9 mln years, that is, total duration of these processes was not higher than 600 thousand years. The narrow formation time range, similar chemical composition and also equal initial isotope ratios 87Sr/S+86S/Sr (reflecting existing of common magmatic source for all young Tyrnyauz rocks) enable to consider these rocks as integral Late-Pliocene volcano-plutonic complex.