Genetic Typification of Zircons from Infrastructure of the Elbrus Subterrane (Caucasus)

摘要

In the crystalline substrate of the Elbrus subterrane intensively metamorphosed, un-stratified, migmatizated infrastructure and less metamorphosed, unmigmatizated stratified supra-structure built up of mainly mica schists are distinguished distinctly. The oldest formations of the Elbrus subterrane infrastructure are represented by garnet-cordierite-sillimanite-K-feldspar bearing gneisses and migmatites that formed under temperature conditions of biotite-sillimanite-K-feldspar bearing and garnet-cordierite-orthoclase bearing facies of metamorphism. Opinions on the age of protoliths of these formations and of regional metamorphism extremely differ and it covers a time span from the Proterozoic to Late Paleozoic. The age of these rocks corresponds to the Late Baikalian and Ordivician phases of folding. At the next stage, in the infrastructure granodiorite gneisses (so-called Adylsu type orthogneisses) were formed. The next stage of regional metamorphism is connected with the Bretonian orophase of folding and covers mainly temperature conditions of facies of biotite-muscovite bearing gneisses. Late Variscan granites that developed in the infrastructure are represented by two varieties -porphyroblastic and equigranular (so-called Ulukami granites) granites. In the Elbrus infrastructure 5 genetic types of zircons are established: 1) detrital; 2) connected with the formation of gneiss-migmatitic complex; 3) developed in granodioritic orthogneisses; 4) formed during the Bretonian regional metamorphism and 5) Late Variscan granites.