Petrology of granites in the Geramdachansky massif (Verkhoyansk-Kolyma orogenic region)

The authors identified several varieties of different ore content of the late-orogenic and anorogenic granites of the North-east of the Verkhoyansk-Kolyma orogenic region. The purpose of the research was a detailed study of petrography, chemistry, crystallization conditions and geochemical features of granites of one of such massifs – Geramdachansky, which is associated with rare-earth mineralization. The belonging of granites to A-type of the late-orogenic or anorogenic geodynamic setting and wide development of dikes of leucogranites and pegmatites is determined. Typomorphic features of the composition of rock-forming and accessory minerals of granites indicate the beginning of crystallization from the high-temperature melt of the lower crust genesis and saturation of the residual melt of water. Isochronous Rb-Sr age of granites 86-94 million years. Considering the value of primary isotopic ratios of strontium (I₀ from 0.71246+/-0.00072 to 0.7213+/-0.0034), granites are derivatives of melting of the crustal material with a model age of protolith on average 2223±151 million years. The presence of almandine-grossular garnet, comparable to eclogite garnets, among the minerals of granites; high-temperature zircon of crust-mantle morphotypes D and J and enriched with native iron chromium can be explained by the presence of the basic rocks within the magma-forming substrates. A rapid increase of the content of all rare earth elements, as well as uranium and thorium from the granites of the main facies to leucogranites and pegmatites up to commercial values, was identified. The melts that formed all these rocks have close and stable high (920-1000°C) temperatures, which suggest the presence of an external heat source, constantly active during the process of formation. These facts and the presence of the dikes of alkaline-basic composition near the granite outcrops with high concentrations of REE allow us to conclude that massif was formed the under the impact of heat and fluids produced by deep mantle magmas.