Osumilite-bearing lavas of the Keli highlands (Greater Caucasus): petrological and geochemical characteristics, mineral composition and conditions of magmatic melts formation

Comprehensive petrological, geochemical and mineralogical studies of osumilite-bearing andesite-dacitic lavas of the volcano Kordieritoviy (Keli Highland, Greater Caucasus), erupted at the end of the Pleistocene (about 200 Ka), were carried out. The results of petrographic study of thin sections and microprobe analysis showed that the rocks contain three paragenetic mineral associations: (1) “xenogenic” (metamorphogenic) – garnet (XPrp = 0.42, XAlm = 0.51–0.53, XGrs = 0.04–0.05) + hercynite + sapphire + bronzite + pargasite + ilmenite; (2) early magmatic – hypersthene + hercynite + garnet (XPrp = 0.21–0.31, XAlm = 0.52–0.71, XGrs = 0.04–0.13) + ferro-kersutite + ilmenite; (3) late magmatic – hypersthene-ferrohypersthene + labradorite + garnet (XPrp = 0.04–0.14, XAlm = 0.65–0.81, XGrs = 0.06–0.18) + osumilite-(Mg) + phlogopite + tridymite + ilmenite + apatite. Osumilite-(Mg) (phenocrysts, xenomorphic aggregates in the rock matrix and crystals in miarolic cavities), the average formula of which for dacites of the Kordieritoviy volcano can be written as (K0.73Na0.06Ca0.02□0.20)1.00(Mg1.06Fe2+0.90Mn0.04)2.00(Al2.75Fe2+0.18Fe3+0.06Ti0.01)3.00(Si10.34Al1.66)12O30, formed mainly at late magmatic stages – in intermediate chambers immediately before the rise of the melt to the surface or after its eruption. Accordingly, this mineral in the studied lavas has purely magmatic origin. Thermobarometric calculations and petrological modeling showed that the deep magma chamber of the Kordieritoviy volcano was located at a level of 45–53 km from the surface in near the Moho boundary. The temperature of the melt at the early magmatic stage was no less than 1100°C at 17–23 kbar. Crystallization of osumilite-(Mg) in intermediate magmatic chambers (at depths of 30–40 km) and during the process of lava outpouring occurred at 1030–870°C and pressure progressively decreasing from 14–9 to 1 kbar. A petrogenetic model has been proposed to explain the reasons for the formation of exotic osumilite-containing lavas of the Kordieritoviy volcano. Its main provisions include: (1) enriched upper mantle source (lithospheric mantle metasomatized as a result of permanent interaction at the Moho boundary with the overlying lower crust composed of metamorphosed terrigenous-volcanogenic formations); (2) generation of “dry” basaltic magmas in the source; (3) crystallization differentiation in the source (fractionation of olivine and chrome spinels) with the formation of a “dry” superheated andesitic melt; (4) limited-scale assimilation by highly differentiated andesitic melts rising to the surface of the material of the lower crust, directly below the volcano, composed of leucocratic granulites, with simultaneous fractionation of garnet, orthopyroxene and ilmenite from the melt.