Osumilite-Bearing Lavas of the Keli Highland (Greater Caucasus): Petrological and Geochemical Characteristics, Mineral Composition, and Conditions of Melt Generation

The paper reports comprehensive petrological, geochemical and mineralogical studies of osumilite-bearing andesite-dacitic lavas of Kordieritoviy Volcano (Keli Highland, Greater Caucasus) erupted at the end of the Pleistocene (about 200 ka). The petrographic and microprobe analysis showed that the rocks contain three paragenetic mineral associations: (1) “xenogenic” (metamorphic) association consisting of garnet (X_Prp = 0.42, X_Alm = 0.51–0.53, X_Grs = 0.04–0.05) + hercynite + sapphire + bronzite + pargasite + ilmenite; (2) early magmatic association represented by hypersthene + hercynite + garnet (X_Prp = 0.21–0.31, X_Alm = 0.52–0.71, X_Grs = 0.04–0.13) + ferro-kaersutite + ilmenite; (3) late magmatic association including hypersthene-ferrohypersthene + labradorite + garnet (X_Prp = 0.04–0.14, X_Alm = 0.65–0.81, X_Grs = 0.06–0.18) + osumilite-(Mg) + phlogopite + tridymite + ilmenite + apatite. The osumilite-(Mg) (phenocrysts, xenomorphic aggregates in the rock matrix, and crystals in miarolic cavities), the average formula for dacites of Kordieritoviy Volcano can be written as (K_0.73Na_0.06Ca_0.02\({{\square }_{{0.20}}}\))_1.00(Mg_1.06\({\text{Fe}}_{{{\text{0}}{\text{.90}}}}^{{{\text{2 + }}}}\)Mn_0.04)_2.00(Al_2.75\({\text{Fe}}_{{{\text{0}}{\text{.18}}}}^{{{\text{2 + }}}}{\text{Fe}}_{{{\text{0}}{\text{.06}}}}^{{{\text{3 + }}}}\)Ti_0.01)_3.00(Si_10.34Al_1.66)₁₂O₃₀, formed mainly at late magmatic stages – in intermediate chambers immediately prior to the rise of the melt to the surface or after its eruption. Accordingly, this mineral in the studied lavas has a purely magmatic origin. Thermobarometric calculations and petrological modeling showed that the deep magma chamber of Kordieritoviy Volcano was located at a depth of 45–53 km near the Moho discontinuity. 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 lava ejection occurred at 1030–870°C and pressure progressively decreasing from 14–9 to 1 kbar. A petrogenetic model has been proposed to explain the genesis of exotic osumilite-bearing lavas of Kordieritoviy Volcano. It includes several stages: (1) formation of an enriched upper-mantle source (lithospheric mantle metasomatized by permanent interaction at the Moho discontinuity with the overlying lower crust composed of metamorphosed terrigenous-volcanogenic complexes); (2) generation of “dry” basaltic magmas in the source; (3) crystallization differentiation in the source (fractionation of olivine and chromium spinels) with the formation of a “dry” overheated andesitic melt; (4) limited-scale assimilation of the lower crustal material made up of leucocratic granulites by highly differentiated andesitic melts rising to the surface, with simultaneous fractionation of garnet, orthopyroxene and ilmenite from the melt.