Rhyolitic melts in eastern Transbaikalia and the North Caucasus: chemical composition, volatiles, and admixture elements (from data of study of melt inclusions in minerals)

We studied melt inclusions in quartz phenocrysts from Late Jurassic rhyolites in eastern Transbaikalia (Strel’tsovka caldera) and in quartz, apatite, and plagioclase phenocrysts from rhyolites in the North Caucasus (North Ossetia and Tyrnyauz region). More than 15 uranium deposits and ore occurrences are known in the Strel’tsovka caldera, as well as Pb-Zn deposits in North Ossetia and the largest Mo-W deposit in the Tyrnyauz region. For the studies we used the inclusion homogenization methods and performed electron and ion microprobe analyses of glasses from more than 30 inclusions. The temperatures of the total homogenization of melt inclusions are shown to depend directly on the initial melting points of their glasses, which might be due to the different contents of volatiles (first of all, water). The experiments on determining the time required for the homogenization and heterogenization of melt inclusions showed that the melts in the inclusions are of different viscosity and homogenize at different temperatures. The rhyolitic melts from eastern Transbaikalia and the North Caucasus have similar contents of rock-forming elements (average, wt.%): 75.0 and 74.6 SiO2, 0.08 and 0.05 TiO2, 11.0 and 12.3 Al2O3, 0.74 and 0.40 FeO, 0.04 and 0.05 MgO, 0.29 and 0.64 CaO, 4.09 and 3.79 Na2O, and 4.30 and 4.24 K2O, respectively. But these melts differ significantly in the contents of Cl (on average, 0.20 and 0.08 wt.%, respectively), F (1.07 and 0.09 wt.%), and some admixture elements. The melts from the first region are much richer in Zr, Nb, La, Ce, Th, and U but poorer in Sr, Ba, and Eu than the melts from the second region, which evidences their deeper differentiation. The revealed high contents of Th and U in the melts from the Strel’tsovka caldera agree with the literature data of the French geologists on the same caldera.