Publishing House SB RAS:

Publishing House SB RAS:

Address of the Publishing House SB RAS:
Morskoy pr. 2, 630090 Novosibirsk, Russia



Advanced Search

Russian Geology and Geophysics

2011 year, number 11

RHYOLITIC MELTS IN EASTERN TRANSBAIKALIA AND THE NORTH CAUCASUS: CHEMICAL COMPOSITION, VOLATILES, AND ADMIXTURE ELEMENTS ( from data of study of melt inclusions in minerals )

V.B. Naumov
V.I. Vernadsky Institute of Geochemistry and Analytical Chemistry, ul. Kosygina 19, Moscow, 119991, Russia
Keywords: Melt inclusions, volatiles, admixture elements, eastern Transbaikalia, North Caucasus
Pages: 1368-1377

Abstract

We studied melt inclusions in quartz phenocrysts from Late Jurassic rhyolites in eastern Transbaikalia (Strel'tsov caldera) and in quartz, apatite, and plagioclase phenocrysts from rhyolites in the North Caucasus (Northern Ossetia and Tyrnyauz region). More than 15 uranium deposits and ore occurrences are known in the Strel'tsov caldera, as well as Pb-Zn deposits in Northern 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 Northern 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'tsov caldera agree with the literature data of the French geologists on the same caldera.