The results of study of chemical composition, mineral-forming medium, P-T conditions of crystallization, and the age characteristics of subvolcanic felsic rocks that are spatially associated with rare-metal granite massifs in the ore units of Transbaikalia (Sherlovaya Gora, Khangilay, Bukuka, Belukha, and Shumilovka) give grounds for defining cogenetic volcanoplutonic associations. These associations within the studied region consist of rare-metal granites, ongonites, rhyolites, ongorhyolites, and trachyrhyodacites, which have much in common, but also many differences. The common chemical features of these rocks are their peraluminium signature, low mafic index and basicity, as well as enrichment (as compared to crust) in trace lithophile elements (Li, Rb, Nb, Ta, Sn, W, and F), the low contents of Zr, REE, and Sr, and the similar distribution of trace and refractory elements. At the same time, these rocks differ in the proportions of sodium and potassium, levels of concentrations of lithophile trace and refractory elements, REE distribution patterns, P-T regimes of crystallization, and the volatile composition. The composition of melts from all types of the studied rocks and trace element distribution between melts and rocks were studied on the basis of ion-microprobe analysis of rehomogenized glasses of melt inclusions in quartz. The highest concentrations of lithophile trace elements in the melt, including Cs (up to 300 ppm), Rb (up to 1002 ppm), U (up to 42 ppm), and Th, were found in the trachyrhyodacites of the Bukuka-Belukha ore unit; in terms of Li content this melt is comparable with the Ary-Bulak ongonites (690 and 715 ppm Li, respectively), and differ by an order of magnitude in the contents of refractory and rare-earth elements (total REE 94.4 and 5.44 ppm, respectively), which is indicative of a lower differentiation degree of this melt as compared to ongonites. Potassic rhyolites are peculiar in the low content of lithophile trace elements, but residual melt reveals notable enrichment in Li (up to 130 ppm) and Nb (up to 120 ppm). The accumulation of U in the residual melt of the trachyrhyodacitic and rhyolitic magmas of Eastern Transbaikalia may indicate their high potential for postmagmatic uranium ore formation. Isotope-geochronological studies (Rb-Sr isotope system) of the Sherlovaya Gora ore unit showed that the entire complex of volcanoplutonic association (granites, ongonites, rhyolites, and ongorhyolites) formed almost simultaneously within an interval of 4 Ma: from 145.7 ± 1.3 Ma at IR Sr = 0.70507 ± 20 and MSWD = 0.48 to 141.5 ± 1.0 Ma at IR Sr = 0.70359 ± 63 Ma and MSWD = 0.24. A spatial association of the subvolcanic rock complex with rare-metal granite massifs, their formation within a common age interval, geochemical features, and P-T conditions of crystallization suggest that they are genetically related but were derived from variably evolved sources, which originated from a single protolith under the action of mantle plume that existed beneath Central Asia at that time (Yarmolyuk and Kovalenko, 2003).
Предметные области Scopus
- Геохимия и петрология