Using electron and ion microprobe analysis, we investigated inclusions of mineral-forming media in minerals from the rare-metal (Nb, Zr, and REE) comendites of the Late Paleozoic bimodal volcanic association of the Dzarta Khuduk occurrence (Central Mongolia). Primary crystalline and melt inclusions were found in quartz from the comendites. Amphibole (riebeckite), ilmenite, and the rare diorthosilicate of REE chevkinite were identified among the crystalline inclusions. The melt inclusions contain glass, a gas phase, and a fine-grained aggregate consisting of villiaumite (NaF) and gricite (LiF). The melt inclusions were homogenized at 1030oC.
It was shown that the latest magmatic products of the Dzarta Khuduk bimodal association were formed from highly differentiated peralkaline silicic melts strongly enriched in trace (Li, Rb, Zr, Nb, and Y) and rare earth elements and volatile components (H2O, F, and Cl). The observed rare-metal melts are in fact ore magmas, which reflect the geochemical peculiarity of the massif.
The evolution of the comendite melts is controlled by crystal fractionation and silicate-salt liquid immiscibility. The observed phase relations in melt inclusions in quartz of the comendites provide evidence for the occurrence of immiscibility phenomena between silicate and salt melts. These results indicate the possibility of separation of fluoride melts enriched in Na and F during alkaline magma differentiation.