Crystallization conditions, magma compositions, and genesis of silicate rocks of the Mushugai-Khuduk carbonatite-bearing alkalic complex, southern Mongolia: Evidence from melt inclusions

Inclusions of mineral-forming media were studied using various techniques i ncluding X-ray spectral and ion microanalysis. Based on this study, the com positions, formation conditions (temperature and pressure), and magma evolu tion were evaluated in the silicate rocks of the Mushugai-Khuduk carbonatit e-bearing alkalic complex in southern Mongolia. It was found that the rock- forming minerals of melanephelinite, leucite phonolitc, shonkinite, therali te, quartz syenite, and rhyolite crystallized from silicate, salt-silicate, and salt melts. The silicate melts form a continuous series from basic to rhyolitic compositions With SiO2 contents from 47 to 77 wt %. These melts s how high concentrations of phosphorus, Co-2, F, S, trace (Ba and Sr), and r are-earth elements. The salt-silicate melts are silicate-phosphate in compo sition containing up to 10-20 wt % of P2O5. In fact, they can be regarded a s ore magmas. The salt melts show phosphate-carbonate compositions. A disti nctive feature of the phosphate-carbonate melts is the elevated content of rare earth elements and fluorine. The crystallization of magmas of the alka lic rocks occurred in a wide temperature range, from 1220 to 850 degreesC a t a pressure of 3 kbar. The analysis of changes in melt composition during the crystallization of the complex allowed us to distinguish the processes that were responsible for the formation of specific rocks and ores and thei r evolution. It was established that the process of crystallization differe ntiation of magma compositionally similar to melanephelinite played a leadi ng role in the genesis of the volcanic and plutonic rocks. This resulted in the generation of a wide spectrum of silicate melts. In addition, liquid i mmiscibility and mixing of chemically contrasting igneous melts contributed to the formation of the rocks. The processes of liquid immiscibility were probably crucial for the genesis of the ore-bearing rocks.