Magma compositions and genesis of the rocks of the Mushugai- Khuduk carbonatite-bear ing alkaline complex (southern Mongolia): evidence from melt inclusions

Inclusions in mineral-forming media were studied usi ng various techniques including X-ray spectral and ion microanalysis. Based on this study, the compositions, formation conditions, and magma evolution were evaluated in the silicate rocks and ores of the Mushugai-Khuduk carbonatite-bear ing alkaline complex in southern Mongolia. It was found that the rock -forming minerals of melanephelinite, leucite phonolite, shonkinite, theralite, quartz syenite, rhyolite, magnetit e-apatite and cel estite -fluorite rocks crystallized from silicate, salt-silicate, and salt melts. The silicate melts form a continuous series from basic to rhyolitic compositions with Si02 contents from 4 7 to 77 wt %. The salt-silicate melts are silicate-phospha te in composition containing up to 10-20 wt% of P205. The salt melts show phosphate­ carbonate, phosphate-sulpha te, fluoride-sulphat e, and chloride-sulphat e compositions. The analysis of changes in melt compos ition during the crystallization of the complex allowed us to distinguish the processes that were responsible for the formation of specific rocks and ores and their evolution. It was established that the process of crystal fractionation played a leading role in the genesis of the vol canic and plutonic rocks. In addition, liquid immiscib ility contrib ute to the formation of the rocks. The processes of liquid immiscibility were probably crucial for the genesis of the ore-bearing rocks.