CRYSTALLIZATION CONDITIONS, MAGMA COMPOSITIONS, AND GENESIS OF SILICATE ROCKS OF THE MUSHUGAI-KHUDUK CARBONATITE-BEARING ALKALIC COMPLEX, SOUTHERN MONGOLIA: EVIDENCE FROM MELT INCLUSIONS

Abstract

Inclusions of mineral-forming media were studied using various techniques including X-ray spectral and ion microanalysis. Based on this study, the compositions, formation conditions (temperature and pressure), and magma evolution were evaluated in the silicate rocks of the Mushugai-Khuduk carbonatite-bearing alkalic complex in southern Mongolia. It was found that the rock-forming minerals of melanephelinite, leucite phonolite, shonkinite, theralite, 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 SiO 2 contents from 47 to 77 wt %. These melts show high concentrations of phosphorus, CO 2, F, S, trace (Ba and Sr), and rare-earth elements. The salt-silicate melts are silicate-phosphate in composition containing up to 10-20 wt % of P 2O 5. In fact, they can be regarded as ore magmas. The salt melts show phosphate-carbonate compositions. A distinctive feature of the phosphate-carbonate melts is the elevated content of rare earth elements and fluorine. The crystallization of magmas of the alkalic rocks occurred in a wide temperature range, from 1220 to 850°C at 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 their evolution. It was established that the process of crystallization differentiation of magma compositionally similar to melanephelinite played a leading 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 immiscibility 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.