Composition and Formation Conditions of Urtite Magmas of the Chik Intrusion of Alkaline Rocks (Southeastern Tuva): Data from the Study of Melt Inclusions

Based on the method of melt inclusion study, the magma composition and formation conditions of melts involved in the formation of urtites of the Chik alkaline intrusion (southeastern Tuva) have been examined and mechanisms of their formation are shown. Primary melt inclusions are studied in nepheline and garnet from the urtite. Amphibole, sphene, garnet, sodalite, banalsite, calcite, and zeolite are identified among the daughter minerals of the melt inclusions. The nepheline was found to have crystallized in the temperature range of 1100—1120°C. It is shown that the glasses of homogenized melt inclusions in nepheline are characterized by a wide variations in SiO2 contents ranging from 37 to 46 wt %, Al2O3 from 10 to 18 wt %, and CaO from 5.6 to 19 wt %, with increased concentrations of alkalis (11–15 wt %), FeO (11–13 wt %), TiO2 (up to 3.4 wt %), and P2O5 (up to 2.5% wt %). The chemical composition of the glasses of homogenized melt inclusions in garnet is similar to the most differentiated composition of the glasses of melt inclusions in nepheline. It has 47 wt % SiO2, 1 wt % TiO2, 18 wt % Al2O3, 9 wt % FeO, 7 wt % CaO, and 16 wt % alkalis (Na2O + K2O). The results of the study of melt inclusions in urtite minerals, as well as the analysis of the composition of various rocks of the Chik Massif (urtites, melteigites, and ijolites, including their calcite- and garnet-containing varieties), suggest involvement of gravitational and crystallization differentiation of magma in their formation. The data obtained refute the hypothesis of the metasomatic nature of the urtites of the Chik intrusion of alkaline rocks (southeastern Tuva) and provide compelling evidence for their magmatic genesis.