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 intrusion of alkaline rocks (southeastern Tuva) are studied. The mechanisms of their formation are identified. 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. It was found that nepheline crystallized in the temperature range of 1100 - 1120°C. It is shown that the glasses of homogenized mеlt inclusions in nepheline are characterized by wide variations in SiO₂ contents, from 37 to 46 wt. %, Al₂O₃ – from 10 to 18 wt. %, CaO – from 5.6 to 19 wt. %, and 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 contains 47 wt. % SiO₂, 1 wt. % TiO₂, 18 wt. % Al₂O₃, 9 wt. % FeO, 7 wt. % CaO and 16 wt. % of 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 the involvement of gravitational and clystallization 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.