Physico-chemical parameters of mineral-forming processes in pegmatites of the ilmenogorsk alkaline complex (South Urals)
and Altai-Sayany Region (Siberia)
The conditions of mineral formation in non-granite pegmatites of the Urals and Siberia are examined. The physico-chemical parameters of the crystallization of minerals in miaskite-pegmatites of the Ilmenogorsk alkaline complex (South Urals) and pyroxenite-pegmatites from ophiolite associations of the Altai-Sayan region (South Siberia) are determined using inclusions of mineral-forming media in minerals and computational modeling based on data on the composition of melt inclusions in COMAGMAT, PETROLOG and PLUTON programs. The study of multiphase (silicate and salt crystals + vapor and liquid) inclusions showed that nepheline miaskite-pegmatites formed from melts-brines at maximum temperatures of 700–750 °C and pressures of 3100–3600 bar. During further decrease in temperature (up to 500–510 °C) and pressure (up to 1200 bar), cancrinite crystallizes from mostly fuid systems. Sodalite forms at the fnal stages of the pegmatite-forming processes (370–440 °C, up to 1000 bar). The analysis of multiphase fuid inclusions with salt phases indicates the involvement of highly saline fuids with temperatures of 445–455 °C during the formation of pyroxenite-pegmatites in serpentinites. Data on melt inclusions in Cr-spinels from ophiolite ultramafc rocks (including clinopyroxenites) indicate the possible crystallization of pyroxenite-pegmatites (hosted in ordinary pyroxenites) in high-temperature (1315–1245 to 1205–1100 °С) magmatic systems.
Figures 5. Tables 10. References 39.
Key words: miaskite-pegmatites, pyroxenite-pegmatites, nepheline, clinopyroxene, fuid and melt inclusions
МINERALOGY № 2 2020