FERROHÖGBOMITE-2S2N IN THE DIATEXITES OF WESTERN SANGILEN, SOUTH-EASTERN TUVA, RUSSIA
In the Western Sangilen (Tuva-Mongolia microcontinent, Central Asian Orogenic Belt, Russia) rare mineral ferrohögbomite-2S2N, for the first time, was found in a deep-seated contact aureole of the early Paleozoic Bayan-Kol gabbro-monzodiorite intrusion. The rocks are diatexites of roof pendant, where metamorphics contact gabbronorite. Diatexites formed as a result of strong melting and desilication of quartz-bearing kyanite-staurolite schists (M1) during progressive thermal metamorphism (M2) near the Bayan-Kol intrusion with peak at 950 °C, 7-8 kbars. Ferrohögbomite-2S2N grains up to 50 μm in size found in ilmenite-corundum-magnetite-hercynite pseudomorphs. The later formed during progressive stage of thermal metamorphism M2 after regional staurolite M1 and are in cordierite-gedrite-plagioclase restite blocks of diatexites. Ferrohögbomite also appeared as thin (1-5 μm) rims around individual grains of hercynite, corundum and magnetite in a rock matrix. According to microtextural features ferrohögbomite grains formed during post-peak metamorphic stage after minerals of pseudomorphs. A univariant mineral reaction of ferrohögbomite-2S2N formation calculated quantitatively in Fe-Al-Ti-Zn-H2O system with program Mathematic 9.0 using electron microprobe chemical composition of minerals of pseudomorphs. The reaction is 0.96Spl+0.194Ilm+0.116Mag+0.036Crn+0.163H2O=0.163Hgb. A local equilibrium value during formation of ferrohögbomite was probably correlated with pseudomorphs size and thus do not transcend 1 mm3. Thermobarometry yields metamorphic temperature and pressure of ferrohögbomite formation as T=665 °C, P=5.8 kbar, which correspond to late retrograde stage of thermal metamorphism of diatexites. To present day in Russia, högbomite group mineral was known only on South Urals and Aldan Shield in Eastern Siberia.