Cristobalite Clinker and Paralavas of Ferroan and Melilite–Nepheline Types in the Khamaryn-Khural–Khiid Combustion Methamorphic Complex, East Mongolia: Formation Conditions and Processes

Abstract —Rock samples from the Khamaryn-Khural–Khiid combustion metamorphic (CM) complex, including cristobalite clinker, ferroan tridymite–sekaninaite and cristobalite–fayalite paralavas, which are rock types new to the complex, as well as clinker xenoliths in melilite–nepheline paralava, have been studied in terms of chemistry and mineralogy. The obtained data on rock-forming, minor, accessory, and rare phases (silica polymorphs, cordierite-group minerals, fayalite, Fe and Ti oxides, ferrosilite, etc.) have implications for the formation conditions and processes of the CM rocks. The Raman spectra of sekaninaite, indialite, ferroindialite, mullite, and anhydrous Fe–Ca–Mn phosphate, presumably from the graftonite group, have several specific features. The diversity of mineral assemblages in the CM rocks is due to heterogeneous lithology of the sedimentary protolith and to local effects in the multistage history of the Khamaryn-Khural–Khiid complex. According to geochemical data, all CM rocks of the complex are derived from the Early Cretaceous Dzunbain Formation, their protolith molten to different degrees. The cristobalite clinker and tridymite–sekaninaite and cristobalite–fayalite paralavas were produced by partial melting of pelitic rocks containing different amounts of iron in a wide temperature range. The formation of mullite developed from dehydration–dehydroxylation and incongruent partial melting of amorphous pelitic matter. Large-scale crystallization of mullite in clinker, occurred from the high-silica potassic aluminosilicate melt at >850 °C. Combustion of subsurface coal seams heated the overburden to >1050 °C or locally to >1300–1400 °C (melting point of detrital quartz) or even, possibly, to >1470 °C corresponding to the stability field of β-cristobalite. Melilite–nepheline paralava was formed by incongruent melting of silicate (pelitic) and carbonate (calcite) components of marly limestone under elevated CO2 partial pressure. Oxygen fugacity (fO2) during combustion metamorphism changed from strongly reducing conditions favorable for crystallization of Fe phosphides (barringerite, schreibersite) and metallic iron from silica-undersaturated melts parental to melilite–nepheline paralava to high fO2 values that can maintain the formation of hematite in Fe-rich CM rocks.