Genesis of the Khaluta alkaline-basic Ba-Sr carbonatite complex (West Transbaikala, Russia)

Abstract

The Khaluta carbonatite complex comprizes fenites, alkaline syenites and shonkinites, and calcite and dolomite carbonatites. Textural and compositional criteria, melt inclusions, geochemical and isotopic data, and comparisons with relevant experimental systems show that the complex formed by liquid immiscibility of a carbonate-saturated parental silicate melt. Mineral and stable isotope geothermometers and melt inclusion measurements for the silicate rocks and carbonatite all give temperatures of crystallization of 915–1,000°C and 890–470°C, respectively. Melt inclusions containing sulphate minerals, and sulphate-rich minerals, most notably apatite and monazite, occur in all of the lithologies in the Khaluta complex. All lithologies, from fenites through shonkinites and syenites to calcite and dolomite carbonatites, and to hydrothermal mineralisation are further characterized by high Ba and Sr activity, as well as that of SO3 with formation of the sulphate minerals baryte, celestine and baryte-celestine. Thus, the characteristic features of the Khaluta parental melt were elevated concentrations of SO3, Ba and Sr. In addition to the presence of SO3, calculated fO2 for magnetites indicate a high oxygen fugacity and that Fe+3>Fe+2 in the Khaluta parental melt. Our findings suggest that the mantle source for Khaluta carbonatite and associated rocks, as well as for other carbonatites of the West Transbaikalia carbonatite province, were SO3-rich and characterized by high oxygen fugacity.