Carbonate-silicate equilibria in the high-magnesia ultrapotassic volcanics of the Toro-Ankole Province (Eastern African rift zone)

摘要

This work reports new data on phenocryst composition and carbonate-silicate equilibria in the volcanic rocks of kamafugitic affinity in the Toro-Ankole Province (East African rift zone). Inclusions of primary carbonates (calcite and dolomite) were found in olivines from ugandite and mafurite of the Bunyaruguru volcanic field. The initial compositions of melt inclusions in olivine from the ugandite were calculated from microprobe analyses and correspond to carbonatites. The find of barite and dolomite-barite inclusions in the olivine from the mafurite indicates that the melt contained sulfate sulfur, which is typical of oxidizing conditions. The calculation of the olivine-spinel equilibrium (T - f^sub O2) showed that crystallization of phenocrysts in mafurite occurred at oxygen fugacities above the NNO buffer (ΔQFM ~ 2.5) in a wide temperature range (1230-750 deg C).