Mafic ultrapotassic volcanics (kamafugites, mafurites) are the youngest eruptives in the Bunyaruguru volcanic field, which is part of the Toro Ankole Province at the northernmost reach of the West Branch of the East African Rift Valley. We obtained new data using LA-ICP-MS on the trace element contents (rare earth, large ion lithophile, high field strength and compatible elements) in clinopyroxene phenocrysts from mafurite lava of the Bunyaruguru volcanic field. The clinopyroxenes are notable for their anomalously high zirconium contents (up to 800–1000 ppm), which is unusual in mafic silicate magmas but typical for clinopyroxenes from carbonatite lavas. This distinctive signature is not found in clinopyroxene in coeval mafic lavas from neighboring vents. Carbonate is also found in the Bunyaruguru volcanic rocks in the form of inclusions in high-Mg olivine phenocrysts, suggesting carbonated material exists in the source of kamafugite magmas. Carbonatites are widespread in the neighboring Fort Portal volcanic field, and we interpret the high-zirconium clinopyroxene as evidence for mixing of mafurite magmas with carbonatite melts.
Muravyeva et al. (Fri,) studied this question.