Ngurumanite from Southern Kenya and its bearing on the origin of rocks in the Northern Tanganyika alkaline district

A new alkaline rock, ngurumanite, is described, from the Nguruman Escarpment west of Lake Magadi in the Rift Valley of Kenya. This essentially nepheline-pyroxene rock is unique in containing iron-rich mesostasis and vug-like patches of primary calcite, analcime, and zeolites; it intrudes ankaratrite and represents the hypabyssal equivalent of the lava.These rocks form part of a strongly alkaline series of the northern Tanganyika alkaline district; a second series of mildly alkaline. Character is also recognized. The rocks of the strongly alkaline series were evidently derived from a parental magma of ankaratritic composition, whilst mildly alkaline types are differentiates from a magma of alkali olivine-basalt composition. The intimate association of rocks of the two series indicates derivation from a common source, and an ultimate peridotitic origin is envisaged.Late enrichment in soda and lime during crystallization of ngurumanite parallels the trend of differentiation shown by the strongly alkaline rocks of the district, where the ultimate derivative is carbonatite. © 1964 Oxford University Press.