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 NgurumanEscarpment west of Lake Magadi in the Rift Valley of Kenya.This essentially nepheline-pyroxene rock is unique in containingiron-rich mesostasis and vug-like patches of primary calcite,analcime, and zeolites; it intrudes ankaratrite and representsthe hypabyssal equivalent of the lava. These rocks form part of a strongly alkaline series of the northernTanganyika alkaline district; a second series of mildly alkaline.Character is also recognized. The rocks of the strongly alkalineseries were evidently derived from a parental magma of ankaratriticcomposition, whilst mildly alkaline types are differentiatesfrom a magma of alkali olivine-basalt composition. The intimateassociation of rocks of the two series indicates derivationfrom a common source, and an ultimate peridotitic origin isenvisaged. Late enrichment in soda and lime during crystallization of ngurumaniteparallels the trend of differentiation shown by the stronglyalkaline rocks of the district, where the ultimate derivativeis carbonatite.