Petrographic Sub-provinces in the Tertiary to Recent Volcanics of Kenya

Abstract

AbstractThree sub-provinces are recognized in the volcanics of the southern half of Kenya as follows:—(a) the Miocene phonolite sub-province, (b) the Mount Kenya phonolite/basanite sub-province, and (c) the Rift Valley trachyte/olivine basalt sub-province. Mela-nephelinitic lavas in the two phonolitic sub-provinces are shown to differ significantly in mineralogy, chemistry, and associated volcanics. The type of volcanic association likely to be related to carbonatite occurrences is briefly discussed.Areas of exposed Miocene phonolite and Plio-Pleistocene trachyte are much greater than those of associated basic lavas, while intermediate types are almost lacking. Calculations show that assimilation of gneissic basement rocks by basanite (or olivine mela-nephelinite) and by olivine basalt may contribute to the development of phonolite and trachyte respectively. Available supporting evidence is discussed, both from Kenya and from similar volcanic provinces elsewhere. The additional amount of salic derivatives “which can be produced in this way is small, however, relative to the volume of basic parent involved.