MINERALOGY AND PETROLOGY OF MOUNT SUSWA,KENYA

Mount Suswa, a Quaternary volcano in the Rift Valley of Kenya, is composed of sodalite-trachytes, sodalite-phonolites, and phonolites, the majority of which are mildly peralkaline. The lavas are predominantly feldspathic with phenocrysts of alkali feldspar and, less commonly, slightly sodic augite and fayalitic olivine. The groundmass typically contains alkali feldspar, augite, titanomagnetite, and may in addition contain sodalite, nepheline, alkali amphibole, aenigmatite, and glass. The lavas of four stratigraphically distinct episodes can be distinguished on the basis of mineralogy and chemical composition. These lavas are the products of at least three parental magmas, none of which appears to be a derivative of the other. Each 'magma type' represents an independent episode of magma generation, emplacement, and eruption. The order of eruption in the final episode corresponds to increasing peralkalinity and undersaturation with respect to silica, and indicates that these lavas were generated via the tapping of a differentiating magma, with the first lavas being the least differentiated. Utilizing coexisting feldspar, residual glass, and bulk rock compositions, the derivation of peralkaline phonolitic residual liquids from a trachytic parent is shown to be a process controlled by feldspar fractionation. © 1969 Oxford University Press.