Volcanism in the western part of the rift valley in Southern Kenya

The stratigraphy, tectonic history, petrography and major oxide petrochemistry of the volcanic sequences in the western part of the rift valley in southern Kenya is summarised. Volcanism and rift faulting began 15 and 7 m.y. ago respectively. A recurrent feature of Miocene and Pliocene volcanism was the tendency for salic magmas to be preferentially erupted in the northern part of the area: the accumulation of a northward-thickening Pliocene trachyte pile is particularly notable. Transitional-mildly alkalic basalts and trachytes were erupted only after the onset of rift faulting, from sites within the rift structure, and so can be considered «rift dependent». Nephelinites, melanephelinites, limburgites and phonolites were erupted before and after onset of rift faulting, from sites within and outside the rift structure, and so can be considered «rift independent».