Summary

The Kenyan and N Tanzanian volcanic province contains sodic alkaline rocks ranging from melilitites and melanephelinites to transitional alkali basalts and their differentiates. Individual volcanoes display three principal magmatic suites: (i) nephelinitic; (ii) alkali basaltic; (iii) transitional basaltic. However, some large volcanoes contain more than one of these suites implying that parental magmas of variable alkalinity were available at certain times and places. A general decrease in alkalinity with time is detectable in the rift zone and for any time period there was a tendency for the least alkaline magmas to be erupted within the central and deepest part of the rift zone.

Compositional variation within the suites was largely controlled by low-pressure crystalliquid fractionation. Extended fractionation produced salic differentiates. Liquid fractionation caused upward segregation of phonolitic and trachytic magmas which were erupted in preference to more mafic magmas. Isotopic data suggest that crustal contamination did not occur on a large scale.