Geochemistry of the carbonatite complexes in East Africa

Abstract

Carbonatite centres are characterized by the common occurrence of carbonatites and alkalic igneous rocks, both as volcanic and as intrusive rocks. In the eastern rift zone, the carbonatite complexes are sodium-rich while in the western rift zone they are potassium-rich. The complexes are undersaturated in silica content and enriched in alkalis. The two suites of alkaline rocks have variable K Rb ratios as well as variable K and Rb contents. The K Rb ratio (approximately 287) of lavas from the western rift zone is very close to estimates for the average continental crust while lavas from the eastern rift zone have higher K Rb ratios. The ratios 87 Sr 86 Sr of the carbonatite complexes show a distinct variation between those of the eastern rift zone and those of the western rift zone. The carbonatite complexes of the eastern rift zone have lower 87 Sr 86 Sr ratios than those of the western rift zone. The 87 Sr 86 Sr ratios of the potassium-rich alkaline rocks of the western rift zone vary considerably and are thought to have resulted from mixing of crustal and upper mantle material. The carbonatite complexes are greatly enriched in trace elements-Sr, Ba, Nb and REE. Most of these trace elements are concentrated in the carbonatites rather than the associated alkaline rocks.