Geochemistry of carbonatities and related rocks from South Nyanza, Kenya.

The distribution of 25 trace elements and selected major elements have been investigated in carbonatites, peralkaline silicate rocks and fenites from Homa Mountain, N. Ruri and Wasaki in W. Kenya. Comparison has been made with the distributions of these elements in similar rocks from Budeda, Toror and Tororo carbonatitic complexes, E. Uganda. The earliest sovitic carbonatites were found to contain greater concentrations of Sr, and generally less Ba, Nb, REE (both Ce- and Y-earths), Fe, Mn, Zn, Zr, Ti, and V than the later alvikitic carbonatites. The later ferruginous alvikites were characteristically enriched in Fe, Mn, Ba, Zn, and REE (Ce-earths) relative to other carbonatites. All carbonatites were typically strongly Ce-earth enriched. The peralkaline silicate rocks and fenites were similarly enriched in Sr, Ba, REE, Nb and Ti, and contained generally more Zr and Ga than the carbonatites. These rocks were also Ce-earth enriched. The concentrations of Sr, Ba, Zr, REE, Ga, and Nb in the fenites indicate that these elements are introduced into the country rock by Na-fenitising solutions. The similarity in trace element content between fenites and nepheline-syenite suggests a genetic relationship between these rock types. The greater concentration of Cr, Ni and Co in the pyroxenites suggest that these rocks formed early in the crystallisation history of the carbonatite complexes, leading to crystallisation of the ijolitic suite. The relationship between the carbonatites and silicate rocks remains controversial. However, the concentration Ba, REE, and Nb in the carbonatites indicates that these rocks were formed late in the history of the complexes. A mechanism involving fractional crystallisation of a carbonated silica-undersaturated magma, leading in the later stages to liquid immiscibility of carbonate and silicate liquids, is favoured for the origin of these rocks.