Abstract

New petrological, geochronological, and geochemical data on basalts, mugearites, peralkaline trachytes, and phonolites from the Baringo–Bogoria Basin, central Kenya Rift, are presented. K–Ar dating indicates that the volcanic rocks were emplaced between 894 ± 13 and 92±5 ka. 87Sr/86 Sr ranges from 0.70304 to 0.70692, 143Nd/144Nd from 0.51237 to 0.5 1 295, 206Pb/204 Pb from 18.4 to 19.8, 207Pb/204 Pb from 15.46 to 15.70, and 208Pb/204 Pb from 38.2 to 40.5. Despite a rather restricted sampling area and a relatively short time span (≤ 820 ka), the mineralogical and geochemical variations are not consistent with a simple cogenetic link between the lavas. The studied area is located in a transition zone between two different lithospheric domains (Tanzanian Craton and Panafrican Mobile Belt). We propose that the petrological and geochemical variations of the studied lavas are essentially linked to the nature of the underlying lithosphere. Some basaltic products underwent carbonate contamination, possibly within the crust. Trachytes and phonolites are derived from different basaltic parents through crustal assimilation coupled with fractional crystallization. One phonolite sample contains primary calcite-rich veinlets. Textural relations and geochemical evidence suggest that there is a direct cogenetic link between these carbonate and phonolite melts. The veinlets are the modal expression of a carbonate component included in all the phonolites from the Baringo–Bogoria Basin.

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