A geochemical study of silali volcano, Kenya, with special reference to the origin of the intermediate-acid eruptives of the central rift valley

Abstract

The Rift Valley zone in East Africa may be regarded as an aborted ocean of long existence but never significantly opened up. Distensive regimes of tectonic up-doming, eruptivity and subsidence on normal faults, and median positive gravity anomalies complicating the wide negative gravity anomaly of Bullard must now be fitted into any model for the Rift Valley. The petrological and geochemical nature of the eruptives is equally important: it is common to equate the abundance of salic volcanics in any section with thickness of the sialic crust there, assuming sialic remelting or sialic contamination. The results of a detailed geochemical study of a single Quaternary caldera volcano, Silali, and its Plio-Pleistocene volcanic foundation are summarised here, and used as a model for differentiation in the central Gregory Rift Valley. A monistic differentiation theory must be sought to explain the peralkaline sialic eruptives, associated with alkalic basalts in great volumes, from the Miocene onwards. Relative to the basalts there is an increase in Si, Na, K and depletion of Mg, Ca, Ti; total iron shows slight depletion and the iron shows oxidation to the ferric oxide. Of the minor elements, Rb, Zn, Y, La, Zr, Pb, and Nb are increased; Cu, Ba and Sr depleted; and Cr, Co and Ni show slight depletion. The trends seen in trachytes, etc., are incipiently evident in hawaiites and mugearites. The consistent bimodality, peralkaline character of the intermediate to acid rocks, major and minor element contents, and intimate intermingling of the two suites in space and time, can only be interpreted in terms of complex differentiation process affecting a mantle-derived basalt parent. Comparisons are drawn with the volcanic suites of Tenerife and with a differentiated intrusion in Australia which shows bimodality as a product of differentiation. It is suggested that a chain of cupolas underlie the Rift Valley, and in these differentiation follows an agpaitic regime, under the control of volatile concentration. Such cupola reservoirs probably reach up close to the floor of the Rift Valley. The change from phonolite dominance to trachyte-comendite dominance in the intermediate suites is the only significant time progressive change evident from Miocene to Recent. Notwithstanding this, the intermediate-acid rocks, and basic rocks associated with them plot consistently on FMA and alkali silica diagrams throughout this period.