The geochemical evolution of Quaternary volcanism in the south-central portion of the Kenya rift

Peter Ayodo Omenda, University of Texas at El Paso


The southern portion of the Kenya rift is characterized by voluminous eruption of mafic to felsic magmas during the Quaternary. These include transitional basalts, basaltic trachyandesites, trachyandesites, phonolites, trachytes, pantellerites, and comendites within the rift axis and silica undersaturated basanites on the eastern flanks. To understand the petrogenesis of these lavas, the basanites from the eastern flank volcanic field at Chyulu are used to represent the most primitive magma associated with rifting. Results of modeling imply that the Chyulu basanites are products of 1-2% partial melting of a chondritic to primitive mantle. The transitional basalts of the rift axis represent 5-14% partial melting of a similar mantle. The lower value is preferred since it is consistent with the generation of the magmas that evolve into the transitional basalts of the rift axis.^ The second phase of this study was to interpret the relationship between the mafic and felsic magmas. The characteristics of pantellerites from Eburru, comendites from Olkaria, and phonolites from Suswa were studied. Major and trace element models show that the pantellerites, trachytes, and associated transitional basalts are cogenetic, and that evolution was dominated by polybaric fractional crystallization processes. Results obtained from stable oxygen isotopes $(\delta\sp{18}o=+5.7$ to 6.6$\perthous)$ indicate that the rocks are mantle-derived and that crustal assimilation was not important in their genesis. Trace elements indicate that initial crystallization of the rift axis magmas occurred at the Moho. A subsequent low pressure crystallization is indicated by the observed phenocrystic mineral assemblage that includes olivine, augite, and plagioclase. The models show that the transitional basalts are highly evolved and experienced 67 to 80% pre-eruption crystallization compared to 20 to 40% for the Chyulu basanites. The phonolites, trachytes and pantellerites represent more than 95% crystallization of basaltic parents. Least squares major element models reveal that the resultant cumulates have sufficiently high densities ($\leq$2.90g cm.$\sp{-3})$ to cause the gravity anomalies observed under the volcanoes in the rift axis.^ Suswa is in a transition zone where the lithosphere begins to thicken southward. The volcano displays earliest shield building lavas of transitional basalt - trachyte association similar to the others in the rift. However, the evolution of the syn-caldera trachytes and post-caldera phonolites require more alkaline parental magma. Comendites from Olkaria are not part of the fractional crystallization evolutionary paths.^ The distribution of the Quaternary lavas associated with rifting can be used to infer the tectonic evolution of the Kenya rift. The occurrence of basanites on the eastern flank of the rift indicates thicker lithosphere than that under the rift axis where Quaternary transitional basalts and their felsic derivatives dominate. Geophysical models indicate thin lithosphere under the rift. The eruption of phonolites at Suswa implies a change to lower degrees of partial melting in the mantle under the volcano during the Holocene time. This is primarily caused by the thickening lithosphere south of Suswa which results in deeper partial melting in the mantle. (Abstract shortened by UMI.) ^

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Recommended Citation

Omenda, Peter Ayodo, "The geochemical evolution of Quaternary volcanism in the south-central portion of the Kenya rift" (1997). ETD Collection for University of Texas, El Paso. AAI9819585.