GEOCHEMISTRY OF PLIOCENE TO QUATERNARY ALKALI BASALTS FROM THE HURI HILLS, NORTHERN KENYA

Pliocene to Quaternary basaltic lavas from the Huri Hills show a conti nuous change in eruptive style from earlier fissure-type eruptions pro ducing extensive lava flows to later central-type activity, resulting in cinder cones and associated small lava flows. This change in erupti ve style was accompanied by systematic variations in chemical composit ion and isotopic signature. With decreasing age, rock compositions cha nge from alkali basalt to basanite (CIPW-normative nepheline increases from 3% to 22%). Whereas chondrite-normalised La abundances increase from approximately 40 to 200, Yb stays almost constant at approximatel y 10 x chondritic. Concurrently, samples with Mg# > 64 exhibit an incr ease in chondrite-normalised Tb/Yb and Zr/Y, as well as a decrease in Sc and Ti/Zr. Since no correlation between any of these parameters and the CaO/Al2O3 ratio or Mg# is observed, the systematic temporal varia tions in trace-element ratios of slightly fractionated magmas can best be explained by an increasing amount of garnet in the residue of the melts. With decreasing age and increasing CIPW-normative nepheline, Hu ri Hills lavas show decreasing Sr-87/Sr-86 ratios (0.7033-0.703) and i ncreasing Nd-143/Nd-144 (0.5129-0.51295) and Pb-206/Pb-204 (18.7-19.3) ratios. Concurrently, ratios such as Ba/Th, K/La and Sr/Nd decrease. These covariations suggest binary mixing of two chemically and isotopi cally distinct end-member compositions. The first end-member, with hig h Pb-206/Pb-204 ratios, is most probably derived from a plume source w ith HIMU affinities. The second end-member, showing low Pb-206/Pb-204, and high Sr-87/Sr-86, Ba/Nb and Sr/Nd ratios, is tentatively attribut ed to the lithospheric mantle but could also be a second plume compone nt.