Different evolution trends in alkaline evolved lavas from the northern Kenya Rift

Abstract

Miocene and Pliocene evolved lavas (SiO 2 > 52 wt%) from the Samburu Hills (SH), northern Kenya Rift, exhibit significant variations in major and trace element compositions, and in Sr-Nd isotopic ratios. The evolved volcanics that erupted during 20-15 Ma consist of mugearite, latite and trachyphonolite. The 10–15 Ma evolved volcanics consist of mugearite and trachyte, whereas Pliocene (4.1-3.6 Ma) rocks are comprised of benmoreite and trachyte. The Pliocene evolved lavas have higher SiO 2, CaO, CaO/Al 2O 3 and Zr/Nb, and lower alkalis, Al 2O 3 and Nb, than the Early-Miocene equivalents. The 10–15 Ma evolved volcanics in many element plots are transitional between the Pliocene and Early Miocene, which is related to compositional differences in their respective mafic magmas. The major-trace element compositions of the evolved rocks can partly be explained by fractional crystallisation from associated mafic rocks, which are dominated by olivine, clinopyroxene, apatite and Fe-Ti oxide, and by plagioclase fractionation in intermediate rocks, whereas fractionation of plagioclase, alkali feldspar, zircon and sphene may control in strongly evolved rocks. In all rock series, intermediate rocks feature higher Ba, Sr, Zr/Nb ratios and much lower Nd isotopic ratios than those of strongly evolved rocks, which have lower Ba and Sr and much higher initial 87Sr/ 86Sr ratios than their mafic equivalents although their initial 143Nd/ 144Nd ratios lie within the same range. The Sr-Nd isotopic compositions of the evolved rocks suggest the importance of combined assimilation and fractional crystallisation processes (AFC) in their evolution.