K-Rb relationships in some continental alkalic rocks associated with the East African Rift Valley System

Abstract

Two suites of alkalic rocks closely associated with the East African Rift Valley System, from both eastern and western Uganda, have variable K and Rb abundances as well as variable K/Rb ratios. Lavas from the western region (including the Birunga and Toro-Ankole fields) have an average K/Rb ratio of 287, very close to estimates of average continental crust. In contrast, lavas from several older volcanic centres in eastern Uganda have lower K and Rb contents, and slightly higher K/Rb ratios around 377. A suite of plutonic rocks from a single volcano, Napak, has a higher K/Rb ratio (578) and higher K content than its supposed volcanic equivalent. Although there is little correlation between K/Rb ratio and K content for the western centres, several of the eastern complexes show a positive co-variance. Although such trends can be produced by either crystal fractionation of a nephelinitic parent magma, or by the mixing of two quite distinct end members, any hypothesis must take into account the additional feature of preferential migration of K during magmatic evolution.