ND, PB AND SR ISOTOPIC DATA FROM THE MOUNT-ELGON VOLCANO, EASTERN UGANDA WESTERN KENYA - IMPLICATIONS FOR THE ORIGIN AND EVOLUTION OF NEPHELINITE LAVAS

Nd, Pb and Sr isotope ratios for nephelinites from the Tertiary Mount Elgon alkaline volcanic centre, eastern Uganda-western Kenya, are high ly variable and indicate open system behaviour. The variation in Nd-14 3/Nd-144 (0.51219-0.51286) and Sr-87/Sr-86 (0.70314-0.70604) ratios sp an almost the entire range documented for carbonatites from several Ea st African alkaline complexes. The whole rock chemical data, mineralog y, composition of diopside phenocrysts, and variation in isotopic rati os from the Mount Elgon nephelinites are similar to those from the nep helinite lavas from the Tertiary Napak volcano, Uganda (Simonetti and Bell, 1994a). The diopside phenocrysts from Mount Elgon nephelinite la vas reveal large core-to-rim compositional variations (which include n ormal, oscillatory and reverse zoning), and their Nd, Pb and Sr isotop ic ratios are not in isotopic equilibrium with their host lavas, Micro probe data along with textural evidence from the Mount Elgon diopside phenocrysts support a model that involves crystallization in an open m agma system that was undergoing continuous chemical and isotopic chang e. The large variation in Pb isotopic ratios (whole rocks-Pb-206/Pb-20 4: 18.45-21.51; Pb-207/Pb-204: 15.61-15.88; Pb-208/Pb-204: 38.62-41.02 ), from the Mount Elgon lavas, best fit a model involving mixing betwe en EM I and HIMU mantle components, and correlations in Pb-Sr and Pb-N d isotopic plots partly support this interpretation. The isotopic data from Mount Elgon and Napak nephelinites suggest complex evolutionary histories involving magma mixing, and support the presence of a hetero geneous sub-continental source beneath eastern Uganda, similar to that documented for various types of peralkaline nephelinite lavas from th e only active carbonatite-nephelinite volcano, Oldoinyo Lengai, Tanzan ia (Bell and Dawson, 1995) and other East African volcanoes (e.g. Voll mer and Norry, 1983). The chemical data and large variation in isotopi c ratios for the Mount Elgon nephelinites suggests that each lava flow may represent a discrete partial melt derived from an isotopically in homogeneous upper mantle. In the cases of the Mount Elgon and Napak la vas, magma mixing may have occurred between newly formed, ascending ne phelinitic liquids and slightly older nephelinitic melts that ponded a t shallower levels.