Nd, Pb and Sr isotopic data from the Mount Elgon volcano, eastern Uganda-western Kenya: Implications for the origin and evolution of nephelinite lavas

Abstract

Nd, Pb and Sr isotope ratios for nephelinites from the Tertiary Mount Elgon alkaline volcanic centre, eastern Uganda-western Kenya, are highly variable and indicate open system behaviour. The variation in 143Nd 144Nd (0.51219-0.51286) and 87Sr 86Sr (0.70314-0.70604) ratios span almost the entire range documented for carbonatites from several East African alkaline complexes. The whole rock chemical data, mineralogy, composition of diopside phenocrysts, and variation in isotopic ratios from the Mount Elgon nephelinites are similar to those from the nephelinite lavas from the Tertiary Napak volcano, Uganda (Simonetti and Bell, 1994a). The diopside phenocrysts from Mount Elgon nephelinite lavas reveal large core-to-rim compositional variations (which include normal, oscillatory and reverse zoning), and their Nd, Pb and Sr isotopic ratios are not in isotopic equilibrium with their host lavas. Microprobe data along with textural evidence from the Mount Elgon diopside phenocrysts support a model that involves crystallization in an open magma system that was undergoing continuous chemical and isotopic change. The large variation in Pb isotopic ratios (whole rocks- 206Pb 204Pb : 18.45-21.51; 207Pb 204Pb : 15.61-15.88; 208Pb 204Pb : 38.62-41.02), from the Mount Elgon lavas, best fit a model involving mixing between EM I and HIMU mantle components, and correlations in Pb-Sr and Pb-Nd 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 heterogeneous sub-continental source beneath eastern Uganda, similar to that documented for various types of peralkaline nephelinite lavas from the only active carbonatite-nephelinite volcano, Oldoinyo Lengai, Tanzania (Bell and Dawson, 1995) and other East African volcanoes (e.g. Vollmer and Norry, 1983). The chemical data and large variation in isotopic ratios for the Mount Elgon nephelinites suggests that each lava flow may represent a discrete partial melt derived from an isotopically inhomogeneous upper mantle. In the cases of the Mount Elgon and Napak lavas, magma mixing may have occurred between newly formed, ascending nephelinitic liquids and slightly older nephelinitic melts that ponded at shallower levels.