ND, PB AND SR ISOTOPIC DATA FROM THE NAPAK CARBONATITE-NEPHELINITE CENTER, EASTERN UGANDA - AN EXAMPLE OF OPEN-SYSTEM CRYSTAL FRACTIONATION

Nd, Pb and Sr isotopic data from nephelinite lavas from the Tertiary n ephelinite-carbonatite complex of Napak, eastern Uganda, show large is otopic variations that can only be attributed to open-system behaviour . Possible explanations of the data include mixing between nepheliniti c melts derived from an isotopically heterogeneous mantle, or interact ion between a HIMU melt and mafic granulites. In both models crystal f ractionation, involving olivine and clinopyroxene, played an important role. Major element chemistry, textural evidence and isotopic data fr om clinopyroxene phenocrysts from the olivine-bearing nephelinites, su ggest that the pyroxenes did not crystallize from their host liquids. The isotopic data from the clinopyroxene phenocrysts support an interp retation of crystal fractionation in an open magma system that was und ergoing continuous isotopic change. This study emphasises the importan ce of using combined isotopic data from both whole rock and mineral ph ases to interpret the evolutionary history of a single eruptive centre .