Variable intra-plate igneous activity in central and north-east Sudan

Four Phanerozoic alkaline igneous complexes from central and north-eastern Sudan are compared petrologically. Despite many similarities in terms of mineralogy, petrography and major element geochemistry significant differences are found when trace element and particularly isotopic data are considered. In particular the complexes from north-eastern Sudan have markedly lower ⁸⁷Sr/⁸⁶Sr ratios and overall RE element content than those occurring in central Sudan. Such differences could be accounted for by variation in the degree of metasomatic alteration associated with non-hydrous volatiles. However, there is no evidence to suggest that the effect of volatiles has been more prominent in any particular geographic location or complex. The chemical character of the north-eastern complexes is similar to the oceanic island leucocratic lavas of St. Helena and Ascension. Further, the complexes of north-eastern Sudan are associated with a prominnent dyke swarm, suggesting a tensional tectonic environment which may have facilitated the passive emplacement of mantle derived magma. In central Sudan there is no direct evidence of rifting and the alkaline rocks are clearly chemically and isotopically similar to anorogenic rocks which are considered to have evolved from mixtures of mantle and crustal derived material. It is suggested that the differences between the alkaline complexes of eastern and central Sudan are due to the charccter of the source material rather than the effect of metasomatic alteration, although mantle source heterogeneity cannot be ruled out.