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The Jabel Sayid Complex comprises two principal rock types which require distinct source regions. An earlier intrusion (572 f 24 Ma) of biotite-hornblende monzogranite displays selective enrichment of LIL elements and depletion of HREE characterizing a calc-alkaline magma derived from a garnet-bearing source within a hydrated mantle wedge. The younger intrusion (534 f 40 Ma) is an aegirine-arfvedsonite peralkaline granite which can be derived neither by fractionation of a calc-alkaline magma, nor by closed-system anatexis of a crustal source. This granite is enriched in HFS elements and strongly depleted in Sr, Ba and Eu, and requires the contribution of a fluorinebearing fluid phase. This volatile phase has resulted in a range of metasomatic rock types including an aplitic rim which carries extreme levels of HFS elements and is a potential economic source of U, and a red granite characterized by widely ranging trace element abundances and strong Eu depletion. The peralkaline granite is typical of siliceous melts derived from an intra-plate tectonic environment, with trace elements strongly controlled by volatiles which are probably mantle-derived. The role of crustal contamination on such granites can neither be proved nor excluded.

The Kadawçb and Homogar Volcanic Groups occur in the Sudanese portion of the Nubian Shield and are late Proterozoic in age (723 Ma and 671 Ma, respectively). The lavas of both groups display a calc-alkaline character, are broadly similar in terms of their lithology, phenocryst mineralogy and major element composition, and geochemically they are similar to modern arc lavas associated with converging plate margins. However, a clear distinction can be made between the two groups on the basis of the trace element geochemistry of the basaltic components of the suites. The Homogar basalts are chemically more evolved and are enriched in large ion lithophile elements (LILE) and rare earth elements (REE) having high La/Nb, Ce/Yb N and 87 Sr/ 86 Sr ratios compared with those of the Kadawçb group. These chemical differences possibly follow an evolutionary trend as the less developed Kadawçb lavas are some 50 Ma older than those of the evolved Homogar group. It is suggested that the older Kadawçb lavas were erupted along an intra-oceanic island arc system, while the younger Homogar lavas were extruded along a more evolved transitional arc or possibly a continental margin. The differences in light/heavy REE and LIL/high-field strength element ratios may reflect variation in the degree of alteration of an overlying mantle wedge, possibly due to the introduction of LILE- and REE-enriched fluids derived from dehydrating subducted oceanic lithosphere.

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