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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.

Granitoid plutons are common throughout the late Proterozoic basement complex of north-east Sudan, frequently accounting for 60% of the outcrop. Geological, petrological and isotopic studies indicate that two distinct types of granitic plutonism can be recognized. An older diorite to granodiorite assemblage was emplaced between c. 815 and 724 Ma and a younger predominantly alkali feldspar granite assemblage consisting of numerous discrete high level intrusions was emplaced between c. 717 and 555 Ma. These granitoids have a calc-alkaline character similar to the volcanic rocks of the basement complex to which they are spatially and temporally related. The geochemical and isotopic data suggest a destructive plate margin rather than a within-plate palaeotectonic association. It is suggested that the granitoids represent an evolutionary trend from early diorite to granodiorite intrusions emplaced above a late Proterozoic subduction zone to later, post-tectonic intrusions emplaced during a late stage of volcanic arc development. The magmas of the early granitoids were probably mantle-derived while the later post-tectonic intrusions may have been derived from magmas produced by partial refusion of the volcanic arc protolith. Early granitoidsGranitoids with no simple intrusive form are common within the Red Sea Hills of Sudan. Their heterogeneous nature and the occurrence of rare internal contacts suggest that the 635