Abstract

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/YbN and 87Sr/86Sr 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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