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Proterozoic Volcanic Suites of Africa |
Department of Geology, Imperial College, London
The geochemistry of the Kadew
b and Homogar Volcanic Groups from the Red Sea Hills of north-east Sudan is discussed. Rb-Sr studies show that these lavas are Upper Proterozoic in age (720 Ma and 670 Ma respectively). The lavas of both groups display a calcalkaline character and are broadly similar in terms of their lithology, phenocryst mineralogy and major element chemistry. Geochemically the Kadawb and Homogar volcanic rocks are similar to modern arc lavas associated with converging plate margins, although they tend to have higher concentrations of some HFS elements. Despite overall similarity, a distinction can be made between the two groups on the basis of the geochemistry of the basaltic components of the suites. The Homogar basalts are enriched in LIL and REE, having high La/Nb, (Ce/Yb)N and Ta/Yb ratios compared with those of the Kadawb group. It is suggested that the older Kadawb lavas were erupted along an intra-oceanic island arc system, while the younger Homogar lavas were extruded along a more evolved, possibly continental, arc margin. Differences between these Upper Proterozoic rocks and modern arc lavas may be explained as resulting from differences in the nature of the Proterozoic mantle.
