New geochronological data on volcanic rocks from northeast Sudan and their implication for crustal evolution

The basement volcano-sedimentary rocks of northeast Sudan form part of the Nubian Shield of northeast Africa. Volcanic rocks from the Kadawēb area yield Rb—Sr wholerock isochron ages of 718 and 722 Ma and initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7027 and 0.7029. In the Homogar area, 150 km to the south, volcanic rocks yield a Rb—Sr whole-rock isochron age of 671 Ma and an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7034. Although all of these lavas have been altered by a low-grade greenschist facies event, isotopic and geochemical evidence indicates limited open system behaviour. Thus these dates most probably represent extrusive ages indicating two episodes of volcanic activity during the evolution of the Nubian Shield. These results place some important constraints on the nature of crustal evolution in northeast Africa.