Precambrian basement around Wadi Halfa, Sudan: a new perspective on the evolution of the East Saharan Craton

Abstract

This paper provides new geochemical and isotopic data on the evolution of the western foreland to the Nubian shield of north-east Africa. There is abundant evidence for early to middle Proterozoic crust west of the River Nile, but this was severely affected by the Pan-African (~ 500-900 Ma) orogenic cycle. The results are reported of Rb-Sr whole rock and zircon evaporation geochronological studies and whole rock Sm-Nd and feldspar Pb isotopic analyses for four rock units around Wadi Halfa in northern- most Sudan. These results indicate the presence of heterogeneous pre-Pan-African crustal components, preserved in mylonitic gneisses and in conglomerates that unconformably overlie the gneisses. Several episo- des of crust formation, inferred from zircon ages, are preserved in the gneisses: 2.6, 2.4, 2.0, 1.7, 1.2 and 0.72 Ga. Nd model ages for the same units are inva- riably older than the zircon ages, yet still record a predominantly late Archaean and Palaeoproterozoic history, with depleted mantle model ages between 1.3 and 2.8 Ga. The earliest recorded Pan-African magma- tic event is about 720 Ma and dates the beginning of collisional deformation. A younger Pan-African volca- nic sequence (~ 650 Ma) has isotopic compositions of Sr and Nd compatible with derivation from late Prote- rozoic asthenospheric mantle. A ~ 530 Ma anorogenic 'A-type' granite also has isotopic compositions sug- gesting derivation from a primitive source. The in- ferred tectonic evolution began with rifting to form an oceanic re-entrant. This was followed by subduction