PRECAMBRIAN BASEMENT AROUND WADI-HALFA, SUDAN - A NEW PERSPECTIVE ON THE EVOLUTION OF THE EAST SAHARAN CRATON

This paper provides new geochemical and isotopic data on the evolution of the western foreland to the Nubian shield of north-east Africa. Th ere is abundant evidence for early to middle Proterozoic crust west of the River Nile, but this was severely affected by the Pan-African (al most-equal-to 500-900 Ma) orogenic cycle. The results are reported of Rb-Sr whole rock and zircon evaporation geochronological studies and w hole rock Sm-Nd and feldspar Pb isotopic analyses for four rock units around Wadi Halfa in northern-most Sudan. These results indicate the p resence of heterogeneous pre-Pan-African crustal components, preserved in mylonitic gneisses and in conglomerates that unconformably overlie the gneisses. Several episodes of crust formation, inferred from zirc on 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 invariably older than the zircon ages, yet still record a predominantly late Archaean and Palaeo proterozoic history, with depleted mantle model ages between 1.3 and 2 .8 Ga. The earliest recorded Pan-African magmatic event is about 720 M a and dates the beginning of collisional deformation. A younger Pan-Af rican volcanic sequence (almost-equal-to 650 Ma) has isotopic composit ions of Sr and Nd compatible with derivation from late Proterozoic ast henospheric mantle. A almost-equal-to 530 Ma anorogenic 'A-type' grani te also has isotopic compositions suggesting derivation from a primiti ve source. The inferred tectonic evolution began with rifting to form an oceanic re-entrant. This was followed by subduction leading to coll ision at about 700 Ma, accompanied by post-orogenic rifting at about 6 50 Ma.