Precambrian events at Sabaloka, near Khartoum, and their significance in the chronology of the basement complex of North-East Africa

Abstract

A large inlier of basement complex rocks surrounded by Cretaceous sediments north of Khartoum comprises a grey gneiss assemblage intruded by late-tectonic adamellitic batholiths and a shallow-level granitic ring complex. The gneiss assemblage contains evidence that an early granulite facies metamorphism was followed by retrogression into the amphibolite facies and widespread migmatization. There are close affinities with basement rocks of Archaean age in northern Uganda. Amphibolite facies metasediments which overlie the grey gneiss to the north and northeast of the Sabaloka inlier probably equate with the Early and Middle Proterozoic cover sequences which in the east African sector of the Mozambique belt are strongly interfolded with thoroughly reworked Archaean basement. The adamellite batholiths are in places strongly discordant to their country rocks and may be coeval with the ‘batholithic granites’ of the Red Sea Hills, emplaced in late Proterozoic times, during the last stages of deformation in the Red Sea belt. The whole area was later affected by reheating and limited tectonism during the Pan-African event (550 ± 100 Ma) with consequent resetting of K-Ar apparent ages. Following a period of deep erosion, the Sabaloka ring complex and associated volcanic rocks were emplaced in mid-Palaeozoic times. It is suggested that the concepts embraced by the terms ‘Pan-African event’, ‘Red Sea belt cycle’ and ‘Mozambique belt cycle’ are best kept distinct from each other since their affects appear to differ in spatial and temporal distribution. One new analysis of a retrogressed and metasomatized amphibolite is presented.