Crustal development in West Africa

Summary

This chapter begins with the important concept of crustal reactivation, outlining how particular areas of continental crust can be involved in repeated orogenic events of deformation and regional metamorphism. The age of a particular geological province represents the time of the last of these events.

Regional patterns of the West African Precambrian show some striking similarities, irrespective of age. In many areas a three-fold subdivision can be recognised comprising varying proportions of (a) a high-grade granodioritic gneiss-migmatite basement, (b) elongate synclinorial belts of lower-grade supracrustal greenstones, schists and phyl-lites, representing original sedimentary and volcanic rocks and (c) syntectonic to late-tectonic intrusive granites. Regional structural trends are almost everywhere between N-S and NE-SW.

These fundamental patterns lead to the suggestion that the West African crust has experienced several repeated cycles of essentially similar events. The nature of those events remains an outstanding problem. There is still no consensus about such things as, for instance, the original continuity of supracrustal sediments and volcanics, the way in which they have been downfolded or downfaulted into the basement, and whether they were originally deposited on oceanic or continental crust. Repeated reactivations involving migmatisation and granitisation have progressively obliterated older supracrustal sequences, of which only remnants are now preserved, so that at least the upper continental crust retains a bulk composition near that of granodiorite. The chapter ends with a summary of the main events in the Precambrian evolution of the West African continental crust.