Niger-Nigerian alkaline ring complexes: a classic example of African Phanerozoic anorogenic mid-plate magmatism

Abstract

Summary The Pan-African orogeny, regarded as a major event in the construction of Gondwanaland, played a dominant role in providing a variety of source materials for mineralization, melting or assimilation as components of the Benin-Nigerian shield. Although the parent magmas of the silica-oversaturated alkaline rocks which constitute the Niger-Nigerian anorogenic province were initiated in the asthenosphere, it was the exploitation and reactivation of the Pan-African shear zones and transcurrent faults during the fragmentation of Gondwanaland that controlled the locations of Phanerozoic intra-plate magmatism in W Africa. The Niger-Nigerian anorogenic province represents one of those regions where progressive uplift has been accompanied by periodic sequential development of chains of volcanoes now exposed as ring complexes with Palaeozoic and Mesozoic ages. The magmatic lineage comparable with other alkaline provinces can be established from preserved volcanic sequences. Associated andesite compositions can be attributed to magma mixing. In Niger, an important petrogenetic parameter is the occurrence of leucogabbros, anorthosites and monzoanorthosites as part of the Palaeozoic sub-volcanic association with syenites, peralkaline granites and biotite granites. The partially eroded volcanic cover of rhyolitic ignimbrites has provided the source for substantial uranium deposits. The southern centres of upper-Silurian-lower-Devonian age are mineralized with columbite and cassiterite. The general geological and geochemical features of the Nigerian Triassic-Jurassic anorogenic centres are well known. While their magmatic derivation from mantle and crustal sources can be conclusively demonstrated, petrological and geochemical research has shown that many of the Nigerian anorogenic centres demonstrate substantial evidence for post-magmatic metasomatism linked to mineralization. There are certain parallels between alkali metasomatism in alkaline granite ring complexes and fenitization associated with carbonatites.