African anorogenic alkaline magmatism and mineralization — a discussion with reference to the Niger-Nigerian Province

Abstract

Thc close of the Pan-African was characterized by a change from subduction and arc related magmatism to post tectonic alkaline magmatism in Saudi Arabia and West Africa. For the remainder ol the Phanerozoic, folding and tectonism was more restricted in Africa than on any other continent and was limited to the Caledonian, Hercynian and Alpine orogenies in the extreme north, and the Cape fold belt at the southern tip of the continent, with an equivalent orogenic belt on the extreme east of the Arabian peninsula. Thus Phanerozoic magmatism was dominantly anorogenic in nature and alkaline in character throughout the African Plate. Such magmatism was initiated by reactivation of deepseated lineaments with thc peak of activity during the Mesozoic, which can be correlated with fragmentation of Gondwanaland. The Phanerozoic alkaline magmatism is characterised by small centres of auhvolanic t o plutonic nature often in the form of ring complexes. These can be subdivided into two separate associations consisting of Provinces dominated by centres of ovcrsaturated magmatism such as Nigeria and Sudan. Thcre are also provinces with undersaturated complexes and carhonatites with oversaturated magmatism such as in Namibia. Angola, and the East African rift system. Despite the contrasting nature of undersaturated and oversaturated magmatism many comparisons can bc drawn between carbonatites and granitic complexcs. Such comparisons include thc source of the magmas, the mechanisms of emplacement, the cause and effect of alkali metasomatism and processes of hydrothermal alteration, the trace element geochemistry, the abundance of zircon, uranium. thorium and complex Ti-Zr silicates, the importance o f columbite, pyrochiore, fluorine, and rare earth enrichment in mineralization, and in their current and future economic potential. The common feature is the devclopment and reactivation of deep-seated lineaments which were not only controlled the ernplacement of anorogenic complexes but also acted as channelways for hydrothcrmal mineralizing fluids. Thc development of a fluid phase during crystallisation occurred in both undersaturated and oversaturated alkaline rocks. Subsequent hydrothermal modifications, subsolidus reactions. geochemical variations and mineralization were dependent upon whether this fluid phase was retained or expelled.

An Interpretation of Structures from the Aeromagnetic Field over a Region in the Nigerian Younger Granite Province

“…Intrusion of arfvedsonite and biotite granite belonged to the plutonic phase that ended the evolution [2]. While ring fracture and cauldron subsidence are considered as the major controls that governed the emplacement of the Nigerian Younger Granites [2] [4] [19], the sites of the emplacement have been suggested to be influenced by lineaments; a feature whose origin have been differently suggested [20] [21]. [20] relates the emplacement sites of ring complexes with the ancient lineaments that are associated with the separation of Africa from South America.…”

“…[20] relates the emplacement sites of ring complexes with the ancient lineaments that are associated with the separation of Africa from South America. [21], on the other hand, believed that the sites of emplacement and the mineralization that are associated with them are influenced by the NE-SW and NW-SE lineaments that originated from the reactivation of the Pan-African shear zones and transcurrent faults.…”

Three-step continental-crust growth from subduction accretion and underplating, through intermediary differentiation, to granitoid production

“…As Phanerozoic granitoids and high-silica rhyolites characterized by high e Nd (t) and low ( 87 Sr/ 86 Sr) t occur in many areas of the world (e.g., Hensel et al 1985;Novak and Mahood 1986;Kinnaird and Bowden 1987;Samson et al 1989;Eberz et al 1990;Hildreth et al 1991;Coleman et al 1992;Whalen et al 1996;Bryant et al 1997;King et al 1997;Pickett and Saleeby 1999), these issues are of broad global relevance. Past inability to solve some of these problems arises largely from inadequate geochronological data and over-reliance on whole-rock Nd-Sr isotopic data from granitoids.…”

Existence of a possible subsurface ring complex in the northwestern part of the Nigerian Younger Granite province: a contribution from aeromagnetic and remote sensing data

“…Intrusion of annular to stock-like bodies of arfvedsonite and biotite granite later followed (Turner 1989). While ring fracture and cauldron subsidence are believed to be the major controls that governed the emplacement of the Nigerian Younger Granites (Turner 1972;Jacobson and Macleod 1977;Bowden and Turner 1974), the sites of the emplacement have been suggested to be influenced by lineaments; a feature whose origin have been differently suggested (Wright 1970;Kinnaird and Bowden 1987). Wright (1970) relates the emplacement sites with the ancient lineaments that are associated with the separation of Africa from South America.…”

“…They are also known to give rise to prominent magnetic anomalies (Jaques et al 1997). Their emplacement sites have been suggested to be influenced by lineaments (Wright 1970;Kinnaird and Bowden 1987); a feature that also give rise to distinct character on aeromagnetic maps and DEM (Drury 1993;Jaques et al 1997). These lineaments -line features or patterns on the earth's surface that reflects faults or fractures, and ring complexes are investigated with aeromagnetic and remote sensing data with the aim of providing new insight on the setup of the study area.…”