40 Ar/ 39 Ar chronology, petrology and geodynamic setting of Mesozoic to early Cenozoic magmatism from the Benue Trough, Nigeria

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Abstract
The Benue Trough is a continental-scale intraplate tectonic megastructure which is part of Mid-African Rift System. This rift, initiated in the latest Jurassic, was related to the opening of the Central and South Atlantic oceans. Mesozoic to early Cenozoic magmatism accompanied this evolution. Two principal magmatic domains are evident, the Northern and Southern Benue. In the northern domain, magmatism is characterized by transitional alkaline basalts and transitional tholeiitic basalts. Acidic magmatism of peralkaline nature is also present. In the Southern Benue, several magmatic districts exhibit alkaline or tholeiitic affinities. A detailed chronology of emplacement of this volcanism has been established using the 40 Ar/ 39 Ar radiometric method which lead to recognition of three periods of magmatic activity: (1) 147–106 Ma, well expressed in the Northern Benue represefited by transitional basaltic types; (2) 97–81 Ma, occurring only in the Southern Benue, represented exclusively by alkaline intrusive rocks; (3) 68–49 Ma, restricted also to the Southern Benue, with alkaline intrusions followed by tholeiitic subvolcanic rocks. In the light of the general geodynamic evolution, a scenario is proposed, supported by the three chronological periods. The late Jurassic to Albian magmatism occurred when the Equatorial Atlantic was still closed, contemporaneous with the NE Brazilian magmatism. Both magmatism could represent the forerunners of opening of the Equatorial Atlantic. This activity occurred under a general extensional regime during which crustal strike-slip faults gave rise to the emplacement of transitional alkaline basalts. Transitional tholeiitic basalts erupted along normal faults. The second period of activity, Cenomanian to Santonian, restricted to the Southern Benue, occurred after the opening of Atlantic Ocean during a period of decreasing extension. This period ended with a Santonian compressional episode. The last period of activity, from late Maastrichtian to Eocene, is characterized by subsidence, generated as an isostatic response to the Early Cretaceous crustal thinning and post-rift thermal relaxation of the lithosphere, expressed by Tertiary E–W extension.