Age Migrations of Anorogenic Ring Complexes in Northern Nigeria

Forty Rb-Sr isotopic measurements on minerals and whole rock samples from 11 anorogenic complexes in northern Nigeria complement the evidence for systematic age variations along a series of en-echelon ENE-WSW and N-S lineaments following two major fracture directions of extensional deformation in the Precambrian basement. The new ages presented in this paper confirm our earlier discoveries of a general southerly age trend but substantiate for the first time that major local migrations of magmatic activity were concentrated along at least two ENE-WSW zones. From the overlap in analytical error, the age patterns also suggest that contemporaneous anorogenic magmatism may have been locally derived from several simultaneous high-level magma chambers connected to a common deeper source. Different models that have been proposed for the origin of linear mid-plate magmatic activity are critically reviewed. In our opinion magma generation was most likely initiated in the asthenosphere-lithosphere decoupling zone or below. The thermal anomaly was either an episodic, partly mobile asthenospheric plume or successive diapiric intrusion of periodite along propagating fractures in the lithosphere and produced magma source compositions whose disposition was controlled by pre-existing fractures and/or lineaments in the Precambrian continental crust.