Age Migrations of Anorogenic Ring Complexes in Northern Nigeria

Abstract

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...