Drainage and Emplacement of Magmas along an Inclined Transcurrent Shear Zone: Petrophysical Evidence from a Granite-Charnockite Pluton (Rahama, Nigeria)

Abstract

A Pan-African granite-charnockite pluton of northeast Nigeria has been core-sampled at an average grid spacing of 2 km. Detailed petrographic and microstructural observations are combined with an AMS study of 172 stations. The continuum between charnockites and granites is revealed by their constituent minerals and the magnetic susceptibility map. The contemporaneity of emplacement of all the petrographic types is demonstrated by their similar mineral- and magnetic-fabrics. Inclined strike-slip shear zones were active during and slightly after the emplacement of this pluton. Earlier geochemical data indicate that magmas originated from several vertically juxtaposed lower-crust sources. The proposed model of emplacement implies that the shear zones were responsible for the migration, drainage and tapping of magmas from the source region. The inclined attitude of the shear zones would have acted as a roof-guide for the up-rising magmas. Finally, the transcurrent movement most likely enhanced the upward migration of magmas.