Internal fabric and strike‐slip emplacement of the Pan‐African granite of Solli Hills, northern Nigeria

Abstract

The Solli Hills pluton (250 km2) in northern Nigeria is an example of a Pan‐African granite emplaced circa 580 Ma ago in the Jos ‐ Adamawa block. Its micro structures and internal fabrics have been studied in order to help constrain the kinematics of the Pan‐African orogeny in this block. We used field observations and measurements, petrographic examinations, and measurements of the magnetic susceptibility and anisotropy of magnetic susceptibility of 124 regularly spaced stations. Most samples are ferromagnetic, i.e., belong to the magnetite‐series granites, and present predominantly magmatic microstructures. Particularly remarkable are the asymmetrical zoning of the petrographic types, the susceptibility magnitudes, and the planar ‐ linear magnetic fabrics. The magnetic foliations and lineations are very consistent in the pluton, around N 20°E, 60°W and N 205°E, 10°, respectively. Field and microscopic observations reveal that deformation occurred everywhere in the pluton and immediate country‐rock envelope during a dextral strike‐slip synplutonic episode. It is suggested that the Solli Hills pluton was emplaced in a late Pan‐African dextral tear‐fault system. The striking fabric asymmetry of the pluton strongly suggests a shear zone termination emplacement model.