Abstract

Structural data from an area of gneisses, migmatites, and low-grade metasedimentary rocks in northwestern Nigeria suggest the presence of two contrasting structural styles, one simple and monocyclic and the other complex and polycyclic. The simple structural style is coextensive with one of two distinctive metasedimentary formations, the Birnin Gwari Schist Formation, which is interpreted as a sedimentary cover infolded during the Pan-African event into a basement of complex style consisting of the second metasedimentary formation, the Kushaka Schist Formation, and the gneisses and migmatites. This infolding may have occurred within the limits 730 to 650 m.y. ago, and it was followed by the 550- to 530-m.y.-ago epeirogenic uplift and cooling of the basement assemblage and the later emplacement of the Kusheriki granite 500 ± 4 m.y. ago.

The gneissic and migmatitic basement of the Birnın Gwari Schist Formation includes ∼2000-m.y.-old Eburnean rocks, but neither the depositional nor metamorphic ages of the Kushaka Schist Formation are known.

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