Pan-African Orogeny in Northern Nigeria

Abstract

An area of approximately 12,000 sq km of Precambrian to lower Paleozoic rocks around Zaria in northern Nigeria has recently been mapped for the first time. Results from this survey have been combined with limited data from adjacent areas to determine the nature of the Pan-African event in this part of Nigeria. A crystalline basement complex of gneisses, migmatites (Dahomeyan), and relicts of an ancient metasedimentary sequence (Birrimian) underlies most of the region. Infolded into this basement, as north-south-trending synclinoria, is a younger metasedimentary sequence (Ka-tangan) forming the remnants of an earlier, more extensive supracrustal cover. A suite of syntectonic to late tectonic granites and granodiorites (late Precambrian to lower Paleozoic) has intruded both the basement and its cover. The structure of the area is dominated by north-south trends typical over much of Nigeria. There is evidence to suggest that the whole area was affected by two successive phases of intense deformation resulting in tight isoclinal folding about (1) east-northeast-west-southwest, and (2) north-south axes. The crystalline basement was reactivated and the supracrustal cover regionally metamorphosed. Two phases of progressive metamorphism accompanied the deformations, which were separated and followed by static metamorphic phases. Pressure-temperature conditions of metamorphism appear to have remained fairly constant throughout, giving rise to a low pressure facies series similar to the Buchan type. Extensive migmadzation of the crystalline basement is believed to have accompanied the first deformation, followed by further differentiation during the second phase to produce homogeneous gneisses and intrusive granites. Jointing, fracturing, and faulting followed the orogeny, and a major northeast-southwest-trending transcurrent fault system crosses the area. It is obvious from the structural and metamorphic evidence that the Pan-African event reached the status of an orogeny in northwestern Nigeria.