Geochronology of a rhyolite dyke from Nassarawa-Egon in Central Nigeria

Abstract

A 50-m thick rhyolite dyke which cuts the Pan-African crystalline basement complex at Nassarawa-Egon, in central Nigeria, yielded a 10-point Rb-Sr whole-rock isochron age of 171±3 Ma and an initial87Sr/86Sr ratio of 0.71137±0.00020. The petrography and major oxide geochemistry of the rock indicate that it has a calc-alkalic affinity. The dyke occupies the axis of an ancient mylonitic shear zone situated at the edge of the Benue Rift, and both strike at a high angle with respect to the long axis of the rift. We therefore consider that the emplacement of the dyke is related to the doming which predated the formation of the Benue Rift (Olade, 1975). The calculated age of the dyke is much older than those of the Jurassic Younger Granites in the closest neighbourhood (e.g. Mada complex; 147 Ma). Furthermore, the Jurassic Granites and their associated volcanics and subvolcanics are all alkalic. This work provides further evidence that acid, pre-rift volcanism began by mid-Jurassic, although the oldest exposed sedimentary fill is mid-Cretaceous in age.