K-Ar ages of intrusive rocks in the Oban-Obudu massif and their significance for the tectonic and plutonic history of southeastern Nigeria

Abstract

K-Ar ages have been obtained for mineral separates: plagioclases from two dolerites and biotites from one granite and four granodiorites in the Oban-Obudu massif of southeastern Nigeria. Dolerites in the Oban area give K-Ar plagioclase ages of 204.0 ± 9.9 Ma and 219.9 ± 4.7 Ma. The granite of the Obudu area yields a K-Ar biotite age of 507.6 ± 10.1 Ma whilst the granodiorites in the Oban area yield K-Ar biotite ages ranging from 474.6 ± 9.4 Ma to 511.8 ± 10.0 Ma. The dolerites are related to the tholeiitic basaltic magmatism at the early opening of the central Atlantic, and are compatible in age with the formation of the Ring Complexes and the rifting of the Benue Trough of Nigeria. The granites and granodiorites belong to the syntectonic Older Granite series in the Pan-African orogeny. The emplacement timing deduced from the reported Pb-Pb zircon age of 617 ± 2 Ma and the newly obtained biotite ages suggest that these older granites in southeastern Nigeria had a prolonged cooling history of ∼110 Ma.