Lamprophyric volcanism of Cenozoic age on the Sierra Leone Rise: Implications for regional tectonics and the stratigraphic time scale

Abstract

Igneous and sedimentary rocks have been recovered by dredging the upper slopes of Krause Seamount, a feature lying on the Sierra Leone Rise near 9°N, 20°W in the eastern Equatorial Atlantic. The igneous samples consist of lamprophyres and an ankaramite, which are rarely encountered in an oceanic setting. Associated with these rocks are breccias made up of lamprophyric fragments set in a lithified calcareous matrix containing a rich planktonic foraminiferal assemblage of early Eocene (Ypresian Subzone P8a) age. Whole-rock K-Ar dating of the igneous samples proved unsuccessful owing to advanced alteration and consequent argon loss. However, within the breccias are numerous, fresh crystal clasts of Ti-rich amphibole, pyroxene and biotite which have compositions closely similar to phenocrysts observed in the lamprophyric rocks, suggesting that they are derived directly from them. K-Ar dating of separates of the three minerals yielded ages in the range 53.3–55.4 Ma. The concordant results provide a maximum age for the Ypresian Subzone P8a and are consistent with its position within recently established Cenozoic time scales. They indicate that Krause Seamount is appreciably younger than the adjacent Gambia and Sierra Leone Basins. In common with other occurrences of lamprophyric rocks in the oceans, those exposed on Krause Seamount are located in an area of old oceanic basement close to a large fracture zone which has been reactivated by late-stage volcanic activity. These relatively rare alkaline rocks may be generated by remelting of a volatile-rich metasomatized layer which is present in the upper mantle beneath Mesozoic ocean floor.