Petrology of the volcanic island of Annobon, Gulf of Guinea

Abstract

Annobon, the southernmost island of a volcanic chain located in the Gulf of Guinea, is built of palagonitic breccia overlain by basaltic flows which are in turn overlain by alkaline lavas, mostly emplaced between 5.4 and 2.6 m.y. ago. A potassic alkaline trend ranging from basanites to quartziferous trachytes through potassic hawaiites and tristanites can be defined. This trend appears to result from fractional crystallization under high water pressures (1–3 kbar) and high oxygen fugacities which increased during differentiation. The presence of oversaturated trachytes results from intense fractionation of amphibole (kaersutite) at the level of intermediate magmas. Similarities with the Nuku Hiva island magmatic series (Central Pacific) are emphasized since both Annobon and Nuku Hiva Islands may occupy identical structural positions above large fracture zones. The geographic trend of the Gulf of Guinea volcanic islands is not well explained by hot spot activity. The islands do not appear related to drift, but instead they seem to be connected with an intraplate extension fracture system which may have been initiated along a NE—SW strike-slip fault.