Petrological and chemical variability of peridotite xenoliths from the Cameroon volcanic line, West Africa: an evidence for plume emplacement

Abstract

Peridotite xenoliths from the Cameroon volcanic line (CVL), which is a 1600-km-long volcanic chain on the continental margin of West Africa and the Gulf of Guinea, exhibit regional variations in petrography, mineral chemistry, and equilibrium temperature. The xenoliths are mainly composed of spinel lherzolite, spinel harzburgite, dunite, and small amounts of wehrlite and websterite. Lherzolite and harzburgite are similar in texture, and the MgO and Cr₂O₃ components in these minerals increase with a decrease in the Al₂O₃ and Na₂O components and the modal abundance of clinopyroxene. This feature can be explained by a process of the partial melting of pyrolitic lherzolite and subsequent melt extraction in shallow mantle at a depth of 30-80 km. The Bioko, which is located near the center of the CVL, is distinctive from other regions (Annobon, Principe, Mt. Cameroon, Oku, and Ngaoundere) because of the higher degree of partial melting of residual peridotites. The estimated degree of partial melting in Bioko was about 25-30%, while in other areas it was less than 25%. Bioko was also characterized by the amount of cumulus dunite and wehrlite. The equilibrium temperatures of the xenoliths, as determined by a two-pyroxene geothermometer, were higher at the center of the CVL (e.g., about 1000 °C in Bioko) and gradually decreased toward the margins of the CVL (e.g., they were about 850 °C in Annobon, the margin of the oceanic area). These observations indicate that extensive igneous activity occurred at the uppermost mantle under the Bioko area, near the center of the CVL. Based on our study and previous isotope studies of volcanic rocks, we conclude that the regional variations in the CVL mantle were formed by the upwelling and emplacement of a deep hot plume about 120 Ma, which caused the opening of the Atlantic Ocean.