Strontium isotope geochemistry and potassium-argon studies on volcanic rocks from the Cameroon Line, West Africa.

Abstract

The Cameroon line is a chain of Tertiary - Recent transitional to strongly alkaline volcanoes extending for 1600Km from the Atlantic island of Pagalu to the northern part of the Cameroon Republic. North of the Oku Massif the line splits into two branches. One runs northwards into N. E. Nigeria while the other runs eastwards through the Tchabal Mbabo and Ngaoundere Plateaux of eastern Cameroon. Eleven volcanic centres have been investigated: four islands in the Gulf of Guinea; six volcanoes in Cameroon; and the Biu Plateau in Nigeria.

The geology of these volcanoes is reviewed and new K-Ar and Rb-Sr age determinations are presented from Principe, Sao Tome, Etinde, Manengouba, Bambouto, Oku, Mandara, and Biu. Volcanism has been continuous from 66Ma ago up to the present day. Activity began with - central volcanoes on the continental sector composed mainly of oversaturated salic rocks and voluminous continental basaltic rocks have only been erupted in the last IOMa. The oldest dated volcanism on the oceanic sector occurred 31Ma ago and most of the island lavas are basaltic with lesser quantities of evolved phonolites and trachytes. There is no marked age trend along the chain and recent activity is evident in most centres.

87Sr/86Sr ratios of nephelinites, basanites, basalts and hawaiites from the oceanic sector (0.7028-0.7037) fall within the range observed from most other islands in the central-south Atlantic Ocean and demonstrate the lack of involvement with seawater - altered lithosphere. An almost identical Sr isotopic (0.7029-0.7038) and chemical spectrum exists for the uncontaminated mafic rocks from the continental centres. In particular the relatively primitive, ultramafic xenolith bearing Biu Plateau basalts show very similar, fine scale isotopic heterogeneities to those recorded in mafic lavas from the Gulf of Guinea islands. These inhomogeneities consistently correlate with degree of alkalinity whereby alkaline lavas have lower 87Sr/86Sr ratios relative to more transitional basalts. These data illustrate that all these volcanoes, on both sides of the continental margin, share the same heterogeneous mantle source region common to many terrestrial anorogenic volcanic fields and oceanic islands. Magmas are considered to have been generated by a thermal anomaly/hotzone in the lower mantle which had previously given rise to the Benue trough. Liquids subsequently migrate to the surface undergoing diffusive zone refining and mixing with the overlying depleted MORB reservoir.

A few basalts and intermediates from several of the Cameroon volcanoes have slightly elevated initial 87Sr/86Sr ratios (0.7039- 0.7047) and are shown to reflect small degrees of crustal contamination superimposed on mantle derived heterogeneities. The major element chemistry of these lavas indicate that such contamination is not necessarily restricted to selective Sr migration from the crust. Phonolites and trachytes from the islands of Sao Tome and Principe are cogenetic with their mafic counterparts although very small levels of interaction with oceanic sediment during fractionation is reflected in the initial 87Sr/86Sr ratios of some evolved rocks from Säo Tome. On the continental sector peralkaline trachytes, rhyolites and ignimbrites have all undergone varying degrees of crustal contamination with the most salic rocks representing 80-90% crustal melts. Where crustal interaction has not occurred during fractionation, liquids evolve to uncontaminated phonolites by identical processes to those giving rise to the oceanic sector fractionates. The sample population is sufficiently large to conclude that the initial 87Sr/86Sr ratios of uncontaminated within-plate basalts from this region are very unlikely to be higher than 0.7037-0.7038 or lower than 0.7027.