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

• Main Author: Dunlop, Hugh Mirams
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: University of Edinburgh 1983
• Subjects: Geochemistry; Four Islands; South Atlantic Ocean
• Online Access: http://hdl.handle.net/1842/7087

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 ...