Emmanuel Njonfang, Ecole Normale Supérieure (Cameroon)
Alexandre Nono, Ecole Normale Supérieure (Cameroon)
Pierre Kamgang, Faculty of Sciences (Cameroon)
Vincent Ngako, I.R.G.M. (Cameroon)
Félix Tchoua Mbatcam, Faculty of Sciences (Cameroon)
The Cameroon Line forms one of the major geological structures of the African plate and continent. The fact that it displays a continental part and an oceanic part, makes it a unique feature in Africa and even in the world. Since the reviews of various hypotheses of its tectono-magmatic origin by Déruelle et al. (1991), the following views are still discussed: (1) a (failed) continental rifting (e.g.: Wilson and Guiraud, 1992); (2) rejuvenation of Precambrian faults (e.g. Moreau et al., 1994); (3) hot line or thermally anomalous linear zone in the earth's mantle (Meyers et al., 1998). Recently available petrological, geochemical and structural data are consistent with: (1) the mantle origin of mafic rocks as petrographically evidenced by their richness in ultramafic xenoliths and isotopically marked by Sri ratios (0.7030-0.7045) in the range of mantle values; the derivation of felsic rocks from mafic ones through fractional crystallization with more or less crustal contamination during ascent; (2) the absence of geochemical discrimination between oceanic and continental sectors; however, rocks are mostly undersaturated in the oceanic sector and saturated to oversaturated in the continental sector (3) the existence of volcanoes with transitional affinities in the continental sector (e.g. Mt Bangou; Fosso et al., 2005) which contrasts with the entire alkaline nature formerly suggested (Déruelle et al., 1991); (4) The older age of plutonic complexes (67-30 Ma) relative to volcanic massifs, but volcanic activity ranges from 45 Ma to Present and not from 30 Ma established for the distribution of volcano-capped swells along the Line (Burke, 2001); (5) the absence of any age migration associated to the SW-NE linear trend. The striking point is the SW younging volcanism in the oceanic sector from Principe (31 Ma) to Annobon (5 Ma) islands (Lee et al., 1994). This lack of a steady time-space migration and the SW-NE trending have also been observed both in the magmatic provinces of Nigeria and Benue Trough which else share similar geochemical features, suggesting similar origin for the three magmatic provinces. We have recently explained such a particular distribution of alkaline magmatism in terms of complex interaction between a hotspot and Precambrian faults (Ngako et al., 2006). Somes References
Burke, K., 2001. Origin of the Cameroon Line of volcano-capped swells. J. Geol. 109, 349-362.
Fosso, J. et al., 2005. Les laves du mont Bangou: une première manifestation volcaniceéocène à affinité transitionnelle, de la Ligne du Cameroun. C.R. Géoscience 337, 315-325.
Meyers, J.B. et al., 1998. Deep-imaging seismic and gravity results from offshore Cameroon Volcanic Line and speculation of African hot-lines. Tectonophysics 284, 31-63.
Ngako, V. et al., 2006. The North-South Paleozoic to Quaternary trend of alkaline magmatism from Niger-Nigeria to Cameroon: complex interaction between hotspots and Precambrian faults. J. Afr. Earth Sci. 45, 241-256.