The Cameroon Line: Analysis of an intraplate magmatic province transecting both oceanic and continental lithospheres: Constraints, controversies and models

Abstract

The near 1700km long Cameroon Line (CL) is an African intraplate ‘fan-shaped’ alkaline volcano-plutonic rift zone of variable width (<200km), with a ca. 66Ma history of magmatic activity without any systematic internal pattern of age variation. Although the CL features prominently in discussions about active rift tectonics and magmatism during the growth of the African Plate and the opening of the Central Atlantic Ocean, there is a significant diversity of opinions about its local geometry, origin and evolution. Here, we review various geodynamic models for the CL based on large data-sets of field geology, petrology, geochemistry and geochronology. Results indicate that the Line traverses both oceanic and continental domains, extending NE–SW for about 700km on oceanic lithosphere of the Gulf of Guinea (ca. 80–120Ma), cutting obliquely across oceanic transform faults and across the ocean–continent boundary (OCB), and then continuous for ~1000km (to Kapsiki), with a significant bifurcation at about 320km, to the Adamawa and the Biu Plateaus along complex Neoproterozoic continental lithosphere of Central and West Africa (500–800Ma, with embedded fragments of Paleoproterozoic–Archean crust). The continental lithosphere is transected by a dense network of major late Neoproterozoic shear zones (500–650Ma) that were linked to conterminous extensions in South America before the opening of the Atlantic Ocean. The shear zones define a “Cameroon Block” to which the continental sector of the Cameroon Line is confined, but neither this continental tectonic anisotropy, nor its oceanic transform faults, appear to have influenced the geometry of the CL. Geochemical data confirm that the mostly alkaline CL magmas originate from deep mantle upwelling with variable input from at least four different mantle end-members (DMM, HIMU, FOZO and EM-I), and with different degrees of partial melts derived from above the 410-discontinuity. These melts then interacted diversely with lithospheric mantle, but without extensive crustal contamination. The oldest igneous activity is documented on the continent at the far end of the CL by the Late Cretaceous–Early Paleocene (on the Kapsiki Plateau at Golda Zuelda), and by the Miocene, contemporaneous volcanic activity spread along the oceanic and continental sectors. The intraplate magmatism along the CL is coincident with continent-wide extensional tectonics across central and northern Africa during the Cretaceous and throughout the Cenozoic, a time during which the African Plate experienced rotation of ~7° and slow but variable rates of convergence with Europe, involving several episodic changes in the pole of rotation between the African and South American plates that affected the opening of the South-Central Atlantic. The CL reflects a unique long-lived geodynamic setting and there appears to be no simple explanation for its origin.