The Pan-African Kekem gabbro-norite (West-Cameroon), U-Pb zircon age, geochemistry and Sr-Nd isotopes: Geodynamical implication for the evolution of the Central African fold belt

Maurice Kwekam (1) , Pascal Affaton (2) , Olivier Bruguier (3) , J. P. Liegeois (4) , Gerald Hartmann (5) , Emmanuel Njonfang (6)

1 Université de Dschang
2 CEREGE - Centre européen de recherche et d'enseignement des géosciences de l'environnement
3 Manteau et Interfaces
4 Musée royal de l'Afrique centrale
5 GZG - Geowissenschaftliches Zentrum Göttingen
6 UY1 - Université de Yaoundé I

The Kekem shoshonitic gabbro-norite association is part of the high-K calc-alkaline (HKCA) post-collisional magmatism, a major feature of the Pan-African Belt in Cameroon. LA-ICP-MS U-Pb zircon analyses provide an age of 576 ± 4 Ma for the Kekem complex. This age is interpreted as dating the emplacement of the massif during the waning stage of the Pan-African orogeny. The latter is related to dextral movements along the Central Cameroon Shear Zone (CCSZ). The REE patterns display enriched LREE (LaN/YbN = 14.2-23.5) while HREE present a nearly flat profile (DyN/YbN = 1.3-1.7), and the La/Sm and Sm/Yb ratios led to propose that the Kekem gabbro-norites have been derived from the partial melting of a garnet-spinel lherzolite mantle source. The negative Nb-Ta and Ti anomalies and the positive Pb anomalies indicate that this mantle source was modified by contribution of a subduction-related material. The low Ce/Pb (2.6-10.4) and Th/Yb ratios associated to high Ba/La ratios, indicate that source enrichment could be related to slab derived fluids. As a whole, the Kekem geochemical features suggest that primary gabbro-noritic magmas derived from a subduction-modified mantle source (metasomatised lithospheric mantle). Moderately high 86Sr/87Sr initial ratios (0.7068-0.7082), low εNd (−5 to −9) and old Nd TDM model ages (1.6-1.8 Ga) are interpreted to result from contamination of Neoproterozoic mantle by the Paleoproterozoic crust. The ca. 576 Ma movements along the CCSZ are related to a Neoproterozoic metacratonization of the northern margin of the Congo craton during the Pan-African orogeny. This metacratonization led to vertical planar lithospheric delamination along lithospheric transcurrent faults, asthenospheric uprise and partial melting of the Paleoproterozoic lithospheric mantle.

Mots clés: West-Cameroon; Subduction-related fluid metasomatism; Late post-collision period; Shoshonitic gabbro-norite; Garnet-spinel peridotite

Domaines: Géochimie

Maurice Kwekam, Pascal Affaton, Olivier Bruguier, J. P. Liegeois, Gerald Hartmann, et al.. The Pan-African Kekem gabbro-norite (West-Cameroon), U-Pb zircon age, geochemistry and Sr-Nd isotopes: Geodynamical implication for the evolution of the Central African fold belt. Journal of African Earth Sciences, 2013, 84, pp.70-88. ⟨10.1016/j.jafrearsci.2013.03.010⟩. ⟨hal-00903665⟩