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

Abstract

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±4Ma 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 (1.6–1.8Ga) are interpreted to result from contamination of Neoproterozoic mantle by the Paleoproterozoic crust.The ca. 576Ma 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.