Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
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.
ISSN: 0899-5362

hal-00903665
https://hal.archives-ouvertes.fr/hal-00903665
DOI : 10.1016/j.jafrearsci.2013.03.010

Éditeur(s) : HAL CCSD Cambridge University Press (CUP)
Résumé : International audience
The Amalaoulaou Neoproterozoic island-arc massif belongs to the Gourma belt in Mali. The metagabbros and pyroxenites forming the main body of this arc root show the pervasive development of garnet. In the pyroxenites, the latter has grown by reaction between pyroxene and spinel during isobaric cooling. By contrast, in the metagabbros, garnet textures and relations to felsic veins exclude an origin through solid-state reactions only. It is proposed that garnet has grown following dehydration and localized melting of amphibole-bearing gabbros at the base of the arc. The plagioclase-saturated melts represented by anorthositic veins in the metagabbros and by trondhjemites in the upper part of the massif provide evidence for melting in the deep arc crust, which locally generated high-density garnet clinopyroxene rutile residues. Garnet growth and melting began around 850 degrees C at 10 kbar and the tonalitic melts were most probably generated around 1050 degrees C at P >= 10 kbar. This HT granulitic imprint can be related to arc maturation, leading to a P-T increase in the deep arc root and dehydration and/or dehydration-melting of amphibole-bearing gabbros. Observation of such features in the root of this Neoproterozoic island arc has important consequences, as it provides a link to models concerning the early generation of continental crust.
ISSN: 0016-7568

hal-00420960
https://hal.archives-ouvertes.fr/hal-00420960
DOI : 10.1017/S0016756808005499