Major, trace elements and Sr-Nd-Pb isotopes systematics of mafic dykes from the Figuil (northern Cameroon) and Lr (southwestern Chad) areas

In this study, field descriptions of the dolerite dykes cross-cutting the Pan-African basement (DPB) and the dolerite and gabbroic dykes cross-cutting the Cretaceous sedimentary basin (DGCSB) are used together with petrographic, bulk-rock geochemistry, as well as Sr-Nd-Pb isotopic data to constrain the geodynamic setting and origin of these mafic (basanite, basalt and basaltic andesite in composition) rocks that outcrop in the Figuil-Lere area. The DPB dykes are alkaline with SiO2 of 46.63-49.03 wt% and MgO of 3.93-7.11 wt%. In contrast, the DGCSB dykes are subalkaline with SiO2 ranging between 50.24 and 53.16 wt% and MgO fluctuating between 5.22 and 7.90 wt%, comparable to dykes described in other regions of the Cameroon Line (CL). The rocks studied belong to the tholeiitic series and were emplaced in within-plate basalts (WPB) and mid-ocean ridge basalts (MORBs) setting. The variations in major and trace elements from the basanite to basaltic andesite compositions are dependable with fractional crystallization (FC) of different mineral phases such as opaque minerals, olivine, clinopyroxene, amphibole and alkali feldspar. Most samples exhibit negative Nb-Ta and positive Sr anomalies, which together with the negative correlation between MgO and Sr-87/Sr-86, and isotopic ratio of Pb-206/Pb-204 <19.5 (18.27-19.49) suggests interactions of crustal components. Nevertheless, this contamination seems limited because there is a positive correlation between SiO2 and Nd-143/Nd-144 plot and low Ba contents (72-210 ppm) of the samples compared to the mid-continental crust (259-628 ppm). Trace element modelling proposes a derivation of the Figuil-Lere dykes from parental melts generated by 4%-15% partial melting of a source containing garnet peridotite in the mixed lithospheric-asthenospheric mantle. The isotopic data of the dykes studied with hybrid magma composition are characterized by more radiogenic Sr-87/Sr-86, Nd-143/Nd-144 and Pb-206/Pb-204 isotopic ratios with a dominant mixing of depleted MORB mantle (DMM)-enriched mantle 1 (EM1)-enriched mantle 2 (EM2), like those of other mafic dykes of the CL.