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Petrography and Geochemical Characterization of Dolerites from Figuil (Northern Cameroon) and Léré (Southwestern Chad)

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Document pages: 24 pages

Abstract: This work presents the petrographic and geochemicaldata of the dolerite dykes crosscutting thePan-African basement of Figuil (North-Cameroon) and Léré (South-WestChad) in order to approach their petrogenesis and their emplacement context.Two groups of dolerites have been highlighted by petrographic and geochemical studies. These groups werediscriminated by their TiO2, Fe2O3,REE, Ba, Nb, Zr, La and Hf contents which are relatively higher in group I;group II, on the other hand, has higher MgO, Mg#, Sc, Ni and Cr contents. Themineralogical assemblage of these dolerites is made up by plagioclases,pyroxenes, olivine, oxides, amphibole, biotite and sometimes pyrite, calcite,apatite, epidote and chlorite. The behaviour of the major and trace elements suggestthat studied dolerites have an evolution dominated by fractionalcrystallization. Most dolerite samples show higher REE concentrations and(La Yb)N > 8.7, (Tb Yb)N > 1.9 and Dy Yb > 2ratios characterizing a garnet-bearing mantle. The difference in incompatibleelements between the two groups is explained by the degree of partial meltingof the same source which becomes more important over time. Low (Ce Yb)N values (3.3 - 11.58) also suggest relativelylow partial melting degree of the source. Fractional crystallizationprocess was possibly combined with minor crustal contamination as shown byenrichment of Th Yb from group II to Group I that might be due to turbulentmagma emplacement. The chemical compositions of these dolerites are similar tothat of continental tholeiites with slightly moderate negative Nb-Ta anomalies which are attributed tocrustal contamination of magmas. As other dolerites of Cameroon,continental tholeiitic signature of the studied dolerites is evidenced ingeotectonic discrimination diagrams with Group II dolerite compositions fallingwithin the field of tholeiitic basalts and group I within the field of alkalibasalts.

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