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

This work presents the petrographic and geochemical\r\ndata of the dolerite dykes crosscutting the\r\nPan-African basement of Figuil (North-Cameroon) and Lere (South-West\r\nChad) in order to approach their petrogenesis and their emplacement context.\r\nTwo groups of dolerites have been highlighted by petrographic and geochemical studies. These groups were\r\ndiscriminated by their TiO2, Fe2O3,\r\nREE, Ba, Nb, Zr, La and Hf contents which are relatively higher in group I;\r\ngroup II, on the other hand, has higher MgO, Mg#, Sc, Ni and Cr contents. The\r\nmineralogical assemblage of these dolerites is made up by plagioclases,\r\npyroxenes, olivine, oxides, amphibole, biotite and sometimes pyrite, calcite,\r\napatite, epidote and chlorite. The behaviour of the major and trace elements suggest\r\nthat studied dolerites have an evolution dominated by fractional\r\ncrystallization. Most dolerite samples show higher REE concentrations and\r\n(La/Yb)N \u003e 8.7, (Tb/Yb)N \u003e 1.9 and Dy/Yb \u003e 2\r\nratios characterizing a garnet-bearing mantle. The difference in incompatible\r\nelements between the two groups is explained by the degree of partial melting\r\nof the same source which becomes more important over time. Low (Ce/Yb)N values (3.3 - 11.58) also suggest relatively\r\nlow partial melting degree of the source. Fractional crystallization\r\nprocess was possibly combined with minor crustal contamination as shown by\r\nenrichment of Th/Yb from group II to Group I that might be due to turbulent\r\nmagma emplacement. The chemical compositions of these dolerites are similar to\r\nthat of continental tholeiites with slightly moderate negative Nb-Ta anomalies which are attributed to\r\ncrustal contamination of magmas. As other dolerites of Cameroon,\r\ncontinental tholeiitic signature of the studied dolerites is evidenced in\r\ngeotectonic discrimination diagrams with Group II dolerite compositions falling\r\nwithin the field of tholeiitic basalts and group I within the field of alkali\r\nbasalts.