Abstract
This work presents the petrographic and geochemical data of the dolerite dykes crosscutting the Pan-African basement of Figuil (North-Cameroon) and Lere (South-West Chad) in order to approach their petrogenesis and their emplacement context. Two groups of dolerites have been highlighted by petrographic and geochemical studies. These groups were discriminated 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. The mineralogical 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 suggest that studied dolerites have an evolution dominated by fractional crystallization. Most dolerite samples show higher REE concentrations and (La/Yb)N > 8.7, (Tb/Yb)N > 1.9 and Dy/Yb > 2 ratios characterizing a garnet-bearing mantle. The difference in incompatible elements between the two groups is explained by the degree of partial melting of the same source which becomes more important over time. Low (Ce/Yb)N values (3.3 - 11.58) also suggest relatively low partial melting degree of the source. Fractional crystallization process was possibly combined with minor crustal contamination as shown by enrichment of Th/Yb from group II to Group I that might be due to turbulent magma emplacement. The chemical compositions of these dolerites are similar to that of continental tholeiites with slightly moderate negative Nb-Ta anomalies which are attributed to crustal contamination of magmas. As other dolerites of Cameroon, continental tholeiitic signature of the studied dolerites is evidenced in geotectonic discrimination diagrams with Group II dolerite compositions falling within the field of tholeiitic basalts and group I within the field of alkali basalts.
Highlights
Mafic dyke swarms provide the most complete record of short-lived, mantlegenerated magmatic events through time and space [1] [2]
Two groups of dolerites have been highlighted by petrographic and geochemical studies. These groups were discriminated 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
The two groups of dolerites are distinguished by their REE, Ba, Nb, Zr, La and Hf contents which are relatively higher in group I than in group II
Summary
Mafic dyke swarms provide the most complete record of short-lived, mantlegenerated magmatic events through time and space [1] [2]. The crystallization of basaltic magmas can occur on the subsurface in the form of sills and dolerite dykes. These intrusions are often formed and located in areas of high orogenic activity (shear and collision zones) and are intrinsically linked to the dynamics of mountain ranges. In the Pan-African Central African Fold Belt (CAFB), petrographic, geochemical and geochronological data were obtained from several magmatic intrusions The Dolerite investigated in the present study is located in the northern domain of CAFB in Cameroon and southwest of Chad. Its preliminary geochemical data of some samples in Cameroon domain of CAFB were presented in Ngounouno et al [10]. The aim of this study is to characterize, constraint the petrogenesis and define geotectonic context of the Figuil (Cameroon) and Léré (Chad) dolerites using new major, trace and REE compositions data
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