ABSTRACT
This work presents the petrographic and geochemical
data of the dolerite dykes crosscutting the
Pan-African basement of Figuil (North-Cameroon) and Léré (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.