Petrographic and Geochemical Characterization of Dolerites East of Léré in Mayo Kebbi (South-West of Chad): Geodynamic Implication

The study area located in Mayo Kebbi belongs to the Northern domain of the Pan-African Range of Central Africa. This work is a contribution to the geological and geochemical study of dolerites in Mayo Kebbi. The main objective of our study is to carry out petrological and geochemical studies of dolerites around Léré and to determine their geodynamic context of emplacement. The methodology of this study is based on an integrated approach combining field work, petrographic and geochemical analyses. The field work revealed that dolerite dykes outcrop in the form of balls and blocks of light to dark gray shades cutting the Pan-African basement. These dolerite dykes have thicknesses varying from 5 to 20 m and lengths of the order of decameters to kilometers; and show main orientations of E-W and N-S. The petrographic study identified three facies: pyrite and basement enclave dolerites, pyrite and calcite dolerites, and amphibole and pyroxene dolerites, including doleritic and ophitic textures. The primary mineral assemblage consists of plagioclases, pyroxenes, oxides, hornblende, apatite and biotite, sometimes pyrite and calcite. Whole rock geochemical analyses using ICP-MS and ICP-AES methods determined the compositions of different facies of the dolerites: pyrite and basement enclave dolerites and pyrite and calcite dolerites are basic to intermediate hypovolcanic rocks (SiO2: 45.2% - 58.2%), with alumina (Al2O3: 14.65% - 17.85%). Amphibole and pyroxene dolerites (SiO2: 60.3% - 65.1%) are classified in the acid rock field. Transition element concentrations vary (Ni: 4 - 68 ppm and V: 73 - 322 ppm) showing a fractionation of iron-titanium oxides. The geodynamic context of emplacement of these rocks is typical of late to post-orogenic (compressive to distensive) to orogenic (compressive) intracontinental zones.