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Petrographic and structural analysis of the Precambrian rocks in the Zalbi Sector, northwest Léré, Chad

The northwestern part of southwest Chad is one of the best regions to find exposed Precambrian terrains and their Phanerozoic cover. In this area, it is easy to establish the lithostratigraphic and structural relationships between sedimentary and granitic rocks, greenstone belts and dolerite dykes. Based on petrographic and structural studies, we distinguished the greenstone belts that belong to the Neoproterozoic Zalbi Series. The Neoproterozoic greenstone belts that are intruded by three generations of Pan-African granitoids (gabbro-diorite, biotite monzogranite and leucogranite) host the mineral resources in the region. The Precambrian rocks are covered by Cretaceous shale, sandstone and microconglomerate in the Kebbi Basin, and intruded by dolerite dykes. These late dolerite dykes, have a continental tholeiitic composition, which is supported by the presence of orthopyroxene in the mineralogical assemblage. Structurally, two major deformation events are present in the Zalbi Series. The earliest D1 event is the most intensive and is associated with vertical to subvertical north-trending S1 foliation, a vertical L1 lineation and P1 folds; the metamorphism associated with this phase is equilibrated in greenschist facies conditions. The second D2 event is discrete and is characterised by large-amplitude folds and fractures.