Late Permian mafic rocks identified within the Doba Basin of southern Chad and their relationship to the boundary of the Saharan Metacraton

The Doba gabbro was collected from an exploration well through the Cretaceous Doba basin of southern Chad. The gabbro is composed mostly of plagioclase, clinopyroxene and Fe-Ti oxide minerals and displays cumulus mineral textures. Whole-rock 40Ar-39Ar step-heating geochronology yielded a Late Permian plateau age of 257±1 Ma. The major and trace elemental geochemistry shows that the gabbro is tholeiitic in composition and has trace element ratios (i.e. La/YbN>7; Sm/YbPM>2.4; Nb/Y>1; Zr/Y>5) indicative of a basaltic melt derived from a garnet-bearing mantle source. The moderately enriched Sr-Nd isotopes (i.e. ISr=0.70495 to 0.70839; εNd(T)=-1.0 to -1.3) fall within the mantle array (i.e. OIB-like) and are similar to other Late Permian plutonic rocks of North-Central Africa (i.e. ISr=0.7040 to 0.7070). The enriched isotopic composition of the Doba gabbro contrasts with the more depleted compositions of the spatially associated Neoproterozoic post-Pan-African within-plate granites. The contrasting Nd isotope composition between the older within-plate granites and the younger Doba gabbro indicates that different mantle sources produced the rocks and thus may mark the southern boundary of the Saharan Metacraton.