Late Neoproterozoic granitic magmatism in the Doba Basin of Southern Chad: Implications for the crustal evolution of North-Central Africa

The Doba Basin is one of four oil producing basins of southern Chad that developed within the West and Central Africa Rift System during the Late Cretaceous opening of the Central Atlantic Ocean. The Cretaceous basins were built upon older Ediacaran basins that formed after the collision between the Congo-São Francisco Craton and the continental crust of North-Central Africa. Oil exploration drill wells in the Doba Basin (Mouroumar-1, Benoy-W2, Kiagor-1, Djabi-1, Bebalem-1) encountered granitic rocks at depths from 2250 m to 3230 m. The granitic rocks were dated by zircon U[sbnd]Pb methods and yielded ages of 594 ± 4.4 Ma (Benoy), 594 ± 4.2 Ma (Kiagor), 595 ± 4.3 Ma (Bebalem), and 579 ± 4.1 Ma (Mouroumar). The older granitoids are magnesian and metaluminous to weakly peraluminous, and compositionally similar to volcanic-arc granites. The younger granites are magnesian to ferroan and classify as within-plate or post-collisional granite. The magmatic zircon Hf isotopes (ε_Hf(t) = −12.5 to 0.0) and whole rock Nd isotopes (ε_Nd(t) = −3.3 to −8.2) show that all granitoids were derived from an isotopically enriched source. Crystallization pressure estimates from biotite and titanite indicate that they were emplaced in the upper crust (1–4 kbar). The volcanic-arc granitoids likely had lower parental magma temperatures (∼800 °C) than the post-collisional granitoid (∼900 °C). The results from this study along with previously published results demonstrate that contemporaneous (620–590 Ma) volcanic-arc magmatism stretched 1000–1500 km from the Doba Basin through the Guéra Massif to the Ouaddaï Massif and was followed by post-collisional magmatism at ≤580 Ma. The implication is that the southeastern and eastern portions of the Saharan Metacraton were not structurally contiguous with the western and northwestern portions until after ∼590 Ma. Consequently, the continental crust of North-Central Africa is not a coherent craton, but rather it is a composite shield terrane similar to the Arabian Nubian Shield or Central Asian Orogenic Belt.