Two series of Ediacaran collision-related granites in the Guéra Massif, South-Central Chad: Tectonomagmatic constraints on the terminal collision of the eastern Central African Orogenic Belt

The Guéra Massif of South-Central Chad is one of the least studied regions of the Saharan Metacraton. The oldest known rocks were emplaced during the Ediacaran and have geochemical characteristics of collision-related granites. The granites can be subdivided into two series on the basis of their compositions: 1) ferroan and alkalic to alkali calcic, and 2) magnesian and alkali calcic to calc-alkalic. The ferroan rocks (589 ± 6 Ma, 589 ± 6 Ma, 593 ± 7 Ma, 590 ± 8 Ma) tend to be younger than the magnesian rocks (595 ± 8 Ma) indicating there was a secular compositional change. The chemical variability within each series can be explained by fractional crystallization, but the ferroan series requires lower water content (~1 wt%) and relatively reducing conditions (ΔFMQ −1) compared to the magnesian series (H₂O = ~2 wt%; ΔFMQ + 0.7). The older magnesian series is isotopically chondritic to moderately enriched (ISr = 0.7037 to 0.7073; ε_Nd(t) = −3.2 to + 0.2) and was likely derived from a subduction-modified mantle source at an ‘Andean-type’ margin. In contrast, the ferroan series is isotopically enriched (ISr = 0.7048 to 0.7176; ε_Nd(t) = −9.4 to −14.1) and has a significant amount of inherited Mesoproterozoic zircons (²⁰⁶Pb/²³⁸U = 1039 ± 24 Ma), indicating that it was likely derived by partial melting of an older crustal source. The changing nature of the granites likely records the transition from an active margin to a collisional zone between the Congo- São Francisco Craton and the Saharan Metacraton.