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

The Guera 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 (similar to 1 wt%) and relatively reducing conditions (AFMQ -1) compared to the magnesian series (H2O = similar to 2 wt%; Delta FMQ + 0.7). The older magnesian series is isotopically chondritic to moderately enriched (ISr = 0.7037 to 0.7073; epsilon(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; epsilon(Nd)(t) = -9.4 to -14.1) and has a significant amount of inherited Mesoproterozoic zircons (Pb-206/U-238 = 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- Sao Francisco Craton and the Saharan Metacraton.