Crystalline inliers near Lake Iro (SE Chad): Post-collisional Ediacaran A2-type granitic magmatism at the southern margin of the Saharan Metacraton

Abstract

The structure and evolution of the continental crust in the southern part of the Saharan Metacraton (central Africa) is poorly known due to extensive laterite and sediment cover as well as geographic isolation. We report on a series of five crystalline inliers newly discovered near Lake Iro (south-eastern Chad) that offered the opportunity to unravel the nature of the local basement in this area. Two spatially determined petrographic types are exposed in the inliers: a medium-grained porphyritic amphibole–biotite granite and a biotite microgranite with typical embayed (rhyolitic) quartz phenocrysts, both containing fine-grained melanocratic igneous enclaves. Some porphyritic granites display evidence for low temperature deformation along N60 shear zones. The Lake Iro igneous rocks are typified by their ferroan mineralogy, define a ferroan alkali-calcic metaluminous to weakly peraluminous potassic association, and exhibit elevated HFSE (Zr, Nb, Y) and REE contents. Therefore, they are best classified as A2-type granites. They are variably ferromagnetic with total magnetization lower than 1 A/m. The Lake Iro granites most likely correspond to a single subvolcanic–plutonic silicic complex because: (i) first-order geochemical modelling indicates that the microgranites can be generated by 30% fractional crystallization of a two feldspar–amphibole assemblage from a porphyritic granite melt and (ii) both granite types yield zircon U–Pb emplacement ages consistent within a 575.3 ± 5.6 to 581.3 ± 3.8 Ma time frame. The Lake Iro granites thus represent the oldest of a series of post-collisional igneous associations exposed in southern Chad and nearby countries which, collectively, show that the amalgamation of the constituent blocks of the southern Saharan Metacraton is older than 580 Ma.