Is the continental crust of North-Central Africa a metacraton or a shield terrane? the view from Chad

The nature of the continental crust of North-Central Africa has been a subject of debate for decades. Assessments of the geology, geochronology, and isotope geochemistry of igneous and metamorphic basement rocks have led to two broad-based models that offer very different views on the crustal evolution of the region. The Saharan Metacraton model suggests that the continental crust between the Tuareg Shield, Arabian-Nubian Shield, and the Central African Orogenic Belt was a single craton that was collectively reworked during the Late Neoproterozoic. In contrast, the Central Sahara Shield model proposes that there was a Great Central Gondwana Arc that extended ∼2400 km from the Tibesti Massif to Cameroon. It was along the arc system that the continental crust of North-Central Africa was sutured during the Late Neoproterozoic. The remnant collisional belt structure is identified as an arcuate gravity anomaly within the crust of central Chad and is known as the Chad Lineament. However, new observations from central, eastern, and southern Chad indicate that neither model is correct, but rather that the continental crust is composed of three or more distinct lithotectonic terranes that were sutured together. In southern Chad, Neoproterozoic granitic rocks of the Adamawa-Yadé domain, Guéra Massif, and Ouaddaï Massif have enriched Nd isotopic values (ε_Nd(t) < 0) and Archean to Mesoproterozoic (3400−1000 Ma) depleted mantle model ages that indicate they were derived from or interacted with ancient cratonic crust that was affected by a Mesoproterozoic tectonothermal event. In central Chad, the Neoproterozoic granitoids of the Poli domain, Mayo Kebbi Massif, and the Late Fitri Inlier are isotopically juvenile (ε_Nd(t) > +1) and have Mesoproterozoic to Neoproterozoic (1600−600 Ma) depleted mantle model ages indicating that this region is a sliver of juvenile continental crust in the middle of the Saharan region. In northern Chad, the Neoproterozoic granitic rocks of the Tibesti Massif represents a third, but poorly defined terrane. The granitic rocks in the Tibesti Massif have enriched Nd isotopic values (ε_Nd(t) < −0.5) and Paleoproterozoic to Mesoproterozoic (2170−1100 Ma) depleted mantle model ages. The amalgamation of the southern and central terranes occurred over a protracted period from the Early Cryogenian to Late Ediacaran with final suturing with the northern terrane during the latest Ediacaran to early Cambrian. The nature and origin of the North-Central Africa continental crust is best viewed from the perspective of Ediacaran ‘terrane amalgamation’ and that it likely resembles an accretion-collision belt similar to the Central Asian Orogenic Belt.