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The Guera Massif of South-Central Chad is composed of collisional and post-collisional intrusive rocks related to the Central African Orogenic Belt. Collisional granites were emplaced at 595–590 Ma...

The southern Saharan Metacraton is one of the least geologically constrained regions in the world as bedrock exposures are rare. To the west of Lake Fitri near the communities of Ngoura and Moyto in south-central Chad there are two granitic inliers that form a series of lenticular to ellipsoidal low laying hills. Very little is known about the Lake Fitri inliers and their regional correlation to larger massifs in Chad is undetermined. The granites yielded weighted-mean zircon 206Pb/238U ages of 554 ± 8 Ma and 546 ± 8 Ma indicating they were emplaced ~45 million years after the cessation of arc-related magmatism and the subsequent collision between the Congo-Sao Francisco Craton and the Saharan Metacraton. The rocks have distinct groupings of inherited zircons with ages of ~580 Ma and ~635 Ma suggesting they are at least in part derived by recycling of older crustal rocks. The biotite mineral chemistry, whole rock compositions and petrological modeling indicate the granites were derived by melting of crustal lithologies but the whole rock Nd isotopes (eNd(t) = +1.3–+2.9) are characteristic of a mantle source. The contrasting inheritance-rich nature of the granites with a juvenile Nd isotopic signature is likely due to mixing between magmas derived from juvenile (Neoproterozoic) arc-related crust and asthenospheric magmas. Asthenospheric upwelling was probably a response to post-orogenic lithospheric delamination related to fault movement along the Chad Lineament, a possible extension of the Tchollire-Banyo shear zone that extended to the interior of the Saharan Metacraton. The implications are that lithospheric delamination may not have occurred immediately after collision but rather propagated along a narrow belt that extended well into the central regions of the Saharan Metacraton.

Precambrian crust within southern Chad and eastern Cameroon preserves rocks that were remobilized and emplaced during and after the Neoproterozoic (∼650 Ma to ∼610 Ma) collision between the Congo, Sao Francisco, West African cratons and the Saharan Metacraton. The Guera Massif of south-central Chad and granites located near Lake Fitri are inferred to be of Neoproterozoic age but there are no radio-isotopic dates available to confirm their association with the Pan-African Orogeny (Central African Fold Belt). New zircon U/Pb geochronology of granitic rocks from the Guera Massif, Lake Fitri region and the Doba Basin of the southern Chad yielded weighted-mean 206Pb/238U ages from 595 ± 8 Ma to 545 ± 6 Ma. The oldest inherited zircons indicate that the Guera Massif is either built upon Meso- to Paleoproterozoic (1000 Ma–1900 Ma) continental crust or that pre-Neoproterozoic rocks were the source of the silicic Ediacaran rocks. The identification of ∼590 Ma volcanic-arc granites within the Guera Massif suggest that subduction continued in the eastern regions of Chad after collision had occurred (≥610 Ma) in the west (Cameroon). The younger granites (≤570 Ma) are post-collisional in nature and are closely linked to deformation cycles recorded in eastern Cameroon after the main continental collision episode between the Congo Craton and the Saharan Metacraton. The post-collisional granites located in the Lake Fitri region likely represent a westward extension of the Guera Massif as there are temporal and compositional similarities between the granitic rocks. The age and composition of the diorite from the Doba Basin of southern Chad is similar to post-collisional granites from the Central African Fold Belt of Cameroon and implies that the southern boundary of the Saharan Metacraton may be north of the Doba Basin.

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 (~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; eNd(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; eNd(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- Sao Francisco Craton and the Saharan Metacraton.

The Saharan Metacraton is largely covered by the Sahel and Sahara with only sparse bedrock exposure making it one of the most poorly studied cratons in the World. Consequently, detrital zircon geochronology is invaluable to discern the lithotectonic development of the Saharan Metacraton in the absence of outcrop. Detrital zircons from the northern Chari River, the largest river system of North-Central Africa, have ages between ~700 Ma and ~500 Ma with a small Cambrian to Ordovician (486 ± 11 Ma and 422 ± 11) population, a few Devonian to Middle Jurassic zircons (394 ± 9 Ma–159 ± 4) and a single Mesoproterozoic (959 ± 19 Ma) zircon. The absence of ancient detrital zircons suggests that the crust in the vicinity of Lake Chad is primarily juvenile. A significant (~17%) proportion of zircons yielded ages of ~580 Ma that are broadly coeval with nappe and wrench faulting, and granulite exhumation in the Central African and Trans-Saharan orogenic belts. Moreover, we identified a gabbro from the Guera Massif of south-central Chad that yielded an in situ zircon weighted-mean 238U/206Pb age of 580 ± 6 Ma. The age of the gabbro is similar to the Kekem gabbro-norite complex (576 ± 4 Ma) in west Cameroon, inherited zircons from silicic rocks near Lake Fitri and Lake Chad, and the detrital zircons from the Chari River. Our results unequivocally demonstrate that there was magmatism at ~580 Ma and that it was contemporaneous with deformation along the Tchollire-Banyo and Central Cameroon shear zones indicating there was a major regional tectonomagmatic episode at that time. We suggest the mafic magmatism was related to post-orogenic mantle upwelling that may have been responsible for or contributed to lithospheric delamination of the southern Saharan Metacraton.