SHRIMP dating and Nd isotope geology of the Archean terranes of the Uweinat-Kamil inlier, Egypt–Sudan–Libya

The Precambrian terranes of north east Africa are still poorly known. It is generally believed that the oldest rocks in the region are the charnockitic gneisses of the Uweinat massif, located at the triple junction between Libya–Egypt–Sudan, because they have yielded Rb–Sr ages around 2.6 Ga and Nd TDM model ages around 3.0–3.2 Ga. Here we confirm that these rocks are indeed Archean with SHRIMP U–Pb zircon ages as old as 3.0 Ga, but we also report older rocks to the east, in the neighboring Gebel Kamil region. This area contains a metaigneous complex formed of tonalite–trondhjemite–granite (TTG) and gabbro-diorite (GbD) gneisses with a whole-rock Sm–Nd isochron age of 3.16 ± 0.16 Ga, average Nd TCR of 3.17 ± 0.04 Ga, and average ɛNd(3.2 Ga) of 3.4 ± 0.3. The oldest TTG gneisses, which are also the oldest rocks found to date in north east Africa, contain large magmatic zircons with SHRIMP U–Pb crystallization ages peaking at 3.22 ± 0.04 Ga, closely matching the Sm–Nd and the Nd TCR model ages. These ages are interpreted to represent arc-magmas produced between 3.1 Ga and 3.3 Ga. Other TTGs have younger Archean zircon ages that form a continuum between 3.1 Ga and 2.55 Ga, with three peaks at around 2.97 Ga, 2.85 Ga, and 2.6–2.7 Ga; these rocks were apparently generated from the older TTGs during repeated events of crustal recycling. The crust was stable from 2.55 Ga to 2.0 Ga, when an intense thermal event generated rims of variable thickness over the Archean zircons. Neither the reworking from 3.1 Ga to 2.55 Ga, nor the metamorphism at 2.0 Ga involved the addition of juvenile material to the crust of this area, which behaved as an almost closed system from 3.1 Ga to 0.75 Ga, when the intrusion of Pan-African I-type granitoids with a juvenile component began.

► This zone is one of the less known in Earth because its remoteness and safety risks. ► The oldest crustal rocks were generated between 3.3 and 3.1 Ga, and form a TTG complex. ► These are the oldest rocks found so far in NE Africa. ► The crust was intensely reworked from 3.1 to 2.55 Ga, and suffered an intense thermal event at 2.0 Ga, during the Eburnian orogeny. ► The Mesoarchean mantle of this region was already depleted in LREE before 3.3 Ga, perhaps due to an earlier event of crust formation.