Abstract
The Awsard-Tichla zone, in the Moroccan sector of the Reguibat Rise, comprises three lithodemic units, the TTG Aghaylas Suite, the Tichla greenstone belt, and the intrusive feldspathoidal syenites of the Awsard-Lechuaf group. The Aghaylas suite consists of Mesoarchean tonalites and trondhjemites with minor granodiorites and granites that are locally migmatised. These rocks are notably depleted in HREE, Y, Nb, Ta, and U, with elevated Th/U and K/Rb, 87Sr/86Sr(t) from 0.7003 to 0.7030, ɛNd(t) from +2 to +5, and Nd model ages in the range TCR=3.03–2.92Ga. Eight TTG gneisses yielded SHRIMP U–Pb zircon crystallisation ages between 3.04Ga and 2.92Ga. One TTG leucocratic gneiss from the neighbouring Oulad Dlim sector has a TCR=3.09 and a zircon crystallisation age of 2.94Ga but also contains a 3.11Ga population of pre-magmatic zircons. The Tichla greenstone belt, on the other hand, was formed between 3.03Ga and 3.01Ga, though Nd model ages suggest the contribution of an older crust. Field evidence and the chemical and isotopic compositions of the TTGs are consistent with crustal recycling of slightly younger juvenile TTG. After 2.92Ga there is no trace of magmatic activity until the intrusion of the Awsard feldspathoidal (kalsilite and nepheline) syenites at 2.46Ga. This reveals that, for reasons that are not yet well understood, the Archean crust in this sector of the Reguibat Rise stabilised long before than any other surrounding cratonic area. Furthermore, the crust stayed stable until the Archean-Proterozoic transition that, here, was marked by a peculiar high-K silica-subsaturated magmatism more extreme in composition than in any other known Archean terrane.
Published Version
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