Two Mesoarchaean terranes in the Reguibat shield of NW Mauritania

Two domains have previously been recognized in the Archaean Reguibat Shield of NW Mauritania, based primarily on their gross lithological differences (Rocci et al., 1991). New fieldwork has identified a major ductile shear zone (Tâçarât -Inemmaûdene Shear Zone) se-parating these domains and new geochronological studies show that the two domains record different Mesoarchaean histories. As such, the two domains are redefined as the Choum-Rag el Abiod Terrane and Tasiast-Tijirit Terrane. Previous isotopic studies of metamorphic litholo-gies of the eastern Choum-Rag el Abiod Terrane indicate a succession of crustal growth from about 3.5-3.45 Ga to between about 3.2 and 2.99 Ga. Isotopic data presented in this contribu-tion from the Tasiast-Tijirit Terrane indicate that emplacement of major calc-alkaline plutons occurred at c.2.93 Ga after volcanism (preserved as greenstone belts) that included late felsic eruptive centres dated at c.2965 Ma. This Mesoarchaean intrusive and extrusive magmatism was confined to the Tasiast-Tijirit Terrane where it was emplaced through migmatitic or-thogneisses that are the oldest lithodemic unit of the Tasiast-Tijirit Terrane. Widespread bi-modal, post-tectonic magmatism in both terranes included major granitic magmatism dated at c.2730 Ma. The N- to NNE-trending curvilinear Tâçarât -Inemmaûdene Shear Zone that separates the two terranes records late intense transpressive ductile shearing. It has a flower structure over a horizontal distance of about 70 km across its southern portion with unquantifiable sinistral horizontal offset, and east-directed thrusting on its eastern side where it cuts into the Choum-Rag el Abiod Terrane. A new U-Pb zircon age of 2954±11 Ma is presented for a deformed granite confined within the central part of this shear zone. A minimum age for the shearing is provided by a previously determined c. 2.73 Ga age for a post-tectonic granite that cuts across the easternmost part of the shear zone in the Choum-Rag el Abiod Terrane.