ALBERT

Description: Two domains have previously been recognized in the Archaean Reguibat shield of NW Mauritania, based primarily on their gross lithological differences. New fieldwork has identified a major ductile shear zone (Tacarat-Inemmaudene Shear Zone) separating 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 lithologies 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 contribution 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 orthogneisses that are the oldest lithodemic unit of the Tasiast-Tijirit Terrane. Widespread bimodal, post-tectonic magmatism in both terranes included major granitic magmatism dated at c. 2730 Ma. The north-south- to NNE-SSW-trending curvilinear Tacarat-Inemmaudene 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{+/-}111 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.