Petrogenesis and crustal evolution of the Tasiast TTG suite (SW Reguibat Shield, Mauritania). Implication for crustal growth in the West African craton

Abstract

The Mauritanian Tasiast unit in the Southwestern Reguibat Archean Shield (North of the West African craton $=$ WAC) consists mainly of gneiss dated between 3.07 and 2.91 Ga. We present new field and petrographic observations combined with whole-rock geochemical data on gneisses of Tasiast to understand their petrogenesis, tectonic setting, and the evolution of the continental crust of WAC. These data provide firm evidence of two distinct orthogneisses: trondhjemite and high-K granite affinities. Geochemical characters suggest that (1) trondhjemites magma source were originally derived from polybaric partial melting at the thickened crust occurred over a range of $P$–$T$ conditions, covering the stability fields of garnet amphibolite and rutile eclogite. The trondhjemite composition is attributed to mixing of two major melts: one originating from eclogitic facies source region (high Nb/Ta) and the other from a garnet-amphibolite facies (low Nb/Ta) leaving garnet, amphibole, and rutile in the residue. (2) Compared to trondhjemite, the granites with high K2O and Rb contents, and low Na2O, Al2O3 and Sr contents suggest that basaltic oceanic crust was not their source material. Moreover, field relationships suggest that the granites were derived from partial melting of trondhjemites. Hence, the trondhjemite and high-K granites marked together two distinct stages during the growth of the continental crust in the Tasiast area.

Keywords

• Trondhjemite
• Granite
• Amphibolite
• Eclogite
• Tasiast
• Mauritania
• West African craton