ReOs and UPb geochronological constraints on the eclogite–tonalite connection in the Archean Man Shield, West Africa

Abstract

Mantle-derived eclogite xenoliths and tonalite–trondhjemite–granodiorites (TTG) that occur in the Man Shield, West Africa, sample different levels of Archean lithosphere. Chemical and oxygen isotope systematics indicate that low MgO eclogites from the Koidu kimberlite are ancient remnants of subducted oceanic crust that may have been involved in regional Archean crust formation. ReOs whole rock isotopic data for these eclogites scatter about a line with slope corresponding to an Archean age of 3.44±0.76 Ga (2 σ), with ReOs model ages ranging from 1.4 to 3.9 Ga. This wide range of model ages overlaps with the age range for crust formation and metamorphism in the Man Shield. In situ UPb ages of zircons from crustal rocks have been measured by laser ablation ICP-MS. A tonalitic gneiss has discordant zircons with rare old cores (∼3.6 Ga) and an upper concordia intercept at 2890±9 Ma (2 σ). Zircons from a mafic lower crustal granulite xenolith are concordant at 2686±32 Ma. Our results, together with previously published ages for Man Shield rocks, indicate an early Archean crust formation event followed by major crustal growth at 2.9–3.0 Ga and a last major metamorphic event at 2.7 Ga. These data show that the eclogites and the continental crust of the West African Craton overlap in time of formation (but only at the very broad age uncertainty provided by the eclogite ReOs results). They are permissive of Archean crustal growth by melting of the protoliths of the materials now sampled as the Koidu eclogite xenoliths. If so, this suggests that Archean crustal growth in the Man Shield occurred in a convergent margin setting.