Geochemistry and petrogenesis of Archean mafic rocks from the Amsaga area, West African craton, Mauritania

Abstract

Determining the extent of mantle mixing during the Hadean-Archean is an integral factor in our understanding of early geodynamic processes as well as the speed and degree of mantle homogenisation. If a dynamic convecting mantle was the predominant regime it would be expected that early formed isotopic anomalies would be quickly homogenised, whilst in a stagnant lid regime these anomalies might last for hundreds of millions of years. Archean terranes are dominated by felsic tonalite, trondhjemite and granodiorite rocks, as such these rocks have been the main rock type studied, even though they are not formed by primary mantle melts. Therefore, in this study, we have focused on mafic rock samples as tracers of primary mantle melting, from the relatively unknown West African craton. The mafic rocks have undergone metamorphism to granulite facies with the majority of samples retrogressed to amphibolite facies. Major element compositions reveal that the protolith was tholeiitic basalt. The majority of trace elements display limited mobility, as primary correlations with Zr are still present. Trace element patterns are typical for Archean mafic rocks, with Nb-Ta and Ti depletions. The meta-basalts yield a 147Sm-143Nd errorchron with an age of 3300 ± 300 Ma, with initial εNd of +4.4. The Nd compositions indicate that the meta-basalts have experienced minimal crustal assimilation and retain primary geochemical attributes. A positive initial εNd for the meta-basalts suggests that the magmatic protolith evolved from an already differentiated source. Nevertheless, 142Nd compositions are within error of modern day terrestrial, as such, the protolith for these samples either did not experience any early differentiation, or that mantle convection had homogenised any anomalous signal. These results highlight the lack of large (>±8 µ142Nd) 142Nd anomalies detected from outside the North Atlantic and Superior Cratons, suggesting that Archean mantle dynamics was complex, with different processes occurring on different parts of Earth, similar to today.