Paleoproterozoic hydration of gabbronorite from the Mesoarchean Lechana layered complex, eastern Botswana

Abstract

Amphibole in mafic rocks of Archean layered complexes could be magmatic, metamorphic or metasomatic in origin. In this study, we use a combined field, petrography, mineral chemistry, phase equilibria modeling and 40Ar/39Ar geochronology approach to characterize amphibole in gabbronorites from the Mesoarchean Lechana layered complex in eastern Botswana. To get a better understanding of the overprint events, the characteristics of amphibole in the spatially associated amphibolite are also included. The gabbronorite preserves magmatic cumulate texture, with brownish amphibole replacing pyroxene as widespread thin (up to 80 μm) rims. In comparison, the greenish amphibole in amphibolite form part of granoblastic polygonal texture. Amphibolite preserve evidence for anatectic and metasomatic overprints, while the gabbronorite only exhibit effects of the later. Phase equilibria modeling provide P-T conditions and H2O content for the amphibole formation in the gabbronorite. The 40Ar/39Ar geochronology gave an age of 2037 ± 27 Ma for this process. We relate the Paleoproterozoic hydration to form amphibole in gabbronorite to the residual aqueous fluids expelled during crystallization of an anatectic melt in the spatially associated amphibolite. The results highlight an example of the formation of amphibole in Archean layered mafic rocks via a post-magmatic hydration event.