Constraints on Paleoproterozoic crustal growth from Birimian Supergroup lavas of the Bui belt (Ghana) in the West African Craton

Abstract

The volcano-metasedimentary Bui belt in Ghana is one of the lesser known Birimian belts of the West African Craton that provide constraints on Paleoproterozoic crustal growth. This study presents the first major, trace element and Nd-Hf isotope data for the mafic suite of the Bui belt to constrain their petrogenesis and geodynamic setting. Although alteration is prevalent, the volcanic, volcaniclastic and plutonic rocks show tholeiitic signatures, with Mg# 35–65. Chondrite-normalised REE patterns are mainly flat, with slight LREE enrichment or depletion, and multi-element diagrams show Th and LREE depletion relative to other immobile incompatible trace elements. These features suggest derivation from a moderately depleted mantle source. Whole rock Sm-Nd isotope data yields an errorchron at ca. 2.2 Ga, with an initial εNd of +3.4 but the Lu-Hf isotopic system appears to be disturbed. The lack of arc related geochemical signatures (absence of negative Nb-Ta and Ti peaks, a nearly flat to depleted REE pattern) in the Bui mafic suite, together with the volcanic-sedimentary rock association, point towards magma generation in an oceanic plateau or divergent setting similar to a mid-ocean ridge, contrasting with the subduction setting that has been proposed for most other instances of Birimian magmatism in the West African Craton.