Geochemistry of mafic-ultramafic rocks of the 3.33 Ga Kromberg type-section, Barberton greenstone belt, South Africa: Implications for early Earth geodynamic processes

Abstract

In conjunction with major, trace and rare earth element data, this study presents the first whole-rock LuHf isotope analyses of mafic-ultramafic rocks of the c. 3.3 Ga Kromberg type-section in the Barberton greenstone belt of South Africa. Three compositionally distinct volcanic rock types are identified namely Group 1 metabasalts, Group 2 metabasalts and komatiitic metabasalts. Group 1 metabasalts are FeMg tholeiitic with compositions that plot close to the Primitive Mantle composition, have isochron intercept εHf(t) value of 0.01 ± 0.4 and Eoarchean Hf model ages. Group 2 metabasalts have very high Al2O3/TiO2 ratios (c. 50), small negative Nb anomalies, LREE-depleted slightly concave up REE patterns, and negative εHf(t) values. The komatiitic basalts variably record small negative Nb anomalies in a primitive mantle normalized plot. Group 1 metabasalts and cumulate peridotites (wehrlites) yield a LuHf isochron age of 3.340 ± 0.190 Ga (95% confidence level). The highly variable major and trace element geochemistry of the rocks in the sequence implies highly dynamic and variable mantle melting environments. The Group 2 metabasalts have geochemical characteristics similar to that of boninites and are interpreted to have formed from a shallow, refractory mantle source. Group 2 metabasalts are argued to have formed in a proto-plate tectonic back-arc basin involving crustal contamination, whereas Group 1 metabasalts likely formed from within a deep mantle plume.