The Petrogenesis of Volcaniclastic Komatiites in the Barberton Greenstone Belt, South Africa: a Textural and Geochemical Study

Abstract

The Onverwacht Group of the 3·5^3·2 Ga Barberton greenstone belt, South Africa contains multiple stratigraphic units that include laterally extensive beds of komatiitic ash, accretionary lapilli, and lapilli. These units have been affected by pervasive silicification, serpentinization, or, less commonly, carbonate metasomatism. Silicification resulted in SiO2þK2O485 wt % and depletion of most other major and trace elements. Most of these tuffs have prominent high Hf/Hf* and Zr/Zr* (0·5^12), which cannot result from normal magmatic processes but are due to the typically immobile rare earth elements migrating during post-silicification fluid^rock interaction. Similarly, their low Ce/Ce* values do not reflect Archean surface redox conditions but the circulation of later oxidizing fluids. Despite this intense alteration, ratios of Al2O3 and TiO2 remain uniform and coherent within single volcanic units. These ratios indicate that most silicified tuffs are not petrogenetically related to the underlying or overlying komatiitic flow rocks and that each originated from either separate mantle sources or different partial melting conditions. Serpentinized tuffs retain komatiitic element abundances but Al2O3 fails to define a tight linear array with the demonstrably immobile elements Ti and Zr.We speculate that this is due to post-depositional mixing of Al-depleted and Al-undepleted tuff layers by aqueous currents. Excellent textural preservation of the silicified tuffs shows they are characterized by a dearth of phenocrysts, low particle vesicularity and abundance of fine vitric ash, suggesting the eruption and rapid quenching of superheated or near-liquidus anhydrous magmas. Minor assimilation of hydrated basaltic or ultramafic crust within the dry magma may have enhanced the surface phreatomagmatic explosivity while still allowing the magma to rise close to an adiabatic ascent path. However, textural and geochemical evidence for such a process is scarce. Temporal and compositional constraints show that the diversity in the types of komatiites throughout the Onverwacht Group can be accounted for by variations in plume^mantle dynamics and that komatiitic tuffs were deposited during intervals of volcanism characterized by low effusive eruptive volumes and/or low emplacement rates.