Fragmentation and dispersal of komatiitic pyroclasts in the 3.5-3.2 Ga Onverwacht Group, Barberton greenstone belt, South Africa

Abstract

The 3.5–3.2 Ga Onverwacht Group of the Barberton greenstone belt, South Africa, contains thick sequences of komatiitic tuff, lapilli tuff, and lapillistone representing proximal to distal volcanic settings. Volcaniclastic komatiites in the southern part of the belt include: (1) areally extensive sheets of silicified, massive to normally graded ash and accretionary lapilli interpreted as pyroclastic fall deposits, and (2) thick, widespread, carbonatized beds of lapilli and minor ash deposited by subaqueous sediment flow and pyroclastic fallout. Lithofacies 1 clasts are aphyric and poorly to nonvesicular, implying rapid quench rates and fragmentation primarily through magma-water contact. Low water/magma ratios, initial melt fracturing through minor volatile exsolution, and high magma fluxes were probably the main factors that enabled the production of buoyant eruption columns and widespread ash distribution. These tuffs were generated during vent shoaling in water that was probably