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 <10 m deep and were preferentially preserved in distal settings on komatiitic platforms. Lithofacies 2 contains both coarse, blocky microphyric and porphyritic grains indicative of low-energy hydroclastic fragmentation and fluidal, ragged clasts formed during vigorous subaqueous explosions. These were deposited mainly in proximal to medial settings prior to vent emergence. In the northern part of the belt, 2–60-m-thick sections of serpentinized, fine-grained komatiitic tuff were deposited by pyroclastic fallout into subaqueous environments and were widely reworked by moderate- to low-energy currents. Komatiites throughout the Onverwacht Group were produced in multivent volcanic fields in marine settings that were starved of clastic input, making explosive komatiitic volcanism one of the main sediment-generating processes.

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