Volcanic evolution of the upper Onverwacht Suite, Barberton Greenstone Belt, South Africa

Abstract

The volcanic stratigraphy of the upper Onverwacht Suite in the southeastern part of the Paleoarchean Barberton Greenstone Belt has been investigated in 18 sections through parts of the Hooggenoeg, Kromberg, and lower Mendon Complexes. The ca. 2700m thick volcanic sequence of the lowest tectonostratigraphic unit – the Hooggenoeg Complex (HC) – can be subdivided into 9 stratigraphic units representing major eruptive phases, each separated by silicified sedimentary and volcaniclastic rocks. The thicknesses of the units vary from <100m to ∼700m. Some units are wedge-shaped and die out over distances of a few kilometre along strike. The volcanic rocks are predominantly basaltic lavas, with minor basaltic komatiite and komatiite in the middle part of the HC. The basaltic lavas of the overlying tectonostratigraphic unit, the Kromberg Complex (KC), occur as screens within intrusives, or are in tectonic contact with adjacent rocks. The basal part (Ncakini section) of the tectonostratigraphically uppermost Mendon Complex (MC), consists of Mg-rich basalt lava.The lavas of the HC and KC are predominantly pillowed and massive flows, whereas those of the Ncakini section are exclusively massive. The massive and pillowed lava are commonly organized in cyclic units from 3m to 32m thick, and consisting of massive lava or large pillows that are succeeded by progressively smaller pillows. Cyclic units are inferred to have resulted from individual eruptions with decreasing rates of effusion. The pillow lavas of both the HC and KC contain <3–5% vesicles indicating eruption at greater depths than ∼2000m. In general vesicular pillows occur in the lower part of both the HC and KC, and non-vesicular (and variolitic) lavas are found at the top, suggesting increasing water depths, and hence eruption in a subsiding basin. The chert layers within the HC include silicified tuffs derived from explosive subaerial or shallow-marine eruptions distant from the deep-water lavas of the HC and KC.