VOLCANIC EVOLUTION OF THE UPPER ONVERWACHT SUITE, BARBERTON MOUNTAIN LAND, SOUTH AFRICA

The volcanic stratigraphy of the Early Archean upper Onverwacht Suite of the Barberton Greenstone Belt has been investigated in 18 sections through parts of the Hooggenoeg-, Kromberg-, and Mendon Complexes. The ca. 2700 m thick volcanic sequence of the tectonostratigraphic lowest unit - the Hooggenoeg Complex (HC) – can be subdivided into 8 or 9 stratigraphic units representing distinct eruptive phases, each separated by chert layers. The thicknesses of the units vary from < 100 m to ~ 700 m, and can change from several hundred meters to nearly zero over distances of a few km along strike. The lavas are predominantly basalt, 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 of various sizes within intrusives, or are in tectonic contact with adjacent rocks. The basal part of the Mendon Complex (MC), tectonostratigraphically overlying the KC, contains basaltic komatities. The geochemical signatures of the basaltic lavas indicate generation from a subduction-influenced MORB source.

The lavas of the HC and KC are predominantly pillowed and massive units, whereas those of the MC are exclusively massive. In several sections the massive and pillow lavas are organized stratigraphically in a cyclic manner. Cyclic units vary in thickness from 3 m to 32 m, and consist of massive flows or lava flows with large pillows that are succeeded by progressively smaller pillows. They are inferred to have resulted from individual eruptions. The pillow lavas of the HC and KC contain < 3-5 % vesicles indicating eruption at depths of > ~ 2000 m. In general vesicular pillows occur in the lower part of both the HC and KC, and non-vesicular (and variolitic) at the top, suggesting increasing water depths, and hence a subsiding basin. The chert layers separating the eruptive phases (of the HC) include silicified tuffs derived from explosive, subaerial or shallow-marine, Plinian eruptions unrelated to the deep water lavas of the HC and KC. They may be products of island arc volcanic activity.