EVOLUTION OF THE ONVERWACHT SUITE OF THE PALEOARCHEAN BARBERTON GREENSTONE BELT (SOUTH AFRICA) AS OCEANIC CRUST AND ISLAND ARCS

The Paleoarchean Onverwacht Suite of the Barberton Greenstone Belt (BGB) comprises seven separate complexes of volcanic and intrusive rocks ranging from komatiite to dacite, with subalkaline basalt as the predominant type. The complexes have tectonic contacts, are tectonically stacked with a total tectonostratigraphic thickness of ~15 km and have an age range of ~120 million years [1]. From tectonically lowest to highest they are: Sandspruit- (lowest), Theespruit-, Komati-, Hooggenoeg-, Noisy-, Kromberg- and Mendon (highest) Complexes. Some of the complexes have ophiolite-like lithological associations, whereas others are more akin to island-arc assemblages [1]. Detailed logging of the volcanic stratigraphy of two of the complexes (Hooggenoeg and Kromberg) shows a dominance of non- to slightly-vesicular pillows, indicating eruption in deep (2000-4000m) water [2]. A major unconformity separates two of the complexes, demonstrating rapid uplift (~ 2-4 mm/year) of the deep-water lavas (of the Hooggenoeg Complex) to above sea level, and erosion and deposition of shallow water/fluvial/glacial (?) deposits (of the Noisy Complex).

Compilation of available data for the mafic extrusive and extrusive rocks from the tectonostratigraphically lowest to highest complexes of the BGB shows a large compositional spread from komatiites to dacites. The various complexes, however, show their own special geochemical characteristics, but their trace element patterns invariably indicate a subduction fingerprint. The combined geological and geochemical relationships indicate that the magmatic rocks of the BGB formed as oceanic crustal units, associated with subduction and islands arc formation.