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

The Paleoarchean Onverwacht Suite (OS) of the Barberton Greenstone Belt (BGB) comprises tectonically stacked complexes of predominantly volcanic and intrusive rocks. From tectonically lowest to highest the OS is defined as: Sandspruit-, Theespruit-, Komati-, Hooggenoeg-, Noisy-, Kromberg- and Mendon Complexes. The total tectonostratigraphic thickness of the OS is ~15 km, and the rocks, ranging from komatiite to dacite, with subalkaline basalt as the predominant type, have an age range of ~120 million years between ~3.48 and 3.36 Ga [1]. 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 deep water pillow lavas [2]. 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, with subalkaline basalts as the predominant type. 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, which had amalgamated by 3.23 Ga.

[1] de Wit et al. Precamb. Res. (2011); [2] Furnes et al. Precamb. Res. (2011)