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Characterization, provenance, and tectonic setting of Fig Tree greywackes from the Archaean Barberton Greenstone Belt, South Africa

Abstract:

In this study, mineralogical and geochemical data, as well as Sr and Nd isotopic compositions of early Archaean greywackes from the Fig Tree Group are presented to provide further information about the evolution of the Barberton Greenstone Belt (BGB) of the Kaapvaal Craton, South Africa. The chemical data on the collected greywackes from the BGB suggest limited chemical modification of the source rocks. The Eu/Eu(*) anomaly is negative with an average value of 0.76 which is slightly higher than that of Eu-depleted crustal rocks (0.65). High Cr and Ni concentrations and a Cr/Ni ratio of about 1.6 indicate derivation from ultramafic sources without fractionation. Although rounded zircons occur, the combination of CIW/CIA (Chemical Index of Weathering/Chemical Index of Alteration) values and Eu/Eu(*) ratios suggests little transportation until final deposition, whereas the juvenile crustal materials comes from local sources. The siliciclastic material seems to be of mixed felsic and mafic-to-ultramafic origin. The tectonic setting of the source areas appears to reflect a continental island arc of Andean type, supporting the existence of an active plate-tectonic geodynamic system in early Archaean time. Sr isotopic data indicate an alteration during a post-depositional event, most probably at 2.6-2.7 Ga, which disturbed the Sr isotopic system. The Sm-Nd isotopic system must also have been altered since deposition of the rocks, as suggested by ε(Nd) values both lower and higher than expected. The calculated depleted-mantle model ages of 3.0 to 3.8 Ga are in the range of BGB sedimentary rocks. The higher model ages appear to reflect the underlying belt, which is represented by the Onverwacht Group and the surrounding intrusives, as being the major source of the greywackes.