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

In this study, mineralogical and geochemical data, as well as Sr and Nd iso topic compositions of early Archaean greywackes from the Fig Tree Group are presented to provide further information about the evolution of the Barber ton Greenstone Belt (BGB) of the Kaapvaal Craton, South Africa. The chemica l data on the collected greywackes from the BGB suggest limited chemical mo dification of the source rocks. The Eu/Eu* anomaly is negative with an aver age 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. Althoug h rounded zircons occur, the combination of CIW/CIA (Chemical Index of Weat hering/Chemical Index of Alteration) values and Eu/Eu* ratios suggests litt le transportation until final deposition, whereas the juvenile crustal mate rial comes from local sources. The siliciclastic material seems to be of mi xed felsic and mafic-to-ultramafic origin. The tectonic setting of the sour ce areas appears to reflect a continental island are of Andean type, suppor ting the existence of an active plate-tectonic geodynamic system in early A rchaean time. Sr isotopic data indicate an alteration during a post-deposit ional event, most probably at 2.6-2.7 Ga, which disturbed the Sr isotopic s ystem. The Sm-Nd isotopic system must also have been altered since depositi on of the rocks, as suggested by epsilon(Nd) values both lower and higher t han expected. The calculated depleted-mantle model ages of 3.0 to 3.8 Ga ar e in the range of BGB sedimentary rocks. The higher model ages appear to re flect the underlying belt, which is represented by the Onverwacht Group and the surrounding intrusives, as being the major source of the greywackes. ( C) 1999 Elsevier Science B.V. All rights reserved.