Contrasting source components of clastic metasedimentary rocks in the lowermost formations of the Barberton greenstone belt

Abstract

Rare occurrences of clastic metasedimentary rocks within the lowermost stratigraphic units of the Barberton greenstone belt represent one of the few lines of evidence that argue for the existence of some component of evolved granitoid crust prior to formation of the lower greenstone belt sequence. This study has examined the field occurrence, petrography, geochemistry, and geochronology of these unique sedimentary rocks in order to place constraints on their provenance and tectonic setting. On the basis of their field occurrence and mineralogy, two types can be distinguished. The most common type is a medium-grained, planar-bedded, and compositionally banded rock displaying a broadly mafic geochemical affinity. It locally displays graded bedding and occurs as thin, discontinuous layers within the predominantly mafic and ultramafic sequences. The second type is a pink, medium- to coarse-grained metaarkose that locally contains well-preserved sedimentary structures such as trough cross-bedding and that occurs in close proximity to the previous rock type. Most of the planar-bedded rocks are depleted in incompatible elements such as Rb and Ba but contain high concentrations of CaO, Fe2O 3, and MgO and some of the transitional trace elements, including Cr, Ni, and Co. Samples of the cross-bedded meta-arkoses are enriched in K-feldspar and contain up to 4.3 wt% K2O. U-Pb dates of rounded detrital zircons from this unit range between ca. 3521 and 3540 Ma, indicating that at least two protoliths for these sediments predate the formation of the bulk of the greenstone belt. Aminimum age of 3431 ± 11 Ma for the formation of the sediments is indicated by the age of a trondhjemitic gneiss that is intrusive into one of the greenstone remnants investigated. Combined major and trace element geochemistry indicates at least two different sources for the sediments, including reworked continental crust as well as mafic to ultramafic rock types. The major and trace element geochemistry of the sediments is highly variable, and this variability is strata-bound. This is interpreted to reflect the variety of rock compositions in the sources as well as different degrees of chemical weathering in the source regions. Consequently, the data are best explained by a model involving contemporaneous deposition of relatively mature, continentally derived sediments and sediments derived from volcanic/ volcaniclastic rocks from syn-depositional volcanic centers in an active tectonic environment such as an island arc.