Weathering and sediment generation in the Archean: An integrated study of the evolution of siliciclastic sedimentary rocks of the 3.2 Ga Moodies Group, Barberton Greenstone Belt, South Africa

Abstract

Alluvial and braided fluvial deposits of the 3.2 Ga Moodies Group in the Barberton greenstone belt, South Africa, are analyzed petrographically and geochemically to identify their source rocks (provenance) and assess the degree and type(s) of weathering required to produce the observed Moodies sediment compositions. Conglomerate, sandstone, and shale data give somewhat differing pictures of provenance and weathering due to derivation from different components within the source terrane and size and compositional fractionation during transport. The results suggest that shale geochemistry provides the most accurate estimate provenance, although both rare-earth element and trace element (Th, Sc, Zr, Cr, and Ti) data must be used in combination to give the best results. The source area for Moodies Group sediments was dominated by tonalite, felsic volcanic rock, komatiite-basalt, and granite. Based on mineralogical and major-element divergence from estimated source area composition, the Moodies Group sediments are remnants of an aggressive weathering environment. Labile materials, such as komatiite, basalt, and coarse plagioclase grains, decomposed almost entirely to clays and solutes, and the chemical index of alteration for Moodies shale is well above the global average. An aggressive weathering environment in the Archean may have been achieved by increased rainfall, higher temperatures, and/or higher atmospheric P C O 2 . More likely, a combination of these conditions worked to offset the inhibitory weathering effects of a plant-free environment.