Geochemistry of shales from the Archean (~3.0 Ga) Buhwa Greenstone Belt, Zimbabwe: Implications for provenance and source-area weathering

Abstract

Phyllites from the Archean (~3.0 Ga) Buhwa Greenstone Belt, Zimbabwe, were deposited on a stable cratonic platform. Analyses of the phyllites generally define a single geochemical group based on major-and trace-element abundances. The phyllites are strongly depleted in CaO, Na 2O, and Sr with respect to average Archean upper crust. By contrast, K 2O, Ba, and Rb are enriched several times relative to average Archean upper crust, reflecting basin-scale K metasomatism. Transition metals are somewhat depleted, whereas high field strength elements are typically enriched relative to average Archean upper crust. Samples generally have fractionated LREE patterns (average Ce N/Sm N = 2.4) with small negative Eu anomalies (average Eu/Eu * = 0.78) and generally flat HREEs (average Gd N/ Yb N = 1.3). Such geochemical characteristics suggest that the source dominantly consisted of tonalite with less common occurrences mafic volcanic rocks and granite. Mixing calculations, which massbalance the REEs and Th/Sc, suggest 70% tonalite, 15% mafic volcanic rocks, and 15% granite as possible proportions for the source of the phyllites. A nearby early Archean (~3.5 Ga) continental nucleus preserves similar lithologies and is the probable source. Intense chemical weathering of the source terrane is indicated by premetasomatized chemical index of alteration values of 95–100, nearly complete depletion of CaO and Na 2O, and high Al 2O 3/Na 2O ratios. Fine-grained sediments of comparable age and presumed tectonic setting elsewhere in southern Africa show similar geochemical characteristics, implying that source-area compositions and weathering intensities were similar. These mature platformal deposits suggest a tectonically stable environment where intense chemical weathering took place at ~3.0 Ga across southern Africa.