GEOCHEMISTRY OF SHALES FROM THE ARCHEAN (SIMILAR-TO-3.0 GA) BUHWA GREENSTONE-BELT, ZIMBABWE - IMPLICATIONS FOR PROVENANCE AND SOURCE-AREA WEATHERING

Phyllites from the Archean (similar to 3.0 Ga) Buhwa Greenstone Belt, Zimbabwe, were deposited on a stable cratonic platform. Analyses of th e phyllites generally define a single geochemical group based on major - and trace-element abundances. The phyllites are strongly depleted in CaO, Na2O, and Sr with respect to average Archean upper crust. By con trast, K2O, 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 held strength elements are typically enriched relative to average Archean upper crust. Samples g enerally have fractionated LREE patterns (average Ce-N/Sm-N = 2.4) wit h small negative Eu anomalies (average Eu/Eu = 0.78) and generally fl at HREEs (average Gd-N/Yb-N = 1.3). Such geochemical characteristics s uggest that the source dominantly consisted of tonalite with less comm on occurrences mafic volcanic rocks and granite. Mixing calculations, which mass-balance the REEs and Th/Sc, suggest 70% tonalite, 15% mafic volcanic rocks, and 15% granite as possible proportions for the sourc e of the phyllites. A nearby early Archean (similar to 3.5 Ga) contine ntal nucleus preserves similar lithologies and is the probable source. Intense chemical weathering of the source terrane is indicated by pre metasomatized chemical index of alteration values of 95-100, nearly co mplete depletion of CaO and Na2O, and high Al2O3/Na2O ratios. Fine-gra ined sediments of comparable age and presumed tectonic setting elsewhe re in southern Africa show similar geochemical characteristics, implyi ng that source-area compositions and weathering intensities were simil ar. These mature platformal deposits suggest a tectonically stable env ironment where intense chemical weathering took place at similar to 3. 0 Ga across southern Africa.