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Geochemistry and origin of the Archaean Beit Bridge complex, Limpopo Belt, South Africa

Department of Geoscience, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA

The Beit Bridge complex (BBC) in the Central Zone of the Limpopo Belt in Southern Africa includes metasedimentary rocks of Archaean age (chiefly metapelites, quartzites and carbonates), amphibolites (chiefly mafic dykes and sills) and granitoids which comprise 50% of the complex. The amphibolites are tholeiites with relatively unfractionated REE patterns and pronounced subduction-zone geochemical signatures in terms of relatively immobile incompatible elements. Tonalites and granites, which also exhibit such a signature, can be formed by batch melting of a mafic source with residual garnet or amphibole. Most granites have negative Eu anomalies and appear to represent residual liquids.

BBC metapelites have negative Eu anomalies and other immobile element distributions similar to Phanerozoic shales. One group of metapelites that shows significant depletion in Ca, Na and Sr appears to have been derived from intensely weathered sources. The negative Eu anomalies in BBC metapelites suggest intracrustal melting of their sources and thus the presence of evolved continental crust in this part of Southern Africa by about 3000 Ma. BBC quartzites are quartz-rich sandstones that are depleted in HFS elements, Ba and Sr, relative to Phanerozoic counterparts; a feature that probably reflects less recycling during the early Archaean. BBC banded iron formation (BIF) is similar to other Archaean BIF and shows large positive Eu anomalies. BBC carbonates are dolostones depleted in REE, HFS elements, U, Th, Ba, Sr, Ni, Sc, V and Y and enriched in Fe, Mn, Na, Rb and Cs relative to Phanerozoic marine carbonates.

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