Petrology and Geochemistry of Early Precambrian Graywackes from the Fig Tree Group, South Africa

Abstract

Graywackes of the Fig Tree Group in South Africa were derived from a diverse source area composed principally of chert, volcanics, granitic-metamorphic rocks, and some ultramafics. Chert and quarts pseudomorphs after shard-like fragments and well-formed plagioclase and K-feldspar crystals in some graywackes indicate that some detritus was derived from contemporary volcanism. Immature textures indicate mild source-area weathering and rapid erosion, deposition, and burial. An unusually large amount of dolomite in the graywackes was probably deposited by carbonate-rich pore fluids during diagenesis. A relative Sr depletion (compared to K, Rb, Ca, and Ba) in graywackes of the lower Fig Tree Group (Sheba Formation) is interpreted in terms of an abundance of Sr-depleted igneous rocks in their source area. Large amounts of Ni in all Fig Tree graywackes and shales appear to have been derived from ultramafic source rocks. A progressive stratigraphic increase in granitic components in the graywackes suggests progressive unroofing of a granitic metamorphic terrane which was initially covered by a thick sequence of Onverwacht or Onverwacht-like volcanic rocks. This source appears to have been located in central Swaziland. The relatively abundant granitic detritus in the graywackes indirectly records one or more pre-Fig Tree (3.4-4.0 b.y.) granite-forming events in southern Africa.