Geochemistry and Provenance of Sediments from the Pongola Supergroup, South Africa: Evidence for a 3.0-Ga-Old Continental Craton
The ~3.0 Ga Pongola Supergroup of South Africa is one of the oldest well-preserved supracrustal successions. The sequence is divided into the Nsuze (chiefly volcanics, quartzites, and pelites) and the overlying Mozaan Groups (chiefly quartzites and pelites). Relative to North American Shale Composite (NASC), most Pongola pelites have similar La/Yb and Eu/Eu* ratios and Large Ion Lithophile Elements (LILE), High Field Strength Elements (HFSE), REE, V and Sc contents. Ni (30-300 ppm) and Cr (100-800 ppm) contents are greater than those of NASC, while Cr/Ni ratios are generally higher than shales and pelites of all ages. These trends may reflect residual Cr enrichment during intense chemical weathering of komatiitic source rocks. Local populations of Nsuze pelites, based on major and trace element data, may result from source heterogeneities and/or contemporaneous volcanic input into the sedimentary basin. Pongola quartzite compositions and detrital modes are similar to those of sandstones from Phanerozoic passive-margin and basement uplift tectonic settings. REE, LILE and HFSE contents in pelites suggest that Pongola source rocks were remarkably similar in composition to average Phanerozoic upper-continental crust. Trace element, geographic and isotopic constraints favor 3.2 to 3.0 Ga hood granites as a major source for Pongola detritus. Moderate contributions from mafic sources are also required, whereas relatively minor contributions from tonalite and komatiite are allowed. These results indicate the existence of a chemically evolved (felsic) craton in southern Africa by 3.0 Ga. This craton appears to have been more evolved than sources for either older Moodies (~3.3 Ga) or most of the younger Witwatersrand sediments (2.8-2.7 Ga). Pelites from the Kaapvaal craton record at least two mafic to felsic source-area compositional cycles in pelites deposited from the mid to late Archean. Additional cycles may be masked by a depositional or erosional hiatus between the Moodies and Pongola sequences. These sedimentary patterns suggest that crustal development in southern Africa was episodic in nature during the early Precambrian.
D. J. Wronkiewicz and K. C. Condie, "Geochemistry and Provenance of Sediments from the Pongola Supergroup, South Africa: Evidence for a 3.0-Ga-Old Continental Craton," Geochimica et Cosmochimica Acta, vol. 53, no. 7, pp. 1537-1549, Elsevier Limited, Jul 1989.
The definitive version is available at http://dx.doi.org/10.1016/0016-7037(89)90236-6
Geosciences and Geological and Petroleum Engineering
Keywords and Phrases
Felsic Craton; Geochemistry; Pelite; Pongola Supergroup; Provenance; South Africa
International Standard Serial Number (ISSN)
Article - Journal
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