Stratigraphic Interpretation of Age Measurements in Southern Africa

Abstract

A review of 60 isotopic age measurements on southern African rocks indicates that many of these data have a bearing on important Precambrian stratigraphic problems. In 1948 Arthur Holmes first used age measurements in deciphering the structural history of this region. His study emphasized dated pegmatite bodies in metamorphic terrains as indicating ancient orogenic belts of different ages. Recent work has focused on dating the relatively flat-lying and unmetamorphosed Precambrian systems, and a revised stratigraphic time scale for the Transvaal Province is proposed here. Certain relatively flat-lying systems in southern Africa have ages greater than 2000 millio n years, exceeding the ages determined in the high-grade metamorphic terrains. These old systems include the Ventersdorp, Witwatersrand, and Dominion Reef. The Transvaal System may be ˜2000 m.y. old. The “ancient orogenic belt” theory implies near-contemporaneous stablep l atformdeposition and possible formation of post-orogenic arkose troughs. Reliable times of deposition for the adjacent undeformed systems can thus supply critical evidence concerning this theory. The region paralleling the Orange River in northernCape Province provides an example: there are many age measurements on pegmatites and metamorphic rock of ˜950m.y., and Holmes postulated the existence of a 1000 m.y.-old orogenic belt; this implies the existence of ˜1000 m.y.-old stratigraphic systems, both as a geosynclinal sequence and as the “stable-platform” equivalent. However, the accepted geologic column for this region does not reveal stratigraphic systems that can be clearly linked to the history of the hypothetical ˜ 1000 m.y.-old orogen. Either the accepted geologic column is in error and part of the Kheis System is much younger than hitherto supposed, or a widespread regional metamorphism and pegmatization affected the Kheis rocks some 2000 m.y. after their deposition. There are many determinations close to 500 m.y. in a belt extending through South-West Africa, the Central African Federation, and Mozambique. Widespread metamorphism, with emplacement of granitic rocks, took place in parts of southern Africa at the close of the Precambrian and during the Cambrian Period. Arcuate synclinal belts of low-grade metavolcanic rock w ith an age > 2500 m.y. are characterized by the “Steinmann” geosynclinal association of rock types; rocks and related mineral deposits of this association were evidently not formed i n the subsequent history of southern Africa. Younger Precambrian metamorphic terrains have a different tectonic style, showing frequent alternation of metasedimentary units with massive and gneissose granites, and occasional highgrade regional metamorphism. The characteristic differences i n structure may be related to thermal history o f the crust. Many of the age measurements impose restraints on stratigraphic correlations between the various isolated Precambrian areas.