THE CR/TH RATIO IN PRECAMBRIAN PELITES FROM THE KAAPVAAL CRATON AS AN INDEX OF CRATON EVOLUTION

The Cr/Th ratio in Precambrian pelites from the Kaapvaal Craton in Southern Africa correlates with pelite Th/Sc and Co/Th ratios and appears to monitor changes in source composition. It is a more sensitive indicator of provenance than other commonly used ratios including Eu/Eu*, La/Yb, and Th/U. Pelite Cr/Th ratios are controlled most strongly by the effects of local provenance and local tectonics. Secular decreases in this ratio, the most prominent of which occurred at 2.4-2.3 Ga in pelites of the Kaapvaal Craton, record rapid uplift exposing granite. Decreases in the ratio reflect increases in the abundance of komatiite-basalt in sediment sources in response to arc collisions, rift/hotspot volcanism, or intracratonic uplift of older greenstones. Relatively constant Cr/Th ratios appear to require widespread uplift or/and efficient sediment mixing during tectonic stability. Changes in Cr/Th ratio of Kaapvaal pelites reflect a complex tectonic history of the craton from about 3.3 Ga to < 1.8 Ga, and do not favor single-stage cratonization in the Late Archean. © 1990.