Geochronology of the Proterozoic Hartley Basalt Formation, South Africa: constraints on the Kheis tectogenesis and the Kaapvaal Craton's earliest Wilson Cycle

The Hartley Basalt Formation in the Northern Cape of South Africa comprises dominant Fe rich tholeiitic basalt and tuff, with porphyritic andesite found at only one locality. Geological evidence points to a rift-related setting which evolved into a passive continental margin as the early Kaapvaal Craton was split up. This marks a change in the stratigraphical record of the Archaean Kaapvaal Craton, from successively larger Witwatersrand, Ventersdorp and Transvaal intracratonic basins, to localised Waterberg Rifts, following the 2050 Ma Bushveld event. These Waterberg rifts are the earliest feature on the craton which can plausibly be related to a plate tectonic Wilson Cycle. The Hartley Formation now crops out within a linear mountain belt, formed at the end of the Wilson Cycle as rocks of the Kheis Tectonic Province overthrust the Archaean Kaapvaal Province along the Kheis Front. Variable degrees of low grade metamorphism are seen in samples from different localities. A 1928 +/- 4 Ma Pb-Pb age by the Kober method on clear magmatic zircons from a porphyry sample is consistent with, and represents a substantial refinement of, the 1984 +/- 78 Ma Rb-Sr isochron age for the Hartley basalts. This is the most precise age determination for a rock formed in any of the Waterberg basins. Ancient Pb loss from large zircons after 1906 Ma is thought to represent a maximum age for Kheis metamorphism. A 1750 +/- 60 Ma Rb-Sr biotite age on a dolerite postdates the Kheis deformation in the foreland region and is a minimum age for the end of the Kheis Wilson Cycle. (C) 1998 Elsevier Science Limited.