GEOCHRONOLOGY OF THE PROTEROZOIC HARTLEY BASALT FORMATION, SOUTH-AFRICA - CONSTRAINTS ON THE KHEIS TECTOGENESIS AND THE KAAPVAAL CRATONS EARLIEST WILSON CYCLE

The Hartley Basalt Formation in the Northern Cape of South Africa comp rises dominant Fe rich tholeiitic basalt and tuff, with porphyritic an desite found at only one locality. Geological evidence points to a rif t-related setting which evolved into a passive continental margin as t he early Kaapvaal Craton was split up. This marks a change in the stra tigraphical 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. Th ese Waterberg rifts are the earliest feature on the craton which can p lausibly be related to a plate tectonic Wilson Cycle. The Hartley Form ation now crops out within a linear mountain belt, formed at the end o f 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 localiti es. A 1928 +/- 4 Ma Pb-Pb age by the Kober method on clear magmatic zi rcons from a porphyry sample is consistent with, and represents a subs tantial refinement of, the 1984 +/- 78 Ma Rb-Sr isochron age for the H artley basalts. This is the most precise age determination for a rock formed in any of the Waterberg basins. Ancient Pb loss from large zirc ons after 1906 Ma is thought to represent a maximum age for Kheis meta morphism. A 1750 +/- 60 Ma Rb-Sr biotite age on a dolerite postdates t he Kheis deformation in the foreland region and is a minimum age for t he end of the Kheis Wilson Cycle. (C) 1998 Elsevier Science Limited.