Precise U-Pb baddeleyite age dating of the Usushwana Complex, southern Africa - Implications for the Mesoarchaean magmatic and sedimentological evolution of the Pongola Supergroup, Kaapvaal Craton

The Usushwana Complex of the south-eastern Kaapvaal Craton (South Africa and Swaziland), intrudes the ca. 3.6-3.1. Ga basement of the craton, as well as the Mesoarchaean volcanic and sedimentary cover succession of the Pongola Supergroup. New high-precision U-Pb dating of gabbros belonging to the Piet Retief Suite of the Usushwana Complex yield ages of 2989 ± 1 Ma, 2990 ± 2 Ma and 2978 ± 2 Ma. The Piet Retief Suite represents part of an intricate magmatic feeder to a major volcanic event which gave rise to the oldest known continental flood basalts on Earth, the Nsuze volcanic rocks. Broadly coeval SE-trending dolerite dykes of the Barberton-Badplaas Dyke Swarm in the larger region of the south-eastern Kaapvaal Craton formed along the same structural trend as the Usushwana Complex. One such dyke is dated herein to 2980 ± 1. Ma. Using the high-precision U-Pb geochronological data, the Nsuze volcanic rocks can now be resolved into at least two magmatic episodes which can be correlated with parts of the Pongola Supergroup. The first episode at ca. 2.99-2.98 Ga is broadly coeval with the Pypklipberg (Nhlebela) volcanic rocks, whereas the second at ca. 2.97-2.96 Ga was near synchronous to the Agatha volcanic rocks. A dolerite sill intruding into the Mozaan Group of the Pongola Supergroup, thought to be part of the Usushwana Complex, was dated to 2869 ± 5 Ma, and is instead coeval with the Hlagothi Complex further to the south, and provides a new minimum age for deposition of the Mozaan Group.