REARRANGING VAALBARA WITH NEW GEOCHRONOLOGICAL AND PALEOMAGNETIC DATA FROM THE NEOARCHEAN VENTERSDORP SUPERGROUP OF SOUTHERN AFRICA

The Ventersdorp Supergroup of southern Africa and the Fortescue Group of Australia are two rare examples of well-preserved Archean large igneous provinces. It has been suggested that the Kaapvaal and Pilbara Cratons were joined together in the crustal block or continent of “Vaalbara” at the time of extrusion of the Ventersdorp and Fortescue lavas. Testing of the Vaalbara hypothesis, however, has been hampered by lack of robust age data from the Kaapvaal Craton and age uncertainties in existing paleomagnetic data or lack of reliable data from the relevant rock units on both cratons.

New SHRIMP U-Pb zircon ages of 2735 ± 3 Ma and 2724 ± 6 Ma for formations in the Platberg Group of the Ventersdorp Supergroup are used to advance a modified correlation of strata on the Kaapvaal Craton, and a novel comparison with strata from the Pilbara is proposed. The ~2.73 Ga ages cast doubt on the previous 2714 Ma age (discordant SHRIMP U-Pb) for the Klipriviersberg Fm., which constitutes the base of the Ventersdorp Supergroup. The possibility that the Klipriviersberg Fm. is a correlative of the 2.78 billion year old Derdepoort volcanics is promoted. The Platberg Group, which has traditionally been correlated (based on lithostratigraphy) with the Hardey Sandstone, the Bamboo Creek/Spinaway porphyry and the Kylena basalt of the Fortescue Group, now correlates in age with the overlying Tumbiana Fm (some overlap with the Kylena Fm remains possible).

Although previous paleomagnetic studies have tried to document the existence of primary magnetizations within the Ventersdorp Supergroup, none were successful in doing so. New paleomagnetic data from the Allanridge lavas result in a ~2.7 Ga paleopole for the Kaapvaal Craton (Q-value of seven). This paleopole, constrained by the new age data, provide a crucial tool for validating the existence of Vaalbara during the Neoarchean. Combined with paleomagnetic data from Western Australia, it is used to suggest a rearrangement of earlier proposed connections between the Kaapvaal and Pilbara Cratons in the Neoarchean.