New SHRIMP U–Pb zircon ages from the Hartswater Group, South Africa: Implications for correlations of the Neoarchean Ventersdorp Supergroup on the Kaapvaal craton and with the Fortescue Group on the Pilbara craton

Abstract

The Neoarchean Hartswater Group of the western Kaapvaal craton is a bimodal volcanic and sedimentary cratonic cover succession traditionally correlated with the Platberg Group of the ∼2.71Ga Ventersdorp Supergroup, South Africa. Correlation between exposures of the Platberg Group equivalents across the Kaapvaal craton is complicated, because they were deposited within isolated grabens, they display lateral facies changes over short distances, and they are extensively covered by calcrete and sand. Such correlation is important, since these units constitute one of the oldest unconformity-bounded sequences originally compared with sequences from the Pilbara craton (northwestern Australia) for reconstructing the ancient continent “Vaalbara”. Present age constraints, however, imply a ∼50 million year discrepancy in the shared geological histories of the cratons. Here we report SHRIMP U–Pb zircon ages from a prominent pyroclastic surge and ash fall deposit in the lower Hartswater Group (2733.4±3.4Ma) and from variable quartz–feldspar porphyry in the upper Hartswater Group (2724.3±5.8Ma). The new constraints significantly improve correlations of the Platberg Group equivalents on the western Kaapvaal craton, and present a clear solution to the apparent enigma in cross-craton correlation. The data cast doubt on the 2714Ma age for the Klipriviersberg Group of the east central Kaapvaal craton, and strengthen lithostratigraphic correlations with units from the Pilbara craton (i.e., the Hardey Sandstone, the Bamboo Creek and Spinaway porphyries, the Kylena Basalt, and in part the Tumbiana Formation). When our ages are placed within paleogeographic context a systematic picture of a shared long-lived extensional event emerges.