The Ongeluk basaltic andesite formation in Griqualand West, South Africa: submarine alteration in a 2222 Ma proterozoic sea

Abstract

The Ongeluk lavas form part of the Palaeoproterozoic Transvaal-Griqualand West supracrustal sequence of the Archaean Kaapvaal Craton of South Africa. They form a thick shallow-marine volcanic sequence of pillow lava, massive flows and hyaloclastite, which together with their comagmatic subaerial Hekpoort correlate, once covered most of the Craton. In this study the magmatic composition and alteration features of Ongeluk samples were distinguished using geochemistry and radiogenic isotopes. The Ongeluk-Hekpoort magma was a basaltic andesite derived from anomalous Kaapvaal Craton lithosphere, with a long history of high U Pb , high Rb Sr and CHUR-like Sm Nd ratios. Little evidence for magmatic fractionation was found, although contamination of the lower lavas with Cr-rich material is evident in both Hekpoort and Ongeluk sequences. The alteration of Ongeluk lavas is characterised using diagrams which compare pillow core-rim pairs, hyaloclastite and massive lava samples with the estimated original magma composition. Alteration is ascribed to a shallow (< 400 m) marine process with both high- and low-temperature stages. This has some common features, but differs in several respects from modern ocean-floor alteration. 87 Sr 86 Sr ratios decreased from magmatic 0.7071 towards seawater 0.7025 during submarine alteration; the Rb Sr system also reveals a much later (∼ 1200 Ma?) phase of Ca and Rb mobility in a few samples. In contrast the submarine alteration affected Pb isotope ratios so little that a 2222 ± 13 Ma isochron age is regarded as a refinement of earlier dates for the Ongeluk-Hekpoort extrusion. The Ongeluk hyaloclastites acted as a sink for K and Rb in the overlying sea, while the Mn and Ca entering the sea could have contributed to the overlying Kalahari manganese deposits.