THE ONGELUK BASALTIC ANDESITE FORMATION IN GRIQUALAND WEST, SOUTH-AFRICA - SUBMARINE ALTERATION IN A 2222 MA PROTEROZOIC SEA

The Ongeluk lavas form part of the Palaeoproterozoic Transvaal-Griqual and West supracrustal sequence of the Archaean Kaapvaal Craton of Sout h Africa. They form a thick shallow-marine volcanic sequence of pillow lava, massive flows and hyaloclastite, which together with their coma gmatic subaerial Hekpoort correlate, once covered most of the Craton. In this study the magmatic composition and alteration features of Onge luk samples were distinguished using geochemistry and radiogenic isoto pes. The Ongeluk-Hekpoort magma was a basaltic andesite derived from a nomalous 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 wit h 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-temperat ure stages. This has some common features, but differs in several resp ects from Sr-87/Sr-86 ratios decreased from magmatic 0.7071 towards se awater 0.7025 during submarine modem ocean-floor alteration. alteratio n; the Rb/Sr system also reveals a much later (similar to 1200 Ma?) ph ase 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 is ochron age is regarded as a refinement of earlier dates for the Ongelu k-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 cou ld have contributed to the overlying Kalahari manganese deposits.