Abstract
Western South Africa between Cape Town and the Namibian border hosts many dyke swarms that strike broadly to the NW and NNW, towards the reconstructed plume-centre of the c. 130 Ma Parana-Etendeka Large Igneous Province (LIP) but are undated. U-Pb ID-TIMS age determinations on baddeleyite from two NNW-striking dykes (~100 km long) parallel to the Namaqualand coast yield 487 ± 8 Ma and 482 ± 7 Ma as age determinations. These results demonstrate that these two dykes represent a magmatic event independent of, and significantly older than the Parana-Etendeka LIP with which they had previously been correlated. Major and trace element compositions are uniform along-these two dykes but distinct from each other; each is inferred to be fed by different magma batches. The inland Garies dyke has the stronger asthenospheric signature, comparable to E-MORBs, while the Namaqualand dyke has a lithospheric overprint, which could have been derived from a metasomatised sub-continental lithospheric mantle and/or assimilation of upper continental crust. The magmatism coincides with a tectonic shift from the compressional Pan-African Saldania-Gariep orogeny to a pull-apart rift system hosting Klipheuwel-Piekenierskloof deposits. This magmatism could potentially be the remnant of a hitherto unrecognized LIP. Within uncertainty, the 485 Ma timing overlaps with the end of the Cambrian period.
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