Identification of a new 485 Ma post-orogenic mafic dyke swarm east of the Pan-African Saldania-Gariep Belt of South Africa

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.