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New age constraints on the tectogenesis of the Kheis Subprovince and the evolution of the eastern Namaqua Province

Council for Geoscience, P.O Box 775, Upington 8800, South Africa, e-mail: gmoen{at}lantic.net

R.A. Armstrong

Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia, e-mail: richard.armstrong{at}prise.anu.edu.au

Mapping and stratigraphic analysis of the rocks adjacent to the Kaapvaal Craton east of Upington has suggested that the area previously defined as the Kheis Subprovince was deformed and metamorphosed not during the so-called Eburnean "Kheis Orogeny" but as the result of a considerably younger event. U-Pb zircon SHRIMP ages on sedimentary as well as volcanic and plutonic rocks show that the arenitic successions that dominate the area were derived from a provenance similar in geochronological make-up to the Kaapvaal Craton and laid down no earlier than ~1.8 Ga ago. Deposition of the volcanosedimentary Wilgenhoutsdrif Group is dated at ~1.3 Ga, confirming its unconformable relationship to the arenites and its co-magmatic relationship with the similarly-aged Kalkwerf Gneiss. The regional dynamothermal event is concluded to be early Namaquan, and its age is revealed by its post-Wilgenhoutsdrif character and Rb-Sr muscovite ages clustering around ~1.2 Ga. These findings confirm that the Kheis Subprovince contains no Eburnean orogenic signature and is, in fact, an integral part of the Namaqua Province. The post-tectonic Koras Group lies entirely within the subprovince and its <~1.17 Ga age is consistent with the area’s early Namaquan tectonic signature. It is separated from the ~1.1 to 1.0 Ga old Gordonia Subprovince by the Trooilapspan shear zone, a terrane boundary thought to indicate significant lateral displacement.