Abstract

The bimodal volcanic and coarse siliciclastic sedimentary rocks of the Koras Group are virtually undeformed and weakly metamorphosed, and overlie the highly metamorphosed and strongly deformed rocks of the Kheis subprovince of the Namaqua—Natal Metamorphic Province with a sharp erosional unconformity. Zircons from the Swartkopsleegte quartz porphyry, close to the base of the Koras Subgroup, have been dated at 1171 ±7 Ma by SHRIMP. This age is in good agreement with, but far more precise than, previously published ages for the Koras Group. The new age defines the onset of bimodal volcanism in the Koras Group and also indicates that the compressional Kibaran Orogeny in the Namaqua—Natal Province ended before or at ~1170 Ma. The main phase of Kibaran deformation, uplift, and erosion in the Namaqualand sector of the orogen took place in the period between the extrusion of the Wilgenhoutsdriflavas at ~1330 Ma and deposition of the Koras Group at 1171 Ma. Younger ages for peak metamorphism and post-tectonic intrusions (1060 to 1030 Ma) in the Namaqua—Natal Province clearly post-date Kibaran compressional tectonics and magmatic activity by more than 100 Maand are related to a subsequent magmatic event in the orogen. The Koras Group in Namaqualand is widely regarded as a typical representative of the ~1105 Ma Umkondo igneous province, a group of volcano—sedimentary successions and igneous bodies which post-date the Kibaran Orogeny in southern Africa. However, extrusion of the Koras lavas predates the Umkondo suite in Zimbabwe and coeval lavas in Botswana andNamibia by 60 Ma.

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