Ion-probe dating of 1.2 Ga collision and crustal architecture in the Namaqua-Natal Province of southern Africa

Abstract

The Namaqua-Natal Province of southern Africa formed a part of the Kalahari craton, possibly linked to the ~1.0 Ga supercontinent Rodinia, but the timing of assembly and its positioning relation to other components is still debated. Thorough ion-probe zircon dating combined with strategic field observations in the tectonic front of a metamorphic belt can clarify some of these issues. In this study, the age of two "pretectonic" units, constrains the timing of collision and clarifies the role of the Koras Group as a tectonostratigraphic marker. The volcanosedimentary Wilgenhoutsdrif Group contains Archaean and Paleoproterozoic material, showing that it probably formed in a continental rift or a passive margin setting, before its involvement in the Namaqua collision event. At 124112 Ma the Areachap island arc magmatism was in progress, followed by a collision event around 1200 Ma which at 116510 Ma gave rise to migmatites in the island arc terrane. At the same time (117312 Ma) in the adjoining Kaaien terrane the first sequence of Koras Group bimodal magmatism formed in a fault basin, invalidating the concept that this Group is a tectonostratigraphic marker of the end of tectonism in the whole Namaqua Province. A time of little activity followed, with yet another pulse of magmatism at 1100-1090 Ma, giving rise to a second sequence of sedimentation and volcanism in the Koras Group, as well as correlated intrusive rocks.This second pulse is not related to any significant regional deformation and may have been thermally induced. It is in part coeval with the Umkundo large igneous province of the Kaapvaal and Zimbabwe Cratons. These formations preserve an important record for reconstructing Rodinia and our 10937Ma U-Pb age of the uppermost volcanic formation of the Koras Group, should be used as the age for the Kalkpunt formation, frequently cited as a Kalahari Craton paleopole.