Neoproterozoic tectono-thermal evolution of the Gariep Belt and its basement, Namibia and South Africa

Within the Pan-African Gariep Belt in southwestern Africa a para-autochthonous. predominantly sedimentary sequence, deposited in a rift graben and subsequently on a passive continental margin with a Kibaran basement, is distinguished from an allochthonous, predominantly oceanic terrane that was thrust on top of the former during collision of the Rio de la Plata and the Kalahari plates. 40Ar/39Ar mineral cooling ages presented here set the following constraints: after Kibaran high-grade metamorphism, the consolidated basement cooled through similar to 300 degrees C at 1006 Ma. Rifting between 781 and 741 Ma was followed by the formation of oceanic crust, the lower parts of which cooled through 500 degrees C sometime between 630 and 600 Ma. After inversion from extension to compression, some of the oceanic crustal material was shredded off and incorporated into an accretionary wedge-a process that is indirectly dated between 573-576 Ma by hornblende plateau ages. Continent-continent collision and thrusting of the internal onto the external zones of the tectonic belt culminated between 547-543 Ma as indicated by further hornblende plateau ages. Muscovite cooling ages for the internal zone ale between 526 and 529 Ma, those for the tectonically deeper, external zone are 483 Ma in the north and between 495-506 Ma in the south of the belt. In conjunction with published data, these results suggest successive closure of first the northern Adamastor ocean (Kaoko Belt), followed by the Khomas sea (intracontinental Damara Belt), and finally the southern Adamastor ocean (Gariep Pelt). (C) 1998 Elsevier Science B.V.