NEOPROTEROZOIC TECTONOTHERMAL EVOLUTION OF THE GARIEP BELT AND ITS BASEMENT, NAMIBIA AND SOUTH-AFRICA

Within the Pan-African Gariep Belt in southwestern Africa a para-autoc hthonous. predominantly sedimentary sequence, deposited in a rift grab en and subsequently on a passive continental margin with a Kibaran bas ement, is distinguished from an allochthonous, predominantly oceanic t errane that was thrust on top of the former during collision of the Ri o de la Plata and the Kalahari plates. Ar-40/Ar-39 mineral cooling age s presented here set the following constraints: after Kibaran high-gra de metamorphism, the consolidated basement cooled through similar to 3 00 degrees C at 1006 Ma. Rifting between 781 and 741 Ma was followed b y the formation of oceanic crust, the lower parts of which cooled thro ugh 500 degrees C sometime between 630 and 600 Ma. After inversion fro m extension to compression, some of the oceanic crustal material was s hredded off and incorporated into an accretionary wedge-a process that is indirectly dated between 573-576 Ma by hornblende plateau ages. Co ntinent-continent collision and thrusting of the internal onto the ext ernal zones of the tectonic belt culminated between 547-543 Ma as indi cated by further hornblende plateau ages. Muscovite cooling ages for t he internal zone ale between 526 and 529 Ma, those for the tectonicall y 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 r esults suggest successive closure of first the northern Adamastor ocea n (Kaoko Belt), followed by the Khomas sea (intracontinental Damara Be lt), and finally the southern Adamastor ocean (Gariep Pelt). (C) 1998 Elsevier Science B.V.