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Archaean to Neoproterozoic magmatic events in the Kaoko belt of NW Namibia and their geodynamic significance

Abstract:

Age relationships in the N-S trending Neoproterozoic (Pan-African) Kaoko belt of northwestern Namibia are still poorly constrained. U-Pb and Pb-Pb zircon age determinations by single grain evaporation, conventional multigrain fraction analyses and ion microprobe (SHRIMP) from a profile along the E-W Hoanib River reveal various episodes of zircon growth, ranging from late Archaean to late Neoproterozoic. From the eastern part of the profile we report crystallization ages of gneiss protoliths from 2645 to 2585 Ma, the oldest ages so far found in Namibia. Sm-Nd isotope data suggest the involvement of still older crustal material in the generation of these gneisses. A thermal event at ca 2285 Ma ago caused new zircon growth in the Archaean rocks, and major igneous activity between 1985 and 1960 Ma is represented by granitoid intrusions. This demonstrates that these gneisses were part of the Congo craton since Palaeoproterozoic times. An emplacement age of ca 1500 Ma was determined for the protolith of a granitic orthogneiss within the central part of the Hoanib River profile. This is the first record of a magmatic event of this age in this region. The western part of the profile consists of granitoid rocks that were generated during the Neoproterozoic Damara orogeny. This terrain has previously been interpreted as a Neoproterozoic metasedimentary domain and subsequently as pre-Damaran basement, but our study proves these rocks to be igneous in origin with emplacement ages ranging from 552 to 656 Ma. Sm-Nd isotope data and zircon xenocrysts demonstrate the involvement of older crustal material in the generation of these granitoids. We interpret a coastal granite, dated at ca 564 Ma, as being emplaced during the last peak of high-grade metamorphism in the Kaoko belt, which is approximately synchronous with the last metamorphic event in the Damara belt of central Namibia.