A ∼ 700 Ma Sm-Nd garnet-whole rock age from the granulite facies Central Kaoko Zone (Namibia): Evidence for a cryptic high-grade polymetamorphic history?

Continental collision of the Kalahari and the Congo craton in Africa and the Rio de la Plata Craton in South America resulted in a structurally complex Neoproterozoic belt system, the Kaoko-Dom Feliciano-Ribeira belt. It is uncertain whether these three cratons collided more or less simultaneously during one single orogenic event at ∼ 580-550 Ma or whether the belt owe its structural and metamorphic features to several so far poorly constrained events. The Kaoko Belt (NW Nambia), representing the belt system between the southern Congo Craton and the Rio de la Plata Craton, is an ideal object to study these complexities. Within this belt, high-grade meta-igneous and metasedimentary rocks of the sillimanite-K-feldspar zone contain large garnet porphyroblasts that grew at ca. 730 ± 10 °C at 6.7 ± 1.2 kbar during peak metamorphic granulite-facies conditions. A Sm-Nd garnet-whole rock errorchron (n = 7, MSWD = 6.0) obtained on a siliceous metasediment yielded an unexpected old age of 692 ±14 Ma which is interpreted as an inherited metamorphic age of an older granulite facies event. It is evident that the dated garnets survived the younger high-grade granulite facies metamorphism that occurred between ca. 570 and 520 Ma and preserved their old Sm-Nd isotope systematics implying that the closure temperature for Sm-Nd in garnet is higher than ca. 730 °C in this case. These results imply fast cooling rates at different times during the Pan-African orogeny that prevent isotopic homogenization at sample scale. Moreover, it is suggestive that trace element (REE) diffusion in garnet is considerably slower than major element diffusion. From a regional point of view, it is possible that these specific siliceous metasedimentary rocks may be unrelated to the Pan-African metamorphic evolution of the Kaoko Belt between 570 and 520 Ma and may represent lithological units that belong to so far unrecognized terranes in the Kaoko Belt.