Isotope constraints on the origin of Pan-African granitoid rocks in the Kaoko belt, NW Namibia

Abstract

The geodynamic evolution of the Kaoko belt, northwest Namibia, which is part of the late Neoproterozoic mobile belt system of western Gondwana, is considered to be the result of collision between the Congo (Africa) and Rio de la Plata (South America) cratons. In the coastal area of this belt, Pan-African dioritic to granitic melts intruded at ~650 Ma and between ~580 and ~550 Ma into sedimentary sequences of the Kuiseb Formation. Field observations suggest that the later intrusive episode coincided with the peak of high-grade metamorphism in this region, which is characterised by granulite facies conditions and anatexis. Franz et al. (1999) state that both intrusive episodes can be connected with high-grade metamorphic events. The δ18O values of the Pan-African granitoids range from +8.5 to +10.3‰ and tend to be slightly higher than those of the Palaeoproterozoic and late Archaean basement gneisses (+7.6 to +9.9‰ and +7.4 to +9.0‰, respectively). The Pan-African granitoids have low to moderate initial 87Sr/86Sr ratios between ~0.7036 and ~0.7121. Sm-Nd isotopic data of the Pan-African granitoids have moderate initial ϵNd values ranging from −1.2 to −6.8. They are distinct from Palaeoproterozoic as well as late Archaean orthogneisses from the adjacent Congo Craton, which reveal much lower ϵNdvalues between −18 and −28 during the time of emplacement of the Pan-African granitoids. Thus, the derivation of these granitoid melts exclusively from much older crust can be precluded. Although the Pan-African granitoids and the Pan-African high-grade metasedimentary rocks have similar initial ϵNd values of −1.2 to −6.8 and also similar mean crustal residence ages between 1.2 and 1.6 Ga, the derivation of the granitoids from these metasedimentary rocks alone seems also improbable since the granitoids reveal significantly lower initial 87Sr/86Sr ratios than the metasediments. It is here suggested that the Nd, Sr and O isotopic features of the Pan-African granitoids reflect mixing between upper crustal material (metasediments) and remelted basement gneisses from lower crustal levels.