Major and trace element and isotopic (Sr, Nd, O) constraints for Pan-African crustally contaminated grey granite gneisses from the southern Kaoko belt, Namibia

Light-grey to dark-grey, fine-grained to medium-grained granitic to granodioritic gneisses are substantial part of the Nk Formation in the southern Kaoko belt, Namibia. Based on field relations, these rocks can be subdivided into two distinct groups. One group belongs to a pluton with an areal extent of approximately 6 km2, the other group represents gneissic granitic veins. For all samples, SiO2 varies between 68.1 and 74.7 wt.%. The LREE-enriched granite gneisses of the Nk pluton are slightly peraluminous (A.S.I.=1.05–1.25) with low Rb concentrations (86–133 ppm) and high concentrations of Sr and Ba (187–683 and 644–1535 ppm, respectively). The gneisses have initial εNd (500 Ma) values ranging from −4.1 to −10.7 and initial 87Sr/86Sr(500 Ma) ratios ranging from 0.70695 to 0.71108 and whole rock oxygen isotope values ranging from 9.4‰ to 10.7‰. The gneissic granitic veins can be subdivided into three different groups (groups A, B, C). Most geochemical data are similar to those obtained for the pluton, only the δ18O values for group B and the Rb/Sr ratios for group C are distinct from the pluton.