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

摘要

Light-grey to dark-grey,fine-grained to medium-grained granitic to granodioritic gneisses are substantial part of the Nk Fonnation 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 km~2,the other group represents gneissic granitic veins.For all samples,SiO_2 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 epsilon_(Nd(500Ma)) values ranging from -4.1 to -10.7 and initial ~(87)Sr/~(86)Sr_((500Ma)) ratios ranging from 0.70695 to 0.71108 and whole rock oxygen isotope values ranging from 9.4per thousand to 10.7per thousand.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 delta~(18)O values for group B and the Rb/Sr ratios for group C are distinct from the pluton.Linear variation of major and trace elements in the Nk granite gneisses from the pluton can be explained by igneous fractionation involving plagioclase (10 wt.%),biotite (1 wt.%),alkali feldspar (13 wt.%) and LREE-enriched accessory minerals.The ~(87)Sr/~(86)Sr ratios become more radiogenic and delta~(18)O values increase with increasing SiO_2 indicating that in addition to crystal fractionation processes,the Nk granite gneisses were affected by assimilation processes involving contaminants similar to basement derived granites from the Central Damara orogen which show lower epsilon_(Nd(500Ma)) ratios.The protolith of the Nk granite gneisses is most likely a biotite- and/or amphibole-bearing but muscovite-free rock,most probably an intermediate igneous rock (i.e.diorite to granodiorite) or a metagreywacke.Concerning the isotopic differences between the granite gneisses from the pluton and the gneissic granitic veins,partial melting probably took place in a heterogeneous compositionally layered middle to lower crust at temperatures of 845+-10degC as calculated by monazite and zircon saturation equations.The final stages of continental collision during westward dipping subduction of the Kaoko belt beneath the Ribeira belt took place between ca.590 and 520 Ma.In view of such a Pan-African subduction zone related geotectonic situation,the Nk granite gneisses are products of melting events in arc-derived rocks,produced in an earlier stage of the Pan-African orogeny.