Petrographische, geochemische und isotopengeochemische Untersuchungen an panafrikanischen Granitoiden des Kaoko Belts, Namibia

The Kaoko Belt is situated in the north-west of Namibia and is part of the late proterozoic to early paleozoic mobile belt system in the western part of Gondwana. It was formed in the course of the collision of the Congo Craton (Africa) and the Rio de la Plata Craton (South America). During this orogeny different kinds of granitoids were intruded. These granitoids contain information about the composition of lower parts of the crust in this region, which otherwise can be explored by indirect geophysical methods, only. In this work those granitoids intruded near Torra Baai are focused in order to document the processes of partial melting and the evolution of these melts with respect to petrographical and particularly to geochemical investigations. The principal aims are the characterization of the source rocks of the melts (continental crust Û lithospheric enriched mantle or a mixture of both) on the base of isotopic investigations and the processes of partial melting to get information about the geodynamic evolution of the Kaoko Belt which is discussed controversially. Near Torra Baai two groups of melts intruded at different times: · In an early stage of the evolution of the Kaoko Belt peraluminous granodiorites and granites of the Nk-Formation intruded as a pluton and as dykes. All these granitoids were deformed during the Pan-African orogeny and now are granitic gneisses (= Nk-granite gneisses). · Afterwards melts of the Torra Baai Granitoids intruded syn- to posttectonically. Subaluminous diorites and peraluminous biotite granites intruded as plutons into the granitic gneisses and metasediments of the Nk-Formation. Besides peraluminous SiO2-rich granites intruded as stocks and dykes. While ascending through the continental crust the melts were contaminated by Pan-African metasedimentary material in the case of the biotite granites and by S-type granites that are derived most likely from basement material in the case of the Nk-granite gneisses which intruded as pluton. The uniform isotopic composition of the diorites is caused by a homogenous crustal source region. Crystal fractionation doesn?t play an important role for the diorites and biotite granites but it is an essential process in the evolution of the Nk-granites. The granitic dykes which are restricted to high SiO2-contents are modified neither by crystal fractionation nor by contamination. These granites are a mirror of the heterogenous composition of their source rocks. The source region of all investigated melts is developed less (87Sr/86Sr(500Ma): 0,70695 to 0,71663; eNd (500 Ma): -4,1 to -10,2) and according to this relatively young. Basement rocks like those cropping out in the north of the Kaoko Belt don?t have significant influence on the formation and evolution of the melts. Also the participation of a component from the mantle cannot be proofed in one of the investigated melts. In comparison with the Zentral Damara Orogen the granitoids of the Kaoko Belt show similar Nd isotope signatures, but less radiogenic 87Sr/86Sr ratios and lower d18O values. In contrast to the Zentral Damara Orogen the source region of the Pan-African granitoids of the Kaoko Belt is dominated by rocks of magmatic origin. The occurrence of other Pan-African magmatic complexes in the Kaoko Belt (near the Hoanib river in the north and the Bandombaai-complex in the south, where dioritic to granitic melts were generated by recycling of continental crust also) indicates a thermal event of regional appearance instead of a local melting event. The geotectonic situation in the Pan-African Kaoko Belt is still discussed controversially. The results of this study in addition to the lack of unequivocal mantle melts favour a model of continent-continent-collision after a short intracontinental rifting instead of a complete wilson cycle with subduction of oceanic crust.