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Barrovian and Buchan type metamorphism in the Pan-African Kaoko belt, Namibia: implications for its geotectonic position within the framework of Western Gondwana

Institut für Mineralogie, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany, e-mail: thomas.will{at}mail.uni-wuerzburg.de; okrusch{at}mail.uni-wuerzburg.de

B.B. Gruner

Institut für Mineralogie, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany, Present address: Lehrstuhl für Allgemeine, Angewandte und Ingenieur-Geologie, Technische Universität München, Arcisstrasse 21, D-80290 München, Germany, Birgit.Gruner{at}helmholz.de

The polyphase deformed Pan-African Kaoko belt, Namibia, which is part of the late Neoproterozoic mobile belt system of western Gondwana, consists of pre-Pan-African basement gneisses, Pan-African volcano-sedimentary cover sequences and large granitoid intrusions.

The eastern part of the Kaoko belt comprises a nearly complete Barrovian sequence of medium to high temperature/medium pressure metapelitic rocks, which range in grade from greenschist- to upper amphibolite-facies conditions. From east to west, north-northwest to northwest trending subparallel garnet-, staurolite-, kyanite-, and kyanite-sillimanite-muscovite metamorphic zones can be distinguished, whereas the western Kaoko belt is dominated by a high temperature/low pressure Buchan-type metamorphism with sillimanite-K-feldspar- and garnet-cordierite-sillimanite-K-feldspar metamorphic zones. The two metamorphic terranes are tectonically separated by the Puros shear zone.

The pressure-temperature path for each metamorphic zone was inferred by using observed mineral assemblages and reaction textures, a combination of various geothermobarometers and, in particular, pseudosections. Peak metamorphic conditions of the Barrovian and Buchan zones indicate peak pressure-temperature ratios of ~20°C/km and 50°C/km, respectively. The Barrovian-type terrane is probably linked to a fold-and-thrust belt, the Buchan-type terrane may be linked to a magmatic arc setting. Similarities between the Kaoko belt in northwest Namibia and the central Ribeira belt in South America might point to a common geological history.

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