METAMORPHIC EVOLUTION OF THE PAN-AFRICAN KAOKO BELT, NAMIBIA: CONSTRAINTS FROM PETROGENETIC GRIDS OF METAPELITIC ASSEMBLAGES

The Kaoko Belt of NW Namibia, part of the late Neoproterozoic mobile belt system of western Gondwana, consists of a Pan-African volcano-sedimentary sequence and underlying basement rocks of Archean and Mesoproterozoic age. During the Pan-African Orogeny, basement and cover experienced polyphase deformation and metamorphic overprint, increasing in grade from greenschist to granulite-facies conditions. From E to W, the following metamorphic zones are distinguished, based on mineral assemblages in metapelites: (i) garnet, (ii) staurolite, (iii) kyanite-staurolite, (iv) kyanite-sillimanite, (v) sillimanite-muscovite, (vi) sillimanite-K-feldspar, and (vii) garnet-cordierite-sillimanite-K-feldspar zone.

The metamorphic evolution in each of these zones were reconstructed using conventional geothermobarometers, petrogenetic grids in the systems KFMASH and KMnFMASH as well as PT pseudosections for the bulk rock composition of typical metapelites. These pseudosections, in combination with prograde growth zoning of garnet, observed peak assemblages, and retrograde reactions, enables a detailed reconstruction of clockwise PT path segments experienced in these rocks. Peak metamorphic PT conditions are 7-9 kbar/500-550°C for the eastern, 7-9 kbar/550-650°C for the central, and 4-6 kbar/650-750°C for the western Kaoko Belt.

Consequently, two different baric types of the metamorphic evolution can be distinguished in the Kaoko Belt: a medium to high-T/medium-P, Barrovian-type evolution in the eastern and central, and a high-T/low-P, Buchan-type evolution in the western segment of the belt. The boundary between these two metamorphic types coincides with the Puros Lineament, a NNW-SSE trending structure ranging over a few kilometers in the central Kaoko Belt. The geodynamic significance of these results is discussed.