Tectono-metamorphic evolution of the Pan-african Kaoko belt, NW-Namibia

Abstract

Abstract provided by author:

A general increase of the metamorphic peak conditions from east to west is conspicuous in the Kaoko belt, with the rocks of the EZ going through upper greenschist to lower amphibolite facies conditions (490-550°C, 8-10 kbar). The CZ is characterised by kyanite-staurolite-bearing metapelites in the eastern (560-630°C, 7-9 kbar) and sillimanite-bearing metapelites in the western part (590-630°C, 4-6 kbar). The WZ shows upper amphibolite to granulite facies conditions with peak temperatures between 750 and 800°C and pressures between 6 and 8 kbar. This area is also characterised by abundant pre- to post-kinematic Pan-African granitoids

A geodynamic model for the Kaoko belt includes delamination of the lower crust in the western part of the Kaoko belt during an early stage of the Pan-African collision between the Kongo Craton of southern Africa and the Sao Francisco Craton of South America. Delamination gave rise to an increasing geothermal gradient due to elevated hot astenospheric material, which caused the intense granitic intrusions in the WZ. Additionally, the sinking crustal slab forced shortening in the middle to upper crust producing a transpressional flower structure

Geochemical patterns of Pan-African metabasites suggest a continental setting for the EZ and an intra plate setting with an oceanic component for metabasites of the CZ and WZ. Similarities with T- and E-type MORBs have been observed in a few samples. This supports the interpretation of the Kaoko belt as a rather small intra-cratonic basin with rare oceanic crust including some seamounts. The deformation history of the Kaoko belt is subdivided into four deformation phases (D1 to D4), of which D1 to D3 are believed to have been progressive and represent a tectonic continuum. The major ductile tectonic phase D2 shows a partitioning of movement directions with ESE directed thrusting in the EZ and NNW-SSE oriented sinistral strikeslip movement in the CZ indicating an overall transpressive deformation regime. Metamorphic peak conditions were reached syn- to post-kinematic with respect to the D2 deformation event