Microfabrics indicating granulite-facies metamorphism in the low-pressure central Damara Orogen, Namibia

Abstract

The reconstruction of the fluid evolution during progressive metamorphism in the central Damara orogen by Hoernes and Hoffer indicated that fluid-present conditions were only periodically attained among these rocks and that the highgrade metamorphism along the west coast of Namibia occurred under essentially water-absent conditions. The characteristic mineral assemblage in the metapelites is: plagioclase, quartz, alkali-feldspar, biotite, cordierite, garnet ± sillimanite, apatite, zircon, ore. Orthopyroxene-bearing parageneses in the metapelites are missing and granulite-facies metamorphism may not be detected on the basis of mineral assemblages. A detailed study along a 100 km traverse across the migmatite area in the central Damara Orogen (Namibia) reveals several microfabric features which indicate, in analogy to many other well known granulite-facies terrains where similar microfabrics have been described, that the Pan-African regional metamorphism in this area culminated at conditions of low-pressure granulite facies. The observed features are: 1. (a) The activation of basal 〈 a〉- and prism 〈 c〉-glide systems in quartz under conditions of plastic deformation, and the development of equivalent subgrain boundaries, parallel prism faces, and parallel basal planes. 2. (b) The orientated exsolution of rutile needles in quartz. 3. (c) The exsolution of string-perthites, mesoperthitic string-perthites and antiperthites in feldspars. 4. (d) In addition, the homogeneous distribution of major elements in garnet crystals and garnet-biotite thermometry suggest granulite-facies conditions.