Serpentinites in central South West Africa/Namibia

Abstract

Abstract provided by author:

The study set out to investigate the relationships: (a) Between the rock types which occur at each body (b) Between the lensoidal and circular bodies with pre-Damaran countryrock and between circular and lensoidal bodies with Damaran country rock (c) Between the ultramafics and the Damara orogeny

On the basis of the investigation it is concluded: (a) The serpentinite was probably derived from harzburgite, which contained lenses of spinel harzburgite (now represented by chlorite schist) and lenses of lherzolite (now represented by amphibole and carbonate bearing rocks). The talc was derived by the reaction of serpentinite with silica bearing country rocks and possibly by MgO loss from serpentinite (b) The serpentinites from all four types of field occurrences are derived from ultramafic Alpine-type material which has been chemically depleted to varying degrees (c) The serpentinites were emplaced during the Damaran deformation. A plate tectonic model provides both ultramafic rocks of suitable composition and a mechanism for serpentinite emplacement

The ultramafics consist of olivine serpentinite, "hobnail" serpentinite, talcose serpentinite, carbonate amphibole serpentinite, talc schist, chlorite schist and amphibole fels. The bodies exhibit a zonation. Talc schist forms a rim around the bodies and grades into talcose serpentinite which grades into hobnail serpentinite. Minor lenses of carbonate serpentinite, amphibole serpentinite and amphibole carbonate may be found in the talcose serpentinite. Occasional lenses of chlorite schist are present throughout the bodies. In the core of the bodies olivine serpentinite may be developed. Possible amphibolite "dykes" or xenoliths may also be present. Ca-metasomatism has led to the development of epidote in plagioclase bearing country rocks near the serpentinites. Mg-metasomatism has resulted in the development of chlorite in biotite and amphibole bearing country rocks

Analyses of the serpentinites show that they have low incompatible element content, low FeOt, A12O3, Na20, K20, CaO content relative to SiO2 and high Ni/Co. This suggests that they were derived from chemically depleted material. A principal component analysis and discriminant analysis on serpentinite samples of known origin from literature classified the serpentinites with depleted Alpine-type material