Tourmaline in meta-evaporites and highly magnesian rocks: perspectives from Namibian tourmalinites

Tourmaline from meta-evaporitic tourmalinites of the Duruchaus Formation of central Namibia reveal a common compositional trend that occurs in tourmaline from other meta-evaporite localities. The meta-evaporitic tourmalines are generally sodic, magnesian, moderately-to-highly depleted in Al, and enriched in Fe3+ and WO2- (calculated). They typically follow this trend along a join between "oxy-dravite" [Na(Mg2Al)(Al6) (Si6O18) (BO3)3(OH)3(O)] and povondraite [Na(Fe33+) (Fe43+Mg2) (Si6O18) (BO3)3(OH)3(O)]. Similar trends occur in the meta-evaporites at Alto Chapare (Bolivia), Challenger Dome (Gulf of Mexico), and Liaoning (China). This chemical feature is attributed to the influence of oxidizing, highly saline, boron-bearing fluids that are associated with these lithologies. In the Namibian tourmalines there are some deviations from this trend, which are considered to be a consequence of later overprints related to sulfate-silicate interactions and/or influx of reactive fluid. Tourmalines occurring in the highly magnesian high-pressure rocks (whiteschists and pyrope-coesite rocks) are distinctly more magnesian and fall close to the dravite and "oxy-dravite" compositions. These latter tourmaline compositions likely reflect the metasomatic processes that produced these unusual bulk compositions and/or the influx of a reactive fluid that eliminated any earlier chemical signatures of meta-evaporitic fluids or protoliths.