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European Journal of Mineralogy; September/October; v. 20; no. 5; p. 889-904; DOI: 10.1127/0935-1221/2008/0020-1879
© 2008 E. Schweizerbart'sche Verlagsbuchhandlung Science Publishers
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From field observation to experimental petrology and back - A special issue to honour Werner Schreyer

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

Darrell J. Henry1,*, Haiting Sun1, John F. Slack2 and Barbara L. Dutrow1

1 Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803, USA
2 US Geological Survey, National Center, Mail Stop 954, Reston, VA 20192, USA

* Corresponding author, e-mail: glhenr{at}lsu.edu

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.

Key-words: tourmaline, mineral chemistry, tourmalinites, Namibia, evaporite, magnesian.






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