CARBONATE-RICH TOURMALINITES FROM NEOPROTEROZOIC META-EVAPORITES OF CENTRAL NAMIBIA

Carbonate-rich tourmalinites from the Neoproterozoic Duruchaus Formation in the Gurumanas area of central Namibia show evidence for partial fluidization and mobilization of carbonate minerals related to south-vergent emplacement of the Naukluft Nappe Complex. The Duruchaus Formation is considered to be a playa-lake evaporite sequence. The mineral assemblage of the tourmalinites includes tourmaline, quartz, albite (An_0-1), dolomite [Mg/(Mg+Fe+Mn) =0.87-0.99)], calcite, and biotite [Mg/(Mg+Fe) = 0.75]. Optical cathodoluminescence (CL) imaging of the carbonate minerals and quartz reveals a complex growth history in open space during brecciation, which was associated with a compositionally variable and dynamic fluid. Preliminary geothermometry using Ti concentrations in biotite (Henry, 2005) suggests a maximum temperature of metamorphism for this tourmalinite of ~600°C. Electron microprobe analyses of the tourmalines reflect the bulk composition of the meta-evaporite and display complex chemical zoning. All of the tourmalines fall in the alkali subgroup [Na/(Na+Ca+X-vacancy) = 0.67-0.86] and are classified as dravite to “oxy-dravite” with most showing a range of Mg/(Mg+Fe) = 0.62-0.77 and Al/(Al+Fe+Mg) = 0.64-0.68. There is a trend toward the composition of povondraite [(NaFe³⁺₃(Fe³⁺₄Mg₂)(BO₃)₃Si₆O₁₈(OH) ₃O)] with the significant substitution primarily related to a homovalent Fe³⁺-Al³⁺ exchange. Some minor substitutions exist between (R+OH^-) and (Al+O^2-), Al, and (Na+R) where R represents (Mg+Fe). Combined with previously published boron isotope (δ11B) values of -4.2 and -8.1 ‰ (Palmer and Slack, 1989), it is suggested that the abundant B in the carbonate-rich tourmalinites originated from B-rich fluids associated with the non-marine evaporites, which were mobilized during diagenesis and possibly also during later carbonate remobilization related to emplacement of hot nappes of the Naukluft Nappe Complex.