Chemical and boron-isotope variations in tourmalines from an S-type granite and its source rocks: the Erongo granite and tourmalinites in the Damara Belt, Namibia

Abstract

Tourmaline is widespread in metapelites and pegmatites from the Neoproterozoic Damara Belt, which form the basement and potential source rocks of the Cretaceous Erongo granite. This study traces the B-isotope variations in tourmalines from the basement, from the Erongo granite and from its hydrothermal stage. Tourmalines from the basement are alkali-deficient schorl-dravites, with Bisotope ratios typical for continental crust (δ 11 B average -8.4‰ ±1.4, n=11; one sample at -13‰, n=2).Virtually all tourmaline in the Erongo granite occurs in distinctive tourmaline-quartz orbicules. This "main-stage" tourmaline is alkali-deficient schorl (20-30% X-site vacancy, Fe/(Fe+Mg) 0.8 to 1), with uniform B-isotope compositions (δ 11 B -8.7‰ ±1.5, n=49) that are indistinguishable from the basement average, suggesting that boron was derived from anatexis of the local basement rocks with no significant shift in isotopic composition. Secondary, hydrothermal tourmaline in the granite has a bimodal B-isotope distribution with one peak at about -9‰, like the main-stage tourmaline and a second at -2‰. We propose that the tourmaline-rich orbicules formed late in the crystallization history from an immiscible Na-B-Fe-rich hydrous melt. The massive precipitation of orbicular tourmaline nearly exhausted the melt in boron and the shift of δ 11 B to -2‰ in secondary tourmaline can be explained by Rayleigh fractionation after about 90% B-depletion in the residual fluid.2

The granite system near Betliar village (Gemeric Superunit, Western Carpathians): evolution of a composite silicic reservoir

“…2c). Tourmaline nodules are known from a number of localities worldwide and their formation was suggested to be related to magmatic crystallization and penetration of hydrothermal fluids (e.g., Samson and Sinclair 1992;Buriánek and Novák 2007;Trumbull et al 2008). Tourmaline nodules at Moslavačka Gora in Croatia (Balen and Broska in press) have been interpreted as indicating decompression and ascent of fluids during shallow emplacement.…”

Matrix effects during laser ablation MC ICP-MS analysis of boron isotopes in tourmaline

“…In contrast to LA MC ICP-MS, the SIMS boron isotope data acquisition and reduction followed the procedures routinely used in the Cameca 6f SIMS laboratory of the Helmholtz Centre in Potsdam. 26,27,[38][39][40][41] Data from SIMS were corrected for mass discrimination and checked for instrumental dri using the mass discrimination factor (D inst ) calculated as a ratio between the measured and reference Compared to the LA MC ICP-MS procedure, this approach typically results in a larger repeatability estimate and represents a somewhat more robust method. We appreciate the rationale of different strategies for data acquisition and reduction in B isotopic analysis by LA MC ICP-MS and SIMS, but we have also recalculated both datasets using the SIMS approach as described above (see Discussion).…”

Textural and compositional evidence for a polyphase saturation of tourmaline in granitic rocks from the Třebíč Pluton (Bohemian Massif)

“…2d-g; 12a) and repre- sents a typical example of evolution from magmatic to hydrothermal stage (e.g. Pichavant and Manning 1984;Trumbull et al 2008). Granophyric and comb textures in the border zone of the MBG near the contact with surrounding durbachites (Fig.…”

“…It is difficult to explain the spatial segregation of tourmaline-rich nodules by exsolved aqueous fluids because they can hardly accumulate such high concentrations of Fe and/or Mg from internal sources, i.e. from the surrounding granite (Němec 1975;Barbey 2007;Trumbull et al 2008). Instead, mass-balance calculations for TN and surrounding MBG (Fig.…”

“…Greenfield et al 1996Greenfield et al , 1998Trumbull et al 2008). Tourmalines occur in peraluminous granite-pegmatite systems as disseminated and nodular in granites, or a wide spectrum of textural and compositional types in granitic pegmatites and associated quartz veins (e.g.…”