期刊
CONTRIBUTIONS TO MINERALOGY AND PETROLOGY
卷 155, 期 1, 页码 1-18出版社
SPRINGER
DOI: 10.1007/s00410-007-0227-3
关键词
SIMS; Tourmaline orbicules; Tourmalinite; Boron isotope fractionation; Liquid immiscibility; Granite
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 B-isotope ratios typical for continental crust (delta(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) 0.8-1), with uniform B-isotope compositions (delta(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 delta(11) B to -2% in secondary tourmaline can be explained by Rayleigh fractionation after about 90% B-depletion in the residual fluid.
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