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首页> 外文期刊>Contributions to Mineralogy and Petrology >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
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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

机译：S型花岗岩及其源岩在电气石中的化学和硼同位素变化：纳米比亚达马拉地带的Erongo花岗岩和电气石

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摘要

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 (δ11B 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 (δ11B −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 δ11B to −2‰ in secondary tourmaline can be explained by Rayleigh fractionation after about 90% B-depletion in the residual fluid.
机译：电气石广泛存在于新元古代达马拉带的变质岩和伟晶岩中，形成了白垩纪埃龙戈花岗岩的基底和潜在烃源岩。这项研究追踪了地下室，埃龙戈花岗岩及其热液阶段碧玺中B同位素的变化。来自地下室的电气石是缺碱的鼠尾草，大陆壳典型的B同位素比（δ 11 B平均为-8.4‰±1.4，n = 11；一个样品为−13‰， n = 2）。实际上，埃隆戈花岗岩中的所有电气石都存在于独特的电气石-石英小行星中。这种“主要阶段”的电气石是缺碱的鼠尾草（X位置空位为20％至30％，Fe /（Fe + Mg）0.8-1），具有均匀的B同位素组成（δ 11 B -8.7‰±1.5，n = 49）与基底平均数没有区别，这表明硼是从当地基底岩石的无烟矿中获得的，同位素组成没有明显变化。花岗岩中的次生热液电气石具有双峰B同位素分布，与主电气石一样，一个峰在约-9‰处出现，而另一个峰在-2‰处发生。我们建议富含结晶石的碧玺是在不混溶的富含Na-B-Fe的含水熔融液中形成的。圆形电气石的大量沉淀几乎耗尽了硼中的熔体，并且次生电气石中δ 11 B向-2‰的位移可以用剩余流体中约90％的B损耗后的瑞利分馏来解释。 。

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