The genesis of LCT-type granitic pegmatites, as illustrated by lithium isotopes in micas
The genesis of LCT-type granitic pegmatites, as illustrated by lithium isotopes in micas
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DOI：
10.1016/j.chemgeo.2015.06.029
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发表时间：
2015-09
期刊：
影响因子：
3.9
通讯作者：
Sarah Deveaud;R. Millot;A. Villaros
中科院分区：
文献类型：
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作者：
Sarah Deveaud;R. Millot;A. Villaros
蒙茨丹巴扎克伟晶岩区（法国中央地块）的同位素组成显示出云母δ 7 Li值范围很窄，介于− 3.6至+ 3.4‰之间。从主岩圣西尔维斯特花岗岩福尔斯中获得的黑云母的δ 7 Li值就在这个范围内（δ 7 Li = − 1.5‰）。锂浓度与每种伟晶岩类型的岩浆演化程度一致：从II型的630 ppm到更演化的VI型伟晶岩的13，500 ppm。虽然稀有元素含量，例如，云母中Li、Cs、Ta与伟晶岩分异一致，δ 7 Li（‰）与岩浆分异程度无关（与伟晶岩类型无关），与Li及Be、Cs等助流元素含量无关。在沙班讷地区的伟晶岩V中取样的白云母是唯一一个δ 7 Li从中间单元（− 1.7‰）变化到内部伟晶岩单元（+ 3.4‰）的伟晶岩。这种δ 7 Li变化的性质表明，在伟晶岩的固结过程中有广泛的分离结晶。δ 7 Li（‰）的演化与岩浆演化程度无关，整个伟晶岩区存在明显的主要含稀土相，这表明云母中记录的δ 7 Li（‰）值继承自花岗岩和伟晶岩形成熔体所共有的地壳源岩。我们建议，不同的伟晶岩亚型（绿柱石铌铁矿与锂云母透锂长石亚型）观察整个蒙茨d 'Ambazac伟晶岩领域反映了地壳原岩的不同贡献。缺乏周围蚀变的证据，加上寄主花岗岩中Li含量没有增加，这倾向于证实在该伟晶岩场中获得的δ 7 Li值是主要的，并且没有Li扩散过程和/或混合驱动的Li同位素分馏叠加这些同位素组成。根据这些结果，富含稀有元素相的原岩的部分熔融过程，云母和石榴石，似乎是更负责锂同位素分馏比锂扩散或分离结晶的温度下伟晶岩固结。最后，我们讨论了使用锂同位素组成，以确定最高度发展伟晶岩系统。
Isotopic compositions in the Monts d'Ambazac Pegmatite Field (French Massif Central) exhibit a narrow range of mica δ7Li values, ranging from − 3.6 to + 3.4‰. The value obtained in biotite from the host Saint Sylvestre granite falls within this range (δ7Li = − 1.5‰). Lithium concentrations are consistent with the degree of magmatic evolution of each pegmatite type: from 630 ppm in Type II up to 13,500 ppm in the more evolved Type VI pegmatite. Although the rare-element contents e.g., Li, Cs and Ta of the micas are consistent with pegmatite differentiation, δ7Li (‰) are firstly, independent of the degree of magmatic differentiation (independent of pegmatite type) and secondly, independent of the content of Li and other flux-elements such as Be and Cs. Muscovite sampled in pegmatite V from the Chabannes locality is the only pegmatite to exhibit a δ7Li variation from an intermediate unit (− 1.7‰) to an internal pegmatitic unit (+ 3.4‰). The nature of this δ7Li variation suggests that there was extensive fractional crystallization during the pegmatite's consolidation. The independence of δ7Li (‰) evolution from the degree of magmatic evolution and the presence of distinct major rare-element bearing phases throughout the pegmatite field tend to confirm that the δ7Li (‰) values recorded in mica are inherited from crustal source rocks common to the granite and pegmatite-forming melts. We propose that the distinct pegmatite subtypes (beryl columbite vs lepidolite–petalite subtypes) observed throughout the Monts d'Ambazac Pegmatite Field reflect the diverse contributions of crustal protoliths. The lack of evidence of surrounding alteration combined with the absence of increasing Li-content within the host granite tends to confirm that the δ7Li values obtained within this pegmatite field are primary, and that no Li-diffusional process and/or mixing-driven Li-isotope fractionation has overprinted these isotopic compositions. In light of these results, the process of partial melting of protoliths enriched in rare-element bearing phases, e.g., mica and garnet, seems to be more responsible for Li-isotope fractionation than Li-diffusion or fractional crystallization at the temperature of pegmatite consolidation. Finally, we discuss the use of Li isotopic compositions to identify the most highly evolved pegmatitic systems.