Pegmatites often exhibit regional as well as internal zonation. In this study, we use major and trace element and Li–B isotopic composition of muscovite and tourmaline from internally zoned and un-zoned S-type pegmatites and their host granites to characterize the geochemical and isotopic fractionation associated with their formation. The internally zoned pegmatites comprise three distinct textural zones, namely wall, intermediate and core. Muscovite and tourmaline occur in all the zones of these pegmatites. The trace element concentrations/ratios of muscovite from the three zones form well-defined differentiation trends marked by enrichment of incompatible elements such as Rb, Cs, Sr, B, Zn, Nb, Ta, P and the depletion of Ni, Co, V, Sc, Ti, Ba from the wall zone, through the intermediate zone to the core zone. This is suggestive of a strong role of fractional crystallization in producing the compositional diversity in the internally zoned pegmatites. Alkali element ratios such K/Rb and K/Cs in muscovite exhibit near exponential decline from the wall to the core zone which is suggestive of Rayleigh-type fractional crystallization. Fractional crystallization modelling reveals that the formation of the wall zone requires < 69% crystallization, the intermediate zone 85–95% and the core zone ca. 99% crystallization, leading to extreme enrichment of Rb and Cs and other incompatible elements. Muscovites and tourmaline from the un-zoned pegmatites display similar compositional trends as the internally zoned ones, but with a significant compositional gap between the host granite and the pegmatite. Lithium isotopic composition of muscovites and B-isotopic composition of tourmalines become progressively lighter from wall to core zone of the internally zoned pegmatite and from the granite to the pegmatite for the un-zoned pegmatites. This is suggestive of an important role of vapour exsolution in the formation of the pegmatites. Taken together, the geochemical and isotopic trends in the pegmatites can be explained by Rayleigh fractional crystallization operating in tandem with vapour exsolution.

中文翻译：

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S型伟晶岩地球化学分异：来自白云母和电气石的微量元素和Li-B同位素组成的约束

伟晶岩通常表现出区域分带和内部分带。在这项研究中，我们使用来自内部分带和未分带的 S 型伟晶岩及其寄主花岗岩的白云母和电气石的主要和微量元素以及 Li-B 同位素组成来表征与其形成相关的地球化学和同位素分馏。内部带状伟晶岩包括三个不同的构造带，即壁、中间和核心。白云母和电气石出现在这些伟晶岩的所有区域中。来自三个区域的白云母的微量元素浓度/比率形成了明确的分化趋势，其特征是不相容元素如 Rb、Cs、Sr、B、Zn、Nb、Ta、P 的富集和 Ni、Co、V 的消耗、Sc、Ti、Ba从壁区，通过中间区到达核心区。这表明分步结晶在产生内部分带伟晶岩的成分多样性方面发挥了重要作用。白云母中的 K/Rb 和 K/Cs 等碱元素比从壁到核心区呈指数下降，这表明存在瑞利型分步结晶。分数结晶模型表明，壁区的形成需要 < 69% 的结晶，中间区需要 85-95%，核心区大约需要 69%。99% 结晶，导致 Rb 和 Cs 以及其他不相容元素的极度富集。来自未分带的伟晶岩的白云母和电气石显示出与内部分带的相似的成分趋势，但在主体花岗岩和伟晶岩之间存在显着的成分差距。白云母的锂同位素组成和电气石的 B 同位素组成从壁到内部分带伟晶岩的核心区和从花岗岩到未分带伟晶岩的伟晶岩逐渐变轻。这暗示了蒸汽出溶在伟晶岩形成中的重要作用。综上所述，伟晶岩中的地球化学和同位素趋势可以通过瑞利分级结晶与蒸汽出溶协同作用来解释。