河北曲阳地区中佐伟晶岩脉中电气石原位硼同位素分析及其意义

摘要

Tourmalines are discovered in different types of deposits and are often used to investigate diagenesis and mineralization environment.For the significant isotope fractionation, the tourmalines have been widely used in geochemical tracer study of granite,pegmatite and various hydrothermal deposits.On the basis of polarizing microscope identification,the in-situ boron isotopic analysis of tourmalines from Zhongzuo pegmatite veins in Quyang area of Hebei was carried out by LA-MC-ICPMS.The results show that δ11 B values from Zhongzuo pegmatite veins are - 10.5‰-- 7.3‰, which are located in the variation ranges of peak values of pegmatite;δ11 B values from Zhongzuo pegmatite veins are almostly the same to Tanco pegmatites in Manitoba,Canada;according to the field and micropetrography characteristics of tourmalines, the δ11 B values from pegmatite with macrograined tourmalines are lower 2‰ than those of pegmatite veins with fine-grained tourmalines,because the former is formed during magma degassing and the latter may be crystallized before boron fractionation;B of tourmaline from Zhongzuo pegmatite veins mainly derives from the exsolution of magma-hydrothermal fluid,and Fe and Mg in the surrounding rock provide necessary sources for the forming of tourmalines.%电气石在各类矿床中均有分布,常用来探讨成岩、成矿环境.鉴于不同矿床类型中电气石硼同位素的组成差别较大,硼同位素已被广泛应用于花岗岩、伟晶岩和各类热液型矿床的地球化学示踪研究中.在偏光显微镜鉴定基础上,利用激光剥蚀等离子质谱对河北曲阳地区中佐伟晶岩脉中电气石开展了原位微区硼同位素分析.结果表明:中佐伟晶岩脉中电气石硼同位素δ11 B值为-10.5‰~-7.3‰,处于伟晶岩中电气石δ11 B值的峰值变化区间,与加拿大马尼托巴坦科伟晶岩的δ11 B值基本一致;根据电气石野外产状及显微岩相特征,粒度粗大的伟晶岩脉中电气石δ11 B值明显偏负约2‰的原因在于其形成过程中岩浆脱气引发硼同位素分馏,而粒度细小的伟晶岩脉中电气石可能在硼同位素分馏前就已晶出;中佐伟晶岩脉电气石中B主要来源于岩浆-热液流体的出溶,围岩中Fe、Mg可能为电气石的形成提供了必要的物质来源.