TIAN Run1,2, ZHANG Hui1*, LYU Zheng-hang1, TANG Yong1
(1. Key Laboratory of High-Temperature and High-Pressure Study of the Earth’s Interior, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China;
2. University of Chinese Academy of Sciences, Beijing 100049, China)
Abstract: In this study, the major and trace elements of mica minerals in different textural zones, altered host rocks, metasomatic rocks, and lepidolite veinlets from the Xiaohusite No. 91 pegmatite dyke were analyzed by using electron microprobe analysis (EMPA) and laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS), respectively, in order to reveal the magmatic-hydrothermal evolution of the pegmatite system. The results show that Mg-rich biotite, muscovite, Li-rich muscovite, and lepidolite respectively occurred in subsequent zones from the alteration zone of host rock, to edge zone, outer zone, intermediate zone, and the core zone of the Xiaohusite No.91 pegmatite dyke. Chemical features of micas from the pegmatite dyke had shown the evolution trend of micas varying from muscovite to lepidolite. The substitution mechanism of Li in muscovites is obviously different from that in Li-rich micas (Li-muscovite and lepidolite), as the substitution of Li in muscovite could be in form of 2SiIV+LiVI？3AlTot or Si + Li？AlTot + Fe(Zn, Mn, Mg) while the substitution of Li in Li-rich micas could be in form of 3LiVI？AlVI + 2？VI (where ？ represents a vacancy). The No. 91 pegmatite dyke had experienced the magmatic-hydrothermal evolution process. As the evolution process proceeds, the K/Rb and Mg/Li ratios of micas are obviously decreased but the contents of Li, Rb, Cs, and F in micas are obviously increased, indicating that fractional crystallization is the main mechanism for the enrichment of Li, Rb, Cs, and F in late-stage melt. However, characteristics of major and trace elements of micas show that the No. 91 pegmatite dyke has no obvious characteristics of the magmatic-hydrothermal transition stage, as the evolution of pegmatite system could be quickly transformed from the magmatic stage to the hydrothermal stage.
Keywords: Chemical composition; trace element compositions; magmatic-hydrothermal evolution; micas; pegmatite
ACTA MINERALOGICA SINICA Vol. 41, No.6, 2021, page 593-609
