Volatile accumulation for the mineralization of Li–Be pegmatites in the northeastern Pamir, Western Kunlun, China

ABSTRACT

Li–Be mineralization in the northeastern Pamir is the most westerly portion of the newly defined Western Kunlun–Songpan–Ganze rare metal belt in China. This paper discusses the geochemistry and isotopic U–Pb–Sr–Nd–Hf systems of two Triassic granites and Li–Be pegmatites, trying to identify their genetic relationship. Our study shows that although the biotite monzogranites, the muscovite granites, and the Li–Be pegmatites have similar zircon and coltan U–Pb ages of 210 Ma, continuous crystallization could not completely account for their compositional and Sr–Nd–Hf isotopic diversity. In comparison with the biotite monzogranites, the muscovite granites exhibit strongly fractionated characteristics, with higher levels of SiO₂, K₂O, P₂O₅, Li, Be, Cs, Rb, and Ta, and are sourced from a partial melt of meta-pelite rocks, showing a genetical relationship with the Li–Be pegmatites. A model of volatile accumulation is an attempt to interpret some characteristics of the strongly fractionated muscovite granites and Li-Be pegmatites, which show dramatically elevated contents of P₂O₅ of 0.20–0.23% to Li–Be pegmatites of 0.4–2.64%, respectively. P₂O₅ might be an agency of the accumulation of fluxing compositions also including B, F, and H₂O. Succeeding pegmatite-forming fluid exsolution and selective removal of Ca–Na–K to precipitate feldspar minerals far away from the parent granites should be accountable for some strongly fractionated characteristics in the residual melt, e.g. the enrichment of Rb, loss of Sr and Ba, and elevated negative Eu anomaly. Furthermore, this selective extractant of alkalis metal triggered by the accumulation of flux notably enhanced the contents of Li and Be in the remaining melts by three times, which might be also an important mechanism for the rare metal concentration in the Li–Be pegmatites by thousands of times.

KEYWORDS:

• P anomaly
• volatile accumulation
• Li–Be pegmatite
• Western Kunlun

Acknowledgments

We are grateful to Ke Huang, Banglu Zhang and Fangping Shi for their assistance with the field work, and to Di Zhang, Lihui Jia, Peng Xiang, and Shuangrong Zhang for their assistance with the experimental analysis. Thorough and constructive comments by Patrick Phelps and an anonymous reviewer have been very helpful in our revision and are gratefully acknowledged. This work is supported by the Second Comprehensive Scientific Survey of the Qinghai–Tibet Plateau (2019QZKK0802).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/00206814.2022.2087109

Additional information

Funding

This work was supported by the Second Comprehensive Scientific Survey of the Qinghai–Tibet Plateau [2019QZKK0802].