Geochemical, thermodynamic and Li isotope constraints on the genesis of rare-metal pegmatites: Implications for crustal Li cycling

Abstract

<p>Rare-metal pegmatite deposits are widespread in collisional settings around the world and are increasingly important to the global requirement for critical metals. However, their genesis and rare-metal enrichment processes remain enigmatic, particularly concerning whether they originate from highly fractionated granitic magmas or anatexis of metasedimentary rocks. In this study, we integrate zircon U[sbnd]Pb geochronology, whole-rock and mineral chemistry and Li isotopic compositions of pegmatites and associated granitoids from the West Kunlun Orogenic Belt (WKOB) in northern Tibetan Plateau, to investigate these issues. Zircon U[sbnd]Pb dating yields weighted mean ages of 223 Ma for the pegmatites and 213–222 Ma for the adjacent granitoids. Compared with the granitoids, the pegmatites exhibit distinct geochemical signatures, including high SiO₂ (72.9–74.5 wt%) and Al₂O₃ (16.2–17.2 wt%), low TiO₂ (<0.01 wt%), ^TFe₂O₃ (0.09–0.21 wt%) and MgO (<0.02 wt%), and strongly peraluminous signatures (A/CNK = 1.04–2.56). δ⁷Li values of Li-rich pegmatites and Li-poor pegmatites are relatively uniform (+2.11 to +0.8 ‰), but for muscovite the δ⁷Li values extend to lower values in Li-poor than Li-rich pegmatites (−2.93 to +2.11 ‰ and + 0.37 to +2.98 ‰, respectively). The absence of coherent fractionation trends between pegmatites and granitoids (e.g., Rb, Cs), along with a lack of mineralogical evidence for extreme magmatic differentiation, argues against a fractionated granite origin. Instead, Li isotope models of partial melting, thermodynamically-defined phase equilibria and associated elemental calculations all demonstrate that the West Kunlun Li-rich pegmatites were likely generated by melting of Li-rich claystone under amphibolite facies conditions, whereas the Li-poor pegmatites originated from typical metasedimentary sources. We propose that the involvement of Li-rich claystone into the middle to lower crust during collisional orogenesis was a key control on the formation of Li-rich pegmatites in the WKOB. This mechanism highlights the metallogenic significance of crustal-scale Li cycling and may be broadly applicable to rare metal pegmatite systems in other collisional orogenic belts.</p>