Petrogenetic link between leucogranite and spodumene pegmatite in Lhozhag, eastern Himalaya: Constraints from U–(Th)–Pb geochronology and Li–Nd–Hf isotopes

Abstract

Rare-metal pegmatite represents the most differentiated magmatic system on earth, containing high concentrations of rare metals (Li, Rb, Cs, Be, Ta, Nb, and Sn) and fluxing components. Recent studies have shown that the Himalayan leucogranite–pegmatite system has considerable potential for rare-metal mineralization. A new spodumene pegmatite-type Li deposit (Gabo) was recently discovered in the Lhozhag region of the eastern Himalaya. However, the lack of precise geochronological and isotopic constraints on the crystallization timing and chemical affinity between spodumene pegmatites and nearby leucogranite has precluded definitive statements on their petrogenetic link. In this study, systematic monazite, xenotime, and columbite group mineral U–(Th)–Pb dating and Li–Nd–Hf isotope analysis were performed on the spodumene pegmatite and nearby leucogranite (Lhozhag pluton). Columbite group mineral LA-ICP-MS UPb dating of spodumene pegmatite indicates that the timing of lithium mineralization occurs at ca. 20–18 Ma, which is consistent with the magmatic crystallization age of the Lhozhag pluton (ca. 19 Ma) within the error margin, as constrained by monazite and xenotime U–(Th) Pb dating. The NdHf isotope analysis also indicates a chemical affinity between the Lhozhag pluton and spodumene pegmatite. Given the consistency in crystallization ages, NdHf isotopic compositions, and close field spatial relationship, the leucogranite probably represents the parent magma of spodumene pegmatites. The continuous evolutionary trend in the composition of spodumene pegmatites, as observed on the spodumene (Spd)– albite (Ab)–orthoclase (Or)–quartz (Qz)–H2O phase diagram, suggests that the enrichment of Li in pegmatite is result from fractional crystallization. Li isotopic analysis shows that the spodumene pegmatite has a δ7Li value of −2.5‰ to +3.3‰, slightly higher than that of the Lhozhag pluton (−2.4‰ to +1.0‰), providing additional support for their fractional crystallization links. These findings warrant a more comprehensive assessment of the lithium potential within the Himalayan orogenic belt.