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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 U Pb 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 Nd Hf isotope analysis also indicates a chemical affinity between the Lhozhag pluton and spodumene pegmatite. Given the consistency in crystallization ages, Nd Hf 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)–H 2 O 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. [Display omitted] • Lhozhag leucogranite pluton formed at ca. 19 Ma. • Gabo Spodumene pegmatites formed at ca. 20–18 Ma. • A genetic link between Lhozhag pluton and Gabo Spodumene pegmatites was revealed. • The spodumene pegmatites formed by multi-stage fractional crystallization from Lhozhag pluton. [ABSTRACT FROM AUTHOR]

The role of collisional belts in the global carbon budget remains controversial. Collisional orogens have traditionally been considered a net carbon sink, but recent studies have highlighted significant CO 2 fluxes. The present contribution combines comprehensive field mapping with detailed petrography, pressure-temperature determination, geochemistry, and geochronology along three transects of the South Tibetan Detachment System (STDS) in the Mount Everest and Nyalam regions of the central Himalaya. The results outline a previously unrecognized carbon source in the orogen: Buchan-type metamorphic decarbonation of carbonate-bearing lithologies in the hanging wall of the STDS driven by juxtaposition against underlying migmatite. Specifically, calc-silicates and calc-schists within the STDS underwent Buchan-type metamorphic overprinting at P-T conditions of 630–400 °C and 5–3 kbar (36–48 °C/km). Monazite and titanite U(-Th)-Pb petrochronology indicate that metamorphism occurred between ca. 23 and 19 Ma, contemporaneous with early deformation along the STDS. Movement along the STDS continued to at least 17–16 Ma, causing deformation and resetting of titanite U-Pb systematics in some calc-silicates. Detrital zircon geochronology shows that the calc-silicates and calc-schists in the STDS have an affinity to the Tethyan Himalayan Sequence and were incorporated into the shear zone and metamorphosed during its upward expansion. Based on decarbonation reactions, protolith restoration, and decarbonation efficiency studies, the metamorphic CO 2 degassing from the metamorphism of carbonate-rich rocks is estimated to be ∼0.6 Mt. C/yr. Recognition of the upward expansion of the STDS and associated metamorphic decarbonation provided new information key to both understanding the development of orogen-scale low-angle normal-sense faulting in the Himalaya and whether the orogen was a net CO 2 source or sink during middle Miocene time. [Display omitted] • The STDS metasediments underwent Buchan-type metamorphism at 650–400 °C and 5–3 kbar. • Metamorphism and deformation in the STDS were contemporaneous during 23–19 Ma. • The STDS metasediments have an affinity to the Tethyan Himalayan Sequence. • Juxtaposition of hot GHC migmatites with cold THS carbonates caused decarbonation. • New carbon source in the Himalaya degassed CO 2 by 0.6 Mt. C/yr. [ABSTRACT FROM AUTHOR]

• Rapid crystallization of anhydrous minerals can form H 2 O+Li-rich, mobile boundary layers. • Shearing along the STDS preferentially segregates inviscid, H 2 O+Li-rich, boundary layers, producing Li-pegmatites. • The ∼2200 km-long STDS along the Himalayan orogen has the potential to host large lithium deposits. Lithium is a key strategic metal with important reserves in pegmatites. However, the formation processes of Li-pegmatites remain doubtful, as most previous models are either qualitative or require large numbers of cyclic fractionation-extraction events that have little direct field evidence. Here, we propose that highly Li-enriched melts in the Himalaya may separate via an unusual combination of rapid crystal growth relative to diffusion of Li and water, and deformation in the context of the South Tibetan Detachment System (STDS). Models of boundary layer formation for felsic liquids show that rapid crystallization (1×10−7 to 1×10−8 m/s; c. 1 mm/day) of anhydrous minerals forms H 2 O+Li-rich, mobile boundary layers, whereas H 2 O+Li-poor, immobile boundary layers form adjacent to micas. Shearing can preferentially segregate inviscid, H 2 O+Li-rich, boundary layers, producing extreme compositions and compositional gaps relative to parent magmas. The rock record of some magmas may not always reflect homogeneous liquid fractionation, rather preferential extraction of hydrous boundary layer liquids that develop adjacent to anhydrous minerals. This model explains whole-rock chemistry and field observations from the STDS, where Li-rich pegmatites and potential coeval source magmas occur with a distinct compositional gap. If our model is correct, the c. 2200 km-long STDS of Himalaya may represent an important zone of Li enrichment and site for future strategic Li-pegmatite reserves. Preferential extraction of boundary layers implies some evolved felsic magmas may not record crystallization of hydrous minerals. [ABSTRACT FROM AUTHOR]