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Several giant industrial-grade rare-metal deposits have been discovered in the Himalaya most recently. However, the mechanism of how rare-metals mineralization formed in the prototype of the collisional belt is poorly understood, mainly due to insufficient studies on mineralogy, petrology and emplacement conditions. In this study, we have reported the whole-rock and mineral compositions of spodumene pegmatites, leucogranite, skarns and P-T paths of metapelites from Lhozhag, eastern Himalaya. Four spodumene pegmatite veins were studied that are enriched in lithium, beryllium, cesium and tantalite (Li-Cs-Ta subtype), mainly hosted by minerals of spodumene, petalite, elbaite, lepidolite, zinnwaldite, and margarite. Li-Be margarite is first reported from the Himalaya, and its lithium and beryllium come from the breakdown of spodumene and beryl. The surrounding skarns are enriched in lithium, beryllium, and tin, mainly hosted by minerals of vesuvianite, scapolite and garnet. Petrography and P-T estimates from the andalusite-staurolite phyllites suggest two stages of metamorphism that earlier Barrovian metamorphism was superimposed by Buchan metamorphism at conditions of 3.1 ± 0.3 kbar and 535°C or 580°C (52–56°C/km) during leucogranite emplacement. Combined with zircon and monazite saturation temperatures and conditions of Buchan metamorphism, the leucogranites should have undergone a first-stage isothermal decompression to crystallization conditions at ∼3.1 kbar and 650–720°C and a second-stage isobaric cooling at ∼3.1 kbar. The pegmatite probably underwent a similar P-T evolution supported by the appearance of two-stage growth of spodumene (Spd-1 and Spd-2 + Qz), petalite corona around spodumene, and cymatolite. The leucogranites/pegmatites have emplaced into the surficial Tethyan Himalayan Sequence simultaneously, causing contact metamorphism, skarnization and undercooling, which may have facilitated rare-metals mineralization in the pegmatites and skarns during the emplacement stage. We suggest that the pegmatites and skarns in Lhozhag, eastern Himalaya have enormous potential for rare-metals exploration. [Display omitted] • Detailed mineralogy and petrology of the spodumene pegmatites in Lhozhag are presented. • Li-Be margarite is reported for the first time in the Himalaya. • Leucogranites/pegmatites emplaced into THS, causing contact metamorphism and undercooling. • P-T conditions of contact metamorphism were ∼ 3.1 kbar and 535–580°C. • Lhozhag in eastern Himalaya has enormous potential for rare-metals exploration. [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]