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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]

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]

Recent studies evoke dispute whether the Greater Himalayan Crystalline Complex (GHC) was exhumed during more than one phase. This contribution investigates the timing of ductile shear along the South Tibetan Detachment (STD), Main Central Thrust (MCT), and hidden discontinuities within the GHC of the central Himalaya. New data from the Nyalam transect, southern Tibet, suggest that ductile shear along the STD was active from ~ 27–25 Ma to ~ 17–15 Ma, whereas ductile shear along the MCT was active from ~ 19–16 Ma to ~ 10–8 Ma. Pseudosection modeling results and published metamorphic ages indicate a P–T–t path discontinuity within the GHC and the upper GHC reached higher peak metamorphic temperatures (ΔT 100 °C), similar peak pressures and older metamorphic ages (~ 5–10 Myr), in favor of the presence of the Nyalam Discontinuity. Summarized results from literature suggest that timing of ductile shear along the STD was ~ 5–10 Myr earlier than that along the MCT across the Nepal, Sikkim and Bhutan Himalayas. In addition, the GHC in Nepal can be separated into two portions by a regional-scale (> 800 km) in-sequence ductile shear zone — the High Himalayan Discontinuity. This shear zone was active coevally with the STD during 25–16 Ma, but ~ 5–10 Myr earlier than the MCT. The High Himalayan Discontinuity could possibly extend to the Sikkim Himalayas, but ends in Bhutan. The identified new architecture suggests that both in-sequence and out-of-sequence tectonometamorphic discontinuities are important components that formed the duplexing structure of the GHC in the central and eastern Himalaya, thus calling for more sophisticated numerical simulation than the current models such as the channel flow and critical taper. [ABSTRACT FROM AUTHOR]

The timing of crustal melting and cooling has been investigated across the migmatites of the Greater Himalayan Crystalline Complex (GHC) in the Nyalam region, central Himalaya. Monazite U-Pb ages vary from 32 to 14 Ma and are linked to metamorphic conditions on the basis of monazite internal zoning, mineral inclusions, and changes in heavy rare earth element and Y composition. Metamorphic temperatures were estimated by Zr-in-rutile thermometry and cooling rates were further constrained by rutile U-Pb ages. The results reveal two distinct blocks within the GHC of the Nyalam region. The upper GHC experienced higher peak metamorphic temperatures (730-750°C) and a higher degree of melting (15-25%). Partial melting was dominated by muscovite dehydration melting, which lasted from ~32 to 25 Ma, possibly until ~20 Ma. The lower GHC experienced lower peak metamorphic temperatures (640-675°C) and a lower degree of melting (0-10%) mainly via H2O-saturated melting from 19 to 16 Ma. At different times, both upper and lower blocks experienced initial slow cooling (rates 35 ± 8 and 10 ± 5°C Ma-1, respectively) followed by rapid cooling (100 ± 20°C Ma-1). The documented diachronous metamorphism implies the presence of the 'High Himalayan Thrust' that was active at ~25-16 Ma within the GHC of the central Himalaya. Different degrees and durations of partial melting in the investigated section suggest that a channel flow process dominated the exhumation of the upper GHC migmatites at 25-16Ma, whereas a critical taper process dominated the exhumation of the relatively lower-grade lower GHC rocks and cooled upper GHC migmatites at 16-10 Ma. We suggest that propagating thrusts along large tectonic boundaries together with low-viscosity lateral crustal flow could contribute to exhumation of high-grade metamorphic rocks in the Himalaya and other similar collisional orogens. [ABSTRACT FROM AUTHOR]

• UHT metamorphism is reported for the first time in the Himalaya. • The heat source was an over-thickened crust associated with lithospheric thinning. • Cold vs. granulitized eclogites formed during infant vs. mature collisional stages. • 2.0–1.8 Ga eclogites have formed by a Himalaya-type global collisional network. Modern-style plate tectonics, often characterised by subduction, is a fundamental dynamic process for planet Earth. Subduction related eclogites are widely used to indicate initiation of plate tectonics or whether different tectonic regimes dominated Earth history. However, such markers are commonly overprinted in ancient metamorphic terranes and rarely preserved even in most Phanerozoic mountain belts. This study tries to reveal the detailed burial and exhumation processes that formed granulitized eclogites in the Everest east region, central Himalaya, so as to explore the tectonic regimes recorded by similar rocks on early Earth. Robust Pressure-Temperature-time paths were achieved by studying the mineral relicts (Omp, Jd ∼29%), high-temperature mineral textures (Sil-Crd-Qz-Spl-Mesoperthite assemblage, rutile exsolution in biotite), and multiple thermobarometry and petrochronology of eclogites and metapelites. Results show that these eclogites underwent eclogitization at conditions of 730–770 °C and ∼20 kbar (∼11 °C/km) at ∼30 Ma and were overprinted by a heating and decompression path to ultrahigh temperature (UHT) conditions of 6–11 kbar and 900–970 °C (∼40 °C/km) during 25–15 Ma. The resulting exhumation rate (2–3 mm/yr) is slow and prolonged (10–15 Myr) (U)HT favoured re-equilibration of the eclogitic mineral assemblage and textures. The obtained UHT conditions, the first time ever reported for the Himalaya, were induced by combined effects of over-thickened (∼60 km) radioactive felsic crust and thinning of lithosphere to <90 km. This case study provides a critical example to understand the heat sources and timescale of UHT condition during continental collision. By comparing with the western Himalaya eclogites, we suggest that formation of cold vs. granulitized continental eclogites during the Himalayan orogeny is caused by different crustal thickness (normal ∼30 km vs. over-thickened ∼60 km) due to different collisional stages (infant vs. mature). In a wider perspective, ancient eclogites were commonly granulitized by stacking into the over-thickened orogenic crust during mature continental collision. According to similar granulitized eclogites preserved on early Earth, Himalaya-type continental subduction/collision should have become a global pattern during the Paleoproterozoic (2.0–1.8 Ga). [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]