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Plots of the chemical compositions of tourmaline from experiments. (a) Diagram of Xv/(Na+Xv) vs. Mg/(Mg+Fe); (b) Ti vs. Mg/(Mg+Fe); (c) Mn vs. Mg/(Mg+Fe); and (d) Na/(Na+Ca) vs. Mg/(Mg+Fe). The gray areas (NTHL) represents the compositional fields of tourmaline from the Himalayan leucogranites (our unpublished data).
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The genetic relationship between different types of granite is critical for understanding the formation and evolution of granitic magma. Fluid-rock interaction experiments between two-mica leucogranite and boron-rich fluids were carried out at 600–700°C and 200 MPa to investigate the effects of boron content in fluid and temperature on the reaction...
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... comparing the compositional difference between experimental tourmaline and natural tourmaline in Himalayan leucogranites, we plot the compositional data of natural tourmaline (our unpublished data) in Figure 5. The diagram shows that the compositions differ between natural tourmaline in leucogranites and experimental tourmaline. ...
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... Major elements (Figure 5 a) and B isotope compositions (Figure 10b) suggest that tourmalines in the Himalayan leucogranites are mainly magmatic in origin Han et al. 2020;Cheng et al. 2021). In addition, our previous experimental study shows that tourmaline leucogranite can be formed by the reactions between boron-rich fluid and two-mica granite (Cheng et al. 2019), which could explain the formation of tourmaline leucogranite veins or dikes in two-mica leucogranites (e.g. Visonà and Lombardo 2002). ...
Contamination is a common scenario of intracrustal magmatic processes that may significantly change the compositions of involved anatectic melts. Here, we present major element and B isotope analyses on tourmalines from the Guowu leucogranite and their host rocks, which allow us to unravel the potential contaminations of the Himalayan leucogranites by metapelite host rocks. Three types of tourmaline have been identified in our samples. Tur-M in the host schist has the highest Mg/(Mg+Fe) ratio (0.52–0.62) and low Al content, suggesting its metamorphic character. Tourmalines in the leucogranite (Tur-L) and in the contact zone between leucogranite and schist (Tur-C) have an identical core-rim texture, and their cores are characterized by the lowest Mg/(Mg+Fe) ratio (0.13–0.18) and high Al content, which are consistent with the composition of magmatic tourmaline in the Himalayan leucogranites. The rims of Tur-L show higher Mg/(Mg + Fe) ratio (0.32–0.45) and moderate Ca, Ti, F contents, reflecting the contribution of the host schist via contamination. The composition of Tur-C rims is similar to that of Tur-M, suggesting a more significant contribution from contaminated schist than that for Tur-L. The different types of tourmaline share consistent B isotope compositions with δ11B ranging from -14 to -12‰. The compositional characteristics of the contaminated tourmaline and the mineral assemblage in the contaminated zone suggest that the involved components derived from the host schist, probably produced by the decomposition of tourmaline, biotite, plagioclase, primarily include Ca, K, Mg, Ti, B and F. In addition, contamination by tourmaline-bearing metapelite may be an alternative interpretation responsible for the peraluminous character of the Himalayan leucogranites. The occurrence of contaminated tourmaline in leucogranites suggests that contamination of host rocks is a possible way promoting the formation of tourmaline in the Himalayan leucogranites, and tourmaline is useful for deciphering the related contamination processes.
... Morgan and London, 1989;Orlando et al., 2017). Cheng et al. (2019) suggest tourmaline granite from the Himalayas can be produced by reactions between boron-rich fluid and two-mica granite. However, no detailed work is published on the geochemistry/mineral chemistry of the sapphire-bearing pegmatites from the Paddar area. ...
Tourmaline of rare elbaite-dravite series has been reported for the first time from the sapphire bearing pegmatite from the Paddar area, Kishtwar District (Jammu and Kashmir), India. The sapphire-bearing pegmatites are hosted by ultramafic rocks of the Higher Himalayan Crystalline metamorphic complex. These pegmatites are dominantly composed of sodic-plagioclase, perthite, tourmaline, and sapphire, whereas microcline, biotite, and muscovite make up the minor phases. The tourmaline from these pegmatites contains high Mg, Ca, Na, and F content as well as low K, Fe, and Cr content. They are classified as alkali tourmaline, characterized by higher Mg/Mg + Fe and Na/Na + Ca ratios. They display complex substitution mechanisms during the crystallization (Li + Al↔Mg, Fe↔Mg, Na↔Ca, B + Al↔Si) and post crystallization alteration by the hydrothermal fluids. The geochemistry of tourmaline suggests that the tourmaline is crystallized from peraluminous, silicious pegmatitic melt (enriched in Al, Li, B, and H2O). This melt was most likely formed through partial melting of the metasediments of Higher Himalaya Crystalline during the M2 phase of regional metamorphism. The pegmatitic melt was injected in the ultramafics, resulting in bimtasomatism reaction evidenced by the development of systematic alteration zones next to the contact. Bimetasomatism results in the diffusion of the Ca and Mg from host rock to pegmatitic melt, and removal of silica from the pegmatitic melt, which leads to the increase of Al in pegmatitic melt. Finally, a saturation of the Al in the melt with a high Mg influx from host rock results in the favourable environment for crystallization of Mg-rich tourmaline and sapphire.
... Tourmaline was measured for Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, F, and B using a 2-stage analytical routine. The first stage used a 15 kV accelerating voltage, 15 nA beam current, and 2 μm spot diameter to analyze major elements other than B and F. The second stage used the same accelerating voltage and spot size, but a 100 nA beam current to analyze B and F. A PC3 crystal was used for detecting boron (Cheng et al., 2019). The following standards were used for calibration: jadeite (Na), MgO (Mg), orthoclase (K), wollastonite (Ca and Si), TiO 2 (Ti), Fe 2 O 3 (Fe), Mn 3 O 4 (Mn), kyanite (Al), SrF 2 (F), and dravite (B). ...
... The texture and composition of these tourmalines are similar to magmatic tourmaline in other Himalayan leucogranite bodies (Gou et al., 2017;Dai et al., 2019;Zhou et al., 2019;Han et al., 2020). Therefore, it can be excluded that these tourmalines formed via interaction between hydrothermal fluids and ferromagnesian minerals (e.g., biotite) (Cheng et al., 2019). Three types of tourmaline have been identified in the GMTL: Tur-Z type, Tur-M type, and Tur-II type. ...
The late-stage evolution of leucogranite magmas and related magmatic-hydrothermal processes which could lead to rare-metal mineralization are still poorly studied in the Himalayas. Since tourmaline is among the minerals that are present in evolved granites of the Himalayas, the geochemistry of this mineral, and in particular the boron isotopic composition, is a powerful tool to trace evolution processes and sources of these magmas. This study focuses on the analysis of tourmalines in the Gurla Mandhata tourmaline leucogranites (GMTL) from the northwest of the Himalayan orogen. Two generations of tourmaline show clear differences in texture, major element, and B isotopic compositions. Early-stage tourmalines have high-Mg# [Mg/(Mg + Fe) 0.39–0.45] and occur as inclusions in late-stage tourmalines [Mg# <0.34] and other minerals. The δ¹¹B values are in the range of −7 ~ −8‰ for the early-stage tourmaline and in the range of −12 ~ −15‰ for the late-stage tourmaline. The occurrence of tourmalines with contrasting B isotopic compositions from the same leucogranite body (even in a same specimen) can be explained either by mixing of magmas from different sources with different B isotopic compositions or by remarkable B isotopic fractionation during magma differentiation. As indicated by mineral textures and compositions, the mixing of different magma batches can be ruled out as a major cause in the case of GMTL. Alternatively, the bimodal distribution of tourmaline δ¹¹B most likely suggests the occurrence of multiple-stage B isotopic fractionation during the magma evolution. Our modeling based on Rayleigh fractionation shows that this fractionation may have been induced mainly by extraction of fluid at a late magmatic stage, but crystallization of tourmaline may have also played a minor role. Interestingly, a compilation of δ¹¹B values (>260 data points) in magmatic tourmaline from eight Himalayan leucogranite bodies systematically shows a similar bimodal distribution (i.e., with peaks at −7‰ and − 13‰), implying that this significant B isotopic fractionation may be a common scenario during magma evolution of the Himalayan leucogranites. However, multiple magma sources with contrasting B isotopic compositions cannot be fully ruled out for explaining the bimodal distribution of B isotopic composition. Combining our results with SrNd isotopic and other geochemical characteristics of the Himalayan leucogranites, we propose that the granitic magmas with high δ¹¹B originated either from fluid-present melting involving boron-rich fluids, or more probably from dehydration melting of the Greater Himalayan Sequence that had been metasomatized by boron-rich fluids. The geological context implies that these fluids were probably derived from dehydration of mica-rich rocks from the Lesser Himalayan Sequence underlying the Greater Himalayan Sequence.
... It is important to note that these authors proposed that a possible explanation for this complex, metasomatic style of mineralization is that it took place at the stage where fluids and melts coexisted and fluids could have transported mineral constituent elements (Mn, Fe, and Sn) to Ta-rich melts to crystallize magmatic Ta minerals. This has yet to be tested experimentally, although Cheng et al. (2019) propose that tourmaline concentrated at the roofs of granite intrusions may be the result of "self-metasomatism" where boron-rich fluids interact with magma. In the environment of coexisting melt and fluid the ore minerals can be considered to consist of dominantly magmatic elements (HFSE such as Nb and Ta) and elements that are potentially more mobile (fluid-mobile elements, FME, such as Mn, Fe, Sn, and Ca). ...
Niobium and tantalum, rare metals and high field strength elements (HFSEs) that are essential to modern technologies, are concentrated among others in lithium-cesium-tantalum (LCT) pegmatites and rare metal granites. The most important hosts for Nb-Ta in these types of deposits are the columbite group minerals (columbite-tantalite), but at some ore deposits significant Ta is also contained in wodginite, microlite, and tapiolite. Previous solubility experiments of HFSE minerals have been limited to high temperatures because of the slow diffusivities of HFSEs in granitic melts. An experiment protocol is described herein that allows HFSE mineral solubilities to be determined at lower temperatures, more in line with the estimated solidus temperatures of LCT pegmatites and rare metal granites. This is achieved through the interaction of a melt that is enriched in high field strength elements (e.g., P and Nb or Ta) with a fluid enriched in a fluid-mobile element (FME, e.g., Mn). A starting glass enriched in a slow diffusing HFSE was synthesized, and HFSE mineral saturation is obtained via the diffusion of a FME into the melt via interaction with a fluid. This interaction can occur at much lower temperatures in reasonable experimental durations than for experiments that require diffusion of niobium and tantalum. The solubility product of columbite-(Mn) from the fluid-melt interaction experiment in a highly fluxed granitic melt at 700 °C is the same as those from dissolution and crystallization (reversal) experiments at the same P-T conditions. Thus, both methods produce reliable measurements of mineral solubility, and the differences in the metal concentrations in the quenched melts indicates that the solubility of columbite-(Mn) follows Henry's Law. Results show that columbite-(Mn) saturation can be reached at geologically reasonable concentrations of niobium in melts and manganese in hydrothermal fluids. This experimental protocol also allows the investigation of HFSE mineral crystallization by fluid-melt interactions in rare-metal pegmatites. Magmatic origins for columbite group minerals are well constrained, but hydrothermal Nb-Ta mineralization has also been proposed for pegmatite-hosted deposits such as Tanco, Greenbushes, and granite-hosted deposits such as Cínovec/Zinnwald, Dajishan, and Yichun. This study shows that columbite-(Mn), lithiophilite, and a Ca-Ta oxide mineral (that is likely microlite) crystallized from experiments in fluid-melt systems at temperatures as low as 650 °C at 200 MPa. It is important to note that HFSE minerals that crystallize from fluid-melt interactions texturally occur as euhedral crystals as phenocrysts in glass, i.e., are purely magmatic textures. Therefore, crystallization of HFSE minerals from fluid-melt interactions in rare metal granites and pegmatite deposits may be more widespread than previously recognized. This is significant because the formation of these deposits may require magmatic-hydrothermal interaction to explain the textures present in deposits worldwide, rather than always being the result of a single melt or fluid phase.
... First, field observations show, in some cases, a tourmaline-rich zone appears in the boundary between the GMTL and surrounding rock. The tourmaline-rich zone can be potentially formed by fluid-rock interaction between boron-rich fluid produced by the degassing of highly evolved magma and biotite in the surrounding rock (Cheng et al., 2019;London and Manning, 1995). Therefore, the presence of a tourmaline-rich zone supports that the GMTL is highly evolved and probably in a fluid-saturated condition. ...
... Second, the GMTL is a typical tourmaline leucogranite with a mineral assemblage of tourmaline + muscovite + plagioclase + K-feldspar + quartz ± garnet (Fig. 2). Although two types of tourmaline have been recognized in these leucogranites, most tourmalines (Tur I) are considered to have crystallized at a late stage of magma evolution on the basis of the abundance of mineral inclusions (quartz, plagioclase, K-feldspar, and zircon), the euhedral and prismatic in shape (Fig. 2a, b, f) (Cheng et al., 2019;London and Manning, 1995) and the enrichment in Fe (Fig. 8) (Benard et al., 1985). Furthermore, Mn-rich magmatic garnet (MnO N 10 wt%, FeO~24-30 wt%, CaO b 1 wt%, MgO b 0.5 wt%) has been found in several leucogranite samples (unpublished data of authors), which contains abundant mineral inclusions (Fig. 2d) and was thought to have preferentially crystallized in highly evolved magma with high Mn content (Gao et al., 2012). ...
The Himalayan leucogranites, which were derived from crustal anatexis during the India-Asia continental collision, have recorded the tectono-magmatic evolution process of the Himalayan orogenic belt. In the Gurla Mandhata area of the western Himalaya, tourmaline leucogranites accompanied by variable deformation are observed along the Gurla Mandhata detachment fault. Whole-rock compositions show that these leucogranites have high SiO2, alkaline elements and low CaO, MgO, FeO, TiO2 contents, and are strongly peraluminous with A/CNK value of 1.13–1.27. For trace elements, these leucogranites are characterized by low Ba Sr, Nb, Nd concentrations and relatively high Rb, U concentrations with significant negative Eu anomalies. Zircon UPb dating on the tourmaline leucogranites reveals a crystallization age of 11–12 Ma. Field observations, mineral assemblage, and geochemical features indicate that the Gurla Mandhata tourmaline leucogranites experienced high-degree fractional crystallization, which mainly induced by the long-distance migration from generation to emplacement, and the enrichment of volatile component (B, F, Cl, and H2O) facilitates the transportation ability of the leucogranite magma. Two types of tourmaline, Tur I and Tur II, are identified in the tourmaline leucogranites, enclosing relationships and mineral compositions suggest that these tourmalines should have formed at late and early stage in terms of magma evolution, respectively. The occurrence of early-stage tourmaline suggests that the primitive magma of the Gurla Mandhata tourmaline leucogranites is enriched in boron and H2O, which is most likely derived from fluid-flux melting of metasedimentary rocks from the Greater Himalayan Crystalline. The existence of the inherited zircon with ages of Paleoproterozoic and Neoproterozoic in combination with previous whole-rock SrNd isotopic study implies that the source region of the Gurla Mandhata tourmaline leucogranites is a two-component mixture between the Greater Himalayan Crystalline and the Lesser Himalayan Sequence.
Leucogranitic rocks, mainly including leucogranite-pegmatite systems, have been found to be widely distributed in the South Tibetan Himalaya, and they have received considerable interest because of their significance in crustal evolution and associated rare-metal mineralization. Although the nature and geodynamic setting of the Himalayan leucogranites have been well documented by numerous studies, the pegmatites spatially associated with these leucogranites are still poorly understood. Tourmaline is a ubiquitous phase from the leucogranite to the pegmatite. We have therefore conducted in situ major and trace element and boron isotope investigations of tourmaline from the Gyirong pegmatite, synthesizing published data on the Gyirong leucogranite, to document the origin of tourmaline and its genetic implications. Two types of tourmaline (Tur-Ⅰ & Tur-Ⅱ) have been identified in this contribution and they are enriched in Fe, Si and Al but depleted in Mg and Ca, with Mg/(Mg+Fe) ratios ranging from 0.22 to 0.45. Accordingly, the tourmalines belong to the alkali group and have schorl composition. Trace elements, such as Zn, Ga, V, Sc, Li, Sn, Sr, and Co in the tourmalines are relatively enriched, whereas, other trace elements record low concentrations less than 10 ppm. The trace element concentrations of tourmaline are mainly controlled by melt composition. Morphological and geochemical characteristics reflect that the tourmalines from the Gyirong pegmatite are magmatic in origin. The Gyirong pegmatitic tourmalines have S-type granitoids and pegmatites boron isotopic signatures with a tight range of δ¹¹B values between −11.8 and −9.7‰, which is consistent with the magmatic tourmalines (Mg-poor) of the Gyirong leucogranite. This study suggests that the Gyirong pegmatite was the product of crustal anatexis and that the crustal metapelitic rocks within the Greater Himalayan Crystalline Complex were the most likely source components.
