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Introduction

Magma-fluid interaction plays a pivotal role in granite evolution, element transport, and rare metal mineralisation (Wu et al., 2020

Wu, F.-Y., Liu, X.-C., Liu, Z.-C., Wang, R.-C., Xie, L., Wang, J.-M., Ji, W.-Q., Yang, L., Liu, C., Khanal, G.P., He, S.-X. (2020) Highly fractionated Himalayan leucogranites and associated rare-metal mineralization. Lithos 352–353, 105319. https://doi.org/10.1016/j.lithos.2019.105319

; Zheng and Gao, 2021

Zheng, Y.-F., Gao, P. (2021) The production of granitic magmas through crustal anatexis at convergent plate boundaries. Lithos 402–403, 106232. https://doi.org/10.1016/j.lithos.2021.106232

). Geochemical and petrological studies reveal that granite formation is a result of multiple processes, such as partial melting of source rocks, magma fractional crystallisation, fluid-magma interaction, and wall rock assimilation (Powell, 1984

Powell, R. (1984) Inversion of the assimilation and fractional crystallization (AFC) equations; characterization of contaminants from isotope and trace element relationships in volcanic suites. Journal of the Geological Society 141, 447–452. https://doi.org/10.1144/gsjgs.141.3.0447

; Cao et al., 2022

Cao, H.-W., Pei, Q.-M., Santosh, M., Li, G.-M., Zhang, L.-K., Zhang, X.-F., Zhang, Y.-H., Zou, H., Zai, Z.-W., Lin, B., Tang, L., Yu, X. (2022) Himalayan leucogranites: A review of geochemical and isotopic characteristics, timing of formation, genesis, and rare metal mineralization. Earth-Science Reviews 234, 104229. https://doi.org/10.1016/j.earscirev.2022.104229

). However, the complexity and overprinting of geochemical signatures often limit the effectiveness of traditional elemental tracers in uniquely constraining granite genesis (Bartoli, 2021

Bartoli, O. (2021) Granite geochemistry is not diagnostic of the role of water in the source. Earth and Planetary Science Letters 564, 116927. https://doi.org/10.1016/j.epsl.2021.116927

; Cao et al., 2022

Cao, H.-W., Pei, Q.-M., Santosh, M., Li, G.-M., Zhang, L.-K., Zhang, X.-F., Zhang, Y.-H., Zou, H., Zai, Z.-W., Lin, B., Tang, L., Yu, X. (2022) Himalayan leucogranites: A review of geochemical and isotopic characteristics, timing of formation, genesis, and rare metal mineralization. Earth-Science Reviews 234, 104229. https://doi.org/10.1016/j.earscirev.2022.104229

). In contrast, stable metal isotopes (e.g., Mg, Fe, Zn, K) provide robust tracers for these processes (Teng et al., 2017

Teng, F.-Z., Dauphas, N., Watkins, J.M. (2017) Non-Traditional Stable Isotopes: Retrospective and Prospective. Reviews in Mineralogy and Geochemistry 82, 1–26. https://doi.org/10.2138/rmg.2017.82.1

), making them powerful tools for deciphering multiple processes during granite formation.

Strontium (Sr), which is compatible in feldspar but incompatible in mica, exhibits contrasting partitioning behaviour that makes it particularly useful for investigating melt generation and evolution (Harris et al., 1995

Harris, N., Ayres, M., Massey, J. (1995) Geochemistry of granitic melts produced during the incongruent melting of muscovite: Implications for the extraction of Himalayan leucogranite magmas. Journal of Geophysical Research: Solid Earth 100, 15767–15777. https://doi.org/10.1029/94JB02623

). To data, the stable Sr isotopic composition (δ^88/86Sr, defined as [(⁸⁸Sr/⁸⁶Sr)_sample/(⁸⁸Sr/⁸⁶Sr)_SRM987 − 1] × 1000 (‰)) is poorly constrained in terrestrial rocks. The δ^88/86Sr values for the Bulk Silicate Earth (BSE) vary from +0.27 to +0.30 ‰ (Moynier et al., 2010

Moynier, F., Agranier, A., Hezel, D.C., Bouvier, A. (2010) Sr stable isotope composition of Earth, the Moon, Mars, Vesta and meteorites. Earth and Planetary Science Letters 300, 359–366. https://doi.org/10.1016/j.epsl.2010.10.017

; Chen et al., 2022

Chen, X.-Q., Zeng, Z., Yu, H.-M., Sun, N., Huang, F. (2022) Precise measurements of δ^88/86Sr for twenty geological reference materials by double-spike MC-ICP-MS. International Journal of Mass Spectrometry 479, 116883. https://doi.org/10.1016/j.ijms.2022.116883

), whereas intermediate to felsic crustal rocks exhibit a broader range from −0.19 to +0.30 ‰ (Ohno et al., 2008

Ohno, T., Komiya, T., Ueno, Y., Hirata, T., Maruyama, S. (2008) Determination of ⁸⁸Sr/⁸⁶Sr mass-dependent isotopic fractionation and radiogenic isotope variation of ⁸⁷Sr/⁸⁶Sr in the Neoproterozoic Doushantuo Formation. Gondwana Research 14, 126–133. https://doi.org/10.1016/j.gr.2007.10.007

; Moynier et al., 2010

Moynier, F., Agranier, A., Hezel, D.C., Bouvier, A. (2010) Sr stable isotope composition of Earth, the Moon, Mars, Vesta and meteorites. Earth and Planetary Science Letters 300, 359–366. https://doi.org/10.1016/j.epsl.2010.10.017

; Charlier et al., 2012

Charlier, B.L.A., Nowell, G.M., Parkinson, I.J., Kelley, S.P., Pearson, D.G., Burton, K.W. (2012) High temperature strontium stable isotope behaviour in the early solar system and planetary bodies. Earth and Planetary Science Letters 329–330, 31–40. https://doi.org/10.1016/j.epsl.2012.02.008

). Such processes as seawater alteration and slab dehydration are thought to preferentially remove isotopically lighter Sr (Amsellem et al., 2018

Amsellem, E., Moynier, F., Day, J.M.D., Moreira, M., Puchtel, I.S., Teng, F.-Z. (2018) The stable strontium isotopic composition of ocean island basalts, mid-ocean ridge basalts, and komatiites. Chemical Geology 483, 595–602. https://doi.org/10.1016/j.chemgeo.2018.03.030

; Klaver et al., 2020

Klaver, M., Lewis, J., Parkinson, I.J., Elburg, M.A., Vroon, P.Z., Kelley, K.A., Elliott, T. (2020) Sr isotopes in arcs revisited: tracking slab dehydration using δ^88/86Sr and ⁸⁷Sr/⁸⁶Sr systematics of arc lavas. Geochimica et Cosmochimica Acta 288, 101–119. https://doi.org/10.1016/j.gca.2020.08.010

; Kani et al., 2023

Kani, T., Misawa, K., Morikawa, N., Kazahaya, K., Kusuhara, F., Yoneda, S., Terakado, Y. (2023) Strontium Isotope Characteristics (δ^88/86Sr, ⁸⁷Sr/⁸⁶Sr) of Arima-Type Brines Originated From Slab-Fluids. Geophysical Research Letters 50, e2022GL100309. https://doi.org/10.1029/2022GL100309

). However, the fractionation coefficients of stable Sr isotopes in crystal-melt-fluid systems remain unknown, as they have not yet been measured in experimental studies or examined from first principles calculations. The mechanisms governing stable Sr isotopic fractionation during metamorphism, partial melting and magma-fluid evolution therefore require further investigation.

In this study, we report high precision stable Sr isotope data for the anatectic rocks (primarily granitic veins and dikes), melanosomes, and their potential source rocks (mica schists and granitic gneisses) from the Cona area, eastern Himalaya, China (Figs. S-1, S-2). In this area, widespread crustal anatexis and migration of felsic melts result in the extensive exposure of migmatites and anatectic rocks including granitic veins, dikes, and leucogranites (Ji et al., 2022

Ji, M., Gao, X.-Y., Zheng, Y.-F. (2022) Geochemical evidence for partial melting of progressively varied crustal sources for leucogranites during the Oligocene–Miocene in the Himalayan orogen. Chemical Geology 589, 120674. https://doi.org/10.1016/j.chemgeo.2021.120674

, 2023

Ji, M., Gao, X.-Y., Zheng, Y.-F. (2023) The petrogenetic relationship between migmatite and granite in the Himalayan orogen: Petrological and geochemical constraints. Lithos 444–445, 107110. https://doi.org/10.1016/j.lithos.2023.107110

). These anatectic rocks were mainly produced by incongruent partial melting of mica schists and granitic gneisses in the late Oligocene to Miocene (Ji et al., 2021

Ji, M., Gao, X.-Y., Zheng, Y.-F., Meng, Z.-Y., Gao, P. (2021) Metapelites record two episodes of decompressional metamorphism in the Himalayan orogen. Lithos 394–395, 106183. https://doi.org/10.1016/j.lithos.2021.106183

). Detailed geological setting and petrological descriptions of the studied samples are provided in Supplementary Information. The exposure of anatectic rocks, migmatites, and their potential source rocks thus make them well suited for investigating partial melting and granitic magma evolution processes. Our results reveal ∼1 ‰ variation in δ^88/86Sr values for these anatectic rocks from the Cona area in eastern Himalaya. These findings offer new insights into stable Sr isotope fractionation during partial melting and granite magma evolution and provide a novel approach for understanding the petrogenesis of Himalayan leucogranites and associated rare metal mineralisation.

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Results

The analytical procedures for stable Sr isotopes and the data set are provided in the SI. Uncertainties in the stable Sr isotope measurements are expressed as 2 s.d. (standard deviation), with long term external precision ≤0.03 ‰ for δ^88/86Sr values, based on repeated analyses of in-house reference USTC-Sr and GB-Sr. Data quality was monitored by analysing four USGS reference materials (G-2, AGV-2, RGM-1 and BHVO-2), which yielded results consistent with recent literature values (Chen et al., 2022

Chen, X.-Q., Zeng, Z., Yu, H.-M., Sun, N., Huang, F. (2022) Precise measurements of δ^88/86Sr for twenty geological reference materials by double-spike MC-ICP-MS. International Journal of Mass Spectrometry 479, 116883. https://doi.org/10.1016/j.ijms.2022.116883

), as shown in Table S-1.

All country rocks, including mica schists and granitic gneisses, show a narrow range of δ^88/86Sr values from +0.14 ‰ to +0.40 ‰ (mean = +0.24 ± 0.15 ‰, n = 8) (Fig. 1a). The melanosomes have slightly higher δ^88/86Sr values from +0.20 ‰ to +0.69 ‰ (mean = +0.36 ± 0.34 ‰, n = 7), although their overall variations remain limited (Fig. 1a). In contrast, the anatectic rocks exhibit much lower and more variable δ^88/86Sr values, ranging from −0.81 to +0.21 ‰ (mean = −0.15 ± 0.59 ‰, n = 22) (Fig. 1a). Based on distinct correlations between δ^88/86Sr values and SiO₂ contents, the anatectic rocks are subdivided into two groups (Fig. 1a). These groups also differ in both Sr contents and δ^88/86Sr values (Fig. 1b). Group 1 has a wide range in the Sr content (36 to 230 μg/g) but a narrow range in δ^88/86Sr values (−0.12 to +0.21 ‰). In contrast, Group 2 shows a relatively narrow range in the Sr content (generally <100 μg/g, typically <30 μg/g) but a broad range in δ^88/86Sr values (−0.81 to −0.18 ‰).

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Discussion

Stable Sr isotopic fractionation due to chemical weathering and source heterogeneity. The distinct geochemical characteristics of the two groups of anatectic rocks suggest the effects of diverse petrological and geochemical processes. Chemical weathering and source heterogeneity should be considered first to evaluate the origin of stable Sr isotopic variations. The chemical index of alteration (CIA = mol. Al₂O₃/(Al₂O₃ + CaO + Na₂O + K₂O)) (Nesbitt and Young, 1982

Nesbitt, H.W., Young, G.M. (1982) Early Proterozoic climates and plate motions inferred from major element chemistry of lutites. Nature 299, 715–717. https://doi.org/10.1038/299715a0

) is an important indicator of the degree of chemical weathering. All studied samples have very low CIA values, indicating minimal weathering. The CIA values show no correlation with the δ^88/86Sr values (Fig. S-4). In addition, petrological observations confirm that the studied rocks were not strongly overprinted by later alteration. Therefore, the observed δ^88/86Sr variations are not attributable to chemical weathering. In the Himalayan orogen, initial Nd isotope compositions calculated at 20 Ma, expressed as ɛ_Nd(t), effectively distinguish source rock types: mica schists exhibit low ɛ_Nd(t) values (−19.9 to −13.0), whereas granitic gneisses have higher values (−13.3 to −8.1) (Ji et al., 2022

Ji, M., Gao, X.-Y., Zheng, Y.-F. (2022) Geochemical evidence for partial melting of progressively varied crustal sources for leucogranites during the Oligocene–Miocene in the Himalayan orogen. Chemical Geology 589, 120674. https://doi.org/10.1016/j.chemgeo.2021.120674

; Zeng et al., 2005

Zeng, L., Asimow, P.D., Saleeby, J.B. (2005) Coupling of anatectic reactions and dissolution of accessory phases and the Sr and Nd isotope systematics of anatectic melts from a metasedimentary source. Geochimica et Cosmochimica Acta 69, 3671–3682. https://doi.org/10.1016/j.gca.2005.02.035

). Despite variability in ɛ_Nd(t) values, both mica schists and granitic gneisses in this study exhibit narrow ranges in of δ^88/86Sr values and initial radiogenic Sr isotope compositions calculated at 20 Ma, expressed as (⁸⁷Sr/⁸⁶Sr)_i (Fig. 1c,d). Notably, anatectic rocks with both high and low δ^88/86Sr values display overlapping ɛ_Nd(t) values. No correlation between δ^88/86Sr and ɛ_Nd(t) values is observed (Fig. 1c). Thus, source heterogeneity cannot explain the observed variations in δ^88/86Sr values. Other processes, such as partial melting of the source rocks, magma fractional crystallisation, and magma-fluid interaction, must therefore be considered.

Stable Sr isotopic fractionation during crustal melting. Anatectic rocks in the Cona area were produced mainly by melting of mica schists and granitic gneisses through the incongruent breakdown of muscovite and biotite (Knesel and Davidson, 2002

Knesel, K.M., Davidson, J.P. (2002) Insights into Collisional Magmatism from Isotopic Fingerprints of Melting Reactions. Science 296, 2206–2208. https://doi.org/10.1126/science.1070622

; Patiño Douce and Harris, 1998

Patiño Douce, A.E., Harris, N. (1998) Experimental Constraints on Himalayan Anatexis. Journal of Petrology 39, 689–710. https://doi.org/10.1093/petroj/39.4.689

), leaving the melanosome residue (Ji et al., 2021

Ji, M., Gao, X.-Y., Zheng, Y.-F., Meng, Z.-Y., Gao, P. (2021) Metapelites record two episodes of decompressional metamorphism in the Himalayan orogen. Lithos 394–395, 106183. https://doi.org/10.1016/j.lithos.2021.106183

). The mineral proportions during this process are dependent on temperature, pressure, and H₂O activity (Patiño Douce and Harris, 1998

Patiño Douce, A.E., Harris, N. (1998) Experimental Constraints on Himalayan Anatexis. Journal of Petrology 39, 689–710. https://doi.org/10.1093/petroj/39.4.689

). Sr contents in melts are mainly controlled by feldspar, which is significantly richer in Sr than mica (Fig. S-5a). We note that higher (⁸⁷Sr/⁸⁶Sr)_i values correspond to greater δ^88/86Sr values (Fig. 1d). Although different melting reactions of heterogeneous source rock types can account for variable geochemical characteristics, such as (⁸⁷Sr/⁸⁶Sr)_i and Sr contents, of the Himalayan leucogranites (Knesel and Davidson, 2002

Knesel, K.M., Davidson, J.P. (2002) Insights into Collisional Magmatism from Isotopic Fingerprints of Melting Reactions. Science 296, 2206–2208. https://doi.org/10.1126/science.1070622

; Gao et al., 2017

Gao, L.-E., Zeng, L., Asimow, P.D. (2017) Contrasting geochemical signatures of fluid-absent versus fluid-fluxed melting of muscovite in metasedimentary sources: The Himalayan leucogranites. Geology 45, 39–42. https://doi.org/10.1130/G38336.1

), the calculated (⁸⁷Sr/⁸⁶Sr)_i values do not exceed 0.8, even for melting through the breakdown of biotite (see SI). Therefore, the extremely high (⁸⁷Sr/⁸⁶Sr)_i values in the anatectic rocks cannot be attributed to partial melting process.

To investigate the stable Sr isotopic fractionation during partial melting, we conducted phase equilibrium modelling of mica schist and granitic gneiss using the Perple_X software combined with geochemical modelling based on mass balance equation (see SI). The mineral assemblages and model proportions of minerals and melt under different anatectic conditions were directly obtained from the phase equilibrium modelling, whereas the Sr contents in the melt and residue were calculated according to batch melting equations (see Table S-4). The average δ^88/86Sr value of mica schists (δ^88/86Sr = +0.29 ‰) and granitic gneiss (δ^88/86Sr = +0.22 ‰) were selected as the primary values, and the estimated fractionation between minerals and melt (Δ^88/86Sr_Pl-melt = +0.02 ‰; Δ^88/86Sr_Kfs-melt = −0.13 ‰; Δ^88/86Sr_Ms-melt = +0.21 ‰; Δ^88/86Sr_Bt-melt = −0.07 ‰) based on the observations in this study were used (see SI). The modelled results indicate that partial melting would produce limited Sr isotopic fractionation (Δ^88/86Sr_residue-melt < +0.10 ‰ in most cases) (Fig. 2a,b,c). This agrees with observations that the δ^88/86Sr values for most melanosomes and some leucosomes overlap with the values for their source rocks. Only fluid-present melting of mica schist under high pressure and low temperature yields larger fractionation (Δ^88/86Sr_residue-melt = +0.21 ‰), which is able to explain the heaviest δ^88/86Sr value of 0.69 ‰ for one melanosome (Figs. 1b, 2a,b,c). Therefore, partial melting cannot account for the extremely low δ^88/86Sr values for the studied anatectic rocks.

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Stable Sr isotopic fractionation during fractional crystallisation. Fractional crystallisation has a significant impact on geochemical characteristics of the Himalayan leucogranites (Wu et al., 2020

Wu, F.-Y., Liu, X.-C., Liu, Z.-C., Wang, R.-C., Xie, L., Wang, J.-M., Ji, W.-Q., Yang, L., Liu, C., Khanal, G.P., He, S.-X. (2020) Highly fractionated Himalayan leucogranites and associated rare-metal mineralization. Lithos 352–353, 105319. https://doi.org/10.1016/j.lithos.2019.105319

; Cao et al., 2022

Cao, H.-W., Pei, Q.-M., Santosh, M., Li, G.-M., Zhang, L.-K., Zhang, X.-F., Zhang, Y.-H., Zou, H., Zai, Z.-W., Lin, B., Tang, L., Yu, X. (2022) Himalayan leucogranites: A review of geochemical and isotopic characteristics, timing of formation, genesis, and rare metal mineralization. Earth-Science Reviews 234, 104229. https://doi.org/10.1016/j.earscirev.2022.104229

). Since Eu is strongly compatible in plagioclase and Ba is more compatible than Rb in feldspar, the Ba/Rb ratio and europium anomaly (Eu/Eu^*) are useful indicators of fractional differentiation. In high-δ^88/86Sr anatectic rocks, Ba/Rb ratios decrease while δ^88/86Sr values remain relatively constant. In contrast, low-δ^88/86Sr anatectic rocks exhibit significant stable Sr isotopic variations with little change in Ba/Rb ratios (Fig. 3a). Although Sr removal via feldspar separation is supported by the correlation between Sr contents and Eu/Eu^*ratios (Fig. S-6d), δ^88/86Sr values show no correlation with Eu/Eu^* in samples with Eu/Eu^* < 1 (Fig. 3b). This indicates that the δ^88/86Sr variations are not controlled by fractional crystallisation.

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Anatectic rocks with Eu/Eu^* > 1, representing cumulates, generally exhibit overlapping δ^88/86Sr values (Fig. 3b). To investigate stable Sr isotope fractionation during fractional crystallisation, we performed phase equilibrium modelling using the Perple_X software (see SI). Note that Sr contents in the primary melts would have a large range during partial melting of mica schist and granitic gneiss (Table S-4). Two representative anatectic rocks with different Sr contents (C_Sr = 161 μg/g and δ^88/86Sr = +0.18 ‰; C_Sr = 36 μg/g and δ^88/86Sr = −0.05 ‰) were selected in the modelling as representative compositions of the primary melts. The mineral assemblage and model proportion of minerals and melt under different crystallisation conditions were directly estimated from the phase equilibrium modelling (Table S-5). The same fractionation factors between minerals and melt as in the partial melting modelling were used. Rayleigh fractionation models were depicted in the Figure 2d. The modelling results show that fractional crystallisation would not produce large variations in δ^88/86Sr values for the differentiated melts, consistent with high-δ^88/86Sr anatectic rocks (Fig. 2d). Thus, the extremely low δ^88/86Sr values for anatectic rocks cannot be explained by fractional crystallisation.

Stable Sr isotopic fractionation during magma-fluid interaction. The most plausible process responsible for the extremely low δ^88/86Sr and high (⁸⁷Sr/⁸⁶Sr)_i values for the Sr-poor anatectic rocks is the interaction of their magma with high temperature fluids. Metasomatic textures and fluid inclusions in quartz provide direct evidence for such interactions (Fig. S-3). Rare earth elements (REE) form complexes with non-bridging oxygen, F⁻, and Cl⁻ in fluids, generating tetrad effects (expressed as TE_1,3) and reducing REE content during magma-fluid interaction (Bau, 1996

Bau, M. (1996) Controls on the fractionation of isovalent trace elements in magmatic and aqueous systems: evidence from Y/Ho, Zr/Hf, and lanthanide tetrad effect. Contributions to Mineralogy and Petrology 123, 323–333. https://doi.org/10.1007/s004100050159

; Irber, 1999

Irber, W. (1999) The lanthanide tetrad effect and its correlation with K/Rb, Eu/Eu^*, Sr/Eu, Y/Ho, and Zr/Hf of evolving peraluminous granite suites. Geochimica et Cosmochimica Acta 63, 489–508. https://doi.org/10.1016/S0016-7037(99)00027-7

). The correlations between δ^88/86Sr values and these sensitive proxies further support the influence of magma-fluid interaction on stable Sr isotope compositions (Fig. 3c,d). In addition, the elevated (⁸⁷Sr/⁸⁶Sr)_i values for the anatectic rocks cannot be explained by partial melting or fractional crystallisation processes, indicating infiltration of external fluids (Fig. 1d). Fluids are known to preferentially carry light Ba isotopes (δ^138/134Ba) (Guo et al., 2020

Guo, H., Li, W.-Y., Nan, X., Huang, F. (2020) Experimental evidence for light Ba isotopes favouring aqueous fluids over silicate melts. Geochemical Perspectives Letters 16, 6–11. http://dx.doi.org/10.7185/geochemlet.2036

). Since Ba and Sr share similar geochemical properties, light Sr isotopes are similarly favoured by fluids (Amsellem et al., 2018

Amsellem, E., Moynier, F., Day, J.M.D., Moreira, M., Puchtel, I.S., Teng, F.-Z. (2018) The stable strontium isotopic composition of ocean island basalts, mid-ocean ridge basalts, and komatiites. Chemical Geology 483, 595–602. https://doi.org/10.1016/j.chemgeo.2018.03.030

, Klaver et al., 2020

Klaver, M., Lewis, J., Parkinson, I.J., Elburg, M.A., Vroon, P.Z., Kelley, K.A., Elliott, T. (2020) Sr isotopes in arcs revisited: tracking slab dehydration using δ^88/86Sr and ⁸⁷Sr/⁸⁶Sr systematics of arc lavas. Geochimica et Cosmochimica Acta 288, 101–119. https://doi.org/10.1016/j.gca.2020.08.010

; Kani et al., 2023

Kani, T., Misawa, K., Morikawa, N., Kazahaya, K., Kusuhara, F., Yoneda, S., Terakado, Y. (2023) Strontium Isotope Characteristics (δ^88/86Sr, ⁸⁷Sr/⁸⁶Sr) of Arima-Type Brines Originated From Slab-Fluids. Geophysical Research Letters 50, e2022GL100309. https://doi.org/10.1029/2022GL100309

). Therefore, fluid influx is inferred to introduce light stable Sr isotopes.

According to the above discussion, it is clear that external fluids with low δ^88/86Sr are required to explain light Sr isotope compositions in the anatectic rocks. To quantify this effect, a simplified two end member mixing model was applied (SI). The Sr contents in Himalayan granites are generally less than 100 μg/g (Cao et al., 2022

Cao, H.-W., Pei, Q.-M., Santosh, M., Li, G.-M., Zhang, L.-K., Zhang, X.-F., Zhang, Y.-H., Zou, H., Zai, Z.-W., Lin, B., Tang, L., Yu, X. (2022) Himalayan leucogranites: A review of geochemical and isotopic characteristics, timing of formation, genesis, and rare metal mineralization. Earth-Science Reviews 234, 104229. https://doi.org/10.1016/j.earscirev.2022.104229

). Since partitioning coefficient of Sr between fluid and melt (D_Sr^fluid/melt < 0.3) is low (Borchert et al., 2010

Borchert, M., Wilke, M., Schmidt, C., Rickers, K. (2010) Rb and Sr partitioning between haplogranitic melts and aqueous solutions. Geochimica et Cosmochimica Acta 74, 1057–1076. https://doi.org/10.1016/j.gca.2009.10.033

), a fluid exsolved from a magma reservoir is predicted to be extremely poor in Sr (<30 μg/g). Alternatively, the external fluids can also be supplied via metamorphic dehydration dominated by decomposition of mica, whose Sr content is low. In this regard, we reasonably set the Sr content in the fluids to be 10 μg/g. In addition, since the high-δ^88/86Sr anatectic rocks represent products of partial melting and fractional crystallisation, two initial points with the average δ^88/86Sr value and different Sr contents were selected as the end members in modelling (M1: C_Sr = 140 μg/g, δ^88/86Sr = +0.04 ‰; M2: C_Sr = 25 μg/g; δ^88/86Sr = +0.04 ‰). If 20–40 % fluids, which is carrying capacity in granitic magma (Thomas and Davidson, 2016

Thomas, R., Davidson, P. (2016) Revisiting complete miscibility between silicate melts and hydrous fluids, and the extreme enrichment of some elements in the supercritical state — Consequences for the formation of pegmatites and ore deposits. Ore Geology Reviews 72, 1088–1101. https://doi.org/10.1016/j.oregeorev.2015.10.004

; Deng et al., 2024

Deng, G., Jiang, D., Li, G., Xu, Z., Huang, F. (2024) Barium isotope evidence for a magmatic fluid-dominated petrogenesis of LCT-type pegmatites. Geochemical Perspectives Letters 31, 14–20. https://doi.org/10.7185/geochemlet.2426

), was adopted in the modelling, then the modelling results in the δ^88/86Sr value of −4.0 ‰ in fluids. Such an extremely light isotopic composition in fluids was also determined for Ba isotope system (Deng et al., 2024

Deng, G., Jiang, D., Li, G., Xu, Z., Huang, F. (2024) Barium isotope evidence for a magmatic fluid-dominated petrogenesis of LCT-type pegmatites. Geochemical Perspectives Letters 31, 14–20. https://doi.org/10.7185/geochemlet.2426

). It is worth mentioning that using higher Sr contents in the modelling will make slightly heavier Sr isotope in fluids. In a nutshell, stable Sr isotopic compositions for the anatectic rocks with low Sr content are more susceptible to be reduced by interaction between melts and fluids (Fig. 2d).

Implications for stable Sr isotopic variation in high silica granites and magmatic evolution. Understanding the differentiation processes of felsic magmas is crucial for elucidating the origin of granites and related rare metal mineralisation. Highly fractionated leucogranites of the Himalayan orogen are enriched in Be-, Nb-, Ta-, and Li-bearing minerals showing significant ore forming potential (Wu et al., 2020

Wu, F.-Y., Liu, X.-C., Liu, Z.-C., Wang, R.-C., Xie, L., Wang, J.-M., Ji, W.-Q., Yang, L., Liu, C., Khanal, G.P., He, S.-X. (2020) Highly fractionated Himalayan leucogranites and associated rare-metal mineralization. Lithos 352–353, 105319. https://doi.org/10.1016/j.lithos.2019.105319

). The formation of these leucogranites involved partial melting of source rocks, magma fractional crystallisation, and magma-fluid interaction. Notably, rare metal mineralisation is often closely associated with pegmatites that formed during late stage magmatic evolution with voluminous participation of fluids (Wu et al., 2020

Wu, F.-Y., Liu, X.-C., Liu, Z.-C., Wang, R.-C., Xie, L., Wang, J.-M., Ji, W.-Q., Yang, L., Liu, C., Khanal, G.P., He, S.-X. (2020) Highly fractionated Himalayan leucogranites and associated rare-metal mineralization. Lithos 352–353, 105319. https://doi.org/10.1016/j.lithos.2019.105319

). Thus, recognising and tracing fluid activity is essential for understanding both magmatic evolution and associated ore mineralisation. However, quantifying the extent of fluid activity remains challenging. While tetrad REE patterns are widely considered qualitative indicators of magma-fluid interaction, they provide limited constraints on fluid volume or other quantitative information (Irber, 1999

Irber, W. (1999) The lanthanide tetrad effect and its correlation with K/Rb, Eu/Eu^*, Sr/Eu, Y/Ho, and Zr/Hf of evolving peraluminous granite suites. Geochimica et Cosmochimica Acta 63, 489–508. https://doi.org/10.1016/S0016-7037(99)00027-7

). Our study is the first to reveal substantial stable Sr isotopic variations between anatectic rocks (Fig. 4). Importantly, neither partial melting nor fractional crystallisation produces significant variation in stable Sr isotopes, despite their effects on Sr contents in melts. Only melt-fluid interaction could induce substantial stable Sr isotopic variations (Fig. 4).

Full size image

Thus, stable Sr isotopes emerge as a novel geochemical tracer for identifying fluid activity in the granitic magma systems. Furthermore, systematic correlations among Sr, Ba, and Rb contents and their corresponding stable isotopes may provide a powerful framework for constraining the evolution of felsic magmas (Hu et al., 2022

Hu, X., Nan, X., Liu, X., Huang, F. (2022) Rubidium isotope compositions of the average upper continental crust and the Himalayan leucogranites: Implications for magmatic-fluid interaction. Geochimica et Cosmochimica Acta 336, 165–176. https://doi.org/10.1016/j.gca.2022.09.015

; Deng et al., 2024

Deng, G., Jiang, D., Li, G., Xu, Z., Huang, F. (2024) Barium isotope evidence for a magmatic fluid-dominated petrogenesis of LCT-type pegmatites. Geochemical Perspectives Letters 31, 14–20. https://doi.org/10.7185/geochemlet.2426

).

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Acknowledgements

This study was supported by funds from the National Key R and D programme of China (2024YFF0807302), the National Science Foundation of China (42472076 and 42330806), the Fundamental Research Funds for the Central Universities, and joint Russian Scientific Foundation – National Science Foundation of China (RSCF-NSFC) project (W2412052 and 25-47-00073). We are grateful to two anonymous reviewers for their thorough and constructive comments that greatly improved the paper. We also express our appreciation to Editor Raul O. C. Fonseca for his editorial handling.

Editor: Raul O.C. Fonseca

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References

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Supplementary Information

The Supplementary Information includes:

• Geological Setting and Samples
• Analytical Methods
• Mineral Sr Contents and Sr Isotopic Compositions
• Details of Geochemical Modelling
• Figures S-1 to S-6
• Tables S-1 to S-13
• Supplementary Information References

Download the Supplementary Information (PDF)

Download Tables S-1 to S-13 (.xlsx)