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<p>The role of fractional crystallization in the formation of unzoned Li-mineralized pegmatites remains poorly understood. Apatite crystallizes throughout the cooling history of pegmatitic melts and can preserve critical information on the properties of the initial melt and its subsequent evolution. In this study, we report in situ element and Nd isotope compositions of apatite from unzoned Li-mineralized pegmatites in the Altyn Tagh Orogen, northern Tibetan Plateau, to investigate their formation and mineralization processes. These pegmatites have Nd isotope compositions similar to those of coeval S-type granites, suggesting that pegmatitic melts are likely fractionated products of granitic melts. Texturally early apatite (Ap1) has positive Eu anomalies (Eu/Eu<sup>*</sup> = 1.0–93), resulting from the depletion of Sm and Gd in the initial pegmatitic melts. This depletion may be due to fractionation of monazite and zircon within the nearby parent granitic melts prior to pegmatitic melt extraction, as supported by phase equilibrium modelling. Ap1 has elevated Li concentrations of &gt; 6 μg/g, which can be taken to discriminate Li-mineralized from barren pegmatites, indicating that fractionation of granitic melts leads to initial Li enrichment. Compared to Ap1, which is closely associated with plagioclase and contains information about the properties of the initial pegmatitic melts, Ap2 formed in a late stage and coexists with spodumene, montebrasite, and muscovite. Texturally late apatite (Ap2) has negative Eu anomalies (Eu/Eu<sup>*</sup> = 0.001–0.9). The shift from positive to negative Eu anomalies in pegmatitic melts recorded in Ap1 and Ap2 reflects feldspar crystallization within the pegmatitic melts. Ap2 has higher Li concentrations than Ap1, indicating that in situ fractional crystallization further enriched Li, culminating in spodumene crystallization. Our study demonstrates that unzoned pegmatites experience significant in situ fractional crystallization after the emplacement of melts, which is crucial for their formation and Li-mineralization, akin to zoned pegmatites.</p>

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The role of fractional crystallization in the formation of unzoned Li-mineralized pegmatites: insights from in situ apatite geochemistry

  • 期刊论文

  • 发表日期：

    2026年8月3日

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  • Shuaijie Liu,
  • Mei-Fu Zhou,
  • Huan Chang,
  • Bin Chen,
  • Hui Zhao

摘要

The role of fractional crystallization in the formation of unzoned Li-mineralized pegmatites remains poorly understood. Apatite crystallizes throughout the cooling history of pegmatitic melts and can preserve critical information on the properties of the initial melt and its subsequent evolution. In this study, we report in situ element and Nd isotope compositions of apatite from unzoned Li-mineralized pegmatites in the Altyn Tagh Orogen, northern Tibetan Plateau, to investigate their formation and mineralization processes. These pegmatites have Nd isotope compositions similar to those of coeval S-type granites, suggesting that pegmatitic melts are likely fractionated products of granitic melts. Texturally early apatite (Ap1) has positive Eu anomalies (Eu/Eu* = 1.0–93), resulting from the depletion of Sm and Gd in the initial pegmatitic melts. This depletion may be due to fractionation of monazite and zircon within the nearby parent granitic melts prior to pegmatitic melt extraction, as supported by phase equilibrium modelling. Ap1 has elevated Li concentrations of > 6 μg/g, which can be taken to discriminate Li-mineralized from barren pegmatites, indicating that fractionation of granitic melts leads to initial Li enrichment. Compared to Ap1, which is closely associated with plagioclase and contains information about the properties of the initial pegmatitic melts, Ap2 formed in a late stage and coexists with spodumene, montebrasite, and muscovite. Texturally late apatite (Ap2) has negative Eu anomalies (Eu/Eu* = 0.001–0.9). The shift from positive to negative Eu anomalies in pegmatitic melts recorded in Ap1 and Ap2 reflects feldspar crystallization within the pegmatitic melts. Ap2 has higher Li concentrations than Ap1, indicating that in situ fractional crystallization further enriched Li, culminating in spodumene crystallization. Our study demonstrates that unzoned pegmatites experience significant in situ fractional crystallization after the emplacement of melts, which is crucial for their formation and Li-mineralization, akin to zoned pegmatites.