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Two stages of Sn enrichment in granitic magmatism: insights from Sn isotopes of cogenetic minerals
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AI
J
李
R
L
C
Z
S
C
DOI：10.1016/j.gca.2026.07.020.png)
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Abstract
En 中文
The enrichment of tin (Sn) in high-silica (high-Si) granites is commonly linked to magmatic-hydrothermal processes, yet the respective roles of mineral crystallization versus fluid activity remain debated. Direct constraints on how individual minerals partition Sn and fractionate Sn isotopes during granite differentiation are particularly scarce. Here we report Sn concentrations and stable Sn isotope compositions (δ122/118Sn3161a) for cogenetic minerals (spodumene, tourmaline, muscovite, biotite, feldspar, quartz, and cassiterite) and whole rocks from the compositionally zoned Pusila leucogranite pluton in the Himalaya. Tin is heterogeneously distributed among minerals: muscovite acts as the principal host and is isotopically heavier than the host rocks, whereas feldspar and quartz contain only minor Sn and are generally isotopically lighter. Cassiterite from pegmatites is also isotopically light, with a mean δ122/118Sn3161a of 0.036‰. Whole‑rock δ122/118Sn3161a values increase systematically from −0.035‰ in biotite granite to + 0.337‰ in albite granite, whereas pegmatites show larger Sn isotope variations (0.292‰ to 0.816‰). These observations reveal two distinct stages of Sn behavior. During the magmatic stage, progressive crystallization of feldspar preferentially removes light Sn isotopes from the melt, driving the residual magma toward heavier isotopic compositions while allowing Sn to behave incompatibly. In the pegmatite stage, low Nb/Ta, Zr/Hf, and K/Rb ratios, pronounced REE tetrad effects, and widespread sericitization indicate significant fluid activity. An exsolved Sn-bearing aqueous fluid precipitates isotopically light cassiterite in the pegmatites, leaving the residual melt isotopically heavier. Together, these results support a model in which Sn enrichment involves early magmatic pre‑concentration followed by fluid‑mediated redistribution and cassiterite precipitation. Our findings delineate the mechanisms of Sn enrichment in evolved granitic systems and demonstrate the potential of Sn isotopes as a tracer of magmatic-hydrothermal processes.
Keywords:
Sn isotopes
High-Si granites
Mineral
Fluid
Cassiterite
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