Decomposition of phengite from residual subducted slabs enhancing lithospheric tin mineralization: Insights from K–Li–Nd isotopes

The sources and roles of volatiles in tin (Sn)-related magmatic systems remain controversial. We present integrated K–Li–Nd isotopic, trace element, and volatile data from Late Cretaceous Xishan Sn-mineralized granites in the southern margin of the South China Block to constrain the origin of volatiles and the mechanisms driving mineralization. The δ41K values of the Xishan granites (−0.80‰ to −0.47‰) are systematically lower than those of coeval barren granites (−0.67‰ to −0.27‰) and positively correlated with δ7Li. Their high F (2350–10,850 ppm) and H2O (2.8–4.6 wt%) contents exhibit negative correlations with δ41K, indicating a genetic link between volatile enrichment and light K isotope signatures. These isotopic variations cannot be explained by post-magmatic alteration, crustal contamination or fractional crystallization. Instead, they record the incorporation of isotopically light, phengite-bearing eclogitic materials recycled from residual subducted slabs during slab rollback. The breakdown of phengite released F- and K-rich hydrous melts with light K–Li isotopic compositions, which mixed (~6% on average; range ~1%–12%) with crustal melts to generate hydrous, volatile-rich magmas. Geochemical and geophysical evidence further suggests that slab rollback induced asthenospheric upwelling, producing extensional basins and widespread granitic magmatism. The recycled hydrous melts, rich in F and Sn, effectively mobilized tin from rutile and transferred it into granitic melts under reduced conditions, where Sn2+ remained incompatible and accumulated. The combination of recycled phengite-derived volatiles, rutile dissolution, and reduced F-rich magmas provided the key physicochemical environment for Sn enrichment and mineralization.