Garnet as an indicator of magmatic evolution and rare-metal mineralization in the granite−pegmatite system: Insights from the Mufushan complex (Southeast China)

Pegmatites are crucial sources of rare metals, and their formation is closely related to granites. However, the magmatic evolution process of granite−pegmatite systems, including the changes in environmental characteristics and the rare-metal enrichment mechanism, remains controversial. The Mufushan complex of Southeast China, which formed through multiple episodes of large-scale magmatism, comprises seven lithological units: biotite granite, two-mica granite, muscovite granite, microcline pegmatite, microcline−albite pegmatite, albite pegmatite, and spodumene−albite pegmatite. These units record multistage magmatic evolution and are closely related to rare-metal mineralization. Garnets in these units, primarily occurring as accessory minerals, are magmatic garnets belonging to the almandine−spessartine solid solution series. Chemical analyses of these garnets provide insights into the evolution of the granite−pegmatite system and rare-metal mineralization. Linear trends in the MnO, rare earth element (REE), Y, and Sc contents and the (Gd/Yb)N versus Y/Yb ratios in garnets suggest that the granites and pegmatites of the Mufushan complex originated from the continuous evolution of a homologous magma. Moreover, the variation of garnet δ56Fe values reflects a decreasing-then-increasing trend in oxygen fugacity during the evolution of the granite−pegmatite system. However, the evolution of the granite−pegmatite system was not merely a gradual transformation. An abrupt event occurred in the late stage. Significant H2O enrichment in spodumene−albite pegmatites is evidenced by the garnet H2O, REE, Y, and Sc contents, Y/Ho ratio, and δ56Fe values, marking a shift from the magmatic stage to a magmatic fluid-rich stage. Petrographic evidence further indicates that garnets crystallized earlier than spodumenes, suggesting that the pegmatite melt was already significantly H2O-enriched before spodumene formation began. Additionally, the MnO content in garnets is closely correlated with pegmatite evolution and rare-metal mineralization, making MnO-rich garnets indicators of highly fractionated spodumene pegmatites. Multiple factors contribute to the excellent mineralization potential of the Mufushan complex. Multiple episodes of magmatism facilitate continuous magma evolution and rare-metal enrichment. The initial granitic magma in the Mufushan complex was fertile, with the Lengjiaxi Group providing a substantial source of materials for rare metal mineralization. Moreover, the significant enrichment of H2O before the crystallization of spodumene during the late pegmatite stage played an important role in the formation of spodumene and lithium mineralization.