Source-Inherited Redox Constraints on Triassic Granitoid Diversity and Implications for Li Enrichment in the NE Tibetan Plateau

Abstract The oxidation state of granitic magma is a key parameter in granitoid petrogenesis, with important implications for related mineralization; however, the mechanisms underlying redox variations in magma sources remain poorly understood. In the Songpan–Ganzi terrane of the northeastern Tibetan Plateau, Late Triassic granitoids exhibit striking variations in oxygen fugacity (fO2), reflecting diverse source compositions and melting conditions. These variations provide critical insights into the petrogenetic processes reflected in magmatic redox conditions. Integrating whole-rock element geochemistry, Sr–Nd–Hf–O isotopes, zircon and apatite trace elements reveals that oxidized granitoids (FMQ + 0.05 to + 1.71, where FMQ is the fayalite-magnetite-quartz buffer) were derived from partial melting of the Neoproterozoic Yangtze basement, while reduced granitoids with A/CNK 1.1 (FMQ − 2.50 to − 0.17) formed through mixing between the Neoproterozoic Yangtze basement and Paleo-Tethyan metasedimentary components. The reduced granitoids with A/CNK ≥ 1.1 (FMQ − 4.75 to − 1.06), closely linked to lithium mineralization, formed through anatexis of Paleo-Tethyan metasedimentary rocks at high temperatures (~700–850°C), as constrained by Ti-in-zircon thermometry. These redox variations are mainly inherited from the source rather than controlled by magmatic differentiation. The Li- and rare-metal-rich metasedimentary sources, coupled with high temperatures and reduced conditions, provided favorable physicochemical conditions for Li enrichment during the late-stage evolution of granitic magmas. This study provides new constraints on the magmatic redox evolution in contemporaneous diverse granitoid magmas and highlights the contribution of a metasedimentary source to lithium enrichment, shedding light on the broader geodynamic evolution of the northeastern Tibetan Plateau.