Amphibole fractionation as a dual filter in arc crust maturation: Evidence from a zoned intrusive complex in the Gangdese batholith, Tibet

Amphibole is one of the most important rock-forming minerals in arc magmatic differentiation; however, the mechanisms through which amphibole governs arc magma maturation remain debated. A critical unresolved question is whether the formation of amphibole-rich cumulates can simultaneously drive both the chemical shift toward felsic compositions and the progressive oxidation of residual arc magmas. The Late Triassic (ca. 210 Ma) Qinu intrusive complex in the central Gangdese batholith provides an ideal case study to address this issue. This complex constitutes a cogenetic suite that ranges from amphibole-rich hornblendites and gabbros to granodiorites and granites and displays a vertical zonation characterized by upward-decreasing amphibole content. We conducted integrated petrological, geochronological, and geochemical analyses of these intrusive rocks with variable amphibole abundances. Our study demonstrates that the amphibole-rich rocks are cumulates formed mainly by accumulation of primary magmatic amphibole, whereas the granitoids crystallized from complementary residual melts. Whole-rock geochemical trends and calculated amphibole-equilibrium melts further support the role of amphibole accumulation in removing mafic components and driving melt evolution toward felsic compositions. Furthermore, zircon oxybarometry reveals a systematic increase in magmatic oxygen fugacity from the cumulates to the granitoids, indicating progressive oxidation of residual melts during differentiation. We therefore propose that amphibole fractionation acts as a dual filter: a redox filter that elevates the fO2 of evolving melts and a geochemical filter that controls the compositional evolution toward felsic magmas. This study highlights amphibole as an active mineralogical filter that regulates both the compositional maturation and redox evolution of continental arc crust.