The mechanism of crustal recycling in subduction zones has been a heated debate, and Mg–Fe isotopes may provide new constraints for this debate. This study reported the Fe–Mg isotope data for mafic plutonic rocks from the eastern and central Gangdese arc and their associated trench sediments in southern Tibet. The δ26Mg (–0.32 to –0.20‰) and δ56Fe (0.04 to 0.12‰) values of the eastern Gangdese arc rocks show negative and positive correlations with (87Sr/86Sr)i and (206Pb/204Pb)i values, but positive and negative correlations with εNd(t) and εHf(t) values, respectively. The Mg and Fe isotopic compositions (δ26Mg = –0.28 to –0.15‰; δ56Fe = 0.02 to 0.12‰) of the central Gangdese arc rocks are comparable with the eastern ones, but they are not covariant with Sr–Pb–Nd–Hf isotopes. More importantly, the Fe–Mg isotopes for most of the arc rocks fall in between local trench sediments (δ26Mg = –0.61 to –0.30‰; δ56Fe = 0.00 to 0.17‰) and the normal mantle. Integrated qualitative analyses and quantitative simulations suggest that while the Mg–Fe isotope variations in the eastern Gangdese arc rocks revealed the important role of source mixing between sediment-derived melts and peridotite, their variations in the central Gangdese arc rocks reflected the controlling effects of source mixing between carbonated serpentinite-derived Mg-rich fluid and peridotite and source melting. The good covariant relationships between Mg–Fe isotope and traditional geochemical tracers provide further evidence for the recycling of crustal materials in subduction zones via various types of slab-derived fluids and melts.