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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.

Partial melting of ultrahigh‐pressure (UHP) metamorphic rocks is common during collisional orogenesis and post‐collisional reworking, indicating that determining the timing and processes involved in this partial melting can provide insights into the tectonic evolution of collisional orogens. This study presents the results of a combined whole‐rock geochemical and zirconological study of migmatites from the Sulu orogen in eastern China. These data provide evidence of multiple episodes of crustal anatexis and geochemical differentiation within the UHP metamorphic rocks. The leucosomes contain higher concentrations of Ba and K and lower concentrations of the rare earth elements (REE), Th and Y, than associated melanosomes and granitic gneisses. The leucosomes also have homogenous Sr–Nd–O isotopic compositions that are similar to proximal (i.e. within the same outcrop) melanosomes, suggesting that the anatectic melts were generated by the partial melting of source rocks that are located within individual outcrops. The migmatites contain zircons with six different types of domains that can be categorized using differences in structures, trace element compositions, and U–Pb ages. Group I domains are relict magmatic zircons that yield middle Neoproterozoic U–Pb ages and contain high REE concentrations. Group II domains represent newly grown metamorphic zircons that formed at 230 ± 1 Ma during the collisional orogenesis. Groups III, IV, V, and VI zircons are newly grown anatectic zircons that formed at 222 ± 2 Ma, 215 ± 1 Ma, 177 ± 2 Ma, and 152 ± 2 Ma, respectively. The metamorphic zircons have higher Th/U and lower (Yb/Gd)N values, flat heavy REE (HREE) patterns with no significantly negative Eu anomalies relative to the anatectic zircons, which are characterized by low Th/U ratios, steep HREE patterns, and negative Eu anomalies. The first two episodes of crustal anatexis occurred during the Late Triassic at c. 222 Ma and c. 215 Ma as a result of phengite breakdown. The other two episodes of anatexis occurred during the Jurassic period at c. 177 Ma and c. 152 Ma and were associated with extensional collapse of the collision‐thickened orogen. The majority of Triassic anatectic zircons and all of the Jurassic zircons are located within the leucosomes, whereas the melanosomes are dominated by Triassic metamorphic zircons, suggesting that the leucosomes within the migmatites record more episodes of crustal anatexis. Both metamorphic and anatectic zircons have elevated εHf(t) values compared with relict magmatic zircon cores, suggesting that these zircons contain non‐zircon Hf derived from material with more radiogenic Hf isotope compositions. Therefore, the Sulu and Dabie orogens experienced different episodes of reworking during the exhumation and post‐collisional stages.

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