Coupled evolution of Neoproterozoic arc mafic magmatism and mantle wedge in the western margin of the South China Craton

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Subduction of oceanic slabs can physically and chemically modify mantle wedges, but how mantle wedges are temporally evolved is difficult to be constrained. In this study, we use in situ zircon U–Pb ages and Hf–O isotopes of mafic intrusions to examine a coupled evolution of magmas and mantle sources above a subduction zone. Neoproterozoic mafic intrusions in the western margin of the Yangtze Block are mostly composed of gabbros that were formed in an arc system during 870–750 Ma followed by generation of voluminous slab-derived granitoids. The mantle wedge was progressively modified by slab fluids, sediment melts and altered oceanic crust (AOC) melts. Gabbros from the 870-Ma intrusion have normal δ18O (4.79‰ to 6.07‰), high eHf (+ 10.4 to + 15.0) and eNd (+ 4.0 and + 6.4) and were derived from a mantle source enriched by slab fluids. Gabbros from the 860–840-Ma intrusions have overall relatively high δ18O (5.61‰ to 7.42‰), but variable eHf values (− 3.5 to + 15.0) that are decoupled from eNd (+ 1.85 to + 3.87). These features are clearly suggestive of a mantle source modified by sediment melts. However, gabbros from the 820–780-Ma intrusions have relatively low δ18O (4.22‰ to 5.49‰), and constant eHf (+ 4.7 to + 9.6) that are decoupled from eNd (− 0.52 to + 1.92). Such features can be explained by a mantle source contaminated by 18O-depleted AOC melts. The widespread younger TTG-type granitoids were partial melts of the subducted oceanic slab and terminated the mafic magmatism in the region, indicating that slab break-off probably resulted in slab melting after a long period of subduction. Similar Neoproterozoic magmatism also occurred in Greater India and Madagascar, suggesting a giant Andean-type arc system along the western margin of Rodinia. In this arc system, mantle-derived magmas were dominated by an arc affinity in the earlier stage before 820 Ma and became rift-related after that, reflecting regional slab tearing and break-off. Our study also provides evidence for the linkage between the marginal subduction and the internal rifting in Rodinia, and suggest that slab break-off probably triggered its break-up.