Subduction Retreating Caused the External Breakup of Rodinia: Constraints from the Neoproterozoic Igneous Rocks in the Western Yangtze Block, South China

The western Yangtze Block is an ideal place for investigating the Precambrian tectonic evolution of the South China Craton and the geodynamic mechanism responsible for the breakup of Rodinia. However, the Neoproterozoic tectonic evolution of the western Yangtze Block has not been fully elucidated; in particular, the driving mechanism has changed from an arc setting to a rift setting. Here, we present new zircon U-Pb geochronology and O-Hf isotopic compositions, whole-rock geochemistry, and Sr-Nd isotope data for Yanbian andesites and Shimian gabbros from the western Yangtze Block, South China. The ca. 850-840 Ma Yanbian andesites are high-Mg andesites with moderate SiO2 (56.75-60.21 wt%) and high MgO (4.24-6.44 wt%) and Mg# (52-66). These rocks show enrichments in large-ion lithophile elements (LILEs; e.g., Rb, Ba, Sr, and K) and light rare earth elements (LREEs) and depletions in high-field strength elements (HFSEs; e.g., Nb, Ta and Ti) and display decoupled Nd-Hf isotopes (epsilon Nd(t) _ +3.9 to + 5.0, epsilon Hf(t) _ +9.7 to + 13.1) and relatively high zircon 518O values (5.82-7.72 %o); these findings indicate that these rocks were derived from a mantle wedge enriched by slab fluids and sediment melts. The ca. 820-810 Ma Shimian calc-alkaline gabbros are characterized by enriched LREEs and LILEs (e.g., Rb, Sr, and K) and depleted HFSEs (e.g., Nb, Ta and Ti). These rocks show decoupled Nd-Hf isotopes (epsilon Nd(t) _ +3.2 to + 4.2, epsilon Hf(t) _ +3.1 to + 7.5) and relatively low zircon 518O values (4.28-5.31 %o), indicating that they were formed by partial melting of mantle wedge metasomatized by sediment and oceanic slab melts. Compiled geochronological data suggest that Neoproterozoic subduction-related magmatism along the western Yangtze Block formed a long-term active arc system. Slab retreat in this subduction zone led to a shift in the tectonic setting from a compressional setting to an extensional setting in the Yangtze Block during the middle to late Neoproterozoic. Moreover, the temporal link between retreating subduction zone and extensional tectonics in the Yangtze Block suggests that protracted subduction retreating resulted in external rifting in Rodinia.