U–Pb zircon geochronology, geochemistry, and Sr–Nd–Hf–O isotopic study of Middle Neoproterozoic magmatic rocks in the Kangdian Rift, South China: slab rollback and backarc extension at the northwestern edge of the Rodinia

Abstract The Neoproterozoic tectonic setting of the Yangtze Block in present-day southern China and its paleogeographic location in the Rodinia supercontinent remain enigmatic. In this contribution, we report Middle Neoproterozoic diabases and rhyolites in the Kangdian Rift along the western margin of the Yangtze Block. The diabases (ca. 837 Ma) are tholeiitic and exhibit arc-like geochemical affinities (Nb–Ta–Ti depletions and elevated Th/Yb ratios). Isotoptic data (eHf(t) = −1.0 to +5.3; δ18O = 5.41‰ to 8.36‰; eNd(t) = +1.9 to +2.4) indicate that these diabases were probably derived from partial melting of subduction-modified lithospheric mantle. The rhyolites (ca. 839 and 836 Ma) mark the oldest A-type granitoids in the Kangdian Rift and have relatively small positive zircon eHf(t) (+2.6 to +8.1) and negative whole-rock eNd(t) (−0.7 and −2.2) values. Their zircon δ18O values (4.64‰ to 5.59‰) overlap but are a little lower than those of typical mantle-derived zircons. They were probably generated by partial melting of hydrothermally-altered Mesoproterozoic crust. Integrating previous studies with the data presented in this contribution, we interpret the Kangdian Rift to be formed by slab rollback and backarc extension in an oceanic subduction background. The Yangtze Block may be located at the northwestern edge of the Rodinia supercontinent in the Middle Neoproterozoic and closed to the Madagascar, Seychelles, and NE India.