Evidence for continental rifting from two episodes of mid-Neoproterozoic silicic magmatism in the northeastern Yangtze Block, China

Abstract Mid-Neoproterozoic (850–720 Ma) magmatism around the periphery of the Yangtze Block records an intense extensional event related to the breakup of Rodinia. However, the detailed geodynamic mechanisms of this extensional tectonism are still poorly constrained. To address this issue, we undertook a whole-rock geochemical and Sr–Nd isotopic, and zircon U–Pb–Hf–O isotopic study of mid-Neoproterozoic silicic meta-igneous rocks from the Feidong Complex and Zhangbaling Group in the northeastern Yangtze Block. These rocks can be classified into two groups: Group A (granitoids) and Group B (volcanic rocks). Zircon U–Pb dating revealed that Group A (808–786 Ma) pre-dates Group B (762–750 Ma). All these rocks are characterized by A-type granite features, with high heavy REE, Zr, and Hf contents, (K2O + Na2O)/CaO ratios, Fe index values, and zircon saturation temperatures, and formed during a mid-Neoproterozoic extensional event. However, the extremely enriched whole-rock Sr–Nd and zircon Hf isotopic compositions (ISr = 0.7024–0.7068, eNd(t) = −19.3 to −14.1, and eHf(t) = −25.7 to −11.8) of Group A suggest derivation by reworking of ancient crustal basement. Group B is characterized by relatively depleted Sr–Nd–Hf isotopic compositions (ISr = 0.7015–0.7063, eNd(t) = −1.6 to +3.6, and eHf(t) = −4.8 to +4.1), indicating a significant contribution from juvenile-crust-derived material. Group A contains zircons with δ18O = 4.1‰–7.1‰, whereas Group B contains zircons with δ18O = 1.6‰–3.1‰. These chemical and isotopic data, along with the gradual change in residual mineralogy from garnet to plagioclase (Group A to B), indicate that magmatism transitioned from an ancient lower crustal source to a juvenile middle–upper crustal source that had experienced intense high-temperature meteoric water–rock interactions. Our results provide new insights into the tectonic evolution from early extension to peak rifting driven by upwelling asthenosphere, due to rifting of the Yangtze Block from Rodinia during the mid-Neoproterozoic.