Initial separation of the South Qinling Terrane from the Yangtze Block during the Ediacaran: Insights from sequence correlation and zircon Hf isotope of tuff

Abstract

The seldom investigated Ediacaran strata in the South Qinling Orogen contain precious sedimentary records that can provide critical insights into the tectonic evolution of the South China Block during the late Neoproterozoic. Here, we present U-Pb dating and Hf isotopic analyses on zircons obtained from a tuff layer of the Heyu section in the South Qinling Orogen. U- Pb dating of zircons gave concordia ages of 633.4 ± 3.1 Ma and 631.4 ± 3.9 Ma (by LA-ICP-MS and SHRIMP, respectively), which are in good agreement with published dates of Ediacaran Cap carbonate from the Yangtze Platform and other regions worldwide. Among the zircons group dated by LA-ICP-MS, 28 grains had markedly positive εHf (t) values ranging from 7.8 to 14.4 with a weighted mean of 11.3 ± 0.8, implying that they were derived from depleted mantle. The zircon εHf (t) values were very similar to those of contemporaneous mafic-ultramafic intrusion swarms and A-type granites in the South Qinling Orogen, suggesting an intimate provenance affinity to Yaolinghe Group volcanic rocks. This indicates that the South Qinling Terrane was dominated by an intra-plate rift setting during the late Neoproterozoic. The Ediacaran succession in the Heyu section is comparable to that in the Yangtze Platform, comprising an initially deepening- and then shallowing-upward sequence in the lower part, and a shallowing-upward sequence in the upper part. The South Qinling Terrane and Yangtze Platform had similar shallow-water environments during the early Ediacaran but evolved in different directions during the late Ediacaran with a relative stability in the Yangtze Platform and a deep-water basin environment in the South Qinling Terrane. It is proposed that the South Qinling Terrane experienced intense extension during the Ediacaran, likely in response to the final breakup of the Rodinia supercontinent or slab pull induced by subduction during the assembly of Gondwana.