Early Neoproterozoic accretionary assemblage in the Cathaysia Block : Geochronological, Lu-Hf isotopic and geochemical evidence from granitoid gneisses

The South China Block (SCB) is composed of the Yangtze Block in the northwest and the Cathysia Block in the southeast. The earliest Neoproterozoic granitic rocks in the SCB are important for our understanding of the tectonic regime and location of this ancient continent in relation to the Rodinia supercontinent. However, such rocks have rarely been reported for the Cathaysia Block so far. In this study, six granitic gneisses from the Wuyi-Yunkai domain gave zircon U-Pb ages of 985-913Ma, indicating the presence of the early Neoproterozoic granitic magmatism in the Cathaysia interior. These granitic gneisses are peraluminous with high FeOt, MgO, TiO₂ and CaO/Na₂O but low SiO₂, Al₂O₃/TiO₂, Al₂O₃/(MgO+FeOt) and Rb/Sr ratios. They show steep REE chondrite-normalized patterns with δEu values of 0.35-0.70. These samples have strongly negative Ba, Sr, Nb-Ta, P and Ti anomalies in primitive mantle-normalized diagrams, and e{open}Nd(t) values of -9.54 to -3.81, similar to those of the Kwangsian granitic rocks and Precambrian volcanosedimentary package in the Cathysia Block. The zircon grains with the earliest Neoproterozoic age gave e{open}Hf(t) values of -9.64 to +2.78 with the peak values at -2.5 and -7.5, and Hf model ages of 1.35-2.00Ga with the two-peaks of 1.57Ga and 1.75Ga, respectively. These data suggest that the granitic gneisses originated dominantly from a binary mixing of metapelitic and metaigneous components, with insignificant input of juvenile materials. We consider that these rocks are closely related to the closure of the earliest Neoproterozoic (~980Ma) Wuyi-Yunkai back-arc basin in an accretionary continental margin setting. In combination with other data, it is proposed that the previously-defined Cathaysia block might consist of various sub-blocks separated by the arc-back-arc basin prior to the early Neoproterozoic period. The Wuyi-Yunkai arc-back-arc system was finally closed at ~920Ma, significantly younger than the classical Grenvillian Orogenic event, and thus the SCB was located on the margin of the Rodinia supercontinent.