New geochemical constraints on the development of active continental margin in Southeast China during the Middle Permian and its tectonic implications

Abstract

Two competing hypotheses that passive or active continental margins developed in South China during the Late Paleozoic are still under debate. Previous studies were mainly focused on the remnant magmatic rocks, but little attention has been paid to the contemporary sediments deposited in the back-arc basin. Therefore, a deeper knowledge of the tectonic link between the continental arc and regional basins is not only vital for understanding their tectono-sedimentary evolution, but also provide the potential to evaluate these hypotheses. Here two Middle Permian sections of bedded chert-mudstone sequence (Gufeng Formation) and an overlying siliciclastic succession (Yinping Formation) from the Lower Yangtze region, were analyzed to understand their provenance, tectonic and depositional setting, and to provide new constraints on the Late Paleozoic tectonics of the South China. Discrimination of tectonic setting indicates that the Middle Permian sedimental source rocks of the Lower Yangtze basin probably originated in a transitional setting from continental island arc to active continental margin, consistent with the development of an active margin magmatic arc along Southeast China during that time. Provenance analysis also showed that, the Gufeng Formation was mainly derived from an Andesitic arc source with mixed mafic and felsic source, whereas the Yinping Formation received detritus from a more felsic source. This provenance switching could reflect a possible tectonic transition related to the continental arcs’ evolution that from Andean-type normal/high-angle subduction to flat-slab subduction, which weakened the arc volcanism and intensified the bedrock uplift in South China. The depositional environments of the Lower Yangtze basin also evolved from an open deep ocean environment to a continental margin setting during the deposition of Gufeng Formation, and then transformed into a coastal/neritic shallow water setting during the deposition of Yinping Formation. This sedimentary evolution can be attributed to the tectonic transition during the Middle Permian.