Provenance of the siliciclastic rocks in the Lower Yangtze region: Implications for Paleozoic tectonic switching of the northeastern South China Block

Abstract

• Permian zircon U-Pb age spectra are different from those of Silurian and Devonian . • The Silurian-Devonian detritus was from the proximal igneous rocks. • Increased ca. 430 Ma zircons indicates the onset of foreland stage. • The Permian detritus was from the basement of the Cathaysia Block. • Foreland basin was replaced by intraplate basin in Late Paleozoic. Integrated analysis of zircon U-Pb dating, in-situ Lu-Hf and whole-rock geochemical data was performed on Silurian-Permian siliciclastic rocks from the Lower Yangtze region (LYZ), northeastern South China Block, to understand the tectono-sedimentary evolution during the Paleozoic. These rocks have a high chemical index of alteration (CIA = 71.3–91.8) and variable index of compositional variability (ICV = 0.2–3.5), indicating intensive weathering. The high Zr/Sc ratios (12.1–75.3) indicate that their sources contain recycled old sediments. The Silurian and Devonian samples contained abundant detrital zircons with ages of 1.0–0.8 Ga and 460–420 Ma. However, the Permian samples show bimodal age spectra with peaks at ∼ 1800 Ma and 280–260 Ma. Integrated with published geochronological data, the proportion of the 460–420 Ma zircons drastically increased from the Early to Mid-Silurian and decreased in the Early Carboniferous, indicating two tectonic transformations, respectively. Based on these variations and relative paleogeographic data, we suggest three evolution stages with different provenances for the LYZ: (1) from Cambrian to Ordovician, the LYZ region inherited the Cryogenian tectonic framework of South China Block, receiving detritus from East Gondwana. (2) from Silurian to Devonian widespread uplift in South China Block led to the development of an isolated foreland basin with independent sedimentary sources and (3) from Carboniferous to Permian, subduction of the Paleo-Pacific plate exposed the Paleoproterozoic basement of the Cathaysia Block, providing detritus for the LYZ intraplate basin.