Sedimentary records reveal an early Cambrian active margin setting in the NW Yangtze Block, South China

The Ediacaran to Cambrian sedimentary succession in the Yangtze Block (South China) has long been interpreted as forming in a passive margin setting. However, some recent studies on Cambrian geological records challenge this view. Here, we investigated a relatively complete Cambrian section in the central Longmenshan fold-thrust belt, NW Yangtze Block. Composed primarily of fine-grained siliciclastic rocks with subordinate carbonate rocks, this section was analyzed to constrain its depositional age, sedimentary environments, provenance and tectonic setting, and to address the ongoing controversies. The Cambrian succession exhibits coarsening-upward trends and indicates deep-water basin, slope and fan delta settings with hydrothermal inputs. The depositional ages were constrained as the Cambrian Terreneuvian to Epoch 2 interval (ca. 524–509 Ma), based on correlation of carbonate stable carbon isotope (δ¹³C) results with the global Cambrian δ¹³C reference curve and maximum depositional age constraints from detrital zircon U-Pb geochronology. Sandstones from the Qiongzhusi Formation have abundant volcanic lithic fragments and detrital zircons therein are dominated by Ediacaran–Cambrian ages. Nevertheless, sandstones from the Canglangpu Formation consist of various lithic fragments (including chert, metamorphic, volcanic and sedimentary clasts) and present diverse detrital zircon U-Pb ages (predominant 1000–500 Ma with minor 2600–2100 Ma). Both petrographic and geochemical results reveal that the Cambrian sediments are compositionally immature and were derived from proximal sources. Detrital zircon (635–500 Ma) trace elemental compositions denote a continental arc origin. Collectively, these results support proximal arc-related orogenic sources for the early Cambrian clastic sediments, contradicting previous proposals of distal orogenic sources. We propose that the Cambrian depositional succession was formed in an active margin setting in response to the Proto-Tethys Ocean subduction beneath the NW Yangtze Block. Our findings represent a significant advance towards understanding the Ediacaran–Cambrian transition and the role of the South China Craton in the Gondwana assembly process.