Sedimentary provenance analysis unravels the Early Cambrian orogeny in the NW Yangtze Block, South China

The Ediacaran to Cambrian transition witnessed a key interval in the Earth’s history for biological revolution, environmental change and tectonic evolution. Wells and seismic data show that a Lower Cambrian thick siliciclastic rock succession occurs in the NW Yangtze Block, South China. The provenance and tectonic setting for the thick Cambrian sedimentary successions are very crucial for understanding the Ediacaran-Cambrian transition. Here, a Cambrian outcrop section (ca. 800-m-thick) in the middle Longmenshan fold-thrust belt, NW Yangtze Block, which is composed of mixed carbonate-siliciclastic strata, was investigated to constrain depositional ages, sedimentary environments, provenance and tectonic settings, through integrating field-based sedimentology, petrography, whole-rock element geochemistry, detrital zircon U-Pb dating, zircon trace elements and Hf isotopes, and carbonate stable C isotope data. The sedimentary succession is thought to accumulate during the Terreneunian to Epoch 2 time (i.e., Age 2 to Age 4, ca. 524-509 Ma) by comparison of our new carbonate stable C isotopic data and the global Cambrian δ13C curve and by the detrital zircon U-Pb age evidence (the youngest age peaks ranging from 538 to 518 Ma). The investigated strata have coarsening-upward trends and indicate complex, variable sedimentary environments (including slope-basin, deep-water shelf and fan delta settings) with hydrothermal inputs. Sandstones from the lower Qiongzhusi Formation have abundant volcanic lithic fragments and detrital zircons therein are dominated by Ediacaran to Cambrian ages. However, sandstones from the upper Canglangpu Formation consist of variable lithic fragments (including chert, metamorphic, volcanic and sedimentary clasts) and indicate diverse detrital zircon U-Pb ages (1000-500 Ma and minor 2600-2100 Ma). Both geochemical data of fine-grained sedimentary rocks and petrographic data of the sandstones reveal the deposits were relatively immature and were derived from proximal sources. Additionally, trace elements of the zircon grains with 650–500 Ma ages indicate a continental arc origin. All the results point to nearby arc-related source terranes for the Cambrian clastic records, rather than distant orogen sources as previously proposed. The geological and geochemical evidence, combining published geophysical data, imply the development of an early Cambrian orogen northwest to the study area. We suggest that the underestimated arc settings were formed in response to the subduction of the Proto-Tethys ocean beneath the NW Yangtze Block, which resulted in continental collision and uplift of northwest microterranes that provided siliciclastic sediments to fill the foreland basin southeastward.