The Paleozoic subduction-splicing-extension process of the North China and South China Blocks: Constraints from sedimentary provenance of the Devonian Liuling Group and surrounding strata in the South Qinling Belt, China

Much controversy exists regarding the timing of the Paleozoic amalgamation between the North China Block (NCB) and South China Block (SCB). The northern region of South Qinling belt (N-SQB) provides massive information about the Paleozoic subduction-splicing-extension process of the NCB and SCB, in which the volcanic-sedimentary successions of fore-arc sedimentary system (FAS) and the sedimentary successions of Devonian Liuling Group (DLLG) provide an ideal location to constrain the closure time of the Shangdan Ocean. Systematic field-based sedimentology, sandstone model composition, whole-rock elemental composition, and detrital zircon U-Pb dating were employed in this study. Our results reveal that there are four types of sedimentary systems, including wave-dominated littoral, tide-dominated littoral, storm-dominated epeiric sea shelf, and abyssal-bathyal systems in DLLG, and the sedimentary sequence indicates that the water depth during the deposition of DLLG deepened and then shallowed, which are the characteristic of an extensional basin. The FAS, DLLG and Carboniferous of N-SQB, and Devonian of the middle region of South Qinling belt (M-SQB), mainly consist of sedimentary tuff and lithic sandstone, quartz sandstone and lithic sandstone, and feldspathic quartz sandstone, in which the rock debris is mainly volcanic and sedimentary rock debris, metamorphic quartz and sedimentary rock debris, and sedimentary rock debris, respectively, indicating that the source material comes from the magmatic rock material of the recycled orogenic belt, basement material of the recycled orogenic belt, and craton area, respectively. The whole-rock elemental composition results suggest that the tectonic setting of the provenance of the N-SQB were continental island arc and active continental margin, whereas the tectonic setting of the Devonian of M-SQB and passive continental margin. LA-ICP–MS detrital zircon U–Pb dating analysis shows that the provenances of the N-SQB were the North Qinling belt (NQB) and NCB, whereas the provenances of the Devonian strata of the M-SQB were the South Qinling belt (SQB) and SCB. An age population with single peak at 420 Ma from andesitic lithic tuff in the FAS suggests that the island arc existed at 420 Ma, hinting the subduction did not finish and the Shangdan Ocean did not close until Late Silurian. The DLLG sediments were mainly sourced from the basement material of the NQB, indicating that the Shangdan Ocean closed before the Middle Devonian, which implies that the NQB and SQB were spliced in Early Devonian. This fast transition from subduction to extension and the absence of molasse hint that no full orogeny occurred in the splicing process. Therefore, we propose that the Liuling Group formed in a post-splicing extensional rift basin.