ObjectiveThe Qinling Orogenic Belt, positioned between the Yangtze and North China cratons, has undergone a multi-stage evolution from the Paleozoic to the Early Mesozoic, fully documenting the collisional orogenic history between both cratons. Substantial research achievements have been accumulated in fields such as provenance, lithogeochemistry, magmatic activity, and tectonics. However, key issues regarding the Early Paleozoic tectonic framework and Late Paleozoic ocean basin evolution of the Qinling Orogenic Belt remain controversial, primarily including the following aspects: The Shangdan Ocean may have closed during the Early Silurian, Carboniferous, or Indosinian periods; the subduction of the Mianlue Ocean may have initiated in the Early Carboniferous, Late Permian, end of the Late Permian, or later than the Early Triassic. The Zhaishang area in Minxian County, located within the Western Qinling Orogenic Belt, shows extensive Paleozoic strata and serves as a critical window for studying the tectonic evolution history of the Qinling Orogenic Belt.
MethodsSystematic sampling was conducted on the exposed Devonian, Carboniferous, and Permian strata in this area. The procedures included zircon single-mineral separation, target preparation, polishing, and cathodoluminescence (CL) imaging, followed by zircon U–Pb isotope analysis using LA–ICP–MS (Laser Ablation Inductively Coupled Plasma Mass Spectrometry). Based on these experimental results, detrital zircon U–Pb chronology was performed for the Devonian, Carboniferous, and Permian strata exposed in the Zhaishang area of Minxian County, Western Qinling, to constrain the Paleozoic tectonic evolution history and sedimentary processes of the Qinling Orogenic Belt.
ResultsResults of detrital zircon U-Pb chronology show that the Shuanglanggou Formation of the Xihan Shui Group and the Badu Formation in the Zhaishang area of Minxian County exhibit similar detrital zircon age compositions. Detrital zircons from the Shuanglanggou Formation (Devonian Xihanshui Group) are characterized by the dominant age peak at 794 Ma, belonging to a Neoproterozoic age group (880~746 Ma), and the youngest age peak at 448 Ma. Detrital zircons from the Lower Carboniferous Badu Formation display Neoproterozoic age groups (901~750 Ma), the dominant age peak at 818 Ma, and the youngest age peak at 390 Ma. Detrital zircons from the middle member of the Lower Permian Shilidun Formation exhibit the dominant and likewise youngest age peak at 443 Ma, belonging to an Early Paleozoic age group (464~409 Ma).
ConclusionThrough comparing the relative probability distribution curves of detrital zircon U–Pb ages of the Devonian, Carboniferous, and Permian strata, as well as the correlation diagrams between detrital zircon age peaks and tectonic settings (including the northern Qinling Orogenic Belt, North China Craton, and Yangtze Craton as comparison targets), it is revealed that: The Devonian Shuanglanggou Formation and the Carboniferous Badu Formation in the Zhaishang area share a consistent detrital provenance, primarily the Yangtze Craton, followed by the northern Qinling Orogenic Belt; both tectonic settings are classified as rift basins. The detrital materials of the middle member of the Permian Shilidun Formation stem predominantly from the northern Qinling Orogenic Belt and correspond to a back-arc basin tectonic setting. By integrating the research results with the relative positions of the Qinling Orogenic Belt, the North China Craton, the Yangtze Craton, the Shangdan Ocean, and the Mianlue Ocean, it is concluded that the Zhaishang area in Minxian County was in a rift basin environment from the Late Devonian to the Early Carboniferous, and transitioned to a back-arc basin in the Early Permian. This transition marks the completion of the Mianlue Ocean’s evolution from oceanic expansion to subduction and demise. Additionally, the study constrains that: The Mianlue Ocean opened after the Early Carboniferous; the Shangdan Ocean closed before the Late Devonian. Significance This study provides new evidence and chronological constraints for the tectonic evolution of the Qinling Orogenic Belt, as well as the closure time of the Shangdan Ocean and the initial subduction time of the Mianlue Ocean, helping to reconstruct and restore the tectonic evolution process of the Qinling Orogenic Belt.
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