U-Pb Zircon Ages and Geochemistry of the Metasedimentary Rocks from the Foping Area in the South Qinling Belt: Evidence for Early Devonian Amalgamation between North China and South China Blocks

The provenance and tectonic implications of Late Paleozoic sedimentary rocks in the South Qinling Belt (SQB) provide important clues for understanding the timing and mechanism of the collision between North China Block (NCB) and South China Block (SCB). Here we report new LA-ICP-MS zircon U-Pb ages and geochemical composition of metasedimentary rocks from the Foping area in the SQB. The results indicate that the depositional age of the Wenquan Group can be limited to the Early Devonian by the youngest U-Pb age of 398 Ma, whereas those of the quartz schist from the Changjiaoba Group could be constrained after the Carboniferous by the youngest Pb-206/U-238 peak age of 306 Ma. Therefore, much of the previously-assumed "Neoarchaean or Paleoproterozoic" strata, including the Wenquan and Changjiaoba groups, were actually deposited in the Late Paleozoic. Based on analysis and comparison of the detrital zircon ages of the Devonian tectonic units in the SQB, we found that most prominent peak ages of detrital zircons from metasedimentary rocks in the Foping area shared similar characteristics with others. They mainly derived from the North Qinling Belt and South Qinling Belt-Yangtze Block, with a minor source probably from the North China Block, implying that the Shangdan Ocean between NCB and SCB had closed in the Early Devonian. Combined with regional geological background and geochemical data, we inferred that these Devonian strata formed in the foreland basin after the amalgamation of the North China Block and South China Block.