Paleozoic Tectonic Setting and Paleogeographic Evolution of the Qin‐Fang Region, Southern South China Block: Detrital Zircon U‐Pb Geochronological and Hf Isotopic Constraints

Abstract The thick Silurian siliciclastic sedimentary rocks in the Qin‐Fang region contrasting with their absence in surrounding regions have long been an enigma for the early Paleozoic geology of South China. Here we employ detrital zircon U‐Pb geochronology and Lu‐Hf isotope analysis of Cambrian to Devonian samples from the Qin‐Fang and adjacent regions to trace sedimentary provenance and to further constrain its tectonic setting. The similar age spectra of the Cambrian samples from both sides of the Qin‐Fang region imply that they should be derived from a common source(s), arguing against the presence of a Cambrian ocean in the Qin‐Fang region. However, the U‐Pb age patterns of these Cambrian samples, as well as from Silurian and Ordovician sandstones both from within and outside the Qin‐Fang region, differ significantly from those of a middle Devonian sample from the Qin‐Fang region, and Ordovician to upper Triassic samples from the Indochina Block. These results, in conjunction with regional geological information, indicate that the Yunkai Mountain, which is located adjacent to the Qin‐Fang region, must have been emerged above sea level in the middle Devonian. Meanwhile, it can also imply that the Qin‐Fang region should be a part of South China rather than being a segment of the Indochina Block. A two‐stage evolutionary history of the Qin‐Fang region is proposed, (1) an intracontinental basin mainly filled by materials eroded from east Gondwana and/or eastern Cathaysia Block during the early Paleozoic and (2) a foreland basin in the Yunkai Mountain region at least since the middle Devonian.