Late Ordovician Tectono-Sedimentary Evolution of the Changma Area, Forearc Basin in the Western North Qilian Orogen

The paleogeography and tectonic evolution history of the Early Paleozoic archipelago-type North Qilian Ocean remain controversial. A set of Early Paleozoic deep-water clastic-volcanic succussion are well exposed in the Changma Area between the north and south ophiolitic belt of the North Qilian Orogen. Previous geologic mappings generally assigned a Cambrian to Early Ordovician stratigraphic age for these strata. In this study, we conducted systemic field-based investigations on outcrops in the Yingzuishan and Chelugou Section in the western Changma, and conducted zircon LA-ICP-MS U-Pb datings on collected turbidites and volcanics. Geochronologic results indicate that the sandstone sample 2307NQL-13 from the previously termed Cambrian Number a unit of the Yingzuishan Section yielded a well-defined weighted mean age of (456±4) Ma for the youngest detrital zircons. An andesite sample 2307NQL-06 from the previously termed Early Ordovician Yingou Formation yielded a well-defined weighted mean age of (450±4) Ma. These new data solidly demonstrated the existence of Late Ordovician sedimentary strata in the Changma Area, requiring further consideration of previous mapping proposals. Detrital zircons from sandstones are mostly of Cambrian-Ordovician ages, indicating major sedimentary derivations form newly-formed magmatic arcs. Appearance of 32 Neo- to Paleoproterozoic aged zircons imply additional supply form metamorphic basement complexes, revealing depositor of detritus from the Central Qilian. This clue hints at that the southern branch of the North Qilian Ocean should have been closed in Late Ordovician to allow sedimentary materials from the Central Qilian to be transported to and deposited in the Changma Area. The subduction-related depositional setting indicates that ongoing subduction of the oceanic crust of the northern branch should sustain until then. These clues provide new geochronologic data and related observations on stratigraphy and sedimentary evolution history of the Changma Area, and yield further constraints on the composite Early Paleozoic subduction-closure history of the North Qilian Ocean.