A study of the material source of Dacaotan Group in the northern margin of West Qinling orogenic belt: LA-ICP-MS U-Th-Pb age evidence of detrital zircons

With clastic rocks of Late Devonian Dacaotan Group on the north margin of West Qinling orogenic belt in eastern Gansu as the study object, the authors used LA-ICP-MS zircon U-Pb isotope geochronologic method to explore the material source of Dacaotan Group. The results show that the age spectra of Dacaotan Group detrital zircon can be evidently divided into four groups：422- 518Ma, 756-887Ma, 1100-1231Ma and 2417-2736Ma, of which, the 422-486Ma age data possess the largest proportion, accounting for about 67% of the total data, having relatively concentrated ages, and showing a strong peak. The other ages possess a smaller proportion. The group of 422-518Ma indicates that its material sources were mainly from northern margin of western Qinling orogenic belt and Qilian orogenic belt in adjacent regions. The compositional ages suggest a sedimentary response to the orogenic erosion of a series of igneous rocks caused by the northward subduction and collision of mid-southern Qilian and western-Qinling micro-blocks in the Caledonian period. Detrital zircon U-Pb isotopic age group of 756-887Ma suggests that the material sources might have mainly come from the Qilian orogenic belt and northern margin of western Qinling orogenic belt. The age group of 1100-1231Ma implies that the material sources might have come from Qilian orogenic belt, and 2417-2736Ma age group indicates that the material sources were probably derived from the basement of north Qilian orogenic belt and northern margin of western Qinling orogenic belt, with part of the material sources probably derived from the North China plate basement. A comprehensive analysis shows that the material sources of Dacaotan Group were much more complicated and characterized by obvious diversity, derived from the northern margin of western Qinling orogenic belt, Qilian orogenic belt and the basement of North China plate. The Qilian orogenic belt and the northern margin of western Qinling orogenic belt made significant source contribution to the sedimentation of Dacaotan Group, with the Qilian orogenic belt possessing the dominant role.