U-Pb ages of detrital zircons from Taiyangsi Rock Group in Shujiba area of West Qinling Mountains and their geological significance

Abstract：With clastic rocks of Taiyangsi Rock Group in Shujiaba area of West Qinling Mountains as the research object, the authors used the LA-ICP-MS zircon U-Pb isotopic geochronologic methods to study the formation age and material source of Taiyangsi Rock Group in Shujiaba area. The results show that the lower limit of the deposition of Taiyangsi Rock Group is (437±4)Ma, suggesting that it was formed during the Silurian, as also evidenced by the previous research on the surrounding geological setting. Detrital zircon age spectra of Taiyangsi Rock Group in Shujiaba area can be clearly divided into four age segments: (1) 437-526 Ma, with the peak value of 458 Ma, characterized by relatively concentrated ages and a strong peak, indicating that the source might have come from the northern margin of West Qinling tectonic zone and North Qilian orogenic belt; (2) 601-978 Ma, which can be divided into two subgroups of 601-651 Ma and 738-978 Ma, with the peak values of 650 Ma and 814 Ma, assuming concentrated ages. During this period, due to the influence of the aggregation and breakup of the Neoproterozoic super-continent Rodinia, a series of orogenic magmatic activities occured in the Qinling and Qilian orogenic belts, which resulted in a possibility for providing source materials for the area; (3) 1032-1245 Ma, with the peak value of 1092 Ma, suggesting that the material source was probably mainly derived from the North Qilian orogenic belt; (4) 1668-2546 Ma, which can be divided into two subgroups of 1668- 2019 Ma and 2304-2546 Ma, with the peak values being 1920 Ma and 2418 Ma respectively, implying that the crystalline basement of both the northern margin of West Qinling tectonic zone and the North Qilian tectonic belt and the crystalline basement of the North China plate were present in the study area to provide the possibility of provenance. A comprehensive analysis shows that the material sources of Taiyangsi Rock Group is much more complicated and characterized by obvious diversity. Its material sources might have come 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 Taiyangsi Rock Group, with the Qilian orogenic belt playing the more dominant role.