Neoproterozoic Longtanhe Formartion, cropping out along the Dabashan Fault on the northern margin of Yangtze Plate in China, consists primarily of volcanogenic sedimentary rocks with primary volcaniclastic rock interlayers. The Longtanhe Formation and overlying sandstone-conglomerate succession (termed the Muzuo Formation) which were regarded as Sinian tillite, have been interpreted as the fills of a passive margin, or rifted basin on the northern margin of Yangtze Plate. The Longtanhe Formation composed of greywacke, pebble-bearing sandstone, conglomerate, tuffaceous siltstone and several (breccias-bearing) tuff interlayers。Sandstone detrital clasts analysis suggests a dominance of volcanic relative to sedimentary and metamorphic source rocks due to the high volcanic lithic clasts and feldspars concentrations. This observation is also supported by the high concentration of volcanic pebbles and volcanogenic clasts within the conglomerates. A small amount of quartz clasts also supports a minor contribution from felsic rocks. A conglomerate, primarily composed of volcanic rock-pebble, was studied for zircon LA-ICP-MS U-Pb dating. The results show two age populations with peaks at ~770 Ma and ~705 Ma, indicating that two periods of magmatism in the source area. Not only the breccias in tuff but also the pebbles, lithic and zircon grains in sandstones are characterized by poor rounding, suggesting that the Longtanhe Formation was deposited near-source. All these lines of evidence suggest that a ~705 Ma magmatic arc, primarily consisting of basic-intermediate volcanic rocks and volcanogenic sedimentary rocks, was the main source for the Longtanhe Formation. Contemporaneous arc magmatism was reported in Ankang area, Shannxi Province. Based on the source rocks’ island arc affinity, the Longtanhe Formation is proposed to have been deposited in a backarc setting, indicating that the subduction-related arc magmatism in North Daba Mountain, South Qinling Orogenic Belt had not ended before 705 Ma.