Abstract: Tuff in stratigraphic sequences of sedimentary basins is a direct product of the orogenic activity of the source region,and is closely related to orogenic activity.The successive discovery of the Lower Jurassic tuff in Ordos Basin and its periphery is very important for constraining the key timing of tectonic evolution and tectonic background.In this paper,we determined the critical time of tuff eruption for the Lower Jurassic using zircon U-Pb geochronology.In conjunction with the tuff geochemistry in the key areas,we discussed the tectonic background for tuff eruption.The following results were obtained.The distribution of the Lower Jurassic tuff show obvious regional differences.They mainly occur in the Pingliang and Ankou counties in the southwestern Ordos Basin and in the Ningwu-Jingle Basin and Guangling Basin of the northeastern Ordos Basin,and are closely related to the evolution of the North Qinling Orogenic belt and the Yanshan Orogenic belt.The eruption of tuff is not completely synchronous in different regions.The tuff on Cedi section and Ankou section of the southwestern Ordos Basin are dated at 175.2 Ma±2.4 Ma and 168.4 Ma±1.3 Ma respectively,the volcaniclastic rocks on the Chenjiabangou section of Ningwu-Jingle Basin at 179.2 Ma±0.79 Ma,and the tuff on the Bantasi section of Hunyuan-Guangling Basin at 171.0 Ma±1.0 Ma.Overall,the eruption of tuff is date at the late Early Jurassic to early Middle Jurassic.On the Th/Hf-Ta/Hf diagrams,all the points fall into the continental rift zones or the initial rift zones,which indicates the extensional tectonic setting from the Early Jurassic to early Middle Jurassic in Ordos Basin and its periphery.The timing of tuff eruption may provide a direct chronological evidence for study of the Mesozoic thermal evolution of the Ordos Basin.

Key words: tuff, zircon U-Pb geochronology, geochemistry, Lower Jurassic, Ordos Basin