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 pa-per,we determined the critical time of tuff eruption for the Lower Jurassic using zircon U-Pb geochronology .In conjunc-tion with the tuff geochemistry in the key areas ,we discussed the tectonic background for tuff eruption .The following re-sults 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 volca-niclastic 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 e-volution of the Ordos Basin .%沉积盆地地层序列中的凝灰岩是源区造山活动的直接产物，与造山带的活动性密切相关。鄂尔多斯及周缘盆地下侏罗统凝灰岩的相继发现，对于探讨早侏罗世构造演化的关键时限和大地构造背景提供了证据。以凝灰岩的锆石U-Pb年代学为线索，厘定了早侏罗世凝灰岩喷发的期次及关键时限，同时辅助以重点区域凝灰岩的地球化学特征，探讨了凝灰岩喷发的大地构造背景。研究认为：①下侏罗统凝灰岩分布具有明显的区域性，集中分布在鄂尔多斯盆地西南缘平凉、安口一带，鄂尔多斯盆地东北缘的宁武-静乐盆地及广灵盆地内，与北秦岭造山带和燕山造山带的演化密切相关；②不同地区凝灰岩的喷发时限并不完全具有等时性，鄂尔多斯西南缘平凉策底、安口地区凝灰岩的时代分别为175.2 Ma ±2.4 Ma和168.4 Ma ±1.3 Ma，宁武-静乐盆地陈家半沟剖面火山碎屑岩的时代为179.2 Ma ±0.79 Ma，浑源-广灵盆地板塔寺剖面凝灰岩的年龄为171 Ma ±1.0 Ma，总体上处于早侏罗世晚期至中侏罗世早期；③在凝灰岩微量元素Th/Hf-Ta/Hf构造环境判别图解中，所有投点全部落入了大陆裂谷带或者初始裂谷玄武岩区，反映了早侏罗世至中侏罗世早期鄂尔多斯及周缘地区整体上处于陆内拉张的构造背景。凝灰岩喷发时限的确定，也为鄂尔多斯盆地中生代热演化年代学的研究提供了直接的证据。
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