[Objective]In the Early Permian,Junggar Basin was an intracontinental successive rift basin formed af-ter the closure of the Paleo-Asian Ocean. During the syn-rifting stage,the most important source rock,the Lower Permian Fengcheng Formation alkaline lake source rock,filled in the Mahu Sag on the northwestern margin of the Junggar Basin,but the exact depositional age remains unclear.[Methods]A detailed petrological analysis of a piece of tuffaceous lithic sandstone was conducted by scanning electron microscopy and energy dispersive analysis.[Results and Conclusions]It showed that the sample contained a large amount of tuffaceous clasts and volcanic glass (52％),intermediate-felsic lithic clasts (38％),and a small amount of quartz,feldspar,and hornblende minerals (10％). The published LA-ICP-MS zircon U-Pb dating of this sample shows that the average age of the five youngest detrital zircons and the youngest age peak are consistent,with an age of 282±4 Ma,suggesting that the Early Permian syn-sedimentary volcanic rocks are the major source area,and the mountains in west Junggar and the Zhongguai-Luliang uplifts also have been source areas. Combined with the published zircon U-Pb results of two tuffaceous samples at the bottom and top of the alkaline lake deposition,the average ages of the five youngest detrital zircons were 284±4 and 279±4 Ma,respectively. The age gradually becomes younger from the bottom to top,which is the syn-sedimentary record of episodic volcanic eruptions in the syn-rift stage of the basin,indicating that the sedimen-tary age of the Lower Permian Fengcheng Formation alkaline lake is 284-279 Ma.
Fengcheng Formationdepositional agezircon U-Pb datingalkaline lakeLower PermianMahu Sag
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