Abstract: A suite of intermediate volcanic rocks in the Jarud Qi area of Inner Mongolia was defined as the Permian Dashizhai Formation and Qingfengshan Formation by previous researchers, but they were lack of paleontological and dating data. In the recent 1:250 000 regional geological survey, the isotopic dating samples are collected from the andesites of Dashizhai and Qingfengshan Formations defined in the former 1:200 000 geological survey. The chronology of volcanic rock samples is studied based on the laser ⁴⁰Ar/³⁹Ar dating method. The result shows that the age is(172.7±2.0)-(165.2±1.3) Ma. In consideration of the clear characteristics of stratigraphic rock assemblages, sufficient time basis and certain extensibility and mapping possibility, this suite of volcanic strata is redefined as the Middle Jurassic Tamulangou Formation.