The 40Ar/39Ar dating of metamorphic schists from the basement of the West Siberian Platform exposed by Lenzitskaya well no. 77 on the Taz Peninsula was performed. The samples collected at depths of 3503 and 3515 m yielded age dates of 251.6 ± 3.5 and 271.2 ± 3.5 Ma. The age of ~271 Ma reflects the age of metamorphism of schists, which was induced by a strong tectonomagmatic event, associated with the onset of emplacement of rift zones and outflow of basalts in the Permian–Triassic. The age of ~252 Ma coincides with the period of the most intense emplacement of submeridional rifts in the basement of the West Siberian Platform. This process was accompanied by extension and thinning of the Earth’s crust in this region, which resulted in the exhumation of rocks from the mid-crystal level. The age boundary of ~250 Ma was established in the metamorphic and intrusive complexes of western Siberia and the Urals. This age is fixed almost everywhere and is correlated with the age of formation of the Siberian Trap Province.