K2O prograde zoning pattern in clinopyroxene from the Kokchetav diamond-grade metamorphic rocks: Missing part of metamorphic history and location of second critical end point for calc-silicate system

► We report the finding of potassium-bearing clinopyroxene with increase of K2O from core to rim zone. ► Prograde stage of metamorphism is deducted for diamond-grade metamorphic rocks. ► P-T parameters of second critical end point are estimated for calc-silicate system.