New indications of ultrahigh-pressure metamorphism in the mica schists of the Kulet site of the Kokchetav Massif (North Kazakhstan)

The first finding of coesite in the garnet-mica schist of the Kulet site of the Kokchetav Massif is described. This fact indicates that the upper eclo gite-bearing unit of the Kokchetav Massif was metamorphosed at pressures hi gher than 26 kbar. The estimated equilibrium temperatures for eclogites and garnet-amphibole-zoisitic and garnet-kyanite-biotite-amphibolic rocks show that the temperature of the high-pressure stage of metamorphism did not ex ceed 650 degrees C. Taking into account that the rocks of the lower and upp er tectonic units are characterized by the same metamorphism age, the autho rs conclude that the Zerenda series is a tectonic mixture of the combined f ragments of the thin continental crust of a passive margin, subduced to dif ferent depths in the Lower Cambrian and rapidly (less than for a million ye ars at the first stage) extracted to the level corresponding to the amphibo lite facies of metamorphism. This may be indicated by the finding of kineti cally unstable coesite in rocks differing in composition and metamorphism d egree.