TWO EPISODES OF METAMORPHISM IN THE ROCKS OF THE EASTERN KITOI BLOCK (SHARYZHALGAI UPLIFT OF THE SIBERIAN PLATFORM) ACCORDING TO THE GARNET AMPHIBOLITE DATA

The Archean metamorphism in the southwestern Siberian craton was confirmed by the studies of the Kitoy and Irkutsk blocks. However, the parameters of the metamorphism process are still poorly investigated. The article presents the first results of studying the metamorphism conditions of garnet amphibolites sampled from the Kitoy sillimanite deposit. The reaction relationships of the studied minerals give grounds to distinguish two episodes of the regional metamorphism. At the end of the first episode, (Т=710–770 °С and Р=8.3–8.8 kb), the pressure reduced to 1.3–2.5 kbar at T=700 °C at the retrograde stage, and amphibole-plagioclase rims formed around garnet grains. During the second episode of metamorphism, the temperature reached 890 °С (granulite facies), and Cpx+Opx paragenesis replaced hornblende. The second episode of metamorphism is not evident in all the samples (considering the same bulk rocks composition of the rocks), which suggests its local character.