TECTONO-METAMORPHIC EVOLUTION OF PRECAMBRIAN AL-RICH METAPELITES DURING THRUST LOADING (TRANSANGARIAN REGION OF THE YENISEY RIDGE, EASTERN SIBERIA, RUSSIA)

In the Transangarian region of the Yenisey Ridge Al-rich metapelitic schists show field and petrological evidences of two superimposed metamorphic events. An early low-pressure event produced andalusite-bearing assemblages of the andalusite-sillimanite facies series (Ð=3.5-4 kbar; Ò=540-560 °Ñ). Subsequently, these rocks underwent a gradual pressure increase in the vicinity of the thrust (P=4.5-6.7 kbar; Ò=540-600 °Ñ), as a result of which low-pressure metamorphic effects were overprinted by medium-pressure regional metamorphic mineral assemblages and textures of kyanite-sillimanite type, as witnessed by kyanite + staurolite pseudomorphs after andalusite in metapelitic rocks. Based on the results of geothermobarometry and P-T paths calculation, the later metamorphic event was not accompanied by significant temperature increase and took place under the conditions of very low geothermal gradient (1-7 °Ñ/km) resulting in nearly isothermal loading. Thermo-physical computational results have shown that the temperature of the rocks beneath the trust plane remains relatively constant at a depth lower than 18-20 km, provided that overlapping rocks is distinguished from rocks beneath by its thermo-physical and heat-producing properties. The proposed model for tectono-metamorphic evolution of the study area, calculated within a framework of the crustal thickening accomplished by southwestward thrusting, explains a number of features (the gradual change in recorded pressure between low- and medium-pressure rocks, lack of a temperature increase and very low thermal gradient) associated with this phenomena and confirms a possibility of nearly isothermal loading during overthrusting.