Fault tectonites of the Yenisei ridge. Article 2: P-T-t-d evolution and possible tectonic models

Abstract

Metapelites and metabasites subjected to intensive deformations with recrystallization of the substrate and the formation of tectonites have been studied within the Yenisei regional shear zone of the Yenisei Ridge. The revealed differences in P-T metamorphism parameters between strongly and weakly deformed rocks and P-T-t-d trend configurations, calculated using mineral geothermobarometry and pseudosections in the NCKFMASH system, were discussed within geodynamic models controlled by different tectonic mechanisms. The polychronous dynamometamorphism of the gneisses of the Angara-Kan block in the south and the formation of the main mass of the blastomylonites of the northern segment of the YRSZ occurred with an increase in pressure of 1,5–3 kbar with a slight increase in temperature with a low metamorphic gradient dT/dH<10°C/km in comparison with the background values of the earlier regional metamorphism. This was probably due to the thickening of the earth's crust as a result of rapid thrust/subduction, followed by a rapid rise in the rocks. The maximum excess of the thermodynamic parameters of met-amorphism was established in the tectonites of the basite composition with relic glaucophane schist associations that underwent metamorphism with a simultaneous significant increase in pressure by 3–5 kbar and in temperature by 180– 240°C with a higher gradient dT/dH=15–20°C/km. Such excesses of P-T parameters could be associated with progressive metamorphism, complicated by local heating of rocks under viscous deformations and exceeding the oriented tectonic pressure over lithostatic in the zones of plastic shear. The obtained data are in good agreement with the results of numerical experiments, which confirms the concept of the role of tectonic stress as an additional thermodynamic factor of metamorphic transformations in the suture zones of the Earth's crust.