Blastomylonite Complexes of the Western Yenisei Ridge (Eastern Siberia, Russia): Geological Position, Metamorphic Evolution and Geodynamic Models

Abstract

Metapelites and metabasites within the Yenisei regional shear zone of the Yenisei Ridge underwent strong deformations with substrate recrystallization and blastomylonite formation during the Ediacaran. The geostructural, petrological, and isotopic–geochronological characteristics of development of the Late Neoproterozoic blastomylonite complexes marking the junction zone of the paleocontinental and paleoceanic sectors of the Yenisei Ridge have been analyzed. Two zones of heterogeneous blastomylonite complexes have been recognized from west to east, in order of increasing distance from the paleoceanic structures: (I) high-baric complexes of the suture zone and (II) frontal (above-suture) medium-baric and medium- to low-temperature complexes. The established differences in the Р–Т parameters of metamorphism between strongly and weakly deformed rocks were interpreted using well-known geodynamic models based on different tectonic mechanisms. As a result of the analysis, we found that the polychronous dynamic metamorphism of gneisses of the Angara–Kan block in the south and formation of the bulk of blastomylonites in the northern segment of the shear zone in the Northern Yenisei Ridge occurred with a 1.5–3 kbar increase in pressure along with insignificant increase in temperature and low metamorphic gradient dT/dH < 10°C/km compared to the background values of earlier regional metamorphism. This was probably due to crustal thickening as a result of rapid thrusting/subduction, followed by rapid uplift. The maximum excess values of the thermodynamic parameters of metamorphism were established in apometabasite tectonites of the suture zone with relict glaucophane schist associations that underwent metamorphism with a simultaneous significant increase in pressure by 3–5 kbar and temperature by 180–240°C with a higher dT/dH gradient of 15–20°C/km. Such excess Р–Т values could be due to progressive metamorphism, complicated by local heating of rocks during viscous deformations and excess oriented tectonic pressure over lithostatic in ductile shear zones. The data agree well with the results of numerical experiments, and this confirms the role of tectonic stress as an additional thermodynamic factor of metamorphic alterations in crustal suture zones.