Kyanite–sillimanite metamorphism of the Precambrian complexes, Transangarian region of the Yenisei Ridge

Four Precambrian metamorphic complexes in the vicinity of regional faults in the Transangarian region of the Yenisei Ridge were examined. Based on geothermobarometry and P-T path calculations, our geological and petrological studies showed that the Neoproterozoic medium-pressure metamorphism of the kyanite–sillimanite type overprinted regionally metamorphosed low-pressure andalusite-bearing rocks at about 850 Ma. A positive correlation between rock ages and P-T estimates for the kyanite-sillimanite metamorphism provide evidence of the regional structural and tectonic heterogeneity. The medium-pressure metamorphism was characterized by (1) the development of deformational structures and textures, and kyanite-bearing blastocataclasites (blastomylonites) with sillimanite, garnet, and staurolite after andalusite-bearing regional metamorphic rocks; (2) insignificant apparent thickness of the zone of medium-pressure zonal metamorphism (from 2.5 to 7 km), which was localized in the vicinity of the overthrusts; (3) a low metamorphic field gradient during metamorphism (from 1–7 to 12 °C/km); and (4) a gradual increase in lithostatic pressure towards the thrust faults. These specific features are typical of collisional metamorphism during overthrusting of continental blocks and are evidence for near-isothermal loading. This event was justified within the framework of the crustal tectonic thickening model via rapid overthrusting and subsequent rapid uplifting and erosion. The results obtained allowed us to consider medium-pressure kyanite-bearing metapelites as a product of collision metamorphism, formed either by unidirectional thrusting of rock blocks from Siberian craton onto the Yenisei Ridge in the zones of regional faults (Angara, Mayakon, and Chapa areas) or by opposite movements in the zone of splay faults of higher orders (Garevka area).