Collision-related metamorphic complexes of the Yenisei Ridge: their evolution, ages, and exhumation rate

In the Transangarian part of the Yenisei Ridge, rocks near the thrusts in area of the Tatarka deep fault underwent the medium-pressure kyanite-sillimanite grade metamorphism, which resulted locally in the progressive replacement of andalusite by kyanite, the development of new mineral assemblages and deformation structures. A number of features special to kyanite-sillimanite grade metamorphism, such as a relatively small measured thickness of the medium-pressure zones (from 2.5 to 7 km) and a gradual increase in pressure towards the thrust faults from 4.5–5 kbar to 6.5–8 kbar with slightly increasing temperature, suggest a low metamorphic field gradient with dT/dH ranging from 7 to 12 °C/km. These specific features are typical of collisional metamorphism during overthrusting of continental blocks and suggest a near-isothermal loading in accordance with the transient emplacement of thrust sheets and subsequent rapid exhumation and erosion. Based on geothermobarometry and 40Ar-39Ar mica ages, the proposed model suggests that, given an estimated exhumation rate of 0.368 mm/yr for a number of areas, the peaks of collision-related metamorphism occurred at 849–862 and 798–802 Ma. The older metamorphic complexes (Angara, Mayakon, Teya, and Chapa areas) are interpreted to have formed by thrusting of Siberian cratonal blocks onto the Yenisei Ridge, as indicated by geophysical observations and regional provenance studies. A later phase of the repeated collisional metamorphism appears to have been associated with reverse motion of some smaller blocks along higher-order splay faults in the eastward direction (Garevka area). On a regional scale, this may result from collision and accretion of a microcontinent split off the craton at the Early–Middle Riphean boundary onto the Central Angara terrane.