Collisional metamorphism as a result of thrusting in the Transangara region of the Yenisei Ridge

Abstract

The Yenisei Ridge is one of the most interesting regions in the southwestern folded framework of the Siberian Craton with respect to geodynamics. The regional structure of the Transangara sector of the Yenisei Ridge is traditionally displayed as a NW-trending system of tectonic sheets divided by faults characterized by the collision of blocks and thrusting. Therefore, this region was subject to pressure-variable regional metamorphism expressed in the juxtaposition of lowand moderate-pressure metamorphic facies. Collisionrelated moderate-pressure metamorphism is locally superimposed on the low-pressure (presumably, younger) metamorphic rocks. As a result, andalusite is replaced with kyanite with the formation of new mineral assemblages and deformational structures. The prograde replacement of andalusite with kyanite in the Yenisei Tange is a rare phenomenon, because the stationary continental geotherm commonly does not intersect the andalusite‐kyanite equilibrium line. Such replacements are usually referred to the retrograde stage of metamorphism, but this interpretation comes into conflict with the regional geological situation. Only a few examples are known in the literature (Northwest Cordillera in the United States and Canada, Dalradian in Scotland, central and northwestern Appalachians in the United States, and the Kola Peninsula and Yenisei Ridge in Russia), where prograde transformation of andalusite into kyanite is assigned either to the metastable state of andalusite in the PT stability field of kyanite or to an increase in pressure as a result of thrusting or magmatic loading characterized by different PT trends. While studying collisional metamorphism in the Transangara sector of the Yenisei Ridge, we selected three (Chapa, Mayakon, and Angara) areas composed of Paleoproterozoic, Middle Riphean, and Upper Riphean rocks (Fig. 1). The Chapa and Mayakon areas are located in the Central uplift between the Ishimbino and Tatarka deep faults. The Angara area covers the junction of the Transangara structural units and the Angara‐ Kan block. The Angara area is situated at the interfluve of the Angara, Belokopytovka, and Malaya Sploshnaya rivers. The reference sections are exposed in the Tatarka shear zone along the right bank of the Angara River between the mouths of the Babkin and Polovinkin creeks. The area is composed of the Upper Riphean low-pressure metasedimentary rocks (rhythmic intercalation of quartzites and phyllites of the Sukhoi Ridge Formation). In the study area, this sequence is largely made up of phyllites of the greenschist facies represented by quartz (Qtz), muscovite (Ms), chlorite (Chl), and ilmenite (Ilm). These rocks underwent high-pressure collisional metamorphism with the formation of new (kyanite-bearing) mineral assemblages. Metamorphism occurred simultaneously with the development of steep (80 ° ‐85 ° NW and SE) near-meridional cleavage. The increase in the metamorphic grade in the near-latitudinal direction is marked by the successive formation of chloritoid (apparent thickness 0.5‐0.8 km) and kyanite (~1.5‐1.7 km) zones. These minerals correspond to the conditions of kyanite schist facies. The eastern boundary of these rocks is hidden beneath unmetamorphosed Paleozoic rocks of the Pogromnino basin. In the Kulakovo uplift on the left bank of the Angara River, collisional metamorphism is expressed in the crystallization of kyanite (Ky), chloritoid (Cld), and ilmenite in metapelites consisting of staurolite (St), plagioclase (Pl), Ms, biotite (Bt), Qtz, and garnet (Grt) [2].