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

Russian Geology and Geophysics 52 (2011) 1256–1269 www.elsevier.com/locate/rgg

Collision-related metamorphic complexes of the Yenisei Ridge: their evolution, ages, and exhumation rate I.I. Likhanov a,*, V.V. Reverdatto a, P.S. Kozlov b a

b

V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia A.N. Zavaritskii Institute of Geology and Geochemistry, Ural Branch of the Russian Academy of Sciences, Pochtovyi per.7, Yekaterinburg, 620151, Russia Received 29 October 2010; accepted 5 April 2011

Abstract 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. © 2011, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: metapelites; geothermobarometry;

40 39 Ar- Ar dating; collisional metamorphism; Yenisei Ridge

Introduction Generally, the two key issues that arise in linking metamorphic and tectonic processes include: (1) the relationship between different types of metamorphism and specific tectonic settings in orogenic belts and (2) the problem of tectonic transport during exhumation of metamorphic complexes and evaluation of parameters relevant to these processes, including exhumation mechanisms and rates (e.g., Dobretsov, 1995). To elucidate these aspects, we shall discuss an example of the zoned collision-type metamorphism associated with thrusting. Craton margins memorize essential information about the lithospheric geodynamics and the accretionary-collision orogens always demonstrate a close association with different

* Corresponding author. E-mail address: [email protected] (I.I. Likhanov)

types of metamorphism. This explains the keen interest of many researchers in problems related to the processes forming accretionary-collisional structures within continents and the tectonic evolution of fold-and-thrust belts at the craton margins (e.g., Khain, 2001, 2010). Understanding the geological history of the Yenisei Ridge developed as an accretionary-collision orogen at the western margin of the Siberian craton will not only set constraints on the tectonic evolution of mobile belts along the margins of the older cratons but will help to address a widely debated question of the possible incorporation of the Siberian craton into the Meso- to Neoproterozoic Rodinia supercontinent (Pisarevsky et al., 2003). The Yenisei Ridge is one of the most geodynamically interesting parts of Siberia. In contrast to many other areas, it includes the most complete and representative Precambrian stratigraphic sequences, ranging in age from Paleoproterozoic to upper Neoproterozoic (Ediacaran/Vendian). The close spatial association of diverse igneous and metamorphic complexes

1068-7971/$ - see front matter D 201 1, V . S. S o bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2011.09+.015

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records a complicated tectonic history of the region. In particular, all the metamorphic complexes in the Transangarian part of the Yenisei Ridge display the heterogeneity of the metamorphic pressures reflected in the development of two metamorphic facies, the andalusite-sillimanite (low-pressure) and kyanite-sillimanite (medium-pressure) baric types. The medium-pressure metamorphism postdated the low-pressure event and occurred locally near thrust faults, resulting in the prograde replacement of andalusite by kyanite and the development of newly formed mineral assemblages and deformation microtextures (Likhanov et al., 2004). This is obviously of particular interest, since the replacement of andalusite or kyanite by sillimanite is known to be common prograde reactions among the Al2SiO5 polymorphs in low- to intermediate-P zoned metamorphic complexes. The replacement of andalusite by kyanite observed in the Yenisei Ridge during prograde metamorphism is unusual, because a normal stable continental geotherm never crosses the andalusite–kyanite equilibrium curve (Kerrick, 1990). Several examples are, however, available in literature (northwestern U.S. and Canadian Cordillera; Dalradian Highlands, Scotland; Central and Northwestern Appalachians, USA; Kola Peninsula and Yenisei Ridge; Altai) where the transition of andalusite- to kyanitebearing assemblages along a single prograde development are attributed to pressure increase either due to (1) thrust loading (Baker, 1987; Beddoe-Stephens, 1990; Bel’kov, 1963; Clarke et al., 1987; Crawford and Mark, 1982; Likhanov and Reverdatto, 2011b,c; Likhanov et al., 2004; Spear et al., 1990, 2002; Sukhorukov, 2007; Ushakova, 1966) or (2) magma loading, i.e. the increase in lithostatic pressure due to the emplacement of an intrusive body (Brown, 1996; Brown and Walker, 1993; Whitney et al., 1999). The study of specific geodynamic processes that operate within different tectonic settings is based on available information on the depth and thermal structure of their evolution over time. The isotopic ages of rocks and their geodynamic interpretation are the key to unraveling the time relations between various stages of metamorphism, tectonic and magmatic activity in the region. However, no solution to these problems is possible without using high-precision isotopic age determinations from metamorphic rocks of the Yenisei Ridge which still remain very scarce (Likhanov et al., 2008a; Nozhkin et al., 2008b). In this context, detailed 40Ar-39Ar geochronology of kyanite/sillimanite-bearing metapelites was used to obtain the ages and exhumation rates of metamorphic complexes and constrain the timing of discrete stages of metamorphic evolution and their relationship with tectonic and magmatic activity in the region.

Geologic framework anp the Riphean evolution of the region The Yenisei Ridge is a fold-nappe belt, some 700 km long and 50–200 km wide, extending SSE–NNW, and located at the southwestern margin of the Siberian platform (Fig. 1). This large accretionary-collision structure is well seen from geo-

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logical and geophysical data to be separated from adjoining areas of the Siberian platform and West Siberian plate (Staroseltsev et al., 2003). The NW-trending structures of the Yenisei Ridge are divided into two segments (South Yenisei and Transangarian), separated by the ENE-trending strike-slip Angara Fault (Kheraskova, 1999). Two allochthonous units have been recognised south of the Angara Fault (Fig. 1, inset), the Paleoproterozoic Angara–Kan terrane and the Neoproterozoic Predivinsk terrane, which lies along the eastern bank of the Yenisei River. North of the Angara Fault, the Transangarian part of the Yenisei Ridge is composed mainly of west-verging thrust sheets containing mainly Meso-Neoproterozoic rocks, comprising the East Angara passive continental margin terrane, the Isakovka island arc terrane and the intervening Central Angara granite-metamorphic terrane (Lobkovskii et al., 2004; Vernikovsky and Vernikovskaya, 2006). All of the terranes mentioned above represent crustal segments 200–500 km long and 50–80 km wide (Fig. 1), which are separated by the largest known thrust faults in the region (Smit et al., 2000). A number of higher-order faults, which splay off theses regional faults, suggest a dip slip displacement forming thrust faults (Egorov, 2004; Konstantinov et al., 1999; Sal’nikov, 2009). These processes brought about a regionally heterogeneous pressure field of metamorphism and, consequently, a combination of two facies series: andalusite–sillimanite (low-pressure) and kyanite–sillimanite (mediumpressure) (Likhanov and Reverdatto, 2011b,c; Likhanov et al., 2004). Collision-related medium-pressure metamorphism that locally overprints the low-pressure metamorphic rocks is thought to be younger. We examined the fields of kyanite/sillimanite-bearing metapelites, which record the late mediumpressure metamorphic events. From north to south, we distinguished five typical localities (Chapa, Garevka, Teya, Mayakon, and Angara), in which rocks of Paleo-, Meso-, and Neoproterozoic age are exposed (Khabarov, 1994; Nozhkin et al., 1999, 2008b; Volobuev et al., 1968, 1973, 1976) (Fig. 1). Tectonically, four northern areas are located within the Central Angara terrane in the vicinity of the Tatarka deep fault. The Angara area is confined to a junction zone between the Transangarian structures and the Anagara–Kan block. Some peculiar features of metamorphism for each of the above areas were described in detail in previous publications (Korobeinikov et al., 2006; Likhanov and Reverdatto, 2009, 2011b; Likhanov et al., 2001a, 2006a, 2008b, 2009, 2010e). Note that the P-T conditions and P-T paths are calculated by a uniform technique using a set of the same internally consistent geothermobarometers with appropriate mixing models, internally consistent thermodynamic datasets, and the two computer programs, THERMOCALC (Powell and Holland, 1994) and PTPATH (Spear et al., 1991). A great advantage of this approach is that it allows the results to be compared with one another and with data reported on metamorphic rocks from other overthrust terranes (Fig. 2). The ophiolites and calc-alkaline island arc volcanic rocks comprise, along with terrigenous and carbonate rocks, the Riphean section in the west of the study area. Increasing predominance of carbonate-terrigenous deposits exhibiting

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Fig. 1. Sketch tectonic map of the Yenisei Ridge (Vernikovsky et al., 2007) showing locations of the kyanite-sillimanite metamorphism and P-T estimates. 1, cover; 2, molasse; 3, dominantly carbonate rocks; 4, ophiolitic and island-arc complexes of the Yenisei belt; 5, granitoids; 6, ophiolites of the Rybinsk-–animba belt; 7, greenschist to amphibolite facies metamorphic rocks; 8, Early Precambrian granulite-amphibolite rocks; 9, regional faults (a) and geologic boundaries (b). Letters in circles denominate major faults: I, Ishimba, T, Tatarka, Y, Yenisei, A, Ankin. Numerals in circles on inset stand for terranes: 1, Central Angara; 2, East Angara; 3, Isakovka; 4, Predivinsk; 5, Angara–Kan.

considerable facies variation and the presence of volcanics in local rift structures are characteristic of the central block. The eastern amagmatic zone of the Yenisei Ridge is represented mostly by sedimentary units of the passive margin of the Siberian craton (Nozhkin et al., 2008b, Vernikovsky et al, 2009).

The Central Angara terrane consists of metamorphosed terrigenous and terrigenous-carbonate rocks of the Teya (including Garevka), Sukhoi Pit, Tungusik, Oslyan, Changasan, and Chapa Groups with a cumulative thickness of 10–15 km (Nozhkin et al., 2008b; Postel’nikov, 1990); these are underlain by metamorphic rocks of the Nemtikha and

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Malaya Garevka units (Kachevskii, 1998). Based on our current knowledge of the supercontinent cycles (Khain, 2000; Khain and Goncharov, 2006), the study area was mostly a continental environment through pre-Riphean times and underwent weathering and peneplanation (Nozhkin et al., 2003). The Paleo-Mezoprotezoic boundary is marked by the onset and development of subtly expressed rift depressions filled with Fe- and Al-rich terrigenous and argillaceous-carbonate deposits of the Teya Group. A subsequent episode of crustal extension and rifting during the early Middle Riphean led to the development of a pericratonic trough, the eruption of picrobasalts of the Rybinsk–Panimba volcanic belt, the emplacement of the rapakivi plutons and the formation of rift-related blastomylonites (Nozhkin, 2009; Likhanov et al., 2010c). These events were broadly contemporaneous with the emplacement of the 1360–1380 Ma plagiogranite intrusions (U-Pb SHRIMP II zircon age) (Likhanov and Reverdatto, 2009; Likhanov et al., 2010e; Popov et al., 2010). Syn-rift and particularly post-rift subsidence in pericratonic domains led to the deposition of deep-water passive margin sequences of the Sukhoi Pit Group which continued into Grenvillian collisional events. The closure of this basin in the early Neoproterozoic was accompanied by formation of an accretionary-collision orogen, intense folding, deformation and metamorphism of the Teya and Sukhoi Pit rocks. This epoch was also marked by the formation of two linear belts of the Teya-type granitic and gneiss domes (Nozhkin et al., 1999), which contain LP/HT metamorphic rocks. Regional LP/HT metamorphism of the andalusite-sillimanite type occurred over a wide area, with temperatures from 400 ºC in the biotite zone to 640 ºC in the sillimanite zone and pressures from 3.5 to 5.1 kbar, indicating a normal metamorphic field gradient with dT/dH of about 30–40 ºC/km, where T is the temperature and H is the burial depth (typical lithostatic pressure-depth distribution for continental crustal rocks corresponds to dP/dH =1 kbar/3.5 km (Spear, 1993). The link between these processes and Grenvillian orogenic events, which may have occurred at approximately the same time in several other lithospheric blocks of the Asian continent (Ernst et al., 2008), was strongly supported by the previously described U-Th-Pb, Rb-Sr and K-Ar geochronology of the Teya granite-gneiss dome (900–1000 Ma) (Nozhkin et al., 1999; Volobuyev et al., 1976) and supplemented by more recent results from single zircon grains (U-Pb SHRIMP II) and 40Ar-39Ar dating of metapelites from the Teya polymetamorphic complex (953–973 Ma) (Likhanov and Reverdatto, 2009; Likhanov et al., 2010e, 2011a,b; Nozhkin et al., 2008a). The deepest blocks in the Garevka area underwent medium-pressure metamorphism under the amphibolite facies conditions at P = 7.7–8.6 kbar and T = 580– 630 ºC. During a later collision stage of orogeny, these amphibolites underwent the Late Riphean (900–880 Ma) dynamometamorphism in the epidote-amphibolite facies along a low metamorphic field gradient with dT/dH < 10 ºC/km accompanied by the formation of blastomylonites within the narrow zones of both brittle and plastic deformations (Likhanov et al., 2011b). Synchronous with those processes, Late Riphean

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Fig. 2. P-T conditions and paths calculated for metapelites from Mt. Garevskii Polkan in comparison with the P-T evolution of collision metamorphism in other overthrust terranes. Each cross indicates P-T estimates and the spread of P-T data calculated for four samples from the Garevka area using various geothermobarometers, without correction for error (Likhanov and Reverdatto, 2011b). The small numbers near crosses are sample numbers in Fig. 3. P-T paths are derived from chemical zoning patterns in minerals using the PTPATH program (Spear et al., 1991). Arabic numerals in arrow head of P-T paths (dark gray thick arrows) correspond to study areas: 1, Angara area, Yenisei Ridge (Likhanov et al., 2006a), 2, Mayakon area, Yenisei Ridge (Likhanov et al., 2004), 3, Garevka area, Yenisei Ridge (Likhanov et al., 2009), 4, Chapa area, Yenisei Ridge (Likhanov et al., 2008b), 5, Teya area, Yenisei Ridge (Likhanov et al., 2011a). Roman numerals in arrow head of P-T paths (light gray thick arrows) correspond to the following regions where replacement of andalusite by kyanite is observed: I, Bellows Falls, Appalachians, USA (Spear et al., 2002); II, Mascoma-Orfordville, Appalachians, USA (Kohn et al., 1992); III, Nason terrane, Canadian Cordillera (Whitney et al., 1999); IV, Piedmont Plateau, USA (Crawford and Mark, 1982). The prograde segment of P-T trajectories for continental collision (light gray dashed arrows) are calculated using 2D modeling (V, (Jamieson et al., 2002); VI, (Huerta et al., 1999)). The coordinates of the aluminum silicate triple point and univariant equilibrium curves of Al2SiO5 polymorphs are given after Holland and Powell (1985) and Pattison (1992).

thermal metamorphism was developed at large depths as a result of emplacement of granitoids and low-alkali granite plutons (Vernikovsky and Vernikovskaya, 2006). Thermal contact aureoles in rocks show a distinct zonation pattern from the chloritoid to sillimanite–K-feldspar zones, over a wide range of temperatures, T = 450–650 ºC, at constant pressures of 2.5–3.5 kbar, indicating that the conditions of metamorphism were along a high metamorphic field gradient with dT/dH ≥ 100 ºC/km (Likhanov, 2003; Likhanov et al., 2001b). The 40Ar-39Ar age of contact metamorphism (861–864 Ma; Likhanov et al., 2010c) is in close agreement with U-Pb dating of granitoids from the Kalama, Middle Tyrada and other massifs (Nozhkin et al., 2009b; Vernikovsky et al., 2007). These events are therefore contemporaneous with a mediumpressure metamorphism of the kyanite-sillimanite type, which was locally developed in the vicinity of the Tatarka deep fault. Although isotopic dates of Middle and Upper Proterozoic

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rocks are still scarce, particularly those for metamorphic rocks, our recent 40Ar-39Ar dates on biotite from metapelites of the Korda Formation, Sukhoi Pit Group, suggest that the kyanitesillimanite type of metamorphism culminated at about 848– 851 Ma (Likhanov et al., 2007), i.e., younger metamorphic overprinting was of a Neoproterozoic age. The final phase of accretionary-collision orogeny within the Yenisei Ridge is marked by regional uplift and erosion of Riphean sediments and subsequent pulses of rift-related and within-plate magmatism between 780 and 650 Ma (Nozhkin et al., 2008a, 2011). The Late Neoproterozoic rifting and within-plate magmatic episodes may be attributed to mantle plume activity that caused the breakup of Rodinia and the opening of the Paleo-Asian Ocean (Li et al., 2008; Maruyama et al., 2007; Nozhkin et al., 2008a; Santosh et al., 2009).

Brief description of the study areas The Garevka area, located on the interfluve between the Chapa–Garevka–Tis Rivers, in the vicinity of Mt. Garevskii Polkan (Fig. 3), is confined to the anticlinorium of the NW-trending Karpinsky Range. The regional geology of the study area consists of the Polkan anticline represented by Lower Proterozoic (1650–1500 Ma) (Volobuyev et al., 1973) regional grade rocks of the Karpinsky Range Formation, which have a gentle (10–15°) or nearly horizontal dip, and a thickness of about 3 km. The regional folded structure is complicated by faults of NW strikes attributed to the Uvolga fault zone in the northern sector of the Tatarka deep fault (Fig. 1). Away from the overthrust, the low-pressure metapelitic rocks containing the assemblage Ms + Chl + Bt + Cld + And + St + Qtz + Pl + Ilm were formed under greenschist and epidoteamphibolite facies conditions (Dobretsov et al., 1972). Mineral symbols are from Whitney and Evans (2010). Closer to the Garevka overthrust, the rocks underwent medium-pressure collisional metamorphism of the kyanite-sillimanite type. The apparent thickness of these rocks bounded in the west by an overthrust and in the east by NW-trending faults is some 4–5 km (Fig. 3). The transition from the low-pressure regionally metamorphosed rocks to those of higher metamorphic grades is marked by the simultaneous appearance of kyanite and sillimanite (kyanite isograd). The critical assemblage Ky + St + Grt + Ms + Bt + Qtz + Pl + Sil with andalusite and chloritoid relics was formed under conditions of the kyanite schist facies (Dobretsov et al., 1974). Three metamorphic zones parallel to this overthrust are distinguished based on variations in the texture and compositional characteristics of the metapelites. Zone (I) contains metapelites of the andalusite-sillimanite type unaffected by the later kyanite-sillimanite overprint. The mineral assemblages in the interior of this zone are spatially coincident with the kyanite isograd. Close to the overthrust, the outer (II) and inner (III) zones of the medium-pressure metamorphic overprint are distinguished based on mineralogical changes associated with the newly generated mineral assemblages and relict mineral grains as well as the intensity of deformation in

Fig. 3. Schematic geological map of Precambrian metamorphic rocks around Mt. Garevskii Polkan and cross-section along A–B line (modified after Kachevskii et al., 1998). 1, Lower Riphean (Korda Fm., biotite-quartz, biotite-feldspar-quartz schist with graphite (kd); 2, Lower Proterozoic (Karpinsky Range Fm., quartzite and high-alumina, two-mica gneiss with andalusite, kyanite, sillimanite, staurolite, and garnet, locally blastomylonitic (hk)); Archean– Lower Proterozoic unstratified rocks: 3, Malaya Garevka Fm., gneiss, marble, calciphyre, quartzite, and micaceous schist (gr); 4, Nemtikha Fm., biotite, two-mica, and biotite-amphibole plagiogneiss, two-mica schist with staurolite, kyanite, sillimanite, and garnet (nm)); 5, major (a) and minor (b) faults; 6, sample locations; all dated samples appear in boldface as a circle with a dot inside; 7, metapelites of andalusite-sillimanite (I) and kyanite-sillimanite (II and III) metamorphic types; 8, onset of the And-Ky transition (a) and boundaries between metapelite zones of kyanite-sillimanite metamorphism (b); 9, major thrusts. The arrow in vertical section A–B shows the direction of overthrusting.

metapelites. Zone (II) is recognized by the appearance of a new mineral assemblage with kyanite in the rocks. This zone is characterized by partial resorption and local replacement of the andalusite porphyroblasts by Ky–St–Ms–Qtz at the periphery. In thin section, the cataclastic grains with a square and prismatic cross-sectional area exhibit a rhombic or ovoid shape, with the long axes of mineral grains parallel to the foliation surface. The nucleation and growth of kyanite, biotite, staurolite, and garnet in Zone II are accompanied by a simultaneous decrease in the modal abundance of chloritoid, chlorite, muscovite and andalusite (Zone I). Zone (III), adjacent to the thrust (Fig. 3), is marked by the development of the new kyanite- and sillimanite-bearing assemblage. This

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Fig. 4. A model of crustal thickening in the vicinity of overthrusts. Heavy black line depicts the thrust fault plane; direction of motion is shown by arrow. Mantle heat flow (Q) is assumed constant. White line A–B corresponds to the location of the cross section along A–B line in Fig. 3.

zone is defined by the complete recrystallization of minerals in the rocks leading to the formation of blastocataclasites and blastomylonites. In the outer part of this zone, the last traces of relict andalusite disappear and kyanite is the dominant Al2SiO5 polymorph with subordinate sillimanite. Aggregates of recrystallized quartz form strain shadows around lenses of rhomb-shaped pseudomorphs of kyanite after andalusite. In blastocataclasites, garnet grains are disrupted by cracks and shifted along strain-slip cleavages or exhibit locally a helicitic (snowball) structure. In contrast to the idioblastic garnet crystals in other zones, garnet in Zone III often occurs as porphyroblasts that exhibit a flattened or discoid shape, reflecting growth under stress. In areas adjacent to the thrust, many minerals show numerous strain-related features such as cataclastic deformation, boudinage, kink bands and undulose extinction. Intense deformation is also obvious from lenticular-nodular structure of rocks, the presence of granular quartz veins and the decrease in grain size of minerals (Likhanov and Reverdatto, 2011b). Geothermobarometry and P-T path calculations suggest a progressive pressure increase towards the overthrust from 4.0–4.5 kbar in Zone I (metapelites of the andalusite-sillimanite type) through 5.0–6.0 kbar in the outer Zone II (metapelites of the kyanite-sillimanite type) to 6.0–7.3 kbar in the inner Zone III (predominantly kyanite blastomylonites of the kyanite-sillimanite type) with a small increase in temperature (from 575 to 645 °C) (Likhanov et al., 2009). In all metapelites from the Karpinsky Range Formation, the above data document an increase in pressure (from 2.2 to 2.5 kbar) from east to west associated with only minor heating, which is indicative of near-isothermal subsidence of rocks along a low metamorphic field gradient with dT/dH of 8–9 °C/km (Fig. 2). These characteristic features are typical of collisional metamorphism during overthrusting of continental blocks (with no subduction influence) (Reverdatto and Sheplev, 1998) and suggest nearisothermal load of a “cold” overthrust plate (Likhanov et al.,

2008b). The proposed model for tectonometamorphic evolution of the study areas due to crustal thickening at high thrusting rates, subsequent rapid exhumation and erosion can explain these tectonic features (Likhanov and Reverdatto, 2011b). The original low-pressure metamorphic rocks are located at a depth of ~14–16 km before thrusting (Fig. 4). Thrust loading caused further subsidence. During thrusting, these rocks experienced a pressure increase of 2.2–2.5 kbar, that is equivalent to an increase in burial depths or crustal thickening by 7–8 km (given the normal lithostatic pressuredepth distribution for continental crustal rocks, 1 kbar/3.5 km), thus implying the overthrusting of a rock stratum, which must have been removed by subsequent erosion. This tectonic thickening could be related to collision of two blocks, which resulted in the eastward thrusting of the western block of the near-thrust structure onto the eastern one. Unlike other collision-type metamorphic areas in the Transangarian region where higher grade conditions occur in the northeast, the Garevka rocks display an increase in metamorphic grade in a southwest direction. The Angara area is located on the interfluve between the Angara, Belokopytovka, and Tatarka Rivers (Likhanov et al., 2006a). The sedimentary sequence is well exposed on the right bank of the Angara River, within the Tatarka shear zone. The geological framework of this area consists of Upper Riphean (1000–900 Ma) sedimentary and low-pressure metamorphic rocks, such as rhythmically intercalating quartzites and phyllites of the Sukhoi Range Formation (Shirokaya Group) (Kachevskii, 1998; Khomichev, 2007). These regionally metamorphosed rocks are mostly phyllites belonging to the greenschist facies assemblage Qtz + Ms + Chl + Ilm (Dobretsov et al., 1972). These rocks underwent regional metamorphism at higher pressure conditions resulting in the development of new kyanite-bearing assemblages. Metamorphism occurred with concurrent development of cleavage with a steep dip (80°) towards the northwest and southeast. An increasing metamor-

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phic grade from east to west is indicated by the successive development of chloritoid (apparent thickness 0.5–1 km) and kyanite (thickness about 1.5–2 km) zones, which corresponds to chloritoid-kyanite subfacies conditions of the kyanite schist facies (Dobretsov et al., 1974). The eastern boundary of the field is concealed beneath a nappe of unmetamorphosed Phanerozoic rocks of the Pogromnaya depression. Rocks from the Kulakov uplift on the left bank of the Angara River show evidence of Neoproterozoic kyanite-sillimanite medium-pressure metamorphism, which is characterized by the replacement of andalusite by kyanite in the St + Pl + Ms + Bt + Grt + Qtz metapelites (Likhanov et al., 2008b). Geothermobarometry and P-T path calculations (Fig. 2) indicate a gradual increase of pressure from 4.1 to 5.6 kbar towards the thrust with a slight increase in temperature from 530 to 560 °C, which suggests near-isothermal subsidence of the rocks and implies a metamorphic field gradient with dT/dH as low as 10 ºC/km (Likhanov et al., 2009). These data are consistent with the petrogenetic grid for Fe- and Al-rich metapelites (Likhanov et al., 2005) and the P-T diagram for typical metapelites (Fed’kin, 1970), in which the univariant Ky + St + Pl + Ms + Bt + Grt + Qtz mineral assemblage in the K2O–FeO–MgO– Al2O3–SiO2–H2O system is stable at temperatures up to 590 ºC and pressures up to 6 kbar, with the staurolite stability field diminishing towards lower pressures and temperatures. The Mayakon area is situated between the Yeruda and Chirimba Rivers (Likhanov et al., 2001a) where Mesoproterozoic (1350–1250 Ma) (Nozhkin et al., 2008b) sedimentary rocks of the Korda Formation underwent low- and mediumpressure metamorphism. In the study area, the low-pressure metapelitic rocks containing Ms + Chl + Bt + Cld + And + Qtz + Ilm ± Crd formed in the greenschist and epidote-amphibolite facies (Dobretsov et al., 1972). The medium-pressure rocks typically contain Ms + Chl + Bt + Qtz + Ky + St + Grt + Ilm + Pl with sporadic sillimanite and relics of andalusite formed under conditions of the kyanite schist facies. They comprise a 5–7-km-wide and some 20 km-long zone bounded to the east by the NW-trending Panimba thrust fault, which is followed to the northeast by Lower Proterozoic metacarbonates of the Teya Group. Three zones of metamorphic overprinting parallel to the Panimba overthrust are distinguished based on mineralogical changes associated with the newly generated mineral assemblages and relict mineral grains as well as the intensity of deformation in metapelites. Geothermobarometry data suggest a progressive pressure increase towards the Panimba thrust from 3.5–4 kbar in regionally metamorphosed metapelites, 4.5–5 kbar in the outer zone, up to 5.5–6 kbar in the intermediate zone, and up to 6.2–6.7 kbar in the inner zone near the overthrust over a relatively narrow temperature range (from 550 to 580 °C) (Likhanov et al., 2004). These data show good agreement with the calculated mineral reactions at the kyanite isograd, which are characterized by the very large volume and small entropy effects (Likhanov and Reverdatto, 2002). The P-T trajectory documents a regular increase in pressure (from 1 to 2.2 kbar) in the Korda metapelites towards the thrust fault associated with only minor heating (not more than 20 ± 15 °C), suggesting

near-isothermal loading conditions along a low metamorphic field gradient with dT/dH of 5–7 °C/km (Fig. 2). For such near-isothermal loading, we proposed a model for the tectonometamorphic evolution of the study area (Likhanov et al., 2004) based on crustal thickening due to southwestward thrusting of the 5–7 km-thick Teya metacarbonates over Korda metapelites. A small temperature increase during thrusting was explained by specific behaviour of steady-state geotherms calculated using lower radioactive heat production and higher thermal conductivities of metacarbonates as compared with metapelites (Likhanov et al., 2004). The Chapa area is located in the middle reaches of the Chapa River, between the mouths of its two tributaries, the Nizhnyaya Veduga and Yelovaya Rivers (Likhanov et al., 2008a,b). It is essentially composed of Lower Proterozoic (≥1650 Ma) (Volobuyev et al., 1973) metasedimentary rocks of the Teya Group. These rocks were folded into the Chapa anticline whose hinge plunges to the northwest at angles of 15–30°. The core of the anticline consists of quartzites and crystalline schists of the Karpinsky Range Formation; its limbs are composed of metaterrigenous-carbonate rocks (marbles with quantitatively subordinate crystalline schists) of the Penchenga Formation. Away from the overthrust, the lowpressure metapelites of the Penchenga and Karpinsky Range Formations are composed of, respectively, Ms + Chl + Bt + Qtz + Pl and And + St + Grt + Ms + Bt + Qtz + Chl + Pl varieties, formed under greenschist and epidote-amphibolite facies conditions (Dobretsov et al., 1972). Closer to the overthrust, these rocks are affected by medium-pressure metamorphism of the kyanite-sillimanite type. The simultaneous occurrence of kyanite and sillimanite (kyanite isograd) marks the transition from a low- to medium-pressure facies series. The critical assemblage Ky + St + Grt + Ms + Bt + Qtz + Pl + Sil with relics of andalusite was formed under conditions of the kyanite schist facies (Dobretsov et al., 1974). A 5–7 km wide zone comprising these rocks is truncated in the east by a NW-trending overthrust. Three metamorphic zones parallel to this overthrust are distinguished based on variations in the texture and compositional characteristics of the metapelites. The results of geothermobarometry indicate a gradual increase in peak metamorphic pressure towards the thrust from 3.9–4.9 to 5.5–5.8 kbar in Zone I (metapelites of the andalusite-sillimanite type) to 6.7–7.4 kbar in Zone II (metapelites of the kyanite-sillimanite type), and further to 8.1–8.4 kbar in Zone III (predominantly kyanite blastomylonites of the kyanite-sillimanite type) (Likhanov et al., 2008a). A slight increase in temperature from 630 to 710 ºC towards the thrust implies a low geothermal gradient. Compared to rocks from other regions of the Transangarian Yenisei Ridge, the metapelites of the Chapa area yield the highest P-T parameters (Fig. 2). Higher temperatures can be accounted for by the widespread occurrence of sillimanite in Zone II. In the kyanite-sillimanite facies sillimanite is either absent in the other metamorphic domains (Angara area) or occurs only in the vicinity of granites (Mayakon area). This can be well explained by additional local heat transfer from the intrusive body (Likhanov, 2003; Likhanov et al., 1999, 2001b). Peak

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pressures can be accounted for by the generally increasing content of grossular component in metapelitic garnets from core to rim. The calculations of PT-paths for the metapelites from the Karpinsky Range Formation support a general increase in pressure of 2.0–2.5 kbar with slightly increasing temperature towards the overthrust from southwest to northeast, which suggests burial along a low metamorphic field gradient with dT/dH no greater than 12 ºC/km (Likhanov et al., 2009). The Teya area is located in the middle course of the Teya River, on the interfluve between the Kurepa and Uvolga Rivers, north of the Teya granitoid massif. It is essentially composed of Proterozoic regional metamorphic rocks of the Teya and Sukhoi Pit Groups (Bovin, 1982). The core of the Teya anticline, which is overturned and dipping at 50º–65° southwestwards, consists of the oldest, strongly metamorphosed metacarbonate-terrigenous rocks of the Lower Proterozoic Teya Group intruded by granitoids of the Kalama massif. Both limbs of the anticline are composed of weakly metamorphosed rocks from the Korda and Gorbilok formations of the Lower–Middle Riphean Sukhoi Pit Group. Five metamorphic zones can be mapped from SW to NE, based on the distribution of isograds. Metamorphic grade increases in the same direction with the following characteristic zonal sequences: (1) Bt + Ms + Chl + Qtz + Pl (biotite zone); (2) Grt + Bt + Ms + Chl + Qtz + Pl (garnet zone); (3) St + Grt + Bt + Ms + Chl + Qtz + Pl ± Crd (staurolite zone); (4) And + St + Grt + Bt + Ms + Qtz + Pl ± Crd (andalusite zone), and (5) Sil + St + Grt + Bt + Ms + Qtz + Pl ± And ± Crd (sillimanite zone) (Likhanov et al., 2011a). For the staurolite zone, gedrite and cummingtonite may occur in a stable association with garnet and cordierite in K2O poor, low-aluminous metaterrigenous rocks of the Ryazanov Formation. Regional metamorphism in the Teya anticline is characterized by strong symmetrical zonation with increasing grades from biotite to sillimanite zones towards the anticline core. This prograde metamorphic zonation in the studied rocks suggests a shallow-level, Buchan-type metamorphism of the andalusitesillimanite type (Dobretsov et al., 1972). Its P-T values correspond to the transition from the greenschist facies to the boundary between the epidote-amphibolite and amphibolite facies. Close to the Teya overthrust, the andalusite and sillimanite zones contain rocks affected by metamorphic overprint. These rocks comprise a 4–5-km-wide zone bounded by the NW-trending fault, indicating only local effects of metamorphic overprinting. The transition from regional grade to high pressure assemblages is characterized by the simultaneous occurrence of kyanite and sillimanite in the absence of cordierite. The critical assemblage Ky + St + Grt + Ms + Bt + Qtz + Pl + Sil with relics of andalusite must have formed under conditions of the kyanite-staurolite subfacies of the kyanite schist facies (Dobretsov et al., 1974). The appearance of kyanite suggests high pressure metamorphic overprint (Barrovian-type metamorphism) (Dobretsov et al., 1974). Regional metamorphism of the andalusite-sillimanite type occurred over a wide range of temperatures, from 510 ºC in the biotite zone to 640 ºC in the sillimanite zone and pressures

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of 3.9–5.1 kbar, indicating a typical metamorphic field gradient with dT/dH of about 30–40 ºC/km for orogenic belts These assemblages were subsequently overprinted by kyanitesillimanite grade metamorphism with a gradually increasing pressure (from 5.65 to 7.15 kbar) and temperature (from 660 to 698 ºC) towards the overthrust (Fig. 2), suggesting a low geothermal gradient of about 8 ºC/km (Likhanov et al., 2011a). The studied rocks are homogeneous metapelites with a narrow range of high Fe- (XFe = FeO/(FeO + MgO + MnO) = 0.60–0.75 whole rock on a mole basis) and high Al- (XAl = Al2O3 – 3K2O/(Al2O3 – 3K2O + FeO + MgO +MnO) = 0.35 – 0.45) bulk compositions with respect to the average pelite whole-rock compositions (XFe = 0.52 and XAl = 0.13) (Shaw, 1956). Detailed reconstructions of the composition and nature of their protolith based on whole-rock major and trace element geochemistry have demonstrated that these rocks represent a re-deposited and metamorphosed product of Precambrian kaolinite weathering crusts (Likhanov and Reverdatto, 2007, 2008, 2011a; Likhanov et al., 2006b, 2008a).

Analytical techniques and results Metapelite samples used for geochronological determinations were collected at equal distances from the thrust from four localities in the outer (samples 284 and 252) and inner (samples 250 and 244) collisional metamorphic zones of the Garevka area (Fig. 3) and four localities within other areas. 40 Ar/39Ar measurements were performed using conventional techniques previously described by Likhanov et al. (2007). The extracted mineral fractions with a size not less than 0.15 mm together with the MCA-11 and LP-6 biotite standards applied as monitors were wrapped in Al foil and vacuum sealed in fused silica tubes. Irradiation was performed in the Cd-shielding in the VVR-K research reactor in the Nuclear Physics Research Institute at Tomsk Polytechnical University (Russia). The neutron gradient did not exceed 0.5%. Step-heating experiments were performed using a quartz vial heated by an external furnace. Typical 40Ar blanks (10 min at 1200 °C) did not exceed 5 × 10–10 ncm3. Released argon was doubly purified by exposure to Ti and ZrAl SAES getters. The isotopic composition of argon was measured with a Micromass 5400 noble gas mass spectrometer at the Analytical laboratory of the Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk (analyst A.V. Travin). All errors are quoted at ±1σ. 40 Ar-39Ar data for mica from different samples from the Garevka area are given in Fig. 5. Mica separates from all of these samples developed plateau ages of 772.9 ± 8.3 Ma (sample 284), 782.6 ± 8.4 Ma (sample 252), 786.8 ± 8.2 Ma (sample 250), and 794.8 ± 8.8 Ma (sample 244). All ages calculated by the plateau method are interpreted to be meaningful estimates of the cooling ages of these rocks below the closure temperature of the K–Ar system in biotite and muscovite (330–360 ºC) (Hodges, 2004), suggesting much lower temperatures than the peak metamorphic conditions during collision. The integrated ages are contributed by

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Fig. 5. 40Ar-39Ar mica ages for collision-related metamorphic rocks of the Garevka area. Either the plateau (arrows) or integrated age is given for each sample.

low-temperature steps, which normally exhibit ages lower than the plateau. It should be noted that the plateau ages for these samples do not overlap within error limits and intermediate values lie in this range in accordance with the distance from the thrust. To determine the rate of exhumation and the age of collisional metamorphism we analyzed the possible thermal history of these rocks during exhumation from the depths of 25.55 km (sample 284: 645 ºC, 7.28 kbar), 21.88 km (sample 252: 617 ºC, 6.25 kbar), 19.25 km (sample 250: 596 ºC, 5.56 kbar), and 17.5 km (sample 244: 575 ºC, 5.04 kbar) (Likhanov and Reverdatto, 2011b). The calculations are based on the previous tectonothermal model for metamorphic evolution of the region, which was developed using the mechanism of tectonic thickening of the Earth’s crust as a result of rapid thrusting with subsequent rapid exhumation and erosion (Likhanov et al., 2009). In this model, the low geothermal gradients were linked to relatively short-lived events and the lack of thermal equilibrium between the blocks of rocks at the respective depths because of a strong thermal inertia relative to pressure. The proposed model can explain a number of features of the metamorphic evolution of metapelitic rocks (e.g., the progressive replacement of andalusite by kyanite, the growth of new mineral assemblages and development of medium-pressure deformation structures, the increase in grossular content from core to rim in garnet grains, the gradual increase in lithostatic pressure with slightly increasing temperature, etc.). The analysis of this model suggests that crustal

thickening in both subducting and overriding plates produces a combined geotherm with the different geothermal gradient in the rocks. The closure temperature of the K–Ar system corresponds to the crustal depth of 15 km for the geotherm calculated for thickened crust. This implies that the metapelites buried to different depths during the post-collisional stage have experienced maximum exhumation of 10.5 km (sample 284), 6.9 km (sample 252), 4.3 km (sample 250), and 2.5 km (sample 244) (Fig. 4). The rapid subsidence, exhumation and erosion of medium-pressure metamorphic rocks should favor the preservation of pre-existing medium-pressure metamorphic assemblages under disequilibrium conditions of the middle and upper crust. Using the difference in K-Ar closure ages (∆t) and depths (∆H) between the deeply and shallow buried blocks which are closest to or farthest from the thrust (present-day erosion surface), we obtain the rate of exhumation: V = ∆H/∆t = 8.05 km/21.9 Myr = 368 m/Myr or 0.368 mm/yr. This value is in good agreement with published 40Ar-39Ar geochronological data (Corsini et al., 2010) and ages from apatite fission tracks (Leech and Stockli, 2000), corresponding to an average exhumation rate of 0.3–1.5 mm/yr. It is also compatible with the rate of overthrusting calculated with a thermophysical model (Likhanov et al., 2004). Taking into account our estimates of the exhumation rate, we can then calculate the time required for metapelites to pass to the 330–360 ºC isotherm: t = 10.5 km/0.368 km/Myr = 28.7 Myr (sample 284), 6.9/0.368 = 18.7 Myr (sample 252), 4.3/0.368 = 11.5 Myr (sample 250), and 2.5 km/0.368 km/Myr = 6.8 Myr

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Fig. 6. 40Ar-39Ar biotite ages for collision-related metamorphic rocks from the Angara (a), Chapa (b), Mayakon (c, d), and Teya (e, f) areas. Either the plateau (arrows) or integrated age is given for each sample.

(sample 244). Collisional metamorphism in these metapelites preceded later uplift and cooling, as evidenced by 40Ar-39Ar results. The timing of peak metamorphism is then the sum of the above values and mica K-Ar cooling ages. Such ages for samples 284, 252, 250, and 244 are no older than 801.4, 801.3, 798.3, and 801.5 Ma, respectively. The results altogether indicate Upper Riphean ages in a tight cluster of 798 to 802 Ma. These age estimates suggest the absence of any significant lag time between exhumation of the metamorphic complexes and the peak of collisional metamorphism in the region, which is quite consistent with previous metamorphic dates reported for collisional orogens (Oliver et al., 2000; Sklyarov, 2006). Thus, exposure of metamorphic complexes

associated with collision was usually governed by the interplay between different tectonic mechanisms, with erosional denudation playing a dominant role. The effect of individual mechanisms may vary at different stages of rock evolution, and metamorphic complexes may present examples of various combinations of these mechanisms (Sklyarov, 2006; Teyssier and Whitney, 2002). 40 Ar-39Ar results on biotite from three other areas are summarized in Fig. 6. Age spectra of biotite from these samples have well-defined plateau ages of 849.1 ± 9.1 Ma (Angara area), 814.9 ± 8.3 Ma (Chapa area), 826.1 ± 8.4, and 829.3 ± 8.4 (Teya area). Biotite ages from two Teya samples overlap within error, indicating a nearly contemporaneous

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Fig. 7. Sequence of magmatic, metamorphic and deformation events throughout the Transangarian Yenisei Ridge during Precambrian time. 1, rift-related plagiogranitic blastomylonites (Popov et al., 2010); 2, Teya-type granite gneisses (Likhanov et al., 2011b; Nozhkin et al., 1999); 3, postcollisional, within-plate and rift-related granitoid plutons with contact metamorphic aureoles and alkaline rocks (Nozhkin et al., 2008a, 2011; Vernikovsky and Vernikovskaya, 2006; Vernikovsky et al., 2007); 4, regional andalusite-sillimanite metamorphism (Likhanov and Reverdatto, 2009, 2011c; Likhanov et al., 2010c, 2011a); 5, collisional kyanite-sillimanite metamorphism caused by westerly directed overthrusts (Likhanov et al., 2010b); 6, intermediate-pressure collisional metamorphism caused by easterly directed overthrusts (Likhanov et al., 2010d); 7, main folding phases during the Grenville orogeny (Rivers, 2008).

onset of rapid exhumation and cooling of all areas within a single block, which experienced collision-related metamorphism. The age of collisional metamorphism was determined from analysis of the thermal history of metamorphic rocks exhumed from depths of 19.6 km (Angara area), 29.4 km (Chapa area), 25.0 and 22.4 km (Teya area) (Likhanov et al., 2009, 2011a). Our interpretation of the above model implies that uplift of metapelites buried to different depths during the post-collisional stage is calculated to be 4.6 km (Angara area), 14.4 km (Chapa area), 10.0 and 7.4 km (Teya area). Taking into account the exhumation rate for the Garevka area (V), we can obtain the time (t) required for metapelites to pass to the 330 ºC isotherm: t = H/V, where H is the distance from the depths of rock formation. For the metapelites from the Angara, Chapa, and Teya areas, these lag times are, respectively, 15–3; 48–10; 33–7, and 25–5 Ma. Given the maximum duration of exhumation, the age of rocks cannot be older than 864 Ma (Angara area), 853 Ma (Chapa area), and 854–859 Ma (Teya area). All these results point to an Upper Riphean age, which agrees well with the age of peak kyanite-sillimanite grade metamorphism in the Mayakon area (851–854 Ma) (Likhanov et al., 2007). These dates obtained from four areas overlap within analytical error. In view of the uniform character of the P-T evolution with a northeastward increase in metamorphic grade across the areas and the apparent synchroneity between ages, all of these metamorphic complexes of the Transangarian Yenisei Ridge can be related to one single geodynamic process.

Conclusions and geodynamic interpretations Our interpretation of isotope and petrological data on the P-T-t evolution of polymetamorphic complexes in the Transangarian Yenisei Ridge allowed us to identify several distinct

metamorphic phases that can be related to different geodynamic settings. The first phase is marked by LP/HT zoned metamorphism of the andalusite-sillimanite type at 953–973 Ma (Likhanov et al., 2010c, 2011a). In the second phase, rocks closest to the thrusts in the vicinity of the Tatarka deep fault underwent the medium-pressure kyanite-sillimanite grade metamorphism, which resulted in the progressive replacement of andalusite by kyanite, the development of new mineral assemblages and deformation structures. A number of features typical of kyanite to sillimanite grade metamorphism, such as a relatively small thickness of the medium-pressure zones (from 2.5 to 7 km) and a gradual increase in pressure towards the thrust from 4.5–5 kbar to 6.5–8 kbar with slightly increasing temperature and weak zoning, suggest a low metamorphic field gradient with dT/dH ranging from 1–7 to 12 °C/km. These features testify to collision-related metamorphism due to crustal thickening as a result of thrusting, rapid tectonic exhumation and erosion. Based on geothermobarometry and 40Ar-39Ar mica ages, the proposed model suggests that, given an exhumation rate of 0.368 mm/yr for a number of areas, the peak of collisionrelated metamorphic conditions occurred at 849–862 and 798–802 Ma. This apparent (~ 50 Ma) difference in metamorphic ages and the features specific to collisional metamorphism (e.g., an opposite direction of increasing metamorphic grade) also indicate that the formation of the studied areas may be related to different geodynamic processes (Fig. 7). 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 supported by geophysical observations (Egorov, 2004; Konstantinov et al., 1999) and regional provenance studies (Likhanov and Reverdatto, 2007, 2008, 2011b; Likhanov et al., 2010a; Maslov et al., 2008; Nozhkin et al., 2009a;

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Vershinin et al., 2007). The above works suggest that the likely provenance during deposition of Riphean sediments in the Transangarian Yenisei Ridge was from Early Proterozoic (1.9–2.0 Ga) granite-gneiss blocks (complexes) of the Siberian craton which represent geochemically mature weathered crust. This is further supported by a detailed study of Sm-Nd isotopic systematics of Precamrbian type sections of the Yenisei Ridge. The above works suggest that the likely provenance during deposition of Riphean sediments in the Transangarian Yenisei Ridge was from Early Proterozoic (2.0 Ga) granite-gneiss blocks (complexes) of the Siberian craton, which represent strongly chemically weathered crustal rocks. This is further supported by a detailed study of Sm-Nd isotopic systematics of Precambrian type sections of the Yenisei Ridge (Nozhkin et al., 2008b). 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 and its subsequent thrusting over the Siberian craton, which is in agreement with the Neoproterozoic accretionarycollision model for the Yenisei Ridge (Vernikovsky et al., 2009) and geophysical observations (Sal’nikov, 2009). Additional evidence for easterly directed thrusting at that time is provided by the structural position of the Rybinsk–Panimba ophiolites (1050–950 Ma) (Nozhkin, 2009; Nozhkin et al., 2011) within the Central Angara terrane, which in turn was thrust over by the ophiolitic nappes and slices as a result of these processes (Vernikovsky and Vernikovskaya, 2006; Vernikovsky et al., 2009). The above two phases of metamorphism were coeval with the final stage of the Grenvillian orogenic events that peaked at 960 and 850 Ma, whereas repeated metamorphism due to thrusting took place at 798–802 Ma in a post-Grenville epoch.

Acknowledgements This study was supported by the Presidium of the Siberian Branch, Russian Academy of Sciences (integration project no. 20) and the Russian Foundation for Basic Research (project no. 11-05-00321). Constructive reviews and comments from A.D. Nozhkin and P.Ya. Azimov are gratefully acknowledged.

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