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

Russian Geology and Geophysics 50 (2009) 1034–1051 www.elsevier.com/locate/rgg

Kyanite–sillimanite metamorphism of the Precambrian complexes, Transangarian region of the Yenisei Ridge I.I. Likhanov *, V.V. Reverdatto, P.S. Kozlov, N.V. Popov Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia Received 19 November 2008; received in revised form 14 January 2009

Abstract 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). © 2009, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: metapelites; geothermabometry; P-T-paths; collisional metamorphism; Yenisei Ridge

Introduction Close mapping of metamorphic rocks on the basis of a metamorphic facies scheme allowed us to establish their relationship with tectonic features. Relationships between metamorphism and geologic setting were revealed from the first mapping results summarized in the Metamorphic Facies Map of the USSR, which was compiled under the leadership of V.S. Sobolev (Dobretsov et al., 1966). Information obtained from interpretation and mapping of the metamorphic rocks has come to be used in most geodynamic reconstructions, shaping our understanding of the physical causes and conditions of metamorphism. This paper attempts to examine whether information on metamorphic complexes could provide a deeper insight into their tectonic evolution. In addition, this is

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

the first study in Russia to evaluate the collisional metamorphism related to overthrusting of continental blocks. Marginal parts of cratons provide valuable information on the evolution of the lithosphere: the growth and rifting of the continental crust, opening of ocean basins, and many other geodynamic events. This may explain the keen interest of many authors to the problems related to the processes forming accretionary-collisional structures within continents and the tectonic evolution of fold-and-thrust belts at the boundaries of cratons (Khain et al., 2003; Vernikovskaya et al., 2007; Volkova and Sklyarov, 2007; and others). The southwestern framing of the Siberian craton comprises heterogeneous blocks of the Yenisei Ridge and the northern slopes of East Sayan, which are part of the Central Asian orogenic belt. Comparison of these blocks with other portions of the belt enables the intraregional correlations, which are important for reconstruction of the complicated tectonic structure of Central Asia. An elucidation of the structure and tectonic history of Siberian Meso-Neoproterozoic continental margins is required to address the question of the possible incorporation of the Siberian

1068-7971/$ - see front matter D 2009, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2009.11.003

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craton into the Meso- to Neoproterozoic Rodinia supercontinent and its subsequent Neoproterozoic breakup involving opening of the Paleoasian ocean (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 Vendian. The close spatial association of diverse igneous and metamorphic complexes suggests that the region had a complicated tectonic history. In particular, a very important feature of metamorphic complexes in the Yenisei Ridge realm is the heterogeneity of the metamorphism in terms of pressure, which was manifested in regional metamorphism of the andalusite-sillimanite (low-pressure) and kyanite-sillimanite (medium-pressure) facies series (Kozlov and Lepezin, 1995). The medium-pressure metamorphism postdated the low-pressure event and occurred locally near thrust faults. It resulted in the prograde replacement of andalusite by kyanite and the development of newly formed mineral assemblages and deformation microtextures (Likhanov et al., 2004). These phenomena and processes are of special petrological interest, since the progressive transformation of andalusite- or kyanitebearing to sillimanite-bearing assemblages are known to be common reactions between the Al2SiO5 polymorphs within low- and medium-pressure metamorphic complexes. The replacement of andalusite by kyanite during the progressive metamorphic stage observed within the Yenisei Ridge is unusual because normal static continental geotherms never cross the andalusite/kyanite stability curve (Kerrick, 1990). Few examples for the transition of andalusite- to kyanite-bearing assemblages are available in the literature (northwestern US and Canadian Cordillera; Dalradian Highlands, Scotland; Central and Northwestern Appalachians, USA; Kola Peninsula and Yenisei Ridge), and these are attributed to pressure increase due to either: (1) thrusting (Baker, 1987; BeddoeStephens, 1990; Bel’kov, 1963; Clarke et al., 1987; Crawford and Mark, 1982; Spear et al., 1990, 2002; 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). P-T paths enable us to discriminate the first type of geodynamic environment from the second one (Jamieson et al., 1998, 2002). In this context, we combined geological and petrological data from four metamorphic complexes of the Transangarian region of the Yenisei Ridge to reveal specific events in their metamorphic histories and to discuss the implications of these facts with respect to the geodynamic processes in the region.

Regional geological setting The Yenisei Ridge is a fold-nappe belt, some 700 km long and 50–200 km wide, extending SSE-NNW (Fig. 1) and located within the southwestern framing of the Siberian platform. This large accretionary-collisional structure is mappable using geological and geophysical data and can be

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separated from adjoining areas of the Siberian platform and West Siberian plate (Starosel’tsev et al., 2003). The Yenisei Ridge is subdivided into two large segments, the South Yenisei and Transangarian, separated by the ENE-trending Angara regional strike-slip fault (Kheraskova, 1999). Two allochthonous units have been recognized south of the Angara Fault, the Paleoproterozoic Angara–Kan terrane and the Neoproterozoic Predivinsk terrane. North of the Angara Fault, the Yenisei Ridge is composed mainly of Meso-Neoproterozoic rocks of the East Angara, Isakovka and Central Angara terranes (Lobkovskii et al., 2004; Vernikovsky et al., 2003). All the terranes are crustal blocks and slices, 200–500 km long and 50–80 km wide (Fig. 1), separated by large deep faults (Smit et al., 2000). A number of higher-order faults, which splay off these regional faults, suggest a dip slip displacement forming thrust faults (Egorov, 2004; Konstantinov et al., 1999). These processes brought about a regionally heterogeneous pressure field of metamorphism and, consequently, a combination of two facies series: andalusite–sillimanite (lowpressure) and kyanite–sillimanite (medium-pressure). Collision-related medium-pressure metamorphism locally overprints the low-pressure metamorphic rocks. We examined the fields of these metapelites, which reflect the late medium-pressure metamorphic processes. From north to south, we distinguished four typical localities (Chapa, Garevka, Mayakon, and Angara), in which Early Proterozoic, Middle and Late Riphean rocks are exposed (Khabarov, 1994; Khabarov et al., 2004; Nozhkin, 2004; Nozhkin et al., 2003a; Volobuev et al., 1968, 1973, 1976) (see Fig. 1). Tectonically, three 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 Angara–Kan block. The present study provides more detailed information on the Garevka area whereas the metamorphic evolution of the other areas has been previously examined in a number of studies (Korobeinikov et al., 2006; Kozlov and Lepezin, 1995; Likhanov and Reverdatto, 2007, 2008; Likhanov et al., 2001a, 2004, 2006a, 2007, 2008a,b). The Central Angara terrane is made up mostly of metamorphosed terrigenous and terrigenous-carbonate rocks of the Teya (including Garevka), Sukhoi Pit, Tungusik, Oslyanka, Chingasan, and Chapa sequences. The gneisses and crystalline schists of the Garevka Sequence are exposed in the western part of the terrane. Farther east, the western and central parts of the terrane comprise metaterrigenous-carbonate complexes of the Teya Sequence that were formed at the Early–Late Precambrian boundary. The Late Precambrian deposits contain the most complete Riphean section that is about 15 km thick (Nozhkin et al., 2003b; Postelnikov, 1990). During the Grenville orogeny, these rocks underwent an early Neoproterozoic low-pressure metamorphic event. The grade of regional metamorphism reached amphibolite and epidote– amphibolite facies in the lower part of the section (Teya Sequence and the lower parts of the Sukhoi Pit Sequence, including the Korda and Gorbilok Formations), however, it did not exceed greenschist facies in most of the Sukhoi Pit, Tungusik, and Oslyanka sequences and prehnite-pumpellyite

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Fig. 1. Schematic tectonic map of the Yenisei Ridge (after: Vernikovsky and Vernikovskaya, 2006) and locations of areas where metamorphism of the kyanite–sillimanite type is observed. 1, cover; 2, molasse; 3, mainly carbonate rocks; 4, ophiolitic and island-arc complexes of the Pre-Yenisei belt; 5, granitoids; 6, ophiolites of the Rybinsk–Panimba belt; 7, greenschist to amphibolite facies metamorphic complexes; 8, Early Precambrian granulite–amphibolite complexes; 9, regional faults (a) and geological boundaries (b). Encircled letters, major thrust faults: I, Ishimba, T, Tatarka, P, Pre-Yenisei, A, Ankinovka. P-T parameters of the kyanite–sillimanite metamorphism are given below the names of the areas. Encircled numbers, terranes: 1, Central Angara, 2, East Angara, 3, Isakovka, 4, Predivinsk, 5, Angara–Kan.

facies in the Chingasan and Chapa sequences. Epidote–amphibolite and amphibolite facies rocks were mapped within the zonal metamorphic complexes attributable to the andalusite– sillimanite and kyanite–sillimanite metamorphic facies types. Isotopic data on Upper Proterozoic rocks, especially metamorphic ones, are still scarce. The available results do not allow precise age constraints on the zonal metamorphic complexes of the andalusite–sillimanite type, which are supposed by many researchers to be genetically related to the onset of the Teya granite-gneiss dome at 1000–950 Ma

(Nozhkin et al., 1999, 2008; Volobuev et al., 1976), i.e., during the Grenville orogenic event that may have occurred at approximately the same time in other lithospheric blocks of the Asian continent (Ernst et al., 2008). More recent SHRIMP II U-Pb data on zircons from the Teya granitoids indicate that these rocks were emplaced at 880–865 Ma (Vernikovsky et al., 2007). Our latest Ar-Ar dates for biotite from metapelites of the Korda Formation of the Sukhoi Pit Sequence suggest with regard to exhumation rates that the culmination of the kyanite–sillimanite metamorphism may be no older than

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848–851 Ma (Likhanov et al., 2007), i.e., late medium-pressure metamorphic overprinting is of Neoproterozoic age. These events may be synchronous with late stages of the Grenville orogeny (Nozhkin et al., 2008; Turkina et al., 2007). A very small time gap between the regional low-pressure metamorphic processes and collisional medium-pressure metamorphic overprinting obviously testifies to the successive character of these events. These estimates and the succession of events are consistent with the Precambrian geological history of the Yenisei Ridge, where two long granite-gneiss belts with numerous swarms of pegmatite veins and zonal low-pressure metamorphic complexes are thought to have been formed due to accretionary–collision processes at the Middle and Late Riphean boundary (950–900 Ma). During the Late Riphean, the southwestern part of the Siberian continent was developed in a continental margin setting replaced then by orogenic setting at 850–870 Ma until eventually it evolved into a folded-nappe belt (Nozhkin, 2004).

Brief description of the Angara, Mayakon, and Chapa areas The Angara area is located on the interfluve between the Angara, Belokopytovka, and Tatarka rivers (Likhanov et al., 2006a). The reference sections are well exposed on the right bank of the Angara River, within the Tatarka shear zone. The geological framework of this area consists of Late Riphean (1000–900 Ma (Khabarov et al., 2004)) sedimentary and low-pressure metamorphic rocks, such as rhythmically intercalating quartzites and phyllites of the Sukhoi Range Formation (Tungusik Sequence). These regionally metamorphosed rocks are mostly phyllites of greenschist facies represented by Qtz* + Ms + Chl + Ilm mineral assemblage. They underwent regional metamorphism under elevated pressures, which produced kyanite-bearing metapelite assemblages. This metamorphic event occurred simultaneously with the development of cleavage dipping steeply (80°) to the northwest and southeast. The increase in metamorphic grade from east to west is marked by the successive development of chloritoid (apparent thickness 0.5–1 km) and kyanite (thickness about 1.5–2 km) zones. Mineral assemblages of rocks from the latter zone show that they are well within the chloritoid-kyanite subfacies of kyanite schists (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 on the Kulakov uplift on the left bank of the Angara River show signs of Neoproterozoic kyanite–sillimanite mediumpressure metamorphism (792–856 Ma (Zvyagina, 1989)), which is characterized by replacement of andalusite by kyanite in the metapelites of St + Pl + Ms + Bt + Grt + Qtz * Symbols of minerals: Alm, almandine; Als, aluminosilicate; An, anorthite; And, andalusite; Bt, biotite; Chl, chlorite; Cld, chloritoid; Crd, cordierite; Grs, grossular; Grt, garnet; Ilm, ilmenite; Ky, kyanite; Ms, muscovite; Pl, plagioclase; Prp, pyrope; Qtz, quartz; Sil, sillimanite; Sps, spessartine; St, staurolite.

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composition (Likhanov et al., 2008b). Geothermobarometry and P-T path calculations indicate a gradual increase in pressure from 4.1 to 5.6 kbar toward the thrust with a slight increase in temperature from 530 to 560 °C, which suggests that the rocks underwent near-isothermal subsidence with a metamorphic field gradient as low as 10 °C/km (Likhanov et al., 2006a). These data are in agreement with the petrogenetic grid for Fe- and Al-rich metapelites (Likhanov and Reverdatto, 2005) and the P-T diagram for typical metapelites (Fed’kin, 1970), in which the univariant mineral assemblage Ky + St + Pl + Ms + Bt + Grt + Qtz in the system K2O–FeO–MgO–Al2O3–SiO2–H2O is stable at temperatures up to 590 °C and pressures up to 6 kbar, with the staurolite stability field diminishing towards lower pressures and temperatures as its Fe/(Fe + Mg) ratio increases. The Mayakon area is situated between the Yeruda and Chirimba Rivers (Likhanov et al., 2001a), where the Middle Riphean (1350–1250 Ma (Nozhkin, 2004; Nozhkin et al., 2003b)) Korda Formation rocks underwent low- and mediumpressure metamorphic events. Within the study area the low-pressure metapelites containing the mineral assemblages Ms + Chl + Bt + Cld + And + Qtz + Ilm ± Crd were formed under greenschist and epidote-amphibolite facies conditions. The medium-pressure rocks are typically Ms + Chl + Bt + Qtz + Ky + St + Grt + Ilm + Pl varieties with sporadic sillimanite and a relict andalusite formed under conditions of the kyanite schist facies. These rocks make up 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 Penchenga Formation. Three zones of metamorphic overprinting parallel to the Panimba overthrust were distinguished based on the relationship between newly formed mineral assemblages and the intensity of deformation in the 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 within a relatively narrow temperature range (from 550 to 580 °C) (Likhanov et al., 2004). These data agree 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 the gradual 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), thus suggesting near-isothermal loading with a metamorphic field gradient as low as 5–7 °C/km. For the observed metamorphic evolution, we proposed a tectonic model (Likhanov et al., 2004) and performed thermophysical calculations using the actual physical properties of metapelites and metacarbonates, such as radioactive heat production and thermal conductivity coefficients. A gradual pressure increase is explained by crustal thickening within the Panimba thrust, which resulted in thrusting of the 5–7 km thick Penchenga metacarbonates over the Korda metapelites. An insignificant increase in tempera-

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ture during thrusting is explained by specific behavior of steady-state geotherms calculated for various rocks with different heat generation and thermophysical properties (Likhanov et al., 2004). The Chapa area located in the middle reaches of the Chapa River, between the mouths of its two tributaries, the Nizhnyaya Veduga and Elovaya Rivers (Likhanov et al., 2008a,b), comprises Lower Proterozoic (1650–1500 Ma (Volobuev et al., 1973)) deposits of the Teya Sequence. These rocks have been 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 metaterrigenouscarbonate rocks (marbles with quantitatively subordinate crystalline schists) of the Penchenga Formation. Away from the overthrust, the low-pressure metapelites of the Penchenga and Karpinsky Range formations consist of Ms + Chl + Bt + Qtz + Pl and And + St + Grt + Ms + Bt + Qtz + Chl varieties, respectively, which were formed under greenschist and epidote-amphibolite facies conditions. Closer to the overthrust, the rocks are affected by medium-pressure metamorphism of the kyanite-sillimanite type. 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 relics was formed under the conditions of kyanite schist facies (Dobretsov et al., 1974). A 4–5 km wide zone comprising these rocks is truncated in the east by a NW-trending overthrust. Three metamorphic zones parallel to this overthrust were distinguished based on variations in the texture and compositional characteristics of the metapelites. The results of geothermobarometry indicate that peak metamorphic pressure gradually increased toward the thrust from 3.9–4.9 to 5.5–5.8 kbar in Zone I (metapelites of the andalusite–sillimanite type of the Penchenga and Karpinsky Range Formations, respectively) to 6.7–7.4 kbar in the outer Zone II (metapelites of the kyanite–sillimanite type), and further to 8.1–8.4 kbar in the inner Zone III (predominantly kyanite blastomylonites of the kyanite–sillimanite metamorphic type). In approaching the thrust the mean maximum temperatures increase only slightly from 630 to 710 °C, indicating a surprisingly low metamorphic field gradient. Compared with rocks from other regions of the Transangarian part of the Yenisei Ridge, metapelites of the Chapa area yield higher P-T values. Higher temperatures can be accounted for by the ubiquitous occurrence of sillimanite in Zone II. In other kyanite–sillimanite metamorphic complexes sillimanite is either absent (Angara area) or located only in the vicinity of granites (Mayakon area), that can be well explained by additional local heat transfer from the intrusive body (Likhanov, 2003; Likhanov et al., 2001b). Peak pressures can be accounted for by a maximum increase in garnet grossular content from core to rim. The PT-path calculations indicate that in approaching the overthrust, an overall pressure in metapelites from the Karpinsky Range Formation gradually increases from southwest to northeast by 2.0–2.5 kbar with a slight increase in temperature, suggesting

that the rocks underwent subsidence with a metamorphic field gradient as low as 12 °C/km (Likhanov et al., 2008b).

Garevka area: metamorphic zones, mineral assemblages, and microtextural observations The Garevka area located on the interfluve between the Chapa–Garevka–Tis Rivers, in the vicinity of Mt. Garevsky Polkan (Fig. 2), 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 structures 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 associated with the Uvolga fault zone in the northern sector of the Tatarka deep fault (Fig. 2). Away from the overthrust, the low-pressure metapelites of the study area contain the mineral assemblage

Fig. 2. Geological sketch map of Precambrian metamorphic rocks in the vicinity of Mt. Garevsky Polkan (Garevka area) (modified after Kachevskii et al. (1998)). Sedimentary-metamorphic units: 1, Lower Riphean (Korda Formation, biotite-quartz, biotite-feldspar-quartz crystalline schists with graphite (kd); 2, Lower Proterozoic (Karpinsky Range Formation, quartzite and aluminous two-mica schists and gneisses with andalusite, kyanite, sillimanite, staurolite and garnet, locally blastomylonitized (hk)); Archean–Lower Proterozoic poorly defined (3, Garevka stratum, mesocratic and leucocratic gneisses, marbles, calciphyres, quartzites and mica schists (gr); 4, Nemtikha stratum, biotite, two-mica and biotite-amphibole plagiogneisses and two-mica crystalline schists with staurolite, kyanite, sillimanite and garnet (nm)); 5, major faults (a), overthrusts with toothing in declination (b) and minor faults (c); 6, sample locations; 7, metapelites of andalusite-sillimanite (I) and kyanite-sillimanite (II, III) metamorphic facies type; 8, isograd And-Ky (a) and boundaries between metapelites of kyanite-sillimanite facies type (b).

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Ms + Chl + Bt + Cld + And + St + Qtz + Pl + Ilm formed under greenschist and epidote-amphibolite facies conditions. Closer to the overthrust, the rocks are affected by mediumpressure metamorphism of the kyanite-sillimanite type. 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 ± Cld with andalusite relics was formed under conditions of the kyanite schist facies (Dobretsov et al., 1974). The Garevka metapelites, unlike other rocks from the Transangarian Yenisei Ridge, contain more abundant chloritoid and blastocataclasites with a zoned, highly fractured garnet grains. A 4–5 km wide zone of these rocks is truncated in the east by a NW-trending overthrust (Fig. 2). Three metamorphic zones parallel to the overthrust were distinguished in this zone based on variations in the texture and compositional characteristics of the metapelites. The rocks of Zone I are typically andalusite- and kyanitebearing metapelites, which were unaffected by medium-pressure metamorphism of the kyanite-sillimanite type. From the inner side of Zone I, mineral parageneses are marked by the kyanite isograd. Away from the overthrust, the metapelites consist of dark gray rocks with a fine- to medium-grained lepidoblastic or lepidogranoblastic Ms-Chl-Bt-Qtz-Pl matrix, comprising large andalusite and small chloritoid and staurolite porphyroblasts. Idioblastic crystals of andalusite are fresh and chiastolitic. In thin section, these crystals show hourglass growth structures, up to 1 cm across, indicated by graphitebearing and graphite-free areas. Staurolite is present as small idioblastic grains (1–2 mm in length) in the matrix, with abundant microscopic inclusions of chlorite and muscovite. Prismatic and tabular crystals of chloritoid, up to 2–3 mm in length, occur in direct contact with andalusite and biotite. Quartz forms idioblastic as well as elongate and ovoid grains up to 0.01–0.2 mm in diameter with regular and irregular boundaries. Muscovite occurs as isolated flakes, 0.03 to 0.1 mm in length, or as aggregates between quartz grains. Biotite and chlorite are found as flakes up to 0.1 mm in diameter. Plagioclase is found as irregular grains (0.07 mm in diameter) or grains elongated parallel to foliation. Graphite is the most abundant minor mineral in all rocks. As the thrust is approached, the outer (II) and inner (III) zones of medium-pressure metamorphic overprinting are recognized. All rocks of Zones II and III differ in abundances of the relict and newly-formed minerals and the degree and style of deformation. Ky-St-Bt pseudomorphs after andalusite are conspicuous in these rocks. The matrix consists of the Grt + St + Bt + Ms + Qtz + Pl ± Sil ± Cld ± Chl mineral assemblages (without Ky and And). Towards the thrust, the abundance of andalusite decreases in rocks until it disappears at the boundary between Zones II and III, while the amount of staurolite and muscovite decreases with increasing kyanite and garnet. Zone II is distinguished by the appearance of a new mineral paragenesis with kyanite and sillimanite. Rocks of this zone represented by Ky + Sil + Grt + St + Ms + Bt + Qtz + Pl

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± And ± Cld ± Chl assemblage can be attributed to the kyanite–sillimanite–staurolite subfacies of the kyanite schist facies (Dobretsov et al., 1974). This zone is characterized by partial cataclastic breaking of andalusite porphyroblasts locally replaced by Ky-St-Bt pseudomorphs along margins. The former square and prismatic cross-sections of cataclastic grains are seen in thin section to take on a rhombic and ovoid habit, with the long axes of grains being oriented parallel to foliation. Prismatic crystals and radial sheaves of kyanite nucleate with random crystallographic orientation along grain boundaries and defects in andalusite. The mineral transformations in the matrix could occur simultaneously with the pseudomorph development: sillimanite appears as fibrolitic aggregates or rare crystals of a prismatic habit. Fine-grained mineral aggregates, which form the matrix, are oriented along the cleavage plane and secondary foliation and locally wrap around andalusite and garnet. Two generations of staurolite crystals are distinct in these samples: large idioblastic porphyroblasts (up to 1.5 mm in size) and small crystals (0.03–0.1 mm in size). The large staurolite grains are characterized by inclusions of vermicular quartz, which forms a veiny texture on the surface. They also contain abundant small inclusions of chlorite and biotite relics, which were captured during the growth of staurolite crystals at the early stages of metamorphism (andalusite–sillimanite type). Biotite and muscovite flakes exhibit kink-bands. At their rims, smaller fragments are rotated in the direction of secondary foliation. Garnet is also present as small idioblastic grains (0.1–1.5 mm across) only in the matrix, with abundant inclusions of chlorite, muscovite and quartz in its core, with rare biotite and chloritoid. The modal amounts of kyanite, sillimanite, biotite, staurolite, and garnet in Zone II increase with decreasing chloritoid, chlorite, muscovite, and andalusite contents in Zone I. Zone III, adjacent to the trust (Fig. 2), is characterized by complete recrystallization of minerals with the development of blastocataclasite and blastomylonite zones. An abrupt decrease in sillimanite and disappearance of relict andalusite define the outer part of this Zone, where kyanite is the only Al2SiO5 polymorph present. Rhomb-like pseudomorphs form lenses, with their long axis oriented parallel to the foliation. In some samples, they exhibit microboudinage, with the necks of the boudins in the granolepidoblastic matrix invaded by well-developed strain shadows with recrystallized quartz. Rocks of Zone III represented by Ky + St + Grt + Ms + Bt + Qtz + Ilm + Pl assemblage can be attributed to the kyanite– staurolite subfacies of the kyanite schist facies (Dobretsov et al., 1974). In blastocataclasites, garnet grains of up 1.0 mm in size are disrupted by cracks and shifted along strain-slip cleavages or show locally spiral-shaped inclusion fabrics (snowball structures). In contrast to the idioblastic garnet crystals in other zones, garnet in Zone III forms lenticular porphyroblasts (Fig. 3, c), reflecting growth under stress (Passchier and Trouw, 1996). Prominent structural features in areas adjacent to the thrust are cataclastic deformation and boudinage. Intense deformation is also obvious from lenticular–nodular structure of rocks, the presence of granular quartz veins and a decrease in grain size of minerals.

1040

I.I. Likhanov et al. / Russian Geology and Geophysics 50 (2009) 1034–1051

Fig. 3. Photomicrographs of garnet crystals in quartz–plagioclase–muscovite–biotite matrix from metapelite samples 280 (a), 282 (b), and 284 (c) collected within the Karpinsky Range Formation and chemical zoning profiles (sample 280, d; sample 282, e; and sample 284, f) through line A–B. Open dots in the profiles show garnet compositions used for P-T path modeling.

Mineral chemistry of the Garevka rocks Previous reconstructions of the composition and nature of the protolith of Fe- and Al-rich metapelites from the Karpinsky Range Formation based on whole-rock major and trace element geochemistry have demonstrated that these rocks are the redeposited and metamorphosed products of Precambrian kaolinitic weathering crusts (Likhanov and Reverdatto, 2008; Likhanov et al., 2006b, 2008a). Chemical analyses of all minerals in the Garevka metapelites were obtained using a Jeol JXA-8100 electron microprobe at the Institute of Geology and Mineralogy, Siberian Branch of the RAS (Novosibirsk). The results are given in Table 1. The presence of graphite and nearly pure ilmenite in every rock indicates that the oxidation ratio was low and that Fe3+ should be minor (Holdaway et al., 1988; Likhanov et al., 1994). Assuming this to be the case, we calculated mineral stoichiometries (Table 1). Considered below are the main tendencies in the variations in the chemical compositions of rock forming minerals. Andalusite, kyanite and sillimanite are pure Al2SiO5 within detection limits of the electron microprobe, and thus must have low Fe and Mg contents of <0.01 atoms per formula unit. Garnet compositions are essentially in the narrow range of Alm72–81, Prp8–14, Grs4–10 with slightly more variable Sps1–13 and with XFe = Fe/(Fe + Mg) values in the range of 0.85–0.90.

Generally, garnets from Zone III are characterized by an increase in grossular component and a decrease in spessartine component from core to rim, and by a conspicuously flat zoning profile with nearly constant almandine and pyrope component (Table 1, Fig. 3). Compared with garnets from other localities subject to collisional metamorphism in the Yenisei Ridge, whose compositions vary from Alm73–76, Prp6–8, Sps8–15, Grs2–6 for the Angara area, through Alm83–85, Prp7–9, Sps3–7, Grs4–8 for the Mayakon area, to Alm70–73, Prp9–13, Sps6–14, Grs5–11 for the Chapa area, the garnets from the Garevka area are intermediate in their end-member composition between the Chapa and Mayakon garnets. Staurolite compositions are very similar in all analysed samples. MnO and ZnO contents are very low and vary only slightly (0.11–0.38 and 0.18–0.99 wt.%, respectively). The staurolite is relatively unzoned and remarkably iron-rich, with XFe in the range 0.83–0.90. Chloritoid is chemically homogeneous, with similar XFe in the narrow range of 0.83–0.88. Biotite is unzoned, and the composition of biotite grains in contact with garnet does not differ significantly from the composition in grains away from garnet. The most iron-rich biotite (XFe = 0.63–0.68) occurs in metapelites nearby the thrust and at the boundary between Zones II and III.

Si Al Fe Ca Na K XAn

SiO2 Al2O3 FeO CaO Na2O K2O Total

Component

Si Ti Al Fe Mn Mg Ca XAlm XPrp XGrs XSps XFe

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Total

Component

56.80 26.83 0.04 9.00 6.79 0.09 99.63 8(O) 2.56 1.43 0.00 0.44 0.59 0.005 0.42

244

Plagioclase

37.25 0.00 21.60 33.00 4.23 2.76 2.52 101.4

37.68 0.00 20.75 32.22 5.75 2.68 1.72 100.8 12(O) 3.000 0.00 1.965 2.250 0.219 0.362 0.216 0.738 0.119 0.071 0.072 0.861

2.57 1.42 0.00 0.43 0.60 0.005 0.42

57.12 26.79 0.07 9.01 6.93 0.10 99.99

245

2.966 0.00 2.027 2.197 0.285 0.328 0.215 0.726 0.108 0.071 0.094 0.870

245

244

Garnet

2.60 1.38 0.00 0.39 0.66 0.004 0.37

57.84 26.07 0.07 8.02 7.52 0.08 99.61

250

3.016 0.00 1.969 2.205 0.284 0.315 0.208 0.732 0.105 0.069 0.094 0.893

37.76 0.00 20.92 33.01 4.20 2.65 2.44 101.0

250

2.57 1.42 0.00 0.43 0.60 0.003 0.42

57.11 26.77 0.06 9.02 6.95 0.04 99.98

252

2.968 0.002 1.974 2.239 0.197 0.386 0.271 0.724 0.125 0.088 0.064 0.853

37.01 0.03 20.89 33.39 2.90 3.23 3.15 100.7

252

2.64 1.36 0.01 0.34 0.65 0.006 0.34

59.01 25.70 0.14 7.17 7.44 0.11 99.57

253

2.73 1.27 0.00 0.29 0.69 0.007 0.30

60.95 24.15 0.10 6.03 7.94 0.12 99.35

269

3.043 0.001 1.893 2.496 0.037 0.270 0.265 0.813 0.088 0.086 0.012 0.902

37.85 0.02 19.98 37.13 0.55 2.25 3.08 100.9

253

2.73 1.27 0.00 0.28 0.71 0.006 0.29

60.99 24.04 0.00 5.93 8.14 0.11 99.22

271

3.036 0.002 1.967 2.106 0.382 0.322 0.158 0.710 0.108 0.053 0.129 0.867

37.75 0.04 20.75 31.31 5.61 2.68 1.83 100.1

269

Table 1 Chemical composition (wt.%) and structural formulas for minerals from metapelitic rocks

2.58 1.42 0.01 0.43 0.53 0.004 0.45

57.07 26.70 0.15 8.80 6.01 0.06 98.95

274

2.970 0.00 2.000 2.170 0.370 0.370 0.130 0.714 0.122 0.043 0.122 0.854

36.57 0.00 20.86 31.96 5.42 3.06 1.52 99.49

271

2.58 1.41 0.01 0.42 0.62 0.004 0.40

57.18 26.50 0.16 8.82 7.14 0.07 99.87

280c

2.958 0.003 1.984 2.218 0.194 0.389 0.294 0.717 0.126 0.095 0.062 0.851

35.98 0.04 20.48 32.26 2.79 3.18 3.34 98.15

274

2.63 1.37 0.00 0.35 0.64 0.004 0.35

58.62 25.92 0.08 7.32 7.39 0.08 99.41

280

2.978 0.00 2.009 2.255 0.305 0.297 0.167 0.746 0.098 0.055 0.101 0.884

36.05 0.00 20.64 32.64 4.35 2.41 1.89 98.07

280c

2.70 1.30 0.01 0.31 0.69 0.008 0.30

57.19 26.51 0.11 6.41 8.05 0.13 98.41

282c

3.051 0.001 1.888 2.430 0.078 0.257 0.299 0.793 0.084 0.097 0.026 0.904

37.98 0.02 19.94 36.18 1.15 2.15 3.47 100.9

280

2.75 1.26 0.00 0.25 0.72 0.012 0.26

61.32 23.99 0.12 5.29 8.48 0.24 99.74

282

2.997 0.00 1.980 2.267 0.327 0.300 0.137 0.748 0.099 0.045 0.108 0.883

36.53 0.00 20.48 33.04 4.71 2.45 1.56 98.85

282c

2.56 1.42 0.00 0.43 0.61 0.003 0.41

57.14 26.81 0.09 8.89 7.04 0.05 99.98

284c

2.66 1.34 0.00 0.33 0.66 0.009 0.33

59.34 25.52 0.12 6.87 7.58 0.18 99.62

284

2.973 0.00 1.975 2.430 0.043 0.442 0.173 0.787 0.143 0.056 0.014 0.846

37.14 0.00 20.94 36.31 0.63 3.71 2.02 100.8

282

2.72 1.28 0.00 0.28 0.70 0.010 0.28

61.02 24.23 0.07 5.74 8.07 0.20 99.40

264

3.049 0.00 1.887 2.434 0.119 0.326 0.188 0.794 0.106 0.061 0.039 0.882

37.94 0.00 19.92 36.22 1.75 2.72 2.18 100.8

284c

2.75 1.26 0.00 0.24 0.74 0.008 0.25

61.64 23.95 0.11 5.08 8.52 0.14 99.44

266

3.062 0.026 1.865 2.369 0.099 0.246 0.312 0.783 0.081 0.103 0.033 0.906

37.63 0.42 19.45 34.82 1.44 2.03 3.58 99.41

284

I.I. Likhanov et al. / Russian Geology and Geophysics 50 (2009) 1034–1051 1041

SiO2 TiO2 Al2O3 FeO MgO Na2O K2O Total 11(O) Si Ti AlIV AlVI Fe Mg Na

Component

Si Ti AlIV AlVI Fe Mn Mg Na K XFe XAnn XPhl

SiO2 TiO2 Al2O3 FeO MnO MgO Na2O K2O Total

Component

35.48 2.97 19.77 19.15 0.15 9.79 0.37 9.01 95.93

35.41 1.16 19.13 18.26 0.08 10.27 0.22 9.24 94.02 11(O) 2.718 0.067 1.282 0.448 1.172 0.005 1.175 0.033 0.922 0.50 0.41 0.41

47.80 0.96 36.00 1.36 0.41 0.36 8.68 95.57

3.124 0.047 0.876 1.897 0.074 0.040 0.046

3.078 0.040 0.922 1.877 0.074 0.034 0.057

245

46.45 0.81 35.85 1.34 0.35 0.45 10.12 95.36

244

Muscovite

2.640 0.170 1.360 0.380 1.190 0.010 1.090 0.060 0.860 0.52 0.42 0.38

245

244

Biotite

Table 1 (continued)

3.111 0.019 0.889 1.924 0.048 0.046 0.071

47.75 0.39 36.63 0.88 0.47 0.56 9.42 96.09

250

2.720 0.140 1.280 0.440 1.210 0.004 1.010 0.020 0.910 0.55 0.43 0.36

35.71 2.54 19.21 19.07 0.07 10.39 0.19 9.31 95.02

250

3.098 0.027 0.902 1.870 0.067 0.039 0.137

47.29 0.56 35.91 1.22 0.40 1.08 9.93 96.40

252

2.722 0.078 1.278 0.452 1.184 0.002 1.157 0.033 0.876 0.51 0.41 0.40

35.80 1.36 19.31 18.62 0.03 10.21 0.22 9.04 94.70

252

3.108 0.035 0.892 1.855 0.064 0.050 0.092

47.08 0.71 35.30 1.16 0.51 0.72 10.26 95.76

253

2.633 0.093 1.367 0.348 1.602 0.006 0.952 0.017 0.814 0.63 0.53 0.32

34.11 1.60 18.84 24.81 0.09 8.27 0.11 8.27 96.10

253

3.103 0.035 0.897 1.856 0.054 0.051 0.106

46.84 0.70 35.26 0.98 0.52 0.83 10.31 95.44

269

2.674 0.113 1.326 0.435 1.209 0.002 1.107 0.040 0.890 0.52 0.42 0.39

34.98 1.96 19.55 18.92 0.03 9.71 0.27 9.13 94.56

269

3.150 0.008 0.850 1.930 0.050 0.042 0.103

49.11 0.18 36.77 0.93 0.44 0.83 8.73 97.16

271

2.666 0.107 1.334 0.458 1.205 0.003 1.109 0.034 0.861 0.52 0.42 0.39

34.89 1.86 19.90 18.85 0.05 9.74 0.23 8.83 94.34

271

3.108 0.038 0.892 1.847 0.070 0.053 0.095

47.47 0.77 35.50 1.28 0.54 0.75 10.31 96.62

274

2.713 0.065 1.287 0.453 1.158 0.007 1.193 0.029 0.915 0.49 0.40 0.42

35.21 1.12 19.16 17.96 0.11 1.039 0.19 9.31 93.50

274

3.088 0.037 0.912 1.856 0.067 0.042 0.107

46.30 0.75 35.22 1.20 0.42 0.83 10.17 94.94

280c

2.710 0.150 1.290 0.436 1.231 0.005 0.992 0.016 0.907 0.55 0.44 0.35

35.51 2.62 19.27 19.28 0.07 8.72 0.11 9.32 94.80

280c

3.094 0.018 0.906 1.898 0.058 0.035 0.144

46.57 0.36 35.82 1.04 0.35 1.12 9.50 94.78

280

2.622 0.088 1.378 0.295 1.663 0.005 0.992 0.007 0.804 0.63 0.55 0.33

34.02 1.53 18.42 25.81 0.08 8.64 0.10 8.19 96.79

280

3.152 0.005 0.848 1.957 0.046 0.038 0.062

47.90 0.11 36.17 0.83 0.39 0.49 7.85 94.16

282c

2.714 0.148 1.286 0.436 1.231 0.005 0.994 0.021 0.904 0.55 0.44 0.35

35.58 2.59 19.15 19.30 0.08 8.74 0.14 9.29 94.47

282c

3.129 0.047 0.871 1.932 0.047 0.056 0.044

47.61 0.95 36.18 0.85 0.58 0.35 7.51 94.05

282

2.668 0.104 1.332 0.436 1.203 0.003 1.171 0.029 0.822 0.51 0.41 0.40

35.11 1.82 19.73 18.92 0.05 10.34 0.20 8.48 94.60

282

3.170 0.025 0.830 1.918 0.058 0.038 0.088

49.70 0.52 36.55 1.09 0.40 0.71 8.36 97.43

284c

2.708 0.148 1.292 0.432 1.231 0.004 0.995 0.014 0.910 0.55 0.44 0.35

35.49 2.58 19.16 19.29 0.06 8.75 0.09 9.34 94.76

284c

3.109 0.042 0.891 1.948 0.047 0.052 0.041

47.30 0.85 36.65 0.85 0.53 0.32 7.53 94.07

284

2.634 0.088 1.357 0.302 1.680 0.005 0.952 0.014 0.812 0.64 0.56 0.31

34.12 1.51 18.17 25.93 0.07 8.24 0.09 8.21 96.34

284

3.154 0.053 0.846 1.864 0.049 0.068 0.047

47.46 1.06 34.61 0.87 0.68 0.36 8.90 93.99

268

2.665 0.081 1.335 0.477 1.379 0.006 0.972 0.025 0.831 0.59 0.47 0.33

33.73 1.37 19.46 20.87 0.08 8.26 0.16 8.24 92.24

268

3.172 0.017 0.828 1.894 0.059 0.078 0.077

48.71 0.35 35.47 1.08 0.80 0.61 8.62 95.71

270

2.673 0.101 1.327 0.440 1.200 0.003 1.176 0.031 0.813 0.51 0.41 0.40

35.13 1.76 19.70 18.87 0.05 10.37 0.21 8.38 94.49

270

3.221 0.018 0.779 1.880 0.061 0.079 0.071

50.38 0.38 35.28 1.15 0.83 0.57 8.55 97.26

273

2.663 0.081 1.337 0.472 1.394 0.006 0.968 0.029 0.831 0.59 0.47 0.33

33.75 1.36 19.45 20.84 0.09 8.23 0.19 8.25 92.19

273

3.192 0.018 0.808 1.897 0.057 0.067 0.076

49.51 0.37 35.59 1.06 0.70 0.61 8.59 96.51

248

2.696 0.089 1.304 0.515 1.534 0.004 0.724 0.026 0.825 0.68 0.54 0.25

34.52 1.52 19.77 23.50 0.06 6.22 0.17 8.28 94.20

248

3.154 0.053 0.846 1.864 0.049 0.068 0.047

47.47 1.06 34.62 0.88 0.69 0.35 8.91 93.98

264

2.666 0.081 1.334 0.480 1.380 0.005 0.971 0.023 0.834 0.59 0.47 0.33

33.74 1.37 19.47 20.88 0.08 8.24 0.15 8.27 92.21

264

3.058 0.026 0.942 1.880 0.058 0.060 0.093

45.91 0.52 35.94 1.05 0.61 0.72 10.21 94.96

266

2.704 0.185 1.296 0.364 1.194 0.006 1.030 0.022 0.944 0.53 0.43 0.37

34.61 3.14 18.02 18.58 0.09 8.89 0.14 9.47 92.94

266

1042 I.I. Likhanov et al. / Russian Geology and Geophysics 50 (2009) 1034–1051

0.39

9.22

0.04

0.01

0.00

86.41

MnO

MgO

Na2O

K2O

ZnO

Total

0.04

1.54

0.01

0.00

0.00

0.66

Mn

Mg

Na

K

Zn

XFe

23.17

0.62

0.00

0.00

0.01

1.71

0.03

2.74

2.95

0.00

2.55

88.04

0.00

0.02

0.05

10.61

0.31

30.28

0.59

0.00

0.01

0.00

1.73

0.02

2.53

3.02

0.00

2.58

88.26

0.00

0.07

0.03

10.90

0.21

28.48

24.13

0.03

24.29

270

0.783 0.917 0.945 0.083 0.024 0.819 0.074

250

0.63

0.00

0.01

0.00

1.69

0.03

2.84

2.90

0.00

2.54

88.19

0.00

0.04

0.01

10.45

0.31

31.31

22.68

0.04

23.34

273

0.830 0.858 0.934 0.142 0.033 0.748 0.124

252

0.58

0.00

0.01

0.03

1.82

0.01

2.51

3.02

0.00

2.55

85.80

0.00

0.05

0.14

11.18

0.13

27.42

23.41

0.06

23.32

264

0.864 0.904 0.926 0.096 0.032 0.775 0.082

253

0.61

0.00

0.00

0.00

1.70

0.01

2.67

2.87

0.00

2.66

87.05

0.00

0.02

0.01

10.53

0.11

29.40

22.43

0.04

24.51

266

0.872 0.892 0.930 0.108 0.027 0.771 0.093

269

0.88

0.00

0.00

0.01

0.22

0.07

1.67

4.01

0.00

2.01

12(O)

92.48

0.00

0.01

0.04

1.77

0.97

24.14

41.21

0.00

24.32

248

0.85

0.00

0.00

0.01

0.30

0.04

1.73

3.91

0.00

2.03

92.07

0.00

0.00

0.07

2.37

0.58

24.81

39.81

0.00

24.42

268

0.861 0.901 0.920 0.099 0.035 0.763 0.084

274

Chloritoid

0.714 0.874 0.948 0.126 0.025 0.790 0.114

271

0.83

0.00

0.00

0.10

0.34

0.04

1.69

3.91

0.00

2.03

92.24

0.00

0.01

0.06

2.73

0.61

24.31

40.03

0.00

24.48

270

0.865 0.890 0.927 0.110 0.033 0.765 0.095

280c

0.87

0.00

0.00

0.00

0.26

0.05

1.75

3.90

0.00

2.04

93.60

0.00

0.04

0.08

2.12

0.72

25.51

40.24

0.04

0.630 0.935 0.928 0.065 0.023 0.809 0.056

282

0.85

0.00

0.00

0.01

0.29

0.04

1.73

3.91

0.00

2.03

92.06

0.00

0.02

0.06

2.37

0.59

24.80

39.81

0.00

24.41

264

0.659 0.914 0.956 0.086 0.022 0.604 0.079

282c

24.77

273

0.805 0.848 0.945 0.152 0.029 0.757 0.136

280

0.86

0.00

0.00

0.01

0.28

0.04

1.72

3.92

0.00

2.04

91.52

0.00

0.00

0.06

2.23

0.51

24.54

39.75

0.00

24.43

266

0.681 0.886 0.941 0.114 0.028 0.763 0.098

284c

0.83

0.11

0.00

0.03

0.64

0.07

3.11

18.42

0.11

8.11

48(O)

98.98

0.51

0.01

0.05

1.49

0.28

13.00

54.74

0.52

28.38

250

Staurolite

0.631 0.939 0.933 0.061 0.022 0.817 0.053

284

0.90

0.04

0.00

0.00

0.36

0.03

3.27

18.58

0.03

8.19

97.05

0.18

0.00

0.01

0.84

0.11

13.44

54.19

0.13

28.16

271

0.754 0.941 0.916 0.059 0.024 0.790 0.050

268

0.86

0.21

0.00

0.00

0.39

0.10

3.04

18.69

0.02

8.10

97.91

0.99

0.00

0.01

0.91

0.38

12.61

54.88

0.10

28.03

283

0.717 0.903 0.925 0.097 0.029 0.773 0.083

270

0.00

0.00

0.00

0.00

0.00

0.01

2.00

0.00

1.00

5(O)

100.2

0.00

0.01

0.01

0.02

0.00

0.22

62.94

0.00

36.99

225

And

0.697 0.908 0.922 0.092 0.030 0.772 0.078

273

0.00

0.00

0.00

0.00

0.00

0.00

1.99

0.00

1.00

100.3

0.00

0.00

0.02

0.08

0.00

0.19

62.74

0.00

37.25

282

Ky

0.706 0.903 0.930 0.097 0.028 0.804 0.084

248

0.00

0.00

0.00

0.00

0.00

0.01

1.97

0.00

1.02

100.6

0.00

0.01

0.01

0.08

0.00

0.24

62.33

0.00

37.93

253

Sil

0.754 0.942 0.915 0.060 0.025 0.790 0.050

264

0.00

0.00

0.00

0.00

0.01

1.04

0.00

0.96

0.01

3(O)

99.58

0.00

0.00

0.00

0.06

0.25

48.48

0.11

50.03

0.38

244

Ilm

0.868 0.903 0.929 0.097 0.029 0.779 0.084

266

Note. XAlm = Fe/(Fe + Mg + Mn + Ca), XPrp = Mg/(Fe + Mg + Mn + Ca), XSps = Mn/(Fe + Mg + Mn + Ca), XGrs = Ca/(Fe + Mg + Mn + Ca), XAn = Ca/(Ca + Na + K), XFe = Fe/(Fe + Mg), XAnn = Fe/(Fe + Mg + Mn + Ti + AlVI), XPhl = Mg/(Fe + Mg + Mn + Ti + AlVI); XAlVI = AlVI/(Fe + Mg + Mn + Ti + AlVI); XNa = Na/(Na + K); XMs = (XK)⋅(XAlVI)2; XPg = (XNa)⋅(XAlVI)2. Total Fe expressed as FeO; 0.00,

2.95

2.96

Fe

0.00

Ti

Al

2.51

Si

14(O)

22.48

31.66

Al2O3

FeO

0.05

0.01

23.57

22.52

SiO2

TiO2

0.724 0.940 0.922 0.060 0.036 0.799 0.051

245

268

Chlorite

0.856 0.938 0.927 0.062 0.037 0.806 0.053

244

Muscovite

248

Component

K XK XAlVI XNa XFe XMs XPg

Component

Table 1 (continued)

I.I. Likhanov et al. / Russian Geology and Geophysics 50 (2009) 1034–1051 1043

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I.I. Likhanov et al. / Russian Geology and Geophysics 50 (2009) 1034–1051

Muscovite varies slightly in celadonite content (expressed as (Mg + Fe)/(Mg + Fe + AlVI)) from 0.04 to 0.07, and in paragonite (XNa) content from 0.06 to 0.15. Plagioclase shows a slight increase in the anorthite component from oligoclase (XAn = 0.25–0.28) in the outermost metapelites of Zone I to sodic andesine (XAn = 0.33–0.45) in Zone III adjacent to the thrust. Here, plagioclase is commonly zoned, with the core slightly more anorthitic (XAn = 0.41) than the rim (XAn = 0.33). Chlorite is chemically homogeneous in each sample, with similar XFe in the narrow range of 0.59–0.66. Ilmenite in all samples is close to its ideal end-member composition, with constant amounts of Mn (0.01 atoms p.f.u.).

Thermodynamic conditions for metapelites from the Garevka area Geothermobarometry. Modifications of the Bt-Ms-Chl geobarometer (Bucher-Nurminen, 1987; Powell and Evans, 1983) were used to determine pressures for garnet-free metamorphic rocks from the Karpinsky Range Formation. Temperature estimates were obtained using the calibration according to the mixing model of Green and Usdansky (1986a) for Pl-Ms geothermometer (Green and Usdansky, 1986b). For an independent temperature control, these values were compared with the temperature estimates from an empirical calibration of the Bt-Ms (Hoisch, 1989) and Chl-Cld (Vidal et al., 1999) geothermometers. Temperature and pressure estimates of garnet-bearing rocks were obtained using the calibrations and appropriate mixing models (Hodges and Spear, 1982) for the Grt-Bt geothermometer (Ferry and Spear, 1978) and Grt-Bt-Ms-Pl geobarometer (Ghent and Stout, 1981) with a modification of Hodges and Crowley (1985). For an independent temperature control, these values were compared with the temperature estimates from three calibrations of the Grt-Bt geothermometer (Kaneko and Miyano, 2004; Kleemann and Reinhardt, 1994; Perchuk and Lavrent’eva, 1983) and the above-mentioned Pl-Ms geothermometer (Green and Usdansky, 1986b). For a pressure control we used the calibration and appropriate mixing model of Hoisch (1991) for the Grt-Bt-Ms-Pl geobarometer (Hoisch, 1990). The P-T estimates for these rocks were calculated using the combination of geothermometers and geobarometers in the MATHEMATICA 5.0 computer software with the NullSpace built-in procedure (Wolfram, 2003). The results are presented in Table 2 and Fig. 4. The error of estimates calculated using the Grt-Bt geothermometer (Ferry and Spear, 1978) and Grt-Bt-Ms-Pl geobarometer (Hodges and Crowley, 1985) and in view of the effects of analytical errors and errors in a geothermobarometer reaction enthalpy do not exceed ±30 °C and ±0.5 kbar (Likhanov et al., 2004). This range correlates well with the most frequently cited uncertainties for typical geothermobarometers (Hodges and McKenna, 1987; Kohn and Spear, 1991). The data obtained suggest a progressive pressure increase toward the thrust 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). As the thrust is approached, the mean maximum temperatures increase only slightly from 575 °C to 645 °C, indicating a surprisingly low metamorphic field gradient of 8–9 °C/km. Unlike rocks from the other Transangarian regions, these rocks yielded intermediate P-T values, in a range between the maximum estimates obtained for the Chapa and Mayakon areas. To test the reliability of these P and T estimates, they were compared, in view of their accuracy, with the P-T conditions inferred from the THERMOCALC calculations (Powell and Holland, 1994) using the internally consistent thermodynamic dataset and mixing models of Holland and Powell (1998). The results exhibit a good consistency (Table 2) within the accuracy of the applied geothermobarometers: T = ±50 °C and P = ±1 kbar (Essene, 1989; Kohn and Spear, 1991; Spear, 1989). The P-T paths can be considered as a record of coherent changes in temperature and pressure during the geological history of metamorphic rocks. For this reason these paths are especially powerful tools in understanding the major geodynamic processes. Two computer programs, GEOPATH (Gerya and Perchuk, 1990) and PTPATH (Spear et al., 1991), are most often used to reproduce the P-T trajectories. In this study, we use the PTPATH program package, because all P-T paths for the evolution of metamorphic complexes in the Yenisei Ridge and other overthrust terranes in the world shown in Fig. 4 were calculated by this method using the same thermodynamic data for minerals (Spear et al., 1991). Spear and Selverstone (1983) have developed a technique for estimating P-T paths that uses an analytical formulation of the phase equilibria of a given mineral assemblage in such a way that changes in the composition of coexisting minerals in the assemblage can be monitored as functions of changing P and T. The mathematical expressions and the procedure of the thermodynamic calculations were described in detail by Spear (1993). Note that all necessary differential thermodynamic equations are written regardless of the modal contents of the minerals and the bulk compositions of the rocks. In general, the calculation of metamorphic P-T paths by this approach should be based on information on the textural features of the rocks, their mineral assemblages, succession of mineral reactions, the chemical compositions of minerals, as well as a correlation between the chemical compositions of zonal minerals. P-T paths for three samples collected in Zone III (samples 280, 282 and 284, their chemical compositions and chemical zoning profiles are given in Table 1 and Fig. 3) were calculated using the computer program PTPATH (Spear et al., 1991) and the thermodynamic data base (Berman, 1988). In modeling P-T paths it is necessary to specify the assemblage in which the garnet grew. The most conspicuous textural feature of these rocks is the development of Ky + St + Bt pseudomorphs (without garnet) after andalusite. The adjacent matrix in these samples consists of the Grt + St + Bt +

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I.I. Likhanov et al. / Russian Geology and Geophysics 50 (2009) 1034–1051

Table 2 Summary of pressure and temperature estimates for regional and collision-related metamorphism in the vicinity of Mt. Garevsky Polkan (for zone location see Fig. 2) Sample

T, °C

P, kbar

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

264

—

—

—

—

556

565

559

560

266

—

—

—

—

572

557

556

244

575

574

582

607

—

—

245

581

565

582

620

—

—

[10]

[11]

[12]

[13]

[9]

578 ± 19 —

—

4.2–4.5

4.1–4.4

4.1 ± 1.1

562

581 ± 20 —

—

4.0–4.3

4.2–4.5

4.4 ± 1.2

569

581

—

5.04

4.55

—

—

—

574

585

—

5.05

4.62

—

—

—

Zone I

Zone II

248

—

—

—

—

587

606

—

596

602 ± 14 —

—

5.2–5.4

5.9–6.1

5.5 ± 1.4

250

596

575

571

623

—

—

594

592

—

5.04

—

—

—

5.56

268

—

—

—

—

601

576

—

588

585 ± 46 —

—

4.6–4.8

5.0–5.2

5.1 ± 1.2

269

573

578

587

611

—

—

582

586

590 ± 13 4.98

4.65

—

—

5.7 ± 0.3

270

—

—

—

—

608

578

—

593

—

—

4.8–5.0

5.4–5.6

—

271

608

581

607

641

—

—

595

606

601 ± 61 5.11

4.55

—

—

5.6 ± 0.9

273

—

—

—

—

578

610

—

594

—

—

5.1–5.3

5.8–6.0

—

—

—

Zone III 252

617

587

605

626

—

—

609

609

591 ± 13 6.25

6.05

—

—

5.7 ± 0.8

253

621

604

611

614

—

—

616

613

629 ± 14 6.48

6.77

—

—

6.7 ± 0.6

274

600

585

592

622

—

—

606

601

608 ± 48 6.01

6.07

—

—

6.4 ± 1.7

280

620

605

607

623

—

—

624

616

610 ± 15 6.50

7.18

—

—

6.4 ± 0.3

282

624

591

617

662

—

—

629

625

639 ± 35 6.89

6.53

—

—

6.8 ± 0.8

284

645

611

613

637

—

—

642

630

646 ± 15 7.28

7.80

—

—

7.7 ± 0.3

Note. Geothermometers: [1], Grt-Bt (Ferry and Spear, 1978); [2], Grt-Bt (Kleemann and Reinhardt, 1994); [3], Grt-Bt (Perchuk and Lavrent’eva, 1983); [4], Grt-Bt (Kaneko and Miyano, 2004); [5], Chl-Cld (Vidal et al., 1999); [6], Bt-Ms (Hoisch, 1989); [7], Pl-Ms (Green and Usdansky, 1986); [8], average temperatures obtained from exchange thermometers; [9], THERMOCALC calculations (Powell and Holland, 1994) are shown with ±2σ error. Geobarometers: [10], Grt-Bt-Ms-Pl (Hodges and Crowley, 1985); [11], Grt-Bt-Ms-Pl (Hoisch, 1990); [12], Bt-Ms-Chl (Powell and Evans, 1983); [13], Bt-Ms-Chl (Bucher-Nurminen, 1987).

Ms + Qtz + Pl ± Sil ± Cld ± Chl mineral assemblages (without Ky and And). Based on these textural observations we assumed that the Grt + Bt + Ms + Qtz + Pl + St + Chl + Cld mineral assemblage was responsible for the initial growth of garnet, whereas the final stages of this process were related to the development of the assemblage Grt + St + Bt + Ms + Qtz + Pl + Sil. The local mineral transformations in the matrix responsible for the growth of the garnet cores may be described as the reaction Chl + Ms + Cld + Pl → Grt + St + Bt + H2O, reflecting the character of mineral transformations in the zonation of the andalusite–sillimanite facies series (Whitney et al., 1996). The mineral transformations during the further growth of the garnet could take place according to the reaction Chl + Ms + St + Pl → Bt + Sil + Grt + H2O (Foster, 1986). The studied matrix assemblages consist of eight phases, including fluid (H2O). These two assemblages can be modeled in the ten-component system SiO2–Al2O3–FeO–MnO–MgO– K2O–CaO–Na2O–TiO2–H2O. Because the two model rocks have four degrees of freedom, four differential variables must be specified to solve the system of equations for the others. The variables chosen were the three independent garnet

components (almandine XAlm, spessartine XSps, and grossular XGrs), and the anorthite component of plagioclase (XAn), provided that the garnet and plagioclase compositions are correlated. Garnet and plagioclase components were chosen as the monitors because the partitioning of calcium between garnet and plagioclase is a strong function of pressure (Ghent and Stout, 1981) and because slow diffusion rates promote the preservation of compositional zoning at these metamorphic grades (Cygan and Lasaga, 1985). Activity models of Hodges and Spear (1982) for garnet and plagioclase were used to provide maximum internal consistency between thermobarometric and differential thermodynamic results. Temperature and pressure estimates for the initial garnet growth have been obtained by the simultaneous consideration of Grt-Bt thermometry (Ferry and Spear, 1978) and the Grt-Bt-Ms-Pl geobarometry (Hodges and Crowley, 1985), using the mixing model of Hodges and Spear (1982). It was assumed that the near-rim compositions of garnet were related to the initial growth of garnet whereas changes in the composition of plagioclase were correlated with progressive growth of garnet. All P-T paths have nearly identical slopes, and differ mainly in the length of the recorded P-T growth

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I.I. Likhanov et al. / Russian Geology and Geophysics 50 (2009) 1034–1051

Fig. 4. P-T diagram summarizing the results of pressure and temperature as well as P-T path calculations for metapelites in the vicinity of Mt. Garevsky Polkan compared with the P-T evolution of collision-related metamorphism for other overthrust terranes. Each cross represents pressure and temperature estimates calculated from various geothermobarometers from the same sample; numerals near crosses correspond to sample number in Table 2 regardless of the errors. The generalized P-T paths derived from compositional zoning in garnet and correlated garnet and plagioclase compositions from samples 280, 282, and 284 are shown by black thin arrows in the core-to-rim direction. Arabic numerals in arrow head of P-T paths (dark gray thick arrows) correspond to the study areas in the Yenisei Ridge: 1, Angara area, Yenisei Ridge (Likhanov et al., 2006a); 2, Mayakon area, Yenisei Ridge (Likhanov et al., 2004); 3, Chapa area, Yenisei Ridge (Likhanov et al., 2008b); 4, Garevka area, Yenisei Ridge (this study). Roman numerals in arrow head of P-T paths (light gray thick arrows) correspond to other metamorphic complexes in which andalusite replacement by kyanite was observed: I, Bellows Falls, Appalachians, USA (Spear et al., 2002), II, Mascoma-Orfordville, Appalachians, USA (Kohn et al., 1992), III, Nason terrain, Cordillera, Canada (Whitney et al., 1999), IV, Piedmont Plateau, USA (Crawford and Mark, 1982). The aluminosilicate phase diagram is taken from Holland and Powell (1985).

history (Fig. 4). In all metapelites from the Karpinsky Range Formation, the P-T trajectory documents increasing pressure (from 2.2 to 2.5 kbar) from east to west associated with only minor heating (up to 50 °C), thus suggesting a low geothermal gradient (8–9 °C/km). These results are in agreement with the P-T conditions estimated from thermobarometry and P-T evolution of metamorphic rocks from some other collisional orogens in the world (Fig. 4), where progressive transformation of andalusite into kyanite were explained as a result of crustal thickening due to thrusting (Crawford and Mark, 1982; Kohn et al., 1992; Likhanov et al., 2004; Spear et al., 2002; Whitney et al., 1999).

Geodynamic interpretation of kyanite–sillimanite metamorphism in the Transangarian region of the Yenisei Ridge Based on the results of geological and petrological studies of metamorphic complexes in the Transangarian region of the Yenisei Ridge it can be shown that medium-pressure metamorphism of the kyanite–sillimanite type overprinted region-

ally metamorphosed rocks of low pressures. The medium-pressure metamorphism was characterized by the localized distribution of the metamorphic zones in the vicinity of the overthrusts with a thickness ranging from 2.5 to 6–8 km, the development of deformational structures and textures, and kyanite-bearing blastocataclasites/blastomylonites with sillimanite, garnet, and staurolite after andalusite-bearing mineral assemblages, which were formed under conditions of a gradual increase in pressure towards the thrust faults and a low metamorphic field (as low as 12 °C/km). These specific features are typical of collisional metamorphism during overthrusting of continental blocks (with no relations to subduction) and are evidence for near-isothermal loading in accordance with the transient emplacement of a cold overthrust plate (Reverdatto and Sheplev, 1998). The specific features of collision-related metamorphism will be exemplified below by the Garevka area. Several possible tectonic models have been proposed to explain the mechanisms responsible for the evolution of the metapelites under conditions of near-isothermal loading. 1. Metamorphism of these rocks took place under the pressure of overburden rocks (lithostatic pressure) and the

I.I. Likhanov et al. / Russian Geology and Geophysics 50 (2009) 1034–1051

normal (average terrestrial) heat flow. A subsequent tectonic event is believed to have resulted in the change in mode of occurrence. During metamorphism the difference in pressures between the upper and lower levels of the rock sequence (over a distance of 4–5 km or less) must have been less than 1.5 kbar, whereas the temperature range (given a normal geothermal gradient of 1 kbar/3.5 km) could correspond to 150–175 °C. This is in conflict with P-T range inferred from geothermobarometric results, which documents the pressure of 2.2–2.5 kbar and temperature as low as 60 °C. 2. Metamorphic zoning could have been a result of separate shifting of crustal slices which were sequentially pushed up from different depth levels during thrusting process (Beaumont et al., 2001). However, this model lacks support from geological and thermobarometric evidence for the presence of tectonic contacts and high lateral temperature gradients between adjacent metamorphic zones. 3. The pressure increase could be related to tectonic stress driven by the collision of blocks (Kusznir and Park, 1984), because intense deformation of minerals with traces of their brittle destruction were observed in the rocks near the thrust. However, laboratory experiments indicate that tectonic overpressure in the presence of a fluid cannot exceed the lithostatic pressure for more than 1.0–1.5 kbar (Brace and Kohlestedt, 1980; Etheridge et al., 1984). Recently obtained data on rheology of the rocks show that the average stress level at a depth of 15–20 km is likely to lie within the range 0.2– 0.3 kbar at a geologically realistic strain rate of 10–17– 10−14 s−1 (Strehlau and Meissner, 1987). Such stress is not sufficient to achieve overpressures as high as 2.2–2.5 kbar. 4. The increase in lithostatic pressure could be due to loading by overlying magma (Brown and Walker, 1993) caused by the emplacement of the intrusive granitoid body of the Teya complex, which crystallized at approximately 850– 870 Ma (Vernikovsky and Vernikovskaya, 2006). If this were the case, along with a correlation between pressures in metapelitic rocks and their proximity to the intrusive contact, the temperatures of rocks would significantly increase due to heating by magmatism (Paterson and Tobisch, 1992; Ruppel and Hodges, 1994), in contradiction to metamorphic P-T paths. Since these models are inconsistent with geological and petrological evidence, we invoked a hypothesis of tectonic thickening owing to thrusting (Baker, 1987; Beddoe-Stephens, 1990; Clarke et al., 1987; Crawford and Mark, 1982; England and Thompson, 1984; Likhanov et al., 2004; Loosveld and Etheridge, 1990; Ruppel and Hodge, 1994; Spear et al., 1990). Prior to thrusting, the original low-pressure metamorphic rocks occurred at a depth of ∼14.0–17.5 km. The further subsidence of these rocks occurred after the thrusting. A pressure increase of 2.2–2.5 kbar would require a crustal thickening by 7–8 km, thus implying the overthrusting of a rock stratum, which has been subsequently removed by erosion. This tectonic thickening could be related to collision of two blocks, which resulted in the eastward thrusting of the western block of the nearthrust structure onto the eastern one. The main difficulty in such a model is the nearly isothermal nature of the loading process. Recent attempts at modeling

1047

these processes that had been discussed in the literature can be summarized as follows. 1. In the geodynamic interpretation of some nappe structures in New England, USA, Spear et al. (1989, 2002) have demonstrated that isothermal loading could occur in the middle plate of a multiplate system if both bounding thrusts move simultaneously. In such a middle plate, heating from above is balanced by cooling from below, so that the temperature remains relatively constant. For this model to apply to the Transangarian region of the Yenisei Ridge, however, would require that the rocks beneath the Karpinsky Range Formation be allochthonous and underlain by another thrust, in contradiction to the geological situation in the study area. 2. For the Mayakon area, which is characterized by the lowest metamorphic field gradient among the studied areas (1–7 °C/km), we have built a thermal model, which explains a small temperature increase by specific behavior of steadystate geotherms calculated for lower- and upper-plate metapelites at a thrust rate of about 350 m/Myr (Likhanov et al., 2004). Our calculations were based on the actual but widely differing thermophysical and heat generation properties of metapelites and metacarbonates. This model can operate if the rock blocks on each side of the fault differ significantly in their thermophysical and heat-producing properties, but this is also contradicts to the actual geological situation. One of the most probable mechanisms responsible for the thermal evolution of rocks in the Garevka, Chapa, and Angara areas is the rapid thrusting with subsequent fast uplift and erosion (Huerta et al., 1998, 1999; Jamiesen et al., 1998, 2002; Peacock, 1989). According to this idea, the low geothermal gradients can be related to the instantaneous nature of the events and to the fact that the underthrust slab fails to equilibrate thermally at depth because the temperature usually lags the pressure. The available thermal models of thrusting (Karabinos and Ketchman, 1988; Shi and Wang, 1987) demonstrated that rocks of the lower plate of a thrust system cannot undergo isothermal loading during thrusting unless the rate of thrusting is unreasonably fast (about a few dozen centimeters per year). Similarly to subsidence, the exhumation and erosion of the rocks affected by medium-pressure metamorphism should occur equally rapidly in order to ensure the preservation of mineral assemblages under disequilibrium conditions at the middle and upper crustal levels. With regard for data on the kinetics of reverse metamorphic reactions, this requires high rates (1–20 cm/year) of the exhumation of the subsided rocks (Dobretsov, 1981; Oliver et al., 2000) and the absence of fluid early in the course of rapid exhumation (Dobretsov, 1991), which can be the reason for the absence of a record of retrograde P-T paths in minerals from regions with overthrust tectonics. The recently conducted precise dating of some metamorphic complexes in collision orogens confirms that the time interval between peak metamorphism and the exhumation of the rocks to upper crustal levels is very short, up to 5–10 Ma (Oliver et al., 2000; Sklyarov, 2006). The exhumation mechanisms present a special problem, which is now broadly discussed in the literature (e.g., Ez,

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1997). The assumption on the predominantly erosion-controlled exhumation of metamorphic complexes from depths of (25 km poses the question of the location of large volumes of sedimentary rocks of corresponding age, which are absent from nearby territories. The problem of these sediments was most convincingly solved in application of the exposure of some high-pressure granulite and eclogite complexes. It was assumed that the uplift of granulite complexes and the simultaneous rock subsidence of the greenstone complexes were controlled by the gravity-driven redistribution of rocks in the Precambrian crust via crustal diapirism (Gerya and Maresch, 2004; Perchuk et al., 2001; and others). The exposure of Caledonian eclogites in Norway, which were produced by the collision of Baltia and Laurentia, was explained predominantly by tectonic denudation, i.e., the tectonic removal of overlying complexes in the process of large-amplitude extension (Austrheim, 1990). There are, however, no necessary prerequisites of such process in our study region. At the same time, recently obtained lines of evidence indicate that the exposure of metamorphic complexes whose genesis was related to collision processes was usually an integrated effect of several tectonic mechanisms with a significant contribution of erosion denudation (Sklyarov, 2006). 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, which are considered most exhaustively by Teyssier and Whitney (2002). Thus, the proposed model explains a number of features of metamorphic evolution of metapelites, such as the origin of kyanite after andalusite, the development of deformational structures and textures of medium-pressure, common increase in garnet grossular content from core to rim, the gradual change in recorded pressure with slightly increasing temperature, etc. However, one question still remains to be answered: What became of the overthrust sheet which triggered the development of metamorphic zoning?

the youngest Upper Riphean deposits of the Sukhoi Range Formation (Angara area). The medium-pressure metamorphism of the kyanite-silimanite type is 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 observed 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 in accordance with the transient emplacement of thrust sheet. The proposed model for tectono-metamorphic evolution of the study areas due to crustal thickening at high thrusting rates and subsequent rapid tectonic exhumation and erosion can explain these features associated with this tectonic phenomenon and confirm the possibility of near-isothermal loading during overthrusting. The analysis of data obtained allowed us to consider medium-pressure kyanite-bearing metapelites as a product of collision-related 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). Acknowledgements. The authors thank N.L. Dobretsov and E.V. Sklyarov for their constructive review and comments on the manuscript. The authors also thank O.P. Polyansky, P.O. Polyansky, N.V. Bannikov, and A.E. Vershinin for help in arranging field trips. The work was supported by the Presidium of the Siberian Branch of the RAS (integration project no. 20) and the Russian Federation President Program for Support of Leading Scientific Schools (project no. NSh258.2008.5).

Conclusions

References

The Precambrian crystalline rocks of the Transangarian region of the Yenisei Ridge appear in the vicinity of regional faults as polymetamorphic complexes, which comprise regional low-pressure metamorphic rocks of the andalusite–sillimanite type and locally developed medium-pressure metapelites of the kyanite–sillimanite type. The latter is indicative of higher pressure conditions existing along the axial part of the region. Within these rocks, the highest-pressure and temperature metapelites of the Chapa (P = 5.8– 8.4 kbar, T = 630–710 °C) and Garevka (P = 5.0–7.3 kbar, T = 585–645 °C) areas were observed northward and confined to older Lower Proterozoic strata of the Karpinsky Range Formation. Unlike these, the Mayakon metapelites, which occur southward in younger Middle Riphean rocks of the Korda Formation, have low P-T values (P = 4.5–6.7 kbar, T = 560–600 °C). The lowest P-T metapelites (P = 4.1–5.6 kbar, T = 530–560 °C) were found in the south of the region within

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