Petrology of metamorphic rocks from the Atbashy complex, Southern Tien-Shan, Kyrgyzstan

Geoscience Frontiers xxx (2017) 1e13

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Research Paper

Petrology of metamorphic rocks from the Atbashy complex, Southern Tien-Shan, Kyrgyzstan Maksatbek Satybaev a, b, c, e, *, Lin Ding a, b, Akira Takasu d, Apas Bakirov e, Kadyrbek Sakiev e, Fulong Cai a, b, Rustam Orozbaev e, f, Azamat Bakirov e, Janybek Baslakunov e a

Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences, Beijing 100101, China c University of Chinese Academy of Sciences, Beijing 100049, China d Department of Geoscience, Shimane University, 1060 Nishikawatsu, Matsue 690-8504, Japan e Institute of Geology, National Academy of Sciences, 30 Erkindik, Bishkek 720040, Kyrgyzstan f Research Center for Ecology and Environment of Central Asia (Bishkek), 30 Erkindik, Bishkek 720040, Kyrgyzstan b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 20 February 2017 Received in revised form 19 September 2017 Accepted 3 November 2017 Available online xxx Handling Editor: C. J. Spencer

The high- to ultrahigh-pressure metamorphic rocks of the Atbashy complex were petrologically investigated. The eclogites of the Choloktor Formation show a prograde evolution from epidote-blueschist facies (P ¼ 17e21 kbar and T ¼ 450e515  C) to peak eclogite-UHP conditions (P ¼ 26e29 kbar and T ¼ 545e615  C) with a subsequent epidote-amphibolite and greenschist facies overprint. The micaschists of the Choloktor Formation also show a clockwise P-T path from blueschist/epidote-blueschist facies conditions through peak eclogite facies conditions (P ¼ 21e23 kbar and T ¼ 530e580  C) to retrograde epidote-amphibolite and greenschist facies stages. A comparison of the P-T paths in the eclogites and mica-schists of Choloktor Formation reveal that they may have shared their P-T history from peak to retrograde stages. The mica-schists of the Atbashy Formation record peak metamorphism of P ¼ 10e12 kbar and T ¼ 515e565  C, which indicates that the highest grade of regional metamorphism in the Atbashy Ridge was epidote-amphibolite facies. The newly obtained P-T conditions for the mica-schists of Choloktor Formation indicate that sheets of sedimentary rocks were brought to great depths along the subduction zone and they metamorphosed under eclogite facies HP conditions. The eclogite blocks were amalgamated with mica-schists of Choloktor Formation in the eclogite facies HP conditions and together they experienced isothermal decompression to w40 km. During this path, the eclogites and mica-schists of Choloktor Formation docked with mica-schists of Atbashy Formation at 10e12 kbar and 515e565  C, and from this depth (w40 km) the whole sequence was exhumed together. These new results improve our understanding of high-pressure metamorphism in subduction-related accretionary prism zones and the exhumation processes of deeply-seated rocks in the Atbashy HP-UHP complex. Ó 2017, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

Keywords: Eclogite Mica-schists P-T conditions Atbashy Kyrgyz Tien-Shan

1. Introduction High-pressure (HP) and ultrahigh-pressure (UHP) mafic rocks (e.g. eclogites) and their surrounding rocks can have complex pressure-temperature (P-T) histories. For instance, the eclogites may

* Corresponding author. Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101, China. E-mail address: [email protected] (M. Satybaev). Peer-review under responsibility of China University of Geosciences (Beijing).

not share P-T path similarly to their surrounding rocks, and can be regarded as exotic tectonic mafic blocks within metapelites, with juxtaposition at relatively lower pressure (Moore, 1984; Okay, 1989;  Takasu et al., 1994; Stipská et al., 2006). In contrast, some studies demonstrated that both eclogites and country rock gneisses experienced HP-UHP metamorphism together and have shared their entire P-T history (Katayama et al., 2000; Song et al., 2003; Zhang et al., 2008; Menold et al., 2009; Wei et al., 2009). In addition, b-shaped metamorphic P-T paths formed by distinct metamorphic events are also reported (Kurz and Froitzheim, 2002; Kabir and Takasu, 2010; Orozbaev et al., 2010). Hence, the combined petrological study on

https://doi.org/10.1016/j.gsf.2017.11.005 1674-9871/Ó 2017, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NCND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article in press as: Satybaev, M., et al., Petrology of metamorphic rocks from the Atbashy complex, Southern Tien-Shan, Kyrgyzstan, Geoscience Frontiers (2017), https://doi.org/10.1016/j.gsf.2017.11.005

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the eclogites and their surrounding metapelites is useful in revealing the metamorphic history of both rock types, which can demonstrate whether the peak conditions of the host rocks are consistent with the peak conditions of eclogites or not. In this study, a detailed documentation of the petrography and mineralogy of the eclogites and surrounding mica-schists from the Atbashy Complex in the Southern Kyrgyz Tien-Shan is given. An attempt has been made to use petrographic descriptions and geothermobarometric calculations for a reconstruction of P-T conditions of these rock types, and compare their P-T evolution. We show that eclogites and host mica-schists of Choloktor Fm. were amalgamated at HP conditions, and during exhumation, they were docked with mica-schists of Atbashy Fm. at relatively lower pressure conditions, and then whole sequence exhumed to the surface together. These results will contribute for better understanding the exhumation processes of the deep-seated rocks in Southern TienShan. 2. Geologic setting 2.1. Regional geology Several HP-UHP metamorphic complexes have been described in the Tien-Shan Mountains (Chinese and Kyrgyz Tien-Shan Mts.). In the Chinese Tien-Shan, the eclogites and blueschists have been described from the southwestern Chinese Tianshan metamorphic belt (CTMB), where these rocks experienced HP to UHP metamorphic conditions (Gao et al., 1999; Klemd et al., 2002; Wei et al., 2003; Lü et al., 2009; Tian and Wei, 2013). There are several age

constraints for the timing of peak metamorphism in CTMB, such as 346e331 Ma (Gao and Klemd, 2003), w320 Ma (Su et al., 2010) and 233e226 Ma (Zhang et al., 2007). The Kyrgyz Tien-Shan extends from east to west for about 1000 km and it is divided into three tectonic units: the Northern Tien-Shan (Caledonian folded belt), the Middle Tien-Shan (Caledonian-Hercynian folded belt) and the Southern Tien-Shan (Hercynian folded belt) (Fig. 1a). There are three (U)HP metamorphic complexes in the Kyrgyz Tien-Shan (Fig. 1a). The Makbal HP-UHP and Aktyuz HP complexes occur in the Northern Tien-Shan (Bakirov, 1978, 1989; Bakirov et al., 1998; Togonbaeva et al., 2009; Orozbaev et al., 2010, 2015; Tagiri et al., 2010; Klemd et al., 2014; Meyer et al., 2014) and the Atbashy HP-UHP complex is located in the Atbashy Ridge of Southern Tien-Shan (Bakirov et al., 1974, 1998; Sobolev et al., 1986; Tagiri et al., 1995; Hegner et al., 2010). The Atbashy Ridge is located to the south of the AtbashyInylchek fault, extends NEeSW for more than 130 km, and is up to 15 km wide (Fig. 1a and b). The Atbashy Ridge consists mainly of carbonate, terrigenous sediments, volcanogenic-flinty sediments and mica-schists (Fig. 1b). Cenozoic sediments cover the northern and southern parts. The northern slope of the ridge consists mainly of mica-schists of the Atbashy Formation, upper Paleozoic molasses and volcanic rocks, lower Paleozoic granitic intrusives (457e417 Ma, Glorie et al., 2011) and ophiolitic rocks (Fig. 1b). On the southern slope of the ridge, the Silurian-Carboniferous carbonate deposits, terrigenous and volcanogenic-flinty sediments, and upper SilurianDevonian ophiolitic rocks are widely exposed. This part of the ridge is intruded by upper Paleozoic intrusive rocks (286e282 Ma, Glorie et al., 2011).

Figure 1. (a) Simplified tectonic map of the Kyrgyz Tien-Shan. (b) Geological map of the Atbashy Ridge. The square shows location of the study area.

Please cite this article in press as: Satybaev, M., et al., Petrology of metamorphic rocks from the Atbashy complex, Southern Tien-Shan, Kyrgyzstan, Geoscience Frontiers (2017), https://doi.org/10.1016/j.gsf.2017.11.005

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2.2. Geology of the Atbashy and Choloktor Formations 2.2.1. The Atbashy Formation The Atbashy Formation occurs on the northern slope of the Atbashy Ridge and it is mainly composed of mica-schists with subordinate carbonates (Fig. 1b). The mica-schists are mainly represented by varieties of mica-quartz, garnet-mica-quartz, micaalbite-quartz and albite-mica-epidote schists. Several serpentinite bodies occur along the tectonic faults within the Atbashy Formation (Fig. 1b). The mica-schists of Atbashy Formation experienced epidote-amphibolite facies metamorphism followed by a greenschist facies overprint (Bakirov, 1978), however, no quantitative thermobarometry has been done yet. The geochronological data for the Atbashy Formation is scarce. Whole-rock K-Ar method gave ages of 1100 and 567 Ma for the mica-schists (dated by Firsov L.N. and published in Bakirov et al., 1974). U-Pb zircon ages of metapelite range between 2620 and 329 Ma (Rojas-Agramonte et al., 2014), where the older U-Pb dates are interpreted to be detrital, while the younger dates to be metamorphic. On the central part of northern boundary of Atbashy Formation, occur the outcrops of the eclogite-bearing metamorphic rocks of this study (Fig. 1b). Bakirov et al. (1984) defined this rock suite as a Choloktor Formation (presented below in details) and separated it from the Atbashy Formation, based on lithological features and tectonic contacts between the two units. 2.2.2. The Choloktor Formation The Choloktor Formation is exposed along the Kembel Valley (Fig. 2) and mainly composed of basic schists and mica-schists (Bakirov et al., 1984). The basic rocks are represented by eclogite, garnet-glaucophane schist, glaucophane schist and carbonatechlorite rock. All these rocks are closely associated in the field and gradually change to each other. The mica-schists are intercalated with basic schists and they are represented by quartz-mica schists that sometimes contain garnet, glaucophane, chlorite, clinopyroxene and biotite. Detailed petrological data are not available for the mica-schists of Choloktor Formation. A greywacke

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protoliths for the mica-schists were suggested by the geochemistry data (Hegner et al., 2010). The zircons separated from the mica-schist show wide range of U-Pb ages from 2774 to 224 Ma (Hegner et al., 2010; Rojas-Agramonte et al., 2014; Sang et al., 2017). The eclogites, garnet amphibolites and garnet-glaucophanites occur as boudins and lenses. Earlier petrological works suggested P ¼ 9e15 kbar and T ¼ 500e700  C for the peak metamorphic conditions of the eclogites (Udovkina, 1985; Sobolev et al., 1986; Bakirov, 1989), however later studies revealed higher-pressure conditions up to UHP metamorphism. Tagiri et al. (1995) reported quartz pseudomorphs after coesite in garnet and omphacite and suggested peak metamorphic conditions as P ¼ 25 kbar and T ¼ 660  C. Another evidence of UHP metamorphism (P > 30 kbar; T > 725  C) in the eclogites is based on the occurrence of phengite with high Si (3.41e3.53 p.f.u.) concentrations and locally high content of K2O (0.25e0.30 wt.%) in clinopyroxene (Bakirov et al., 1998). The peak P-T conditions of eclogites as P ¼ 23e25 kbar and T ¼ 510e570  C (Simonov et al., 2008) and a clockwise P-T path with peak pressures of P ¼ 18e24 kbar at T ¼ 520e600  C (Hegner et al., 2010) are also reported. The major, trace and rare-earth elements indicate an N-MORB and/or IAT affinities for the protoliths of eclogites (Bakirov, 1989; Simonov et al., 2008; Hegner et al., 2010; Volkova et al., 2014). Several ages for the eclogites in the Choloktor Formation have been reported: phengite K-Ar ages of 320e288 Ma (Udovkina, 1985); 40Ar/39Ar ages of phengite and glaucophane of 327e324 Ma (Simonov et al., 2008) and phengite 316  3 Ma (Hegner et al., 2010); a Rb-Sr mineral isochron age of 267  5 Ma (Tagiri et al., 1995); a Sm-Nd whole-rockemineral isochron age of 319  4 Ma (Hegner et al., 2010). Recent U-Pb dating of zircons, separated from the eclogites, yielded metamorphic ages of 424  8 to 217  4 Ma and detrital ages of 926e821, 2493  39 and 3420  8 Ma (Sang et al., 2017). In this study, we have collected two eclogite samples (samples A-50 and 15-2) and one mica-schist (sample R-22b) from the Choloktor Formation, and two mica-schists (samples KG-826 and KG-830) from the Atbashy Formation (Fig. 2).

Figure 2. Geological map of the Choloktor Formation (Bakirov et al., 1984).

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3. Petrography 3.1. The Choloktor Formation 3.1.1. Eclogites Eclogites are massive and consist of garnet, clinopyroxene, glaucophane, phengite, paragonite, quartz, and rutile (Fig. 3aed). The minor minerals are epidote, chlorite, plagioclase, apatite and ilmenite.

Garnet occurs as a porphyroblast (<2.5 mm across) (Fig. 3a) and as inclusions in clinopyroxene (Fig. 3b). Porphyroblastic garnets are optically zoned from pale red core to colorless rim. The core of the garnets contain inclusions of quartz, glaucophane, phengite, apatite, epidote and rutile (Fig. 3a), whereas rims have inclusions of quartz, rutile and clinopyroxene. The garnets are partly replaced by epidote and chlorite. Clinopyroxene occurs as inclusions in garnet and as idioblastic to hypidioblastic prismatic grain in the matrix, ranging size up to 2 mm in length (Fig. 3b and c). The matrix

Figure 3. Microphotographs of eclogites (aed) and mica-schist (eef) of the Choloktor Formation. (a) Porphyroblastic garnet in the matrix with inclusions of glaucophane, phengite, quartz, apatite, rutile and epidote in its core. (b) Large crystal of clinopyroxene contains inclusions of garnet, glaucophane and rutile. (c) Phengite inclusion in the core of clinopyroxene. (d) Matrix amphiboles are zoned from bluish core to greenish rim. (e) Garnet rim of the mica-schists contain quartz inclusion with concentric and radial crack textures. (f) Glaucophane and epidote aggregate in the matrix of mica-schist.

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clinopyroxene includes quartz, garnet, glaucophane, phengite and rutile (Fig. 3b and c). Amphibole occur as inclusion (<0.2 mm in length) in porphyroblastic garnet and clinopyroxene (Fig. 3a and b) and as hypidioblastic prismatic grain (<0.7 mm) in the matrix, which has a zoning with core of glaucophane, rim of winchite and outermost rim of actinolite (Fig. 3d). Phengite and paragonite in the eclogites are tabular crystals in the matrix with size is up to 0.5 mm and as inclusion in garnet and omphacite (Fig. 3a,c). Micas in the matrix have mineral inclusions of quartz and rutile. Epidote occurs as hypidioblastic to xenoblastic prismatic crystals in the matrix with size up to 0.3 mm and as inclusions in garnets. Rutile found as inclusions in all constituent minerals of eclogite. Ilmenite is developed after rutile (Fig. 3d). Chlorite occurs as inclusions in garnet and in the matrix replaces garnet and amphibole along their margins and cracks.

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inclusions in the porphyroblastic albite (Fig. 4a and b). Garnet in the matrix contains inclusions of quartz, epidote and rutile (Fig. 4b), and is partially replaced by chlorite along the fractures. The finegrained garnet inclusions in albite contain inclusions of quartz and epidote. Phengite occurs in the matrix as a tabular grain with size up to 1 mm (Fig. 4a and b) and as inclusion in the porphyroblastic albite. Chlorite occurs as up to 1 mm across in the matrix of the rocks and as a replacement product along garnet rims and fractures. Epidote occurs as up to 0.5 mm (hyp)idioblastic grains in the matrix, and as inclusions in albite, garnet and chlorite. Biotite occurs at the margins of phengite and chlorite.

4. Mineral chemistry 4.1. Analytical procedures

3.1.2. Mica-schists Mica-schists in the Choloktor Formation consist of quartz, phengite, garnet, glaucophane, epidote, chlorite, rutile and ilmenite. Schistosity is defined by preferred orientation of phengite. Garnet occurs as idioblastic to hypidioblastic large crystal (<2 mm across) have zoning from pale reddish core to colorless rim. The mineral inclusions in garnets are quartz, rutile and ilmenite. Quartz inclusion with concentric and radial cracks in the garnet rim has been also observed (Fig. 3e). Garnet is replaced by chlorite along the fractures. Glaucophane occurs as prismatic crystal in the matrix with size up to 2 mm in length. It has zoning from blue core to pale-bluish rim (Fig. 3f) and contains mineral inclusions of rutile, epidote, and ilmenite. Locally, glaucophane was found in ca. 1 mm aggregates/intergrowths with epidote (Fig. 3f). Phengite occurs as ca. 2 mm platy crystals in the matrix and contains inclusions of quartz, rutile and ilmenite. Epidote occurs as hypidioblastic to idioblastic prismatic crystals with size about 1 mm in the matrix. Chlorite is developed after garnet and glaucophane. Rutile and ilmenite appear mainly as inclusion phases in the above described minerals. 3.2. The Atbashy Formation 3.2.1. Mica-schists Mica-schists from the Atbashy Formation consist mainly of quartz, phengite, albite, and chlorite with minor amounts of garnet, epidote, calcite, biotite, titanite, rutile and tourmaline. Schistosity is defined by preferred orientation of phengite and chlorite (Fig. 4a). Albite occurs as porphyroblast up to 3 mm across, and it contains garnet, phengite, quartz, epidote, calcite, tourmaline, rutile and titanite as inclusions (Fig. 4a and b). Garnet occurs as hypidioblastic to xenoblastic grain (<1 mm across) in the matrix and as

Chemical compositions of the constituent minerals were analyzed using an electron probe microanalyzer (JEOL JXM-8800M) at the Department of Geoscience, Shimane University. The operation conditions used for all phases were 15 kV acceleration voltage, 20 nA beam current and 5 mm beam diameter. The calculation of ferric iron was based on charge balance, i.e. Fe3þ ¼ 8 e 2Si e 2Ti e Al (for garnet) and Fe3þ ¼ 4 e 2Si e 2Ti e Al þ Na (for clinopyroxene). The classification of amphiboles follows Leake et al. (1997). 4.2. Garnet All garnets from eclogite and mica-schists (Choloktor and Atbashy Formations) are almandine-rich (Fig. 5; Table 1). Garnets from the eclogites have a chemical compositions of Fe2þ ¼ 1.45e1.95 p.f.u. (per formula unit), Mg ¼ 0.19e0.61 p.f.u., Mn ¼ 0.02e0.28 p.f.u. and Ca ¼ 0.71e1.04 p.f.u. They are zoned with Ca discontinuously decreasing from core (1.04 p.f.u.) to rim (0.71 p.f.u.), Mg increasing from core (0.19 p.f.u.) to rim (0.61 p.f.u.), Mn decreasing from core (0.28 p.f.u.) to rim (0.02 p.f.u.) and Fe2þ decreasing from core (1.95 p.f.u.) to rim (1.45 p.f.u.) (Fig. 5). Garnets from the mica-schist of the Choloktor Formation are characterized by Fe2þ ¼ 1.15e1.77 p.f.u., Mg ¼ 0.24e0.72 p.f.u., Mn ¼ 0.10e0.92 p.f.u. and Ca ¼ 0.47e0.61 p.f.u. (Fig. 5), and are zoned from core to rim with increasing Fe2þ (1.15e1.77 p.f.u.) and Mg (0.24e0.72 p.f.u.). Mn (0.92e0.10 p.f.u.) and Ca (0.61e0.47 p.f.u.) decreases from core to rim (Fig. 5). Garnets in the matrix of the mica-schists of the Atbashy Formation have chemical composition of Fe2þ ¼ 1.68e1.85 p.f.u., Mg ¼ 0.08e0.15 p.f.u., Mn ¼ 0.20e0.51 p.f.u. and Ca ¼ 0.58e0.93 p.f.u. (Fig. 5). The chemical compositions of fine-grained garnet

Figure 4. Microphotographs of mica-schists of the Atbashy Formation. (a) Porphyroblastic albite includes fine-grained garnet, phengite, quartz, calcite, epidote, tourmaline, rutile and titanite. (b) Garnets in the matrix contain inclusions of quartz, epidote and rutile, and replaced by chlorite. The alignment of mineral inclusions in porphyroblastic albite is almost parallel to the main schistosity defined by phengite in both (a) and (b).

Please cite this article in press as: Satybaev, M., et al., Petrology of metamorphic rocks from the Atbashy complex, Southern Tien-Shan, Kyrgyzstan, Geoscience Frontiers (2017), https://doi.org/10.1016/j.gsf.2017.11.005

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Figure 5. Mn-Mg-Fe2þ and Mg-Ca-Fe2þ diagrams showing the chemical compositions of garnets.

inclusions in porphyroblastic albite are Fe2þ ¼ 1.73e1.78 p.f.u., Mg ¼ 0.08e0.09 p.f.u., Mn ¼ 0.25e0.27 p.f.u. and Ca ¼ 0.86e0.92 p.f.u. Garnets in the matrix are zoned with Mn decreasing (0.51e0.20 p.f.u.) and Ca increasing (0.58e0.93 p.f.u.) from core to rim. Mg content is almost homogeneous with a slight decrease from core (0.15 p.f.u.) to rim (0.08 p.f.u.). The chemical compositions of fine-grained garnets in porphyroblastic albite are similar to the rim compositions of garnets in the matrix (Fig. 5).

51e59 mol.%, respectively. This omphacite has zoning from core (Jd31Ae16Aug53) to rim (Jd42Ae4Aug54). The omphacite inclusions in garnets have lower jadeite (28e39 mol.%), slightly higher aegirine (4e16 mol.%) and augite (50e62 mol.%) contents. In sample 15-2, the compositions of matrix omphacite are jadeite-rich and vary within the range of Jd ¼ 39e50 mol.%, Ae ¼ 3e11 mol.% and Aug ¼ 44e51 mol.% (Fig. 6). 4.4. Amphibole

4.3. Clinopyroxene Amphiboles in the eclogites are classified mainly as Naamphibole (glaucophane) with minor Na~Ca-amphibole (winchite) and Ca-amphibole (actinolite) (Fig. 7; Table 3). The matrix amphibole is zoned with a glaucophane core [Si ¼ 7.79e7.94 p.f.u., NaB ¼ 1.83e1.94 p.f.u. and XMg ¼ Mg/(Mg þ Fe2þ) ¼ 0.72e0.91]

All analyzed clinopyroxenes of eclogites are classified as omphacite (Fig. 6; Table 2) (Morimoto et al., 1988). In sample A-50, the matrix omphacite is characterized by jadeite (Jd), aegirine (Ae) and augite (Aug) contents ranging from 31e42, 2e15 and Table 1 Representative compositions of garnet (oxides in wt.% and others in p.f.u.) Rock type

Eclogite of Choloktor Fm.

Mode

rim

core

core

rim

rim

core

core

rim

rim

core

core

rim

SiO2 TiO2 Al2O3 FeOa MnO MgO CaO Na2O K2O Cr2O3 Total O ¼ 12 Si Ti Al Fe3+ Fe2+ Mn Mg Ca Na K Cr Total

38.54 0.02 21.67 24.08 0.36 4.67 10.83 0.00 0.03 0.07 100.28

38.31 0.02 21.26 23.69 0.34 3.46 12.29 0.04 0.03 0.02 99.46

38.53 0.04 21.48 24.38 0.38 3.37 12.51 0.00 0.04 0.07 100.80

38.01 0.07 21.06 25.73 0.41 3.33 10.87 0.00 0.04 0.01 99.53

37.68 0.06 21.27 27.78 1.98 4.94 6.44 0.00 0.04 0.02 100.21

37.01 0.11 20.55 20.68 12.72 2.23 6.85 0.04 0.05 0.00 100.24

36.23 0.10 20.41 19.56 13.40 2.23 7.04 0.04 0.05 0.04 99.09

38.24 0.05 21.08 26.73 1.89 4.85 6.26 0.03 0.02 0.02 99.18

38.04 0.14 21.25 26.65 4.07 0.87 11.12 0.00 0.02 0.01 102.16

37.87 0.09 21.51 27.36 7.44 1.07 6.42 0.13 0.02 0.00 101.92

37.48 0.15 21.15 27.51 7.62 0.92 6.79 0.04 0.02 0.00 101.68

38.21 0.10 21.44 27.89 3.03 1.09 10.62 0.01 0.02 0.00 102.41

2.987 0.001 1.979 0.043 1.518 0.024 0.540 0.900 0.000 0.003 0.004 8.000

3.008 0.001 1.967 0.018 1.538 0.022 0.404 1.034 0.006 0.003 0.001 8.003

2.989 0.002 1.964 0.052 1.530 0.025 0.390 1.040 0.000 0.004 0.004 8.000

2.998 0.004 1.958 0.038 1.660 0.027 0.391 0.919 0.000 0.004 0.001 8.001

2.954 0.003 1.966 0.120 1.702 0.132 0.578 0.541 0.000 0.004 0.001 8.001

2.957 0.007 1.935 0.141 1.241 0.861 0.265 0.587 0.006 0.005 0.000 8.003

2.926 0.006 1.942 0.196 1.125 0.916 0.268 0.609 0.006 0.005 0.003 8.003

3.023 0.003 1.964 0.000 1.767 0.126 0.572 0.530 0.005 0.002 0.001 7.995

2.979 0.008 1.962 0.064 1.681 0.270 0.101 0.933 0.000 0.002 0.000 8.000

2.995 0.005 2.006 0.001 1.809 0.499 0.126 0.544 0.020 0.002 0.000 8.007

2.978 0.009 1.981 0.049 1.779 0.513 0.109 0.578 0.006 0.002 0.000 8.003

2.983 0.006 1.972 0.051 1.769 0.201 0.127 0.888 0.002 0.002 0.000 8.001

a

Mica-schist of Choloktor Fm.

Mica-schist of Atbashy Fm.

Total Fe as FeO.

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Figure 6. Chemical compositions of clinopyroxene.

Table 2 Representative compositions of clinopyroxene in eclogites (oxides in wt.% and others in p.f.u.) Rock type

Eclogites of Choloktor Fm.

Mineral

Omp

Omp

Omp

Mode SiO2 TiO2 Al2O3 FeOa MnO MgO CaO Na2O K2O Cr2O3 Total O¼6 Si Ti Al Fe3+ Fe2+ Mn Mg Ca Na K Cr Total a

Omp

Omp

Omp

in Grt

in Grt

in Grt

Omp

Omp

Omp

Omp

Omp

Omp

in Grt

in Grt

54.20 0.03 7.56 4.58 0.01 10.10 16.34 5.41 0.04 0.01 98.28

56.77 0.04 10.76 3.77 0.05 8.84 13.68 7.16 0.08 0.03 101.15

54.07 0.03 8.37 4.84 0.01 9.56 15.02 6.02 0.06 0.07 98.05

55.78 0.07 9.26 7.93 0.05 7.76 12.97 7.08 0.04 0.01 100.94

55.39 0.03 7.63 8.32 0.06 9.03 14.78 6.04 0.04 0.04 101.35

56.24 0.04 9.60 4.99 0.04 9.87 14.70 6.27 0.03 0.07 101.85

55.73 0.07 11.95 4.64 0.05 7.22 11.38 8.05 0.04 0.10 99.21

56.08 0.03 12.25 3.82 0.02 7.85 12.30 7.69 0.03 0.02 100.08

56.11 0.06 12.12 4.11 0.02 7.39 11.58 8.12 0.05 0.03 99.59

55.62 0.00 11.95 3.62 0.00 7.89 12.46 7.77 0.03 0.11 99.45

55.88 0.07 10.80 5.22 0.06 7.97 13.40 6.98 0.02 0.02 100.41

56.08 0.03 10.70 5.42 0.05 7.76 13.24 7.02 0.02 0.04 100.35

1.969 0.001 0.324 0.119 0.020 0.000 0.547 0.636 0.381 0.002 0.000 4.000

1.983 0.001 0.443 0.077 0.033 0.001 0.460 0.512 0.485 0.003 0.001 4.000

1.963 0.001 0.358 0.140 0.006 0.000 0.518 0.584 0.424 0.003 0.002 3.999

1.981 0.002 0.387 0.137 0.099 0.001 0.411 0.493 0.487 0.002 0.000 4.000

1.969 0.001 0.320 0.158 0.089 0.002 0.479 0.563 0.416 0.002 0.001 4.000

1.964 0.001 0.395 0.100 0.046 0.001 0.514 0.550 0.424 0.001 0.002 3.999

1.982 0.002 0.501 0.089 0.049 0.001 0.382 0.433 0.555 0.002 0.003 3.999

1.975 0.001 0.509 0.067 0.046 0.001 0.412 0.464 0.525 0.001 0.000 4.000

1.983 0.001 0.505 0.084 0.038 0.001 0.389 0.439 0.556 0.002 0.001 4.000

1.968 0.000 0.498 0.099 0.008 0.000 0.416 0.472 0.533 0.001 0.003 3.999

1.980 0.002 0.451 0.065 0.090 0.002 0.421 0.509 0.479 0.001 0.000 4.000

1.990 0.001 0.447 0.055 0.106 0.001 0.411 0.504 0.483 0.001 0.001 4.000

Total Fe as FeO

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through winchite (Si ¼ 7.71e7.72 p.f.u., NaB ¼ 0.60e0.62 p.f.u. and XMg ¼ 0.73e0.78) to an actinolite rim (Si ¼ 7.58e7.73 p.f.u., NaB ¼ 0.35e0.45 p.f.u. and XMg ¼ 0.74e0.82). Locally, the outermost rim of glaucophane has a composition of Si ¼ 7.75 p.f.u., NaB ¼ 1.63 p.f.u. and XMg ¼ 0.69. The glaucophane inclusions in garnet and omphacite have similar compositional range to those in the matrix with Si ¼ 7.83e7.92 p.f.u., NaB ¼ 1.82e1.94 p.f.u. and XMg ¼ 0.75e0.85 (Fig. 7). In the mica-schist of Choloktor Formation, the matrix amphibole is Na-amphibole (glaucophane) with Si ¼ 7.81e7.94 p.f.u., NaB ¼ 1.70e1.86 p.f.u. and XMg ¼ 0.80e0.91 (Fig. 7).

of 6.75e6.95 p.f.u. (based on 22 oxygen) and XNa ¼ Na/ (Na þ K þ Ca) ¼ 0.04e0.12. They are zoned with slightly increasing Si content from core (Si ¼ 6.80e6.87 p.f.u.; XNa ¼ 0.04e0.12) to rim (Si ¼ 6.91e6.95 p.f.u.; XNa ¼ 0.04e0.05) and decreasing at the outermost rim (Si ¼ 6.75e6.79 p.f.u.; XNa ¼ 0.05e0.09). Phengite inclusions in garnet and omphacite show less content of Si ¼ 6.70e6.75 p.f.u. and XNa ¼ 0.10e0.11, compare to the matrix phengite. Si and XNa contents of the matrix paragonite varies of 5.96e6.04 p.f.u. and 0.94e0.97, respectively.

4.5. White micas In the eclogites, white micas are represented by phengite and paragonite (Fig. 8; Table 4). The Si content of matrix phengites are

Figure 7. Chemical compositions of amphibole.

Figure 8. Chemical compositions of white micas.

Table 3 Representative compositions of amphiboles (oxides in wt.% and others in p.f.u.) Rock type

Eclogites of Choloktor Fm.

Mineral

Gln

Gln

Brs

Mica-schist of Choloktor Fm. Act

Act

Mode SiO2 TiO2 Al2O3 FeOa MnO MgO CaO Na2O K2O Cr2O3 Total O ¼ 23 Si Ti Al Fe3+ Fe2+ Mn Mg Ca Na K Cr Total a

Wnch

Wnch

Gln

Gln

Gln

Gln

Gln

Gln

rim

in Grt

in Grt

in Grt

in Omp

in Omp

rim

core

Gln core

55.86 0.46 11.00 10.19 0.05 10.69 2.52 6.23 0.07 0.08 97.15

57.23 0.03 12.24 9.03 0.02 10.63 1.02 6.91 0.06 0.00 97.17

52.78 0.08 6.03 10.59 0.06 15.15 9.63 2.28 0.18 0.11 96.88

54.47 0.06 3.48 9.69 0.08 16.66 10.83 1.48 0.12 0.02 96.87

55.21 0.05 3.42 10.57 0.06 17.28 10.48 2.09 0.19 0.00 99.35

54.05 0.05 5.97 9.64 0.06 14.31 9.01 3.06 0.16 0.04 96.36

54.41 0.12 4.05 11.68 0.08 15.11 9.02 2.18 0.17 0.03 96.85

56.39 0.00 11.42 9.87 0.05 10.63 1.63 6.71 0.06 0.01 96.76

58.21 0.05 12.05 6.63 0.06 11.78 0.39 7.40 0.04 0.03 96.64

58.26 0.01 12.15 5.88 0.04 12.59 0.52 7.09 0.04 0.00 96.58

58.16 0.00 11.73 7.52 0.02 11.78 0.44 7.46 0.03 0.00 97.14

57.47 0.04 8.27 9.86 0.15 13.08 1.40 6.68 0.05 0.00 96.99

58.02 0.03 9.10 9.01 0.09 12.92 0.84 7.07 0.05 0.05 97.16

57.43 0.04 8.74 8.84 0.03 13.49 1.45 6.50 0.08 0.00 96.60

7.748 0.048 1.798 0.165 1.016 0.006 2.210 0.375 1.675 0.013 0.009 15.063

7.824 0.003 1.971 0.232 0.801 0.002 2.167 0.150 1.833 0.010 0.000 14.993

7.497 0.008 1.010 0.377 0.881 0.007 3.208 1.465 0.627 0.033 0.012 15.125

7.729 0.006 0.581 0.226 0.923 0.009 3.524 1.647 0.407 0.021 0.002 15.075

7.641 0.005 0.558 0.446 0.778 0.007 3.564 1.554 0.561 0.034 0.000 15.149

7.724 0.005 1.006 0.000 1.152 0.008 3.049 1.380 0.846 0.030 0.005 15.204

7.716 0.013 0.677 0.490 0.895 0.009 3.195 1.371 0.600 0.030 0.003 15.000

7.799 0.000 1.861 0.246 0.896 0.006 2.191 0.242 1.799 0.010 0.001 15.052

7.913 0.005 1.930 0.160 0.594 0.006 2.388 0.057 1.949 0.007 0.003 15.013

7.864 0.001 1.933 0.323 0.341 0.004 2.533 0.075 1.856 0.007 0.000 14.938

7.887 0.000 1.875 0.255 0.598 0.002 2.382 0.064 1.961 0.005 0.000 15.031

7.854 0.004 1.332 0.763 0.364 0.018 2.665 0.205 1.770 0.008 0.000 14.983

7.884 0.003 1.457 0.648 0.376 0.010 2.617 0.122 1.862 0.008 0.005 14.993

7.835 0.004 1.405 0.759 0.250 0.003 2.744 0.212 1.720 0.013 0.000 14.945

Total Fe as FeO

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Table 4 Representative compositions of white micas, chlorite and albite (oxides in wt.% and others in p.f.u.). Rock type

Choloktor Fm.

Mineral

Eclogite

Mode

Phn

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Cr2O3 Total O Si Ti Al Fe Mn Mg Ca Na K Cr Total

50.54 0.25 25.96 1.93 0.00 3.81 0.05 0.46 10.78 0.13 93.92 22 6.855 0.025 4.151 0.219 0.000 0.771 0.008 0.122 1.865 0.014 14.03

Atbashy Fm. Mica-schist

Phn

Phn

in Grt

in Omp

50.25 0.19 28.10 2.36 0.01 3.68 0.02 0.64 10.28 0.03 95.55 22 6.694 0.019 4.412 0.263 0.001 0.731 0.002 0.164 1.747 0.003 14.04

50.81 1.37 26.21 1.51 0.02 4.03 0.00 0.53 11.41 0.01 95.89 22 6.770 0.138 4.116 0.168 0.002 0.801 0.000 0.138 1.939 0.001 14.07

Pg

45.41 0.03 38.38 0.96 0.04 0.12 0.22 7.61 0.36 0.09 93.21 22 5.958 0.003 5.935 0.105 0.004 0.023 0.031 1.935 0.060 0.009 14.06

Pg

46.40 0.06 37.71 0.94 0.02 0.38 0.15 7.63 0.55 0.09 93.92 22 6.043 0.006 5.788 0.102 0.002 0.073 0.021 1.927 0.091 0.009 14.06

Chl

30.53 0.11 19.52 17.66 0.43 16.37 0.12 0.09 1.39 0.07 86.29 28 6.265 0.016 4.720 3.031 0.075 5.007 0.026 0.037 0.363 0.011 19.55

Phn

Phn

Core

Rim

49.54 0.22 26.19 3.88 0.00 3.23 0.00 0.59 10.53 0.14 94.32 22 6.760 0.023 4.212 0.442 0.000 0.657 0.000 0.157 1.834 0.015 14.10

52.30 0.22 25.45 3.76 0.01 3.98 0.00 0.20 8.28 0.14 94.34 22 6.988 0.023 4.007 0.420 0.001 0.793 0.000 0.053 1.412 0.015 13.71

In the mica-schists of Choloktor Formation, white micas are phengite, and their Si range of 6.89e7.04 p.f.u. and XNa of 0.03e0.06 (Fig. 8). Generally, the Si content decrease from core (7.04) to rim (6.89). White micas in the mica-schists of the Atbashy Formation are also phengite (Fig. 8). Phengites in the matrix have Si content of 6.62e7.00 p.f.u. and the XNa range of 0.02e0.09. Phengite inclusions in the porphyroblastic albite show similar range of Si ¼ 6.68e6.99 p.f.u. and XNa ¼ 0.03e0.07.

Mica-schist Chl

26.11 0.00 20.05 22.46 0.33 17.87 0.01 0.00 0.04 0.07 86.93 28 5.477 0.001 4.957 3.941 0.059 5.587 0.003 0.001 0.010 0.011 20.04

Phn

Phn

Phn

Phn

in Ab

in Ab

Core

Rim

50.62 0.21 29.08 2.86 0.05 2.90 0.00 0.47 10.29 0.05 96.52 22 6.677 0.020 4.520 0.315 0.005 0.570 0.000 0.120 1.731 0.006 13.96

53.73 0.10 26.21 2.94 0.01 3.82 0.00 0.19 10.61 0.01 97.62 22 6.992 0.009 4.020 0.320 0.001 0.741 0.000 0.049 1.761 0.001 13.89

52.95 0.12 24.89 2.99 0.03 4.10 0.01 0.09 11.13 0.04 96.33 22 7.019 0.012 3.889 0.332 0.003 0.811 0.001 0.023 1.882 0.004 3.51

49.03 0.19 29.77 2.53 0.00 2.76 0.01 0.42 10.62 0.04 95.37 22 6.559 0.019 4.694 0.283 0.000 0.550 0.002 0.109 1.812 0.005 3.28

Chl

Chl

Ab

Ab

24.70 0.02 19.56 29.57 0.30 13.29 0.04 0.00 0.04 0.01 87.53 28 5.365 0.004 5.009 5.372 0.054 4.304 0.009 0.000 0.012 0.002 20.13

25.49 0.03 19.53 26.85 0.26 14.25 0.04 0.00 0.03 0.02 86.49 28 25.49 0.03 19.53 26.85 0.26 14.25 0.04 0.00 0.03 0.02 86.49

68.80 0.00 19.70 0.03 0.01 0.00 0.13 11.20 0.08 0.04 99.99 8 2.999 0.000 1.012 0.001 0.000 0.000 0.006 0.947 0.004 0.001 4.970

69.23 0.00 19.40 0.00 0.03 0.00 0.10 11.44 0.06 0.00 100.3 8 3.010 0.000 0.994 0.000 0.001 0.000 0.005 0.964 0.003 0.000 4.977

thermodynamic data set (Holland and Powell, 1998) was applied for the mineral assemblages that are interpreted to have equilibrated in each stage of the metamorphic evolution of the studied rocks. The activities of the minerals were calculated using the AX program (Holland and Powell, 1998). The activity of H2O and SiO2 was assumed to be unity. The set of independent reactions used in the calculations and the obtained results of all P-T estimates by THERMOCALC and other available geothermobarometers are summarized in Supplementary Tables S1 and S2, respectively.

4.6. Other minerals The YPs ¼ Fe3þ/(Al þ Fe3þ) content of epidote included within garnet in the eclogites ranges from 0.16 to 0.23. Epidote in the matrix of the eclogites has YPs ¼ 0.14e0.18. Epidote in the matrix of mica-schists of Choloktor Formation has higher YPs value of 0.23e0.31. In the mica-schists of Atbashy Formation, YPs content of matrix epidotes vary from 0.16 to 0.18, whereas those in the porphyroblastic albite have YPs ¼ 0.14e0.16 (Table 4). XMg ¼ Mg/(Fe þ Mg) of chlorite inclusions in garnet of the eclogites varies of 0.53e0.71, whereas those replacing garnets characterized by XMg ¼ 0.34e0.41. Chlorite in the matrix of micaschist of Choloktor Formation has XMg ¼ 0.56e0.66. XMg ratio of chlorite in the matrix of mica-schists of Atbashy Formation range of 0.45e0.49. Chlorite inclusions in porphyroblastic albite have XMg ¼ 0.37e0.47 (Table 4). In the mica-schist of the Atbashy Formation, plagioclase is albite (An0e2). 5. Thermobarometry The mineral paragenesis and metamorphic evolution of each rock type is summarized in Figs. 9 and 10. The eclogites of the Choloktor Formation record prograde, peak and retrograde metamorphic stages, whereas for the mica-schists of the Choloktor and Atbashy Formations preserve only peak and subsequent retrograde stage conditions. The ‘Average P-T calculation’ mode of THERMOCALC v. 3.26 software (Powell and Holland, 1994) with internally consistent

5.1. P-T conditions of eclogites (Choloktor Formation) The prograde stage of the eclogites can be deduced from the mineral inclusions in peak minerals such as garnet and omphacite. Porphyroblastic garnets are prograde zoned from core to rim (Fig. 5). Hence, the mineral inclusions of quartz, glaucophane, phengite (Si ¼ 6.70e6.75 p.f.u.), clinopyroxene (Jd ¼ 28e33 mol.%), epidote and rutile in the core of the garnets (Fig. 3a) are interpreted to have crystallized during progressive metamorphism, whereas those inclusions of quartz, clinopyroxene (Jd ¼ 36e39 mol.%) and rutile at the garnet rims e during a prograde to close to peak stage conditions. Omphacite contain also inclusions of glaucophane, phengite and rutile (Fig. 3b and c). Thus, the prograde stage of eclogites are characterized by mineral assemblage of garnet-core, glaucophane, clinopyroxene (Jd ¼ 28e33 mol.%), epidote, phengite (Si ¼ 6.70e6.75 p.f.u.), quartz and rutile, which represents the mineral paragenesis of blueschist/epidote-blueschist facies metamorphism (Evans, 1990). We have applied THERMOCALC ‘average P-T’ mode calculations in order to estimate the P-T conditions of prograde and peak metamorphic stages of the eclogites and mica-schists. For the prograde stage of eclogites, we have applied the chemical compositions of garnet core (with lowest Mg and highest Mn contents) along with compositions of mineral inclusions of glaucophane, phengite and epidote (sample 15-2) and omphacite, glaucophane and phengite (sample A-50) (Table S2). The P-T conditions estimated for the prograde epidote-blueschist facies stage are

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Figure 9. The mineral paragenesis of the Atbashy metamorphic rocks.

17e21 kbar and 450e515  C (Fig. 10). Similar P-T estimation (P ¼ 21e22 kbar and T ¼ 490e520  C) was also obtained using the Grt-Cpx-Phn geothermobarometer (Krogh Ravna and Terry, 2004) for sample 15-2 (Table S2). The temperature estimates using the garnet-clinopyroxene Fe2þeMg exchange geothermometers of Ellis and Green (1979), Powell (1985) and Krogh-Ravna (2000) at 20 kbar are 500e570  C, 475e545  C and 440e510  C, respectively (Table S2). The calculated temperature of 440e510  C by geothermometer of Krogh-Ravna (2000) is consistent with the temperature (450e515  C) obtained using THERMOCALC ‘average P-T’ application. However, the temperatures estimated by geothermometers of Ellis and Green (1979) and Powell (1985) are 30e60  C greater. We have adopted Krogh-Ravna (2000) estimates, which is based on both experimental and natural data sets, and incorporates the effect of Mn content of garnet, and thus supersedes the previous calibrations. Our estimated P-T conditions (P ¼ 17e21 kbar and T ¼ 450e515  C) for the prograde stage of eclogites are consistent with those of previous studies, i.e. P ¼ 15 kbar and T ¼ 500  C (Tagiri et al., 1995), P ¼ 21 kbar and

T ¼ 515  C (Volkova et al., 2014) and P ¼ 8e19 kbar and T ¼ 300e500  C (Hegner et al., 2010), and lie in the stability field of blueschist/epidote-blueschist facies conditions (Fig. 10). The peak metamorphic stage of the Choloktor eclogites is characterized by the coexistence of garnet, omphacite, glaucophane, phengite (Si ¼ 6.91e6.95 p.f.u.), quartz and rutile in the matrix (Fig. 3aed). The mineral inclusions of omphacite (Jd ¼ 36e39 mol.%), quartz and rutile occurring at the rim part of garnets with highest Mg and lowest Mn content can be also formed at peak metamorphic conditions. These textures of minerals and their compositions suggest mineral assemblage for the peak metamorphic stage of the eclogites to be garnet, omphacite, glaucophane, phengite, quartz and rutile. The application of THERMOCALC ‘Average P-T’ mode calculation yielded P ¼ 26e29 kbar and T ¼ 545e615  C (Fig. 10). The Grt-Cpx-Phn geothermobarometer (Krogh Ravna and Terry, 2004) show consistent P-T estimation of 25e28 kbar and 545e605  C. The garnet-clinopyroxene geothermometers gives the temperature of 590e680  C (Ellis and Green, 1979), 565e655  C (Powell, 1985) and 540e610  C (KroghRavna, 2000) at 25 kbar for the peak eclogite metamorphic conditions (Suppl. Table S2). In this study, the obtained peak pressure conditions (P ¼ 26e29 kbar) is higher than previous estimates, except for studies of Tagiri et al. (1995) and Bakirov et al. (1998), and lies in the coesite stability field (UHP pressure conditions) (Fig. 10). Our pressure estimates is consistent with the earlier finding of quartz pseudomorphs after coesite in garnet and omphacite from the Atbashy eclogites (Tagiri et al., 1995), and below the proposed pressure conditions of 35e40 kbar of Bakirov et al. (1998) (Fig. 10). The temperature calculation of 545e615  C is consistent with the earlier estimated range of 510e600  C (Simonov et al., 2008; Hegner et al., 2010; Volkova et al., 2014) (Fig. 10). The temperature estimates of Tagiri et al. (1995) and Bakirov et al. (1998) show much higher range of 610e790  C (Fig. 10), and their obtained P-T conditions are based on talc-albite intergrowth and two pyroxene lamellae textures found in the eclogites. On the other hand, the estimated temperatures range of 610e705  C at 25 kbar using Grt-Cpx geothermometers (Ellis and Green, 1979; Powell, 1985) by Tagiri et al. (1995) are similar to those obtained in this study (T ¼ 565e680  C) using the same geothermometers (Table S2), suggesting that there are no contradictions in the temperature estimates from both studies. The retrograde stage of the eclogites in the Choloktor Formation is represented by zoning of matrix amphibole from glaucophane cores through winchite to actinolite at the rims, Si content zoning of phengite from rim (Si ¼ 6.91e6.95 p.f.u.) to outermost rim (Si ¼ 6.75e6.79 p.f.u.) and by replacement of garnet by epidote and chlorite. Based on the development of secondary mineral assemblages after the peak minerals we can suggest two stages of retrograde metamorphism for the eclogites, i.e. the epidoteamphibolite facies conditions (epidote, winchite, phengite (Si ¼ 6.75e6.79 p.f.u.) and chlorite) followed by greenschists facies overprint (actinolite and chlorite) (Table S2). In this study, the application of THERMOCALC ‘Average P-T’ mode calculation to the mineral assemblage of epidote-amphibolite facies conditions gives insufficient set of mineral reactions; hence, no absolute P-T estimation can be obtained for retrograde stage. On the other hand, the proposed retrograde stages, passing through epidote-amphibolite facies conditions to greenschists facies, are also suggestive by previous studies (Tagiri et al., 1995) (Fig. 10). 5.2. P-T conditions of mica-schists (Choloktor Formation) The prograde zoned garnet from the mica-schists contain mineral inclusions of quartz and rutile only (Fig. 3e), therefore it is challenging to constrain prograde stage of the rock. However, the

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Figure 10. Obtained P-T conditions for the metamorphic rocks of the Atbashy complex. P-T boundaries of metamorphic facies [greenschist (GS), epidote-amphibolite (EA), blueschist (BS), amphibolite (AMP), granulite (LGR and HGR) and subdivision of the eclogite (EC) into amphibolite (Amp) eclogite, epidote (Ep) eclogite, lawsonite (Lw) eclogite and dry eclogite] are after Liou et al. (2009). Reaction lines: Ab ¼ Jd þ Qtz (Holland, 1983), Qtz 5 Coe (Bohlen and Boettcher, 1982).

presence of glaucophane (Fig. 3f) with inclusions of epidote may suggest epidote-blueschists conditions before the peak metamorphic stage (Fig. 10). The minerals of garnet, glaucophane, phengite (Si ¼ 6.89e7.04 p.f.u.), epidote, chlorite (XMg ¼ w0.66) and quartz are regarded as the peak mineral assemblage. The compositions of these minerals yielded P ¼ 21e23 kbar and T ¼ 530e580  C (Suppl. Table S2), which belongs to eclogite facies conditions (Fig. 10). In earlier work, Hegner et al. (2010) suggested that host mica-schists underwent the same metamorphic development as the eclogite boudins. Their interpretation based on the high-pressure phengite (Si ¼ 6.7e6.9 p.f.u.) relics in the schists, however, no detailed petrologic evidences were given. In this study, we provide geothermobarometric calculations suggesting HP metamorphic conditions for the peak metamorphic stage of Choloktor mica-schists (Fig. 10). The final retrograde stage of schists is represented by greenschist facies conditions, similarly to the previous studies (Bakirov, 1978; Bakirov et al., 1984; Hegner et al., 2010) (Fig. 10).

developed in the matrix of rocks. Furthermore, the slightly curved inclusions trails in porphyroblastic albite are parallel and continuous to the matrix foliation (Fig. 4a and b). These porphyroblastmatrix microstructural relationships implies the syn-kinematic growth of porphyroblastic albite (Johnson, 1999). Hence, the peak mineral assemblage is characterized by garnet, phengite, albite, quartz, epidote, chlorite, tourmaline and rutile (Fig. 9). Using the compositions of these minerals, THERMOCALC ‘Average P-T’ mode calculation gave P ¼ 11e12 kbar and T ¼ 535e565  C (mineral inclusions in porphyroblastic albite) and P ¼ 10e12 kbar and T ¼ 515e560  C (matrix phases) (Suppl. Table S2). These estimated values are similar and, therefore, regarded as peak P-T conditions of epidote-amphibolite facies for the mica-schists of Atbashy Formation (Fig. 10). The development of chlorite after garnet and biotite after phengite suggest greenschist facies overprint.

5.3. P-T conditions of mica-schists (Atbashy Formation)

The eclogites of the Choloktor Formation show a prograde evolution from the epidote-blueschist facies (P ¼ 17e21 kbar and T ¼ 450e515  C) to the peak eclogite facies UHP conditions (P ¼ 26e29 kbar and T ¼ 545e615  C) with subsequent retrograde epidote-amphibolite and greenschist facies overprints (Fig. 10). The mica-schists of the Choloktor Formation also show a clockwise P-T path from blueschist/epidote-blueschist facies conditions through peak eclogite facies conditions (P ¼ 21e23 kbar and T ¼ 530e580  C) to retrograde epidote-amphibolite and greenschist facies stages. The difference in the calculated pressure

In the mica-schists of the Atbashy Formation, it is difficult to obtain P-T constraints for the prograde stage of metamorphism, based on the epidote, quartz and rutile inclusions in garnet. As with the mica-schists of Choloktor Formation, only constraints on the peak metamorphic stage can be given. Porphyroblastic albite contain mineral inclusions of garnet, phengite, quartz, epidote, calcite, tourmaline, rutile and titanite (Fig. 4a and b). The similar mineral assemblage of garnet, phengite, epidote and quartz is also

5.4. P-T evolution of the Atbashy metamorphic rocks and implications

Please cite this article in press as: Satybaev, M., et al., Petrology of metamorphic rocks from the Atbashy complex, Southern Tien-Shan, Kyrgyzstan, Geoscience Frontiers (2017), https://doi.org/10.1016/j.gsf.2017.11.005

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M. Satybaev et al. / Geoscience Frontiers xxx (2017) 1e13

conditions for eclogites and mica-schists of the Choloktor Formation is about 6 kbar, but their temperature estimates are consistent. The similarities between P-T paths inferred for the mica-schists of Choloktor Formation in this study and previous studies on eclogites (Simonov et al., 2008; Hegner et al., 2010; Volkova et al., 2014) are compatible with them having shared their whole P-T history (Fig. 10). However, the presence of eclogites with various peak P-T conditions (this study; Tagiri et al., 1995; Bakirov et al., 1998; Simonov et al., 2008; Hegner et al., 2010; Volkova et al., 2014) (Fig. 10) may imply a tectonic mix of rocks formed at different depths of the crust and upper mantle. The newly obtained P-T conditions for the mica-schists of Choloktor Formation may indicate that some blocks of the originally sedimentary rocks were also brought to great depths along subduction zone and they metamorphosed under eclogite facies UHP conditions. Thus, the eclogite blocks were amalgamated with mica-schists of Choloktor Formation in the eclogite facies HP-UHP conditions and they together experienced isothermal decompression to shallow level of crust (Fig. 10). The mica-schists of the Atbashy Formation record medium pressure peak metamorphism (P ¼ 10e12 kbar and T ¼ 515e565  C) and they lack of evidence for HP-UHP metamorphism (Fig. 10). This may indicate that the highest grade of regional metamorphism in the Atbashy Ridge experienced in the epidote-amphibolite facies conditions. During the exhumation, the eclogites and mica-schists of Choloktor Formation docked with mica-schists of Atbashy Formation at about 10e12 kbar and 515e565  C, and from this depth (w40 km) the exhumation of whole sequence is shared (Fig. 10). Acknowledgments Editorial handling by Associate Editor Dr. C. Spencer and constructive comments by three anonymous reviewers are greatly acknowledged. We thank A. Togonbaeva and I. Bektenov for their help during the field survey. The members of the Geoscience Seminar of Shimane University and the Institute of Tibetan Plateau Research CAS are thanked for their discussion and suggestions. This study was partly supported by the Project ISTC (No KR-712) to the Institute of Geology of NAS KR and JSPS KAKENHI (Grant Nos. JP15H05695 for AT and JP12F02026 for RO). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.gsf.2017.11.005. References Bakirov, A.B., 1978. Tectonic Position of the Tien-Shan Metamorphic Complexes. Ilim, Frunze, 261 pp. Bakirov, A.B., 1989. The peculiarities of the compositions and conditions of the formation of the Tien-Shan eclogite-bearing metamorphic complexes. In: Zharikov, V.A., Fonarev, F.I. (Eds.), Crystallicheskaya kora v prostranstve i vremeni e metamorphicheskie i gidrotermal’nye processy. Nauka, Moscow, pp. 193e203. Bakirov, A.B., Dobretsov, N.L., Lavrent’ev, Y.G., Usova, L.V., 1974. Eklogity Atbashinskogo hrebta, Tyan-Shan. Doklady Academii Nauk SSSR 215, 677e680. Bakirov, A.B., Balbachan, A.R., Kotova, L.S., 1984. The Structural-petrological Map of the Choloktor Formation. Institute of Geology, Academy of Sciences of Kyrgyz SSR, Frunze. Bakirov, A.B., Tagiri, M., Sakiev, K.S., 1998. Rocks of ultrahigh-pressure metamorphic facies in the Tien Shan. Russian Geology and Geophysics 39, 1709e1721. Bohlen, S.R., Boettcher, A.L., 1982. The quartz-coesite transformation: A precise determination and the effects of other components. Journal of Geophysical Research 87, 7073e7078. Ellis, D.J., Green, D.H., 1979. An experimental study of the effect of Ca upon garnetclinopyroxene FeeMg exchange equilibria. Contributions to Mineralogy and Petrology 71, 13e22. Evans, B.W., 1990. Phase relations of epidote-blueschists. Lithos 25, 3e23.

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