Metamorphism and K-Ar White Mica Ages of Pelitic Schists in the Makbal Complex, Kyrgyz Northern Tien-Shan

Earth Science (Chikyu Kagaku) vol. 74, 47-64. 2020 47

Metamorphism and K-Ar white mica ages of pelitic schists in the Makbal Complex, Kyrgyz Northern Tien-Shan

KASYMBEKOV Adil*, TAKASU Akira*,**, KABIR Md Fazle***, ENDO Shunsuke*, BAKIROV Apas B.****, SAKIEV Kadyrbek****, OROZBAEV Rustam****,*****, HIRAJIMA Takao****** and YOSHIDA Kenta*******

Abstract Garnet-free pelitic schist (KG1251) and garnet and chloritoid-bearing pelitic schist (KG1244) of the Neldy Formation in the Makbal HP-UHP Metamorphic Complex, Kyrgyz Northern Tien-Shan are petrologically and geochronologically described. The pelitic schist (KG1251) consists mainly of phengite, chlorite and quartz, with small amounts of albite, titanite, calcite, rutile and carbonaceous matter. The peak metamorphic conditions are roughly constrained as T <630 °C and P = 0.9–1.7 GPa. A K-Ar age for schistosity-forming phengite is obtained as 524 ± 13 Ma, and it is almost identical to the previously reported peak metamorphic ages (ca. 500 Ma) of eclogites and garnet-chloritoid-talc schists in the Makbal Complex. The pelitic schist (KG1244) consists mainly of white mica (phengite-core and muscovite-rim), chlorite and quartz, with minor amounts of garnet, chloritoid, albite, tourmaline, zircon, monazite, titanite, rutile, calcite and carbonaceous matter. The peak metamorphic conditions are estimated as T = 485–545 °C and P = 1.2–1.5 GPa (high-P/T metamorphism), followed by a low-P/T metamorphism of T = ca. 500 °C and P >0.3 GPa, which is probably caused by contact metamorphism of granitic intrusions. The K-Ar white mica age of 474 ± 12 Ma obtained by the present study is similar to the ages of the Ordovician granitic intrusions. The peak metamorphic conditions of the Neldy pelitic schists (KG1251 and KG1244) are significantly lower in pressure than those of the previously reported eclogites and garnet-chloritoid-talc schists located in the lower tectono-structural levels of the Makbal Complex.

Key Words : K-Ar‌ age, pelitic schist, garnet-chloritoid-talc schist, eclogite, white mica, Makbal Complex, Kyrgyz Northern Tien-Shan.

Bakirov et al. 1987; 1998; 2017; Togonbaeva et al. 2009; Orozbaev Introduction et al. 2010; 2015; Tagiri et al. 2010; Hegner et al. 2010; Konopelko The Kyrgyz Tien-Shan Mountains extend from east to west, et al. 2012; Klemd et al. 2015; Satybaev et al. 2018). separating Kazakhstan Plate to the north and the Tarim Plate to the The Makbal Complex (Nikolaev 1933) was divided into south. They are divided into three major tectonic units; Northern two major tectono-stratigraphic units, the structurally lower Tien-Shan, Central (or Middle) Tien-Shan and Southern Tien-Shan, Akdjon Group and the upper Sharkyrak Group, based mainly on bounded by tectonic discontinuities (Fig. 1). Several high-pressure metamorphic grade. The Akdjon Group was further divided into (HP) and ultra-high-pressure (UHP) metamorphic belts have been the Makbal and the Neldy Formations (Medvedeva 1960)(Fig. 2). described in the Kyrgyz Tien-Shan Mountains: the Makbal and the Garnet-chloritoid-talc schists and occasional meta-quartzites and Aktyuz HP/UHP Complexes are located in the Northern Tien-Shan, eclogites in the Makbal Formation contain coesite as inclusion in while the Atbashy HP/UHP Complex is located in the Southern garnet (e.g. Tagiri et al. 2010). The Neldy Formation is composed Tien-Shan (Fig. 1) (Bakirov 1978; Bakirov and Maksumova 2001; mainly of apparently low-grade pelitic schists with lenses and

Received September 27, 2019. Accepted March 19, 2020. Editor TAKEUCHI Keiji * Department of Earth Science, Graduate School of Science and Engineering, Shimane University, 1060 Nishikawatsu, Matsue 690-8504, Japan (e-mail: [email protected]) ** San’in Branch, Professor emeritus, Shimane University, 1060 Nishikawatsu, Matsue 690-8504, Japan (e-mail: [email protected]) *** Fujii Consulting & Associates, Japan **** Institute of Geology, Kyrgyz National Academy of Sciences, Kyrgyzstan ***** Applied Geology Department, American University of Central Asia, Kyrgyzstan ****** Department of Geology and Mineralogy, Graduate School of Science, Kyoto University, Japan ******* Submarine Volcano Research Group, Japan Agency for Marine-earth Science and Technology (JAMSTEC), Japan

（ 1 ） 48 Kasymbekov et al.

72° E 76° E Almaty 80° E Kazakhstan Plate Fig. 2 (6) Bishkek Aktyuz HP Makbal HP-UHP complex complex Issyk-Kul Lake 42° N NTS (1) MTS Aksu Tashkent MTS (2) (4) STS 70° 80° Atbashy HP-UHP N Kazakhstan complex Osh (3) Kyrgyzstan (5) 40° STS Tarim Plate Uzbekistan China Tajikistan 0 50 100 km

Fig. 1 Tectonic outline of the Kyrgyz Tien-Shan. Localities of HP-UHP rocks are indicated by stars. The Kyrgyz Tien-Shan Mountains extend from east to Fig. 1 Tectonic outline of the Kyrgyz Tien-Shan. Localities of HP-UHP rocks are indicated by stars. The Kyrgyz Tien-Shan west, separating the Kazakhstan Plate to the north and the Tarim Plate to the south. They are divided into three tectonic units: NTS-Northern Tien-Shan: MTS- Middle Tien-Shan; STS-Southern Tien-Shan, bounded by major fault zones. They are the Main Tectonic Line (1), the Atbashy-Inylchek Fault (2), the North Tarim FaultMountains (3), the Talas–Fergana extend from eastFault to (4 west,), the separatingSouth Fergana the Fault Kazakhstan (5) and the Plate Djalair-Naiman to the north andSuture the ( 6Tarim). Plate to the south. They are divided

into three tectonic units: NTS-Caledonian Northern Tien-Shan: MTS-Caledonian-Hercynian Middle Tien-Shan; STS-Hercynian

Southern Tien-Shan, bounded by major fault zones are the Main Tectonic Line (1), the Atbashy-Inylchek Fault (2), the North layers of eclogites, amphibolites, marbles and meta-quartzites. The Postscript: Prof. Kadyrbek Sakiev of the Director of the Institute of lithotypesTarim of the Fault Neldy (3), theFormation Talas–Fergana vary from Fault pelitic (4), the schists South with Fergana or Fault (5)Geology, and the Kyrghyz Djalair-Naiman National AcademySuture (6). of Sciences, one of the authors of without garnet to HP eclogites. However, in the Neldy Formation, the present publication, suddenly has passed away on April 17, 2020, after coesites and/or pseudomorphs after coesite have not been found so the acceptance of the manuscript. We would like to express our deepest far. sympathy for the passing of Prof. Sakiev. Several geochronological studies of the Makbal Formation have been performed. CHIME monazite age of garnet-chloritoid-talc Geology and geochronology of the Makbal schists yielded 481 ± 26 Ma (Togonbaeva et al. 2009). SHRIMP Complex U-Pb zircon ages of eclogites gave 509 ± 7 Ma and 498 ± 7 Ma as the peak metamorphic ages (Konopelko et al. 2012). However, The Makbal Complex has been divided into two tectono- with regard to the Neldy Formation, only a Sm-Nd age of 526 ± metamorphic units, the structurally lower Akdjon Group and the 9.5 Ma from a HP eclogite has been reported (Togonbaeva et al. upper Sharkyrak Group. The Akdjon Group consists of HP-UHP 2010a). metamorphic rocks, and it is subdivided into the structurally lower In the Makbal Formation, P-T conditions and geochronology (meta-quartzite-dominant) Makbal Formation and the structurally of the HP-UHP rocks have been well studied. On the other hand, upper (pelitic schist-dominant) Neldy Formation (Fig. 2). no such studies for apparently low-grade pelitic schists of the The Makbal Formation is composed mainly of meta-quartzites Neldy Formation have been carried out. In this paper we reveal and pelitic schists with a minor amount of garnet-chloritoid- metamorphic P-T conditions and K-Ar ages of the Neldy pelitic talc schists, eclogites, mafic schists and marbles (Bakirov schists. We will compare the metamorphic conditions and the 1978; Bakirov et al. 1987, 1998; Tagiri et al. 2010). The garnet- ages of the Neldy pelitic schists, with the eclogites and the garnet- chloritoid-talc schists are concordantly intercalated in meta- chloritoid-talc schists in the Makbal and the Neldy Formations. quartzites. The protolith of the garnet-chloritoid-talc schists were The result suggests a new tectono-metamorphic framework of the formerly regarded as latteric clay (Bakirov et al. 2008) or pelitic Makbal Complex. rock (Tagiri et al. 2010). However, whole rock major and trace Mineral abbreviations used in the text, tables and figures follow element signatures of the garnet-chloritoid-talc schists suggest a Whitney and Evans (2010). metasomatized protolith of altered oceanic crust or volcaniclastic material from a magmatic arc (Meyer et al. 2014).

（ 2 ） Metamorphism and K-Ar ages of the Makbal Complex, Kyrgyzstan 49

Coesite and quartz pseudomorph after coesite have been found the pelitic schists is similar to that of the Neldy garnet-free pelitic as inclusion in garnets from the garnet-chloritoid-talc schists and schists. The Sharkyrak Group is subdivided into the structurally occasional eclogites and meta-quartzites (Tagiri and Bakirov 1990; lower Chymynsai Formation and the upper Kaindy Formation on Tagiri et al. 2010; Orozbaev et al. 2015). The peak metamorphic the basis of lithology. The Chymynsai Formation is composed of conditions of the garnet-chloritoid-talc schists have been estimated various types of marbles and interlayering meta-quartzites, and as 530–580 °C and 2.8–3.3 GPa (Tagiri et al. 2010; Meyer et al. the Kaindy Formation is composed mainly of pelitic schists with 2014; Orozbaev et al. 2015). The pelitic schist and the garnet- a small amount of meta-quartzites, marbles and amphibolites chloritoid-talc schist layers contain lenticular bodies of eclogites (Bakirov et al. 1987, 2017). Garnet amphibolites yielded the peak and garnet amphibolites. The estimated peak metamorphic metamorphic conditions of 620 °C at 1.4 GPa (Rojas-Agramonte conditions of the eclogites are 290–660 °C and 2.0–2.8 GPa (Tagiri et al. 2013). A Lu-Hf age of 470.1±2.5 Ma was suggested as et al. 2010; Meyer et al. 2013). garnet growth age (Rojas-Agramonte et al. 2013). The geochemical A SHRIMP U-Pb zircon age of the coesite-bearing garnet- features of the garnet amphibolites are of continental gabbroic chloritoid-talc schists from the Makbal Formation yielded 502 protolith (Rojas-Agramonte et al. 2013). ±10 Ma (Konopelko et al. 2012). Monazites occurring in the Several granitic bodies occur in the Makbal area (Fig. 2). matrix yield a CHIME age of 481±26 Ma, and those included Granitic body distributed in the southwestern part of the Makbal in porphyroblastic garnets yield the similar age of 480±56 Ma area yielded a SHRIMP U-Pb zircon age of 456±3 Ma (Rojas- (Togonbaeva et al. 2009). These two ages are well consistent Agramonte et al. 2013), and the similar K-Ar amphibole age of with each other, suggesting that the matrix monazite age of 463 Ma was reported (Apayarov 2007). These suggest Ordovician 481±26 Ma may represent the peak metamorphic age of the magmatic age of the granitic body. Tagiri et al. (2010) reported UHP metamorphism in the Makbal area. A Sm-Nd garnet age considerably younger K-Ar orthoclase age of 399±10 Ma and of the garnet-chloritoid-talc schists is 475±4 Ma. This age is they suggested cooling age in the Devonian (closure temperature interpreted as an average growth age of garnet during prograde- of orthoclase is 110–200 °C, Shibata et al. 1990; Kaneoka 1998). to-peak metamorphism (Meyer et al. 2014), but it is younger than This granitic body is geochronologically post-metamorphic and the suggested peak metamorphic age, 500–480 Ma. Tagiri et al. geologically it gives a contact metamorphism to the HP-UHP (2010) reported K-Ar phengite age of 509±13 Ma of the garnet- metamorphic rocks of the Makbal Complex (Figs. 2 and 3) (Bakirov chloritoid-talc schist and they regarded it as cooling age. However, et al. 1987). The roof plane of the granitic body is subhorizontal, this is a little bit older than, or similar to the peak metamorphic and, therefore, the apparent thermal effect is detected considerably age. far from the contact at the land surface. The rims of metamorphic zircon from the Makbal HP eclogite The granitic body in the southeastern part of the Makbal area, yield SHRIMP U-Pb ages of 509±7 Ma and 498±7 Ma, which named the Karajylga granite, is supposed to be Mesoproterozoic are regarded as the peak metamorphic age (Konopelko et al. (Konopelko et al. 2012; Rojas-Agramonte et al. 2013; Meyer et al. 2012). K-Ar paragonite age of 482±17 Ma of the HP eclogite is 2013) or Neoproterozoic in age (Bakirov et al. 1987). TIMS U-Pb interpreted as cooling age (Tagiri et al. 1995). zircon age of 1131±4 Ma (Degtyarev et al. 2011) and 1120 Ma Significantly older K-Ar ages of biotite (769±19 Ma) and and 1087 Ma (Apayarov 2007), and similar SHRIMP U-Pb zircon phengite (717±18 Ma) were reported for pelitic schists, while a age of 1102±7 Ma and 1094±8 Ma (Kröner et al. 2013), suggest K-Ar amphibole age of 881±22 Ma is obtained for a mafic schist Mesoproterozoic age of the granite. Apayarov (2007) reported two (winchite schist), suggesting allochthonous blocks in a stratified different younger K-Ar amphibole ages of 554 Ma and 477 Ma, tectonic melange (Tagiri et al. 2010). and an Rb-Sr age of 473 Ma that is similar to the latter of the K-Ar The Neldy Formation is composed mainly of pelitic schists with ages. Tagiri et al. (2010) reported K-Ar orthoclase age of 389±10 subordinate marbles and meta-quartzites, and lenses and layers of Ma, and it is similar to the K-Ar oligoclase age of granitic body in eclogites and amphibolites. The apparent metamorphic grade of the the southwestern Makbal area. Neldy Formation varies from low-grade garnet-free pelitic schists A deformed granitic body (biotite-amphibole granodiorite) in to HP eclogites. The metamorphic conditions of the eclogites are the northwestern part of the Makbal area shows SHRIMP U-Pb estimated as 550–610 °C and 2.2–2.5 GPa (Togonbaeva et al. zircon age of 514±5 Ma (Fig. 2), which coincides with the 2010b), and Sm-Nd age of 526±9.5 Ma is obtained as the peak peak metamorphic age of the HP-UHP metamorphic rocks of the metamorphic age (Togonbaeva et al. 2010a). Makbal Complex (Konopelko et al. 2012). Therefore, this granite The Sharkyrak Group is composed of low-grade metamorphic is a member of the protolith of the Makbal Complex. The rims of rocks such as garnet-free pelitic schists. The mineral assemblage of the zircon grains yield an age of 447±11 Ma (Konopelko et al.

（ 3 ） 50 Kasymbekov et al. 80

60 72°10'57'' 75 42°51'26'' U-Pb 1131±4 Ma U-Pb 1131±4 65 (Degtyarev et al. 2011) 35 45 K-Ar 477 Ma, 554 Ma and U-Pb 457 Ma (Apayarov, 2007) (Apayarov, Sm-Nd 526±9.5 Ma Rb-Sr 473 Ma (Apayarov, 2007) Rb-Sr 473 Ma (Apayarov, 50 (Togonbaeva et al. 2010a) (Togonbaeva 50 (Apayarov, 2007) (Apayarov, 75 U-Pb 1087 Ma, 1120Ma Lu-Hf 470.1±2.5 Ma K-Ar 482±13 Ma (Tagiri et al. 1995) (Tagiri K-Ar 389±8 Ma (Tagiri et al. 2010) (Tagiri ö (Rojas-Agramonte et al. 2013) Sm-Nd 475±4 Ma (Meyer et al. 2014) 80 55 45 50 SHRIMP U-Pb 1102±7 Ma, U-Pb 1102±7 SHRIMP 1094±8 Ma (Kr ner et al. 2013) 35 K-Ar 509±13 Ma (Tagiri et al. 2010) (Tagiri 502±10 Ma (Konopelko et al. 2012) SHRIMP U-Pb 509±7 Ma, 498±7 SHRIMP 50 K-Ar 463 Ma 60 KG 1244 80 (Apayarov, 2007) (Apayarov, CHIME 480±56 Ma KG 1251 40 (Togonbaeva et al. 2009) (Togonbaeva 45 60 K-Ar 399±10 Ma (Tagiri et al. 2010) (Tagiri SHRIMP U-Pb 456±3 Ma SHRIMP (Rojas-Agramonte et al. 2013) 60 35 SHRIMP U-Pb 514±5 Ma, SHRIMP 447±11 Ma (Konopelko et al. 2012) 447±11 60 60 70 2 km 0

（ 4 ） Metamorphism and K-Ar ages of the Makbal Complex, Kyrgyzstan 51

HP-UHP metamorphic rocks Geological map Geological map ). Black stars stars Black 1987 ). Granites Fig. 2 of the Makbal HP-UHP Complex (after Bakirov al. et indicate the sample locations pelitic garnet-free of the schist (KG 1251 ) and garnet chloritoid-bearing and pelitic schist (KG 1244 ).

Fig. 3 Photograph of the exposures of granitic rocks in the southwestern part of the Makbal area. The broken lineFig. indicates 3 Photograph the boundary of between the exposures granitic rocks of and granitic HP-UHP rocks metamorphic in the rocks.

southwestern part of the Makbal area. The broken line indicates Sample locations Eclogite Granitic rock Garnet amphibolite Pelitic schist (this study) Garnet-chloritoid-talc schist 2012), which is similar to the SHRIMP age of 456 ± 3 Ma (Rojas-Agramonte et al. 2013) for the the boundary between granitic rocks and HP-UHP metamorphic granitic body in the southwestern part of the Makbal area. The granitic body in the northeast part of the area has been suggested magmatic ages of the rocks. Devonian (Bakirov et al. 2017; Konopelko et al. 2012; Meyer et al. 2013; Klemd et al. 2015) or Ordovician (Bakirov et al. 1987). Apayarov (2007) reported TIMS U-Pb zircon age of 457 Ma similar to the age of Ordovician granitic body in the southwestern part of the Makbal area. This granitic body is geochronologically thought to be postmetamorphic, however, the thermal aureoles are not shown in the geologic map (Fig. 2; Bakirov et al. 1987). Amphibolites Neldy Formation Makbal Formation Akdjon Group

Kaindy Formation Chymynsai Formation

Sharkyrak Group Petrography Makbal Complex Makbal Two representative samples of pelitic schists, i.e. garnet-free pelitic schist (KG1251) and garnet and chloritoid-bearing pelitic schist (KG1244), were collected from the Neldy Formation. Garnet-free pelitic schist (KG1251) consists mainly of phengite, chlorite and quartz with small amounts of albite, titanite, rutile, calcite and carbonaceous matter (Fig. 4a, b). A schistosity is defined by preferred orientation of phengite and chlorite. Phengite occurs as subhedral to anhedral tabular crystal up to 0.3 mm across, and it shows compositional zoning with subhedral dark core and relatively bright rim in backscattered electron image (BEI) (Fig. 5a). The dark-core gradually changes to the bright rim. Schistosity-forming chlorite occurs as subhedral to anhedral tabular grain up to 0.3 mm across. Rutile is partially replaced by titanite.

Strike and dip Garnet and chloritoid-bearing pelitic schist (KG1244) consists mainly of white mica (muscovite, phengite), chlorite and quartz, with minor amounts of garnet and chloritoid (Fig. 4c). Albite, tourmaline, titanite, rutile, zircon, monazite, calcite and carbonaceous matter occur as accessory

LEGEND minerals. A schistosity is defined by preferred orientation of white mica, chloritoid and chlorite. Garnet occurs as subhedral porphyroblasts up to 3 mm across and it is commonly associated with Faults Ordovician terrigenous sediments Quaternary sediments Granitoids Lower Paleozoic ophiolite complex Thermal aureole by granite intrusion Geological map of Complex the (after Makbal Bakirov HP-UHP 1987). Black stars indicate the sample locations of the garnet-free pressure shadows consisting mainly of quartz with small amounts of phengite and chlorite. The internal schistosity of garnet is sigmoidal in shape and it is continuous to the external schistosity Fig. 2 pelitic schist (KG1251) and garnet and chloritoid-bearing pelitic schist (KG1244). schist pelitic chloritoid-bearing and garnet and (KG1251) schist pelitic in the matrix (Fig. 4c). The porphyroblastic garnets contain inclusions of quartz, chloritoid, tourmaline, titanite, calcite and rutile. Most of the garnet grains are intensively fractured and the fractures are filled by muscovite, chlorite and quartz. The schistosity-forming white micas

（ 5 ） 52 Kasymbekov et al.

Fig. 4 Photomicrographs showing texture and mode of occurrence of minerals in the garnet-free pelitic schist (KG1251) (a-b) and the garnet and chloritoid-bearing pelitic schist (KG1244) (c-d). (a) Schistosity- forming phengite and chlorite in the garnet-free pelitic schist (Open nicol) (KG1251). (b) Cross nicol photograph of the figure 4a. (c) Photomicrograph showing porphyroblastic garnet, schistosity-forming white mica and chlorite, and matrix minerals of chloritoid, tourmaline and quartz. Garnet contains inclusion trails of chloritoid, chlorite, tourmaline, carbonaceous matter and quartz. Chlorite and white mica occur along the fractures of the garnets, and inclusions in the garnets indicate sigmoidal shape. Compositional profile along the line A–B is shown in Fig. 7a. Chlorite replaces garnet along the rim. (d) Chloritoid inclusion in the porphyroblastic garnet (BEI).

（ 6 ） Metamorphism and K-Ar ages of the Makbal Complex, Kyrgyzstan 53

Fig. 5 Backscattered electron images (BEI) showing texture and mode of occurrence of minerals in the garnet-free pelitic schist (KG1251) (a-b) and the garnet and chloritoid-bearing pelitic schist (KG1244) (c-g). (a) Schistosity-forming white mica showing high-Si phengite core and low-Si phengite rim (KG1251). (b) Separated white mica grain showing a slight zoning (KG1251). (c) Schistosity-forming white mica showing a zoning from bright core (phengite) and overgrown dark rim (muscovite) (KG1244). (d) Schistosity-forming white mica showing a zoning from phengite core to muscovite rim (KG1244). (e)

Separated white mica grains showing zoning from phengite core to muscovite rim (KG1244). (f) Chlorite in the matrix shows a zoning with higher-XMg in the core and lower-XMg in the rim. (g) Light gray chlorite in the garnet fracture is lower in XMg than dark gray chlorite.

（ 7 ） 54 Kasymbekov et al. occur as subhedral tabular crystals up to 1 mm across with a 8530F) installed at the Department of Earth Science, Shimane compositional zoning, brighter resorbed core (phengite), and University. The analytical conditions used for quantitative analysis overgrown darker rim (muscovite) (BEI in Fig. 5c, d). Schistosity- were 15 kV accelerating voltage, 20 nA specimen current and 5 μm forming chlorite occurs as subhedral to anhedral grains up to 0.3 beam diameter. Corrections were carried out using the procedure mm across. It shows a zoning with dark core to bright rim, defined of Bence and Albee (1968). End-member components of garnet by BEI image (Fig. 5f). Two types of chlorite occur in the fracture were calculated on the basis of four components; pyrope (XPrp), of the garnet. Light gray chlorite veins are cross-cut by dark gray almandine (XAlm), spessartine (XSps) and grossular (XGrs). XPyr = Mg 2+ 2+ 2+ chlorite veins (Fig. 5g). Schistosity-forming chloritoid occurs as /(Mg + Fe + Mn + Ca); XAlm = Fe /(Mg + Fe + Mn + Ca); XSps 2+ 2+ subhedral crystals up to 0.2 mm across. Chloritoid inclusion up to = Mn /(Mg + Fe + Mn + Ca); XGrs = Ca/(Mg + Fe + Mn + Ca) 0.04 mm in diameter occurs in the porphyroblastic garnet (Fig. 4d). (Deer et al. 1992). Ferric iron contents in garnet were estimated Albite occurs as subhedral to anhedral grain up to 0.3 mm across, using charge balance Fe3+ = 8 – 2Si – 2Ti – Al (O = 12). Fe3+ and it shows zoning with dark core to bright rim defined by BEI contents in phengite, chlorite and chloritoid are calculated using the image. Rutile in the matrix is partly replaced by titanite. THERMOCAL AX program (Holland and Powell 1998), and the

Mineral chemistry 0.20 Chemical compositions of the minerals were examined using a) White micas in Garnet and chloritoid-bearing thin sections pelitic schist electron probe microanalyzers (JEOL JXA-8800M and JXA- KG 1244 Schistosity-forming Core 0.15 Rim Table 1 Representative chemical compositions of phengite and chlorite In garnet fracture from the garnet-free pelitic schist (KG1251). Garnet-free pelitic schist KG 1251 Schistosity-forming 0.10 8 9 Na/(Na+K) 7 5 10 7 Rim 11 6 6 1-5 0.05 4 3 Core 1 2 0.00 6.0 6.2 6.4 6.6 6.8 7.0 Si 0.15 b) Separated grains Garnet and chloritoid-bearing pelitic schist KG 1244 Core Rim 0.10 Garnet-free pelitic schist KG 1251

Na/(Na+K) Rim 8 6 5 7 65 0.05 4 1-3

Core 4 1 3 2 0.00 6.0 6.2 6.4 6.6 6.8 7.0 Si Fig. 6 Chemical compositions of zoned white micas. (a) White micas in Fig. 6 Chemical compositions of zoned white micas. (a) White thin section. The analytical points are shown in figures 5a and d. (b) White mica grains separated from the rock samples. The analytical points are shownmicas in Figure in the 5 b, thin e. Arrows section. indicate The analytical zoning trends points of are the shownwhite mica in grains. figures 5a and d. (b) White mica grains separated from the （ 8 ） samples. The analytical points are shown in figures 5b and e.

Arrows indicate zoning trends of the white mica grains. Metamorphism and K-Ar ages of the Makbal Complex, Kyrgyzstan 55

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Fig. )LJ8 Compositional&RPSRVLWLRQDOSURILOHRIZKLWHPLFD D VFKLVWRVLW\IRUPLQJ profile of white mica, (a) schistosity-forming white 3US $OP mica from the garnet-free pelitic schist (KG1251) and (b) schistosity- FRUH formingZKLWH white PLFD mica IURP from WKH JDUQHWIUHHthe garnet and SHOLWLF chloritoid-bearing VFKLVW .* pelitic DQG schist E (KG1244). VFKLVWRVLW\IRUPLQJ ZKLWH PLFD IURP WKH JDUQHW DQG FKORULWRLG

EHDULQJSHOLWLFVFKLVW .* 3US ULP $OP Garnet-free pelitic schist (KG1251) Schistosity-forming white mica in garnet-free pelitic schist Fig.)LJ 7 (a) D &RPSRVLWLRQDOSURILOHRIJDUQHWLQWKHJDUQHWDQGFKORULWRLG Compositional profile of garnet in the garnet and chloritoid- 2+ bearing pelitic schist (KG1244). (b) Ternary diagrams showing the (KG1251) are compositionally zoned, Si (6.84–6.40 pfu), Fe 2+ chemicalEHDULQJSHOLWLFVFKLVW .* E 7HUQDU\GLDJUDPVKRZLQJWKH compositions of the garnet in the garnet and chloritoid-bearing + Mg (0.88–0.58 pfu) and XMg (Mg/(Fe + Mg) (0.86–0.80) pelitic schist (KG1244). Arrows indicate compositional zoning from core continuously decreasing and XNa (Na/(Na + K) (0.04–0.08) to FKHPLFDOFRPSRVLWLRQVRIWKHJDUQHWLQWKHJDUQHWDQGFKORULWRLGrim. increasing from core to rim (Fig. 6a). A representative white mica 2+ EHDULQJ SHOLWLF VFKLVW .* $UURZV LQGLFDWH FRPSRVLWLRQDO grain shows continuously decreasing Si (6.70–6.45 pfu), Fe + Mg 3+ Fe contents of these minerals are all negligible. Representative (0.81–0.68 pfu) and XMg (0.86–0.81), and increasing XNa (0.06–0.08) ]RQLQJIURPFRUHWRULP chemical compositions of the constituent minerals in the garnet- from core to rim (Figs. 5a, 6a and 8a; Table 1). Chlorite in the free pelitic schist (KG1251) and the garnet and chloritoid-bearing matrix has high XMg (0.79–0.81). Plagioclase in the matrix is albite pelitic schist (KG1244) are presented in Tables 1 and 2. (An 5) in composition.

（ 9 ） 56 Kasymbekov et al.

0.5 Ab=Jd+Qz (Holland 1983) Isopleth of Si in Ph (Massonne and Schreyer 1987) core KD (Garnet-Chlorite) J2 J1 3J 4J 2.5 KD (Garnet-Phengite) 0.4 J 7 rim 5 J6 The estimated P-T conditions for garnet-free J8 pelitic schist 11 8 Grt-Chl (Grambling 1990) 9 0.3 6 7 Grt-Ph (Green and Hellman 1982) 10 P-T conditions for garnet and chloritoid-bearing 2+ 2.0 pelitic schist

KD 0.11 KD 0.13 Mg/(Fe0.2 +Mg) Grt-in Mg

X (XMg=0.8)

K 5 3 D

4 1 K 21 2 1.5 0.1 Si 6.9 KG 1244 Si 6.8 In matrix In garnet fracture high-X Jd+Qz Pressure (GPa) Pressure EC In garnet fracture low-X Mg KG 1251 Ab 0 Mg (1) 2.5 2.6 2.7 1.0 KG 1244 Si Fig. 9 Chemical compositions of chlorites. Fig. 9 Chemical compositions of chlorites. Analytical points are shown in Figure 5f, g. EA Analytical points are shown in figures 5f and g. GL

O 2 AM 0.5

0.36)

Garnet and chloritoid-bearing pelitic schist (KG1244) ? Mg Si 6.2 Porphyroblastic garnets in garnet and chloritoid-bearing GS GR

Cld+Bt+Qz+H pelitic schist (KG1244) have almandine-rich compositions (XAlm Ms+Chl((2)X 0.68–0.77) with variable amounts of pyrope, spessartine and 0 grossular components (Table 2). The garnet shows a growth zoning 300 400 500 600 700 with decreasing in spessartine (XSps 0.12–0.02) and increasing in Temperature (°C) Fig. 10 Estimated P-T conditions for the garnet free-pelitic schist pyrope (XPyr 0.04–0.08), and slightly decreasing and subsequently Fig. 10 Estimated P-T conditions for the garnet free-pelitic schist (KG1251) and garnet and chloritoid-bearing pelitic schist (KG1244). increasing in almandine from core to rim. Grossular shows slightly Metamorphic facies boundaries are after Takasu (1989). EC, eclogite increasing (XGrs 0.08–0.19) and decreasing (XGrs 0.19–0.16) from facies;(KG1251) GL, glaucophane and garnet schist and facies; chloritoid-bearing EA, epidote-amphibolite pelitic schistfacies; core to rim with slight fluctuations (Fig. 7; Table 2). Schistosity- AM, amphibolite facies; GS, greenschist facies; GR, granulite facies. KD isopleths(KG1244). for Grt-Ph Metamorphic and Grt-Chl facies geothermometers boundaries are after are fromTakasu Green (1989). and forming white mica consists of resorbed phengite core and Hellman (1982) and Grambling (1990), respectively. overgrowing muscovite rim (Figs. 5c and 5d). The chemical EC, eclogite facies; GL, glaucophane schist facies; EA, epidote- compositions of the white mica vary from core to rim (Si = 2+ 6.94–6.10 pfu, XNa = 0.01–0.06, Fe + Mg = 1.13–0.30 pfu, XMg formingamphibolite white facies; micas AM,(Fig. amphibolite 6a; Table 2 facies;). Chloritoid GS, greenschist inclusion facies; in the

0.64–0.33) (Fig. 6a). The core of a representative white mica grain porphyroblastic garnet is similar XMg (0.12–0.17) with schistosity- 2+ GR, granulite facies. KD isopleths for Grt-Ph and Grt-Chl shows decreasing in Si (6.86–6.74 pfu), Fe + Mg (1.08–0.96 pfu) forming chloritoid (XMg 0.12–0.18). Chlorite in the matrix shows and X (0.63–0.60), and increasing in X (0.02–0.03) towards the a zoning with higher X (0.41–0.46) in the core and lower X Mg Na geothermometers areMg from Green and Hellman (1982) andMg rim (Figs. 6a and 8b; Table 2). The rim of the white mica shows (0.36–0.40) in the rim (Fig. 9). Light gray (BEI) chlorite in the 2+ decreasing in Si (6.18–6.10 pfu), Fe + Mg (0.40–0.31 pfu) and garnetGrambling fracture (1990), shows respectively. lower XMg (0.08–0.18) than dark gray chlorite

XMg (0.40–0.33) to the outermost rim. At the rim, XNa fluctuates (XMg 0.28–0.36) (Fig. 9). Albite shows a compositional zoning with between 0.06 and 0.09. There is a large compositional gap between increasing anorthite content from core to rim (An 1-5). the core and the rim, and it is represented by Si (6.74–6.18 pfu) and X (0.03–0.07) (Fig. 6a). Muscovite occurring in the fractures Na Mineral paragenesis and metamorphic conditions of the porphyroblastic garnet shows Si (6.16–6.27 pfu), Fe2+ +

Mg (0.23–0.47) pfu), XNa (0.07–0.09) and XMg (0.24–0.33) similar Garnet-free pelitic schist (KG1251) to the compositions of the muscovite rim of the schistosity- The peak metamorphic conditions of garnet-free pelitic schist

（ 10 ） Metamorphism and K-Ar ages of the Makbal Complex, Kyrgyzstan 57

Table 2 Representative chemical compositions of white mica (phengite/muscovite), chloritoid, chlorite and garnet from the garnet and chloritoid-bearing pelitic schist (KG 1244).

(KG1251) are defined by the schistosity-forming minerals of minimum pressure obtained by maximum Si content in phengite phengite (max. Si 6.84 pfu), chlorite (XMg = 0.79–0.81), albite (Si 6.84; Massonne and Schreyer 1987; Fig. 10). The absence of (An <5), rutile and quartz. The absence of garnet and the presence jadeite and the presence of albite represent the maximum pressure, of chlorite suggest that the peak metamorphic temperature did not and only a rough constraint of the peak metamorphism, T < 630 °C; cross over the garnet-in reaction line (1), P = 0.9–1.7 GPa, is obtained (Fig. 10).

chlorite + quartz = garnet + H2O ------(1) Garnet and chloritoid-bearing pelitic schist (KG1244) Based on the texture and chemical compositions of the Using activity models of garnet (Holland and Powell 1998) and constituent minerals two metamorphic events, i.e. first and second chlorite (Holland et al. 1998) garnet-in line of the continuous metamorphic events, are distinguished in the garnet and chloritoid- reaction (1) for rock consisting of Mg-rich chlorite (XMg = 0.8) bearing pelitic schists (KG1244). and quartz constraints the maximum temperature of 630 °C at the The first metamorphic event is further divided into the prograde

（ 11 ） 58 Kasymbekov et al.

First metamorphic event Second metamorphic event

Prograde to peak stage Retrograde stage

X 0.04-0.08, X 0.12-0.02 Garnet Pyr Sps

Phengite Ph (Si 6.94-6.74) Ms (Si 6.18-6.10; Muscovite ? X Na 0.06-0.09) XMg(0.36-0.40) X (0.41-0.46) X (0.08-0.18) X (0.28-0.36) Chlorite Mg Mg Mg An (1-5) Albite ? X (0.12-0.18) Chloritoid Mg Quartz ?

Rutile ? Titanite ? ?

Fig. 11 Fig. Mineral 11 Mineral paragenis paragenis of the garnet of the and garnet chloritoid-bearing and chloritoid-bearing pelitic schists pelitic (KG schists1244). (KG1244).Solid lines indicateSolid the presence as major constituent and broken lines indicate minor or sporadic occurrence. lines indicate the presence as major constituent and broken lines indicate minor or sporadic

Table 3 occurrence. Representative estimated temperatures at 1.3 GPa for the garnet and chloritoid-bearing pelitic schist (KG1244).

（ 12 ） Metamorphism and K-Ar ages of the Makbal Complex, Kyrgyzstan 59 to peak metamorphic stage and the retrograde stage (Fig. 11). The suggest the metamorphic temperature for this event is lower than prograde to peak metamorphic stage is defined by garnet (XPrp the following temperature-sensitive reaction (2) (Fig. 10),

0.04–0.08, XSps 0.12–0.02), minerals included in the garnet such as chloritoid (XMg = 0.12–0.17), tourmaline, calcite, rutile, ilmenite muscovite + chlorite = chloritoid + biotite + quartz + H2O ----- (2) and quartz, and the matrix minerals of phengite (Si 6.94–6.74), chlorite core (XMg = 0.41–0.46), chloritoid (XMg = 0.12–0.18) and The location of the reaction (2) is calculated for KFMASH albite (An 1–5). system using THERMOCALC ver. 3.33 with an updated version The peak metamorphic conditions of the first metamorphic event of the internally consistent thermodynamic dataset of Holland and were estimated by the rim of garnet and the core of schistosity- Powell (1998). The AX2 program is used to calculate the mineral 2+ forming chlorites. The rim of the chlorite is slightly rich in Fe activity, and the activity of H2O is assumed to be unity. We have compared with the core, and it is supposed to be developed at the used the highest XMg of chlorite (0.36) and the highest Si contents timing of the garnets beginning to decompose just after the peak in muscovite (6.18 pfu) for the calculation of reaction (2). This metamorphism. Therefore, it is considered non-equilibrium with reaction represents the maximum temperature limit of the second (Mg/Fe)grt —————— the rim of the garnet. The KD [ (Mg/Fe)chl ] values between the rim of metamorphic event. The Si content of muscovite (max. Si 6.18 garnet and the core of chlorite range between 0.11 and 0.13. The pfu) without K-feldspar and biotite indicates a minimum pressure maximum Si content of schistosity-forming phengite (Si 6.94) (Massonne and Schreyer 1987), and the intersection with the indicates a minimum pressure, because biotite and K-feldspar reaction (2) represent maximum temperature of ca. 500 °C and the are not identified in the sample (Massonne and Schreyer 1987). minimum pressure of ca. 0.3 GPa (Fig. 10). The absence of jadeite and the presence of albite indicate the In the absence of paragonite, the maximum solubility of 2+ maximum pressure (Holland 1983). The garnet-chlorite Fe -Mg paragonite component into muscovite constrains the minimum exchange geothermometer (Grambling 1990) and the temperature by the muscovite-paragonite solvus. The maximum geobarometers describe above suggest the peak metamorphic XNa of muscovite in the second metamorphic event is 0.09 conditions of 485–545 °C and 1.2–1.5 GPa (Table 3; Fig. 10). indicating a minimum temperature of ca. 500 °C using the solvus According to the petrographic description the rim of the garnet configuration of Parra et al. (2002). Thus, the reaction (2) and coexists with phengite of the core of schistosity-forming white the muscovite-paragonite solvus represent the peak metamorphic (Fe/Mg)grt —————— mica. KD [ (Fe/Mg)ph ] values of garnet and phengite pairs range conditions of this metamorphic event as ca. 500 °C and minimum 2+ between 17 and 21. The garnet-phengite Fe -Mg exchange pressure of ca. 0.3 GPa. geothermometer (Green and Hellman 1982) and the geobarometers described above give 505–540 °C and 1.2–1.5 GPa (Table 3; Fig. K-Ar ages of the pelitic schists from the Neldy 10). Phengite core of the white mica experienced resorption after Formation the peak metamorphic stage (Figs. 5c, d, 6 and 8), and the chemical compositions representing the peak stage were possibly lost or Chemical compositions of separated white micas and obtained modified during retrograde metamorphism. Therefore garnet- K-Ar ages phengite thermometry does not exactly represent the peak White micas were separated from the garnet-free pelitic metamorphic temperatures. schist (KG1251) and the garnet and chloritoid-bearing pelitic The retrograde stage of the first metamorphic event is defined schist (KG1244) for K-Ar age dating. Chemical compositions of by the minerals occurring in the fracture of the porphyroblastic separated white mica grains and K-Ar ages obtained are shown in garnets, i.e. chlorite with lower XMg (0.08–0.18) and quartz. The Tables 4 and 5. rim of the schistosity-forming chlorite (XMg 0.40–0.36) is likely to The chemical compositions of the separated white mica grains be developed at the initial stage of the retrograde metamorphism. were verified by EPMA analysis. The white micas in the garnet- The second metamorphic event is defined by muscovite (Si free pelitic schist (KG1251) are of phengite in composition and Si 2+ 6.18–6.10 pfu, XNa 0.06–0.09), overgrowing on the resorbed peak contents range 6.68–6.48 pfu (XNa 0.05–0.08, Fe + Mg 0.76–0.60 stage phengite. Chlorite with higher XMg (0.28–0.36), muscovite (Si pfu, XMg 0.82–0.79) (Figs. 5b and 6b). A phengite core of the

6.16–6.27 pfu, XNa 0.07–0.09) and quartz occurring in the fracture white mica from the garnet and chloritoid-bearing pelitic schist 2+ of the porphyroblastic garnet also represent the minerals stable (KG1244) has Si ranging 6.89–6.84 pfu (XNa 0.01–0.02, Fe + Mg during the second metamorphic event. 1.07–1.02 pfu, XMg 0.61–0.62), and a muscovite rim has Si ranging 2+ The absence of biotite and chloritoid, and the presence of 6.26–6.23 pfu (XNa 0.08–0.07, Fe + Mg 0.45–0.35 pfu, XMg muscovite and chlorite during the second metamorphic event 0.37–0.39) (Figs. 5c and 6b; Table 4). The compositions of white

（ 13 ） 60 Kasymbekov et al.

Table 4 Representative chemical compositions of separated white micas from garnet-free pelitic schist (KG1251) and garnet and chloritoid- bearing pelitic schist (KG1244).

Table 5 (Kasymbekov et al. ) K-Ar analytical data of white mica for garnet-free pelitic Table 5 K-Ar analyticalschists data (KG1251) of white micas and ofgarnet garnet-free and chloritoid-bearing pelitic schist (KG1251 pelitic) and schi garnetsts and(KG1244) chloritoid-bearing pelitic schist (KG1244).

40 Material Isotophic Rad. Ar %Rad. 40 Ar %K Sample No. analyzed Age (Ma) (scc/gx10-5 )

15.6 97.5 6.59 KG 1251 Phengite 524+13 15.5 98.7 6.58

Phengite/ 17.7 98.5 8.43 KG 1244 Muscovite 474+12 17.8 97.8 8.43

mica grains separated from the samples are highly consistent with (muscovite) ages are generally interpreted to date last cooling the white micas in thin sections, and hence are representative of the through the closure temperature of argon system in the grain (ca. mineral population in the garnet-free pelitic schist and garnet and 350–430 °C; Purdy and Jäger 1976; McDougall and Harrison chloritoid-bearing pelitic schist for the K-Ar dating (Fig. 6). 1988; Blanckenburg et al. 1989; Kirschner et al. 1996). K-Ar ages of white micas were determined by Geospace Science Co., Ltd. The white mica concentrates separated from the Interpretation of the K-Ar ages garnet-free pelitic schist (KG1251) and the garnet and chloritoid- The peak metamorphic temperature of the garnet-free pelitic bearing pelitic schist (KG1244) yielded ages of 524 ± 13 Ma schists (KG1251) is loosely constrained as <630 °C, at pressure and 474 ± 12 Ma, respectively (Table 5). The K-Ar white mica of 0.9–1.7 GPa, and it is similar to or higher than the closure

（ 14 ） Metamorphism and K-Ar ages of the Makbal Complex, Kyrgyzstan 61 temperature of muscovite in the K-Ar system (350–430 °C; Purdy conditions of the former are estimated as 485–545 °C at 1.2–1.5 and Jäger 1976). Therefore, the K-Ar age of 524 ± 13 Ma obtained GPa (Fig. 10). This metamorphic event is likely to be coincided by the present study represents approximate peak metamorphic age with the HP to UHP metamorphism of the eclogites and the or cooling age. garnet-chloritoid-talc schists in the Akdzhon Group at ca. 500 Ma. The white mica from the garnet and chloritoid-bearing pelitic The low-P/T metamorphic event after the ca. 500 Ma regional schists (KG1244) yielded a K-Ar age of 474±12 Ma. This age metamorphism is evidenced by the overgrown of muscovite on is obviously younger than that of garnet-free pelitic schist. The phengite (Fig. 5c,d), and the metamorphic conditions are ca. garnet and chloritoid-bearing pelitic schist experienced two distinct 500 °C and > 0.3 GPa (Fig. 10). This kind of low-P/T regional metamorphic events, high-P/T metamorphism (T = 485–545 °C, metamorphism has never found in the Makbal Complex excepting P = 1.2–1.5 GPa) and subsequent low-P/T metamorphism (T = ca. for the contact metamorphism caused by the Devonian granitic 500 °C, P > 0.3 GPa). The estimated temperature of the low-P/ intrusions (Bakirov et al. 1987; Konopelko et al. 2012). Therefore, T metamorphic event is considerably higher than the closure temperature of white mica. Most of the argon is expected to be released from the white micas during the second metamorphic GCT schist (Ta 2010) event. Therefore, 474±12 Ma for the garnet and chloritoid-bearing pelitic schist is likely to be cooling age after the temperature peak M Ecl (Ta 2010) of the second metamorphic event. N Ecl Coe (To 2010a; b) 2.5 Qz Discussion GCT schist (Me 2014) M Ecl (Me 2013) The peak metamorphic conditions of the apparently low-grade M Ecl (Ta 2010) garnet-free pelitic schist (KG1251) in the Neldy Formation are roughly constrained as T <630 °C and P = 0.9–1.7 GPa. The 2.0 M Ecl (Ko 2012) estimated P-T range wholly includes the P-T conditions of the apparently high-grade garnet and chloritoid-bearing pelitic schist

1244 10 (KG ) (Fig. ). These imply that the garnet-free pelitic schist K Grt amp KG 1244 is not necessarily lower in metamorphic grade than the garnet 1.5 (RA 2013) and chloritoid-bearing pelitic schist. XMg of chlorite in KG1251 is considerably high (0.79-0.81) indicating highly Mg-rich bulk rock (GPa) Pressure Jd+Qz EC chemical compositions. This kind of Mg-rich compositions easily KG 1251 Ab prevents the occurrence of garnet and chloritoid which prefer Fe- rich bulk rock compositions. This is probably reason why garnet 1.0 does not appear even if the possible metamorphic temperature is similar or higher than the garnet and chloritoid-bearing pelitic EA schist (KG1244). GL The determined K-Ar age of 524±13 Ma for the garnet-free 0.5 AM pelitic schist represents the peak metamorphic age or cooling GR ? age after the peak conditions. This age is similar to the peak KG 1244 Contact metamorphism metamorphic age of ca. 500 Ma for the HP and UHP metamorphic GS rocks of the eclogites and the garnet-chloritoid-talc schists in the Makbal and the Neldy Formations (Togonbaeva et al. 2009; 0 2010b; Konopelko et al. 2012). The metamorphic ages of ca. 500 300 400 500 600 700 Ma probably represent the age of HP and UHP metamorphism Temperature (°C) throughout the Makbal and the Neldy Formations in the Akdjon Fig. 12 Estimated P-T conditions for the garnet free-pelitic schist (KGFig.1251 12) Estimated and the garnet P-T andconditions chloritoid-bearing for the garnet pelitic free-pelitic schist (KG schist1244). Group. M Ecl, Makbal eclogite; GCT schist, Garnet-chloritoid-talc schist; N Ecl, The garnet and chloritoid-bearing pelitic schist (KG1244) Neldy(KG1251) eclogite; and K Grt the amp, garnet Kaindy and garnet chloritoid-bearing amphibolite. Ta pelitic2010, Tagiri schist et 2010 2012 2012 2013 2013 experienced two metamorphic events, i.e. high-P/T metamorphism al. ( ); Ko , Konopelko et al. ( ); Me , Meyer et al. ( ); Me 2014, Meyer et al. (2014); To 2010a; b, Togonbaeva et al. (2010a; b); (KG1244). M Ecl, Makbal eclogite; GCT schist, Garnet- and subsequent low-P/T metamorphism. The peak metamorphic RA 2013, Rojas-Agramonte et al. (2013).

chloritoid-talc schist; N Ecl, Neldy eclogite; K Grt amp, Kaindy （ 15 ）

garnet amphibolite. Ta 2010, Tagiri et al. (2010); Ko 2012,

Konopelko et al. ( 2012); Me 2013, Meyer et al. (2013); Me 2014,

Meyer et al. (2014); To 2010a; b, Togonbaeva et al. (2010a; b); RA

2013, Rojas-Agramonte et al. (2013). 62 Kasymbekov et al. muscovite overgrown on phengite in the garnet and chloritoid- the shallower crustal levels (ca. 10 km in depth) at least before the bearing pelitic schists (KG1244) is probably developed during granitic magma intrusions at ca. 460 Ma. reheating due to the granitic magma intrusions. The K-Ar amphibole ages of 463 Ma and TIMS U-Pb zircon age of 457 Ma Conclusions (Apayarov 2007), and SHRIMP zircon age of 456±3 Ma (Rojas- Agramonte et al. 2013) are reported from the granitic bodies. Petrological and geochronological studies for the Neldy pelitic These ages are similar to the K-Ar white mica age of 474± schists in the Makbal Complex, Northern Kyrgyz Tien-Shan are 12 Ma for garnet and chloritoid-bearing pelitic schists. The heat carried out and the following results are obtained: supply from the granitic bodies at ca. 460 Ma caused almost all or 1. The peak metamorphic conditions of the pelitic schsits in the considerable amounts of argon to be released from the white micas, Neldy Formation are constrained as T <630 °C and P = 0.9–1.7 and, therefore, K-Ar age of 474±12 Ma is attained for the age of GPa (garnet-free pelitic schist), and T = 485–545 °C and P = the contact metamorphism. The rim of the zircon grains from the 1.2–1.5 GPa (garnet and chloritoid-bearing pelitic schist). deformed granitic body in the northwest part of the Makbal area 2. The peak to cooling age of the garnet-free pelitic schist is 524 shows an age of 447±11 Ma, and it is overgrown on the core of ± 13 Ma, and it is similar to the previously reported peak 514±5 Ma (Konopelko et al. 2012). The rim of the zircon also metamorphic ages of high grade metamorphic rocks (eclogites grew during the contact metamorphism of ca. 460 Ma granitic and garnet-chloritoid-talc shists) in the Makbal and the Neldy intrusions. Formations. The peak metamorphic conditions of garnet-free pelitic schist 3. The peak metamorphic conditions of the Neldy pelitic schists and garnet and chloritoid-bearing pelitic schist in the Neldy are considerably lower in pressure compared with the previously Formation are <630 °C at 0.9–1.7 GPa, and 485–545 °C at reported eclogites and garnet-chloritoid-talc schists, which are 1.2–1.5 GPa, respectively. The peak metamorphic conditions of located in the lower tectono-structural levels of the Makbal garnet amphibolite of the Kaindy Formation in the Sharkyrak Complex. Group (Fig. 2) have been reported as 620 °C and 1.4 GPa (Rojas- 4. Some metamorphic rocks of the Makbal Complex suffered Agramonte et al. 2013) (Fig. 12). These comprise a group of contact metamorphism by the Ordovisian granitic intrusions at relatively low-pressure metamorphic rocks (Fig. 12). In contrast, ca. 460 Ma. the metamorphic conditions of the HP and UHP metamorphic rocks such as the eclogites and the garnet-chloritoid-talc schists in Acknowledgements: We thank A. Togonbaeva, A. A. Bakirov the Akdzhon Group were estimated to be 530–580 °C and 2.8–3.3 and M. Satybaev for their help during our field survey, and F.H. GPa (garnet-chloritoid-talc schist; Tagiri et al. 2010; Meyer et Apayarov of the Institute of Geology, NAS KR for providing us al. 2014; Orozbaev et al. 2015), ~510 °C and 2.8 GPa (coesite- the data on geochronology of the granitic bodies. We also would bearing eclogite; Tagiri et al. 2010) and 520–610 °C and 2.2–2.5 like to thank M. Tagiri of Ibaraki University and A. Kamei, M. GPa (eclogite; Togonbaeva et al. 2010b; Meyer et al. 2013). These Akasaka, Y. Sampei, A. Auer, B. Roser and the other members of suggest that the peak metamorphic conditions of the Neldy pelitic the Geoscience Seminar and the Metamorphic Geology seminar schists are significantly lower in pressure compared with the of Shimane University for their discussion and useful suggestion. previously reported eclogites and garnet-chloritoid-talc schists, The manuscript was greatly improved by constructive reviews which are located in the lower tectono-structural levels of the by Y. Kouketsu and an anonymous reviewer, and the editorial Makbal Complex, i.e. lowermost parts of the Neldy Formation comments by K. Takeuchi are highly appreciated. This study was and the Makbal Formation. These results clearly indicate that the partly supported by JSPS KAKENHI Grant (No. JP24340123, conventional division of the Akdjon Group, i.e. the Makbal and the JP16K05577 to AT; No. JP17204047, JP21109004, JP22244067, Neldy Formations, has no longer geotectonic significance. JP25257208 to TH; JP12F02016 to RO). After suffered the peak metamorphism, whole Makbal and the Neldy high-grade metamorphic rocks, such as the garnet-chloritoid- References talc schists and the eclogites, were exhumed isothermally (Fig. 12; Togonbaeva et al. 2010b; Meyer et al. 2013; 2014; Orozbaev et Apayarov FH (Editor) (2007) Geological research of 1:200000 scale al. 2015), suggesting rapid exhumation toward the crustal levels. on the near-state boundary territory at the western part of the Kyrgyz Ridge within K-42-XII, To-42-XVIII, To-43-VII, To-43-VIII, To-43- Relatively low-pressure metamorphic rocks of the Neldy pelitic XIII, To-43-XIV map sheets*. Archives of the State Geological Agency schists were likely to be exhumed synchronously. The HP and of the Kyrgyz Republic. Ivanovka, 2007 p. UHP metamorphic rocks of the Makbal Complex were uplifted to Bakirov AB (1978) Tectonic Position of the Tien-Shan Metamorphic

（ 16 ） Metamorphism and K-Ar ages of the Makbal Complex, Kyrgyzstan 63

Complexes*. Ilim, Frunze, 262 p. Gondwana Research, 28: 1-25. Bakirov AB, Kotov VV and Kotova LS (1987) Stratigraphy of Konopelko D, Kullerud K, Apayarov F, Sakiev K, Baruleva O, Ravna metamorphic formation of the western part of the Kyrgyz ridge*. In: E and Lepekhina E (2012) SHRIMP zircon chronology of HP-UHP Korolev V (Ed), Caledonides of the Tien-Shan. Ilim, Frunze, 8-21. rocks of the Makbal metamorphic complex in the Northern Tien Shan, Bakirov AB, Tagiri M and Sakiev K (1998) Rocks of ultrahigh- Kyrgyzstan. Gondwana Research, 22: 300-309. pressure metamorphic facies in the Tien Shan. Russian Geology and Kröner A, Alexeiev DV, Rojas-Agramonte (2013) Mesoproterozoic Geophysics, 39: 1709-1721. (Grenville-age) terranes in the Kyrgyz North Tienshan: Zircon ages Bakirov AB and Maksumova RA (2001) Geodynamic evolution of the and Nd–Hf isotopic constraints on the origin and evolution of basement Tien-Shan lithosphere. Russian Geology and Geophysics, 42: 1359– blocks in the southern Central Asian Orogen. Gondwana Research, 23: 1366. 272-295. Bakirov AB, Sakiev KS, Tagiri M and Takasu A (2008) The intrusive Massonne HJ and Schreyer W (1987) Phengite geobarometry based nature of quartzites of the Makbal Complex, Tien-Shan, Kyrgyzstan. on the limiting assemblage with K-feldspar, phlogopite and quartz. 33rd International Geological Congress, Oslo, Norway, 2008 p. Contributions to Mineralogy and Petrology, 96: 212-224. Bakirov AB, Tagiri A, Takasu A, Sakiev K, Orozbaev R, Bakirov AA, McDougall I and Harrison TM (1988) Geochronology and Togonbaeva A, Satybaev M and Baslakunov J (2017) (U)HP terreins Thermochronology by the Ar/Ar Method. Oxford University Press, of the Tien-Shan*. In: Bakirov AB (Ed), Makbal (U)HP terrain. Ilim, New York, 212 p. Bishkek, 74-151. Medvedeva IE (1960) Genesis of eclogites of the Makbal dome (Northern Bence AE and Albee AL (1968) Empirical correction factors for the Tien Shan)*. Izvestia vuzov, Geologiya i razvedka 11: 41-60 electron microanalysis of silicates and oxides. Journal of Geology, 76: Meyer M, Klemd R and Konopelko D (2013) High-pressure mafic 382-403. oceanic rocks from the Makbal Complex, Tianshan Mountains Blanckenburg FV, Villa IM, Baur H, Morteani G and Steiger RH (1989) (Kazakhstan & Kyrgyzstan): Implications for the metamorphic Time calibration of a PT-path from the Western Tauern Window, evolution of a fossil subduction zone. Lithos, 177: 207-225. Eastern Alps: the problem of closure temperatures . Contributions to Meyer M, Klemd R, Hegner E and Konopelko D (2014) Subduction and Mineralogy and Petrology, 101: 1-11. exhumation mechanisms of ultra-high and high-pressure oceanic and Deer WA, Howie RA and Zussman J (1992) Appexdix 3, End-Member continental crust at Makbal (Tianshan, Kazakhstan and Kyrgyzstan). Calculations, An introduction to the rock-forming minerals. Longman Journal of Metamorphic Geology, 32: 861-884. Scientific & Technical, London, 683-685. Nikolaev VA (1933) On the most important structural line of the Tien- Degtyarev KE, Tretyakov AA, Ryazantsev AV, Kotov AB, Salnikova Shan*. Proceedings of the Russian Mineralogical Society, 2: 347-354. EB, Aleksandrov PA and Anisimova IV (2011) Stenian granitoids of Orozbaev R, Takasu A, Bakirov AB, Tagiri M and Sakiev KS (2010) the west Kyrgyz Ridge (North Tian Shan): position, structure, and Metamorphic history of eclogites and country rock gneisses in the determination. Doklady Earth Science, 441, Part 1, 1484-1488. Aktyuz area, Northern Tien-Shan, Kyrgyzstan: a record from initiation

Grambling JA (1990) Internally-consistent geothermometry and H2O of subduction through to oceanic closure by continent–continent barometry in metamorphic rocks: the example garnet-chlorite-quartz. collision. Journal of Metamorphic Geology, 28: 317-339. Contributions to Mineralogy and Petrology, 105: 617-628. Orozbaev R, Hirajima T, Bakirov AB, Takasu A, Maki K, Yoshida K, Green TH and Hellman PL (1982) Fe-Mg partitioning between coexisting Sakiev K, Bakirov A, Hirata T, Tagiri M and Togonbaeva A (2015) garnet and phengite at high pressure, and comments on a garnet- Trace element characteristics of clinozoisite pseudomorphs after phengite geothermometer. Lithos, 15: 253-266. lawsonite in talc-garnet-chloritoid schists from the Makbal UHP Hegner E, Klemd R, Kroner A, Corsini M, Alexeiev DV, Iaccheri LM, Complex, northern Kyrgyz Tian-Shan. Lithos, 226: 98-115. Zack T, Dulski P, Xia X and Windley BF (2010) Mineral ages and P-T Parra T, Vidal O and Agard P (2002) A thermodynamic model for Fe– conditions of Late Paleozoic high-pressure eclogite and provenance Mg dioctahedral K white micas using data from phase–equilibrium of melange sediments from Atbashi in the south Tianshan orogen of experiments and natural pelitic assemblages. Contributions to Kyrgyzstan. American Journal of Science, 310: 916-950. Mineralogy and Petrology, 143: 706-732. Holland TJB (1983) The experimental determination of activities in Purdy J and Jäger E (1976) K-Ar ages on rock-forming minerals from disordered and short-range ordered jadeitic pyroxenes. Contributions to the central Alps. Memorie degli Instituti di Geologia e Mineralgia dell' Mineralogy and Petrology, 82: 214-220. Universita di Padova, 30: 1-31. Holland TJB and Powell R (1998) An internally consistent Rojas-Agramonte Y, Herwartz D, García-Casco A, Kröner A, Alexeiev thermodynamic data set for phases of petrological interest. Journal of DV, Klemd R, Buhre S and Barth M (2013) Early Palaeozoic deep Metamorphic Geology, 16: 309-343. subduction of continental crust in the Kyrgyz North Tianshan: evidence Holland TJB, Baker J and Powell R (1998) Mixing properties and from Lu–Hf garnet geochronology and petrology of mafic dikes. activity-composition relationships of chlorites in the system MgO- Contributions to Mineralogy and Petrology, 166: 525-543.

FeO-Al2O3-SiO2-H2O. European Journal of Mineralogy, 10: 395-406. Satybaev M, Ding L, Bakirov A, Sakiev K, Cai F, Orozbaev R, Bakirov Kaneoka I (1998) Numerical dating-from decade to 46 hundred million AA and Baslakunov J (2018) Petrology of metamorphic rocks from years ago**. University of Tokyo Press, 315 p. the Atbashy complex, Southern Tien-Shan, Kyrgyzstan. Geoscience Kirschner DL, Cosca MA, Masson H and Hunziker JC (1996) Staircase Frontiers, 9: 1795-1807. Ar/Ar spectra of fine grained white mica: timing and duration Shibata K, Danhara T and Takagi H (1990) Concordance between K-Ar deformation and empirical constraints on argon diffinsion. Geology, ages of K-feldspar and fission-track ages of zircon in Cretaceous 24: 152-168. granitic rocks from Japan. Geochemical Journal, 24: 187-192. Klemd R, Gao J and Li JL (2015) Metamorphic evolution of (ultra)- Tagiri M and Bakirov AB (1990) Quartz pseudomorph after coesite in high-pressure subduction-related transient crust in the South Tianshan garnet from a garnet-chloritoid-talc schist, Northern Tien-Shan, Kirghiz Orogen (Central Asian Orogenic Belt): Geodynamic implications. SSR. Proceedings of the Japan Academy, Series B, 66: 135-139.

（ 17 ） 64 Kasymbekov et al.

Tagiri M, Yano T, Bakirov AB, Nakajima T and Uchiumi S (1995) report of the age of the UHP metamorphism. Journal of Mineralogical Mineral parageneses and metamorphic P-T paths of ultrahigh-pressure and Petrological Sciences, 104: 77-81. eclogites from Kyrghyzstan Tien-Shan. Island Arc, 4: 280-292. Togonbaeva A, Takasu A, Kamei A, Shibata T, Tagiri M, Bakirov AA, Tagiri M, Takiguchi S, Ishida C, Noguchi T, Kimura M, Bakirov AB, Bakirov AB and Sakiev K (2010a) Sm-Nd and K-Ar ages of the HP Sakiev KS, Takahashi M, Takasu A, Bakirov AA, Togonbaeva A and rocks in the Neldy Formation in the Makbal HP-UHP metamorphic Suzuki A (2010) Intrusion of UHP metamorphic rocks into the upper terrane, Northern Tien-Shan, Kyrgyzstan. The 117th Annual Meeting crust of Kyrgyzian Tien-Shan: P-T path and metamorphic age of the of the Geological Society of Japan. The Geological Society of Japan, Makbal Complex. Journal of Mineralogical and Petrological Sciences, Toyama: 136. 105: 233-250. Togonbaeva A, Takasu A, Tagiri M, Bakirov AB, Bakirov AA and Sakiev Takasu A (1989) P–T histories of peridotite and amphibolite tectonic K (2010b) Newly described eclogites from the Neldy Formation, blocks in the Sambagawa metamorphic belt, Japan. In: Evolution Makbal district, Northern Tien-Shan, Kyrgyzstan. Journal of of Metamorphic Belts (eds, Daly JS, Cliff RA and Yardley BWD), Mineralogical and Petrological Sciences, 105: 80-85. Geological Society Special Publication, 43: 533-538. Whitney DL and Evans BW (2010) Abbreviations for names of rock- Togonbaeva A, Takasu A, Bakirov AA, Sakurai T, Tagiri M, Bakirov forming minerals. American Mineralogist, 95: 185-187. AB and Sakiev K (2009) CHIME monazite ages of garnet-chloritoid- *: in Russian talc schists in the Makbal Complex, Northern Kyrgyz Tien-Shan: First **: in Japanese

KASYMBEKOV Adil・高須晃・KABIR Md Fazle・遠藤俊祐・BAKIROV Apas B.・SAKIEV Kadyrbek・ OROZBAEV Rustam・平島崇男・吉田健太．2020．キルギス北部天山マクバル・コンプレックス中の泥質片岩の変成作用と白色雲母 K-Ar 年代．地球科学，74, 47-64． KASYMBEKOV Adil, TAKASU Akira, KABIR Md Fazle, ENDO Shunsuke, BAKIROV Apas B., SAKIEV Kadyrbek, OROZBAEV Rustam, HIRAJIMA Takaso, YOSHIDA Kenta. 2020. Metamorphism and K-Ar white mica ages of the pelitic schists in the Makbal Complex, Kyrgyz Northern Tien-Shan. Earth Science (Chikyu Kagaku), 74, 47-64.

要旨

キルギス北部天山に分布する高圧－超高圧変成作用を受けたマクバル・コンプレックス（Makbal Complex）中のネルディ層（Neldy Formation）のざくろ石を含まない泥質片岩（KG1251）と含ざくろ石－クロリトイド泥質片岩（KG1244）の岩石記載と地質年代の測定を行った．KG1251 の主要造岩鉱物はフェンジャイト，緑泥石と石英であり，その他に少量の曹長石，チタン石，方解石，ルチル及び炭質物を含む．ピーク変成条件は T < 630 ℃，P = 0.9-1.7 GPa が見積もられた．片理を形成するフェンジャイトの K-Ar 年代は 524 ± 13 Ma であり，これはこれまでに報告されているマクバル・コンプレックスのエクロジャイト及びざくろ石－クロリトイド－タルク片岩のピーク変成年代（ca. 500 Ma）と調和的である．KG1244 の主要構成鉱物は白色雲母（コアがフェンジャイト，リムが白雲母），緑泥石，石英であり，その他に少量のざくろ石，クロリトイド，曹長石，電気石，ジルコン，モナザイト，チタン石，ルチル，方解石及び炭質物を含む．ピーク変成条件は T = 485-545 ℃，P = 1.2-1.5 GPa の高圧型変成作用を示し，その後，T = ca. 500 ℃，P > 0.3 GPa の低圧型変成作用を受けた．この変成作用は花崗岩体の貫入にともなう接触変成作用と考えられた．本研究で得られた白色雲母の K-Ar 年代（474 ± 12 Ma）は，この地域に分布するオルドビス紀の花崗岩の年代（ca. 460 Ma）と調和的である．KG1251 及び KG1244 のネルディ層の泥質片岩の変成条件は，マクバル・コンプレックスの構造・層序学的に下部を占めるエクロジャイトやざくろ石－クロリトイド－タルク片岩に比べて有意に低圧であることが明らかになった．

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