U–Pb–Hf zircon study of two mylonitic granite complexes in the Talas-Fergana fault zone, Kyrgyzstan, and Ar–Ar age of deformations along the fault

Journal of Asian Earth Sciences 73 (2013) 334–346

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U–Pb–Hf zircon study of two mylonitic granite complexes in the Talas-Fergana fault zone, Kyrgyzstan, and Ar–Ar age of deformations along the fault D. Konopelko a,⇑, R. Seltmann b, F. Apayarov c, E. Belousova d, A. Izokh e, E. Lepekhina f a

Geological Faculty, St. Petersburg State University, 7/9 University Embankment, St. Petersburg 199034, Russia Center for Russian and Central EurAsian Mineral Studies, Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UK North-Kyrgyz Geological Expedition, Ivanovka 725008, Kyrgyzstan d GEMOC, Department of Earth & Planetary Sciences, Macquarie University, Sydney NSW 2109, Australia e Institute of Geology and Mineralogy, SB RAS, 3 pr. Akademika Koptyuga, Novosibirsk 630090, Russia f Center of Isotopic Research, Russian Geological Research Institute (VSEGEI), 74 Sredny Pr., St. Petersburg 199106, Russia b c

a r t i c l e

i n f o

Article history: Received 31 July 2012 Received in revised form 13 April 2013 Accepted 26 April 2013 Available online 14 May 2013 Keywords: Tien Shan (Tianshan) Talas-Fergana fault Kyrgyzstan U–Pb zircon age Hf isotopes Muscovite Ar-dating

a b s t r a c t A 2000 km long dextral Talas-Fergana strike–slip fault separates eastern terranes in the Kyrgyz Tien Shan from western terranes. The aim of this study was to constrain an age of dextral shearing in the central part of the fault utilizing Ar–Ar dating of micas. We also carried out a U–Pb–Hf zircon study of two different deformed granitoid complexes in the fault zone from which the micas for Ar dating were separated. Two samples of the oldest deformed Neoproterozoic granitoids in the area of study yielded U– Pb zircon SHRIMP ages 728 ± 11 Ma and 778 ± 11 Ma, characteristic for the Cryogenian Bolshoi Naryn Formation, and zircon grains analyzed for their Lu–Hf isotopic compositions yielded eHf(t) values from 11.43 to 16.73, and their calculated tHfc ages varied from 2.42 to 2.71 Ga. Thus varying Cryogenian ages and noticeable heterogeneity of Meso- to Paleoproterozoic crustal sources was established for mylonitic granites of the Bolshoi Naryn Formation. Two samples of mylonitized pegmatoidal granites of the Kyrgysh Complex yielded identical 206Pb/238U ages of 279 ± 5 Ma corresponding to the main peak of Late-Paleozoic post-collisional magmatism in the Tien Shan (Seltmann et al., 2011), and zircon grains analyzed for their Lu–Hf isotopic compositions yielded eHf(t) values from 11.43 to 16.73, and calculated tHfc ages from 2.42 to 2.71 Ga indicating derivation from a Paleoproterozoic crustal source. Microstructural studies showed that ductile/brittle deformation of pegmatoidal granites of the Kyrgysh Complex occurred at temperatures of 300–400 °C and caused resetting of the K–Ar isotope system of primary muscovite. Deformation of mylonitized granites of the Bolshoi Naryn Formation occurred under high temperature conditions and resulted in protracted growth and recrystallization of micas. The oldest Ar–Ar muscovite age of 241 Ma with a well defined plateau from a pegmatoidal granite of the Kyrgysh Complex is considered as a ‘‘minimum’’ age of dextral motions along this section of the fault in the Triassic while younger ages varying from 227 Ma to 199 Ma with typical staircase patterns indicate protracted growth and recrystallization of micas during ductile deformations which continued until the end of the Triassic. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The Central Asian Orogenic Belt (CAOB) formed during two main cycles of collision-accretion in Earlier and Late Paleozoic known in the southern part of the CAOB as the Caledonian and Hercynian orogenies. The Paleozoic orogen was later redeformed in the course of the Cenozoic collision of India and Eurasia. It was shown that the largest displacements along major ⇑ Corresponding author. Tel.: +7 9219950845; fax: +7 8123279776. E-mail address: [email protected] (D. Konopelko). 1367-9120/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2013.04.046

trans-crustal shear zones which significantly reshaped regional tectonic structures of Tien Shan and Kazakhstan occurred after Late Paleozoic collision in the Early Permian and later in the early Mesozoic as a result of convergence of amalgamated terranes and crustal shortening (Pirajno, 2010; Buslov, 2011). The oldest ages constraining deformation along the major regional strike–slip faults show that they were already active in the Early Permian (Laurent-Charvet et al., 2003). However the prolonged history of deformation along major faults remains poorly studied. The heat flow associated with deformation was often accompanied by circulation of fluids which formed a number of shear-related gold

D. Konopelko et al. / Journal of Asian Earth Sciences 73 (2013) 334–346

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Fig. 1. Principal tectonic zones and lineaments of Tien Shan in Kyrgyzstan. Abbreviations: NTS – Northern Tien Shan, MTS – Middle Tien Shan, STS – Southern Tien Shan.

Fig. 2. Schematic geological map of study area. Based on 1:50,000 map of Orlov et al. (1972).

deposits with ages varying from Early Permian to Triassic (Zhu et al., 2007; Zhu, 2011). Because an understanding of the largescale displacements and distribution of the regional heat flow during the post-collisional stage is important for the whole Tien Shan orogen, we addressed this problem using as an example the TalasFergana (Farghona) fault, a major dextral shear zone separating western terranes of the Tien Shan in Uzbekistan and Kyrgyzstan from eastern terranes (Fig. 1). The research was carried out in the central part of the fault and devoted to deformed granitoid intrusions in the area of Karasu lake (Fig. 2). Our methodology included determination of emplacement ages of granites (U–Pb zircon SHRIMP geochronology), investigation of their sources utilizing Hf isotopic systematics in zircons, and constraining thermal history and age of deformation by microtextural studies and Ar-Ar dating of micas. 2. Principal terranes of the Tien Shan The Tien Shan (Tianshan) orogen formed during Late Palaeozoic (Hercynian) collision between the Precambrian microcontinents of

Karakum and Tarim in the south with the Early Palaeozoic Kazakhstan continent in the north (Zonenshain et al., 1990; Bakirov and Maksumova, 2001; Biske and Seltmann, 2010). The western part of the Tien Shan in Kyrgyzstan is composed of three tectonic units (Fig. 1): (1) the Northern Tien Shan, the deformed margin of the Paleo-Kazakhstan; (2) the Middle Tien Shan, a Late Paleozoic volcano-plutonic arc; and (3) the Southern Tien Shan, an intensely deformed fold and thrust belt formed during the final closure of the Paleo-Turkestan ocean (Zonenshain et al., 1990; Biske, 1995; Biske and Seltmann, 2010). An eye-catching feature of the present day Tien Shan geology is a number of major east–west trending left-lateral strike–slip faults, dividing the Tien Shan into a series of linear tectonic blocks. These are cut by the major NW trending Talas-Ferghana fault which demonstrates a total dextral offset of about 200 km and separates the western Tien Shan terranes from the eastern terranes (Fig. 1). The Northern Tien Shan in Kyrgyzstan is represented by an early Palaeozoic continental arc and its Precambrian basement formed as a result of progressive subduction to the north and subsequent closure of the Terskey ocean in the Late Ordovician and accretion of

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the Middle Tien Shan to the Northern Tien Shan. In Kyrgyzstan, the Northern and Middle Tien Shan are separated by the Nikolaev Line, a Late Paleozoic strike–slip fault generally following a Caledonian suture. The development of the Northern Tien Shan arc is documented by continuous Andean type magmatism which created voluminous subduction-related Ordovician granitoids (Kiselev and Maksumova, 2001; Ghes, 2008; Konopelko et al., 2008). The main component of the Middle Tien Shan west of the Talas-Fergana fault is the subduction-related Beltau-Kurama volcano-plutonic belt formed on an Early Paleozic and/or Neoproterozoic Precambrian basement as a result of northward subduction during the closure of the Paleo-Turkestan ocean to the south. The Beltau-Kurama belt is usually considered as a southern active margin of the Palaeo-Kazakhstan (e.g. Seltmann et al., 2011) or as a continental arc that was accreted to the Palaeo-Kazakhstan margin ca. 320 Ma ago (Seliverstov and Ghes, 2001; Yakubchuk et al., 2002). The structure of the Middle Tien Shan east of the Talas-Fergana fault, where the study area is situated (Figs. 1 and 2), is different from that west of the fault (Sengor and Natal’in, 1996). The Precambrian basement of the Middle Tien Shan east of the Talas-Fergana fault is represented by Paleoproterozoic gneisses and schists of the Kuilyu Complex, which crop out in the east, within the Sarydjaz river basin and in the Ak-Shijrak ridge. Discordant U–Pb ages of 2 Ga were obtained for these rocks by previous multigrain zircon dating (Kiselev, 1999) and the ages of ca. 2.3 and 1.8 Ga received recently by single grain SHRIMP dating (Kröner et al., 2011). Neoproterozoic magmatic rocks comprise granites (Sarydjaz Complex) and rhyolites (Bolshoi Naryn Formation) which are dated at 830 Ma and 764 Ma respectively (Kröner et al., 2009, 2011). Ordovician Andean type granites are not characteristic for this part of the Middle Tien Shan. The Precambrian granitoids are overlain by Cryogenian and Ediacarian (Vendian) sanstones and diamictites, which change upsection to Cambrian shales and carbonates, Ordovician cherts and turbidites. Deformations due to amalgamation of the Middle Tien Shan with the North Tien Shan in the Late Ordovician led to the formation of unconformity and uplift. Marine sedimentation resumed in the Middle and Late Devonian and continued until the Pennsylvanian. The Paleo-Turkestan ocean situated to the south of the Middle Tien Shan was eliminated during the Early Permian collision of the Kazakhstan continent with Tarim (Biske and Seltmann, 2010). Pennsylvanian (Late Carboniferous) subduction to the north under the Middle Tien Shan terrane east of the Talas-Fergana fault was amagmatic or its evidence was eroded or hidden under the cover of younger rocks (Alekseev et al., 2009). The Kyrgyz South Tien Shan represents a pile of folded tectonic nappes, which are thrust southward upon sediments and continental basement of the Tarim craton. Traditionally the northern edge of Tarim was considered as a Late Paleozoic passive margine (e.g. Biske, 1995; Allen et al., 1999) but there is a growing evidence that before the Carboniferous the northern edge of Tarim represented an active margin (Wang et al., 2011 and references therein). East of the Talas-Fergana fault the Middle and Southern Tien Shan terranes are separated by the Atbashi-Inylchek suture. The sedimentary units of the Southern Tien Shan include Silurian to Missisippian (Early Carboniferous) pelagic sediments and volcanics of a fore-arc accretionary complex, and thick carbonate platforms, mostly of Upper Devonian – Mississippian age. A typical feature of the Kyrgyz Southern Tien Shan is a thick pile of clastic sediments of the Tarim continental slope varying in age from Middle Devonian to Carboniferous. Thick Pennsylvanian – Early Permian foredeep turbidites and molasses indicate the final closure of the PaleoTurkestan ocean (Biske and Seltmann, 2010). The Late Paleozoic collision was followed by a regional uplift and intrusion of Early Permian post-collisional granitoids across terrane boundaries. Shortening in a north–south direction continued after collision in the Permian and early Mesozoic, and caused displacements along

major strike–slip faults and lateral escape of large crustal blocks (Burtman et al., 1996; De Jong et al., 2009; Buslov, 2011). At least some of these major faults formed already in the Early Permian (Laurent-Charvet et al., 2003) and controlled post-collisional magmatism and important mineralization (Mao et al., 2004; Pirajno, 2010; Konopelko et al., 2011).

3. The Talas-Fergana fault The Talas-Fergana fault is one of the major NW striking faults in the CAOB. The other major NW-striking faults include the Irqiz fault and other NW striking faults of eastern Kazakhstan (Sengor et al., 1993; Dobretsov et al., 1996; Buslov, 2011). The Talas-Fergana fault strikes across the Tien Shan for 2000 km from Kazakhstan to west Tarim and separates the western terranes of the Tien Shan from the eastern terranes (Fig. 1). In its northern part in the Karatau and Talas ranges the Talas-Fergana fault probably inherited an early-middle Paleozoic terrane boundary as indicated by juxtaposition of blocks with different geological histories in its walls (Sengor et al., 1993; Burtman et al., 1996; Sengor and Natal’in, 1996; Charvet et al., 2011). It was shown that major NW-trending faults in the CAOB initiated as sinistral strike–slip faults and were later reactivated by right-lateral motions (Allen et al., 2001; Laurent-Charvet et al., 2003). This early sinistral stage was also suggested for the Talas-Fergana fault (Sengor and Natal’in, 1996; Alexeiev et al., 2009).The dextral displacement along the Talas-Fergana fault from the Early Permian to the Quaternary reached 180–200 km in its central part based on the offset of the South Tien Shan suture in the Fergana range (Burtman, 1964). The amplitude of displacement decreases from the central part of the fault to its NW and SE terminations (Burtman et al., 1996). After Late Paleozoic – Early Mesozoic deformation following the Hercynian collision, the slip rate along the Talas-Fergana fault probably decreased and then again increased in the Cenozoic. Based on paleomagnetic data (Bazhenov et al., 1993) dextral displacement along the Talas-Fergana fault during the Cenozoic is estimated as 60 km, and the slip rate during the Holocene reached 8–16 mm/yr (Burtman et al., 1996). The dextral displacement along the Talas-Fergana fault was accommodated by thrusting of the Chatkal range to the south onto the Cenozoic Fergana basin (Burtman et al., 1987; Trifonov et al., 1990; Bazhenov et al., 1993). In the field the Talas-Fergana fault occurs as a thick (<1 km) subvertical zone of variably mylonitized rocks with numerous tectonic slices of various ages striking in the area of study roughly at 300° (Fig. 2).

4. Geology of study area and sampling The area of study shown in Fig. 2 includes an approximately 60 km long section along the Talas-Fergana fault. The northern wall of the Talas-Fergana fault has a more complex structure compared to the southern wall, and includes a number of Neoproterozoic units, the oldest dated being the Bolshoi Naryn Formation with U–Pb zircon ages of 830–764 Ma (Kröner et al., 2009, 2011). The Neoproterozoic formations overlying the Bolshoi Naryn Formation on a regional scale comprise Cryogenian (Vendian) tillites and tilloids with interlayers of hematite schists. In the area of study the Cryogenian-Ediacarian (Vendian) rocks are represented by the mylonitized shales and sandstones of the Dzhakbolot Formation (Fig. 2) The Paleozoic rocks in the northern wall of the Talas-Fergana fault comprise Cambrian-Late Devonian clastic metasediments and Late Devonian-Carboniferous sediments. In the southern wall of the Talas-Fergana fault the rocks comprise Silurian – Middle Devonian carbonates, and Early Permian conglomerates (Fig. 2).

D. Konopelko et al. / Journal of Asian Earth Sciences 73 (2013) 334–346

In order to characterize the deformed granitoids in the TalasFergana fault zone we chose for sampling two complexes of different ages. Neoproterozoic mylonitized granitoids of the Bolshoi Naryn Formation (Orlov et al., 1972) were sampled on the NE shore of Karasu lake (Fig. 2). Two samples of slightly texturally different mylonites were collected in this locality (Fig. 2). Sample 25 (N41°33.530, E73°15.692) characterizes a ca. 70 m thick band of homogeneous mylonites outcropping along the NE shore of Karasu lake (Fig. 3a). Foliation planes in the mylonites are subvertical and strike at 305° along the Talas-Fergana fault. A close up photograph of mylonite, showing the texture in the plane perpendicular to foliation, demonstrates dextral shearing in the mylonites corresponding to the general dextral slip along the Talas-Fergana fault (Fig. 3b). Sample 24 (N41°33.547, E73°15.765) was taken from a slightly different rock type with a relic porphytitic texture. Orientation of foliation planes in this rock is similar to that in locality of sample 25. In the NW part of the area shown in Fig. 2 indjection migmatites, gneissic granites and deformed pegmatoidal granites

a

337

of the Early Permian Kyrgysh Complex impregnate strongly mylonitized sediments of the Dzhakbolot Formation (Orlov et al., 1972; Osmonbetov et al., 1982; Zhukov et al., 2008). The foliation in gneisses and elongation of granite bodies are parallel to the Talas-Fergana Fault which strikes at 300°and dip subvertically or steeply to the north. A number of veins of pegmatoidal granite are emplaced in Ordovician metasediments south-east of intensely mylonitized rocks of the Dzhakbolot Formation (Fig. 2). Two samples of deformed pegmatoidal granites Nr 26 (N41°37.286, E73°08.437) and Nr 27 (N41°37.156, E73°08.129) were collected from two parallel veins shown on Fig. 3c. The veins with a thickness of 5–10 m strike at 285° and dip 75° north concordant with the schistosity of the surrounding Ordovician sediments. 5. Results The methodology included U–Pb zircon geochronology of the deformed granites, investigation of their sources utilizing Hf isotopic systematics in zircons, and constraining thermal history and age of deformation by Ar–Ar dating of micas. Mineral concentrates of zircon grains recovered from the granites were produced at St. Petersburg University, and selected zircon grains were dated on the SHRIMP-II ion microprobe in the Center of Isotopic Research, VSEGEI, St. Petersburg, Russia. Hf-in-zircon analyses were performed at GEMOC, Macquarie University, Australia. Ar–Ar dating of micas by the stepwise heating method was performed at the Institute of Geology and Mineralogy, Novosibirsk, Russia. Analytical procedures are summarized in the Appendix A, and the data are listed in Tables 1–3. 5.1. Mylonitized granites of the Bolshoi Naryn Formation

b

c

Fig. 3. Field occurrence of deformed granitoids in the Talas-Fergana fault zone: (a) – outcrop of homogeneous mylonites of the Bolshoi Naryn Formation (sample 25), (b) – close up photograph showing the texture of mylonites (sample 25), photograph is taken perpendicular to subvertical foliation plains in mylonites which strike at 305° along the Talas-Fergana fault, note dextral sense of shearing, (c) – veins of deformed pegmatoidal granites of the Kyrgysh Complex and sampling sites 26 and 27.

Sample 25 represents a strongly mylonitized two-mica granite consisting of quartz (25–30%), microcline (25–30%), seriticized plagioclase (10–15%), chloritized biotite (10–15%) and muscovite (5– 10%). The mylonitic fabric is evident through the strong alignment of fragmented quartz and ‘‘augen’’ of K-feldspar. Biotite and muscovite flakes are well-oriented parallel to the main foliation. Fig. 4a shows newly grown biotite in a pressure shadow of an augen K-feldspar. Sample 24 demonstrates relic porphytitic texture. Fragmented quartz (15%), microcline (10%) and seriticized plagioclase (10–15%) represent relic phenocrysts in the matrix of finegrained quartz, microcline, biotite and muscovite (50–55%). The rock is strongly mylonitized. The fragmented and dynamically recrystallized groundmass minerals including micas flow around preserved relic quartz and feldspar grains (Fig. 4b). Accessory minerals in both samples are opaques (mainly Fe–Ti ore minerals), sphene, zircon and apatite. Ductile crystal-plastic deformation in quartz and feldspar becomes important above 300 ± 50 °C and 450 ± 50 °C, respectively (Voll, 1976; Tullis, 1983; Sibson, 1983). Micro-structural observations presented in this paper demonstrate that the mylonitization of the granites of Karasu lake developed under relatively high temperature conditions above the estimated closure temperatures of biotite (335 ± 50 °C) and muscovite (400 ± 50 °C) (Harrison et al., 1985; Hames and Bowring, 1994; McDougall and Harrison, 1999), thus the K–Ar isotope systems of the primary micas were completely reset during the deformation. Zircon grains recovered from samples 24 and 25 make a uniform population of stubby prismatic grains with pronounced oscillatory zoning typical for magmatic zircons (Fig. 5a and b). Eight zircon grains were analyzed in each sample. Analytical data from both samples plot on concordia as tight clusters. However calculated 206Pb/238U concordia ages for samples 24 and 25 are slightly different: 728 ± 11 Ma and 778 ± 11 Ma, respectively (Fig. 6). This may imply that a porphyritic body represented by sample 24

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Table 1 U–Pb analytical data and calculated ages. Isotope ratiosc

Concentrations Pb ppm

f206 %

±1s %

235

±1s %

238

±1s %

err. corr.

206

206

Pb Pb*

206

Sample 24 – granite (N41°33.547, E73°15.765) 24.1.1 893 552 0.64 91.4 24.2.1 219 136 0.64 22.3 24.3.1 315 164 0.54 31.7 24.4.1 343 188 0.57 37.1 24.5.1 232 141 0.63 24 2 24.6.1 255 141 0.57 26.6 24.7.1 605 372 0.63 60.8 24.8.1 358 203 0.59 36.5

0.08 0.50 0.16 0.22 0.04 0.07 0.04 0.17

0.0636 0.0621 0.0625 0.0645 0.06384 0.06387 0.06328 0.0631

0.88 2.8 1.6 2.3 1.5 1.5 1 1.6

1.045 1.009 1.01 1.119 1.068 1.07 1.02 1.03

2.6 3.8 3 3.5 3 3 2.7 3

0.1191 0.1178 0.1172 0.1258 0.1214 0.1215 0.1169 0.1185

2.5 6.6 2.5 2.6 2.5 2.5 2.5 2.5

.943 .669 .839 .745 .855 .861 .926 .846

726 718 714 764 738 739 713 722

17 17 17 19 18 18 17 17

Sample 25 – granite (N41°33.530, E73°15.692) 25.1.1 660 295 0.46 73 25.2.1 435 247 0.59 44.8 25.3.1 1198 752 0.65 134 25.4.1 124 40 0.34 13.4 25.5.1 488 354 0.75 53.8 25.6.1 851 379 0.46 92.9 25.7.1 507 167 0.34 59.3 25.8.1 581 87 0.15 65.3

0.06 0.28 0.09 0.42 1.96 0.11 0.87 0.10

0.0656 0.0646 0.0657 0.0661 0.0646 0.06388 0.0645 0.0653

0.92 1.6 0.73 2.9 3.2 1.4 2.1 1.5

1.163 1.065 1.177 1.145 1.118 1.117 1.199 1.177

2.7 3 2.6 3.9 4.1 2.9 3.3 2.9

0.1286 0.1195 0.1299 0.1257 0.1256 0.1268 0.1349 0.1307

2.5 2.5 2.5 2.6 2.5 2.5 2.5 2.5

.938 .851 .960 .670 .623 .873 .767 .860

780 728 787 764 763 770 816 792

18 17 18 19 18 18 19 19

U ppm

Th ppm

Th/U

*

b

207

Age (Ma) 207

Samplespot #a

206

*

Pb U

*

Pb U

*

d

Pb* U

±1s

238

207

±1s

Disc.e %

728 679 692 757 736 737 718 711

19 61 35 49 33 32 22 34

0 5 3 1 0 0 1 1

794 762 797 808 759 738 757 784

19 33 15 61 67 30 44 31

2 5 1 6 0 4 7 1

Pb* Pb

206

a

In sample-spot numbers (e.g. 24.1.1) the last two digits denote number of grain and number of analytical spot within the grain. f206 Denotes 100(common 206Pb)/(total measured 206Pb). Corrected for 204Pb. d Error correlation 207Pb/235U–206Pb/238U. e Disc. % denotes 100((1 (age 206Pb/238U)/(age 207Pb/206Pb)). Radiogenic Pb. b

c c

*

Table 2 Lu–Hf isotopic data of zircons from deformed granites along the Talas-Fergana fault. Spot

176

176

24.01.1 24.02.1 24.03.1 24.04.1 24.05.1 24.06.1 24.07.1 24.08.1 25-01.1 25-02.1 25-03.1 25-04.1 25-05.1 25-06.1 25-07.1 25-08.1 26-01.1 26-02.1 26-03.1 26-04.1 26-05.1 26-06.1 26-07.1 26-08.1 27-01.1 27-02.1 27-03.1 27-04.1 27-05.1 27-06.1 27-07.1

0.057662 0.036276 0.062278 0.053608 0.039784 0.036551 0.045365 0.044189 0.029860 0.048652 0.029518 0.027060 0.022270 0.056034 0.032249 0.024093 0.051583 0.033624 0.046730 0.084022 0.054851 0.051189 0.045912 0.046915 0.059168 0.054334 0.047585 0.034606 0.066662 0.063946 0.098371

Yb/177Hf

0.001276 0.000808 0.001513 0.001126 0.000915 0.000767 0.000936 0.000968 0.000624 0.000971 0.000648 0.000572 0.000457 0.001555 0.000965 0.000515 0.001389 0.000924 0.001555 0.002214 0.001183 0.001544 0.001490 0.000986 0.001337 0.001192 0.001259 0.000739 0.001671 0.002074 0.002166

Lu/177Hf

176

Hf/177Hf

0.282292 0.282348 0.282330 0.282326 0.282365 0.282314 0.282382 0.282371 0.281972 0.281859 0.281910 0.281868 0.281874 0.281855 0.281927 0.281844 0.282227 0.282270 0.282234 0.282256 0.282252 0.282231 0.282224 0.282257 0.282276 0.282249 0.282260 0.282266 0.282213 0.282201 0.282189

±(1r)

t (Ma)

(176Hf/177Hf)i

eHf(t)

±(1r)

tHfDM (Ga)

tHfc

0.000015 0.000016 0.000015 0.000016 0.000020 0.000011 0.000018 0.000013 0.000021 0.000027 0.000013 0.000019 0.000013 0.000017 0.000014 0.000014 0.000012 0.000008 0.000010 0.000012 0.000009 0.000011 0.000016 0.000013 0.000011 0.000013 0.000008 0.000008 0.000013 0.000017 0.000013

726.0 718.0 714.0 764.0 738.0 739.0 713.0 722.0 780.0 728.0 787.0 764.0 763.0 770.0 816.0 792.0 274.2 279.4 294.4 277.1 289.0 265.4 318.5 277.1 272.1 275.5 288.1 285.0 278.3 283.6 311.8

0.282275 0.282337 0.282310 0.282310 0.282352 0.282303 0.282369 0.282358 0.281963 0.281846 0.281900 0.281860 0.281867 0.281833 0.281912 0.281836 0.282220 0.282265 0.282225 0.282245 0.282246 0.282223 0.282215 0.282252 0.282269 0.282243 0.282253 0.282262 0.282204 0.282190 0.282176

-1.59 0.45 -0.61 0.51 1.43 -0.28 1.48 1.27 -11.43 -16.73 -13.49 -15.44 -15.19 -16.27 -12.42 -15.65 -13.51 -11.80 -12.87 -12.58 -12.28 -13.59 -12.71 -12.32 -11.81 -12.67 -12.03 -11.78 -13.97 -14.36 -14.23

0.53 0.56 0.53 0.56 0.70 0.39 0.63 0.46 0.74 0.95 0.46 0.67 0.46 0.60 0.49 0.49 0.42 0.29 0.35 0.42 0.33 0.39 0.56 0.46 0.39 0.46 0.29 0.29 0.46 0.60 0.46

1.37 1.27 1.32 1.31 1.25 1.32 1.23 1.24 1.78 1.96 1.87 1.92 1.91 1.99 1.86 1.95 1.46 1.38 1.46 1.45 1.42 1.46 1.47 1.40 1.39 1.42 1.41 1.38 1.49 1.53 1.55

1.75 1.62 1.68 1.65 1.57 1.68 1.55 1.57 2.42 2.71 2.55 2.66 2.64 2.71 2.51 2.69 2.15 2.05 2.13 2.10 2.09 2.15 2.14 2.08 2.04 2.10 2.07 2.05 2.18 2.21 2.23

was emplaced some 50 Ma after formation of the main body of coarse-grained granite (later turned into homogeneous mylonite) indicating the prolonged nature of Cryogenian magmatism in the region. The 8 spots in zircon grains from sample 25 dated by SHRIMP were analyzed for their Lu–Hf isotopic compositions, The results

including depleted mantle extraction (tHfDM) and crustal model ages (tHfc) are presented in Table 2 and Fig. 7. Eight analyses yielded eHf(t) values for the measured crystallization ages from 11.43 to 16.73 (Table 2), and their calculated tHfc ages vary from 2.42 to 2.71 Ga (Table 2 and Fig. 7). The variations in isotopic composition and model age are most likely explained by the

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D. Konopelko et al. / Journal of Asian Earth Sciences 73 (2013) 334–346 Table 3 Ar–Ar analytical data for mica separates from deformed granites along the Talas-Fergana fault. Heating T (°C)

t (min)

40

Ar (ncm3)

24 Muscovite 500 750 950 1130

10 10 10 10

3.9 5 11 9.5

25 Biotite 500 600 700 800 900 1000 1130

10 10 10 10 10 10 6

26 Muscovite 500 650 750 850 930 1030 1130 27 Muscovite 500 600 700 800 850 900 1000 1130

40

Ar/39Ar

±1r

38

Ar/39Ar

±1r

37

±1r

36

144.71 48.65 39.77 49.19

11.81 0.86 0.19 0.48

0.13 0.04 0.02 0.026

0.11 0.017 0.0053 0.0077

0.95 0.079 0.045 0.20

0.30 0.11 0.035 0.028

2.9 8.3 15.7 14.7 52.6 23.8 34.4

64.65 39.39 35.37 43.83 149 33.47 46.71

0.72 0.28 0.09 0.15 0.47 0.07 1.58

0.18 0.042 0.01 0.036 0.088 0.017 0.023

0.032 0.0042 0.0038 0.0047 0.0018 0.0009 0.0018

0.58 0.15 0.007 0.14 0.13 0.072 0.12

10 10 10 10 10 10 6

9.7 36.2 61.8 267 162 95.5 122.7

26.99 23.07 33.95 37.26 35.25 34.5 35.38

0.11 0.02 0.02 0.02 0.023 0.021 0.036

0.033 0.019 0.02 0.02 0.02 0.02 0.02

0.0048 0.0005 0.0007 0.0002 0.0002 0.0004 0.0003

10 10 10 10 10 10 10 6

7.6 14.3 61.7 205.5 275.4 132.4 137.2 298.2

72.41 46.80 49.61 43.07 37.91 38.05 38.47 37.85

1.16 0.24 0.07 0.03 0.017 0.021 0.031 0.013

0.07 0.045 0.028 0.023 0.021 0.02 0.021 0.02

0.013 0.0034 0.0012 0.0004 0.0002 0.0004 0.0005 0.0003

heterogeneous isotopic composition of a crustal source from which the mylonitic granite of sample 25 was derived. Such heterogeneity has also been observed in other crustal-derived granitoids (Belousova et al., 2006; Stevens et al., 2007; Kröner et al., 2012; Villaros et al., 2012). The eight analyzed spots in zircon grains from sample 24, which has a ca. 50 Ma younger emplacement age than sample 25, yielded more juvenile eHf(t) values from 1.48 to 1.49 (Table 2), and produced an array of younger tHfc ages from 1.55 to 1.75 Ga (Table 2 and Fig. 7a). As for sample 25 we interpret the variations in isotopic composition as due to derivation from a heterogeneous source. However it is evident that the mylonitized porphyritic granite of sample 24, probably representing a younger crosscutting body, was either derived from a younger Mesoproterozoic crustal source or it was derived from a heterogeneous source including a juvenile mantle component. Thus varying Neoproterozoic ages and noticeable heterogeneity of Meso- to Paleoproterozoic crustal sources were established in mylonitic granites of the Bolshoi Naryn Formation on the NE shore of Karasu lake. Biotite and muscovite mineral concentrates were selected from samples 25 and 24, respectively, for dating by the 40Ar/39Ar stepwise heating method. Both concentrates yielded variable 40Ar/39Ar spectra with similar characteristics (Table 3 and Fig. 8). The lowand intermediate-temperature steps up to 700–850 °C recorded a systematic increase of apparent ages. A plateau age of 199.2 ± 3.4 Ma, calculated for sample 25, is defined by intermediate- and high-temperature steps (except for the last step) over 65% gas released. For sample 24 a poorly defined plateau age of 217.4 ± 7.1 was calculated for the two last high-temperature steps over 75% gas released. The ‘‘staircase’’ 40Ar/39Ar spectra of finegrained micas from mylonites were reported in a number of publications (e.g. Kirschner et al., 1996) and were interpreted to record protracted mica recrystallization and growth during the late stages of low-temperature ductile deformation.

Ar/39Ar

Ar/39Ar

±1r

Ca/K

R39Ar (%)

Age (Ma)

±1r

0.46 0.086 0.031 0.054

0.045 0.0088 0.0046 0.0035

3.41 0.28 0.16 0.73

4.5 21.7 67.8 100.0

66.9 159.2 207.0 223.9

50.1 17.0 9.0 7.1

0.15 0.031 0.008 0.028 0.015 0.0039 0.0065

0.18 0.051 0.013 0.047 0.4 0.016 0.012

0.036 0.0061 0.0048 0.005 0.0038 0.0014 0.0011

2.11 0.53 0.024 0.5 0.46 0.26 0.45

1.6 9.0 24.7 36.5 48.9 74.0 100

69.6 164.5 210.8 200.9 213.7 194.1 282.8

72.8 11.8 9.1 9.5 7 3.3 10.2

1.54 0.01 0.46 0.008 0.13 0.029 0.29

0.68 0.18 0.13 0.027 0.057 0.12 0.074

0.039 0.0061 0.013 0.0039 0.0015 0.0015 0.0023

0.0031 0.0006 0.0007 0.0002 0.0002 0.0004 0.0003

5.55 0.038 1.67 0.03 0.47 0.11 1.03

1.6 8.9 17.2 50.2 71.3 84 100

104.9 142.0 198.3 234.8 226.9 222.3 226.2

6.1 1.8 2.3 2.3 2.2 2.2 2.2

0.28 0.16 0.055 0.01 0.033 0.014 0.0085 0.0038

0.28 0.16 0.055 0.01 0.02 0.014 0.0085 0.0038

0.14 0.036 0.043 0.019 0.0026 0.0041 0.0054 0.0019

0.015 0.0018 0.0017 0.0004 0.0002 0.0005 0.0003 0.0001

0.99 0.58 0.2 0.037 0.12 0.052 0.03 0.014

0.4 1.4 5.8 22.5 47.8 60 72.5 100

211.7 234.9 239.6 243.8 241.6 239.8 240.1 242.5

28.2 4.0 3.8 2.4 2.3 2.4 2.3 2.3

5.2. Deformed pegmatoidal granites of the Kyrgysh Complex Samples 26 and 27, represent strongly deformed foliated coarse-grained pegmatoidal granite. Mineralogically, both samples includes quartz (25–30%), microcline (25–30%), seriticized plagioclase (20%), and muscovite (10–15%). Muscovite forms relatively large deformed flakes (up to 2 cm) oriented parallel to the foliation. Feldspar crystals are fractured, deformed and kinked, quartz is fragmented and locally show recrystallized polygonal grains indicating that growth of new quartz continued after deformation (Fig. 4c and d). Sample 26 is slightly more altered and contains quartz–sericite–chlorite veinlets also aligned the foliation. Accessory minerals are sphene and zircon. Zircon grains separated from samples 26 and 27 were dated by SHRIMP and yielded identical 206Pb/238U concordia ages of 279 ± 5 Ma (Seltmann et al., 2011). In this study we analyzed zircon domains dated by SHRIMP for their Lu–Hf isotopic compositions. Eight spots were analyzed in sample 26 and 7 spots in sample 27 (Table 2). Analyses of both samples yielded identical eHf(t)-values from 14.36 to 11.78, Table 2) and Paleoproterozoic tHfc ages from 2.04 to 2.23 Ga (Table 2 and Fig. 7). The variations in isotopic composition of samples 26 and 27 are explained by derivation from a heterogeneous crustal source. Thus the Early Permian age and derivation from a Paleoproterozoic crustal source was established for pegmatoidal granites of the Kyrgysh Complex. Derivation from a crustal source is also supported by the presence of abundant muscovite, indicating the peraluminous character of the granites. Muscovite mineral concentrates separated from samples 26 and 27 were dated by the 40Ar/39Ar stepwise heating method. Analysis of sample 27 yielded a well defined plateau age of 241.1 ± 2.3 Ma calculated for all steps of heating, except the first low temperature step, over 95% gas released. A plateau age of 227.5 ± 2.2 Ma,

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D. Konopelko et al. / Journal of Asian Earth Sciences 73 (2013) 334–346

a

a

b b

Fig. 5. CL images of analyzed zircon grains from milonitized granites of the Bolshoi Naryn Formation: (a) – sample 24, (b) – sample 25. Ovals define analytical spots. Spot numbers are as in Table 1. Ages are in Ma, errors at 1r level.

c

calculated for sample 26, is defined by intermediate- and hightemperature steps over 80% gas released, while three low- and intermediate-temperature steps up to 750 °C recorded a systematic increase of apparent ages yielding a ‘‘staircase’’ degassing spectrum similar to those for samples 24 and 25 (Table 3 and Fig. 8).

6. Discussion 6.1. Neoproterozoic tectonic slices in the Talas Fergana fault zone

d

Fig. 4. Microphotographs of deformed granitoids in the Talas-Fergana fault zone: (a) asymmetric augen structure, caused by sliding and rotating of feldspar megacrysts, and growth of biotite in a pressure shadow (sample 25), (b) relics of porphyritic texture. The fragmented and dynamically recrystallized groundmass minerals, including micas, bend around preserved relic porphyritic quartz and feldspar crystals (sample 24), (c and d) curved and kinked twins of plagioclase, deformed flakes of muscovite, fragmentation of quartz and plagioclase and recrystallized polygonal quartz in pegmatoidal granites (samples 26 and 27, polarized light).

Tectonic slices in major shear zones are difficult to interpret. The mylonitic granites sampled on the NE shore of Karasu lake were shown on geological maps as Neoproterozoic Bolshoi Naryn Formation (Fig. 2, Orlov et al., 1972), as a part of the Pennsylvanian (Late Carboniferous) Keninbel intrusion (Dodonova, 1964; Dodonova and Korolev, 1966) and as the Cambro-Ordovician Shortor Formation (Zhukov et al., 2008). In this study we confirmed the interpretation of Orlov et al. (1972) by dating two samples of the granites at 778 and 728 Ma. The obtained ages of mylonitic granites and their model crustal ages (tHfc) varying from 1.55 to 2.71 Ga show that formation of Neoproterozoic volcanic cover was accompanied by protracted emplacement of granites derived from heterogeneous Meso-to Paleoproterozoic crustal sources. Neoproterozoic volcanics and intrusions similar in age and composition to the Bolshoi Naryn Formation were described in northern Tarim (Xu et al., 2005), in drill cores of central Tarim (Guo et al., 2005), in the Quruqtagh block of NE Tarim (Shu et al., 2010), and also in the Northern Tien Shan and Kazakhstan (Kiselev, 1999; Kröner et al., 2009, 2011). In particular, similar U–Pb zircon ages of 844–788 Ma and Paleoproterozoic to Archaean crustal model ages (tHfc) were reported for gneisses of the Aktyuz and Kemin complexes in the Kyrgyz Northern Tien Shan (Kröner et al., 2011). An eye-catching similarity of Neoproterozoic volcanic cover of Kazakhstan, Middle Tien Shan and Tarim led

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Fig. 6. Concordia diagrams for zircon U–Pb SHRIMP data for mylonitized granites of the Bolshoi Naryn Formation. Sample numbers as in Table 1.

(a)

(b)

(c)

(d)

Fig. 7. Hf evolution diagrams for zircons from samples 24 (a), 25 (b), 26 (c) and 27 (d). Solid line is depleted mantle evolution after Griffin et al. (2000). Dashed lines are evolution lines for crustal reservoir (176Lu/177Hf = 0.015). Minimum and maximum crustal model ages for the analyzed zircon grains from each sample are shown.

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Fig. 8. 40Ar–39Ar age spectra as a function of 39Ar released for micas from granitoids of the Bolshoi Naryn Formation (samples 24 and 25) and the Kyrgysh Complex (samples 26 and 27). Error boxes for each step are at the 1r level. Errors for plateau ages are at the 1r level. Percentage of 39Ar degassed used in the plateau calculations are listed in Table 3.

D. Konopelko et al. / Journal of Asian Earth Sciences 73 (2013) 334–346

several authors to propose that these terranes were parts of a single continent in Neoproterozoic – Early Cambrian times (Khain et al., 2003; Levashova et al., 2011). The geodynamic setting of the Neoproterozoic magmatism is uncertain. Volcanics of the Bolshoi Naryn Formation were considered as rift-related (Kiselev and Maksumova, 2001). On a global scale Neoproterozoic belts are considered to be a result of the breakup of Rodinia which involved fragmentation of the NW Gondwana terranes and separation of Laurentia from Siberia (e.g. Yarmolyuk et al., 2006). 6.1.1. Ma pegmatoidal granites of Kyrgysh Complex in Talas-Fergana fault zone U–Pb zircon ages of 279 Ma obtained for two samples of pegmatoidal granites (Seltmann et al., 2011) well match the peak of Late Paleozoic post-collisional magmatism that affected the whole Tien Shan between 290 and 275 Ma (Konopelko et al., 2007; Seltmann et al., 2011). The pegmatoidal granites were derived from a Paleoproterozoic crustal source as indicated by their peraluminous compositions and Paleoproterozoic crustal model ages (tHfc). According to the model of Konopelko et al. (2007), the heat flow at the post-collisional stage was controlled by major trans-crustal shear zones. However shear-related intrusions are often tectonically reworked and difficult to interpret. The distribution of granitoid intrusions shown in Fig. 2 shows that their shapes do not preclude shear-controlled emplacement but may be equally a result of subsequent rotation and slicing. 6.2. Age constraints on dextral deformation along the Talas-Fergana fault The ductile deformation of quartz and the brittle deformation of feldspar in mylonitized pegmatoidal granites of the Kyrgysh Complex indicate that mylonitization occurred under greenschist facies conditions, at temperatures of about 300–400 °C and may have caused resetting of the K–Ar isotope system of primary muscovite in the course of deformation (Fig. 4c and d). Thus a well defined plateau age of 241.1 ± 2.3 Ma obtained for sample 27, the oldest of the Ar–Ar ages, is considered as a ‘‘minimum’’ age of dextral deformation along the Talas-Fergana fault in the study area, while the staircase character of plateau age 228 Ma obtained for sample 26 is probably explained by the prolonged low-temperature deformation of pegmatoidal granites during the Triassic. It is commonly accepted that the Talas-Fergana fault was active over a long period in the Mesozoic and Cenozoic (Burtman et al., 1996). Microstructural studies of the mylonites of the Bolshoi Naryn Formation showed that in these rocks the K–Ar isotope systems of the primary micas were completely reset during the deformation. This is supported by younger Ar–Ar ages of samples 24 and 25 from 217 Ma to 199 Ma, respectively, and by staircase patterns of their plateaus. We interpret progressively younger age of samples 26, 24 and 25 to manifest the protracted mica recrystallization and growth during the low-temperature ductile deformation which continued along this section of the Talas-Fergana fault until the end of the Triassic.

343

after formation of the main body of granite. Analyses of zircons, dated by SHRIMP, for their Lu–Hf isotopic compositions yielded eHf(t) values from 11.43 to 16.73, and their calculated tHfc ages vary from 2.42 to 2.71 Ga. Zircons from a younger porphyritic granite yielded more juvenile eHf(t) values from 1.48 to 1.49 and produced an array of younger tHfc ages from 1.55 to 1.75 Ga. Thus varying Cryogenian ages and a noticeable heterogeneity of Mesoto Paleoproterozoic crustal sources was established in these mylonitic granites. Two samples of mylonitized pegmatoidal granites of the Kyrgysh Complex yielded identical 206Pb/238U ages of 279 ± 5 Ma (Seltmann et al., 2011) Zircon domains dated by SHRIMP were analysed for their Lu–Hf isotopic compositions and yielded identical eHf(t)values from 14.36 to 11.78, and Paleoproterozoic crustal Hf model ages from 2.04 to 2.23 Ga. Thus an Early Permian age and derivation from a Paleoproterozoic crustal source was established for pegmatoidal granites of the Kyrgysh Complex. The obtained ages well match the peak of post-collisional magmatism that affected the whole Tien Shan between 290 and 275 Ma. Microstructural studies of the mylonites of the Bolshoi Naryn Formation showed that these rocks were strongly deformed under high temperature conditions and that K–Ar isotope systems of the primary micas in these rocks were completely reset, while deformation of pegmatoidal granites of the Kyrgysh Complex occurred under lower temperature conditions and was less intense. We believe that the oldest and well defined Ar–Ar plateau age of 241 Ma obtained for micas from pegmatoidal granite represents a ‘‘minimum’’ age for dextral deformation along this section of the Talas-Fergana fault. Younger ages varying from 228 Ma to 199 Ma, characterized by staircase plateau patterns, indicate that protracted growth and recrystallization of micas during dextral deformation along the fault continued until the end of the Triassic. Acknowledgements The authors are grateful to the staff of Talas Copper Gold LLC for their generous support and assistance during fieldwork. Anatoly Ilyukhin was in charge for transportation in the field. Sergey Petrov is acknowledged for his assistance in zircon separation. Georgy Biske kindly read and commented on an earlier version of this paper. Kåre Kullerud provided field photographs from his collection. We thank reviewers Dmitry Alexeiev, Jaques Charvet and handling editor Boris Natal’in for their constructive comments. Chris Stanley kindly improved the English language. This study was supported by Federal Grant-in-Aid Program «Human Capital for Science and Education in Innovative Russia» (Governmental Contract No. 14.740.11.0187) and a research grant from Saint Petersburg State University (DK). This is contribution 311 from the ARC Centre of Excellence for Core to Crust Fluid Systems (http://www.ccfs.mq.edu.au) and 877 in the GEMOC Key Centre (http://www.gemoc.mq.edu.au). The Hf analytical data were obtained using instrumentation funded by DEST Systemic Infrastructure Grants, ARC LIEF, NCRIS, industry partners and Macquarie University. The research is a contribution to IGCP-592 sponsored by UNESCO-IUGS.

7. Conclusions Appendix A. Analytical procedures Two mylonitized varieties of granites were sampled along the Talas-Fergana dextral strike–slip fault in Kyrgyzstan in order to determine the timing of emplacement of the granites, to characterize the nature of their sources and to estimate an age of deformation along the fault. Homogeneous mylonites of Bolshoi Naryn Formation yielded slightly different 206Pb/238U ages for two samples: 728 ± 11 Ma and 778 ± 11 Ma. Relics of porphyritic texture in the younger sample may imply that a porphyritic body was emplaced some 50 Ma

A.1. SHRIMP zircon dating and cathodoluminescence imaging Selected zircon grains were handpicked and mounted in epoxy resin together with chips of standard zircon grains. The grains were sectioned approximately in half and polished. Prior to analysis, the zircon grains were investigated in transmitted and reflected light and under a scanning electron microscope equipped with cathodoluminescence (CL) and back-scattered electron (BSE) detectors. The

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U–Th–Pb isotope analyses were made using the SHRIMP-II ion microprobe in the Center of Isotopic Research, VSEGEI, St.Petersburg, Russia. Each analysis consisted of four scans through the mass range. The diameter of spot was about 30 lm, and the primary beam intensity was about 4 nA. Every fourth measurement was carried out on the zircon standard TEMORA 1, with an accepted 206Pb/238U age of 416.75 ± 0.24 Ma (Black et al., 2003). The Pb/U ratios have been normalized relative to a value of 0.0668 for the 206Pb/238U ratio of the TEMORA 1. The zircon standard 91500, with U concentration of 81.2 ppm and an accepted 206 Pb/238U age of 1065 Ma (Wiedenbeck et al., 1995) was applied as ‘‘U-concentration’’ standard. The data were reduced in a manner similar to that described by Williams (1998) and references therein, using the SQUID Excel Macro of Ludwig (2000). Corrections for common Pb were made using 204Pb isotope (measured 204Pb/206Pb) and the present day terrestrial average Pb-isotopic composition (Stacey and Kramers, 1975). Uncertainties given for individual analyses in Table 1 (ratios and ages) and Fig. 5 are at the 1r level, however the uncertainties in calculated concordia ages (Fig. 6) are reported at 2r level. The concordia plots were constructed using ISOPLOT/EX macro (Ludwig, 1999). Ion-microprobe dating of zircons younger than about 1000 Ma is best achieved by using 206 Pb/238U-ages (Black and Jagodzinski, 2003), while the 207 Pb/206Pb isotopic system is suitable for dating of zircons older than 1000 Ma because of the short half-life of 235U producing 207 Pb (Black et al., 2003). In this paper we generally followed this practice in quoting the obtained ages. A.2. Lu–Hf isotopic analysis Hf-isotope analyses on zircons were carried out in GEMOC, Macquarie University, Sydney, using a New Wave/Merchantek LUV213 laser-ablation microprobe, attached to a Nu Plasma multi-collector ICPMS. These analyses were done on the zircons grains dated previously by SHRIMP-II at VSEGEI, St. Petersburg. The analytical spots for the analyses were located so they overlapped the SHRIMP pits to avoid any discrepancy between the U–Pb age data and Hf-isotopic data in case there was any internal zoning or variation in age within a particular grain. The analyses were done with a beam diameter of ca. 50 lm and a 5 Hz repetition rate, and typical ablation times were 100–120 s, resulting in pits 40–60 lm deep. The carrier gas transported the ablated sample from the laser-ablation cell via a mixing chamber to the ICPMS torch. The methodology and analyses of standard solutions and standard zircons are described by Griffin et al. (2000). The Mud Tank zircon, analyzed together with the samples (every 10 unknowns), was used as an independent control on reproducibility and instrument stability. Most of the data and the mean value are within 2 s.d. of the recommended values (176Hf/177Hf = 0.282522 ± 42 (2 s.d.); Griffin et al., 2007). eHf values and model ages used in the figures were calculated using the chondritic values proposed by Scherer et al. (2001). To calculate mantle extraction model ages (tHfDM) based on a depleted-mantle source, a model with (176Hf/177Hf)i = 0.279718 at 4.56 Ga and 176Lu/177Hf = 0.0384 was adopted; this produces a present-day value of 176 Hf/177Hf = 0.28325, similar to that of average MORB (Griffin et al., 2000). tHfDM ages, which are calculated using the measured 176 Lu/177Hf of the zircon, can only give the minimum age for the source material of the magma from which the zircon crystallised. Therefore we also have calculated for each analysis of zircon a ‘‘crustal’’ model age (tHfc; see Table 2), which assumes that its parental magma was produced from an average continental crust (176Lu/177Hf = 0.015) that originally was derived from depleted mantle. The depleted mantle lines in Fig. 7 are defined by present-day 176Hf/177Hf = 0.28325 and 176Lu/177Hf = 0.0384 (Griffin et al., 2000), and the analytical data are presented in Table 2.

A.3.

40

Ar/39Ar age determinations

The isotopic composition of argon was measured on a Noble Gas 5400 mass spectrometer at the Institute of Geology and Mineralogy, Novosibirsk. Minerals for 40Ar/39Ar isotope geochronological studies were extracted by the standard magnetic and density separation techniques. Weighted samples of mineral fractions and reference biotite MSA-11 (standard sample 129-88) were packed into aluminum foil, placed into a quartz ampoule, and, after evacuation, sealed. The biotite MSA-11, prepared as a standard K/Ar sample at the All-Russian Research Institute of Mineral Resources, Moscow, in 1988, was attested as a 40Ar/39Ar reference sample, using international standard samples of muscovite Bern 4 m and biotite LP-6 (Baksi et al., 1996). The averaged calibrated date, 311.0 ± 1.5 Ma, was taken as the integral age of biotite MSA-11. The quartz ampoules with samples were exposed to radiation in the cadmiumplated channel of a VVR-K research reactor at the Tomsk Polytechnic Institute. The neutron flux gradient did not exceed 0.5% of the sample volume. Stepwise heating experiments were carried out in a quartz reactor with an external-heating furnace. The blank sample after 10 min heating at 1200 °C contained no more than 5  10 10 ncm3 of 40Ar. Argon purification was performed with Ti and ZrAl SAES getters. The measurement errors presented in the text, tables, and figures lie within ±1r. References Alekseev, D.V., Degtyarev, E.V., Kotov, A.B., Sal‘nikova, E.B., Tret‘yakov, A.A., Yakovleva, S.Z., Anisimova, I.V., Shatagin, K.N., 2009. Late Paleozoic subductional and collisional igneous complexes in the Naryn segment of the Middle Tien Shan (Kyrgyzstan). Doklady Earth Sciences 42, 760–763. Alexeiev, D.V., Cook, H.E., Buvtyshkin, V.M., Golub, L.Ya., 2009. Structural evolution of the Ural–Tian Shan junction: a view from Karatau ridge, South Kazakhstan. In: Schulmann, K., Scrivener, R., Lardeaux, J.M. (Eds.), Mechanics of Variscan Orogeny: a modern view on orogenic research. Comptes Rendus Geoscience, vol. 341 no. 2–3, pp. 287–297. Allen, M.B., Vincent, S.J., Wheeler, P., 1999. Late Cenozoic tectonics of the Kepingtage thrust zone: interactions of the Tien Shan and Tarim Basin, northwest China. Tectonics 18, 639–654. Allen, M.B., Alsop, G.I., Zhemchuzhnikov, V.G., 2001. Dome and basin refolding and transpressive inversion along the Karatau fault system, southern Kazakhstan. Journal of the Geological Society of London 158, 83–95. Bakirov, A.B., Maksumova, R.A., 2001. Geology and evolution of the Tien Shan lithosphere. In: Seltmann, R., Jenchuraeva, R. (Eds.), Paleozoic Geodynamics and Gold Deposits in the Kyrgyz Tien Shan. IGCP-373 Excursion Guidebook, International Association on the Genesis of Ore Deposits (IAGOD), London, pp. 7–16. Baksi, A.K., Archibald, D.A., Farrar, E., 1996. Intercalibration of 40Ar/39Ar dating standards. Chemical Geology 129, 307–324. Bazhenov, M.L., Chauvin, A., Audibert, M., Levashova, N.M., 1993. Permian and Triassic paleomagnetism of the south-west Tien Shan: the timing and mode of tectonic rotations. Earth Planet Science and Letters 118, 195–212. Belousova, E.A., Griffin, W.L., O‘Reilly, S.Y., 2006. Zircon crystal morphology, trace element signatures and Hf isotope composition as a tool for petrogenetic modelling: examples from eastern Australian granitoids. Journal of Petrology 47, 329–353. Biske, Y.S., 1995. Late Paleozoic collision of the Tarimskiy and Kirghiz-Kazakh paleocontinents. Geotectonics 29, 26–34. Biske, Yu.S., Seltmann, R., 2010. Paleozoic Tian-Shan as a transitional region between the Rheic and Urals-Turkestan oceans. Gondwana Research 17, 602– 613. Black, L.P., Jagodzinski, E.A., 2003. Importance of establishing sources of uncertainty for the derivation of reliable SHRIMP ages. Australian Journal of Earth Sciences 50, 503–512. Black, L.P., Kamo, S.L., Allen, C.M., Aleinikoff, J.N., Davis, D.W., Korsch, R.J., Foudoulis, C., 2003. TEMORA 1: a new zircon standard for U–Pb geochronology. Chemical Geology 200, 155–170. Burtman, V.S., 1964. Talas-Ferghana strike–slip fault (Tien Shan). Geological Institute, Academy of Sciences of the USSR, Transactions 104, Moscow, ‘‘Nauka’’, 143 pp. (in Russian). Burtman, V.S., Skobelev, S.F., Sulerzhitskiy, L.D., 1987. Talas-Fergana fault: recent displacements in the Chatkal region of Tien Shan. ‘‘Doklady’’ of Academy of Sciences of the USSR 296 (5), 1173–1176 (in Russian). Burtman, V.S., Skobelev, S.F., Molnar, P., 1996. Late Cenozoic slip on the TalasFerghana fault, the Tien Shan, Central Asia. GSA Bulletin 108, 1004–1021. Buslov, M.M., 2011. Tectonics and geodynamics of the Central Asian Foldbelt: the role of Late Paleozoic large-amplitude strike–slip faults. Russian Geology and Geophysics 52 (1), 52–71.

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