Isotope Lu–Hf composition of detrital zircon from paragneisses of the Sharyzhalgai uplift: evidence for the Paleoproterozoic crustal growth

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Isotope Lu–Hf composition of detrital zircon from paragneisses of the Sharyzhalgai uplift: evidence for the Paleoproterozoic crustal growth O.M. Turkina a,b,*, N.G. Berezhnaya c, V.P. Sukhorukov a,b a

V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia b Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia c A.P. Karpinsky All-Russian Research Geological Institute, Srednii pr. 74, St. Petersburg, 199106, Russia Received 10 April 2015; accepted 28 August 2015

Abstract We present results of study of the trace-element and Lu–Hf isotope compositions of zircons from Paleoproterozoic high-grade metasedimentary rocks (paragneisses) of the southwestern margin of the Siberian craton (Irkut terrane of the Sharyzhalgai uplift). Metamorphic zircons are represented by rims and multifaceted crystals dated at ~1.85 Ga. They are depleted in either LREE or HREE as a result of subsolidus recrystallization and/or synchronous formation with REE-concentrating garnet or monazite. In contrast to the metamorphic zircons, the detrital cores are enriched in HREE and have high (Lu/Gd)n ratios, which is typical of igneous zircon. The weak positive correlation between 176Lu/177Hf and 176Hf/177Hf in the zircon cores evidences that their Hf isotope composition evolved through radioactive decay in the closed system. Therefore, the isotope parameters of these zircons can give an insight into the provenance of metasedimentary rocks. The Paleoproterozoic detrital zircon cores from paragneisses, dated at ~2.3–2.4 and 2.0–1.95 Ga, are characterized by a wide range of εHf values C (from +9.8 to –3.3) and model age THf = 2.8–2.0 Ga. The provenance of these detrital zircons included both rocks with juvenile isotope parameters and rocks resulted from the recycling of the Archean crust with a varying contribution of juvenile material. Zircons with high C positive εHf values were derived from the juvenile Paleoproterozoic crustal sources, whereas the lower εHf and higher THf values for zircons suggest the contribution of the Archean crustal source to the formation of their magmatic precursors. Thus, at the Paleoproterozoic stage of evolution of the southwestern margin of the Siberian craton, both crustal recycling and crustal growth through the contribution of juvenile material took place. On the southwestern margin of the Siberian craton, detrital zircons with ages of ~2.3–2.4 and 1.95–2.0 Ga are widespread in Paleoproterozoic paragneisses of the Irkut and Angara–Kan terranes and in terrigenous rocks of the Urik–Iya graben, which argues for their common and, most likely, proximal provenances. In the time of metamorphism (1.88–1.85 Ga), the age of Paleoproterozoic detrital zircons (2.4–2.0 Ga), and their Lu–Hf isotope composition (εHf values ranging from positive to negative values) the paragneisses of the southwestern margin of the Siberian craton are similar to the metasedimentary rocks of the Paleoproterozoic orogenic belts of the North China Craton. In the above two regions, the sources of detrital zircons formed by both the reworking of the Archean crust and the contribution of juvenile material, which is evidence for the crustal growth in the period 2.4–2.0 Ga. © 2016, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: Paleoproterozoic; paragneisses; detrital zircon; Lu–Hf isotope composition; crustal growth; Sharyzhalgai uplift; southwestern margin of the Siberian craton

Introduction Metasedimentary complexes that underwent Paleoproterozoic granulite metamorphism are widespread in the exposed areas of the Siberian Platform basement: on the Aldan and Anabar Shields, in the Sharyzhalgai uplift, and in the Angara– Kan terrane of the Yenisei Ridge. The time of formation of

* Corresponding author. E-mail address: [email protected] (O.M. Turkina)

their protoliths was estimated mainly from the model Nd age, whose minimum values determine the probable lower limit for sedimentation. The model Nd age (TNd(DM) = 2.2–2.5 Ga) of terrigenous metasediments in the central part of the Aldan Shield (Chuga, Kholbolokh, Kyurikan, Idzhek, and Fedorovka strata) testifies to their formation in the Paleoproterozoic (Kovach et al., 1999). For the Fedorovka Formation, this is confirmed by the U–Pb zircon age (2006 ± 3 Ma) of volcanites associated with metasedimentary rocks (Velikoslavinsky et al., 2006). Paragneisses of the Khapchan Group of the Anabar

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O.M. Turkina et al. / Russian Geology and Geophysics 57 (2016) 1016–1026

Shield also have a model Nd age of 2.3–2.4 Ga and were metamorphosed at ~1.97 Ga (Rozen et al., 2000), which indicates that sedimentation was limited between 1.97 and 2.30 Ga. A wider range of the model Nd age was established for terrigenous rocks metamorphosed in the granulite facies in the southeastern part of the Irkut terrane of the Sharyzhalgai uplift (TNd(DM) = 2.4–3.1 Ga) (Turkina and Urmantseva, 2009) and in the Kan metamorphic complex of the Angara– Kan terrane (TNd(DM) = 2.4–2.8 Ga) (Nozhkin et al., 2008; Urmantseva et al., 2012). The first U–Pb dating (SHRIMP-II) of detrital zircons from paragneisses of these two regions showed that zircons in the southeast of the Irkut terrane have mostly an age of 2.3–2.4 and 2.0–1.95 Ga and the time of metamorphism is 1.85–1.86 Ga (Turkina et al., 2010). A similar period of deposition of sedimentary protoliths was established for paragneisses of the Angara–Kan terrane, based on the age of the youngest detrital zircons (2.0–1.94 Ga) and metamorphic rims (1.87–1.89 Ga) (Urmantseva et al., 2012). Thus, the Paleoproterozoic age of the protoliths of paragneisses in the Irkut and Angara–Kan terranes casts no doubt. The source provenance of terrigenous material and Paleoproterozoic detrital zircons is still under discussion. The Lu–Hf isotope composition of detrital zircons shows the contribution of juvenile and recycled crust to sedimentation and provides information about the provenances of detrital zircons. Note that granulite-facies metamorphism can influence on the trace-element and isotope compositions of zircons. In high-grade metamorphic rocks, the Hf isotope composition of metamorphic zircons is either inherited from magmatic (detrital) ones or becomes more radiogenic owing to the exchange with Hf isotopes with coexisting matrix minerals having higher Lu/Hf and 176Hf/177Hf ratios (Chen et al., 2010; Gerdes and Zeh, 2009). Indicative features of metamorphic zircons are different degrees of depletion in REE and/or geochemical evidence for the influence of cocrystallizing phases (garnet and monazite), expressed as depletion in LREE or HREE, respectively (Fedotova et al., 2008; Harley et al., 2007; Hoskin and Schaltegger, 2003; Kelly and Harley, 2005; Rubatto, 2002; Turkina et al., 2009, 2012a). These compositional features of metamorphic zircon should be taken into account on the interpretation of isotope-geochronological data for granulite-facies rocks. To estimate the provenances of Paleoproterozoic metasedimentary rocks, we studied the Lu–Hf isotope composition of detrital zircons from orthopyroxene–biotite and garnet–cordierite–biotite paragneisses sampled in the southeast of the Irkut terrane of the Sharyzhalgai uplift. Since the sedimentary protoliths of these rocks underwent granulite-facies metamorphism, which might have changed the Lu–Hf isotope characteristics of zircons, we determined the trace-element composition of the zircons, sensitive to the influence of high-temperature metamorphism. The obtained results give an insight into the Paleoproterozoic processes of crustal growth and recycling in the southwest of the Siberian craton.

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Composition and age of the rocks of granulite–gneiss complex of the Irkut terrane The Irkut terrane is located in the southeastern part of the Sharyzhalgai uplift (superterrane) (Fig. 1, inset). It is separated from the Kitoi granulite–gneiss block by N–S striking zone of the Kitoi fault. The Irkut terrane consists of predominant granite- and charnockite–gneiss domes and strongly deformed interdomal zones (Fig. 1) (Grabkin and Mel’nikov, 1980). This structure resulted from the Paleoproterozoic collisional folding, metamorphism, and granite formation (Aftalion et al., 1991; Hopgood and Bowes, 1990). Based on the rock assemblages, the genesis of metamorphic rocks, and the age of their protoliths, we recognized two associations in the southeast of the Irkut terrane (Nozhkin and Turkina, 1993; Turkina et al., 2012a). The first, metamagmatic, association includes orthopyroxene-bearing gneisses (intermediate and felsic granulites) and mafic two-pyroxene and amphibole–pyroxene granulites, which either alternate or form separate layers. The second, metasedimentary, association is dominated by cordierite-bearing garnet–biotite, orthopyroxene–biotite, and garnet–orthopyroxene paragneisses. The sedimentary nature of gneisses of the second association is evidenced from their spatial co-occurrence or, sometimes, alternation with metacarbonate rocks (marbles and calciphyres). The igneous protoliths of mafic, intermediate, and felsic granulites of the first association formed in the subduction-related setting on the margin of an older continental block at ~2.7 Ga (Turkina et al., 2012a). Formation of the Irkut terrane crust began in the Paleoarchean, as evidenced from rare outcrops of biotite–orthopyroxene and two-pyroxene granulites of intermediate composition with zircons dated at ~3.4 Ga (Poller et al., 2005; Turkina et al., 2011). The model Nd age (TNd(DM) = 2.9–3.3 Ga) of intermediate and felsic Neoarchean granulites and the model Hf age of their igneous C = 3.0–3.3 Ga) testify to the contribution of the zircons (THf Paleoarchean crust to their genesis and, hence, to the presence of a widespread old basement (Turkina, 2010; Turkina et al., 2012a). The protoliths of terrigenous metasediments of the second association varied in composition from graywackes to pelites (Turkina and Urmantseva, 2009). The ages of the detrital zircon cores from paragneisses are 2.7, ~2.2–2.4, and 1.95–2.0 Ga, and the metamorphic rims are dated at ca. 1.85 Ga. Therefore, the deposition of sedimentary protoliths was limited between 1.85 and 1.95 Ga (Turkina et al., 2010). The rocks of the Irkut terrane were twice subjected to high-temperature metamorphism and granitoid magmatism, at 2.55 and 1.85 Ga (Aftalion et al., 1991; Poller et al., 2005; Sal’nikova et al., 2007; Turkina et al., 2012a).

Analytical methods Trace elements in zircon were determined by secondary-ion mass spectrometry (ion probe) with a Cameca IMS 4F in the Yaroslavl Branch of the Institute of Physics and Technology, following the procedure described by Fedotova et al. (2008).

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Fig. 1. Schematic geological map of the southeastern part of the Irkut terrane of the Sharyzhalgai uplift (Turkina et al., 2010). 1, Quaternary deposits; 2, Jurassic sedimentary deposits; 3, Sharyzhalgai complex (undivided); 4, 5, Sharyzhalgai complex: 4, predominant biotite, orthopyroxene–biotite, and biotite–amphibole gneisses and mafic and moderately felsic granulites, 5, garnet–biotite, biotite, garnet–cordierite, and hypersthene–biotite paragneisses; 6, Early Precambrian gabbroids; 7, Early Precambrian granitoids; 8, faults; 9, Circum-Baikal Railroad; 10, sampling localities. Inset shows the structure of the Sharyzhalgai uplift. Terranes: I, Bulun; II, Onot; III, Kitoi; IV, Irkut.

The determination accuracy was <10% for contents of >0.1 ppm and 30–50% for those less than 0.1 ppm. Rare-earth elements in standard zircon 91500 were determined for control. The Lu–Hf isotope composition of zircon was determined by LA–ICP-MS with a 193 nm COMPex 102 ArF laser, a DUV-193 ablation system, and a Thermo Finnigan Neptune ICP multicollector mass spectrometer at the Center of Isotopic Research, following the procedure described by Griffin et al. (2000). The configuration of the collectors permitted the simultaneous recording of 172Yb, 174Yb, 175Lu, 176Hf, 177Hf, 178Hf, and 179Hf isotopes. Correction for mass discrimination was made using one normalizing ratio (178Hf/177Hf). An accurate 176 Hf value was obtained by subtracting 176Yb and 176Lu 172 ( Yb and 175Lu free of superposition were measured). Lu–Hf isotope analysis was carried out for the same spots as U–Pb dating, but the spot diameter was ~50 μm and the crater depth was 20–40 μm. Through the measurement period, the average 176 Hf/177Hf values for the zircon standards were 0.282701 ± 0.000035 (TEMORA, n = 6), 0.282497 ± 0.000027 (Mud Tank, n = 5), and 0.282009 ± 0.000023 (GJ-1, n = 7). During the data processing, the decay constant of 176Lu was taken to be 1.865 × 10–11 yr–1 (Scherer et al., 2001). The Hf values were calculated using of the chondrite values: 176 Lu/177Hf = 0.0332 and 176Hf/177Hf = 0.282772 (BlichertToft and Albarede, 1997). The Hf model age was determined with respect to depleted mantle (DM) with 176Lu/177Hf = 0.0384 and 176Hf/177Hf = 0.28325 (Chauvel and Blichert-Toft, 2001). As the model age of zircon (THf(DM)) is the minimum age of the source of the melt, a more realistic two-stage age C was calculated by projecting the initial 176Hf/177Hf value THf

of zircon onto the line of depleted mantle, with the use of the average crustal value of 176Lu/177Hf = 0.015 (Griffin et al., 2000).

Composition of metasedimentary rocks and conditions of metamorphism Metasedimentary rocks in the southeast of the Irkut terrane contain mineral assemblages of granulite and uppermost amphibolite facies of metamorphism: Grt + Crd + Sill + Bt + Kfs + Pl + Qtz, Grt + Crd + Opx + Bt + Kfs + Pl + Qtz, and Grt + Opx + Bt + Pl + Kfs1. Based on the compositions of rock-forming minerals, the estimated peak PT-conditions of metamorphism are 800 ºC and 6–7 kbar (Sukhorukov, 2013). The reaction mineral microtextures evidence that after the peak PT-conditions, paragneisses underwent retrograde, mainly decompression, metamorphism (Sukhorukov, 2013). The temperatures of metamorphism of the studied rutile- or ilmenite-bearing paragneiss samples were estimated from their metamorphic rims and multifaceted zircon, using Ti-in-zircon geothermometers (Ferry and Watson, 2007; Watson et al., 2006). The obtained temperatures are within 780–816 and 692–860 ºC for zircons from garnet–cordierite–biotite (sample 118-87) and orthopyroxene–biotite (sample 28-84) paragneisses, respectively. The major- and trace-element compositions of paragneisses in the southeast of the Irkut terrane and their Sm–Nd isotope composition were earlier described in detail by Turkina and Urmantseva (2009); therefore, we will describe them briefly. 1

The mineral symbols are given after Kretz (1983).

O.M. Turkina et al. / Russian Geology and Geophysics 57 (2016) 1016–1026

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Table 1. Contents of major and trace elements in paragneisses of the Irkut terrane Component

28-84

187-87

SiO2, wt.%

67.98

53.03

TiO2

0.58

1.01

Al2O3

14.9

21.72

Fe2O3t

5.05

11.84

MnO

0.08

0.15

MgO

1.68

4.62

CaO

3.63

0.93

Na2O

2.8

1.24

K2O

2.55

5.32

P2O5

0.18

0.07

LOI

0.57

0.22

Total

100.02

100.15

Th, ppm

12.8

22

U

1.02

1.9

Rb

66

124

Ba

714

1125

Sr

131

208

La

38

58

Ce

71

109

Pr

8.5

13.5

Nd

30

48

Sm

5

9

Eu

1.12

1.83

Gd

4.1

8.7

Tb

0.55

1.34

Dy

2.6

8.4

Ho

0.49

1.72

Er

1.37

5.3

Tm

0.2

0.88

Yb

1.3

5.7

Lu

0.19

0.82

Zr

209

180

Hf

6.3

5.8

Ta

0.54

1.03

Nb

7.1

15.2

Y

13.4

50

Cr

56

406

Ni

18

115

Co

11

28

Sc

11.5

32

(La/Yb)n

19.7

6.9

Eu/Eu*

0.73

0.62

Note. Fe2O3t, Total iron; Eu/Eu* = Eun/((Smn + Gdn)⋅0.5); n, chondrite-normalized (Boynton, 1984) ratio.

Table 1 presents the contents of major and trace elements in the studied rock samples. The paragneisses show a wide variation in contents of SiO2 (53–72 wt.%) and Al2O3 (12.0–21.9 wt.%) and correspond in major-element composition to graywackes and pelites (Turkina and Urmantseva, 2009). These rocks are characterized by moderately to strongly fractionated REE patterns ((La/Yb)n = 4–20) with a distinct

Fig. 2. Nd–εNd diagram for Paleoproterozoic paragneisses and Neoarchean granulites of the Irkut terrane. Paragneisses: 1, garnet–biotite and orthopyroxene–biotite; 2, garnet–cordierite–biotite; Neoarchean granulites: 3, intermediate and felsic, 4, mafic.

negative Eu anomaly (Eu/Eu* = 0.4–0.8). The Eu minimum indicates a contribution of felsic igneous rocks to the formation of sedimentary protoliths. Since granitoids with a negative Eu anomaly are mostly products of intracrustal melting, the formation of terrigenous sediments must have been followed up metamorphism and granite formation. This is in accord with high-temperature metamorphism and granite magmatism in the Irkut terrane at 2.56–2.54 Ga (Sal’nikova et al., 2007; Turkina et al., 2012a). The studied paragneisses are characterized by a wide variation in model age, TNd(DM) = 2.4–3.1 Ga (Turkina and Urmantseva, 2009). The TNd(DM) value of studied garnet– cordierite–biotite and orthopyroxene–biotite paragneisses is 2.5 and 2.7 Ga, respectively. Comparison of the εNd values and Nd contents in the paragneisses and Neoarchean mafic, intermediate, and felsic granulites of the Irkut terrane shows (Fig. 2) that the Neoarchean granulites could not be the only source rocks. Paragneisses, which are characterized by higher εNd values than the felsic granulites, required the contribution of material with a more radiogenic Nd isotope composition, i.e., rocks of juvenile crust.

Trace-element and Lu–Hf isotope compositions of zircon from paragneisses Zircons from the studied paragneisses show distinct core– rim structure with detrital or rounded cores and rims, which are dark and unzoned in cathodoluminescence (Cl) images. The rims and multifaceted zircon are a metamorphic generation dated at ~1.85 Ga (Turkina et al., 2010). The U–Pb dating revealed two main groups of detrital zircon cores (with ages of 2.3–2.4 and 1.95–2.0 Ga) and rare ca. 2.7 Ga zircon grains in garnet–cordierite–biotite paragneiss (sample 118-87) (Turkina et al., 2010). All detrital zircon

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Table 2. Trace element analyses (ppm) of zircons from paragneisses of the Irkut terrane Component Garnet-cordierite-biotite paragneiss (sample 118-87) 10.2c

17.1c

6.1c

15.1c

6.2r

17.2r

15.2r

1.2r

14.2r

16.2r

19.1r

4.1ca

13.1ca

(2664)

(2347)

(2301)

(1997)

(1857)

(1831)

(1826)

(1853)

(1805)

(1815)

(1868)

(1894)

(1925)

La

0.26

0.84

5.76

0.28

0.22

1.53

0.17

0.21

0.14

0.1

0.08

0.16

0.64

Ce Pr Nd Sm Eu Gd Dy Er Yb Lu Y Th U Hf Ti

7.9 0.49 5.8 7.1 0.46 26.4 65.6 108 179 29.5 686 42 357 6598 16.8

19.06 1.03 7.05 4.26 0.82 18.6 72.7 172 328 55.4 999 79 287 8005 19.0

44.6 3.61 21.5 6.9 0.37 14.7 47.4 89 157.4 25.9 551 50 166 7599 15.4

2.4 0.07 0.5 1 0.12 9.1 67.3 113 154 23.8 939 35 826 10048 13.6

0.6 0.05 0.2 0.5 0.12 4.7 48.3 91 145 23.2 772 4 642 10131 15.5

12.2 1.83 12.3 7.3 1.25 23.3 20.9 10 13.7 2.6 131 33 940 9403 16.9

3.2 0.22 3.3 4.4 0.22 13.9 26.5 26.9 31.1 4.4 290 81 453 9949 19.9

2 0.16 3.2 6 0.27 33.7 67.5 44.8 52.8 9.5 478 41 955 9270 17.6

2.6 0.12 2.6 4.5 0.09 14.23 22 15.1 21 5.6 215 77 533 9774 21.3

3.3 0.12 2.4 3.6 0.02 7.8 7.7 4.3 6.7 0.8 58 99 422 8374 22.5

3.3 0.17 3.7 5.4 0.04 13.7 14.1 8.4 9.2 1.7 102 110 440 8735 22.1

3.5 0.23 4.4 4.1 0.12 9.2 11.6 9.2 12.7 2 90 89 381 9097 18.8

7.8 1.02 8.5 6.6 0.36 14.9 15.9 9.7 12.9 2.1 120 78 444 8984 21.1

(Lu/Gd)n Eu/Eu* Ce/Ce* Lu/Hf Th/U T, °C

9.0 0.10 5.3 0.0045 0.12 –

24.0 0.28 4.9 0.0069 0.28 –

14.2 0.11 2.4 0.0034 0.30 –

21.0 0.12 4.1 0.0024 0.04 –

39.7 0.24 1.4 0.00229 0.01 794

0.9 0.29 1.8 0.00028 0.04 802

2.5 0.09 3.9 0.00044 0.18 818

2.3 0.06 2.6 0.00102 0.04 806

3.2 0.03 4.8 0.00057 0.14 825

0.8 0.01 7.3 0.00010 0.23 831

1.0 0.01 6.8 0.00019 0.25 829

1.7 0.06 4.4 0.00022 0.23 813

1.1 0.11 2.3 0.00023 0.18 825

Component

Orthopyroxene-biotite paragneiss (sample 28-84) 12.1c

17.1c

8.1c

17.2r

12.2r

IIr

6.1r

5.2r

IIIr

(2017)

(1977)

(1915)

–

(1850)

–

(1926)

(1899)

–

La Ce Pr Nd Sm Eu Gd Dy Er Yb Lu Y Th U Hf Ti

0.24 9.1 0.34 5.7 8.9 1.09 50 179.5 336 517 81.2 2118 60 151 6778 –

0.14 8.9 0.19 2.3 3.9 0.72 28.4 138 309 504 81.5 1799 69 212 6480 5.9

0.58 29.9 1.26 7.9 3.7 0.47 15.6 66 153 296 47.7 921 206 417 6574 9.2

0.23 6 0.2 1.2 1 0.08 6.1 38.1 125 330 59.9 667 89 1014 10330 5.0

0.31 19.3 0.14 1.3 2.9 0.39 20.4 118.5 268 461 75.3 1667 170 353 9419 6.8

0.11 11.1 0.05 0.6 1.4 0.07 10.4 66.1 179 348 56 1031 51 259 8372 3.9

0.31 10 0.18 3.3 3.3 0.36 15.2 40.7 53.5 75.8 11.5 430 59 268 8190 21.1

0.11 2.5 0.06 1.2 2.7 0.05 10.4 19.2 15.9 16.6 2.5 173 61 264 8988 10.5

0.09 3.9 0.06 1.4 2.3 0.04 6.5 12.5 23.9 47.8 8.5 158 103 254 7789 18.4

(Lu/Gd)n Eu/Eu* Ce/Ce* Lu/Hf Th/U T, °C

13.1 0.16 7.7 0.0120 0.40 –

23.1 0.21 13.1 0.0126 0.33 –

24.6 0.19 8.4 0.0073 0.49 –

79.0 0.10 6.7 0.0058 0.09 715

29.7 0.16 22.3 0.0080 0.48 742

43.3 0.06 36.0 0.0067 0.20 692

6.1 0.16 10.2 0.0014 0.22 860

1.9 0.03 7.4 0.0003 0.23 784

10.5 0.03 12.8 0.0011 0.41 845

⎯⎯⎯⎯⎯⎯⎯⎯ ⎯⎯⎯⎯⎯⎯⎯ Note. Eu/Eu* = Eun/√ Smn ⋅ Gd⎯n , Ce/Ce* = Cen/√ Lan ⋅ Prn ; n, chondrite-normalized (Boynton, 1984) ratio; c, detrital core; ca, altered core; r, metamorphic rim. Temperatures were calculated by the thermometer described by Watson et al. (2006) (sample 118-87) and Ferry and Watson (2007) (sample 28-84). Parenthesized is the age (Ma).

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Fig. 3. Chondrite-normalized REE patterns of zircons from garnet–cordierite–biotite gneiss (sample 118-84). Chondrite values are from (Boynton, 1984). a, Core (c, core; ca, altered cores); b, rims (r). The point numbers follow Table 2.

cores, independently of their age, show similar REE patterns with enrichment in HREE (Ybn = 740–1570), high (Lu/Gd)n values (9–24), and positive Ce and negative Eu anomalies (Table 2; Fig. 3a). This REE pattern is typical of igneous zircon (Hoskin and Schaltegger, 2003). Zircon with an age of ~2.7 Ga has a lower (Lu/Gd)n ratio, equal to 9 (Table 2, grain 10.2), which might be due to its alteration during metamorphism. Two cores (Fig. 3a, grains 6.1 and 17.1) are enriched in LREE, which is due to the presence of monazite microinclusions identified by scanning microscopy. The detrital zircon cores have Hf = 6600–10,050 ppm and the average Lu/Hf of 0.0043. Compared with the cores, the metamorphic zircon rims are depleted in HREE (Ybn = 32–253) and are characterized by a flat HREE pattern with low (Lu/Gd)n values (0.8–3.2) (Fig. 3b). The high contents of Hf (8370–9950 ppm) in the rims, along with the HREE depletion, lead to a drastic decrease in Lu/Hf (0.0001–0.001). The only exclusion is one rim (Table 2, grain 6.2r) with high values of (Lu/Gd)n (39.7) and Lu/Hf (0.0023) but extremely low value of Th/U (0.01). Two zircon grains have ages (1.89 and 1.92 Ga) intermediate between the ages of the cores and rims and are also depleted in HREE (Ybn = 61–62) and have low values of (Lu/Gd)n (1.1–1.7) and Lu/Hf (0.00022–0.00023) (Fig. 3a, grains 4.1 and 13.1). Their younger (compared with the other cores) age is due to the loss of Pb, accompanied by the depletion in HREE, which probably resulted from alteration during metamorphism. In the orthopyroxene–biotite gneiss (sample 28-84), detrital zircon cores with an age of 2.0–1.9 Ga are predominant, and zircons with an age of ~2.3 and 2.75 Ga are scarcer (Turkina et al., 2010). Zircons of the first group, like igneous zircons (Hoskin and Schaltegger, 2003), are enriched in HREE (Ybn = 1415–2470) and are characterized by a high (Lu/Gd)n value (13–25), Ce and Eu anomalies, moderately high contents of Hf (6480–6780 ppm), and moderately high Lu/Hf ratios (0.0073–0.0126) (Table 2; Fig. 4, grains 12.1, 17.1, and 8.1). These compositional features of the zircon cores indicate their magmatic origin.

Compared with the cores, most of the dark and unzoned in CL rims are depleted mainly in LREE and MREE (Fig. 4, grains 17.2, 12.2, and II) and, consequently, have higher (Lu/Gd)n (30–79) and somewhat lower Lu/Hf (0.0058–0.0080) values. Only three rims (Fig. 4, grains 6.1, 5.2, and III) are depleted both in LREE and HREE (Ybn = 79–363) and are characterized by low values of (Lu/Gd)n (2–10) and Lu/Hf (0.0003–0.0014), like the zircon rims from garnet–cordierite– biotite paragneiss. The Lu–Hf isotope composition was determined mainly in the detrital zircon cores lacking signs of the influence of metamorphism on their REE patterns. The results are given in Table 3 and in Figs. 5 and 6. The zircon cores show a wide range of 176Lu/177Hf values (0.000067–0.001830), which do not correlate with their age. The 176Hf/177Hf values of the zircons also vary over a wide range, and two age groups of the Paleoproterozoic zircons differ little in isotope composition (Fig. 5a). The zircons with an age of 2.2–2.4 Ga are characterized by only positive εHf values (from +8.6 to 0), and those with an age of 1.94–2.0 Ga show a wider range of εHf values ranging from +9.8 to –3.3 (Fig. 6). The only 2.76 Ga grain shows εHf = –5.4. Note that scarce detrital

Fig. 4. Chondrite-normalized REE patterns of zircons from orthopyroxene–biotite gneiss (sample 28-84). c, Cores, r, rims. The point numbers follow Table 2.

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Table 3. Lu-Hf isotope composition of detrital zircon cores from paragneisses of the Irkut terrane T, Ma

176

176

±σ

176

±σ

εHf

±2σ

C , Ma THf

23-1

2411

0.01028

0.000475

0.000002

0.28138

0.00003

3.9

0.9

2693

22-1

2351

0.02351

0.001041

0.000005

0.28136

0.00002

1.2

0.8

2813

17-1

2347

0.00612

0.000267

0.000010

0.28154

0.00003

8.6

1.0

2352

6-1

2301

0.00943

0.000408

0.000001

0.28135

0.00003

0.5

1.0

2814

Point

Yb/177Hf

Lu/177Hf

Hf/177Hf

Sample 118-87

11-1

2297

0.00982

0.000546

0.000007

0.28137

0.00002

1.0

0.8

2784

5-1

2163

0.02067

0.000893

0.000001

0.28144

0.00002

0

0.8

2740

15-1

1997

0.01728

0.000725

0.000008

0.28157

0.00002

1.2

0.7

2541

3-1

1987

0.03319

0.001457

0.000014

0.28149

0.00003

–3.0

1.0

2787

24-1

1968

0.01851

0.000776

0.000005

0.28167

0.00003

4.0

1.0

2346

9-1

1945

0.02412

0.001179

0.000075

0.28174

0.00003

5.4

1.0

2241

14-1

2758

0.01130

0.0005190

0.0000032

0.28090

0.00002

–5.4

0.6

3525

18-1

2283

0.03630

0.0015087

0.0000062

0.28157

0.00002

6.3

0.7

2446

12-1

2017

0.02863

0.0011622

0.0000049

0.28147

0.00002

–2.6

0.6

2786

15-1

2007

0.01504

0.0006507

0.0000014

0.28144

0.00003

–3.3

0.9

2824

9-1

1993

0.00203

0.0000674

0.0000008

0.28147

0.00002

–1.6

0.9

2707

17-1

1977

0.04340

0.0018146

0.0000072

0.28187

0.00003

9.8

1.1

1990

10-1

1968

0.02796

0.0011312

0.0000184

0.28175

0.00003

6.3

1.1

2201

4-1

1961

0.03272

0.0018271

0.0000483

0.28163

0.00002

1.1

0.7

2519

Sample 28-84

11-1

1940

0.00864

0.0005445

0.0000295

0.28159

0.00003

0.8

1.1

2519

8-1

1915

0.01284

0.0006193

0.0000136

0.28154

0.00002

–1.7

0.8

2654

Note. T, Age of zircon, from measured = 0.015.

176Lu/177Hf

207Pb/206Pb;

C , two-stage model age (time of crustal extraction), calculated with the use of crustal average THf

zircons with an age of ≤1.95 Ga overlap by 176Hf/177Hf values with the other Late Paleoproterozoic cores (2.0–1.95 Ga) (Fig. 5a). Hence, their Lu–Hf isotope system was probably not disturbed, in spite of the loss of Pb during metamorphism.

Discussion The influence of metamorphism on the trace-element composition and Lu–Hf isotope characteristics of zircon.

All studied detrital zircon cores are characterized by high (Lu/Gd)n values (13–25) typical of igneous zircons (Hoskin and Schaltegger, 2003). In contrast, the HREE-depleted rims of zircon from garnet–cordierite–biotite gneiss show low (Lu/Gd)n values (0.8–3.2) specific for metamorphic zircons formed in equilibrium with garnet concentrating these elements (Fedotova et al., 2008; Hoskin and Schaltegger, 2003; Rubatto, 2002). The HREE depletion of the rims is responsible for the lower Lu/Hf values (≤0.001) as compared with the core values (Lu/Hf = 0.002–0.007). Since the garnet–cordierite–bi-

Fig. 5. T–176Hf/177Hf (a) and 176Lu/177Hf–176Hf/177Hf (b) diagrams for zircon cores from paragneisses. Zircons from: 1, garnet–cordierite–biotite paragneiss, 2, hypersthene–biotite paragneiss; 3, altered zircon cores.

O.M. Turkina et al. / Russian Geology and Geophysics 57 (2016) 1016–1026

otite gneiss is migmatized and contains a quartz–plagioclase leucosome, the high contents of HREE in one of the rims with an extremely low Th/U ratio (0.01) might be due to the zircon crystallization in the melt. Rubatto (2002) assumed that metamorphic zircon growing in equilibrium with partial melt does not differ in REE pattern from magmatic zircon but shows a low Th/U value (<0.07). This rim with high contents of HREE is strongly depleted in LREE (Cen < 1) and Th (4 ppm), which is probably due to the zircon crystallization together with monazite, the main concentrator of these elements. In orthopyroxene–biotite gneiss, containing garnet only as an accessory phase, some of the metamorphic rims are depleted in LREE and MREE rather than HREE; therefore, (Lu/Gd)n reaches 30–79. Hoskin and Black (2000) assumed that during metamorphism the subsolidus recrystallization of zircon can result in the depletion and removal of elements with a radius strongly different from that of Zr (i.e., mainly LREE and MREE). Another hypothesis for the LREE and MREE depletion of zircon is the competitive growth of monazite. The other rims, like those of the metamorphic zircon from garnet–cordierite–biotite gneiss, are depleted in HREE, which probably reflects their growth/recrystallization in equilibrium with garnet. Thus, the compositional variations of the metamorphic zircon point to the influence of different factors on its trace-element characteristics. Its LREE and HREE depletion might be due to subsolidus crystallization and/or formation together with minerals concentrating REE (garnet and monazite). All studied detrital zircon cores from paragneisses show REE patterns typical of igneous zircons and differ from the rims, whose growth and alteration occurred during high-temperature metamorphism. Despite the resetting of their U–Pb isotope system during metamorphism, magmatic zircons can preserve Lu–Hf isotope characteristics owing to the low rates of diffusion of these elements (Gerdes and Zeh, 2009; Zeh et al., 2009). In other cases, metamorphism can lead to a 176Hf/177Hf increase in zircon through hafnium isotopes exchange with the rock-forming minerals (first of all, garnet) and/or melt, which have a higher Lu/Hf ratio and, hence, a higher 176Hf/177Hf value (Chen et al., 2010; Zheng et al., 2005). The increase in 176 Hf/177Hf at constant or decreasing Lu/Hf as a consequence of the depletion in HREE leads to an inverse correlation between these parameters for zircons affected by metamorphism. In contrast, all studied detrital cores of zircons from paragneisses show a weak positive correlation between 176 Lu/177Hf and 176Hf/177Hf (Fig. 5b), which indicates that their Hf isotope composition evolved through radioactive decay in the closed system. In this case, the initial 176Hf/177Hf and εHf values of zircons of different ages depend only on the isotope composition of melt from which they crystallized and thus can be used as characteristics of the provenance rocks. Note that the detrital zircons from garnet–cordierite–biotite gneiss which underwent partial Pb loss do not differ in Hf isotope composition from other Late Paleoproterozoic zircons,

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Fig. 6. T–εHf diagram for zircons from Paleoproterozoic paragneisses and Archean granulites of the Irkut terrane. Paragneisses: 1, garnet–cordierite–biotite, 2, orthopyroxene–biotite; 3, Neoarchean granulite of intermediate and felsic composition; 4, Paleoarchean granulite; 5, garnet–two-pyroxene mafic granulite of the Angara–Kan terrane. Dash lines mark the field of isotope evolution of the Archean crust of the Irkut terrane at 176Lu/177Hf = 0.015.

i.e., the resetting of their U–Pb system was not accompanied by the change in Hf isotope composition (Fig. 5b). The sources of Paleoproterozoic detrital zircons: juvenile or recycled. The wide variations in isotope parameters of detrital zircons from paragneisses indicate the presence of source rocks with different crust prehistory. Most of the grains of the Paleoproterozoic zircons of two age groups are characterized by positive εHf values. Obviously, zircons with highest εHf values were supplied to the sedimentation basin during the erosion of the Paleoproterozoic juvenile crust. The C (2.0–2.8 Ga) values ranges of εHf (from +9.8 to –3.3) and THf in the detrital zircons suggest the contribution of the Archean crust to their source genesis. The contribution of the ancient continental crust to the formation of paragneiss protoliths is confirmed by the Nd model age of these rocks, reaching 3.1 Ga. The ancient continental crust, which contributed to the formation of terrigenous material and detrital zircons of paragneisses, might have been represented by the Archean complexes of the Irkut and other terranes of the Sharyzhalgai uplift. This suggestion agrees with the location of the points of Paleoproterozoic detrital zircons between the line of depleted mantle and the isotope evolution field of the Archean crust of the Irkut terrane (Fig. 6). The single zircon from paragneisses with an age of ~2.7 Ga is close in Hf model age C = 3.5 Ga) to the zircons from Paleoarchean orthopy(THf roxene-bearing granulites of intermediate composition from C = 3.3–3.5 Ga). Hence, the source of this the Irkut terrane (THf detrital zircon resulted from the recycling of the Paleoarchean crust without addition of juvenile material (Fig. 6). The Neoarchean (~2.7 Ga) intermediate and felsic granulites of the Irkut terrane were probably another older crustal component that participated in the formation of Paleoproterozoic detrital

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zircons. Their magmatic zircons are characterized by a model C = 3.0–3.2 Ga and ε of –0.8 to +2.6 (Turkina et al., age THf Hf 2012a). Thus, the isotope composition of detrital zircons shows that their provenance included both rocks with juvenile isotope characteristics and rocks resulted from the recycling of the Archean crust of the Irkut terrane with the variable contribution of Paleoproterozoic juvenile material. The igneous complexes that might have been the sources of Paleoproterozoic detrital zircons of paragneisses are still debatable. Magmatic and detrital zircons with an age of 2.2–2.4 Ga are extremely rare in the geologic history, and only some of them have juvenile Lu–Hf characteristics (Condie et al., 2005), which indicates the minor continental crustal growth in this period. Igneous rocks of this age have not been revealed within the exposed basement of the Siberian Platform. In general, rocks of the juvenile Paleoproterozoic crust or rocks formed with the participation of juvenile material are extremely rare in the exposed basement of the Siberian Platform. Only Sm–Nd isotope data are available for these complexes. Among such rocks are the intermediate and felsic volcanics of the Fedorovka Group in the Central terrane of the Aldan Shield, with TNd(DM) = 2.0–2.2 Ga and the zircon age of ~2.0 Ga (Velikoslavinsky et al., 2006); the 2.0 Ga granites of the Chuya complex, developed in the Baikal terrane of the Akitkan belt, with TNd(DM) = 2.3–2.4 Ga (Neimark et al., 1998); and the monzodiorites of the Billyakh complex of the Anabar Shield (1.98–1.97 Ga), with TNd(DM) = 2.2– 2.4 Ga (Smelov et al., 2012). According to the Nd model ages, these rocks might have been the sources of terrigenous material for the paragneisses of the Irkut terrane, with the minimum TNd(DM) value of 2.4 Ga. The Paleoproterozoic biotite–orthopyroxene and garnetbearing paragneisses of the Angara–Kan terrane, a basement uplift within the southwestern Siberian Platform (Urmantseva et al., 2012), contain few zircon cores dated at 2.2–2.4 Ga, like the zircons from paragneisses of the Irkut terrane. These high-grade metasedimentary rocks overlap with paragneisses of the Irkut terrane by the Nd model age (TNd(DM) = 2.4–2.8 Ga (Nozhkin et al., 2008; Urmantseva et al., 2012)), which points to the similarity of their provenances. Paragneisses of the Angara–Kan terrane contain subtabular and boudine-like bodies of mafic garnet–two-pyroxene granulites with magmatic zircons dated mainly at ~1.9 Ga, and the age of few cores, probably inherited from the paragneisses, is 2.3–2.4 Ga (Turkina and Sukhorukov, 2015; Turkina et al., 2012b). These Paleoproterozoic zircons from mafic granulites show both positive and negative εHf values and overlap with detrital zircons from the Irkut terrane paragneisses by Hf isotope composition (Fig. 6). In the lower part of section of the Urik–Iya graben located between the Sharyzhalgai and Biryusa uplifts, terrigenous rocks contain dominant detrital zircons with an age of 1.96 Ga and scarcer zircon grains with an age of ~2.3–2.2 Ga (Gladkochub et al., 2014). These sedimentary rocks are similar to paragneisses of the Irkut terrane both in the time of deposition (1.91–1.87 Ga) and in the age of detrital zircons. The above examples evidence that detrital zircons with ages of 2.3–2.4 and 1.95–2.0 Ga are

typical of Paleoproterozoic metasedimentary rocks on the southwestern margin of the Siberian craton, which argues for their common and, most likely, proximal provenances. The available information about the age and Sm–Nd isotope parameters of rocks and the Lu–Hf isotope composition of zircons from the Early Precambrian complexes shows that the continental crust of the Sharyzhalgai uplift formed in two main stages: Paleo- and Neoarchean (Gladkochub et al., 2009; Turkina, 2010; Turkina et al., 2012a, 2013, 2014a,b). The new data on the Lu–Hf isotope composition of detrital zircon from paragneisses of the Irkut terrane provide the first evidence for both the crustal recycling and the growth through the input of juvenile material during the Paleoproterozoic evolution of the southwestern part of the Siberian craton. In the recent decade, numerous isotope-geochronological data on the Paleoproterozoic metasedimentary rocks within the North China Craton have been obtained. Sedimentary deposits that underwent metamorphism in the period 1.88–1.85 Ga are widespread in three orogenic belts of the North China craton: Trans-North China Orogen, Khondalite Belt, and Jiao-Liao-Ji Belt (Wan et al., 2006). These metasedimentary rocks contain detrital zircons mainly of two age groups: 2.57–2.37 and 2.35–2.0 Ga, which indicates that their deposition occurred between 2.0 and 1.88 Ga (Wan et al., 2006). Paleoproterozoic metasedimentary associations of orogenic belts of the North China Craton are similar to paragneisses of the southwestern part of the Siberian Platform in the age of detrital zircons and the time of deposition. Detrital zircons with an age of 1.84–2.32 Ga in metasedimentary rocks of the Wulashan Complex in the Khondalite Belt are characterized by a wide range of εHf values (from –8 to +9), whereas ca. 2.0 Ga zircons from metaquartzites show positive εHf values (from +1 to +9) (Xia et al., 2006). This testifies to the contribution of juvenile material and the recycling of the more ancient crust in the provenance of Paleoproterozoic detrital zircons (Xia et al., 2006). Metasedimentary rocks of the Paleoproterozoic Gantaohe Group in the Trans-North China Orogen contain both predominant ancient detrital zircons (2.6–2.4 Ga) and Paleoproterozoic zircons with age of 2.4 to 1.9 Ga and negative εHf values; their sources resulted from the remelting of the Archean (3.2–2.6 Ga) crust (Liu et al., 2012). In contrast, the Paleoproterozoic (2.4–2.0 Ga) detrital zircons from sedimentary rocks of the Hutuo Group in the Trans-North China Orogen show mainly positive εHf values (from +8 to –4), which indicates that their sources resulted from the reworking of crust with an age of 2.35–3.0 Ga (Liu et al., 2011). In the metasedimentary rocks of the North Liaohe and South Liaohe Groups in the Jiao-Liao-Ji Belt, predominant detrital zircons with an age of 2.0–2.2 Ga are characterized by εHf between +9.2 and –5.7 (Luo et al., 2008). The predominance of zircons with positive εHf values suggests crustal growth through the input of juvenile material in the time interval 2.0–2.2 Ga (Luo et al., 2008). In summary, the above examples show that in the North China Craton, as in the Sharyshalgai uplift, the sources of detrital zircons from metasedimentary rocks of Paleoproterozoic orogens resulted not only from the recycling of the Archean crust but also from its growth. Thus, the isotope

O.M. Turkina et al. / Russian Geology and Geophysics 57 (2016) 1016–1026

data on Paleoproterozoic detrital zircons from metasedimentary rocks of the North China Craton and the southwestern part of the Siberian Craton argue for the growth of continental crust in the time interval 2.4–2.0 Ga, but the scales of this process are difficult to estimate.

Conclusions Zircons in garnet–cordierite–biotite and hypersthene–biotite paragneisses of the Irkut terrane (Sharyzhalgai uplift) have detrital cores of different ages. The metamorphic rims and multifaceted zircons have an age of ~1.85 Ga. The metamorphic zircons are depleted in LREE and HREE as a result of their subsolidus recrystallization and/or synchronous formation with REE-concentrating minerals, such as garnet and monazite. In contrast, the detrital cores are enriched in HREE and have high (Lu/Gd)n ratios, which is typical of igneous zircons. The weak positive correlation between 176Lu/177Hf and 176 Hf/177Hf in the cores proves that their Hf isotope composition evolved through radioactive decay in the closed system. Thus, the isotope parameters of these zircons can give an insight into the provenance of metasedimentary rocks. The Paleoproterozoic detrital zircons from paragneisses, dated at ~2.3–2.4 and 2.0–1.95 Ga, are characterized by a wide range of εHf values (from +9.8 to –3.3) and model age C 2.8–2.0 Ga. The provenance of these detrital zircons THf = included both rocks with juvenile isotope parameters and rocks resulted from the recycling of the Archean crust with a varying contribution of juvenile material. Zircons with high positive εHf values were derived from the juvenile Paleoproterozoic C values crustal sources, whereas the lower εHf and higher THf for zircons suggest the contribution of the Archean crustal sources to their formation. Thus, at the Paleoproterozoic stage of evolution of the southwestern margin of the Siberian craton, both crustal recycling and crustal growth at the expense of supplied juvenile material took place. On the southwestern margin of the Siberian craton, detrital zircons with ages of ~2.3–2.4 and 1.95–2.0 Ga are widespread in Paleoproterozoic paragneisses of the Irkut and Angara–Kan terranes and in terrigenous rocks of the Urik–Iya graben, which argues for their common and, most likely, proximal provenances. In the time of metamorphism (1.88–1.85 Ga), the age of Paleoproterozoic detrital zircons (2.4–2.0 Ga), and their Lu–Hf isotope composition (εHf varies from positive to negative values) the paragneisses of the southwestern margin of the Siberian craton are similar to the metasedimentary rocks of the Paleoproterozoic orogenic belts of the North China Craton. In the above two regions, the sources of detrital zircons formed by both the reworking of the Archean crust and the contribution of juvenile material, which is evidence for the crustal growth in the period 2.4–2.0 Ga. We thank I.N. Kapitonov and S.G. Simakin for isotope and trace-element analyses of zircons as well as A.D. Nozhkin and the anonymous reviewer for useful critical remarks. This work was supported by grants 15-05-02964 and 14-05-00373 from the Russian Foundation for Basic Research.

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