Early Proterozoic postcollisional granitoids of the Biryusa block of the Siberian craton

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ScienceDirect Russian Geology and Geophysics 55 (2014) 812–823 www.elsevier.com/locate/rgg

Early Proterozoic postcollisional granitoids of the Biryusa block of the Siberian craton T.V. Donskaya a,*, D.P. Gladkochub a, A.M. Mazukabzov a, M.T.D. Wingate b a

Institute of the Earth’s Crust, Siberian Branch of the Russian Academy of Sciences, ul. Lermontova 128, Irkutsk, 664033, Russia b Geological Survey of Western Australia, East Perth, WA 6004, Australia Received 25 December 2012; accepted 23 May 2013

Abstract Comprehensive geochemical and geochronological studies were carried out for two-mica granites of the Biryusa block of the Siberian craton basement. U–Pb zircon dating of the granites yielded an age of 1874 ± 14 Ma. The rocks of the Biryusa massif correspond in chemical composition to normally alkaline and moderately alkaline high-alumina leucogranites. By mineral and petrogeochemical compositions, they are assigned to S-type granites. The low CaO/Na2O ratios (<0.3), K2O ≈ 5 wt.%, CaO < 1 wt.%, and high Rb/Ba (0.7–1.9) and Rb/Sr (3.9–6.8) ratios indicate that the two-mica granites resulted from the melting of a metapelitic source (possibly, the Archean metasedimentary rocks of the Biryusa block, similar to the granites in εNd(t) value) in the absence of an additional fluid phase. The granite formation proceeded at 740–800 ºC (zircon saturation temperature). The age of the S-type two-mica granites agrees with the estimated ages of I- and A-type granitoids present in the Biryusa block. Altogether, these granitoids form a magmatic belt stretching along the zone of junction of the Biryusa block with the Paleoproterozoic Urik–Iya terrane and Tunguska superterrane. The granitoids are high-temperature rocks, which evidences that they formed within a high-temperature collision structure. It is admitted that the intrusion of granitoids took place within the thickened crust in collision setting at the stage of postcollisional extension in the Paleoproterozoic. This geodynamic setting was the result of the unification of the Neoarchean Biryusa continental block, Paleoproterozoic Urik–Iya terrane, and Archean Tunguska superterrane into the Siberian craton. Keywords: granites; U–Pb zircon age; geochemistry; Paleoproterozoic; Siberian craton

Introduction Within the southern marginal salients of the Siberian craton basement, there are abundant granitoids dated at 1.84– 1.88 Ga, which intruded at the final stages of the craton formation (Fig. 1). Larin et al. (2003) united these granitoids and rocks of the North Baikal volcanoplutonic belt into the South Siberian postcollisional magmatic belt. Biotite–amphibole granites, assigned to A-granites by geochemical features, are the most widespread among postcollisional granitoids (Donskaya et al., 2002, 2003, 2005; Larin et al., 2000, 2009; Levitskii et al., 2002; Nozhkin et al., 2003; Savel’eva and Bazarova, 2012; Turkina et al., 2003). They are present in almost all marginal salients of the southern Siberian craton. Besides A-type biotite–amphibole granites, granitoids of other geochemical types also formed within this area in the same period (Larin et al., 2006; Turkina, 2005; Turkina et al., 2006).

* Corresponding author. E-mail address: [email protected] (T.V. Donskaya)

These are, e.g., S-type two-mica granites (1846 ± 8 Ma) in the Tonod salient of the Siberian craton basement (Larin et al., 2006). The greatest diversity of Paleoproterozoic postcollisional granites is observed within the Biryusa block of the basement. Tonalites, I-type diorites, and A-type biotite–amphibole granites here were studied in detail (Turkina, 2005; Turkina et al., 2006). Levitskii et al. (2002) also examined A-type biotite–amphibole granites and, in addition, revealed two-mica granites in the Biryusa block, which they assigned to the same rock complex as the A-type granites. Nevertheless, these two-mica granites, in contrast to the biotite–amphibole ones, tonalites, and diorites of the block, have been poorly studied until recently. In this work we present results of geochronological and geochemical studies of two-mica granites of the Biryusa block of the Siberian craton. In addition, we generalize our and literature data on the Paleoproterozoic postcollisional granitoids of the Buryusa block and analyze the possible causes of the nearly synchronous formation of granites of widely varying compositions within a small site of the Siberian craton basement.

1068-7971/$ - see front matter D 201 4, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.201 + 4.06.002

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Fig. 1. Schematic geologic structure of the southern Siberian craton, modified after Didenko et al. (2003). 1, Central Asian Fold Belt; 2, Siberian Platform cover; 3, Riphean deposits of subsided craton sites; 4–6, rocks of the marginal salients of the Siberian craton basement: 4, Paleoproterozoic postcollisional granitoids; 5, Paleoproterozoic North Baikal volcanoplutonic belt; 6, united Archean–Paleoproterozoic metamorphic and igneous complexes; 7, major faults; 8, boundaries of blocks within the cis-Sayan marginal salient of the basement. Inset schematically shows major tectonic elements of the Siberian craton, modified after Gladkochub et al. (2006) and Rosen (2003). 1, superterranes; 2, Paleoproterozoic fold belts; 3, basement salients; 4, suture zones. Encircled numerals: 1, Angara Fold Belt; 2, Akitkan Fold Belt.

Geologic position of the Biryusa block granitoids Paleoproterozoic nonmetamorphosed granitoids are widespread within the cis-Sayan marginal salient of the Siberian craton basement, including the Biryusa block and the zone of its junction with the Urik–Iya terrane (Fig. 2). The Biryusa block is composed of Archean rocks of the Khailama and Monkres Groups discordantly overlain by Paleoproterozoic rocks of the Neroi and Subluk Groups (Belichenko et al., 1988; Dmitrieva and Nozhkin, 2012; Turkina et al., 2006). The Khailama Group includes gneisses (biotite, garnet–biotite, garnet–cordierite, biotite–hornblende), amphibolites, two-pyroxene schists, granulites, and migmatites. The age of the rock metamorphism is estimated at 1.9 Ga (207Pb/206Pb zircon dating of biotite gneisses) (Turkina et al., 2006). The Nd model age of the Khailama Group is 2.6–2.8 Ga (Turkina, 2005; Turkina et al., 2006). The Monkres Group is composed of mafic metavolcanics, amphibolites, gabbroids with interbeds of felsic volcanics, and quartzites (Belichenko et al., 1988). The Paleoproterozoic Neroi Group is formed mainly by metacarbonate-terrigenous deposits, and the Subluk Group, by metavolcanosedimentary deposits (Belichenko et al., 1988; Turkina et al., 2006). The Neroi Group rocks have a model Nd age of 1.9–2.7 Ga (Dmitrieva and Nozhkin, 2012).

The Paleoproterozoic nonmetamorposed granitoids intrude into the above-described Archean rocks and the Paleoproterozoic rocks of the Subluk Group and, together with them, are overlain by Neoproterozoic sedimentary rocks (Fig. 2). The relationships between the granitoids and the Paleoproterozoic Neroi Group rocks are still debatable. All Paleoproterozoic granitoids are post-tectonic postfolding rocks and belong to the Sayan complex subdivided into two phases. Levitskii et al. (2002) recognized the third phase composed of veined rocks. According to these authors, the first phase includes granodiorites, syenites, quartz syenites, granosyenites, quartz diorites, and subordinate pyroxene diorites and tonalites; the second phase is formed by biotite, amphibole–biotite, and two-mica granites. On the geological map (scale 1:1,500,000) (Yanshin, 1983), granites, granodiorites, granosyenites, and granite-gneisses are assigned to the first phase of the Sayan complex, and granites, granosyenites, syenites, aplites, and pegmatites, to the second phase. The legends of some geological maps (Rik et al., 1959) present the Sayan and Biryusa complexes, with the latter being regarded as a possible individual phase of the Sayan complex. Earlier, Levitskii et al. (2002) proposed a geological scheme of the location of the Sayan complex granitoids, where they recognized a group of garnet-bearing hypersthene–biotite granitoids composing the massifs along the Major Sayan Fault.

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Fig. 2. Schematic geologic structure of the cis-Sayan marginal salient of the Siberian craton, modified after Yanshin (1983). 1, Central Asian Fold Belt; 2, Phanerozoic deposits of the Siberian Platform cover; 3, Devonian volcanosedimentary rocks of superposed troughs; 4, Neoproterozoic sedimentary rocks of the cis-Sayan trough; 5, Paleozoic granitoids; 6, Paleoproterozoic postcollisional granitoids; 7, Paleoproterozoic rocks of the Urik–Iya terrane; 8, Paleoproterozoic–Archean rocks of the Biryusa block; 9, Archean rocks of the Sharyzhalgai salient of the Tunguska superterrane basement; 10, Major Sayan Fault; 11, Biryusa Fault (generalized line).

On other geological schemes (Yanshin, 1983), however, the granitoids of these massifs are considered Paleozoic (Fig. 2). At present, there are several U–Pb zircon dates for granitoids of the Sayan complex of the Biryusa block (Fig. 2). The age of biotite–amphibole granites of the Barbitai massif is 1858 ± 20 Ma (Levitskii et al., 2002); the age of tonalites of the Podporog massif is 1869 ± 10 Ma (Turkina et al., 2003); and the age of quartz diorites of the Uda massif is 1859 ± 10 Ma (Turkina et al., 2006). Up to now, the age of two-mica granites of the Sayan complex has not been determined. For study, we chose two-mica granites of the conditionally named Biryusa massif in the Biryusa block. The massif extends along the Biryusa River in its middle reaches (Fig. 3). The contacts of the massif granites with the host metamorphic rocks are overlain by Neoproterozoic sedimentary strata. The studied granites of the Biryusa massif are light gray mediumto coarse-grained rocks composed of plagioclase (35–38%), quartz (30–35%), K-feldspar(25–30%), biotite (3–4%), and

muscovite (1–2%). Muscovite occurs mainly as a primary mineral in the form of plates and flakes. Sericite pseudomorphs after plagioclase and chlorite pseudomorphs after biotite are observed in some varieties. Accessory minerals in the granites are zircon, apatite, and rutile.

Methods The representative samples of two-mica granites from the Biryusa massif were analyzed for major oxides and trace and rare-earth elements. One sample was studied by U–Pb zircon dating, and its Nd isotope composition was determined. The localities of sampling for petrogeochemical and geochronological studies are shown in Fig. 3. The contents of major oxides were determined by silicate analysis at the Institute of the Earth’s Crust SB RAS, Irkutsk (analysts N.N. Ukhova and N.Yu. Tsareva). The contents of Rb, Sr, Y, Zr, Nb, and Ba were measured by the X-ray

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fluorescent method at the Geological Institute SB RAS, Ulan Ude (analyst B.Zh. Zhalsaraev). The concents of rare-earth elements, Th, and U were determined by ICP MS on a VG Plasmaquad PQ-2 (VG Elemental, England) mass spectrometer, following the technique proposed by Garbe-Schonberg (1993), at the Center of Collective Use of the Irkutsk Scientific Center SB RAS (analysts S.V. Panteeva and V.V. Markova). The mass spectrometer was calibrated against the G-2 and GSP-2 International standard samples. Chemical decomposition of the samples for ICP MS analysis was performed by their fusion with LiBO2, following the technique described by Panteeva et al. (2003), which favored the total dissolution of all minerals. The error of the ICP MS determination of trace and rare-earth elements was ≤5%. The contents of Sm and Nd and the Nd isotope composition of sample 02100 were determined on a Finnigan MAT-262 (RPQ) mass spectrometer in the static regime at the Geological Institute of the Kola Scientific Center SB RAS, Apatity. The samples were prepared for isotope analysis following Bayanova’s (2004) technique. The blank sample contained 0.06 ng Sm and 0.3 ng Nd. The errors of determination of Sm and Nd concentrations and isotopic ratios were as follows: Sm and Nd—±0.2% (2σ), 147Sm/144Nd—±0.2% (2σ), and 143Nd/ 144 Nd—±0.003% (2σ). The measured 143Nd/144Nd values were normalized to 148Nd/144Nd = 0.241578, which corresponds to 146Nd/144Nd = 0.7219, and were reduced to 143 Nd/144Nd = 0.511860 in the La Jolla Nd-standard. The weighted average 143Nd/144Nd value in the La Jolla Nd-standard was 0.511833 ± 0.000014 (n = 11). U-Pb zircon dating of two-mica granites from the Biryusa massif (sample 02100) was performed on a SHRIMP-II ion microprobe at the Curtin University of Technology, Perth (Australia). The manually selected zircon grains were implanted into epoxy resin together with grains of the TEMORA zircon standard. Then, the preparation was polished and subjected to gold spraying. The spot (crater) diameter was 30 µm. The U/Pb ratios were normalized to the value of the TEMORA standard zircon (0.0668), corresponding to the zircon age of 416.75 Ma (Black and Kamo, 2003). The measured values were corrected for common lead based on the measured 204Pb content and in accordance with the model values (Stacey and Kramers, 1975). The age was calculated using the generally accepted uranium decay constants (Steiger and Jäger, 1977). The data obtained were processed using the SQUID (Ludwig, 2001) and ISOPLOT (Ludwig, 2003) programs. A diagram with a concordia was constructed according to Tera and Wasserburg (1972).

Isotope and geochemical characteristics of granites Two-mica granites of the Biryusa massif contain 71.9– 74.5 wt.% SiO2 and 7.3–8.5 wt.% alkalies (K2O/Na2O ≥ 1.4) (Table 1). By composition, these are normally alkaline and moderately alkaline leucogranites. According to the classification by Frost et al. (2001), the granites are both magnesian and ferroan (FeO*/(FeO* + MgO) = 0.73–0.86), mainly alkali-

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Fig. 3. Schematic geologic structure of the region in the middle reaches of the Biryusa River, modified after (State..., 2011). 1, Phanerozoic deposits of the Siberian Platform cover; 2, Neoproterozoic sedimentary rocks of the cis-Sayan trough; 3, Neoproterozoic dolerite dikes; 4, Paleoproterozoic postcollisional granitoids; 5, Early Precambrian rocks of the Biryusa block; 6, sampling localities.

calcic rocks (Na2O + K2O–CaO = 7.1–8.3). These are highalumina granites, with ASI varying from 1.17 to 1.49 (Table 1). In the above petrogeochemical features they are similar to the world high-alumina leucogranites (Frost et al., 2001). The granites have high contents of Rb (230–380 ppm) and Th (14–41 ppm), low contents of Sr (41–70 ppm), medium contents of Ba (160–390 ppm), Zr (60–150 ppm), Nb (10– 24 ppm), and Y (8–27 ppm) (Table 1), and ΣREE = 95– 288 ppm. They are characterized by a fractionated REE pattern ((La/Yb)n = 5.6–26.5) and a distinct negative Eu anomaly (Eu/Eu* = 0.16–0.37) (Fig. 4). By mineral and petrogeochemical compositions, the Biryusa massif granites are assigned to S-type granites according to the classification of Chappell and White (1974, 1992). Sm–Nd isotope studies were performed for two-mica granite from the Biryusa massif (sample 02100). The sample is characterized by εNd(1874 Ma) = –6.4 and a Neoarchean model age—tNdDM = 2.74 Ga (Table 2).

Results of geochronological U–Pb studies For dating granite from the Biryusa massif we used sample 02100. The sampling locality is shown in Fig. 3. The separated accessory zircon has fine euhedral and subeuhedral crystals

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Table 1. Chemical composition of two-mica granites of the Biryusa massif Component

01092

01093

01095

01096

01097

01098

01099

01100

01101

01102

01103

0298

0299

02100

SiO2, wt.%

74.53

71.86

72.84

73.22

71.97

73.59

73.60

74.38

73.72

73.56

74.24

73.84

73.47

72.92

TiO2

0.12

0.21

0.17

0.22

0.24

0.25

0.19

0.15

0.19

0.13

0.13

0.22

0.21

0.24

Al2O3

14.45

14.25

14.40

14.20

14.15

14.43

14.20

13.65

14.25

13.72

13.75

13.70

13.90

14.00

Fe2O3

0.31

0.21

0.38

N.f.

0.03

0.01

0.01

N.f.

0.06

0.06

0.00

0.28

0.42

0.36

FeO

1.11

2.20

1.28

2.46

2.78

2.33

2.56

1.80

1.77

2.53

1.95

1.64

1.65

1.77

MnO

0.03

0.05

0.05

0.01

0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

<0.01

MgO

0.51

0.45

0.51

0.63

0.69

0.53

0.42

0.40

0.63

0.67

0.45

0.45

0.66

0.53

CaO

0.19

0.81

0.43

0.29

0.26

0.24

0.22

0.07

0.07

0.10

0.16

0.62

0.42

0.47

Na2O

2.63

3.32

2.85

3.01

3.22

2.77

3.11

2.80

2.85

2.76

3.14

3.36

3.19

3.24 5.23

K2O

4.65

4.70

5.43

5.04

4.83

4.74

4.77

5.47

5.17

5.28

5.28

4.64

5.00

P2O5

0.07

0.10

0.09

0.09

0.08

0.25

0.09

0.06

0.09

0.05

0.08

0.06

0.07

0.07

H2O–

0.07

0.07

<0.01

0.15

0.25

0.05

0.23

0.22

0.18

0.03

0.13

<0.01

<0.01

0.20

LOI

1.22

1.24

1.22

1.18

1.41

1.26

1.09

1.00

1.05

1.43

0.87

0.77

1.33

1.27

CO2

0.06

0.11

<0.06

<0.06

<0.06

<0.06

<0.06

<0.06

<0.06

<0.06

<0.06

0.11

<0.06

<0.06

F

0.16

0.16

0.16

0.04

0.06

0.06

0.03

0.04

0.05

0.03

0.04

0.10

0.11

0.08

–O(F)

0.07

0.07

0.07

0.02

0.03

0.03

0.01

0.02

0.02

0.01

0.02

0.04

0.05

0.03

Total

100.04

99.67

99.74

100.52

99.96

100.49

100.51

100.02

100.06

100.34

100.20

99.75

100.38

100.35

Rb, ppm

260

350

380

310

290

320

310

270

270

230

230

280

280

280

Sr

54

62

57

58

60

54

64

70

67

58

57

59

41

41

Y

16

18

16

15

17

17

15

20

9

23

8

11

23

27

Zr

60

150

95

140

150

130

130

110

100

110

91

140

130

130

Nb

11

24

22

18

24

20

23

10

15

20

20

18

20

19 290

Ba

160

180

200

260

300

300

260

380

370

250

170

380

390

La

17.38

59.80

42.78

–

59.15

59.29

52.93

38.86

–

–

24.17

–

–

45.88

Ce

43.43

130.80

90.61

–

134.28

124.36

112.32

80.41

–

–

55.57

–

–

107.32

Pr

4.39

15.92

11.10

–

15.26

14.79

13.46

9.43

–

–

6.01

–

–

11.60

Nd

15.14

53.27

37.87

–

49.48

52.49

44.64

31.70

–

–

20.77

–

–

37.77

Sm

2.57

10.03

6.94

–

8.60

8.28

8.88

6.93

–

–

4.88

–

–

6.83

Eu

0.30

0.45

0.33

–

0.51

0.48

0.60

0.64

–

–

0.47

–

–

0.37

Gd

2.43

7.08

4.58

–

6.10

6.01

6.63

5.55

–

–

4.11

–

–

4.75

Tb

0.46

0.81

0.62

–

0.70

0.88

0.71

0.67

–

–

0.51

–

–

0.80

Dy

3.09

4.31

3.38

–

4.12

4.55

3.75

3.79

–

–

2.76

–

–

5.00

Ho

0.65

0.70

0.61

–

0.72

0.82

0.65

0.81

–

–

0.51

–

–

1.03

Er

1.98

1.99

1.61

–

2.03

2.13

1.62

2.32

–

–

1.33

–

–

2.85 0.53

Tm

0.34

0.27

0.21

–

0.29

0.24

0.20

0.30

–

–

0.18

–

–

Yb

2.08

2.02

1.12

–

2.09

1.50

1.38

2.23

–

–

1.27

–

–

2.86

Lu

0.36

0.24

0.20

–

0.29

0.25

0.21

0.32

–

–

0.18

–

–

0.40

Th

13.58

51.29

24.03

–

41.41

31.43

45.65

31.61

–

–

35.51

–

–

38.63

U

2.67

7.94

4.16

–

5.15

3.95

8.33

6.22

–

–

8.71

–

–

3.55

C, norm.

4.88

2.43

3.22

3.44

3.30

4.86

3.69

3.09

4.00

3.36

2.74

2.12

2.59

2.28

ASI

1.50

1.20

1.28

1.31

1.30

1.47

1.34

1.29

1.38

1.32

1.24

1.18

1.23

1.19

FeO*/(FeO* + MgO) 0.73

0.84

0.76

0.80

0.80

0.82

0.86

0.82

0.74

0.79

0.81

0.81

0.75

0.80

CaO/Na2O

0.07

0.24

0.15

0.10

0.08

0.09

0.07

0.03

0.02

0.04

0.05

0.18

0.13

0.15

Rb/Sr

4.8

5.6

6.7

5.3

4.8

5.9

4.8

3.9

4.0

4.0

4.0

4.7

6.8

6.8

Rb/Ba

1.6

1.9

1.9

1.2

1.0

1.1

1.2

0.7

0.7

0.9

1.4

0.7

0.7

1.0

Sr/Ba

0.3

0.3

0.3

0.2

0.2

0.2

0.2

0.2

0.2

0.2

0.3

0.2

0.1

0.1

Lan/Ybn

5.6

19.8

25.6

–

18.9

26.5

25.7

11.7

–

–

12.7

–

–

10.7

Eu/Eu*

0.37

0.16

0.18

–

0.22

0.21

0.24

0.32

–

–

0.32

–

–

0.20

Al2O3/TiO2

120

68

85

65

59

58

75

91

75

106

106

62

66

58

T, °C

737

793

761

796

800

797

792

775

772

776

756

788

784

781

Note. C, norm., Content of normative corundum; ASI(mol) = Al2O3/(CaO – 1.67 × P2O5 + Na2O + K2O); FeO* = FeO + 0.8998 × Fe2O3; Eu/Eu* = Eun/√  (Smn x Gdn) ; n, chondrite-normalized (Sun and McDonough, 1989) values; T, °C, temperatures at the initial stages of melt crystallization (zircon saturation temperatures) (Watson and Harrison, 1983). N.f., Not found. Dash, Not analysed.

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50 to 130 µm in size, with elongation of 1 :2 and 1 :3.5. The cathodoluminescence images show a distinct magmatic zoning in the zircon grains (Fig. 5). The results of ion microprobe analysis (SHRIMP-II) of eight zircon grains are presented in Table 3 and in Fig. 6. The contents of U and Th are 118–491 and 52–225 ppm, respectively. The 232Th/238U ratio varies from 0.46 to 0.83. On the U–Pb diagram with concordia (Fig. 6), five isotope composition points of the studied zircon lie on the concordia, and its concordant age is 1879 ± 8 Ma (MSWD = 0.45). The average 207Pb/206Pb age calculated over six points of zircon isotope composition is 1874 ± 14 Ma (MSWD = 2.0). Isotope composition points 2.1, 3.1, and 8.1 were ignored on the calculation of the concordant age, and points 3.1 and 8.1 were not involved in the calculation of the weighted average age (Table 3, Fig. 6). Exclusion of points 3.1 and 8.1 from all calculations was due to the strong discordance of obtained values. In accordance with the morphology of zircon, pointing to its magmatic genesis, the weighted average age of 1874 ± 14 Ma can be interpreted as the time of its crystallization and, correspondingly, as the age of the Biryusa massif granites.

Discussion Petrogenesis and formation conditions of two-mica granites of the Biryusa massif. The mineral composition of two-mica granites of the Biryusa massif and their geochemical features similar to those of S-type granites point to their metasedimentary nature. Experimental data showed that such granites can result from the melting of pelites or psammites (Harris and Inger, 1992; Miller, 1985; Patiño Douce and

Fig. 4. Chondrite-normalized (Sun and McDonough, 1989) REE patterns of two-mica granites of the Biryusa massif.

Johnston, 1991; Skjerlie and Johnston, 1996). One of the criteria for distinguishing between granites melted from compositionally different sources is the CaO/Na2O ratio reflecting different contents of plagioclase and clay minerals in pelites and psammites (Sylvester at al., 1998). The studied Biryusa massif granites show low CaO/Na2O ratios (<0.3), which indicates their pelitic source (Table 1, Fig. 7a) (Sylvester at al., 1998). The metapelitic composition of the source is also evidenced by the high-alumina composition of granites (normative corundum amounts to 2.12–4.88), K2O ≈ 5 wt.%, CaO < 1 wt.%, and high values of Rb/Ba (0.7–1.9) and Rb/Sr (3.9–6.8) (Table 1, Fig. 7b) (Miller, 1985; Sylvester, 1998). This is also confirmed by the arrangement of the figurative

Table 2. Sm-Nd isotope data for granites of the Biryusa massif Sample

Age, Ma

02100

Sm/144Nd

Content, ppm

1874

Sm

Nd

5.60

32.42

147

143

Nd/144Nd ±2σ

εNd(t)

tNd(DM), Ma

tNd(DM-2st), Ma

0.1045

0.511175 ± 14

–6.4

2745

2888

Table 3. Results of U-Pb analysis of zircons from two-mica granite of the Biryusa massif (sample 02100) Crystal, crater

238

U, ppm

232

Th, ppm Th/U

f204, %

Isotope ratios 238

206

U/ (1) 1.1

206

130

0.66

0.070

2.976

Pb*

Age, Ma (± 1σ) 207

206

± 1σ

Pb*/ (1)

0.034

0.11485

Pb*

238

206

± 1σ

U/ (1)

0.00055

1868 ± 19

Pb*

207

Pb*/ (1)

D, % 206

1877 ± 9

Pb*

0.5

2.1

195

91

0.48

0.190

2.986

0.035

0.11296

0.00064

1862 ± 19

1848 ± 10

–0.8

3.1

491

232

0.49

0.046

3.226

0.035

0.11324

0.00046

1741 ± 17

1852 ± 7

6.0

4.1

133

68

0.53

–0.016

2.988

0.036

0.11562

0.00063

1861 ± 19

1890 ± 10

1.5

5.1

161

77

0.50

–0.084

3.017

0.036

0.11494

0.00070

1845 ± 19

1879 ± 11

1.8

6.1

281

225

0.83

0.054

2.966

0.034

0.11484

0.00053

1873 ± 18

1877 ± 8

0.2

7.1

161

80

0.52

0.045

2.967

0.035

0.11450

0.00059

1872 ± 19

1872 ± 9

0.0

8.1

118

52

0.46

0.141

3.183

0.039

0.11285

0.00087

1761 ± 19

1846 ± 14

4.6

Note. f204, Portion of common

206Pb

in total measured

206Pb.

(1), corrected for common Pb, using measured

204Pb.

Pb*, Radiogenic Pb. D, Discordance.

818

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Fig. 5. Cathodoluminescence images of zircon crystals from two-mica granite of the Biryusa massif.

points of the analyzed granites on the Al2O3/(MgO + FeO*)– CaO/(MgO + FeO*) diagram (Altherr et al., 2000) (Fig. 7c). The behavior of Rb, Ba, and Sr in the high-alumina granites permits elucidation of not only the composition of their source (Fig. 7b) but also the crystallization conditions of the melt and the main mineral phases controlling its generation, because these elements are predominant in the major minerals of the granites: plagioclase, K-feldspar, and micas. According to the model calculations by Harris and Inger (1992), leucogranites resulting from the incongruent melting of muscovite (or muscovite and biotite) in metapelites in the absence of a fluid phase can contain K-feldspar, plagioclase, quartz, garnet, and sillimanite among the restite phases. If only muscovite undergoes melting in the source, then the restite can contain biotite as well. The melts thus produced show high Rb/Sr and low Sr/Ba ratios and a negative Eu anomaly. Melting in the presence of fluid, on the contrary, leads to low Rb/Sr and high Sr/Ba ratios, which is due to the absence of plagioclase and K-feldspar from the restite. Thus, the high Rb/Sr (3.8–6.8) (Table 1, Fig. 7b) and low Sr/Ba (0.1–0.3) (Table 1) values and negative Eu anomaly on the REE patterns of the granites (Fig. 4) evidence that the metapelitic-source melting proceeded in the absence of an additional fluid phase (Harris and Inger, 1992). Moreover, the above Rb/Sr and Sr/Ba ratios suggest the presence of both plagioclase and K-feldspar in the restite during the melting-out of two-mica granites of the Biryusa massif (Harris and Inger, 1992). The elevated contents of Y and HREE in the granites (Table 1, Fig. 4) indicate that garnet could not be the main restite phase during their formation. The negative correlation between the SiO2 and (FeO* + MgO + TiO2) contents (Fig. 7d) suggests the presence of biotite in the restite and, correspondingly, the melting of only muscovite in the metapelitic source. To sum it up, the two-mica granites of the Biryusa massif resulted from the

dehydration melting of muscovite in a metapelitic source in equilibrium with the restite containing plagioclase, K-feldspar, biotite, quartz, and sillimanite. The similar Nd isotope compositions of two-mica granites of the Biryusa massif (εNd(1874 Ma) = –6.4, tNdDM = 2.74 Ga) and Archean gneisses of the Khailama Group of the Biryusa block (εNd(1874 Ma) = –3.9 to –6.8, tNdDM = 2.57–2.81 Ga (Turkina, 2005; Turkina et al., 2006)) suggest that the latter rocks were the source of the former. The earlier viewpoint (Levitskii et al., 2002) that the two-mica granites belong to the second phase of the Sayan granitoid complex in the Biryusa block is not confirmed by the available isotope-geochemical data. The amphibole and biotite–amphibole granitoids assigned by the above authors to the first phase of the complex cannot be even potential sources of the two-mica granites because they are characterized by εNd(t) = –0.7 to –2.0 (Kirnozova et al., 2003). Moreover, the Biryusa massif granites resulted from the melting of a metapelitic source; therefore, they cannot be genetically related to amphibole and biotite–amphibole granites of the Sayan complex. The low Al2O3/TiO2 ratio (58–120, average 78) in the two-mica granites of the Biryusa massif testifies to the relatively high temperature of granite melts (Table 1, Fig. 7a) (Sylvester, 1998). This is confirmed by the calculations made with the zircon thermometer of Watson and Harrison (1983). It helps to estimate the degree of melt saturation with zircon depending on temperature and melt composition and the approximate temperatures at the initial stages of granitoid melt crystallization. The calculations yielded crystallization temperatures of 740–800 ºC (average 780 ºC) for the two-mica

Fig. 6. 207Pb*/206Pb*–238U/206Pb* diagram (Tera and Wasserburg, 1972) for zircons from two-mica granite of the Biryusa massif. Gray squares are the isotope composition points of zircons used for calculation of the weighted average age; black squares are the isotope composition points ignored on the calculation.

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819

Fig. 7. CaO/Na2O–Al2O3/TiO2 (Sylvester, 1998) (a), Rb/Ba–Rb/Sr (Sylvester, 1998) (b), Al2O3/(MgO + FeO*)–CaO/(MgO + FeO*) (Altherr et al., 2000) (c), and (FeO* + MgO + TiO2)–SiO2 (d) diagrams for two-mica granite of the Biryusa massif.

granites of the Biryusa massif, which are close to those of some A-type granites (Donskaya et al., 2005). The high contents of Y and Yb in the granites and, correspondingly, the absence of garnet as a restite phase indicate that the pressures during the formation of parental melts must have been <7 kbar (Patiño Douce and Johnston, 1991). Diversity of Paleoproterozoic granitoids in the Biryusa block of the Siberian craton and tectonic consequences. Within the Biryusa block, there are granitoids of different compositions that intruded nearly at the same time (Fig. 2): I-type tonalites and diorites (Turkina, 2005; Turkina et al., 2006), S-type two-mica granites (this paper), and A-type biotite–amphibole granites (Levitskii et al., 2002; Turkina et al., 2006). All these granites form a single magmatic belt stretching along the zone of junction of the Biryusa block with the Paleoproterozoic Urik–Iya terrane and Tunguska superterrane of the Siberian craton (Figs. 1 and 2). In petrogeochemical features the granites of different geochemical types differ from each other. The S-type twomica granites are ferroan and magnesian, mainly alkali-calcic rocks; I-type tonalites and diorites (Turkina, 2005; Turkina et al., 2006) are magnesian calcic or calc-alkalic rocks; and

A-type biotite–amphibole granites (Levitskii et al., 2002; Turkina et al., 2006) are ferroan alkali-calcic or alkalic rocks (Fig. 8a, b). The composition points of the Biryusa block granites are arranged on the FeO*/MgO–(Zr + Nb + Y + Ce) diagram (Whalen et al., 1987) in accordance with the geochemical types, forming individual fields (Fig. 9). Thus, three types of the Biryusa block granitoids are recognized by geochemical features. Here questions arise: How could such compositionally diverse granites form within a single structure nearly at the same time? Are there any distinctive features in their formation conditions? Let us dwell on the A-type biotite–amphibole granites described by Levitskii et al. (2002) and Turkina et al. (2006). Despite their high Fe and alkali contents (Fig. 8) and the localization of their composition points in the field of A-type granites (Fig. 9), they have geochemical features nonspecific for A-type granites (Whalen et al., 1987). In particular, they are similar to I-type granites in high Sr and low Y and Nb contents but are richer in Zr, like A-type granites (Levitskii et al., 2002; Turkina et al., 2006). Note that the anorogenic A-type granites (1747 Ma) in the Biryusa block are more similar to typical A-type granites and are richer in Zr, Nb, and Y and poorer in Sr than the

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Fig. 8. FeO*/(FeO* + MgO)–SiO2 (a) and (Na2O+K2O–CaO)–SiO2 (b) diagrams (Frost et al., 2001) for Paleoproterozoic postcollisional granitoids of the Biryusa block. Circles, S-type two-mica granites (this work); triangles, I-type tonalites and diorites, chemical compositions are borrowed from Turkina (2005) and Turkina et al. (2006); squares, A-type biotite–amphibole granites, chemical compositions are borrowed from Levitskii et al. (2002) and Turkina et al. (2006).

postcollisional granites (Turkina et al., 2006). In other words, the postcollisional biotite–amphibole granites of the Biryusa block should not be regarded as typical A-granites according to the classifications by Bonin (2007) and Whalen et al. (1987). We should emphasize a few else facts that are crucial for reconstructing the mechanism of formation of Paleoproterozoic postcollisional granitoids of the Biryusa block. Turkina (2005) noted that the tonalites of the Podporog massif formed at high pressure (>10 kbar), i.e., in thickened continental crust. Later on, Turkina et al. (2006) showed that the Paleoproterozoic I- and A-type postcollisional granitoids of the Biryusa

Fig. 9. FeO*/MgO–(Zr + Nb + Y + Ce) diagram (Whalen et al., 1987) for Paleoproterozoic postcollisional granitoids of the Biryusa block. A-type, field of A-type granites; FG, field of fractionated granites; OGT, field of S- and I-type nonfractionated granites.

block were generated from mixed mantle–crustal sources. Calculations yielded high temperatures (740–800 ºC) at the initial stages of crystallization of two-mica granites (Table 1, Fig. 10). High temperatures were also estimated for the I-type tonalites and diorites studied by Turkina et al. (2006) (Fig. 10). A-type granites are characterized by high melting temperatures. The temperature ranges of the three recognized groups of the Biryusa block granites overlap (Fig. 10), i.e., these are high-temperature granites. All this brings us to the conclusion that the Biryusa block postcollisional granitoids were generated within a high-temperature collision structure (Sylvester, 1998). Thus, high-temperature granitoids of different compositions were intruded into the rocks of the Biryusa block of the Siberian craton during the postcollisional extension within the thickened crust in the Paleoproterozoic. Their geochemical features, permitting assigning them to I-, S-, and A-type granites, suggest that all these granitoids formed in collision setting resulting from the unification of continental blocks and terranes of different geodynamic nature into a single structure. This diversity of coeval granitoids seems to be specific for areas of junction of several blocks (terranes). The unification of blocks leads to a diversity of substrates undergoing melting, which is one of the key petrologic factors for the formation of granites of different compositions (Donskaya et al., 2005, 2013; Turkina et al., 2006). We think that the crustal thickening in the region might have been caused by the unification of the Neoarchean Biryusa continental block (Turkina et al., 2007), Paleoproterozoic Urik–Iya terrane (Gladkochub et al., 2009), and Archean Tunguska superterrane (Gladkochub et al., 2009; Turkina, 2010; Turkina et al., 2007, 2013; Urmantseva et al., 2012) into the single Siberian craton (Gladkochub et al., 2006; Rosen, 2003; Rosen et al., 1994). The high temperature of generation for all granites was, most likely, due to the inflow of mantle material to the crust

T.V. Donskaya et al. / Russian Geology and Geophysics 55 (2014) 812–823

basement (this is confirmed by the presence of granitoids with mixed mantle–crust characteristics). Formation of chemically different coeval granitoids evidences that basaltic melts were the source of heat (e.g., for S-type two-mica granites of the Biryusa massif) and interacted with the crustal material during the formation of I- and A-type granitoids. That is, the contribution of the mantle material to the granite formation was one else factor responsible for the diversity of the Biryusa block granitoids. Moreover, we admit that the formation of compositionally different granites might have been related to shearing caused by the oblique collision of continental blocks, because it is shear systems that are usually formed by granitoids of different compositions (Luchitskaya, 2012). During the collision of blocks, shears might have spread deep into the crust, thus favoring the penetration of mantle magmas to higher levels and promoting the beginning of their melting. The studied belt of Paleoproterozoic granitoids can be considered a sewing structure uniting several collided blocks. This collision within the Sharyzhalgai salient (southern salient of the Tunguska superterrane of the Siberian craton) is manifested as large massifs of post-tectonic biotite–amphibole granites of the Shumikha and Sayan complexes, coeval with the Biryusa block granites (Didenko et al., 2003; Donskaya et al., 2002; Levitskii et al., 2002). Compared with the Biryusa block, the large massifs of post-tectonic granitoids in the Sharyzhalgai salient are mainly biotite–amphibole granites with geochemical features of A-type granites. The Sharyzhalgai salient granites are richer in indicator elements Zr, Nb, and Y than the A-type granites of the Biryusa block (Donskaya et al., 2002; Levitskii et al., 2002; Turkina et al., 2006). This might be explained by the fact that the Archean Tunguska superterrane (and, correspondingly, Sharyzhalgai salient with large fragments of Paleo-Archean crust) was an ancient continental block as compared with the Neoarchean Biryusa block and Paleoproterozoic Urik–Iya terrane. Collision of the latter two young structures with the large thick Tunguska superterrane led to the formation of typical high-temperature A-type granites in the latter.

Conclusions Based on the results obtained, we have drawn the following conclusions. U–Pb zircon dating of the Biryusa massif granites yielded an age of 1874 ± 14 Ma. This date agrees with the age estimated for the biotite–amphibole granites, tonalites, and quartz diorites of the Biryusa block of the Siberian craton (Levitskii et al., 2002; Turkina et al., 2003, 2006). Two-mica granites of the Biryusa massif correspond in chemical composition to normally alkaline and moderately alkaline high-alumina leucogranites. By mineral and petrogeochemical compositions, they are assigned to S-type granites. The geochemical characteristics of two-mica granites of the Biryusa massif show their formation through the melting of a metapelitic source (possibly, the Archean metasedimentary rocks of the Biryusa block, similar to the granites in εNd(t)

821

Fig. 10. T, ºC–(Na + K + 2*Ca)/(Al * Si) diagram for Paleoproterozoic postcollisional granitoids of the Biryusa block. T (temperature) and coefficient M (Na + K + 2*Ca)/(Al * Si) were calculated from data of Watson and Harrison (1983). Designations follow Fig. 8.

value) in the absence of an additional fluid phase. The granite formation proceeded at 740–800 ºC (zircon saturation temperature). The S-type two-mica granites together with the coeval I-type tonalites and diorites (Turkina, 2005; Turkina et al., 2006) and A-type biotite–amphibole granites (Levitskii et al., 2002; Turkina et al., 2006) form a magmatic belt stretching along the zone of junction of the Biryusa block with the Urik–Iya terrane and Tunguska superterrane. The granitoids are high-temperature rocks, which evidences that they formed within a high-temperature collision structure (Sylvester, 1998). The intrusion of granitoids took place within the thickened crust in collision setting at the stage of postcollisional extension in the Paleoproterozoic. This geodynamic setting was the result of the unification of the Neoarchean Biryusa continental block, Paleoproterozoic Urik–Iya terrane, and Archean Tunguska superterrane into the Siberian craton. The coexistence of nearly coeval granitoids of different geochemical types within a local crust site is a distinctive feature of granite formation in thickened continental crust. We thank T.B. Bayanova, Geological Institute of the Kola Scientific Center RAS, Apatity, for isotope study of Nd. This work was supported by grant 12-05-00749 from the Russian Foundation for Basic Research and by Basic Research Project 79 from the Siberian Branch of the Russian Academy of Sciences.

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Editorial responsibility: A.E. Izokh