Late Paleoproterozoic basites of the northern Baikal area: composition and melt sources

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ScienceDirect Russian Geology and Geophysics 55 (2014) 1278–1294 www.elsevier.com/locate/rgg

Late Paleoproterozoic basites of the northern Baikal area: composition and melt sources T.V. Donskaya *, D.P. Gladkochub, M.N. Shokhonova, A.M. Mazukabzov Institute of the Earth’s Crust, Siberian Branch of the Russian Academy of Sciences, ul. Lermontova 128, Irkutsk, 664033, Russia Received 1 July 2013; accepted 11 October 2013

Abstract We present results of isotope-geochemical studies of Late Paleoproterozoic basites from intrusions located in different parts of a dike swarm traceable for more than 200 km within the Baikal marginal salient of the Siberian craton basement (northern Baikal area). The basites of the southern (Khibelen site) and northern (Chaya site) parts of the dike swarm show both similarity and difference in their sources and formation conditions. For example, the Khibelen basites correspond in chemical composition to basalts and trachybasalts, and the Chaya basites, to basalts and andesite-basalts. Based on petrographic and petrochemical data, the basites of both sites can be referred to as medium-alkali (subalkalic) series. All analyzed basites show distinct negative Nb–Ta and Ti anomalies on element spidergrams, negative εNd(t) values, and indicative geochemical ratios Th/Nbpm, La/Nbpm, and La/Smn > 1. All this points to the formation of basites of both sites from mantle sources contaminated with continental crust. Contamination might have occurred in intermediate magma chambers localized in crust. Differentiated basic varieties of both sites resulted from fractionation of clinopyroxene. For the Khibelen basites, the mantle source (probably, with geochemical parameters close to those of IAB) might have been initially contaminated with middle-crust rocks and then, with lower/upper-crust material. The source of the Chaya basites was probably produced during the interaction of mantle components similar in composition to IAB and N-MORB with a crustal component. The performed studies testify to the heterogeneous composition of the upper mantle beneath different sites of the Siberian craton basement. © 2014, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: basites; intrusions; geochemistry; intracontinental extension; Late Paleoproterozoic; Siberian craton

Introduction Dike swarms formed by small intrusions of basic composition (dikes, sills, and small intrusive bodies) mark large-scale extension within consolidated Earth’s crust areas. Dike swarms can be indicators of the opening of new ocean basins and reflect intracontinental extension that did not cause destruction of cratonized sites (Sklyarov et al., 2000). Most of igneous rocks within each dike swarm show similar geochemical features, which suggests their formation from the same sources (Gladkochub et al., 2007). However, comprehensive isotopegeochemical studies revealed a difference in the sources of rocks of the same dike swarm. This confirms the earlier conclusions about the heterogeneous composition of the upper mantle even beneath consolidated mature sites of the Earth’s crust (Kostitsyn, 2007; Solov’eva et al., 2010).

This work is concerned with study of the Late Paleoproterozoic basites from a large dike swarm of the Baikal marginal salient of the Siberian craton basement. The intrusive bodies of the swarm are regarded as indicators of anorogenic intracontinental extension that did not cause a noticeable destruction of the craton lithosphere (Gladkochub et al., 2007, 2010, 2012). Gladkochub et al. (2010) suggested that this dike swarm, together with the nearly coeval dike swarms of the Anabar and Aldan Shields, is part of the Late Paleoproterozoic Large Igneous Province (Fig. 1a). We consider the isotopegeochemical features of Late Paleoproterozoic basites from different parts of this dike swarm and draw conclusions about their sources and genesis.

Geologic setting of the Late Paleoproterozoic basites

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

Abundant basic intrusions in the northern Baikal area are traceable from the upper reaches of the Lena River in the south to the middle reaches of the Chaya River in the north.

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 + 4.10.003

T.V. Donskaya et al. / Russian Geology and Geophysics 55 (2014) 1278–1294

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Fig. 1. Occurrence of Late Paleoproterozoic dike swarms within the salients of the Siberian craton basement (a), modified after Gladkochub et al. (2010), and geological scheme of the northern Baikal area (b), modified after Donskaya et al. (2009). a: 1, Phanerozoic sedimentary cover of the Siberian Platform; 2, basement salients (A, Anabar Shield; AS, Aldan–Stanovoi Schield; B, Baikal salient; K, Kan salient; O, Olenek salient; S, Sayan salient); 3, large collision belts (AK, Akitkan; AN, Angara); 4, suture zones; 5, Late Paleoproterozoic dike swarms; 6, assumed extension of dike swarms beneath the sedimentary cover; 7, center of the Vilyui Large Igneous Province (radiate dike swarm). b: 1, Central Asian Fold Belt; 2, Phanerozoic sedimentary cover of the Siberian Platform; 3, Meso–Neoproterozoic deposits of the Siberian Platform; 4–8, rocks of the basement of the Siberian Platform: 4, Paleoproterozoic volcanosedimentary deposits of the Akitkan Group of the North Baikal volcanoplutonic belt (postcollisional, 1.85–1.87 Ga), 5, Paleoproterozoic granitoids (postcollisional, 1.85–1.87 Ga), 6, Paleoproterozoic granitoids (precollisional, 2.0 Ga), 7, Paleoproterozoic metamorphic strata, 8, Mesoarchean granitoids; 9, major faults; 10, study areas (a) and sampling localities (b).

They extend for more than 200 km and form a single dike swarm (Gladkochub et al., 2007) (Fig. 1b). Intrusive bodies break through the Early Precambrian strata of the Siberian craton basement, including volcanic and volcanosedimentary deposits of the Akitkan Group of the North Baikal volcanoplutonic belt (1.85–1.88 Ga) (Donskaya et al., 2005, 2007, 2008; Larin et al., 2003; Neimark et al., 1991) as well as the conformly overlying deposits of the Okun’ Formation of the Teptorga Group. The basic bodies are discordantly overlain by the Neoproterozoic deposits of the Baikal Group. Within the swarm, the intrusive bodies are of NE to N–S strike concordant to the regional structure of the Baikal marginal salient of the Siberian craton basement. The first detailed description of intrusive bodies in the northern Baikal area was made by E.V. Pavlovskii, A.I. Tsvetkov, and E.A. Shalek (review by Salop (1964)). They defined the petrography of basites in the Chaya River area and reported that the basites intrude the granitoids of the Irel’

complex and metamorphic strata of Early–Middle Proterozoic age. In the later published schemes of magmatism, the North Baikal basic dikes were assigned to the Chaya or Mogol complex (Bukharov, 1987; Sryvtsev, 1986). According to Sryvtsev (1986), these dikes intruded at the rifting stage of the area evolution. They were dated at the Middle Riphean, based on the assumed age of the host sedimentary rocks of the Okun’ Formation of the Teptorga Group (Sryvtsev, 1986). At present, there are two dates for the basites, obtained for the rocks of the southern and northern parts of the dike swarm. The age of gabbro-dolerites of Cape Khibelen (southern end of the swarm) was estimated at 1674 ± 29 Ma by Sm–Nd dating of clinopyroxene, plagioclase, and the whole rock (Gladkochub et al., 2007). The age of gabbro-dolerites in the middle reaches of the Chaya River (northern end of the swarm) was determined as 1752 ± 3 Ma by U–Pb dating of baddeleyite (Gladkochub et al., 2010).

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Fig. 2. Geological scheme of the Cape Khibelen region, modified after Gladkochub et al. (2007). 1, Quaternary deposits; 2, Neoproterozoic sedimentary rocks (Baikal Group); 3, Late Paleoproterozoic basites; 4, Paleoproterozoic felsic volcanics of the Akitkan Group; 5, Paleoproterozoic volcanosedimentary rocks of the Akitkan Group; 6, Paleoproterozoic metamorphic rocks; 7, Mesoarchean granitoids; 8, faults; 9, geologic boundaries; 10, sampling localities.

Geological, petrographic, and mineralogical characteristics of the studied objects Basites of the southern part of the dike swarm (Khibelen site). The first object of our study was dikes and intrusive basic bodies in the regions of the Khibelen and Svetlyi Brooks and Capes Srednii Kedrovyi and Burgunda (~1.67 Ga). These dikes intrude the Archean granitoids of the Kocheriki complex, Early Proterozoic metamorphic rocks of the Sarma Group, Early Proterozoic granitoids of the Irel’ and Tatarnikovo complexes, and nearly coeval volcanic and volcanosedimentary deposits of the Akitkan Group of the North Baikal volcanoplutonic belt (Figs. 1b and 2). The basic bodies within the studied area are few tens of meters thick (in places, up to 250 m) and up to 8 km long. All bodies are of N–S strike and have sharp contacts with the host rocks and high dips. Despite the great thickness of some intrusive bodies, their identical location with smaller dikes and a significant domination of their length over their width permit them to be regarded as a single dike swarm. The intrusions are differentiated: Their cores are composed of medium- to coarse-grained rocks, and their periphery is formed by cryptogranular varieties. The basites within the dike swarm are mostly dolerites of massive structure and ophitic, poikilophitic, doleritic, porphyritic, and, more seldom, subophitic textures. The rocks of subophitic texture are, most likely, subophitic gabbro. The

main minerals of the basites are plagioclase (45–55%) and pyroxene (20–32%). The minor minerals are primary biotite (1–3%), hornblende (1–4%), quartz (1–5%), ore minerals (ilmenite and titanomagnetite) (1–4%), and, in some samples, K-feldspar (1–5%). In the samples containing primary quartz and K-feldspar, these minerals, together with plagioclase, form granophyric intergrowths. The accessory minerals are sphene, apatite, and zircon. Plagioclase is mostly zoned, partly recrystallized to a saussuritized aggregate. In some basite samples, pyroxene is replaced by secondary amphibole (tremolite–actinolite and uralite), epidote, and chlorite. Primary biotite is subjected to chloritization; some samples contain secondary biotite in cracks. Also, clino- and orthopyroxenes are present, with the former prevailing. According to the classification of pyroxenes (Morimoto et al., 1988), most of these clinopyroxenes correspond in chemical composition to augite (Wo36–45En23–48Fs15–39); one sample contained pigeonite (Wo10En50Fs40) (Table 1). Orthopyroxene is similar in composition to clinoenstatite (Wo4En53Fs43) (Table 1). According to the classification by Leake et al. (1997), primary amphiboles in the basites are Ca-amphiboles meeting the condition CaB = 1.5, (Na + K)A < 0.5 (Table 2). The analyzed amphiboles can be classified as magnesian hornblende (magnesiohornblendite) and ferrous hornblende (ferrohornblendite). Plagioclases are dominated by labradorite (An52–63). The cores

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T.V. Donskaya et al. / Russian Geology and Geophysics 55 (2014) 1278–1294 Table 1. Representative analyses of pyroxenes from basites of the Khibelen and Chaya sites and their crystallochemical formulas Component Khibelen site

Chaya site

0201*

0201*

0201** 0204*

0204*

0204** 0250*

0250** 0252*

0252*

0252** 04104* 04104* 04104** 06392* 06392* 06393*

Opx

Cpx

Cpx

Cpx

Cpx

Cpx

Cpx

Cpx

Cpx

Cpx

Cpx

Opx

Cpx

Cpx

Cpx

Cpx

Cpx

SiO2, wt.% 51.17

50.64

52.03

49.57

49.61

50.51

52.03

52.58

50.92

51.15

51.34

49.61

50.66

51.29

51.67

49.59

51.24

TiO2

0.52

0.00

0.75

0.00

0.00

0.64

0.00

0.34

0.00

0.00

0.55

0.00

0.00

0.39

0.00

0.52

0.62

Al2O3

0.00

0.74

3.10

0.00

1.38

1.99

0.00

1.95

0.70

0.00

2.41

0.00

0.00

2.26

1.80

2.93

1.49

Cr2O3

0.00

0.00

0.12

0.00

0.00

0.00

0.00

0.16

0.00

0.00

0.00

0.00

0.00

0.07

0.00

0.60

0.00

Fe2O3

0.549

0.08

0.58

0.00

0.00

1.69

0.00

0.59

0.00

0.17

0.84

1.71

0.00

2.25

0.00

0.75

0.00

FeO

25.42

12.92

8.87

21.34

22.11

13.96

13.57

9.30

23.93

15.54

9.31

28.18

13.19

9.22

8.13

7.79

11.09

MnO

0.72

0.00

0.22

0.00

0.00

0.38

0.00

0.19

0.00

0.80

0.28

0.50

0.00

0.32

0.00

0.00

0.00

MgO

18.49

12.27

15.96

7.60

8.03

12.66

10.90

16.93

16.90

10.28

13.55

15.82

11.86

14.06

15.37

15.14

13.45

CaO

1.99

20.11

18.87

18.76

16.27

17.74

21.06

17.52

4.56

20.67

21.04

1.90

20.04

20.18

19.92

19.50

19.71

Na2O

0.00

0.00

0.25

0.00

0.00

0.29

0.00

0.23

0.00

0.00

0.26

0.00

0.00

0.26

0.00

0.00

0.00

0.00

K2O

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Total

98.85

96.76

100.74 97.27

97.40

99.86

97.56

99.79

97.01

98.61

99.56

97.72

95.75

100.29

96.89

96.82

97.60

Si

1.977

1.982

1.912

2.004

1.990

1.923

2.024

1.946

1.995

1.997

1.929

1.974

2.006

1.916

1.967

1.899

1.965

Ti

0.015

0.000

0.021

0.000

0.000

0.018

0.000

0.010

0.000

0.000

0.016

0.000

0.000

0.011

0.000

0.015

0.018

Al

0.000

0.034

0.134

0.000

0.065

0.089

0.000

0.085

0.032

0.000

0.107

0.000

0.000

0.099

0.081

0.132

0.067

Cr

0.000

0.000

0.003

0.000

0.000

0.000

0.000

0.005

0.000

0.000

0.000

0.000

0.000

0.002

0.000

0.018

0.000

3+

0.016

0.002

0.016

0.000

0.000

0.048

0.000

0.016

0.000

0.005

0.024

0.051

0.000

0.063

0.000

0.022

0.000

Fe2+

0.821

0.423

0.272

0.721

0.742

0.445

0.442

0.288

0.784

0.508

0.293

0.938

0.437

0.288

0.259

0.250

0.356

Mn

0.024

0.000

0.007

0.000

0.000

0.012

0.000

0.006

0.000

0.026

0.009

0.017

0.000

0.010

0.000

0.000

0.000

Mg

1.065

0.716

0.874

0.458

0.480

0.718

0.632

0.934

0.987

0.598

0.759

0.939

0.700

0.783

0.872

0.864

0.769

Fe

Ca

0.082

0.843

0.743

0.813

0.699

0.724

0.878

0.695

0.191

0.865

0.847

0.081

0.850

0.808

0.813

0.800

0.810

Na

0.000

0.000

0.018

0.000

0.000

0.022

0.000

0.017

0.000

0.000

0.019

0.000

0.000

0.019

0.000

0.000

0.000

K

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

0.000

Total

4.000

4.000

4.000

3.996

3.977

4.000

3.976

4.000

3.989

4.000

4.000

4.000

3.994

4.000

3.992

4.000

3.984

En

53

36

46

23

25

37

32

48

50

30

39

46

35

40

45

45

40

Wo

4

42

39

41

36

37

45

36

10

43

44

4

43

41

42

41

42

Fs

43

21

15

36

39

26

23

16

40

27

17

50

22

19

13

14

18

Note. Crystallochemical formulas were calculated per six oxygen atoms. Conversion of Fe for Fe2+ and Fe3+ was made following Droop’s (1987) procedure. The end-members of En (enstatite), Wo (wollastonite), and Fs (ferrosilite) were calculated following the classification by Morimoto et al. (1988). * Analyses were carried out on a LEO-1430VP electron microscope. ** Analyses were carried out on a modernized MAR-3 microprobe.

of the analyzed zoned grains are composed of labradorite (An52–53), and the periphery, of oligoclase (An25–28). K-feldspars correspond in composition to orthoclase (Ort74–93) (Table 3). Basites of the northern part of the dike swarm (Chaya site). The second object of study was basic intrusions (~1.75 Ga) in the middle reaches of the Chaya River, where they break through the Early Proterozoic sedimentary rocks of the Chaya Formation and the volcanic rocks of the Khibelen Formation of the Akitkan Group of the North Baikal volcanoplutonic belt as well as the sedimentary rocks of the Okun’ Formation of the Teptorga Group (Fig. 3). Basic bodies in this area show different occurrences relative to the host deposits. For example, in the sedimentary deposits

of the Chaya and Okun’ Formations, basites make up sills, whose position is consistent with the gentle and moderate NW dip of bedding. The thickness of basic sheets varies from few meters to 50 m, and the length, from 3–5 to 15 km. In the massive volcanics of the Khibelen Formation, basites compose subvertical to gently dipping bodies up to 5 km long. Large intrusive bodies in the middle reaches of the Chaya River are differentiated: Their cores are formed by coarse-grained rocks richest in primary quartz. Basites of the Chaya site have a massive structure and poikilophitic, porphyritic, doleritic, and subophitic textures. The rocks with a subophitic texture can be probably classified as subophitic gabbro, and the rest rocks, as dolerites. The main rock-forming minerals are plagioclase (50–55%) and pyroxene

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Table 2. Representative analyses of amphiboles from basites of the Khibelen and Chaya sites and their crystallochemical formulas Component

Khibelen site 0204*

Chaya site 0204**

0250*

0252**

0252**

02104*

02104**

06393*

SiO2, wt.%

46.08

43.25

47.90

49.11

49.96

46.81

47.70

47.09

TiO2

0.72

1.75

0.00

0.12

0.37

1.52

1.31

1.25

Al2O3

5.33

7.13

2.97

3.71

3.77

5.57

5.24

5.84

Fe2O3

7.26

8.63

4.53

0.98

6.71

4.62

7.36

5.64

FeO

16.38

19.56

13.53

13.84

9.55

11.37

9.26

10.10

MnO

0.56

0.32

0.00

0.22

0.21

0.00

0.15

0.00

MgO

8.76

5.61

11.16

14.13

14.27

12.98

13.84

13.40

CaO

10.62

10.01

14.02

13.90

11.82

11.21

10.94

10.93

Na2O

1.52

1.72

0.82

0.36

0.63

1.43

1.75

1.60

K2O

0.78

0.88

0.00

0.04

0.51

0.69

0.66

0.80

Total

98.01

98.84

94.93

96.40

97.81

96.20

98.21

96.65

Si

6.998

6.658

7.348

7.328

7.284

7.006

6.975

6.990

Ti

0.082

0.203

0.000

0.014

0.041

0.171

0.144

0.140

Al

0.954

1.293

0.537

0.652

0.647

0.983

0.903

1.022

Fe3+

0.829

1.000

0.523

0.110

0.736

0.520

0.810

0.630

2+

Fe

2.081

2.519

1.735

1.727

1.165

1.424

1.133

1.254

Mn

0.072

0.041

0.000

0.028

0.026

0.000

0.018

0.000

Mg

1.983

1.287

2.552

3.142

3.102

2.896

3.017

2.965

Ca

1.728

1.651

2.305

2.222

1.847

1.798

1.715

1.738

Na

0.448

0.512

0.244

0.104

0.179

0.415

0.497

0.461

K

0.151

0.172

0.000

0.007

0.095

0.132

0.124

0.152

Total

15.327

15.335

15.244

15.333

15.120

15.345

15.335

15.351

CaB

1.728

1.651

2.000

2.000

1.847

1.798

1.715

1.738

CaA

0.000

0.000

0.305

0.222

0.000

0.000

0.000

0.000

(Na + K)A

0.327

0.335

0.244

0.111

0.120

0.345

0.335

0.351

0.49

0.34

0.60

0.65

0.73

0.67

0.73

0.70

2+

Mg/(Mg + Fe )

Note. Crystallochemical formulas were calculated per 23 oxygen atoms. Conversion of Fe for Fe2+ and Fe3+ was made following Droop’s (1987) procedure. Formula quantities of CaB, CaA, and (Na + K)A were calculated by the procedure of Leake et al. (1997). * Analyses were carried out on a LEO-1430VP electron microscope. ** Analyses were carried out on a modernized MAR-3 microprobe.

(26–30%). The minor minerals are primary biotite (1–2%), hornblende (1–4%), quartz (1–7%), K-feldspar (3–6%), and ore mineral (ilmenite) (1–2%). The set of minor minerals is different in the samples; for example, there are varieties free either of K-feldspar or of hornblende. Plagioclase, quartz, and K-feldspar, when coexisting together, form a granophiric texture. The accessory minerals are sphene, apatite, and baddeleyite. In most samples, plagioclase is intensely saussuritized and shows inner zoning. Pyroxenes are replaced by amphibole (tremolite–actinolite and uralite), epidote, and chlorite. Biotite is sometimes replaced by chlorite and leucoxene aggregate. Ore mineral is also replaced by leucoxene aggregate in some samples. Pyroxenes in the basites of the Chaya site are mainly clinopyroxenes corresponding in composition to augite Wo41–43 En35–45Fs13–22 (Table 1). Orthopyroxene is scarcer and is

compositionally similar to clinoferrosillite Wo4En46Fs50. Primary amphiboles are classified as calcic amphiboles (Leake et al., 1997) and are magnesian hornblende (Table 2). Plagioclases are mainly labradorite (An51–61) as well as andesine (An46) and oligoclase (An29) (Table 3). Oligoclase occurs on the periphery of zoned plagioclase, and labradorite, in the core.

Methods The representative samples of basites from the Khibelen site in the southern part of the dike swarm (17 samples) and from the Chaya site in the northern part of the swarm (8 samples) were analyzed for major oxides and trace and rare-earth elements. The compositions of rock-forming minerals were studied in seven samples. The isotopic composition

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T.V. Donskaya et al. / Russian Geology and Geophysics 55 (2014) 1278–1294 Table 3. Representative analyses of plagioclases and K-feldspars from basites of the Khibelen and Chaya sites and their crystallochemical formulas Component Khibelen site

Chaya site

0201*

0201*

0201** 0201*

0204** 0204*

0250*

0250*

0250** 0250*

0252** 04104* 04104* 04104** 06392* 06393* 06393*

Pl c

Pl r

Pl

Pl

Pl c

Pl r

Pl

Pl

Kfs

Kfs

Kfs

Pl c

Pl r

Pl

Pl

Pl

Pl

SiO2, wt.% 54.64

59.65

50.42

62.83

53.83

63.07

53.06

60.69

52.23

62.17

51.68

52.59

59.15

54.37

52.33

54.72

51.84

TiO2

0.00

0.00

0.00

0.00

0.07

0.00

0.00

0.00

0.05

0.00

0.00

0.00

0.00

0.08

0.00

0.00

0.00

Al2O3

27.51

23.49

30.87

17.89

28.48

17.69

27.06

22.67

29.90

17.53

30.04

27.95

23.07

28.41

27.83

26.68

28.70

FeO

0.00

0.55

0.66

0.62

0.58

0.00

0.55

0.50

0.59

0.98

0.52

0.75

0.00

0.39

1.58

0.59

0.00

MgO

0.00

0.00

0.10

0.00

0.00

0.00

0.00

0.00

0.04

0.00

0.00

0.00

0.00

0.00

0.48

0.00

0.00

CaO

10.69

5.78

13.00

0.00

11.30

0.00

10.80

5.15

12.53

0.00

12.74

11.59

5.85

10.79

10.89

9.58

12.02

Na2O

5.35

7.70

3.87

2.95

5.14

2.68

5.10

7.95

4.51

0.70

4.37

4.80

7.13

5.43

4.54

5.97

3.88

K2O

0.00

0.49

0.44

13.07

0.37

12.96

0.40

0.77

0.18

15.30

0.35

0.00

0.95

0.30

0.00

0.37

0.45

Total

98.19

97.66

99.37

97.36

99.76

96.40

96.97

97.73

100.03 96.68

99.70

97.68

96.15

99.76

97.65

97.91

96.89

Si

2.504

2.723

2.317

2.974

2.448

2.998

2.480

2.765

2.376

2.985

2.362

2.442

2.737

2.465

2.434

2.525

2.420

Ti

0.000

0.000

0.000

0.000

0.002

0.000

0.000

0.000

0.002

0.000

0.000

0.000

0.000

0.003

0.000

0.000

0.000

Al

1.486

1.264

1.672

0.998

1.527

0.991

1.490

1.217

1.603

0.992

1.619

1.530

1.258

1.518

1.526

1.451

1.579

Fe

0.000

0.021

0.025

0.025

0.022

0.000

0.021

0.019

0.022

0.039

0.020

0.029

0.000

0.015

0.061

0.023

0.000

Mg

0.000

0.000

0.007

0.000

0.000

0.000

0.000

0.000

0.003

0.000

0.000

0.000

0.000

0.000

0.033

0.000

0.000

Ca

0.525

0.283

0.640

0.000

0.551

0.000

0.541

0.251

0.611

0.000

0.624

0.577

0.290

0.524

0.543

0.474

0.601

Na

0.475

0.682

0.345

0.271

0.453

0.247

0.462

0.702

0.398

0.065

0.387

0.432

0.640

0.477

0.410

0.534

0.351

K

0.000

0.029

0.026

0.789

0.021

0.786

0.024

0.045

0.011

0.937

0.021

0.000

0.056

0.017

0.000

0.022

0.027

Total

4.991

5.000

5.032

5.057

5.024

5.023

5.018

5.000

5.025

5.020

5.033

5.009

4.982

5.020

5.007

5.028

4.979

An

52

28

63

0

54

0

53

25

60

0

60

57

29

51

57

46

61

Ab

48

69

34

26

44

24

45

70

39

7

38

43

65

47

43

52

36

Ort

0

3

3

74

2

76

2

4

1

93

2

0

6

2

0

2

3

Note. Crystallochemical formulas were calculated per eight oxygen atoms. End-members: An, anorthite; Ab, albite; Ort, orthoclase. c, Mineral core; r, mineral rim. * Analyses were carried out on a LEO-1430VP electron microscope. ** Analyses were carried out on a modernized MAR-3 microprobe.

of Nd was determined for ten samples. The sampling localities are shown in Figs. 1b, 2, and 3. The mineral composition was studied on a LEO-1430VP scanning electron microscope coupled with an INCA Energy 350 EDX microanalysis system and on a modernized MAR-3 electron microprobe at the Geological Institute, Ulan-Ude (analysts N.S. Karmanov and S.V. Kanakin). SEM operation conditions: accelerating voltage 20 kV, probe current 0.3– 0.4 nA, probe diameter <0.1 µm, and total analytical error 2–4 wt.%. MAR-3 operation conditions: accelerating voltage 20 kV, probe current 45–50 nA, probe diameter 2–3 µm, and total calculation error 1%. The contents of major oxides were determined by the X-ray fluorescent method at the Analytical Center of the Institute of Geology and Mineralogy, Novosibirsk (analyst N.M. Glukhova), and at the Institute of Geochemistry, Irkutsk (analyst A.L. Finkel’shtein). Samples 0205, 0377, 0381, 0486, 0497, 04104, 04105, and 04107 were analyzed for major oxides by silicate analysis at the Institute of the Earth’s Crust, Irkutsk (analysts E.G. Koltunova, M.M. Samoilenko, and N.Yu. Tsareva). The contents of Co, Ni, Sc, V, and Cr were determined by spectral analysis at the Institute of the Earth’s

Crust, Irkutsk (analysts V.V. Shcherban’, A.V. Naumova, L.V. Vorotynova, and E.M. Sos’ko). The contents of other trace and rare-earth elements were measured by ICP-MS on a VG Plasmaquad PQ-2 (VG Elemental, England) mass spectrometer at the Center of Collective Use of the Irkutsk Scientific Center (analysts S.V. Panteeva and V.V. Markova). The mass spectrometer was calibrated against the BHVO-1, DNC-1, JB-2, and W-2 International standard samples. The error of the ICP-MS determination of trace and rare-earth elements was ≤5%. The isotopic composition of Nd was measured at the Center of Collective Use of the Irkutsk Scientific Center. 100 or 200 mg samples were digested in the mixture of HF, HClO4, and HNO3 in a microwave oven, following a particular technique. Rare-earth elements were separated by ion exchange chromatography, using columns filled with 3.5 g BioRad-AG50WX resin. Pure Nd fractions were separated by extraction chromatography using the LnSpec columns. The isotopic composition of Nd was measured on a Finnigan MAT-262 mass spectrometer in the static regime. The 147Sm/144Nd ratios were calculated from the contents of Sm and Nd determined by ICP-MS. The measured 143Nd/144Nd values were normalized to

0227

0247

0248

Intrusion 3

0250

0251

0252

0257

0259

Intrusion 4

0275

0276

Intrusion 5

0377

0381

Intrusion 6

0486

0497

04104 04105 04107 06387 06392 06393

Intru- Intru- Intru- Intrusion 4 sion 1 sion 2 sion 3

Basites of the Chaya site

–

–

1.15

0.09

2.04

–

Na2O

K2O

P2O5

LOI

H2O–

–

– –

–

0.96

8.20

0.18

0.13

3.81

0.13

0.63

1.44

9.67

0.12

2.70

0.15

0.77

2.86

6.55

0.15

210

440

8.54

460

5.24

310

360

2.46

130

230

140

5.99

230

340

51

150

4.89

140

270

38

100

6.49

110

270

49

200

4.06

590

230

71

17.16

10.49

170

260

46

97

42

(continued on next page)

19.61 17.80 13.63 12.59 8.07

7.46

140

270

31

110

50

21.75 22.21 43.20 22.86 20.10 22.23 22.63 18.55 22.26 22.01 18.32 26.63 26.96 25.92 28.74 21.43 20.72 14.80 8.75

230

220

35

140

61

La

80

13.79 13.84 13.41 15.09 8.95

190

200

45

110

37

361.90 371.46 697.04 406.96 472.11 584.36 342.82 281.34 345.78 328.09 291.86 517.81 496.58 378.87 429.06 283.71 306.94 314.53 230.05 355.59 404.19 540.28 266.19 356.47 382.25

11.42 10.50 9.67

130

240

32

190

44

Ba

12.02 10.34 7.25

340

210

42

220

29

12.31 11.74 22.54 12.89 7.29

360

200

37

130

47

Nb

490

160

41

130

41

102.07 99.71 196.82 105.74 121.18 96.50 91.84 119.23 99.81 98.50 89.74 119.62 111.63 91.50 97.82 89.66 88.30 72.30 38.57 108.07 87.21 74.78 43.71 39.51 110.82

860

170

26

95

54

–

–

2.34

0.11

1.26

3.04

20.52 20.29 34.22 19.78 22.55 20.60 21.27 22.12 19.83 22.36 17.65 22.70 22.66 21.97 24.37 22.25 21.82 20.27 18.01 19.43 17.70 13.91 25.36 21.11 18.19

420

240

35

160

75

–

–

3.81

0.15

0.87

1.35

Zr

94

150

37

200

40

–

–

2.63

0.11

1.56

1.12

10.75 10.30 8.86

Y

91

180

37

200

40

0.99

0.27

4.05

0.08

1.67

2.77

8.98

0.18

10.34 11.07 6.82

0.20

293.91 291.39 268.01 328.55 347.95 279.45 221.49 290.06 282.50 267.14 317.66 278.66 300.85 305.07 306.97 150.60 259.76 165.93 170.47 259.09 259.73 269.69 137.36 217.23 330.17

11

210

37

220

32

<0.06 0.11

0.22

2.69

0.13

1.25

2.91

9.04

5.74

0.10

Sr

120

250

34

240

42

<0.06 <0.06 N.f.

0.08

2.63

0.11

0.73

2.06

4.64

0.13

40.44 34.96 80.72 43.25 62.05 66.61 31.26 30.09 31.57 30.17 28.34 58.86 64.47 90.19 51.28 23.03 44.53 15.28 51.72 32.98 23.96 47.95 106.80 34.42 38.47

160

49

340

41

0.11

0.07

3.15

0.10

0.93

2.07

10.20 9.77

5.88

0.14

–

–

0.96

190

150

31

130

48

–

0.34

4.67

0.12

0.69

3.49

5.33

7.28

0.15

–

–

0.82

11.94 11.65 9.38

–

–

0.61

Rb

310

27

150

45

–

–

1.18

0.11

1.14

2.33

9.26

6.65

0.13

–

5.16

3.14

0.82

Cr

160

30

170

42

–

–

2.16

0.09

1.16

2.56

9.10

8.64

0.16

–

6.63

2.27

0.99

180

32

84

54

–

–

1.62

0.11

1.31

2.33

9.28

9.80

0.19

–

6.19

2.16

1.01

V

34

160

41

–

–

2.01

0.11

1.43

2.32

7.99

0.18

–

8.20

3.59

0.80

180

39

–

–

1.90

0.08

0.80

2.73

8.40

0.18

–

7.50

2.09

0.83

33

43

–

–

1.19

0.11

1.00

2.99

7.65

0.18

–

8.02

2.03

1.10

Sc

47

–

–

1.45

0.11

0.94

2.17

9.26

3.09

1.18

Ni

45

–

–

2.04

0.12

0.97

8.59

0.16

10.45 10.16 10.21 9.70

9.04

0.17

–

–

1.19

47

–

–

2.66

0.10

0.84

2.98

10.35 9.87 2.62

9.48

0.17

–

–

1.06

Co, ppm

<0.06 –

0.17

11.41 9.71

0.19

–

–

1.16

10.90 10.61 10.37 10.81 –

–

–

1.11

–

–

1.46

0.10

1.52

2.02

9.73

8.49

0.17

–

–

0.86

10.33 9.74

–

–

1.01

100.04 99.91 98.57 99.99 100.39 99.69 100.28 100.05 99.73 100.39 100.27 99.92 99.28 99.25 99.36 100.02 100.01 99.91 99.59 99.75 99.96 99.88 100.14 100.12 99.99

–

2.78

0.10

1.74

3.30

8.96

8.02

0.17

<0.01 –

2.48

0.09

1.17

2.64

9.65

6.54

–

–

1.03

Total

–

1.36

0.17

1.87

2.46

7.70

5.79

–

–

1.04

10.60 10.92 10.58 10.11 9.97

–

–

1.15

CO2

2.11

0.10

1.05

3.30

10.23 9.96

2.41

CaO

9.10

0.14

9.34

0.18

MgO

0.16

7.28

0.17

–

MnO

–

–

10.02 10.08 13.17 –

1.86

Fe2O3*

–

FeO

–

1.07

–

1.00

Fe2O3

1.75

13.92 13.60 12.87 15.70 14.81 13.32 11.19 12.85 14.00 13.64 14.57 13.17 13.21 12.79 13.34 13.10 12.50 15.17 16.80 14.63 15.35 14.95 14.10 14.51 14.56

Al2O3

0.95

1.01

TiO2

SiO2, wt.% 49.66 49.50 51.25 51.44 48.84 50.81 49.30 50.20 50.03 50.75 50.63 50.79 51.82 51.38 51.83 48.65 51.04 52.16 46.96 53.50 56.69 51.16 46.78 45.41 52.51

0205

0212

0204

0201

0202

Intrusion 2

Intrusion 1

Component Basites of the Khibelen site

Table 4. Chemical composition of representative basite samples from the northern Baikal area

1284 T.V. Donskaya et al. / Russian Geology and Geophysics 55 (2014) 1278–1294

0259

0275

0276

0377

0381

0486

0497

04104 04105 04107 06387 06392 06393

67.8

6.97

0.78

2.72

1.96

1.38

2.63

3.08

0.85

4.05

0.93

68.5

6.52

0.75

2.76

1.83

1.50

2.68

0.52

0.47

Hf

Ta

Th

U

mg#

(La/Yb)n

Eu/Eu*

(Th/Nb)pm

(La/Nb)pm

(Th/La)pm

(La/Sm)n

Ti/Ti*

Nb/Nb*

0.43

0.51

2.92

1.58

1.99

3.14

0.69

7.91

50.6

2.04

8.44

1.40

5.46

0.54

3.65

0.54

3.86

1.30

6.34

1.25

7.17

1.83

9.14

9.46

0.47

0.52

2.79

1.51

1.84

2.78

0.80

7.14

60.5

0.94

4.27

0.84

3.20

0.31

2.14

0.34

2.29

0.75

3.84

0.76

4.04

1.17

5.06

5.00

0.29

0.61

2.92

1.65

2.86

4.72

0.90

6.75

63.8

0.88

4.10

0.67

2.90

0.32

1.99

0.29

1.98

0.62

3.70

0.63

4.05

1.22

4.25

5.41

0.48

0.58

2.66

1.33

1.92

2.56

0.81

7.58

64.4

0.73

3.67

0.74

2.91

0.34

1.96

0.33

2.32

0.74

3.98

0.77

4.35

1.25

5.15

5.07

0.41

0.47

2.43

1.30

2.27

2.95

0.71

6.79

71.5

0.68

3.64

0.65

3.09

0.37

2.23

0.38

2.60

0.84

4.28

0.82

4.79

1.22

5.75

5.35

0.32

0.60

2.63

1.56

2.66

4.15

0.82

6.67

69.1

0.69

3.59

0.64

2.82

0.30

1.86

0.26

1.90

0.60

3.60

0.60

3.82

1.09

4.36

5.11

0.44

0.48

2.65

1.42

2.02

2.88

0.76

6.86

68.9

0.83

3.92

0.72

2.96

0.34

2.17

0.34

2.35

0.79

4.06

0.76

4.26

1.17

5.19

5.01

0.41

0.57

3.30

1.46

2.18

3.17

0.88

6.88

67.1

0.81

3.97

0.69

2.82

0.31

2.14

0.36

2.34

0.79

3.80

0.79

4.23

1.20

4.11

5.16

0.45

0.49

2.52

1.48

1.97

2.91

0.81

6.06

67.3

0.75

3.36

0.67

2.83

0.29

2.02

0.33

2.02

0.73

3.49

0.66

3.87

1.10

4.49

4.29

0.44

0.48

2.72

1.46

2.00

2.93

0.80

7.15

63.7

1.06

4.81

0.84

3.39

0.37

2.49

0.39

2.66

0.87

4.50

0.86

5.00

1.43

6.05

6.13

0.45

0.51

2.79

1.36

2.02

2.75

0.81

8.08

62.7

0.93

4.54

0.81

3.07

0.35

2.23

0.36

2.73

0.87

4.36

0.88

4.97

1.43

5.96

6.10

0.46

0.49

2.85

1.33

2.01

2.67

0.78

7.53

65.4

0.73

4.27

0.81

2.66

0.33

2.30

0.35

2.46

0.80

4.24

0.85

4.70

1.30

5.61

5.84

0.45

0.50

2.87

1.45

1.98

2.87

0.77

8.15

63.3

1.03

5.16

0.92

2.87

0.39

2.36

0.39

2.92

0.94

4.60

0.90

5.26

1.44

6.18

6.44

0.36

0.66

3.12

1.41

2.49

3.51

0.75

7.46

63.1

0.71

3.74

1.06

2.35

0.28

1.92

0.27

2.19

0.90

3.93

0.69

4.21

1.03

4.24

5.00

0.33

0.63

3.07

1.67

2.52

4.19

0.91

6.33

64.8

0.90

4.27

0.92

2.47

0.33

2.19

0.31

2.32

0.93

4.30

0.68

4.16

1.23

4.16

4.76

0.37

0.62

2.86

0.96

2.93

2.80

1.00

5.05

59.8

0.28

1.75

0.59

1.89

0.30

1.96

0.31

1.92

0.78

3.42

0.48

3.12

1.03

3.19

3.53

0.26

0.71

2.13

1.21

3.69

4.47

1.13

3.13

57.2

0.38

1.31

0.37

1.21

0.29

1.87

0.29

1.84

0.73

3.39

0.45

2.82

0.98

2.53

2.39

0.33

0.57

2.81

1.44

2.73

3.94

0.96

8.04

60.3

0.77

3.50

0.80

3.02

0.23

1.63

0.26

1.54

0.74

3.72

0.56

4.17

1.32

4.31

4.97

0.30

0.57

2.59

1.29

3.08

3.99

1.06

7.88

52.9

0.62

2.85

0.77

2.50

0.24

1.51

0.25

1.62

0.72

3.49

0.56

3.95

1.41

4.24

4.72

0.31

0.62

2.52

1.42

2.89

4.10

1.08

6.65

60.1

0.51

2.39

0.50

2.05

0.20

1.37

0.21

1.33

0.58

2.77

0.45

2.96

1.10

3.34

3.48

0.38

0.43

2.47

1.94

2.01

3.90

0.82

3.26

66.9

0.82

3.02

0.41

1.45

0.40

2.58

0.40

2.68

0.97

4.31

0.64

3.62

0.90

3.15

3.17

0.45

0.68

1.91

1.33

2.06

2.75

1.00

2.50

68.9

0.33

1.33

0.21

1.31

0.36

2.16

0.34

2.16

0.81

3.68

0.50

3.14

0.93

2.61

2.30

0.46

0.60

2.70

1.85

1.70

3.14

0.89

7.36

62.9

0.71

3.93

0.44

3.17

0.23

1.56

0.25

1.69

0.66

3.42

0.55

3.72

1.11

3.92

4.46

    (Turkina and Nozhkin, Smn⋅Gdn ; Ti/Ti* = Tipm/√ Smpm⋅Gdpm ; Nb/Nb* = 0.3618Nb/√ Th⋅La Note. mg# = Mg⋅100/(Mg + Fe2+), where Mg = MgO/40.31, Fe2+ = (Fe2O3*⋅0.8998⋅0.85)/71.85; Eu/Eu* = Eun/√ 2008). n, Chondite-normalized (Nakamura, 1974); pm, primitive-mantle-normalized (Sun and McDonough, 1989). Dash, Not measured. N.f., Not found.

0.46

0.49

0.78

3.80

0.75

2.78

0.30

2.13

0.34

2.34

0.34

Er

0.78

3.85

Lu

2.43

Ho

0.34

0.78

Dy

0.76

2.23

3.91

Tb

1.17

4.04

Yb

0.75

Gd

Tm

1.13

4.29

Eu

5.21

5.10

5.00

0257

Sm

0252

21.64 21.65 39.79 21.87 20.14 22.19 23.56 19.25 22.06 19.79 18.45 26.51 26.46 25.36 28.00 20.54 19.61 14.58 10.86 21.61 21.26 15.68 13.05 10.33 17.66

0251

Intru- Intru- Intru- Intrusion 4 sion 1 sion 2 sion 3

4.94

0250

Intrusion 6

Nd

0248

Intrusion 5

Pr

0247

Intrusion 4

43.41 44.50 83.91 44.89 41.56 44.46 46.33 39.77 44.33 42.46 37.21 54.05 53.84 50.76 57.53 44.27 41.02 26.49 16.98 38.82 35.98 26.49 24.00 17.57 36.13

0227

Intrusion 3

Basites of the Chaya site

Ce

0205

0212

0204

0201

0202

Intrusion 2

Intrusion 1

Component Basites of the Khibelen site

Table 4 (continued)

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Table 5. Sm–Nd isotope data for basites of the Khibelen and Chaya sites Sample

Age, Ma

Sm/144Nd

Content, ppm Sm

Nd

5.21

21.65

147

143

Nd/144Nd ± 2σ

εNd(T)

TNd(DM), Ma

0.1449

0.511708 ± 6

–7.0

3177

Basites of the Khibelen site 0202

1674

0204

1674

9.14

39.79

0.1383

0.511643 ± 7

–6.9

3032

0248

1674

4.36

19.25

0.1363

0.511711 ± 14

–5.1

2820

0250

1674

5.19

22.06

0.1417

0.511710 ± 8

–6.3

3033

0259

1674

5.96

26.46

0.1355

0.511677 ± 5

–5.6

2860

–2.5

2466

Basites of the Chaya site 04104

1752

4.31

21.61

0.1199

0.511628 ± 8

04105

1752

4.24

21.26

0.1200

0.511637 ± 7

–2.3

2453

04107

1752

3.34

15.68

0.1284

0.511682 ± 24

–3.3

2613

06392

1752

2.61

10.33

0.1523

0.511944 ± 7

–3.6

2982

06393

1752

3.92

17.66

0.1338

0.511646 ± 10

–5.2

2856

146

Nd/144Nd = 0.7219 and reduced to 143Nd/144Nd = 0.512100 in the JNdi-1 Nd-standard. The weighted average 143Nd/144Nd value in JNdi-1Nd-standard was 0.512101 ± 8 (n = 20). Calculation of εNd(T) and model age TNd(DM) was made using the modern CHUR values after Jacobsen and Wasser-

Fig. 3. Geological scheme of the Chaya River region, modified after Donskaya et al. (2007). 1, Quaternary deposits; 2, Cambrian deposits; 3, Neoproterozoic sedimentary rocks (Baikal Group); 4, Late Paleoproterozoic basites; 5, Paleoproterozoic rocks of the Okun’ Formation of the Teptorga Group; 6–8, Paleoproterozoic rocks of the Akitkan Group: 6, Upper Chaya Subformation, 7, Lower Chaya Subformation, 8, Khibelen Formation; 9, faults; 10, sampling localities.

burg (1984) (143Nd/144Nd = 0.512638, 147Sm/144Nd = 0.1967) and the DM values after Goldstein and Jacobsen (1988) (143Nd/144Nd = 0.513151, 147Sm/144Nd = 0.2136). Isotope and geochemical characteristics of basites We established that basites from the southern and northern parts of the dike swarm (Khibelen and Chaya sites) contain SiO2 = 48.6–51.8 and 45.4–56.7 wt.% and Na2O + K2O = 3.00–5.04 and 2.07–4.44 wt.%, respectively (Table 4). On the Na2O + K2O–SiO2 classification diagram (Sharpenok et al., 2009), the composition points of the Khibelen basites fall in the fields of basalts and trachybasalts, and those of the Chaya basalts, in the fields of basalts and andesite-basalts (Fig. 4a). According to K2O contents, the basites of both sites are medium- and high-K rocks (Fig. 4b). Based on the above petrochemical data and petrographic features of rocks, namely, the presence of primary biotite and K-feldspar, we assigned these basites to medium-alkali (subalkalic) rocks. The basites of both sites have low and medium contents of TiO2 = 0.86–1.75 and 0.61–1.01 wt.% and P2O5 = 0.08–0.17 and 0.08–0.15 wt.%, respectively (Table 4). All analyzed rocks are differentiated; the mg# values of the Khibelen basites vary from 51 to 72, and those of the Chaya basites, from 53 to 69 (Tables 4 and 5). Note that mg# variations are observed both within the dike swarm and within its constituent bodies. For example, for samples 0201–0205 taken from the same dike at the Khibelen site, mg# varies from 51 to 69; the maximum value is found in the core sample, and the minimum one, in the peripheral sample (Table 4). On the variation diagrams for the Khibelen basites, mg# shows a negative correlation with TiO2, Th, Nb, and La. These correlations are expressed to different degrees and are observed on analysis of all studied samples and in the case when most highly differentiated sample 0204 is eliminated from the analysis. At the same time, the Khibelen basites show no correlation between mg# and P2O5 and between mg# and Zr

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Fig. 4. Na2O + K2O–SiO2 (Sharpenok et al., 2009) (a) and K2O–SiO2 (Le Maitre, 1989) diagrams (b) for Late Paleoproterozoic basites of the northern Baikal area. Basites from the sites: 1, Khibelen, 2, Chaya.

in the case of elimination of sample 0204. In the Chaya basites, mg# shows a slight negative correlation with TiO2, La, and Zr but no correlation with P2O5, Nb, and Th (Fig. 5). Basites of the Chaya site are poorer in TiO2, La, Th, and Nb than basites of the Khibelen site but have the same mg# values (Table 4, Fig. 5). The average Nb contents are low, 7.25 to 22.54 ppm in the Khibelen basites and 2.46 to 10.49 ppm in the Chaya basites. The basites have elevated contents of Th, 3.36 to 8.44 ppm at the Khibelen site and 1.31 to 3.93 ppm at the Chaya site (Table 4, Fig. 5).

Basites of the Khibelen site are richer in LREE than basites of the Chaya site (La = 18.32–43.20 against 7.91–19.61 ppm). All rocks show fractionated REE patterns: (La/Yb)n = 6.06– 8.15 in the Khibelen basites and 2.50–8.04 in the Chaya basites (Fig. 6a, b). The Eu/Eu* value varies from 0.69 to 0.91 in the Khibelen basites and from 0.82 to 1.13 in the Chaya basites (Table 4). The primitive-mantle-normalized (Sun and McDonough, 1989) element spidergrams of basites from both sites show distinct negative anomalies of Nb–Ta, P, and Ti (Fig. 6c, d).

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Fig. 5. mg#–TiO2, P2O5, Nb, Th, La, and Zr variation diagrams for Late Paleoproterozoic basites of the northern Baikal area. 1, trends constructed over all composition points of the Khibelen basites; 2, trends constructed over the composition points of the Khibelen basites except for differentiated sample 0204; 3, trends constructed over the composition points of the Chaya basites. Other designations follow Fig. 4.

For Sm–Nd isotope study, we chose the most representative samples of basites from the Khibelen (five samples) and Chaya (five samples) sites, with varying mg# values and SiO2 contents. The isotopic composition of Nb (Table 5) evidences that the Khibelen and Chaya basites are similar in some features. In particular, they have close 143Nd/144Nd ratios, varying from

0.511643 to 0.511711 in the Khibelen basites and from 0.511628 to 0.511944 in the Chaya ones. All samples show negative initial values of εNd(T). However, the calculated εNd(T) values for the Khibelen basites are somewhat lower (εNd(1674 Ma) = –5.1 to –7.0) than those for the Chaya basites (εNd(1752 Ma) = –2.3 to –5.2) (Table 5).

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Fig. 6. Chondrite-normalized (Nakamura, 1974) REE patterns (a, b) and primitive-mantle-normalized (Sun and McDonough, 1989) spidergrams (c, d) of Late Paleoproterozoic basites of the northern Baikal area.

Discussion Basites of the Khibelen and Chaya sites show distinct Nb–Ta and Ti anomalies on spidergrams (Fig. 6c, d) and negative εNd(T) values (Table 5). On the Th/Yb–Nb/Yb diagram (Dampare et al., 2008; Pearce, 1983) (Fig. 7), the composition points of all basites lie beyond the N-MORB + E-MORB + OIB field; they fall in the field of rocks formed from mantle sources either having a subduction component or contaminated with crustal material. Let us analyze which of the processes could have prevailed during the formation of basites. For this purpose, we will use mainly the ratios of Th, Nb, and La, which serve as indicators for the reconstruction of mantle sources, as well as the ratios of other incompatible elements, in particular, Zr and Y (Safonova et al., 2008; Saunders et al., 1988; Turkina and Nozhkin, 2008). The basites of the Khibelen and Chaya sites show high indicative ratios Th/Nbpm (2.56–4.19 in the Khibelen basites and 2.75–4.47 in the Chaya ones) and La/Nbpm (1.83–2.86 in the Khibelen basites and 1.70–3.69 in the Chaya ones), close to those of continental crust (Fig. 8a). The Th/Nbpm values are usually higher than the La/Nbpm values (the only exclusion is sample 0486 from the Chaya site), and, correspondingly,

Th/Lapm = 0.96–1.94, i.e., is almost always >1, which is a specific feature of continental-crust rocks and distinguishes them from mantle rocks, including IAB (Fig. 8a, b). Moreover, the Khibelen and Chaya basites show high La/Smn ratios (1.91–3.30) close to those in continental-crust rocks (Fig. 8c). The above indicative ratios, together with distinct anomalies of Ti on spidergrams (Ti/Ti* = 0.47–0.66 in the Khibelen basites and 0.43–0.71 in the Chaya ones) (Table 4; Fig. 6c, d) and with negative values of εNd(T) (Table 5), suggest that the mantle sources of the basites were contaminated with crustal material. Contamination processes, which made the major contribution to the formation of the studied basites, are also evidenced from their high Ba/Y ratios. Their average ratios are higher than those in IAB resulted from subduction and are the same as in the rocks of continental crust (Fig. 8d) (Martynov et al., 2007). Note that all analyzed basites, including the most primitive ones with mg# > 65, have geochemical features of rocks contaminated with continental crust (Table 4). Basites of both the Khibelen and the Chaya sites show no distinct correlations between Th/Lapm and Nb/Lapm and between Nb/Nb* and La/Smn (Fig. 8b, c), which distinguishes them from classical

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Fig. 7. Th/Yb–Nb/Yb diagram (Dampare et al., 2008; Pearce, 1983) for Late Paleoproterozoic basites of the northern Baikal area. Composition points of N-MORB, E-MORB, and OIB are given after Sun and McDonough (1989). SZ/CC is the field of rocks resulted from mantle sources either having a subduction component or contaminated with crustal material. Designations follow Fig. 4.

contaminated mafic volcanics (Turkina and Nozhkin, 2008). However, as seen from the above diagrams (Fig. 8b, c), such correlations would be possible only if the mantle source, such

as N-MORB or OIB, was contaminated with crustal material. There is also no correlation between εNd(T) and SiO2 for both types of basites (Fig. 9a). The Chaya basites show no correlation between the degree of Nb anomaly (Nb/Nb*) and crustal components SiO2 and La (Fig. 9b, c) in contrast to the Khibelen basites showing positive correlations between these parameters (Fig. 9b, c). In the latter case, the degree of Nb anomaly decreases as the content of crustal component increases, which contradicts the classical model of mantle source contamination with crustal material. On the εNd(T)– Nb/Nb* diagram (Fig. 9d), one can see a negative correlation between these parameters, i.e., a εNd(T) decrease is accompanied by the weakening of Nb anomaly, which also disagrees with the above model (Patchett et al., 1994). Thus, though all indicative geochemical ratios point to the formation of the Khibelen and Chaya basites from a mantle source contaminated with continental crust, the above correlations between trace elements and Nb isotope composition show that the basites do not obey the classical model of crustal contamination of a mantle source. Anyway, the parental mantle melts of the studied basites should have formed in intermediate magma chambers located in the crust, where they were, most likely, subjected to contamination with crustal material.

Fig. 8. Th/Nbpm–La/Nbpm (a), Th/Lapm–Nb/Lapm (b), Nb/Nb*–La/Smn (c), and Ba/Y–Zr/Y (d) diagrams for Late Paleoproterozoic basites of the northern Baikal area. Composition points of N-MORB and OIB are given after Sun and McDonough (1989), and that of IAB, after Dorendorf et al. (2000). Composition points of the upper continental crust (UCC), middle continental crust (MCC), and lower continental crust (LCC) are taken from Rudnick and Fountain (1995). Designations follow Fig. 4.

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Fig. 9. εNd(T)–SiO2 (a), Nb/Nb*–SiO2 (b), Nb/Nb*–La (c), and εNd(T)–Nb/Nb* (d) diagrams for Late Paleoproterozoic basites of the northern Baikal area. Basites from sites: 1, Khibelen, 2, Chaya; 3, trend constructed over the composition points of the Khibelen basites; 4, trend constructed over the composition points of the Chaya basites. FC, fractional-crystallization trend, AFC, assimilation–fractional-crystallization trend.

To clarify the sources of the Khibelen and Chaya basites, let us analyze the ratios of incompatible elements in these rocks, which do not depend on fractional crystallization and partial melting but are perfect indicators of heterogeneous composition (Sklyarov, 2001). These are Th/Nb, La/Nb, Zr/Y, Y/Nb, and Zr/Nb. The basites of both sites show varying La/Nb, Th/Nb, Zr/Nb, and Y/Nb ratios (Fig. 10). The Zr/Y ratios in the Khibelen basites are close to each other, whereas in the Chaya basites they are different (Fig. 10a, c). The ratios of incompatible elements in the Khibelen basites are close to those in continental crust or IAB (Fig. 10), which suggests the mixed source of the rocks. We must emphasize again that there is no correlation between εNd(T) and SiO2 (Fig. 9a) and there is a negative correlation between εNd(T) and the degree of Nb anomaly (Fig. 9d), which contradicts the classical model of crustal contamination of a mantle source. However, the Th/Nb–La/Nb diagram (Fig. 10b) shows that the middle-crust rocks have higher ratios than the lowerand upper-crust rocks, i.e., show a more distinct Nb anomaly (Nb/Nb* (middle crust) = 0.28, Nb/Nb* (lower and upper crust) = 0.58–0.50 (Rudnick and Fountain, 1995)). Therefore, the revealed regularities of the εNd(T)–Nb/Nb* correlation are

mainly due to the initial contamination of mantle source (with parameters probably close to the IAB ones) with middle-crust rocks, which was followed by the addition of lower/uppercrust rocks. It is likely that the melt migrated from one intermediate chamber to another within continental crust, where it was then contaminated with crustal material of different chemical composition. The presence of a mantle component with geochemical features of IAB in the source of the Khibelen basites is evidenced by the established subductionally enriched lithospheric source of mafic igneous rocks dated at 1.88–1.85 Ga, which intruded near the area of occurrence of the studied basites (Shokhonova et al., 2010). The contents of all incompatible elements in the Chaya basites vary greatly. For example, the basites are similar in La/Nb to IAB and rocks of the middle and lower crust; in Th/Nb, to IAB and rocks of the middle and upper crust; in Zr/Y, to IAB, N-MORB, and rocks of the lower and middle crust; and in Y/Nb, to rocks of the middle and lower crust, shifting to the field of N-MORB (Fig. 10a–d). Thus, all these data evidence that the Chaya basites were generated from a mantle–crust source resulted from the mixing of mantle components compositionally similar to IAB and N-MORB

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Fig. 10. Zr/Y–La/Nb (a), Th/Nb–La/Nb (b), Th/Nb–Zr/Y (c), and Y/Nb–Zr/Nb (d) diagrams for Late Paleoproterozoic basites of the northern Baikal area. Designations follow Figs. 4 and 8.

Fig. 11. CaO/Al2O3–mg# (a) and Eu/Eu*–mg# (b) diagrams for Late Paleoproterozoic basites of the northern Baikal area. Designations follow Figs. 4 and 8.

with a crustal component. The contribution of a primitive mantle component similar in composition to N-MORB is confirmed by the lower contents of Th, Nb, and La and higher εNd(T) values in the Chaya basites as compared with the Khibelen ones (Table 4; Figs. 5 and 9). The composition points of the Khibelen and Chaya basites are almost always arranged far from the composition point of OIB, which excludes the presence of a mantle component with

similar geochemical features in the sources of these basites (Fig. 10a–d). This admits only the minimum effect of mantle plume on the formation of basites. The wide variations in mg# values in the Khibelen and Chaya basites and the location of their composition points on the variation diagrams (Fig. 5) suggest that fractional crystallization played a particular role in the formation of these rocks. The basites of both sites show distinct positive correlations

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between mg# and CaO/Al2O3 (Fig. 11a), which indicates fractionation of clinopyroxene during the formation of differentiated varieties. In the Chaya basites, Eu/Eu* increases as mg# decreases, i.e., as the degree of fractionation grows. This suggests that the stronger fractionated varieties contain more plagioclase.

Conclusions The isotope-geochemical features of Late Paleoproterozoic basites from intrusions of the southern (Khibelen site) and northern (Chaya site) parts of the dike swarm in the Baikal marginal salient of the Siberian craton basement (northern Baikal area) help to recognize both the similarity and the difference in their sources and features of formation. The Khibelen basites correspond in chemical composition to basalts and trachybasalts, and the Chaya basites, to basalts and andesite-basalts. According to the petrographic and petrochemical data, the basites of both sites are referred to as medium-alkali (subalkalic) series. All studied basites show distinct negative Nb–Ta and Ti anomalies on spidergrams, negative εNd(T) values, and indicative geochemical ratios Th/Nbpm, La/Nbpm, and La/Smn > 1, which evidences that the rocks formed from mantle sources contaminated with continental crust. The contamination took place, most likely, in intermediate magma chambers located in the crust. The mantle source of the Khibelen basites, probably similar in geochemical features to IAB, might have been initially contaminated with middle-crust rocks and later with lower/upper-crust rocks. Differentiated basic varieties resulted from the fractionation of clinopyroxene. The mantle–crustal source of the Chaya basites formed during the mixing of mantle components similar in composition to IAB and N-MORB with a crustal component. Differentiated basic varieties resulted from the fractionation of clinopyroxene. The performed studies showed the heterogeneous composition of the upper mantle beneath different, even proximal, sites of the Siberian craton basement. Moreover, some difference in age between the basites of the southern and northern parts of the dike swarm evidences that the crustal extension from north to southwest led to a change in the composition of mantle sources from mixed IAB–MORB to IAB, with the participation of a crustal component in both cases. We thank T.A. Kornilova, Institute of the Earth’s Crust, Irkutsk, for help in petrographic study of rocks. This work was supported by grant 13-05-91173-GFEN_a from the Russian Foundation for Basic Research and project 10.3 from the Department of Geosciences of the Russian Academy of Sciences.

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