Crystallization history of Paleozoic granitoids in the Ol’khon region, Lake Baikal (SHRIMP-II zircon dating)

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Crystallization history of Paleozoic granitoids in the Ol’khon region, Lake Baikal (SHRIMP-II zircon dating) V.A. Makrygina a,*, E.V. Tolmacheva b, E.N. Lepekhina c a

A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia Institute of the Precambrian Geology and Geochronology, Russian Academy of Sciences, nab. Makarova 2, St. Petersburg, 199034, Russia c A.P. Karpinsky Russian Geological Research Institute, V.O., Srednii pr. 74, St. Petersburg, 199106, Russia

b

Received 4 May 2012; accepted 21 February 2013

Abstract At the Center of Isotope Studies of the A.P. Karpinsky Russian Geological Research Institute, the structure and isotope composition of zircons from two granitoid complexes, the age of their sequential growth zones, and the hosted inclusions have been studied using a SHRIMP-II ion mass spectrometer. The zircons consist of deformed cores with crystalline melt inclusions and of shells: inner, with glassy, partly devitrified inclusions, and outer metamorphogene, with fluid inclusions. Judging from the zircon zoning, crystallization of melts of both complexes proceeded in several stages: (1) The generation of melts and the beginning of zircon core growth (505 and 493 Ma) were synchronous with the overthrusting in the Ol’khon region; (2) The rapid ascent of melts (the inner shell, 479 and 475 Ma) together with the host rocks was caused by upthrust faulting and shear dislocations; (3) The metamorphogene shell (456 Ma) reflects the second stage of metamorphism. At the same time, the Shara-Nur migmatite–granite complex corresponds in composition, structures, and textures to syncollisional K-granites, whereas the differentiated Khaidai gabbro-diorite–diorite–granodiorite–granite complex is close in geochemical features (similar to those of the Anga sequence metavolcanics) and the mantle (juvenile) source of substance to the recent island-arc magmatism. It is suggested that the Caledonian island-arc magmatism was close in time to the accretion of the sediments of back-arc basin (Ol’khon Group) to the continental margin, on the one hand, and to the island-arc block, on the other. © 2014, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: granitoids; metamorphism; tectonic processes; zircons; inclusions; age

Introduction The main aspects of the formation of granitoids in the Ol’khon region, including their geologic position, relationship with tectonic processes and metamorphism, composition, and geochronology, are considered elsewhere (Ivanov and Shmakin, 1980; Makrygina and Petrova, 1996; Makrygina et al., 2000a,b; Vladimirov et al., 2004). The ages are given mainly for the granitoids of the Shara-Nur complex and fall in the narrow range 465–486 Ma. The granites of the Khaidai complex are dated at 570 Ma (Rb–Sr) and 484–465 Ma (U–Pb and Ar–Ar) (Makrygina et al., 2010). More comprehensive studies always add new details, sometimes highly crucial, to the history of melting, magma crystallization, and sources of substance for igneous rocks. The relationship of the granitoids of the Shara-Nur and Khaidai complexes with the main stages

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

of the regional geologic evolution and metamorphic processes are still obscure, because the granitoids are localized in different structural blocks and show no direct mutual relationship. The goal of this work was to elucidate the history of zircon crystallization in the granitoids of both complexes and refine their evolution in time.

Geologic position of granitoids and their relationship with metamorphism The Ol’khon region (Ol’khon Island and adjacent area) extends along the western shore of Lake Baikal (Fig. 1). Structure-geological studies (Fedorovskii et al., 1993, 1995) showed that its volcanosedimentary sequences are a collage of tectonic plates resulted from the Caledonian collision and composed of the metamorphic rocks of the Ol’khon and Anga sequences. Two stages of metamorphism are recognized in the Ol’khon regional sequences (Fedorovskii et al., 1993; Makrygina et al.,

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.2013.12.010 +

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V.A. Makrygina et al. / Russian Geology and Geophysics 55 (2014) 33–45

Fig. 1. Location of the Shara-Nur granite-gneiss dome and Krestovsky massif in the Ol’khon region. 1, granitoids of the Khaidai complex; 2, gabbroids of the Ozersky complex; 3, large granite-gneiss domes of the Shara-Nur complex, Ol’khon Group; 4, gneiss and 5, marble-schist tectonic plates; 6, schists and marbles of the Anga sequence; 7, cataclasites and mylonites; 8, faults.

2000a,b; Makrygina and Petrova, 2005). They are both of zonal structure. Early, syncollisional metamorphism developed in the conditions of granulite facies near the marginal suture of the platform (Primorsky Fault) and in the conditions of amphibolite facies in the south of the region. It was accompanied by weak enderbitization in the granulite facies zone and by plagiomigmatization in the amphibolite facies zone (Makrygina and Petrova, 1996). The next stage was superposition of amphibolite facies on granulite and amphibolite facies in the Ol’khon Group area and of epidote–amphibolite facies on amphibolite facies in the Anga sequence in the south. The second stage is wide development of migmatites and granite– gneiss domes. Granitization proceeded in the gneiss plates of the Ol’khon Group, in amphibolite facies, during the transition to the regressive stage. The Anga sequence with low-grade metamorphism lacks granitization. The Shara-Nur complex of granitoids in the Ol’khon region includes migmatites, granite–gneiss domes, and allochthon veins of pegmatoid granites (Pavlovskii and Eskin, 1964). Exposures of these rocks are confined to the Ol’khon Group of metamorphic rocks and occur in tectonic plates composed of garnet–biotite, biotite, and amphibole–biotite gneisses, making up about a half of the bulk group (Fig. 1). These zones have many small unnamed granite–gneiss domes. The dome zones are separated from each other by plates composed of alternating marbles, quartzites, diopside plagioschists, and amphibolites lacking migmatization but containing long dikes of pegmatoid granites assigned to the vein series of the Shara-Nur complex. The Khaidai complex of granitoids includes multiphase intrusions confined to the exposures of the Anga sequence in the Ol’khon region (Makrygina and Petrova, 1996; Makrygina et al., 2010). Their composition varies from gabbro–diorites and diorites to granodiorites and granites.

The Shara-Nur granite–gneiss dome The Shara-Nur granitoid complex was recognized by Pavlovskii and Eskin (1964). The largest granite–gneiss dome near Lake Shara-Nur on Ol’khon Island is its petrotype. The complex is hosted by garnet–biotite and amphibole–biotite gneisses and migmatites grading into para-autochthonous

granites. The granites are highly inhomogeneous, medium- to coarse-grained, with a gneissoid structure and large microcline grains. They have many sites of incompletely altered gneisses. The granitoids contain, on the average (vol.%): plagioclase— ~30, quartz—25, biotite—5, and microcline—>40. The rocks are of cataclastic and granoblastic textures. Microcline develops after plagioclase, as observed from myrmekite rims and relict plagioclase in coarse K-feldspar grains (Fig. 2a). In the thin sections of the dated sample, there are veinlets of finer-grained granite cutting the coarse-grained aggregate (Fig. 2b). The accessory minerals in the massif are apatite, sphene, magnetite, allanite, and abundant zircon; the latter is present as elongate or oval grains with intricate zoning, often with dark cores. Our earlier study of the geochemistry of the Shara-Nur complex granitoids performed throughout Ol’khon Island and the Ol’khon region showed diverse K2O/Na2O ratios and trace-element compositions, which depend on the composition of the host rocks (Makrygina and Petrova, 1996). In the central and northern zones of Ol’khon, there are migmatites, granite-gneisses, and granites enriched in REE and Zr (Table 1), that is why this dome was chosen for zircon study.

The Krestovsky multiphase massif of the Khaidai complex Granitoids of the Khaidai complex are confined to the exposures of the Anga rock sequence. It exposes mostly in the southwest of the Ol’khon region, in the area between the Orso Bay and the Bugul’deika River, where it is truncated by the Primorsky Fault (Fig. 1). The quartzite–dolomite–marble members of the sequence alternate with biotite–amphibole and epidote–amphibole schists corresponding in composition to normal and mafic volcanics (andesites, andesite–basalts, and basalts). In places, these rocks preserve the textures of plagioporphyrites and amphibole porphyrites. In geochemistry they are similar to rocks of developed island arcs (Makrygina et al., 2000a). The Anga sequence abounds in intrusions, from pyroxenite–gabbro–gabbro-diorite massifs of the Ozersky complex to gabbro-diorite–diorite–granodiorite–granite massifs of the Khaidai complex. The rocks of the Khaidai complex are of typical magmatic hypidiomorphic textures and massive

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V.A. Makrygina et al. / Russian Geology and Geophysics 55 (2014) 33–45 Table 1. Granitoids of the Shara-Nur (PO856–PO1252) and Khaidai complexes (PO2538–PO1738) of the Ol’khon region Component

PO856

PO2506

PO2519

PO1239

PO1252

PO2538

PO2539

PO2540

PO1784

PO1738

SiO2, wt.%

70.37

69.56

72.76

75.11

74.11

56.10

58.97

63.60

69.92

71.94

TiO2

0.45

0.47

0.22

0.07

0.08

0.89

0.73

0.65

0.36

0.3

Al2O3

14.66

14.71

14.26

13.38

13.53

16.32

17.08

18.18

15.11

14.36

Fe2O3 tot

1.25

3.49

2.57

1.15

1.84

7.27

6.58

4.17

2.94

2.78

MnO

0.03

0.05

0.02

0.01

0.02

0.15

0.11

0.05

0.05

0.04

MgO

0.55

0.55

0.43

0.16

0.13

4.64

3.57

1.59

1.21

0.77

CaO

1.66

1.65

1.39

0.77

0.71

7.15

5.80

3.67

3.06

1.99

Na2O

3.02

3.16

2.37

2.37

2.48

4.29

3.99

4.95

4.13

3.77

K2O

5.18

4.52

5.24

6.4

6.59

1.87

2.01

2.28

2.2

3.23

P2O5

0.11

LOI

0.13

0.10

0.03

0.05

0.27

0.21

0.22

0.1

0.09

1.54

0.29

0.34

0.15

1.11

1.01

0.61

0.57

0.58

Total

99.79

100.09

100.13

99.76

99.63

100.22

100.19

100.21

99.65

99.85

Li, ppm

30

26

6

2

4

22

18

26

22

12

Rb

86

84

86

99

113

54

52

54

50

74 1100

Ba

2100

3000

7400

2300

2300

1000

1200

1900

773

Sr

280

290

420

340

760

550

690

640

530

420

Pb

18

24

31

23

23

11

10

12

16

11

Zn

60

71

35

5

17

150

86

77

58

29

Sn

1.4

2.2

1.2

0.4

0.9

1.4

1.9

1.5

1.7

1.2

Cu

9.2

5.8

9.3

15

15

9.8

17

14

34

12

Co

5.4

4

3

2.8

2.5

27

32

12

8

5.5

Ni

6.5

5.6

8.8

6.5

10

70

55

18

18

9.4

Cr

7.2

2

2

4.4

9.4

120

2

2

15

9.5

V

36

33

35

7.6

39

170

140

58

38

28

La

83

72

250

31

110

26

17

27

19

12

Ce

135

166

538

71

190

38

23

39

27

26

Nd

54

42.3

92

23

100

14

7.3

9.5

12

10

Yb

2.5

2.9

1.2

0.2

1.2

1.4

1.4

1.3

0.84

0.91

Y

23

25.3

10.5

1.9

9.4

16

14

8.5

5

8.4

Zr

400

447

349

80

290

163

141

380

120

120

F

250

N.d.

N.d.

100

150

N.d.

N.d.

N.d.

450

150

Note. PO856, Biotite granite, north of the Uzur Bay; PO2506–PO1252, Shara-Nur granite-gneiss dome; Krestovsky massif: PO2538—gabbro-diorite, PO2539—diorite, PO2540, PO1784—granodiorite, PO1738—granite. Here and in Table 2, analyses were carried out at the Institute of Geochemistry, Irkutsk: X-ray diffraction (silicate) (analyst A.L. Finkel’shtein), flame photometry (Li, Rb, analyst L.V. Altukhova), and atomic-emission absorption spectroscopy (analysts M.V. Pazhitnykh, O.V. Zarubina, and V.A. Rusakova). N.d., Not determined.

structures. There are widespread varieties with zonal plagioclase phenocrysts (Fig. 3). The dark-colored minerals are hornblende and rare pyroxene (in gabbro-diorites), amphibole and biotite (in diorites), and biotite (in granites). The accessory minerals are prevailing sphene and apatite and subordinate elongate zircon and fluorite grains. In geochemical features (Table 1, Fig. 4) the intrusive rocks are identical to the metavolcanics of the Anga sequence. Therefore, we interpreted both the volcanic and the intrusive rocks of the Anga Formation as igneous derivates of a mature island arc, which we called the Anga–Talanchan arc (Makrygina and Petrova, 2005). The abundance of metasediments accumulated in oxidizing conditions (in contrast to the graphite-containing metasediments of the Ol’khon Group) suggests

that the entire Anga sequence was deposited on the slopes of the island arc. The sequence was metamorphosed in two stages: under conditions of amphibolite facies at the collision stage and under superposed epidote–amphibolite metamorphism at the stage of shear dislocations (Fedorovskii et al., 1995; Makrygina and Petrova, 2005). This is evidenced from the presence of epidote in cataclased diopside–amphibole schists. No rock migmatization is observed here. At the same time, there are widespread multiphase intrusions of the Ozersky and Khaidai complexes, and granite, syenite, granodiorite, and diorite dikes similar in composition to the late phases of the Khaidai complex (Table 2) and being their veined series. The dikes cut both the volcanosedimentary sequence and the massifs of the Ozersky and Khaidai com-

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V.A. Makrygina et al. / Russian Geology and Geophysics 55 (2014) 33–45

Fig. 2. Biotitic granite of the Shara-Nur complex: Micr, microcline; Q, quartz; Pl, plagioclase; Bt, biotite. a, Development of large microcline with myrmekites at the boundary with plagioclase; b, crystallization of fine-grained granite in cracks, the result of late melting. ×32, crossed nicols.

To study the growth of zircon and determine the age of its shells, we chose the Shara-Nur granite-gneiss dome as a reference for the Shara-Nur complex. At the dome center,

200 m from the shore of Lake Shara-Nur, we took a 14 kg granite sample (PO2519) and separated zircon (~350 mg in weight). For dating, optical (in transmitted and reflected light) and cathodoluminescence (CL) images were used, which depict the internal structure and zoning of zircons. The CL images were obtained on a CamScan MX2500 scanning electron microscope. Local U–Pb dating of zircons was made on a SHRIMP-II ion microprobe at the Center of Isotope Studies of the A.P. Karpinsky Russian Geological Research Institute, St. Petersburg, following the standard technique (Larionov et al., 2004; Williams, 1998). The U/Pb ratios were normalized to the Temora standard zircon (Black et al., 2003). The obtained data were processed applying the SQUID software (Ludwig, 2001a); the plots with concordia were constructed using the ISOPLOT program (Ludwig, 2001b). The error ellipses and concordant-age errors are at the 2σ level.

Fig. 3. Granodiorite of the Khaidai complex (sample PO2539), hypidiomorphic texture with zonal plagioclases. ×32, crossed nicols.

Fig. 4. N-MORB-normalized (Taylor and McLennan, 1985) REE spidergram for the Anga sequence metavolcanics (1, 2) and diorite (3), granodiorite (4), and granite (5) of the Khaidai complex (Krestovsky massif); 6, average andesites of the Kamchatka Peninsula (Ivanov, 1990).

plexes and differ strongly from the veins of K-pegmatoid granites of the Shara-Nur complex in a predominance of Na. Gabbro-diorites, diorites, granodiorites, and granites of the Krestovsky complex break through each other, with clear intersections, which suggests their successive intrusion and phase relations (Makrygina et al., 2010). At the same time, there are widespread intersections of intrusions with the formation of magmatic breccias as well as trapping of xenoliths of the host porphyrites (Fig. 5). This points to the unquiet tectonic setting during the intrusion.

The structure of zircons, their U, Th, and Pb isotope composition and age

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V.A. Makrygina et al. / Russian Geology and Geophysics 55 (2014) 33–45 Table 2. Composition of dikes of pegmatoid granites of the Shara-Nur complex (O61–O541) and veined series of the Khaidai complex (PO1632–PO1937) Component

O61

O511

O541

PO1632

PO1628

PO1723

PO1924

PO1937

SiO2

73.3

74.28

72.6

65.59

70.61

72.82

73.17

74.38

TiO2

0.08

0.01

0.55

0.35

0.29

0.14

0.1

0.11

Al2O3

15.3

13.62

13.25

15.63

15.43

14.2

13.19

13.74

Fe2O3

0.1

0.05

0.02

0.1

1.07

0.46

0.62

1.03

FeO

0.39

1.29

0.05

2.87

1.61

2.33

2.78

1.26

MnO

0.01

0.03

0.06

0.04

0.03

0.04

0.05

0.03

MgO

0.15

0.11

0.66

0.91

0.88

0.38

0.33

0.32

CaO

1.06

0.44

1.67

2.81

2.55

1.53

1.22

1.19

Na2O

2.44

2.76

2.21

4.49

4.5

5.08

3.43

3.77

K2O

6.69

6.30

8.62

2.03

1.99

2.5

4.46

3.33

P2O5

0.02

0.10

0.01

0.12

0.1

0.04

0.04

0.04

LOI

0.52

1.04

0.23

0.6

0.62

0.08

0.28

0.53

Total

99.96

100.03

99.93

100.05

100.02

100.02

100.10

99.73

Li

0.1

0.7

0.1

18

20

22

4

6

Rb

113

364

528

46

46

52

62

82

Ba

3600

25

110

870

820

860

913

1500

Sr

300

100

91

610

580

420

290

290

Pb

28

28

57

11

7.9

5.1

9.3

16

Zn

5

0.1

0.1

40

43

49

39

5

Sn

1

1.9

1.2

0.9

1

3.7

1.9

0.5

Cu

9.1

3.5

2.2

21

8.1

23

14

39

Co

4.1

1.9

1

5.7

5.2

5.3

6.3

9.2

Ni

7.9

2

2

10

12

14

12

8.2

Cr

8

9.1

6.6

10

13

16

13

7.1

V

4

2.5

2.5

36

43

3.5

13

11

La

120

5

5

26

20

15

15

5

Ce

120

20

20

55

44

30

20

15

Nd

34

5

5

24

14

13

5.2

6

Yb

0.6

0.5

0.5

0.5

0.5

0.5

0.21

0.5

Y

4.6

2

2

4.8

4.3

4

1.6

5.6

Zr

170

10

5

150

130

140

110

90

F

170

70

90

200

300

290

280

250

Note. Pegmatoid granites: O61, Kuchelga River, O511, O541, Tazheran steppes; PO1632, PO1628, diorite and granite dikes, Ozersky massif; PO1723, plagiogranite-porphyry, Lake Khall; PO1924, granite dike in the Krestovsky massif; PO1937, granite dike at the contact of the Ozersky massif, Khariuzovaya Bay. Bold type marks the difference in the content of alkalies between the two complexes.

Zircon grains from the sample PO2519 are yellowish transparent and semitransparent. They consist of a core and one or two shells. The cores are strongly deformed and sometimes boudinaged as compared with the shells (Fig. 6). Most of the separated zircons are elongate-prismatic crystals with Ke = 2.59–2.85 and bear inclusions of apatite and ore mineral crystals as well as melt and fluid inclusions, which were studied by E.V. Tolmacheva. On CL and optical images, the grains show a thin zoning. Their cores are darker than the shells. The outer shell is much lighter (Fig. 6-1, 6-7). The cores (Zr1) are of different types: nonzonal transparent, often boudinaged (Fig. 6-2), or, more seldom, weakly corroded, with a thin oscillatory zoning (Fig. 6-1). The nonzonal

transparent cores bear occasional completely crystallized melt inclusions, specific for intrusive rocks, and solid inclusions of apatite and ore mineral. They testify to the core crystallization from deep-seated magmatic melt. The cores with thin zoning contain occasional primary (syngenetic, judging from their arrangement in the growth zones) partly crystallized melt inclusions consisting of crystalline phases, glass, and a gas bubble (Fig. 6-3, 6-4). Such inclusions are typical of hypabyssal rocks. The zircon cores might have been xenogenic and of different genesis. Trapped zircon clastics are usually older than zircon shells (Makrygina et al., 2013). But none of the cores yielded a significantly older age, as might be expected in the

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V.A. Makrygina et al. / Russian Geology and Geophysics 55 (2014) 33–45

Fig. 5. Magmatic breccia of xenoliths of host plagioporphyrites (1), gabbro-diorites (2) in diorite (3), and all this in granodiorite (4), Khaidai complex, Shirokaya creek valley.

case of zircon clastics trapping by melt. The cores bear completely crystallized secondary melt inclusions penetrating from the inner shell via cracks (Fig. 6-6). All inclusions have been poorly preserved: Most of them were subjected to natural decrepitation, which indicates superposed tectonic movements and high temperatures. The inner shell (Zr2) growing over the core has a thin growth zoning (Fig. 6-1, 6-2) and contains a lot of completely crystallized primary melt inclusions, apatite crystals, and fluid inclusions. Most of the latter are decrepitated; only fluid submicroinclusions have been preserved. Melt inclusions in this shell often “stick” to the core surface and apatite crystals, forming combined inclusions (Fig. 6-5). Some melt inclusions

contain silicate glass, which suggests the shallow depth of melt crystallization at this stage of zircon growth. The surface of the inner shell bears traces of partial dissolution and corrosion, which, together with the poor preservation of fluid inclusions, testifies to repeated metamorphism. The second (outer) (Zr3) shell of zircons is thin and nonzonal (Fig. 6-1). It is observed not on all grains, but its presence makes zircons more euhedral. This shell contains only submicroscopic fluid inclusions. Chains of fluid inclusions from the outer shell penetrate into the inner shell and, sometimes, into the cores, causing their alteration. The presence of only fluid inclusions in the outer shell points to its metamorphogene nature. But the corrosion and partial transformation of the inner shell at the contact with the outer shell as well as the scarcity of crystallized melt inclusions at these sites testify to the repeated melting of the zircons. The 206Pb/238U, 232Th/238U, 207Pb/235U, and 207Pb/206Pb isotope ratios were measured in different zones of zircons to determine their age (Tables 3 and 4). The contents of U in the zircons are about 1500 ppm, with the minimum of 276–303 ppm and the maximum of 2337 ppm. The contents of Th vary from 37 to 879 ppm and are in weak correlation with U. The 232Th/238U values vary from 0.17 to 0.45, with the average being 0.33 in the cores, 0.35 in the inner shell, and 0.24 in the outer shell. This indicates the magmatic origin of the core and inner shell and the metamorphogene nature of the outer shell. Using the 238U/206Pb and 207Pb/206Pb ratios in the sample PO2519, a narrow age range was calculated (Table 3). The concordant age for the bulk rock is 476 ± 4 Ma (Fig. 7). The age histogram constructed by all points is ideal single-peak (Fig. 8). However, despite the partial overlapping of the age intervals (caused by the analysis error) on the U–Pb age histograms constructed separately for the sample zircon cores

Fig. 6. Structure of zircons from the granite PO2519: 1, 7, zonal zircon grain with core (Zr1), inner and outer shells (Zr2 and Zr3) (optical and CL images); 2, fragment of grain with a boudinaged core (Zr1) enveloped by a shell with stuck crystallized melt inclusions; 3–6, melt inclusions: 3, crystallized, in zircon Zr1, 4, vitrified, partly crystallized, in Zr2, 5, in apatite inclusion in Zr1, 6, in Zr3; 7, arrangement of age measurement points in different zones of zircon grains (CL image).

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V.A. Makrygina et al. / Russian Geology and Geophysics 55 (2014) 33–45 Table 3. U–Pb–Th isotope data for zircons from granite of the Shara-Nur complex Point

206

Th/238U Age, Ma

206

U

Th

143

2175

834

Pbr

PO2519_1.1

232

Pbc, Content, ppm

%

0.01

0.40

Isotope ratios

(1) 206 Pb/238U

(1) ±% 207 Pb*/206Pb*

(1) ±% 207 Pb*/235U

(1) ±% 206 Pb*/238U

476 ± 10

0.0568

0.600

0.0766

1.6

2.8

2.2

K, rel. un.

0.808

PO2519_1.2

0.10

22

351

56

0.17

453 ± 11

0.0568

2.2

0.570

3.3

0.0728

2.4

0.731

PO2519_2.1

0.00

78

1170

390

0.34

482 ± 10

0.0570

1.1

0.610

2.5

0.0776

2.3

0.901

PO2519_3.1

0.00

19

276

44

0.16

505 ± 12

0.0574

2.5

0.645

3.4

0.0815

2.4

0.921

PO2519_3.2

0.00

109

1617

597

0.38

486 ± 11

0.0568

0.95

0.613

2.4

0.0783

2.2

0.696

PO2519_4.1

0.01

137

2116

798

0.39

469 ± 10

0.0563

0.83

0.585

2.4

0.0754

2.2

0.937

PO2519_4.2

0.01

20

303

76

0.26

482 ± 11

0.0570

2.1

0.610

3.2

0.0776

2.4

0.751

PO2519_5.1

0.01

150

2337

954

0.42

465 ± 10

0.0562

0.81

0.579

2.4

0.0747

2.2

0.941

PO2519_5.2

0.00

43

734

103

0.14

428 ± 9

0.0565

1.5

0.535

2.7

0.0687

2.3

0.844

PO2519_6.1

0.03

123

1828

680

0.38

488 ± 11

0.0566

0.92

0.614

2.4

0.0786

2.2

0.925

PO2519_6.2

0.02

97

1484

458

0.32

470 ± 10

0.0565

1.0

0.590

2.5

0.0757

2.3

0.908

PO2519_7.1

0.04

94

1354

416

0.32

502 ± 11

0.0579

1.6

0.647

2.8

0.081

2.3

0.814

PO2519_7.2

0.00

21

313

37

0.12

487 ± 11

0.0570

2.7

0.617

3.7

0.0784

2.4

0.665

PO2519_8.1

0.06

85

1249

41

0.03

492 ± 11

0.0578

1.4

0.633

2.7

0.0793

2.3

0.850

PO2519_8.2

0.14

35

538

111

0.21

476 ± 11

0.0560

2.2

0.591

3.2

0.0766

2.3

0.905

PO2519_8.3

0.00

98

1512

579

0.40

471 ± 10

0.0576

1.1

0.601

2.5

0.0757

2.3

0.724

PO2519_9.1

0.02

102

1587

607

0.40

464 ± 10

0.0570

1.1

0.586

2.5

0.0747

2.3

0.900

PO2519_10.1

0.02

70

1076

368

0.35

473 ± 10

0.0562

1.2

0.590

2.6

0.0761

2.3

0.884

PO2519_11.1

0.05

75

1194

347

0.30

456 ± 10

0.0559

1.2

0.566

2.6

0.0733

2.3

0.882

PO2519_12.1

0.00

107

1657

490

0.31

468 ± 10

0.0564

0.99

0.585

2.5

0.0752

2.3

0.915

PO2519_13.1

0.00

71

1048

286

0.28

489 ± 11

0.0576

1.4

0.626

2.7

0.0788

2.3

0.857

PO2519_14.1

0.00

129

2018

879

0.45

463 ± 10

0.0561

0.93

0.576

2.4

0.0745

2.3

0.924

PO2519_14.2

0.02

127

1965

769

0.40

468 ± 10

0.0570

0.93

0.592

2.5

0.0753

2.3

0.927

Note. Analyst I.P. Paderin. The errors are at the 1σ level (Ma, for age; %, for ratios). K, Correlation coefficient of ratios 207Pb/235U–206Pb/238U. The error of the standard sample calibration is 0.65%. Pbc and Pbr are common and radiogenic lead, respectively. (1), correction for Pbc was made by measured 204Pb. 8.1, 8.2, 8.3, analytical points: 1, core, 2 and 3, inner and second shells.

Table 4. U–Pb–Th data for zircons from diorite PO2539 of the Khaidai complex Point

206

Th/238U Age, Ma

206

U

Th

116

1973

1098

Pbr

PO2539.9.1

232

Pbc, Content, ppm

%

0.12

0.57

Isotope ratios

(1) 206 Pb/238U

(1) ±% 207 Pb*/206Pb*

(1) ±% 207 Pb*/235U

(1) ±% 206 Pb*/238U

426 ± 2.9

0.0547

0.5151

0.0683

1.2

1.4

0.70

K, rel. un.

0.505

PO2539.7.1

0.04

88

1374

224

0.17

464 ± 3.3

0.0555

1.4

0.5709

1.6

0.0746

0.74

0.476

PO2539.8.1

0.05

64

981

148

0.16

474 ± 3.5

0.0561

1.4

0.5899

1.6

0.0763

0.77

0.482

PO2539.6.1

0.17

56

847

329

0.40

477 ± 4.1

0.0577

2.2

0.6100

2.4

0.0768

0.89

0.376

PO2539.1.1

0.05

69

1046

464

0.46

477 ± 3.4

0.0555

1.7

0.5880

1.9

0.0768

0.74

0.397

PO2539.10.1

0.04

164

2481

2322

0.97

478 ± 3.1

0.0566

0.84

0.6007

1.1

0.0770

0.67

0.621

PO2539.2.1

0.04

200

3023

2394

0.82

479 ± 3.0

0.0564

0.69

0.5999

0.95

0.0771

0.64

0.680

PO2539.7.2

0.00

74

1083

254

0.24

492 ± 3.5

0.0574

1.1

0.6280

1.3

0.0793

0.73

0.553

PO2539.5.1

0.09

77

1124

640

0.59

493 ± 3.4

0.0569

1.3

0.6236

1.5

0.0794

0.73

0.496

PO2539.3.1

0.09

97

1408

841

0.62

494 ± 3.3

0.0575

1.1

0.6317

1.3

0.0797

0.70

0.527

Note. Analyst E.N. Lepekhina. The errors are at the 1σ level (Ma, for age; %, for ratios). Pbc and Pbr are common and radiogenic lead, respectively. K, Correlation coefficient of ratios 207Pb/235U–206Pb/238U. The error of the standard sample calibration is 0.53%. (1), correction for Pbc was made by measured 204Pb. 7.2, 5.1, and 3.1—cores Zr1; 2.1 and 10.1—Zr2, 1.1–9.1—shells.

40

V.A. Makrygina et al. / Russian Geology and Geophysics 55 (2014) 33–45

Fig. 7. 206Pb/238U–207Pb/235U diagram with concordia for zircons from the granite PO2519 of the Shara-Nur complex.

and shells, the age obviously decreases from core to outer shell (Fig. 9). We analyzed the sites of isotope measurements in zircon grains to refine the growth zones they belong to and to rule out possible measurement errors caused by fragments of other zones or fluid and melt inclusions. Based on the obtained data, we selected only those of the measurement points (a total of 23) that lack distortions, i.e., showing no signs of the metamict state of zircons (eight points). 1. The most reliable measurements of the age (Ma) for the cores are at points 3.1 (505 ± 12), 7.1 (502 ± 11), 6.1 (488 ± 11), and 13.1 (489 ± 11) (Table 3, Fig. 3-6,7), which probably correspond to the beginning of crystallization of melts generated from granite-gneisses at the regressive stage of the first metamorphism episode. 2. The age of the inner shell was determined at points 2.1 (482 ± 10 Ma) and 7.2 (487 ± 11 Ma). These ages differ insignificantly from the ages of the cores, but the shell contains partly noncrystallized melt inclusions. This is explained by the subsequent rock crystallization at a shallower depth during the complete crystallization of the central zone of the granite–gneiss dome. 3. The age of the second shell was measured at points 1.2 (453 ± 11 Ma) and 11.1 (456 ± 10 Ma). The shell apparently grew over the zircons at the second stage of metamorphism. In the Krestovsky massif we sampled rocks of all phases but succeeded in separating zircons from diorites and granodiorites only (samples PO2539 and PO2540, respectively). Besides long light-yellow zircon grains, the concentrates were rich in apatite and, in contrast to the Shara-Nur granites, fluorite.

Zircons from the diorite sample (PO2539) are of complex structure and are often strongly altered. Their grains consist of a core and one or two shells. The cores are of two types: without and with growth zoning, Zr1 and Zr2, respectively (Fig. 10-1, 10-2); Zr2 are both the cores and the inner shell grown over the cores Zr1. The boundaries between Zr1 and Zr2 have or lack traces of weak corrosion. Both Zr1 and Zr2 bear completely crystallized melt inclusions, which confirm the magmatic genesis of diorite. This agrees with its hypidiomorphic texture and type of intrusion (Fig. 5). Moreover, the melt inclusions in Zr1 are usually decrepitated (Fig. 10-4), and those in Zr2 and Zr3 have been well preserved (Fig. 10-6, 10-7). This might be due to the drastic upwelling of crystallizing magma, leading to the melting and resorption of early zircon cores and their overgrowth with new cores at low pressures. The outer shell (Zr3) is usually thin and bears only fluid inclusions, which mark its metamorphogene origin. The shell often grows over only one or both pyramids of the grains (Fig. 10-1). Metamorphogene fluid penetrates into the cores and the inner shell via cracks and causes the alteration of zircon in the near-crack zones. A specific feature of zircons from the sample PO2539 is the different degrees of deformation and resorption of similar zones, the abundance of mineral (mainly quartz) inclusions in the cores (Fig. 10-5) and of zircon intergrowths (Fig. 10-7), and different sizes of the shells and cores. All this testifies to the rapid nonuniform growth of zircons during the melt movement and the simultaneous crystallization of other minerals. Analysis of the sites of isotope ratio measurements in various zones of zircons (Fig. 10) showed that the concordant age of 493 ± 4 Ma (Fig. 11a) corresponds to the oldest cores (Zr1), and the concordant age of 477 ± 3 Ma (Fig. 11b), to the younger cores (Zr2) or the inner shell over the older grains, that is, to the age of the Krestovsky massif diorites (Table 4).

Fig. 8. Histogram for all U–Pb age measurement points of zircons from the sample PO2519.

V.A. Makrygina et al. / Russian Geology and Geophysics 55 (2014) 33–45

41

The age of zircon from the next phase of the Krestovsky massif, granodiorite, was not determined, though this zircon is of the same habit as zircons from diorites, is also zonal, and bears the same inclusions. The Ar–Ar age of plagioclases from diorite and veined granodiorite is 484 Ma and 465 Ma, respectively (Makrygina et al., 2010).

Discussion Geological explorations showed that high-grade metamorphism in the Ol’khon region was caused by heat inflow during the collisional upthrusting (early stage of metamorphism) and during shear dislocations (late stage) (Fedorovskii et al., 1993; Makrygina and Petrova, 2005). The progressive stage of metamorphism of the Ol’khon Group terminated with weak enderbitization in the granulite facies zone and intense migmatization and formation of granite–gneiss domes in the amphibolite facies zone, which gave way to anatectic melting in the central zones of the domes (Shara-Nur complex). This is confirmed both by the appearance of massive granites at the center of the Shara-Nur dome and by the presence of melt inclusions in the cores of zircons formed in the granites at this stage. The crystalline state of inclusions evidences that the melts were generated at a great depth and underwent calm crystallization. Zircon cores crystallized together with other accessory minerals—apatite and magnetite. Even at that stage, however, zircons were subjected to deformations (Fig. 6-2). All this suggests that the zircons have xenogenic cores formed by the sedimentary protolith. Such cores were recognized in zircons from the Ol’khon gneisses (Gladkochub et al., 2007) and were dated at 1012 and 2573 Ma (SHRIMP-II). Our U–Pb date for the zircon core, 488–505 Ma, corresponds to the age of granulite metamorphism of two-pyroxene schists on the Khadarta Peninsula and Cape Khoboi (Gladkochub et al., 2007), i.e., to the peak of the volcanosedimentary sequence heating, and the zircons are syngenetic with the granite formation. Shear dislocations led to the dramatic heterogenization of stress. The northern shear walls were subjected to increased pressures of up to 10 kbar, which gave rise to eclogite-like rocks (Petrova and Levitskii, 1984). The pressure on the southern walls decreased; as a result, quartzite–marble melanges and dikes of pegmatoid granites feathering the shear zone formed. This is evidence of upthrust faulting and shear dislocations, which caused the joint transfer of partly crystallized magmas and the host sequences to a shallow depth as well as slight heating and new partial melting of granites. Under rapid magma upwelling, the rock-forming minerals underwent cataclasis, and the zircon cores were subjected to boudinage and overgrowing with a new shell containing noncrystallized melt inclusions. These inclusions suggest a dramatic ascent of the crystallized melt and a rapid growth of zircon grains at a shallower depth with lower temperature and pressure. The rapid ascent of the melt is evidenced by the similar ages of the inner shell (482–487 Ma) and zircon cores.

Fig. 9. Histograms for U–Pb age measurement points of the cores and shells of zircons from the sample PO2519.

The outer shell of the zircon grains has an age of 453–456 Ma. It resulted from the corrosion of the inner shell and bears gas–liquid fluid and completely crystallized primary melt inclusions. Granites had crystallized by that time, but the new stage of amphibolite metamorphism led to a new partial melting (Fig. 2b), corrosion, and growth of the outer shell with new holocrystalline melt inclusions at the sites of melt appearance. The abundance of fluid inclusions and Th/U = 0.24 in this shell (with Th/U = 0.33–0.35 in the zircon cores) confirm its metamorphogene formation with the participation of volatiles. By composition, the granite-gneisses and granites of the Shara-Nur complex are assigned to K-granites rich in Ba, LREE, Th, and Zr. Their veined derivates are still K- and Rb-richer (Table 3, Fig. 12). Both the rock structure and the intricate history of zircon crystallization as well as the absence of clear intrusive boundaries indicate that the origin, transfer, and crystallization of the Shara-Nur granitoids proceeded with the participation of the formed at several tectonic and metamorphic stages, which is well fixed in the term “syn-shear stress granites” introduced by Vladimirov et al. (2004). The melting of these granites out of crustal rocks is confirmed by negative values εNd(T) = –5.2 for the sample PO2519 with model age T(DM)Nd = 1257 Ma (Makrygina et al., 2010). A

42

V.A. Makrygina et al. / Russian Geology and Geophysics 55 (2014) 33–45

Fig. 10. Structure of zircons from the diorite PO2539 (optical image): 1–3, cores (Zr1–Zr2), zonal, nonazonal, boudinaged, Zr3, metamorphogene rim; 4–7, types of inclusions in zircons: 4, decrepitated melt inclusion in Zr1, 5, quartz inclusion, 6, melt inclusions in Zr2, 7, crystallized melt inclusion in Zr2, fluid (FI) and melt (MI) inclusions in Zr3; 8, zircons from the diorite PO2539 with the arrangement of age measurement points (CL image).

close model age, 1524 Ma, was obtained for the Cape Khoboi paraschists (Gladkochub et al., 2007). In the Khaidai complex, the rock composition varies from diorites to granites, with a regular increase in Si and K abundance and decrease in Al, Fe, Mg, Ca, Sr, Zn, Co, Ni, and V contents, pointing to the melt differentiation (Table 1). The main difference between these complexes is that all derivates of the Shara-Nur complex, from granite-gneisses and sites of massive granites to abundant veined series of pegma-

toid granites and pegmatites, are granites only. The Khaidai complex is a differentiated series from gabbro-diorites and diorites to granodiorites and granites with intrusive contacts and the textural features of shallow-depth rocks. In composition these rocks are identical to the metavolcanics of the Anga sequence and often contain their xenoliths (Fig. 4). Such relationships between volcanic and intrusive phases are typical of the recent Kuril-Kamchatka arc (Antonov, 2008; Ivanov, 1990; Volynets et al., 1982). The characteristic features of the

V.A. Makrygina et al. / Russian Geology and Geophysics 55 (2014) 33–45

43

Fig. 11. 206Pb/238U–207Pb/235U diagrams with concordia for zircons from the granite PO2539 of the Khaidai complex: a, zircon cores, b, zircon shells.

intrusive rocks of the Khaidai complex, namely, Na > K, high Sr, Zn, and Cu, moderate REE, and low Nb and Th contents (Fig. 12), are also specific for island-arc igneous rocks (Ivanov, 1990; Makrygina et al., 2000a) and strongly distinguish them from the granites of the Shara-Nur complex. The veined series of the Khaidai complex, cutting the metamorphosed metavolcanics and marbles of the Anga Formation, the gabbro massifs of the Ozersky complex, and the Krestovsky gabbro-diorite–diorite–granodiorite–granite massif, is also similar in composition to the granitoids of the late phases of this complex (Fig. 12). Zircons from the diorites and granodiorites of the Krestovsky massif are also of intricate structure, with a core grown in two stages and an outer shell. The core growth was probably interrupted by a drastic melt ascent, which led to the decripitation of melt inclusion in the early cores and to the core deformation. The growth of the second core and shell proceeded together with the crystallization of the bulk rock, as evidenced from abundant quartz and partly crystallized melt inclusions. Granitoids crystallized at a shallow depth, which is reflected in the zonal structure of plagioclases similar to subvolcanic products and in the presence of vitrified melt inclusions. The genesis of the early zircon core is still unclear: This is either zircon of early generation crystallizing in the magma chamber or in the intermediate chamber or zircon inherited from the melting substratum. However, (1) an early grain is present in all zircon grains, though it is strongly resorbed, (2) it contains decrepitated or completely crystallized melt inclusions, and (3) the inner-shell zircon is poorer preserved than the cores. All this suggests that the age of the early cores (493 ± 4 Ma) corresponds to the beginning of magma crystallization and the age of 477 ± 3 Ma marks the completion of crystallization of magma, after its intrusion (the age of diorites). Thus, the magma chamber probably existed for ~16 Myr.

Zircons from the Khaidai granitoids seldom have an outer metamorphogene shell, and it is almost free of fluid inclusions. This agrees with the anhydrous regime and lower metamorphism temperatures in the Anga sequence, as evidenced from the well-preserved primary structures of metavolcanics and no migmatization (Makrygina and Petrova, 1996). It is remarkable that the age of the cores and shell of zircons from the Krestovsky massif diorite (493 ± 3–477 ± 7 Ma) is almost the same as the age of zircons from the Shara-Nur granites (505–488 ± 11 Ma). The model age of the Khaidai granitoids is <1 Ga, and εNd(T) = +3–6. The positive values of εNd(T) and low Sr0 = 0.703–0.704 evidence that the melts were generated from mantle (Makrygina et al., 2010) or a juvenile source. That it, the melts of these complexes crystallized at the same time but in different geodynamic settings (structural blocks) and from different sources. There are no direct relationships between the granite massifs of the two complexes. According to earlier concepts, the abundant dikes cutting the Ozersky, Krestovsky, and

Fig. 12. Upper continental crust-normalized (Taylor and McLennan, 1985) REE pattern for granites of the Shara-Nur granite–gneiss dome (1) and differentiated granitoids of the Krestovsky massif (2) of the Khaidai complex. Composition of dikes of: 3, Shara-Nur, 4, Khaidai complexes.

44

V.A. Makrygina et al. / Russian Geology and Geophysics 55 (2014) 33–45

Table 5. Geochronological data on granitoids of the Shara-Nur and Khaidai complexes Object

Rock, mineral

Method

Age, Ma

References

Ozersky complex

Gabbro

Nd–Sm

531 ± 23

(Bibikova et al., 1990)

Khaidai complex Krestovsky massif

Diorite-granodiorite-granite

Isochron Rb–Sr

570 ± 18

(Makrygina et al., 2000b)

Diorite, zircon

U–Pb SHRIMP-II

493 ± 4, cores 477 ± 3, shell

This work

Diorite Granodiorite

Ar–Ar, plagioclase

484 ± 4.2 465 ± 4

(Makrygina et al., 2010)

Pyroxene schists, garnet-biotite gneisses, zircons

U–Pb SHRIMP-II

498 ± 7 535–1012 2753

(Gladkochub et al., 2007)

Shara-Nur complex

Granites

Isochron Rb–Sr

522 ± 88

(Makrygina et al., 2000b)

Shra-Nur dome

Granite, zircon

U–Pb SHRIMP-II

505–489 ± 11, cores 482–487 ± 11, inner shell 453–456 ± 10, second shell.

This work

Ol’khon unit

Tazheran massifs and the host schists at their contacts are the derivates of the Shara-Nur complex. However, their diverse rock composition (from diorites and syenites to granites), fine-grained structure, and REE composition similar to that of the Khaidai complex granitoids as well as the domination of Na over K in most of the dikes (Table 3, Fig. 12) evidence that they belong to the veined series of the Khaidai complex. Such dikes were also described in volcanoplutonic complexes of the Kuril–Kamchatka island arc (Antonov, 2008). This brings up the question: How could the island-arc igneous rock associations and syncollisional Shara-Nur granites have crystallized at the same time? To answer it, we should examine the large volume of geochronological material obtained by different methods and authors for these complexes (Table 5). All dates fall into two groups: (1) from 530–570 Ma (scarce data in the Ozersky and Khaidai complexes) to 505–465 Ma (voluminous data in both complexes) and (2) 1012–2573 Ma (scarce data for the cores of zircons from the Ol’khon Group metamorphic rocks, obtained by Gladkochub et al. (2007)) and 1890 Ma (U–Pb zircon age of the Ol’khon Group granite-gneiss, obtained by Bibikova et al. (1990)). The old ages of the zircon cores are due to the trapped relics of more ancient crust, the more so that they are close to the model Nd-ages of the host schists. We think that at the stage of reverse faulting and shear dislocations, the back-arc paleobasin plates (Ol’khon Group) with the Shara-Nur granitoids accreted to the collision suture and the block of the Anga–Talanchan island arc. The same scenario of the regional evolution is proposed by Zorin et al. (2009).

Conclusions 1. The geologic structure of metamorphic rocks and granitoids of the Ol’khon region evidences that the thrusting was changed by upthrust faulting and shear dislocations, which are expressed as shearing and melange of the Ol’khon Group

rocks as well as magmatic breccias in the Khaidai complex of the Anga sequence. 2. The thorough study of zircons and inclusions in them and SHRIMP-II dating of the zircon growth zones have shown the multistage crystallization of granitoids of the Shara-Nur (Ol’khon Group) and Khaidai (Anga Group) complexes. 3. The generation of melts of both complexes proceeded synchronously with the thrusting (505–485 Ma). At the next stage of upthrust faulting and shear dislocations, the melts began to crystallize and rapidly ascend (485–475 Ma). Then, the complete crystallization of the melts took place, followed by their influence on the rocks formed at the second stage of metamorphism (456 Ma). The generation and crystallization of the melts of both complexes fall in the interval from 16 to 20 Ma. 4. The Shara-Nur migmatite–granite complex corresponds in composition, structures, and textures and the crustal source of substance to syncollisional K-granites. The Khaidai differentiated gabbro-diorite–diorite–granodiorite–granite complex is close in geochemical features (similar to those of the Anga sequence metavolcanics) and the mantle source of substance to the recent island-arc magmatism. 5. It is suggested that the intrusive island-arc magmatism was close in time to the accretion of the sediments of back-arc basin (Ol’khon Group) to the continental margin, on the one hand, and to the island-arc block, on the other. The initial volcanosedimentary island-arc deposits might have been thrust over the accretionary prism of the back-arc basin sediments, as suggested by the lower-temperature anhydrous metamorphism of the Anga sequence. The magmatism activity in the Ol’khon and Anga blocks increased during the reverse faulting, shear dislocations, and growth of granite-gneiss domes. This work was supported by grant 11-05-00515 from the Russian Foundation for Basic Research, Integration Project ONZ-10.3 from the Geosciences Department of the Siberian Branch of the Russian Academy of Sciences, and Scientific School Project NSh-6153.2012.5.

V.A. Makrygina et al. / Russian Geology and Geophysics 55 (2014) 33–45

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