Alkaline granites of the Syadatoyakhinsky massif (Polar Urals)

Udoratina O.V.

Institute of Geology of Komi SC UB RAS, Syktyvkar, Russia

 

In Polar Urals among numerous acidic magmatic rocks of the Kharbey block of preuralides the alkaline granitoids of the Syadatayakhinsky massif are found. The granitoids are related to the Syadatayakhinsky complex (Correlation , 1985). The rocks of the massif are rare exception - they are poorly affected in this zone by metasomatic silica-alkaline processes and actually magmatic (intrusive formations) (Okhotnikov, 1985).

In this zone the alkaline metasomatites with rare metal specificity are widespread (Taykeyusky ore knot). The ore rocks are related to the formation of alkaline metasomatites of fault zones and have paragenetic relation with magmatites. Recently the material has been collected, which allow relating genetically the rare metal deposits with alkaline source. The magmatic rocks of such bias are also included in the studied massif, which does not yield, however, ore load.

The granitoids, exposing in source of the Zaozerny brook, comprise sheet bodies among green shales metamorphites of the Middle Riphean Nyarovey suite. The absolute age of massif rocks on the basis of Rb-Sr (bulk) and U-Pb (zircon) datings corresponds to Middle Cambrian (Andreichev, 2007). It should be noted that figures of age of rocks, determined by different methods, are close. The initial 87Sr/86Sr ratio makes 0.7049041 and specifies the abyssal nature of the matter.

The massif is composed by arfvedsonite-biotite, biotite and micropegmatite granites, as well as granophyres and granite-porphyries. The ratio with host rocks layered intrusion into metamorphite strata. The arfvedsonite-biotite granites are the deepest, other varieties of rocks are located hypsometrically higher and represent endocontact facies. Lenticular or rhythmical zoning is observed near endocontacts and in apophysis bodies. The observable geological ratios intertransitions of rocks specify their near-simultaneous formation that allows relating them to the products of one phase of intrusion (t 600700) (Okhotnikov, 1985).

The ore load of the granitoids of the Syadatoyakhinsky massif, as N. Okhotnikov emphasizes, does not represent practical value; generally it is a fine scattered sulphidic mineralization (molybdenite, sphalerite, pyrite, pyrrotyne). He also stated that the granites of lower and upper levels have different geochemical specialization - atop the magmatic chamber tin, molybdenum and tungsten, and below - tantalum-bearing granites with niobium (!) accumulated.

The tested rocks of the massif (V. L. Andreichevs collection) are represented by medium-grained, massive alkaline arfvedsonite-biotite and biotite granites. Microscopic studies reveal granite hypidiomorphic-granular texture, locally cataclastic.

The rocks are composed (%) of potassic feldspar (40-45), plagioclase (25-30), quartz (20-25) and biotite (3-5). The constant presence of arfvedsonite is characteristic. Accessory minerals are presented by allanite, sphene, zircon (rarer by apatite). Ore minerals magnetite, pyrite; secondary - epidote, limonite.

Salic minerals are presented by potassic feldspar and plagioclase and quartz. Potassic feldspar is presented by pertitized orthoclase and neogenic microcline replacing orthoclase. Plagioclase - albite-oligoclase (An1015) idiomorphous prismatic, it forms hypidiomorphic-granular texture. It is often sericitized. Quartz is poorly cataclized. Chemical compositions of plagioclase and potassic feldspar are without impurities. Widespread femic minerals are represented by micas (biotite and phengite), and also amphibole (arfvedsonite). In arfvedsonite-biotite reveal taxitic distribution of dark minerals. The amphibole is dark green, pleochroic from dark green to yellowish green; it is optically negative. The biotite is presented by thin scale stilpnomelane. Often biotite is a unique dark mineral in the rocks. At transition to micropegmatitic granites and also granophyres and granite-porphyries muskovite (phengite) is found. Epidote and allanite are observed as small isometric grains forming accumulations. Among accessory the structures of zircons and sphene have been studied. Apatite and garnet, described by V.N.Okhotnikov, are not found. Zircon is observed as small crystals of prismatic yellow-brownish habitus. According to V. L. Andreichev the catode-luminescent survey reveals concentric magmatic zoning (Andreichev, 2007). It is characterized by HfO2 presence, which content vary from 1.75 to 2.51 weight %. Sphene is observed as individual small extended brown crystals.

Petrochemically the rocks of the massif correspond to the family of alkaline granites. The content (table) of silica acid varies (weight %) from 67 to 74, and alkalis from 8.5 to 10.5, at the alumina content at 14-15, but can reach 21 in granosyenites (Okhotnikov, 1985; Andreichev, 2007). The agpaite index varies from 0.7 to 1 except for one test.

The geochemically studied rocks are characterized by the raised content in relation to clarke for acid rocks Rb, Zr, Hf, reduced content of all the light (except for La) and medium rare earths, also Cr, Sr, Ba, Zn, Th, Y, the contents of heavy rare earths Y, Nb and U are observed at the clarke level. The rocks are characterized by low REE content with predominance of light over heavy and distinct europium anomaly, (La/Yb)N varies from 13 to 47. By the substrate type the studied rocks are related to A-granites. Thus, the geochemical features of the rocks, their abyssal nature and age values specify that the geodynamic conditions of formation correspond to postcollision settings prior to riftogenesis in the north Urals.

The work is performed under financial support of project -3763.2008.5 Leading scientific schools.

 

 

Table

Chemical composition (weight %) and content of rare elements (g/t)

Component

368/1

368/3

368/4

368/5

368/7

368/8

368/9

SiO2

66.78

71.12

71.38

71.50

70.94

70.82

73.90

TiO2

0.10

0.15

0.19

0.14

0.18

0.20

0.13

Al2O3

21.85

13.12

14.36

14.18

15.95

15.67

12.72

Fe2O3

0.99

1.34

1.31

1.05

1.58

1.47

1.08

FeO

0.45

0.76

0.73

0.68

0.56

0.76

0.58

MnO

0.02

0.02

0.04

0.03

0.03

0.05

0.02

MgO

0.07

0.80

0.20

0.70

0.33

0.31

0.31

CaO

0.56

0.56

0.69

0.67

0.62

0.48

0.67

Na2O

3.88

4.64

4.60

4.80

3.90

4.62

4.17

K2O

4.59

5.84

5.47

5.39

4.58

5.49

4.95

P2O5

0.03

0.02

0.02

0.01

LOI

0.88

1.27

0.90

0.83

0.85

0.61

1.04

Total

100.17

99.62

99.90

99.97

99.54

100.50

99.58

Na2O+ K2O

8.47

10.48

10.07

10.19

8.48

10.11

9.12

Na2O/ K2O

0.85

0.80

0.84

0.90

0.85

0.84

0.84

P.I.

0.52

1.06

0.94

0.97

0.71

0.86

0.96

La

38.0

23.2

19.7

48.5

95.4

83.4

3.87

Ce

65.3

35.0

30.1

71.0

150.0

138.0

5.0

Pr

6.35

3.08

2.74

6.9

13.1

12.2

0.4

Nd

21.7

9.88

8.52

20.4

40.3

40.2

1.03

Sm

4.76

2.0

1.82

4.2

8.43

8.19

0.19

Eu

0.26

0.046

0.22

0.036

0.53

0.23

0.11

Gd

4.2

2.22

2.02

4.98

11.1

10.1

0.23

Tb

0.53

0.29

0.3

0.71

1.73

1.58

0.037

Dy

2.53

1.6

1.77

4.11

10.1

9.03

0.24

Ho

0.44

0.33

0.39

0.87

2.24

2.0

0.054

Er

1.01

0.82

1.02

2.26

6.26

5.58

0.16

Tm

0.13

0.11

0.15

0.33

0.93

0.81

0.026

Yb

0.56

0.57

0.8

1.71

5.03

4.5

0.14

Lu

0.076

0.085

0.13

0.27

0.85

0.7

0.026

(La/Yb)N

46.75

27.44

16.60

19.12

12.78

12.49

18.63

EuN/Eu*N

0.17

0.07

0.35

0.02

0.17

0.08

0.61

Cs

0.042

0.049

2.4

1.14

Ba

45

20

33

165

125

265

69

Sc

2.57

1.1

1.33

3.31

4.87

4.61

0.48

Cr

3.36

3.23

1.05

1.25

8.93

8.61

0.92

Co

0.54

0.26

0.22

1.69

0.99

4.2

0.13

Ni

410

570

110

260

380

360

140

Zn

46.1

46.4

14.4

Se

2.01

0.53

0.38

0.44

3.91

1.66

2.73

As

6.68

3.37

3.17

5.95

2.35

2.96

Sb

1.17

0.4

0.62

0.37

1.04

0.18

0.35

Th

9.07

4.07

3.04

10.0

14.5

11.2

0.97

U

2.97

0.39

2.57

2.72

4.92

2.17

2.1

Br

1.02

0.5

0.4

2.36

0.89

0.54

0.33

Hf

10.5

3.15

3.12

14.0

17.5

0.92

0.31

Ta

0.58

0.88

4.31

2.12

Au

0.019

0.026

0.05

Ag

1.76

Nb*

13

21

18

17

19

22

19

Zr*

207

450

411

274

428

422

248

Y*

24

37

34

29

39

36

31

Rb,

97.5

95.3

87.8

108.9

98.3

102.7

88.0

Sr,

85.2

75.7

27.0

9.2

25.4

12.3

14.8

Note. Rare and rare earth elements are determined in the Institute of Geochemistry RAS by neutron-activation and X-ray-radiometric (*) methods. The chemical contents and Rb, Sr data are according to [1].

 

References

Andreichev V. L., Larionov A. N., Litvinenko A.F. New Rb-Sr and U-Pb data on the age of granitoids of Syadatayakhinskya intrusion (Polar Urals) // Lithosphera, 2007, 1, pp. 147-154. (in Russia).

Correlation of magmatic complexes of European northeast of the USSR / V.N. Okhotnikov, V.I.Mizin, L.T.Belyakova et al. // Series of pre-prints Scientific recommendations for the national economy. Syktyvkar, 1985. edition 53.24 pp. (in Russia).

Okhotnikov V. N. Granitoids and ore formation (Polar Urals). Leningrad: Nauka, 1985.184 pp. (in Russia).


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