Mineralogy and geochemistry of K-rich basalts of the central part of the Sredinnyi Range, Kamchatka

Babansky A., Solovova I.

Institute of Ore deposits, petrography, mineralogy and geochemistry RAS (IGEM), Moscow, Russia

 

 

ššššššššššššššš The potassium-rich igneous suite of the Tigil anticlinorium in Western Kamchatka includes absarokites, mica absarokites, trachybasalts and their holocrystalline analogues (shonkinites), and more silicic rocks (shoshonites and latites). We studied absarokites, shonkinitesš and shoshonitesš from the region of the Anmanna River at the southeastern base of Kekunaisky Volcano (collection of O.N. Volynets and A.B. Perepelov) and from the Uksichan caldera (collection of A.D. Babansky and V.S. Rodin). The main focus is the compositions and crystallization conditions of magmas trapped as melt inclusions in minerals at different stages of magmatic evolution. The phenocrysts of the rocks are olivine, clinopyroxene (Table 1), phlogopite, Ca-rich plagioclase, and orthoclase. The least evolved absarokite contains olivine and clinopyroxene (En40-44Wo47-51Fs9) with mg-numbers of 0.86–0.67 and 0.81–0.83, respectively. Daughter clinopyroxene from melt inclusions (sample 7041) shows high TiO2 contents of up to 2.5 wt %. The groundmass contains Ti-magnetite and ilmenite.

 

Table 1. Chemical compositions of phenocrysts from the alkaline rocks, wt %

 

1

2

3

4

5

6

7

SiO2

41.79

38.55

51.92

54.29

51.75

55.94

67.42

TiO2

š -

š -

0.64

0.26

0.46

0.08

0.00

Al2O3

š -

š -

3.59

0.65

1.49

27.03

19.74

FeO

22.08

29.52

5.80

16.34

8.06

0.81

0.33

MnO

 

 

 

1.19

0.57

0.04

0.00

MgO

39.38

44.95

14.80

26.85

15.67

0.06

0.00

CaO

0.28

0.08

22.62

1.28

21.15

9.69

0.28

Na2O

š -

š -

0.32

š-

0.45

5.73

3.65

K2O

š -

š -

š -

-

š -

0.51

11.02

SrO

š -

š -

š -

-

š -

0.21

0.00

BaO

š -

š -

š -

-

š -

0.04

0.33

mg

0.76

0.67

š -

šš š-

š šš-

š -

š -

Wo

š -

šš -

46.5

šš š-

45.4

š -

š -

En

š -

š -

44.3

75

46.8

š -

š -

Fs

š -

š -

9.3

25

7.7

š -

š -

An

š -

š -

š -

š -

š -

47

1

Ab

š -

š -

š -

š -

š -

50

33

Or

š -

š -

š -

š -

š -

3

66

 

Note. Anmanna River: (1) olivine from sample 7039, (2) olivine from sample 7042, (3) clinopyroxene from sample 7039. Uksichan caldera: (4) orthopyroxene, (5) clinopyroxene xenocryst, (6) plagioclase, (7) K-feldspar.

 

The olivine-free alkaline basaltoids of the Uksichan caldera contain abundant phenocrysts of Mn-bearing orthopyroxene (up to 2 wt % Mn, mg-number of 0.70–0.75) and feldspars, plagioclase (An47Ab50Or3) and alkali feldspar (An1Ab33Or66) (Table 1). In addition, there are amphibole and Fe–Ti oxide phenocrysts.

ššššššššššššššš The initial magma of the basaltoids is alkali-rich (up to 9 wt % Na2O + K2O at K/Na up to 1.6) and shows high contents of volatile components (up to 0.2 wt % Cl and F), 2O5 (up to 2 wt %), and 2. The bulk analyses of the shonkinites contain up to 4 wt % H2O. The magmas of the Uksichan caldera were probably F-rich, which is indicated by the F-rich composition of apatite (up to 6% wt % F). Also noteworthy are high contents of some incompatible trace elements, especially Ba and Sr, which were detected in substantial amounts in feldspar compositions. Heating experiments with the melt inclusions showed that the melt began crystallizing at relatively low temperatures, no higher than 1180œC, which is in line with the high contents of alkalis and H2O.

 

Table 2. Chemical compositions of rocks (Anmanna River) and melt inclusions in olivine and clinopyroxene, wt %

š 

1

2

3

4

5

6

7

8

9

Sample

7042

7039

7021

7039

7039

7039

7042

7039

7021

SiO2

51.17

52.76

55.26

50.18

54.07

55.07

48.1

45.5

48.9

TiO2

1.73

1.15

1.44

2.3

0.85

0.83

0.68

0.95

0.64

Al2O3

15.2

17.31

19.21

15.36

14.29

14.28

15.98

13.28

12.86

FeO

11.4

9.62

5.1

6.29

6.32

6.08

7.256

8.723

6.77

MnO

- 

-

-

-

-

-

0.16

0.19

0.17

MgO

5.31

4.87

3.27

5.76

5.32

5.17

8.59

12.33

12

CaO

6.66

6.36

4.91

11.06

8.8

8.77

8.02

7.22

5.41

Na2O

3.09

3.39

3.2

3

2.27

3.08

2.92

2.55

2.71

K2O

4.13

5.56

7.16

3.68

5.16

5.27

3.09

2.52

3.44

Total

98.69

101.0

99.55

97.63

97.08

98.55

94.79

93.26

92.90

K2O+Na2O

7.22

8.95

10.36

6.68

7.43

8.35

9.1

7.59

9.59

, œC

1160

1160

1115

1135

1165

1165

-

-

-

 

Note. (1–3) Inclusions in olivine, (4–6) inclusions in clinopyroxene, and (7–9) bulk rock compositions. T is the homogenization temperature of melt inclusions.


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