The typochemical features of pyroxenes from Paleozoic picrite dikes within Spitsbergen Archipelago

 

Burnaeva M.Yu.*, Antonov A.V.**, Sirotkin A.N. ***

 

*FGUP VNIIOkeangeologiya, St.Petersburg, Russia;** FGUP VSEGEI, St.Petersburg, Russia;***FGUNPP PMGRE, Lomonosov, Russia.

 

Picrite dikes are located within the West Spitsbergen Island, in the east and the south of an Andre Land in the area of a Krjusspjunten Cape, a Peterman Cape (western coast of Vejde-Fiord), a Purpurdallen Valley, and also around a glacier Rubinbreen - Haakon VII Land in the Devonian graben filled with redrocks. Geologists of FGUNPP PMGRE discern two types of the dikes: those saturated with xenoliths (type 1) and with mica (type 2). The dikes have been described earlier (Evdokimov, etc., 2006).

A pyroxene is one of the main rock-forming minerals for dikes of the both types. Discerned are three generations of the mineral: phenocrysts, groundmass microlites and xenocrysts. The pyroxene makes 20 to 60 % in the total, consisting mainly of microlites; phenocrysts and xenocrysts being under 1 %. Considered below is the composition of precisely the pyroxenes from dikes without regard for those from xenoliths..

Phenocrysts represent short-prism grains 0.4 to 1.2 mm large, occasionally up to 3 mm long, often colored in tints of brown, being rarely slightly greenish or achromatic. The grains are zonal with the largest showing up to 3 zones.

The groundmass pyroxene is present as long-prism grains colored brown, 0.05 to 1.0 mm long. The grains vary in dike section from small (0.05 to 0.2 mm) at the rim up to elongated (0.2 to 1.0 mm) in the center. Twinned grains and rosettes of needle-shaped units are discerned among microlites.

Microprobe analysis of the grains has been performed to reveal the features of composition of the pyroxenes (analysts: A.V.Antonov (FGUP VSEGEI), Ju.L.Kretser (LLC PC +)). The results are shown in Table 1.

Table 1. Average contents of major elements and calculated Fe3+, Fe2+ and Kok in pyroxenes of ultramafic Paleozoic dikes within Spitsbergen Archipelago by generations

Type 1

SiO2

Al2O3

TiO2

FeOt

MgO

CaO

Na2O

Cr2O3

Fe2+

Fe3+

Microlites (n=11)

48,49

5,08

2,30

6,34

13,54

23,70

0,05

0,16

0,15

0,05

0,26

Phenocrysts (n=18)

50,59

5,16

1,23

6,33

14,61

20,99

0,69

0,11

0,15

0,07

0,32

Type 2

 

 

 

 

 

 

 

 

 

 

 

Microlites (n=9)

45,92

6,55

3,19

7,44

13,06

23,41

0,57

0,08

0,07

0,15

0,66

Phenocrysts (n=16)

49,74

5,57

1,40

7,43

14,11

20,54

1,15

0,03

0,11

0,12

0,52

n quantity of analyses; Kok = Fe3+/(Fe3++Fe2+); FeOt total ferrum defined as FeO; Fe2+ and Fe3+- calculated with stoichiometry.

Started with phenocryst crystallization the process of mineragenesis had been completed by forming groundmass microlites. Along the same direction, the composition of pyroxenes shows decrease in SiO2, MgO, Na2O and increase in TiO2, CaO, Cr2O3 in dikes of the both types; contents of Al2O3 being slightly lower in dikes of type 1 and much higher in dikes of type 2. FeOt shows nearly no variations in quantity, but samples from dikes of type 1 contain for the most part ferrous oxide. A share of ferric oxide increases in pyroxenes from picrites of type 2 (table 1). It should be noted from the latter that oxidation of the environment for pyroxene generation in dikes of type 1 was of lower potential.

As a whole, the elements in pyroxenes of dikes of the both types demonstrate a similar nature. The difference is observed but in the content of Al2O3 that may be a result of its high content in the melt and undersaturation of the melt in SiO2 at final stages of its crystallization.

All the pyroxenes studied concern Ca-Mg-Fe-clinopyroxenes. Three mineral varieties (diopside, augite, fassaite) are discerned by crystallochemical formulae (Mineraly, 1981). Among minals, dominating are diopside (20-77 %), tschermakite (3-25 %), hedenbergite (0-43 %), enstatite (0-27 %). The highest average contents of the first two and the latter two components have been revealed in microlites and in megacrysts of dikes of the both types, respectively.

Large phenocrysts of pyroxenes are characterized by zoning resulted from rapid change in crystallization conditions when the mineral composition has not managed to come to equillibrium with the surrounding melt. To study the zoning, 6 grains from 3 samples (table 2) were analyzed. Three zones are pronounced as a whole. Distribution of the elements within the zones shows a similar pattern in all the samples. The content of SiO2 and MgO is established to increase toward the second zone then decreasing toward the rim. Al2O3 and FeOt in most part of the grains decrease toward the second zone then increasing toward the rim (exception is one grain from dike of type 1 showing subsequent core-to-rim increase in aluminia). TiO2 and CaO increase from core to rim. The composition of the rim zone and in part of the second zone of grains is similar to that of microlites that indicates their syngenetic pattern.

 

Table 2. Average composition of pyroxenes for the zones of dikes of the both types

Type of dike, zone

SiO2

Al2O3

TiO2

FeOt

MnO

MgO

CaO

Na2O

K2O

Cr2O3

Type 1 (n=5)

core

50,74

6,14

0,87

6,39

0,00

14,28

20,15

1,23

0,00

0,00

Zone 2

51,23

3,26

0,77

5,64

0,00

15,32

22,80

0,12

0,00

0,18

rim

47,02

6,19

2,32

6,58

0,00

13,26

23,89

0,10

0,00

0,00

type 2 (n=1)

core

51,24

6,14

0,88

7,30

0,15

15,73

16,80

1,80

0,00

0,00

2 zone

52,82

2,76

0,68

5,45

0,00

17,47

20,62

0,00

0,00

0,00

rim

45,81

7,05

3,27

6,94

0,00

13,15

23,70

0,00

0,00

0,00

 

Fig.1 The correlation between pyroxenes from dikes of Spitsbergen and pyroxenes from kimberlite groundmass for different areas within coordinates MgO/FeOt-CaO.

Pyroxene fields from: 1 picrite dikes of Spitsbergen; 2 kimberlites from the World different occurrencies.

 

 

References:

 

Dobretsov N.L., Kochkin Yu.N., Krivenko A.P., Kutolin V.A. The rock-forming pyroxenes. Moscow: Nauka, 1971. 454 p. (in Russian).

Evdokimov A.N. et al. The first occurrence of accessory minerals of kimberlites in Paleozoic dikes of Spitsbergen //Doklady RAN. V. 407, 2, 2006. P. 275-279

Malkov B.A. The petrology of dike series of alkaline gabbroids of North Timan. Leningrad: Nauka, 1972. 128 p. (in Russian).

Minerals: the Directory. Moscow: Nauka, 1981, V. 3, ed. 2. 613 p. (in Russian).

Nikishov K.N. The petrologic-mineralogical model of kimberlite process. Moscow: Nauka, 1984. 214 p.

Yefimov A.F.The typochemistry of rock-forming dark-color minerals of alkaline rocks. Moscow: Nauka, 1983. 256 p. (in Russian).


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