2013

Abstracts Travel
Program Organizing committee

Chemical composition of phlogopite from devonian lamprophyres (Zhlobin diatreme field, Belarus )

Mikhailov, N.D., Laptsevich, A.G.

Republican Unitary Enterprise Belarusian Research Geological Exploration Institute, Minsk, Belarus, mihailov@igig.org.by

 

In order to classify the host rocks more accurately and to ascertain their belonging to the kimberlite formation we studied the chemical composition of phlogopite from lamprophyres retrieved from the borehole 761 (Zhlobin diatreme field). Primary rock forming minerals of Devonian lamprophyres from Belarus are phlogopite (mainly tetraferriphlogopite with the Mg/Fe ratio >3), olivine, calcite, monticellite?, magnetite, monoclinic pyroxene (diopside?). The rock mass is dominated by brownish phlogopite. It is represented by fine xenomorphic scales in fine crystal and aphanitic rock varieties, but more frequently, by coarser poikilitic phenocrystals with abundant microimpregnations of magnetite. Chemical compositions were determined using microprobe analysis. The results are given in the table.

Table. Contents (weight %) of  the rock forming oxides in phlogopite

Depth, m

 

Si2

Ti2

Al23

Fe

Mn

Mg

Ba

Ca

Cr23

Na2

K2

F

232.0 (n=3)

39.2-40.3

1.3-2.0

7.9-8.9

8.4-9.9

0

23.6-24.8

0-0.8

0

0

0.3-0.4

10.0-10.5

0-1.5

235.0 (n=6)

39.6-41.0

1.2-1.4

7.2-8.4

8.0-9.1

0

24.4-25.0

0-0.6

0-0.2

0

0.3-0.6

10.3-10.6

1.0-2.1

237.5 (n=8)

36.8-41.2

0.4-2.4

7.2-12.6

6.2-10.3

0

22.8-26.4

0-1.1

0-0.2

0-0.42

0-0.5

9.0-10.6

0-1.4

239.4 (n=2)

39.5-39.8

1.9-2.3

7.2-7.9

9.2-10.0

0

23.1-23.7

0

0

0

0.5

10.4-10.5

1.3-1.3

240.5 (n=7)

38.4-40.6

1.7-2.0

7.4-10.9

9.4-10.0

0-0.5

23.1-24.8

0-1.3

0-0.4

0

0.2-0.5

7.9-10.3

0.8-1.3

n number of samples

 

The chemical composition varies in a narrow range. Compared to the average chemical composition of phlogopite (Chirvinskij, 1953) the samples under investigation are depleted in Al2O3 and enriched in TiO2 and FeO (mg = 0.7-0.8). Relatively small alumina contents are consistent with low temperature conditions of crystallization. The contents of FeO and Ti2O are positively correlated. Some phlogopite varieties are enriched in chromium.

In order to determine genetic belonging of phlogopite we used the Ti2O-MgO diagram (Figure 1), in which the majority of micas in lamprophyres from the borehole 761 fall in the field of titan-containing micas, and partly, in the field of micas with normal titan contents, and correspond to the areas characteristic of micas from the type 2 kimberlites (orangeites).

In the Al2O3 - TiO2 and Al2O3 FeO diagrams (Mitchell, 1989) Belarusian phlogopites lie in the areas of micas from orangeites or follow trends of orangeite micas changing (Figure 2).

Thus, the composition of phlogopites in lamprophyres from the borehole 761 is close to that of phlogopites from orangeites. The minerals studied may be classified as titan-containing phlogopites slightly depleted in Al2O3.

 

This research was carried out with financial support from the Belarusian Republican Foundation of  Fundamental Investigations (grant X12P-058).

 

References

Chirvinskij, P.N. The average chemical composition of the main minerals of igneous, metamorphic and sedimentary rocks. Kharkov, 1953 96 p. (in Russian).

Mitchell, R.H. Kimberlites. Mineralogy, Geochemistry and Petrology. Plenum Press. New York and London. 1989.