2013
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Geochemical peculiarities of phlogopite from Devonian alkaline picrite found in the Braginsky and Loevsky saddle (Belarus)
Ignatkevich E.S.*, Varlamov D.A.**
*Belarusian scientific research and designed institute of oil, Gomel, Belarus;
**Institute of Experimental Mineralogy RAS, Chernogolovka, Rassia.
This paper is one of the first reports on the chemical composition of dark-colored rock-forming minerals of late Devonian magmatic rocks of the Braginsky and Loevsky saddle (Belarus). Samples of alkaline picrite from the core of the drill hole Vasilyevskaya 1 (depth 1785 m) were selected for the research.
The mica forms phenocrysts and microcrystals in the picrite. In accordance with the IMA recommended nomenclature [3] the composition of micas from alkaline picrite is approximated to the composition of the phlogopite (table). Attention is drawn to a high content of titanium oxide in the phlogopite (6,77-9,48 wt.% TiO2) as a criteria of high degree of differentiation. A high content of titanium oxide in the melt increases a distribution ratio of barium in the mica [1]. Barium enrichment is significant for the phlogopite in point. Its content reaches about 4 wt. % of BaO, which makes it possible to call the phlogopite one of barium [3]. Magnesium and iron oxides content ranges 6-7 wt. % and, in general, it corresponds to the composition of the phlogopites of ultrabasic rocks (Mg/Fe > 3) [1]. Alumina content is increased in these phlogopites (Al2O3 = 10,9-13,1 wt.%), which suggests high-temperature conditions of their crystallization.
|
Table. Chemical compound (comp. %) and formular coefficientes of phlogopite |
|||||||||||||
|
Oxide |
SiO2 |
TiO2 |
Al2O3 |
Cr2Î3 |
Fe2O3 |
MnO |
MgO |
CaO |
Na2O |
K2O |
NiÎ |
BaÎ |
Sum |
|
Comp. % |
39.9 |
6.6 |
12.5 |
0.0 |
7.5 |
0.1 |
19.8 |
0.2 |
0.4 |
8.8 |
0.0 |
4.0 |
99.8 |
|
38.8 |
6.9 |
13.0 |
0.0 |
7.8 |
0.1 |
19.9 |
0.2 |
0.6 |
9.2 |
0.2 |
3.9 |
100.4 |
|
|
Element |
Si |
Ti |
Al |
Cr |
Fe |
Mn |
Mg |
Ca |
Na |
K |
Ni |
Ba |
Î |
|
Formula coefficient |
3.03 |
0.38 |
1.12 |
0.00 |
0.43 |
0.01 |
2.24 |
0.01 |
0.06 |
0.85 |
0.00 |
0.12 |
12.00 |
|
2.94 |
0.39 |
1.16 |
0.00 |
0.44 |
0.01 |
2.24 |
0.01 |
0.08 |
0.89 |
0.01 |
0.12 |
12.00 |
|
Wide variations of a chemical composition of phlogopite allow its use as an indicator to diagnose rocks. On the correlation graph TiO2 – Al2O3 of R.H. Mitchell [2] the phlogopites in point get in the minette field (figure).
Figure. Diagram Al2O3 – TiO2 for phlogopite
Identified geochemical peculiarities of phlogopite indicate alkaline conditions of crystallization and a high degree of differentiation of magma.
Literature
1. Antipin V.S., Kovalenko V.I., Ryabchikov I.D. Distribution ratios of rare elements in magmatic rocks. Moscow: Nauka, 1984. 253p.
2. Mitchell R.H. Kimberlites, Orangeites and Related rocks // Plenum. Publications Inc. New York. 1995.
3. Rieder Ì., Cavazzini G., D'yakonov Y. S. & others. Nomenclature of the micas // The Canadian Mineralogist. 1998. V. 36. pp.41-48.
