2013

Abstracts	Travel
Program	Organizing committee

Ore pyroxene melilitites from the North Pripyat area of magmatism

Mikhailov, N.D.*, Laptsevich, A.G*., Vladykin, N.V.**

*Republican Unitary Enterprise «Belarusian Research Geological Exploration Institute», Minsk, Belarus,

**A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, Irkutsk, Russia

mihailov@geology.org.by

The Upper Devonian alkaline magmatic formation of Belarus includes petrographically and facially variable rocks. During the last year, one more variety of ultrabasic rocks (ore pyroxene melilitite with specific features of mineral and chemical compositions) was discovered. This suggests a possibility of recognition of rocks with ore specialization within the formation.

Ore pyroxene melilitites were retrieved from a borehole located in the southern part ot the North Pripyat shoulder. They occur at the depth of more than 300 m. In the upper part of the sequence, rocks are represented by volcanoclastic varieties (tuffic sandstones and tuffs). Their petrographic compositions are similar to those of underlying melilitites. Tuffs are strongly changed. In some cases all or most primary minerals are entirely decomposed and substituted by secondary fine dispersed hardly determinable minerals. Melilitites which occur at the depth of 392 m below a unit of melilitite tuffic sandstones are greenish dark-grey rocks with massive structure, hypidiomorphic granular oligophyre texture. They frequently include differently oriented 1-5 mm thick interlayers of carbonate – barite – zeolite composition.

The main mass consists of several minerals. Pyroxenes (diopside and hedenbergite) comprise about 60%, titanomagnetite and magnetite – 15-25%, melilite – 10-15%. Phenocrystals are composed of pyroxene, titanomagnetite, rarely of olivine replaced by secondary serpentine and chlorite. Melilitite of the main mass is substituted by montmorillonite.

By their petrographic characteristics, the rocks under consideration are consistent with melilitites, but are less alkaline and contain increased amounts of iron and titan. Also, the rocks are enriched in some trace elements (Ва, Cu,Ti, V, Li, Ta,Th, Nb, Sr, REE).

Table. Contents of major and trace elements in pyroxene melilitites

Major elements			Trace elements
SiO₂	34.89	34.44	Li	167.9	165.5	Sr	411	414	Tb	2.61	3.73
TiO₂	6.254	6.21	Be	2.76	2.30	Y	37.57	45.74	Dy	12.18	16.07
Al₂O₃	4.29	4.27	Sc	55.9	58.6	Zr	286	280	Ho	1.92	2.46
Fe₂O_{3 .}	20.4	19.35	Ti	40665	44389	Nb	127.79	128.48	Er	4.09	4.99
MnO	0.22	0.21	V	507	516	Sn	3.59	3.81	Tm	0.47	0.54
MgO	12.4	13.93	Cr	174	176	Cs	0.21	0.25	Yb	2.47	2.84
CaO	15.36	14.83	Mn	1662	1613	Ba	509	233	Lu	0.37	0.39
Na₂O	0.67	0.64	Co	40	72	La	71.40	167.72	Hf	10.30	10.28
K₂O	0.17	0.15	Ni	118	125	Ce	207.64	411.48	Ta	14.91	15.58
P₂O₅	2.12	2.05	Cu	247	509	Pr	30.34	54.84	W	0.93	0.39
Ba+Ce	0.08	0.05	Zn	418	112	Nd	132.35	227.41	Tl	0.01	0.01
Sr	0.05	0.04	Ga	14.01	15.58	Sm	26.24	39.34	Pb	2.38	1.99
Zr	0.04	0.04	Ge	2.50	2.37	Eu	7.31	10.96	Th	26.74	27.75
п.п.п.	2.86	3.56	Rb	7.33	4.72	Gd	19.37	29.21	U	1.95	2,87

Analyses were performed in the A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, Irkutsk.

This research was carried out with financial support from the Belarusian Republican Foundation of Fundamental Investigations (grant X12P-058) and Russian Foundation of Fundamental Investigations (grant 12-05-90000).