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Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

 

Mineralogy-geochemical peculiarities of ultrabasic alkaline rocks found in the eastern part of the Prypiat graben (Belarus)

Ignatkevich E.S.*, Varlamov D.A.**, Mikhajlov N.D.***

*Belarusian scientific research and designed institute of oil, Gomel, Belarus;

**Institute of Experimental Mineralogy RAS, Chernogolovka, Rassia;

 *** Belarusian Research Geological Exploration Institute.

e.ignatkevich@beloil.by

 

Devonian basic magmatism of the Prypiat graben appears in the form of dykes, sills, necks, effusive nappes, pyroclastic strata and has been studying from the 60th – 70th of the twentieth centuries. Published papers cover basic petrochemical peculiarities, facial composition, structure, formation belonging, position of alkaline rocks of the Eastern part of the Prypiat graben. Chemical analyses of some rock forming minerals have not been performed and some accessory minerals have not been studied, geochemical specialization of magmatism has not been completely clarified, as well as source and condition of formation of initial magma.

Ultrabasic alkaline rocks of the eastern part of the Prypiat graben are represented by various nephelinites, leucites, alkaline picrites and to different extent they bear traces of influence of postmagmatic processes.

Nephelinite from the well core of Yastrebovskaya 3k (depth 957 m) reveals porphyritic structure. Rock is cracked. Mineral composition is the following: (in % of volume): nepheline – 40, clinopyroxene - 35, mica - 8, leucite – 4, ore minerals – 6, plagioclase – 4, glass – 3. Amphibole and chlorite are developed in matrix and leucoxene and illite are developed in inclusions. Fine grains of grothite, apatite and barite are developed in accessories. Apatite and barite possess significant admixtures of strontium, which indicates alkalinity of Devonian complex. Apart this, inclusions of strontianite in grains of apatite denote strongly alkaline conditions of mineral formation, as well as strongly thorium zircons discovered in rutile aggregate.

Nepheline picrite (khatangite), discovered in the well core Vasilevskaya 1 at the depth of 1785-1788 m represents massive rock of porphyritic structure. Inclusions of olivine and monoclinic pyroxene are singled out in anisomerous matrix. Share of inclusions in the total volume of rock makes up 40 %. Thin sections of rock show glomeroporphyric structure. Mineral composition of khatangite is the following (in % of volume): clinopyroxene - 55, olivine – 19, mica - 10, nepheline – 7, magnetite – 7, glass – 2. Secondary minerals are developed along mineral and inclusions – serpentine, chlorite, carbonate. Accessory mineral are represented by fluoric apatite and copper sulphide. Turnerite, which indicates high differentiation of magma, was discovered in rock in the form of mineral inclusion.

Nepheline of khatangite and nephelinite form microstylous prismatic crystals and it is represented by two generations – inclusions and microlites of matrix. Nepheline of the first generation is rich in clinopyroxenic interpositions. Dimensions of inclusion vary from sub millimeters to first millimeters. Nepheline was substantially altered by means of substitution with clay minerals.

Olivine forms inclusions. Its grains are of idiomorphic shape, which results in sections similar to longitudinal section of short prism. Average dimension of crystal is in the range from 0.5 to 0.8 mm. Olivine is edged with serpentine and chlorite.

Biotite forms inclusions and microchips. It pleochlorises from dark-yellow to pale-yellow colours. Plates are elongated regarding fissility and their dimensions vary from sub millimeters to 1 mm. Mica inclusions contain incorporations of clinopyroxeneа and nepheline.

Pyroxene of the abovementioned rocks are represented with natron augite and aegirine, which are the products of late crystallization of alkaline magma. Clinopyroxene differ morphologically: they form idiomorphic crystals, long prismatic xenocrysts, poikilitic crystals with incorporations of apatite, fine-needled in crystals of biotite, interprosition in defects of nepheline crystals. These facts make it possible to assume various ways of formation and release order of pyroxene in rock, several generations. Needle-shaped incorporations of aegirine-augite in biotite, regarding its structural relations with minerals, are protogenetic, captured by host mineral in the course of growth. Xenocrysts are distinguished among inclusions and it seems that they crystallized in coexisting magmatic chambers. Edges of these crystals are broken and grains themselves are cracked. Zonal crystals with changes in colour from yellowish (in cores) to green are found in thin sections. Occurrence of green edges is conditioned by variations in content of aegerine minal. Dimensions of phenocrysts vary from tenths of millimeters to first units of millimeters and often poikilitically catch micro crystals of apatite. Clinopyroxens of matrix form needle-shaped small crystals and are rich in aegirine component. Aegerine interpositions in crystal defects of nepheline relate to products of late crystallization.

Table 1 gives results of two analyses of clinopyroxene: 1 – natron augite inclusions in biotite, 2 – aegirine of matrix. Compositions of minerals were determined at the Institute of experimental mineralogy (Chernogolovka) by means of digital scanning microscope Tescan VEGA-II XMU with energy-dispersive spectrometer INCA Energy 450.

Due to the fact that this paper is one of the first reports on mineralogy-geochemical peculiarities of alkaline-ultrabasic-alkaline-basalt-like formations of the Prypiat graben, carried out research gives grounds for further detailed examination of rocks of the complex under study. Discovered interpositions of strongly thorium zircons, monazite and strontianite indicate strong differentiation of magma and strongly alkaline conditions of mineral formation. Determination of composition of clinopyroxene interpositions enables to switch to reconstruction of P-T parameters of melts crystallisation.

This research was carried out under support of the Belarusian Republican Fund for Fundamental Investigations (grant no. Х11М-222)

 

Literature

 

1.    B. Ye. Borutskiy. Rock forming minerals of strongly alkaline complexes. – М.: Science, 1988. – 215 p.

2.    Korzun V. P., Machnach A. S. Late Devonian magmatism of the Prypiat aulacogen // Tectonics and magmatism of the Eastern European Platform. М., 1994. P. 195-199.

Petrographic code of Russia. Magmatic, metamorphic, metasomatic and impact formations. Second edition, altered and supplemented SPb: VSEGEI, 2008. 200 p.