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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. |