special properties of the Palaeozoic alkali igneous rocks of Belarus
Laptsevich, N.D. Mikhailov
unitary enterprise “Belarusian Research & Development geological
prospecting institute”, Minsk, Republic of Belarus
The article summarizes the results of petrochemical research study of
the rocks belonging to the Upper Devonian alkali igneous formation of
Belarus with reference to each locality (area, structure) within the
bounds of which these assemblages are identified. The igneous sequences
within them are distinguished by the scale of occurrence, structure, set
or predominance of certain facial or petrographic rock types as well as
by mode of occurrence and stratigraphic position incorporated into the
Pre Upper Famennian part of the platform mantle. To obtain the total
characteristic of the region altogether the paper under consideration
refers to the results of the study of Pripyat graben rocks, first
carried out by V.P. Korzun in the 1960s-1980s (Korzun, 1977 and others;
Korzun, Makhnach, 1977).
Pripyat graben (Pripyat palaeovolcanic area).
In this area igneous rocks occur at the depth of 900-2500 m and they
have the largest distribution areas, scale of occurrence, facial
representativeness (effusive stream flows, nappes, other volcanic,
subvolcanic and hypabyssal bodies) and petrographic variety (represented
by varieties of alkali-ultrabasic, alkali and subalkali basic and medium
igneous rocks). Volcaniclastic assemblages build up bands and beds which
have stable thickness and extensive distribution area and, by
alternating with effusive rocks in the cross-section, they form two
thick rock masses up to 800-900 m each but within multiple buried
palaeovolcanic structures thickness of volcanic rocks reaches 2300 m and
even more. In the make-up of the major rock-forming minerals there are
alkali pyroxenes – aegirine and aegirine-augite. Feldspars in the
majority of cases are represented by an alkali variety of (orthoclase,
anorthoclase, primary albite). Hyperalkalinity of ultrabasic rocks
reflects the presence of biotite, phlogopite, analcime and natrolite in
them. Major rockforming minerals of alkali (feldspathoid) rocks are
nepheline, leucite and aegirine-augite. The following accessory minerals
are most widely spread in the rocks of the formation: titanomagnetite,
sphen and apatite. The main special properties of rock chemism are
markedly increased alkalinity with the predominance of sodium and mixed
potassium-sodium type, low contents of earth silicon – all rock kinds
are undersaturated with silicic acid and have increased titanium level,
mineralogically revealed in high contents of accessory sphen, higher
magnesium and iron oxides contents in alkali and subalkali trachytes
(increased level of mafic minerals). The most alkali rocks of the main
composition (nephelinites, leucitites) as well as trachybasalts,
trachyandesites and trachytes are poor in calcium. Alkalis’ composition
is marked by the prevalence of Na2O over K2O even in the rocks
containing modal leucite (leucitites, leucite nephelinites).
Gomel structural stopping.
Igneous rocks of this structure extend in northern direction of Pripyat
palaeovolcanic area beyond the bounds of Pripyat graben forming effusive
stream flows or domes (trachytes, trachybasalts) and subvolcanic bodies
(nephelinites, syenite-porphyries, fourchites, etc) and they occur at
the depth of 150-300 m (Veretennikov and others, 2000; Mikhailov,
Laptsevich, 2011). The attention is drawn to the fact that there is low
concentration of CaO and increased alkali level in all rock varieties.
K2O markedly prevails in leucitites and tephriphonolites, its content
reaching 10-15%, which broadly corresponds to alkali rocks of potassium
series, containing modal leucite both among porphyry phenocrysts and in
the form of microlites in the rock groundmass. It should be noted that a
low earth silicon content is most typical of trachytes and syenite-porphyries
but basic rocks are characterized by a high titanium content. All the
rocks of this structure are classified as highly aluminiferous and
Within the bounds of Nort-Pripyat shoulder igneous rocks
are identified only in the outermost eastern part coming in contact with
igneous sequences of Gomel structural stopping. They form small central
type palaeovolcanic structures and are mostly represented by
volcaniclastic rocks which sometimes alternate with effusive ones and at
great depths are superseded by massive subvolcanic or vein igneous rocks
(Veretennikov and others, 2000; Mikhailov, Laptsevich, 2011). All the
rocks are considerably modified. They can be subdivided into several
groups. The first group is the rocks of alkali-ultrabasic composition
with feldspathoids (leucite, nepheline) and olivine; the second one is
basic rocks (basaltoids) of subalkali and alkali type with abundance of
pyroxene and feldspars. The next group is formed by the rocks which have
been uncovered by borehole drilling performed beside Gomel structural
stopping. Alkali trachytes and trachybasalts are identified here. All
the examined igneous rocks of North-Pripyat shoulder are characterized
by a higher alkali and titanium content (they mostly belong to alkali
rocks of potassium type) and they are undersaturated with earth silicon.
Ultrabasic rocks have an increased level of earth silicon and a low
level of MgO. The chemical composition of the rocks belonging to the
second group reflects their “transitional nature” from alkali ultrabasic
rocks to basic pycrobasalt ones.
Igneous assemblages are mostly represented by blow holes (diatremes) of
Zhlobin field (Khomich and others, 1993). All the detected diatremes are
formed by ultrabasic volcaniclastic rocks of alkali type (principally
potassium series) with a subordinate evolution of subvolcanic and
hypabyssal igneous rocks of the same composition (Veretennikov and
others, 2001). In terms of the chemical composition the majority of
rocks of the examined diatremes correspond to ultrabasic foidites –
melanephelinite and melaleucitite or they are rather close to them
according to their mineral composition. The rocks of the different
minority part of diatremes by these parameters are closer to alkali
picrites and melilitites. Picrites are characterized by a noticeably
(less than 5%) low modal olivine content which found its way in
respectively low MgO content. Ultrabasic foidites have a slightly higher
MgO and Fe2O3 content and low earth silicon content. Petrochemical
special property of diatreme rocks is a higher alkali level (with
prevalence of potassium over sodium) and low concentration of calcium.
Thus, the Palaeozoic igneous rocks of Belarus, represented by a wide
range of petrographic types and varieties of volcanic, effusive and
hypabyssal rocks, are concentrated in 4 localities. Their petrographic
variety is reflected in the chemical composition of rocks as well. The
classification chart SiO2 – Na2O+K2O depicts the information on the
chemical composition of all the Palaeozoic igneous rocks of Belarus
(Figure). The compositions of rocks of all the localities cover the
margins (patches) from ultrabasic to medium ones with all intermediate
varieties (the continuity of compositional change is clearly seen).
Figure – Position of the Palaeozoic alkali igneous rocks of
Belarus on the classification chart TAS
Rocks: 1 – Pripyat graben, 2 – Zhlobin saddle, 3 – North-Pripyat
shoulder, 4 – Gomel structural stopping.
Together with petrochemical differences ascertained for the rocks of
each locality (area, structure) common petrochemical special properties
can also be observed, they are inherent in all or in the overwhelming
majority of the examined rocks. First of all it corresponds to
hyperalkalinity of rocks which makes it possible to refer them to rocks
of alkali igneous formation with a prevailing potassium type of a
considerable part of the rocks under consideration. Also all the rocks
have a low level of earth silicon and calcium. All this makes it
possible to conclude that all the Upper Devonian igneous formations of
the south-east of Belarus belong to one genetically unique alkali
The paper is executed with financial support from the BRFFR – RFFR
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