Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Petrochemical special properties of the Palaeozoic alkali igneous rocks of Belarus

A.G. Laptsevich, N.D. Mikhailov

Republican unitary enterprise “Belarusian Research & Development geological prospecting institute”, Minsk, Republic of Belarus

laptsevich@geology.org.by

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 rather aluminiferous.

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.

Zhlobin saddle. 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).

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 formation.

The paper is executed with financial support from the BRFFR – RFFR Х12Р – 058.

References:

V.P. Korzun. On the subvolcanic formations in the sedimentary mantle of the eastern part of Pripyat trough // Lithology, geochemistry, palaeogeography of the Upper Proterozoic and Palaeozoic sediments of Belarus. Science and technology. 1977. P. 119-125.

V.P. Korzun, A.S. Makhnach. Upper Devonian alkali volcanogenic formation of Pripyat trough. Science and technology. 1977. P. 164.

P. Z. Khomich, E.A. Nikitin, A. I. Grishko and others. New development area of kimberlite magmatism in the west of the East-European platform // Reports by the Academy of sciences of Belarus. 1993. Vol. 37. #1.p. 83-86.

N.V. Veretennikov, V.P. Korzun, A.G. Laptsevich, N.D. Mikhailov. Blow hole (diatreme) petrology of Zhlobin field (Belarus) //Lithosphere. 2001. #1 (14). P. 46-55.

N.V. Veretennikov, V.P. Korzun, A.S. Makhnach, A.G. Laptsevich. Upper Devonian volcanogenic formations of Uvarovichy locality // Report by the Academy of sciences of Belarus. 2001. Vol. 45, #1. P. 100-102.

N.V. Veretennikov, V.P. Korzun, A.S. Makhnach, A.G. Laptsevich. Upper Devonian volcanogenic formations of Gomel structural stopping. Report by the Academy of sciences of Belarus. 2000. Vol. 44, #5. P. 107-109.

N.D. Mikhailov, A.G. Laptsevich. Petrographic special properties of igneous formations of Gomel structural stopping. // Lithosphere. 2011. #1 (34). P. 94-102.

N.D. Mikhailov, A.G. Laptsevich. Petrographic characteristic of the Devonian alkali magmatism of North-Pripyat shoulder // Lithosphere. 2012. #1 (36). P. 94-102.