Mineralogical and geochemical pyroxene features of Pripyat paleograben alkaline magmatic rocks

Kraineva E.,

Production Association «Belorusneft» BelNIPIneft, Gomel, Belarus.

Upper Devonian alkaline formation located in the south-eastern part of Belarus is composed of petrographically diverse volcanic, subvolcanic and hypabyssal rocks. Pyroxenes occur in different quantities in all rock types. Thus pyroxenes are the ones of the most widespread and at the same time informative minerals of upper Devonian formation. Composition, morphology, crystallization features of rhombic and monoclinal pyroxenes reflect conditions of formation and evolution of magmatic series.

There occurs universal development of secondary mineralization within the formation (among the substitution products zeolites, chlorites, clay intrusions and carbonates are widespread). For surveying hyalopicrites, alkaline picrites, monchikites, nephelinites, feldspathic nephelinites, leucitites, alkaline trachytes, syenite-porphyres were substracted. Pyroxenes from the listed samples are characterized by satisfactory condition; they form phenocrystals in the rocks and are a part of the matrix. Pyroxene crystallization occurred in several stages. This fact is confirmed by the development of three clinopyroxene generations distinguished by crystal morphology: size, form, relation with the other minerals; extinction angle, pleochroic scheme. The first two include porphyres of orto- and clinopyroxenes, the third generation is formed by matrix clinopyroxenes.

Ortopyroxene is contained in single marks, it forms phenocrystals and in thin sections is diagnosed by straight extinction. Clinopyroxenes form isomorphic series of diopside-salites and aegerine-augites. Crystal size changes from centimeters to centimillimeters. Idiomorphic grains of hexagonal, prismatic, tabular, acicular form predominate. In thin sections they have well-defined pleochroism (change tone, depth of color and shade). Extinction angles are 0-3°, 30-50°. Zonal crystals of aegerine-augite possibly carried out by lava from the more deep horizons are met as well. Inner part of zonal grains is composed of greenish augite; periphery has pale-yellow shades.

Material composition of pyroxenes was defined by means of micrologging with the help of Vega\\TESCAN analyzer at the institute of experimental mineralogy.

All the studied ortopyroxenes are magnesian and in magnesium-ferric oxide relation correspond to bronzite composition. It is known that in alkali melts ortopyroxen appears under moderate and high pressure. High concentration of natrium in ortopyroxene (to 0,2 weight %) is also indicative of over-pressured conditions of rhombic pyroxene crystallization (in paragenesis with monoclinic). However in some samples of ultrabasic rocks quantity of natrium oxide in rhombic pyroxene exceeds 0,2 % and even reaches 1,41 % what can be caused by the influence of superimposed postmagmatic processes. Silicium oxide content varies greatly (from 45,19% to 60,39%) and is related to secondary mineralization character and different degree of rhombic pyroxene variation. Chrome and nickel are typical admixtures of rock ortopyroxenes. In rhombic pyroxenes of monchikites and alkali picrites quantity of these oxides is 0,01-0,06 weight %, 0,03-0,07 weight % correspondingly. Other element content in the studied pyroxenes agrees with ortopyroxene classification of U.A. Dir and amounts to: 0,01-0,06% TiO₂; 0,51-4,94% Al₂O₃; 9,3-14,2% FeO; 25,88-32,43% MgO; 0-0,31% K₂O. Linear relations of monoclinal and rhombic pyroxene compositions don’t deviate from the common ones and are indicative of balanced paragenesis character.

Clinopyroxenes are met in all rock types and characterized by wide variation of chemical composition. Thus silicium oxide content changes from 45,01% to 52,51%, titanium – from 0,54% to 3,93%, aluminium – from 0,58% to 6,15%, chrome – from 0% to 0,55%, kalium – from 0% to 0,56%, ferrum –from 6,01% to 29,66%, magnesium – from 0,11% to 16,09%, calcium – from 3,88% to 25,11%, natrium – from 0,16% in augites to 12,24% in aegerines. Clinopyroxenes that can be diagnosed as chromous are widespread. Monoclinal pyroxenes in foiditic rocks are magnesium poor, ferrum abundant and as a result don’t contain chrome or contain this element but in small amount. High concentration of manganese oxide and low nickel content in some analyses can be explained by high ferrum concentration. Isomorphic kalium admixture in pyroxenes crystallized in monoclinal singony makes it possible to estimate thermobaric conditions of rock formation. However abnormally high kalium content in amount 0,54%, 0,56% in aegerine-augites of leucitite and feldspathic nephelinite is related not so much to high pressure but postmagmatic rock reformation. Clinopyroxenes are also characterized by high titanium oxide concentration what is typical for pyroxenes of the main alkali rocks and once again confirms alkaline character of upper Devonian magmatism.

References:

1 Dir U.A., Haui R.А., Zusman J. Rock-forming minerals. Vol 2. Moscow: Mir, 1965. 405 p.

2 Petrographic code. Magmatic and metamorphic formations. Approved by IPC on the 28^th of December, 1994. St. Peterburg: VSEGEI, 1995.