2011

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Ðóäíûé ïîòåíöèàë ùåëî÷íîãî, êèìáåðëèòîâîãî

 è êàðáîíàòèòîâîãî ìàãìàòèçìà

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Petrogeochemical characteristics and genesis of the Paleoproterozoic alkali-mafic effusive-intrusive association of the East European Craton southwest
Aksamentova N.V.

Republican Unitary Enterprise “Belarusian Research Geological Exploration Institute”, Minsk, Belarus, aksam@igig.org.by

All the rocks of the Late Paleoproterozoic effusive-intrusive alkali-mafic association of the East European Craton southwest are characterized by a high ferruginous value, high Zr, Nb, Be, Ba, TR and low Mg, V, Cr, Ni contents. Features of the rock mineral and chemical composition allow us to consider them as products of the alkali-basaltic magma differentiated at two depth levels.

Lava sheets, sills and dikes of trachyandesites, trachydolerites, trachytes and intrusive massifs of alkaline gabbro and syenites are widespread in the East European Craton southwest within the Ovruch depression, in its periphery and in the crystalline basement of the Zhitkovichy horst in the southern part of Belarus. All the rocks are similar in their formation time and can be grouped into two comagmatic series in the composition of the Late Paleoproterozoic effusive-intrusive alkali-mafic association. In the rock variety, presence of effusive and subvolcanic facies along with the intrusive rocks, as well as the intensity and wide-spread occurrence of the alkaline metasomatism this association has no analogues among the coeval igneous rock manifestations in the craton territory.

Effusive series includes effusive and subvolcanic facies rocks distributed within the Upper Zbranky subsuite of the Ovruch depression (Lunko et al., 1971; Danilovich et al., 1983). The effusive facies involves 10–20 m thick trachyandesite sheets separated by volcanomictic sandstones. Sometimes, the upper sheet parts are well preserved and consist of hematitization lava breccias, suggesting their formation in the subaerial environment. Within the Ovruch depression subvolcanic sills are represented by 25–30 m thick trachydolerites, in Belarus – by zonal dikes of 45 m thick trachydolerites and dikes of trachyandesites, their thickness ranging from 0.7–1.6 to 20 m.

Intrusive series is represented by alkaline gabbroids, pyroxene and amphibole alkali-feldspar and alkaline syenites. These rocks form the Nichiporovka, Davidkov, Yastrebetsk and other small massifs, which are 1.5–2.5 km in diameter and frame the Ovruch depression. The isotopic age of the Yastrebetsk massif syenites is 1722+5 (17) Ma (Skobelev, 1987). All the rocks have a complex mineral composition. The main rock-forming minerals in gabbros are two generations of plagioclase (An48–50 and An16–27), potassium feldspar, mafic minerals of high iron content (f, at.) – fayalite (0.91), ferroaugite (0.56), barkevikite (0.73), biotite (0.79). In the association of rock-forming minerals and their uneven distribution gabbroids are similar to feldshpatoidless melanocratic olivine essexites. Syenites consist of alkaline feldspar, ferrohedenbergite, biotite, hastingsite, aegirine or arfvedsonite; titanomagnetite is present.

The chemical composition of rocks of the alkali-mafic association varies in a wide range from basic to moderately acidic. All rocks are characterized by high ferruginous (f = 0.76–0.98), low magnesium and moderate or high alumina content (see Table). Against the background of common petrochemical characteristics, there are some differences in the composition of facies rock types, due to both conditions (depth) of their crystallization, and degree of magmatic differentiation. In particular, unlike effusive rocks, trachydolerites are lower in SiO₂ and higher in TiO₂ and P₂O₅. In trachydolerites and trachyandesites of the Ovruch depression K₂O usually dominates over Na₂O, whereas an inverse ratio of these oxides is observed in similar rocks of Belarus. The degree of iron oxidation in trachydolerite sills of the Zbranky subsuite is higher than in similar rocks of the dikes of the Zhitkovichy horst, which obviously reflects the deeper crystallization conditions of the latter. Lava breccias of the sheet upper parts are mostly oxidated and characterized by enrichment in potassium. In the K₂O and Na₂O ratio trachytes are divided into the potassium and potassium-sodium varieties. The agpaitic ratio in both of them is close to unity (0.95 and 0.85, respectively).

Among intrusive rocks alkali-feldspar syenites are petrochemical analogues of effusive rocks (trachyandesites), whereas gabbros show smaller contents of SiO₂, Al₂O₃, alkalis and higher contents of TiO₂, MgO, reflecting their more melanocratic composition. Rock varieties with increased sodium alkalinity are present both among volcanic, and intrusive rocks. The trace element composition of the rocks is distinguished by very low contents of V, Cr, Ni (<5–10 ppm) at increased participation of rare lithophile (Zr, Nb, Be, Ba) and rare earth elements in both volcanic, and intrusive rocks. This is characteristic of nepheline rocks such as essexites, phonotephrites and tephriphonolites that form parts of the potassium-sodium alkaline series. The lack of feldspathoids in gabbros seems to be due to shallow conditions of their formation (Sharkov, Bogatikov, 1981). The latest members of the intrusive series are fine-grained syenite and selvsbergite dikes that are comparable to mariupolites in their enrichment in zirconium and other rare elements.

The alkali-mafic association of the East European Craton southwest is similar in its rock set to the magmatic series of continental rifts, which associates its formation with the consolidated Earth's crust rifting processes. The greatest similarity was established with the Devonian potassium-sodium alkaline series of the Kuznetsk Alatau region appeared under rifting condition at the final stage of the folded region orogeny.

The chemical composition features of rocks of the effusive-intrusive association allow us to consider them as products of the alkali-basaltic or basanitic magma subject to differentiation, at least at two depth levels – in deep-seated and shallow-level intermediate magma chambers. The appearance of the entire rock spectrum of the volcanic and intrusive series was, obviously, associated with the melt differentiation in just a shallow-level chamber. Because of a small depth of the magma chamber, favourable conditions were apparently created for the separation and removal of volatiles together with sodium and trace elements. These processes resulted in decrease of the sodium alkalinity of the melt and its crystallization products, as well as formation of broad aureoles of alkali-metasomatic rocks showing rare-metal mineralization.

Table – Chemical composition of the alkali-mafic extrusive-intrusive association rocks

of the East European Craton southwest

Note. 1–megaplagiophiric trachydolerites, 2–aphyritic trachyandesites and 3–hematitization lava breccias of trachyandesites of the Zbranky subsuite within the Ovruch depression; 4–megaplagiophiric trachydolerites,  5–aphyritic trachyandesites, 6–potassium and 7–potassium-sodium trachytes from the Zhitkovichy horst, 8–alkaline gabbros of the Nichiporovka massif, 9–pyroxene alkali-feldspar syenites of the Davidkov massif; 10–amphibole alkali syenites, 11–amphibole alkali-feldspar syenites and 12–selvsbergite of the Zhitkovichy horst.

The restriction of the association within the junction of two major long-lived lineament zones enables us to connect the alkaline mafic magmatism manifestation in this area with a mantle plume occurring in this region and generation of alkali-basaltic magma in a deep-seated mantle source. The alkali-mafic association arisen after manifestations of the subalkaline basic magmatism completed in fact the development of the  Paleoproterozoic magmatism in this region.

This study was financially supported by BRFBR –RFBR, grant ¹ Õ10Ð-092.

References:

Danilovich L.G., Bukovich I.P., Polishchuk L.V. Features of petrology of volcanic formations of the Ovruch Graben (northwestern Ukrainian Shield) // Geol. Journ. 1983. Vol. 43. N 6. P. 101–109 (in Russian).

Kogarko L.N. Problems of genesis of agpaitic magmas. Moscow: Nauka, 1977. 294 ð (in Russian).
Lunkov V.F., Drannik A.S., Rolik A.G., Metalidi S.V. The main features of geological structure and metallogeny of the northwestern Ukrainian Shield // Problems of petrology and ore potential of the crystalline basement of the BSSR and the adjacent areas. Minsk: Nauka i technika, 1971. P. 286–313 (in Russian).
Sharkov E.V., Bogatikov O.A. Minerals parageneses of the alkali-gabbroid series of rocks as a reflection mode of magma differentiation // Petrology and ore potential of the indicator magmatic formations. Moscow: Nauka, 1981. P. 21–35 (in Russian).