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Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Paleoproterozoic alkaline-ultramafic magmatism with carbonatites of Kursk block (Central Russia): geology and geodynamic interpretation

Albekov A. Yu., Kuznetsov V.S., Ryborak M.V.

Voronezh State University, Voronezh, Russia



Within a Kursk block, which in modern coordinates is the eastern part of Sarmatia, set several manifestations of alkaline-ultramafic magmatism with carbonatites. They are confined to deep fault NW zones, which mark Tim-Yastrebovskaya and Volotovskaya structures of Alekseevsko-Voronetskaya Paleoproterozoic rift zone.

The rocks of alkaline-ultramafic formation in this region are described in the form of dikes and typical Dubraviskiy massif, which located in the northern closure of Volotovskaya structure. Dubraviskiy massif is a arc-like steep-falling (75-80o) intrusive body length of about 7,0 and a width of 0,6 km, which formed a alternation complex of rocks formed in several stages (Бочаров, Фролов, 1993): 1) the earliest, represented by alkaline (aegirine - augite) pyroxenites and subcalcium diopsidites; 2) carbonatites, with the formation of calcium, at least calcic-dolomite carbonatites silicium-carbonatites and apatite ores; 3) the final stage of the formation granite-syenites intrusion bodies, dikes and veins syenites and alkali granites.

Alkaline pyroxenites have a pan-idiomorphic and sideronitic structure, this rocks consist of sodium-augite, aegirine-augite, diopside and subcalcium ferroavgit, Ti - biotite, high-iron tetraferribiotite, tetraferriphlogopite, are well represented magnetite, ilmenite, sphene and apatite. Among the carbonate rocks are distinguished apatite carbonatites (70-90% carbonate), magnetite-rich varieties - nelsonity (50-70% carbonate) or silicate minerals (50-70%) - silicium-carbonatites. Calcite, at least dolomite are predominantly for mineral composition of carbonatites, rarely carbonatites containing different amounts of apatite, magnetite, ilmenite, sphene, epidote, sulfides, alkali pyroxenes and amphiboles (richterite) and varieties of biotite - tetraferribiotite and tetraferriphlogopite.

Silicium-carbonatites is a rock with unstable contents of calcite, silica and ore components, they are characterized by coarsely banded structure with multiple pattern of relationship the major rock-forming minerals. When the apatite and magnetite clearly predominance over other minerals intrusive bodies becomes into the category of ore bodies with sufficiently sharp boundaries. Apatite in ore has idiomorphic forms and get a solid aggregates with the smallest interstices occupied by xenomorphic calcite, biotite, magnetite, sphene and clinopyroxene. For the quantitative content of magnetite and silicate minerals distinguish two major types of ore: magnetite-apatite and silicate apatite.

The most powerful manifestation of the alkali syenite located in the footwall of the ultramafic-alkaline-carbonatite intrusion, and thin veins and dike are widely distributed throughout the body. The main rock-forming mineral syenite is a latticed microcline, dark-colored minerals are aegirine and aegirine-augite, biotite and, rarely, alkaline amphibole. Accessory minerals are always present calcite, magnetite, sphene, and apatite. Large bodies of alkaline granites (35-40 m) revealed among the syenites in the supine and in contact with pyroxenites in the hanging wall of Dubravinskiy massif. Contacts of these bodies of granite accompanied by metasomatic reaction-formations, confirming the nature of the intrusive granites. Alkaline granites differ from syenites not only rich in quartz, but also simplified species composition of mafic and accessory minerals and the appearance of plagioclase (not more than 3-7% vol.) in the main quartz-microcline matrix. Mafic minerals are present in small amounts (up to 5% vol.), biotite, egirine-augite, sphene.

This rocks belong to the alkaline-ultramafic igneous potassium-sodium series (Бочаров, Фролов, 1993), their distinctive petrochemical characteristics are high agpaitic (over 1.25) and the low value of the ratio of magnesium to the total iron and calcium (less than 1), as well as titanium containing high (more than 2.2 wt.%). All carbonatite rock forms a single syngenetic petrochemical series with increasing concentrations of carbonatites-bearing components while maintaining high and stable potassium agpaitic specialization.

Identified geological and petrographic characteristics (position in the rock sections; contact and reaction relationships, textural and structural characteristics, prevalence, and the affinity of the metasomatic mineralization) show: 1) the genetic relationship of alkaline pyroxenites, carbonatites and syenite and legitimacy of combining them into a single igneous complex; 2) the temporal sequence of the three main manifestations of intrusive phases - from the alkaline carbonatites and pyroxenite to granite-syenites 3) magmatic genesis of the main types of mineralization - magnetite-apatite, with slave metasomatic development of mono-and polymineral apatite mineralization.

In addition Dubravinskiy massif on the Kursk block allocated a significant amount of dikes of biotite-carbonate and alkali-amphibole-carbonate composition, widely located on the region of bridge between Volotovskaya and Tim-Yastrebovskaya structures, as well as within them. In regard to mineral and petrochemical composition this formation is identical to the carbonatite dykes and silicium-carbonatites of Dubravinskiy massif, that is the basis for combining them into a single magmatic complex.

Currently, there is a definite pattern of relationships with carbonatites and large igneous provinces (LIPs), which is manifested both in time and space, suggesting the existence of LIP - carbonatite associations, components can be considered as different evolutionary paths of the same magmatic process are the different parts of a magmatic system (Ernst, Bell, 2010). About 50% of all known manifestations of carbonatites associated with extension environment, such as a system of rift valleys, and the rest are associated with major faults and large-scale dome-shaped elevations (Ernst, Bell, 2010). Although the various models of carbonate-rich magma origin (Carbonatites ..., 1989), there is increasing evidence that many carbonatites directly or indirectly connected with the rise of deep mantle material in the plumes and hot spots (Carbonatites ..., 1989; Simonetti et al., 1998 ; Tolstikhin et al., 2002; Kogarko, 2006, etc.), which implies a rift nature of their manifestation in the lithosphere.

Assuming the genetic relationship between large igneous provinces and carbonatites, the authors attempted to set their position in the Precambrian tectonic stages of evolution of Kursk block. Given the high degree of erosion, is also powerful enough to cover sedimentary rocks, uniquely reconstruct the likely areas of distribution of large igneous provinces of relics is extremely difficult in the Kursk block.

However, based on available geological information on the distribution and composition of known volcanic and intrusive formations, as well as the correlation with the model of Precambrian cratons are invited to a selection of three different ages of the Precambrian manifestations (epochs), probably related to large igneous provinces of mantle-plume origin (Альбеков, Рыборак, 2012): 1) Neoarchaean high magnesian komatiite-basalt magmatism in formation with age in the range of 2600-2500 Ma (TIMS U-Pb isochron for zircon and Sm-Nd erohron ages), 2) Paleoproterozoic rift rock associations (from the age of about 2066 Ma - U-Pb isochron), formed by the active influence of a mantle plume (expressed in the formation of Belgorod-Michaylovskiy, and Alexeevsko-Voronetskiy rift structures) and the formation of a wide range of ultramafic-mafic igneous formations peridotite-gabbronorite formation; 3) the Paleoproterozoic platform manifestations of plateau basalts (with the age of about 1790 Ma - U-Pb isochron), preserved in a well-manifested in a modern block of Kursk northern part of hypabyssal troctolite-gabbrodoleritic sills.

Due to the current lack of isotope-geochronological data on the alkaline-ultramafic formations with carbonatites on the Kursk block, the age of their formation is unknown, and for this reason there is a possibility that they belong, as the second group of manifestations, and the third. However, the indirect geological features can be attributed to the second phase of plume magmatism of Kursk block area and assume their rift nature, it is: 1) the spatial association and the proximity of the material composition of the Dubravinskiy intrusive massif and dike complex, and 2) the location of intrusions and dikes on the bridge between the two rifting depressions (Tim-Yastrebovskaya and Volotovskaya) within a Alexeevsko-Voronetskaya rift zone, and 3) an arc-like shape of Dubravinskiy massif that replicates in terms of the closure Volotovskaya structure, which indicates the time of the introduction of intrusion pre-folding 4) a significant spatial isolation structures of alkaline-ultramafic formations with carbonatites from the distribution ranges of plateau-basalt formations, developed mainly in the northern part of the Kursk block.



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