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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
sashaalb@list.ru
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.
References:
Альбеков А.Ю., Рыборак М.В. Крупные
изверженные провинций (LIPs) и их отражение в геологической эволюции
Воронежского кристаллического массива //Вест. Воронеж. ун-та. Сер.
Геология. 2012. №1. (в печати)
Бочаров В.Л., Фролов С.М. Апатитоносные
карбонатиты КМА. Воронеж: Изд-во ВГУ, 1993. 122 с.
Ernst R.E., Bell K. Large
igneous provinces ( LIPs) and carbonatites // Min. Petrol. 2010. V.98.
N.1. P.55-76.
Carbonatites: genesis and
evolution / Bell K. London: Unwin Hyman. 1989. 618 p.
Simonetti A., Goldstein
S.L., Schmidberger S.S., Viladkar S.G. Geochemical and Nd, Pb, and Sr
isotope data from Deccan alkaline complexes-inferences for mantle
sources and plumelithosphere interaction // Jour. Petrol. 1998. V.
39. P. 1847–1864.
Tolstikhin I.N., Kamensky
I.L., Marty B. Rare gas isotopes and parent trace elements in
ultrabasic-alkaline-carbonatite complexes. Kola Peninsula:
identification of lower mantle plume component // Geochim Cosmochim
Acta. 2002. V.
66. P. 881–901.
Kogarko L.N. Enriched
mantle reservoirs are the source of alkaline magmatism // Proceedings of
VI International Workship «Alkaline Magmatism, its sources and plumes».
– Irkutsk and Napoli. 2006. P. 46–58. |