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Rare-metal granites and
rare-earth syenites in the context of anorthosite-rapakivigranite formation of Ukrainian
Shield
Sheremet Ye.M.*,
Krivdik S.G.**, Sedova Ye.V.***
*Ukrainian
State Research and Design Institute of Mining Geology, Rock Mechanics
and Mine Surveying, NAS of Ukraine (UkrNIMI, NAS of Ukraine), Donetsk,
Ukraine; **Institute
of Geochemistry, Mineralogy and Ore Formation, NAS of Ukraine, Kiev,
Ukraine, ***Donetsk National Technical University, Donetsk, Ukraine.
EvgSheremet@yandex. ru
In Ukrainian Shield
anorthosite-rapakivigranite formation (1.85‑1.7 milliard years) is
represented in three megablocks: in Priazovie – gabbro-syenite south
Kalchik complex; in Ingul-Ingulets ‑
Korsun-Novomirgorod
anorthosite-rapakivigranite pluton; in Volyn megablock –
Korosten
anorthosite-rapakivigranite pluton.
Rare-metal granites of
lithium-fluorine type in the context of the above formations of
anorthosite-rapakivigranite formation
are known respectively as kamennomogilskie (Priazovie), russkopolyanskie
(Ingul-Ingulets megablock); leznikovskie and perzhanskie (North-West
megablock).
By their petrochemical
composition granites of all massifs of the kamennomogilsky complex is no
different from similar petrographic differences of south Kalchik complex
and rapakivi granites and are identical to rare-metal leukogranites of
Ukrainian Shield (US):
russkopolyanskie, leznikovskie and perzhanskie granites.
Comparison of chemical
composition of biotites of granites of south Kalchik complex and
rapakivi granites and rare-metal granites of US by the level
metal-alkalinity environment (Marakushev-Tararin diagram) (Markushev,
Tararin, 1965) shows that completely overlaid in the fields of groups II
and III of metal-alkalinity environment of the diagram are biotite
compositions of moderately leucocratic granites with content of SiO2
in the range of 71.0‑72.5 % of Korsun-Novomirgorod, Korosten massifs and
massifs of south Kalchik complex. Biotites of ultra-acid granites of the
kamennomogilsky complex, of
leznikovskie and perzhanskie with content of SiO2
in rocks in the range of 73.2‑76.9 % fall compactly into group I of
metal-alkalinity environment. There is gradual transition of granite
biotite compositions from a moderate group of metal-alkalinity
environment to ultra-acid group.
For
kamennomogilskie rare-metal granites of
the Priazovie the following trend of crystallization differentiation of
parent melt is determined: granosyenites → amphibolic
granites → amphibole-biotite granites → biotite granites and
leukogranites → biotite-muscovite and muscovite leukogranites with topaz
and fluorite (+pegmatites). In the process of differentiation the melts
were enriched in F, Ta, Nb, Li,
Ве, Rb, Sn, W
и
Mo, the content of which increased in residual melts by a factor of 2‑5
in comparison with less differentiated, and depleted in Ba, Sr, Y, and
REE. Maximum content of Y and REE has been registered in amphibolic
differences (enriched in calcium). By the level of content of light
lanthanides La and Ce granite massifs differ rather strongly reflecting
different in depth conditions of development of intrusions and
consequently different levels of their erosional truncation. For
distribution of REE in
kamennomogilskie granites especially specific are: sharp deficiency of
Eu typical to all rare-metal granites (Eu/Eu* in the range
of 0.02‑0.27) and high content of REE: from 500 to 1700 g per ton (Shcherbakov,
2005).
Comparison of ranges for
distribution of REE in
kamennomogilskie granites and in perzhanskie and leznikovskie rare-metal
granites of the Korosten complex speaks for similar small-depth and
hypabyssal conditions for formation of these rare-metal granites with
rather intense feldspar fractionation, which specifies low content of
barium and strontium and negative europium anomalies in ranges of REE.
In
massifs of kamennomogilsky complex widely manifested are
processes of alkali-type metasomatism (biotitezation, microclinezation,
albitization) and processes of acid metasomatism – partial and more
rarely complete granite greysening. Formation of ore bodies with
yttrium‑rare-earth mineralization is closely connected with zones of
alkali-type metasomatism, whereas for greisenized rocks rare-metal
mineralization is typical.
Syenite Azov intrusion and
rare-earth deposit of the
same name of south Kalchik complex of the Priazovie (Azov Sea Region)
are typical indications of the multistage formation of sub-alkaline-
alkali-type metasomatism.
According to Melnikov
V. S., Voznyak D. K. et al. (2000)
syenites of the Azov deposit are products of crystallization of residual
melt formed as a result of fraction crystallization of
basic melt of subcrustal
origin. Hypersolvus composition of alkali feldspars, extremely
ferruginous composition of femic minerals containing
H2O, F and CO2 and
negative europium anomaly (Shcherbakov, 2005) in distribution of
REE are indications of high level of melt differentiation.
The Azov deposit is the upper part of
thick fractional column where accumulation of fluorophilous rare
elements and enrichment in fluid components took place. High temperature
of the melt enriched in fluid components contributed to high amount of
concentration of rare elements. Magma pocket contained residual melt
enriched in rare elements (Zr, RRE) and volatile components (water,
halides, carbonic oxides). Formation of zirconium and
zirconium-lanthanide ores at the Azov deposit took place in the process
crystallization differentiation of syenite melt in
magma pockets by a mechanism of
stratified intrusions.
Syenite intrusion of
Yastrebetsky stock (and zirconium
deposit of the same name) is located in Sushchano-Perzhanskaya tectonic
zone of Volyn megablock of US and is classified as derivatives of
Korosten anorthosite-rapakivigranite pluton. Yastrebetsky
syenite massif with its rich
zirconium ores is almost similar in every way to the Azov deposit.
Britholite, orthite, and bastnasite, which are typical minerals of
rare-earth and zirconium‑rare-earth ores of the Azov deposit, both by
their gross chemical composition and ranges of the content of rare
earths and yttrium were found identical or close at those two deposits.
There is a number of differences caused by both different level of
differentiation of parent melts, and may be by different directions of
their fractioning.
Syenites of
Velyka Vyska massif are one of
those of the final differentiates of anorthosite-rapakivigranite
Korsun-Novomirgorod pluton. High iron content of these syenites and
minerals in them as well as considerable concentration of such
incompatible elements as Zr, TR, Y, Nb and low concentration
of Sr and Ba speak for residual nature of the melts from which these
rocks were formed. By their petrological and geochemical features
Velikoviskovsk syenites occupy
intermediate position between fayalite-hedenbergite syenites of South-Kalchik
massif and similarly-named rocks of the Azov deposit.
Syenites occur in many anorthosite-rapakivigranite plutons, however,
within the Ukrainian Shield syenite trend in the evolution of these
plutons is most pronounced. For example,
south Kalchik massif in
Priazovie is essentially
syenite analog of such plutons. The
weight of evidence suggests that this trend might have been worked in
abyssal conditions at reduced fugacity of oxygen and thus it could
manifest itself in more eroded plutons. Korsun-Novomirgorod pluton and
South Kalchik massif, as realized by us, differ by higher
erosional truncation (Krivdik., Tkachuk, 1990).
All characteristics of the
main elements and rare-earth components of rare-metal granites and
rapakivi granites of US indicate distinctively enough that they belong
to A-type granitoids. Belonging to A-type granitoids is the
determinative circumstance both with relation to their geochemistry and
metallogenic specialty and with relation to melt evolution and formation
paragenesis.
To our opinion genesis of
formations of US of anorthosite-rapakivigranite formation under
consideration is attributable to their formation in rift zones attached
to the up-going plumes of convective currents. The rising mass of the
substance in the form of soft diaper, which by its composition
corresponds to alkali-type pyrolite, while raising has been
differentiating and enriching in low-melting alkali and other
lithophylic elements and volatiles in its upper part and has been
depleting of them in its low part.
At the bottom of
lithosphere, which is responsible for conditions of melting of alkali
and alkali-basic magmas (Zonnenshain
et al.,
1976), in the upper
part of diaper a shell saturated with alkali where pockets of molten
rock occurred has been forming. While raising additional differentiation
of alkali magmas could take place. Heat currents and flows of
lithophylic elements and volatiles from diaper came into continental
lithosphere that resulted in occurrence of local sources of melting with
formation of eutectic granite magmas. Simultaneous rise to the surface
of alkali melts from diaper and acid crust melts resulted in the
occurrence of bimodal series, which are anorthosite-rapakivigranite
formations.
Rare-earth specialty of
syenite magmas and rare-element
specialty of acid eutectic magmas are the result of
differentiation of magma of integrated geotectonic process in which
differentiates of alkali basaltoid magmas possessed rare-earth specialty
and eutectic melts aroused as residual magmas with accumulation of rare
lithophylic elements.
References:
Krivdik S. G., Tkachuk
V. I. Petrology
of Ukrainian Shield alkaline rocks. Kiev: Naukova Dumka, 1990.
408 p. (in Russian).
Maraqkushev A. A.,
Tararin I. A. On
mineralogic kriteria of granitoid alkalinity // Proceedings of the
Academy of Sciences of the USSR. Series:
Geology.
1965. No.
3. P. 20-37.
(in Russian).
Melnikov V. S., Voznyak
D. K., Grechanovskaya Ye. Ye. et al.
The Azov zirconium‑rare-earth deposit: mineralogical peculiarities //
Mineralogichesky zhurnal (Mineralogical Magazine).
2000.
Vol. 22, No. 1. P.
42-61 (in Russian).
Shcherbakov I. B.
Petrology of Ukrainian Shield. –Lvov: ZUKC, 2005. 366 p. (in
Russian).
Zonnenshain L. P., Kuzmin M. I., Moralev V. M.
Global tectonics, magmatism and metallogeny. Moscow: Nedra, 1976.
231 p. (in
Russian). |