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Rb-Sr,
Sm-Nd, U-Pb, Lu-Hf
isotope
systems and geochemical specificity
Il’meno-Vishnevogorsky Alkaline-Carbonatite Complex (Urals, Russia)
Nedosekova I.L.
Institute of Geology and Geochemistry UB RAS, Ekaterinburg, Russia
vladi49@yandex.ru
Il’meno-Vishnevogorsky
Alkaline Carbonatite Complex
(IVAC) is one
of the largest alkaline complexes of miaskites, fenites and carbonatites
with REE-Zr-Nb mineralization located within the Urals Fold Belt. We
have dated by different isotope methods including Rb-Sr and Sm-Nd
isochrons on WR and rock-forming minerals, SHRIMP U-Pb zircon and U-Pb
pyrochlore dating, the main rock types of IVAC and studied Lu-Hf isotope
systematics and trace element patterns of various components of the
complex. The isotope dating has detected numerous age clusters for the
IVAC rocks – 446-410, 390-360, 335-325 and 280-230 Ma. These age
clusters correspond to the major stages of tectonic development of the
Ural fold belt (rifting, early and late collisions, postcollisional
extension) [Puchkov, 2010] and recorded by isotopic dating in other
structural-material complexes of the Southern Urals [Echtler et al.,
1997].
The isotope
signatures of the IVAC main rock types: miaskite-carbonatite complex
(epsilon Sr = -6...-10, epsilon Nd = +3…+6, epsilon Hf = +4…+6) and
Buldym ultrabasic massif and related carbonatites (87Sr/86Sr
= 0.70421–0.70470, εSr
(+6…+8), εNd
(+1…-3) и
εHf
(0…-2)
show
moderately depleted to moderately enriched (type EM1) isotopic
compositions and require the depleted mantle source [Kramm et al, 1983;
Nedosekova et al, 2009; Nedosekova et al, 2010].
A close resemblance exists between the isotope characteristics and lines
of isotopic systems of the IVAC and carbonatites of rift platform
ultramafic-alkaline carbonatite complexes (UACP)
– Kola, Maymecha-Kotuy Alkaline
Provinces and other, which are located within Precambrian cratons. This
suggests that the origin of these alkaline-ultrabasic complexes is
connected with deep seated mantle sources, possibly to an upwelling
mantle plume and/or further mixing plume component with enriched
component EM1[Bell, 2001;
Kramm 1993; Kramm, Kogarko, 1994;
Kogarko et al., 2010
and other].
Therefore it is very possible the IVAC has the similar source. But at
the present the origin of alkaline magmas of the IVAC due to melting of
the UACP rocks or oceanic crust rocks
with the same isotope signatures is not ruled
out.
Close
resemblance of the IVAC and UACP complexes according to isotope and
geochemical characteristics are fixed
(divergent). IVAC
have
much in common with
the rift
UACP and
with the collision
carbonatite-alkaline complexes formed in the final stages of development
of orogens during postcollisional extension. Thus, IVAC miaskites and
carbonatites have significant content of HFSE
– Nb, Zr, etc., which are similar to the rifting
UACP complexes and differs from the
collision carbonatite complexes, which are usually depleted in HFSE.
However, IVAC
miaskites and carbonatites are enriched large ion lithophile elements (LILE)
– primarily Sr, to a lesser extent Ba,
K, Rb, LREE, which is characteristic for a collision alkaline and
carbonatite rocks, localized in linear
post-collision zones. It should be noted that collision
alkaline-carbonatite complexes in contrast to the IVAC have more
enriched ("crust ") isotopic compositions [Hou, 2006].
Isotopic-geochemical
divergence is probably due to a long history of IVAC formation.
Geochronological data indicate the IVAC has been introduced in the
crystalline basement rocks (PR1) in the lower Paleozoic and
has undergone a significant transformation during the evolution of the
Ural folded region. The geochronological data for the IVAC specify the
intrusion of alkaline rocks and carbonatites (3) and long-lasted
metamorphic stage of the IVAC formation, with which are connected the
processes of anatexis, pegmatite genesis, metasomatose, and re-loading
of carbonatites, wide developed within the IVAC at collision (D2-3,
C1) and post-collision (P-T) stages of the Urals Belt
evolution.
This study was financially supported by
programm of interdisciplinary projects Ural Division, Siberian Branch
and DO of the Russian Academy of Science № 12-С-5-103
and by president programm № 12-П-5-2015.
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Ufa: Design Polygraph Service, 2010. 280 p. |