ISOTOPE GEOCHEMISTRY (Rb-Sr, Sm-Nd, U-Pb, Lu-Hf) AND GEOCHRONOLOGY OF
IL’MENO-VISHNEVOGORSKY ALKALINE-CARBONATITE COMPLEX (URALS)
Nedosekova1 I.L., Belyatsky2 B.V. and Sharygin3 V.V.
1A.N.Zavaritsky Institute of Geology and Geochemistry UB RAS, Ekaterinburg, e-mail: vladi49@yandex.ru
2VNII Okeangeologia, St. Petersburg, e-mail:bbelyatsky@hotmail.ru
3V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, e-mail:sharygin@igm.nsc.ru
The 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 used different isotope methods for dating of IVAC: Rb-Sr and Sm-Nd isochrons on bulk rock compositions and rock-forming minerals – TIMS, U-Pb zircon and U-Pb pyrochlore dating - TIMS, Lu-Hf isotope systematics for zircon – SHRIMP and LA-ICP MS. The isotope dating has detected some 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 complexes of the Southern Urals [Echtler et al., 1997].
The isotope signatures of the IVAC main rock types are: 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). These data show moderately depleted to moderately enriched (type EM1) isotopic compositions and suggest the depleted mantle source [Kramm et al, 1983; Nedosekova et al, 2009; Nedosekova et al, 2010]. There are some analogies in the isotope characteristics and lines of isotopic systems for the IVAC and carbonatites of rift-related ultramafic-alkaline carbonatite complexes (UACP – Kola, Maymecha-Kotuy Alkaline Provinces and others), which are located within the Precambrian cratons. It is suggested that these alkaline-ultramafic complexes have mantle-derived sources, possibly due to upwelling mantle plume and/or mixing of plume component with enriched component EM1[Bell, 2001; Kramm 1993; Kramm, Kogarko, 1994; Kogarko et al., 2010 and other]. Therefore it is quite possible the IVAC has a similar source. However, the appearance of alkaline magmas of the IVAC due to melting of the oceanic crust rocks, low crust continental rocks or the UACP rocks, which have similar isotope signatures, is not excluded.
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. Isotopic-geochemical divergence of IVAC is probably related to a complex history of its formation. The geochronological data show evidences that IVAC has been intruded in the crystalline basement rocks (PR1) in the lower Paleozoic and has undergone a significant transformation during the evolution of the Ural folded region. These geochronological data for the IVAC indicate the intrusion of alkaline rocks and carbonatites (О-S1) and long-timed metamorphic stage of the IVAC formation, which induced the processes of anatexis, metasomatism, pegmatite and ore formation, wide developed within the IVAC at the collision (D2-3, C1) and post-collision (P-T) stages of the Urals Belt evolution.
This study was financially supported by projects № 12-С-5-103 and № 12-P-5-2015.
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