Age, types of sources and genesis of the Il'meno-Vishnevogorsky complex in light of new isotope data (Urals, Russia)
Nedosekova I.L.
Zavaritsky Institute of Geology and Geochemistry, Yekaterinburg, Russia
Il'meno-Vishnevogorsky miaskite-carbonatite complex – one of the world's largest alkaline complexes with unique rare and rare-earth mineralization and deposits Nb, Zr and REE. Despite of a significant level of scrutiny of a complex, a number of the important genetic questions remains till now debatable. First of all it is spatial and genetic link between carbonatites, miaskites and fenites outside of series of the ultramafic-alkaline rocks (traditional for carbonatites complexes of the ultramafic-alkaline formation) and, as consequence, a question of parent magmas for Il'meno-Vishnevogorsky complex.
Recently, the carbonatites with Nb-REE mineralization have been opened in ultramafic massifs (Buldym, Haldihin, Spirihin and other), which are located near to contacts with Vishnevogorsky and Il'menogorsky miaskite intrusions [Levin et al., 1997; Nedosekova, 2007]. Now there is a question on genetic link between carbonatites and ultramafic rocks of the Buldym massif and also miaskite-carbonatite complex.
The Sm-Nd and Rb-Sr isotopic compositions in carbonatites and ultramafic rocks of the Buldym massif and their minerals were measured. U-Pb isotopic age determination on samples of monazite and zircon from late dolomitic carbonatites of Buldym massif were performed. Thus, the first data about age and sources of substance ultramafic rocks and carbonatites of the Buldym massif are received by us.
Sm-Nd isochron diagram for the rocks and minerals of the Buldym massif is constructed (fig. 1). Six samples of this rocks yielded isochron age of 602 + 24 Ma with an initial Nd ratio 0.511853 + 0.000016, MSWD = 1.3. Isotopic compositions of ultramafic rocks (olivinites, olivinites with amphibole and phlogopite) and early dolomite-calcite pirochlore-bearing carbonatites of the Buldym massif are presented on this isochron diagram. Late dolomite monazite-bearing carbonatites of the Buldym massif have other isotopic compositions which are not belonging these isochron (fig. 1; sample №1-54, №10-21). According to U-Pb datings of monazite the age of formation dolomite carbonatites makes 254 + 30 Ma.
Fig. 1. Sm-Nd diagram for the samples of rocks and minerals from the Buldym ultramafic massif, Il'meno-Vishnevogorsky complex, Urals.
505-27 – olivinite, 15-22 – olivinite with amphibole and phlogopite, 97-1 amphibole-olivinite rock, 3311 – early dolomite-calcite carbonatite I; 3311К, 915К – calcites from early dolomite-calcite carbonatites I; 1-54, 10-21 – late dolomite carbonatites II.
Thus, the first Sm-Nd isotopic data who testify about Pre-cambrian (V) age of ultramafic rocks and early carbonatites of the Buldym massif are obtained. Based on U-Pb and Rb-Sr data, the introduction and crystallization of miaskite-carbonatite complex has occured on the boundary Ordovician (O) and Silurian (S) times – 410–440 million years ago [Kononova et al., 1979; Kramm et al., 1993]. The subsequent processes of metamorphism and metasomatism of miaskite-carbonatite complex, related to the Hercynian orogeny (360–320 Ma) and postcollisional extension (260–240 Ma), also are dated by U-Pb and Rb-Sr methods [Kononova et al., 1979; Kramm et al., 1983; Tchernyshev, etc., 1987].
Initial ratio of isotopes Sr and Nd in ultramafic rocks and early dolomite-calcite carbonatites of Buldym massif are characterized by uniform sizes ((87Sr/86Sr)602 = 0.7040–0.7048, (143Nd/144Nd)602 = 0.511842–0.511914, εNd = –0.37… +1.03), that testifies about possible comagmatic genesis and uniform sources of their substance. Initial Sr and Nd isotopic data for rocks of Buldym massif correspond to isotopic parameters enriched mantle EM1. Miaskite-carbonatite complex has other isotopic parameters (87Sr/86Sr)440 = 0.7034–0.7038, (143Nd/144Nd)440 = 0.512219–0.512372, εNd = 2.9…+5.9, corresponding moderately depleted mantle and, possibly, other sources of substance.
Conclusion. The fragments of the ultrabasic rocks and carbonatites more ancient in comparison with miaskite-carbonatite complex are established in Il'meno-Vishnevogorsky complex. Age of these rocks (~600 Ma) corresponds to a stage continental riftogenes, previous disclosing of the Urals paleoocean. Isotopic composition of ultrabasic rocks and carbonatites from Buldym massif is very close to enriched mantle EM1 and differes from those for rocks of miaskite-carbonatite complex. Similar sources of substance have carbonatite complexes ultramafic-alkaline formations of East-African rift, in particular carbonatite lavas, nephelinites, phonolites and xenolites of pyroxenites and ijolites from carbonatite volcano Oldoinyo Lengai [Kramm and Sindern, 1998].
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This study was financially supported by the integration project (2008-2011)between branches of the Russian Academy of Science «К-alkaline volcano-plutonic complexes of different structure of the Earth, processes of differentiation passing in them and the division of magmas leading formation ore-bearing lamproites and carbonatites; their deep sources».