Il'meno-Vishnevogorsky Alkaline Complex (IVAC) from Urals, Russia : age, sources and origin

Abstract

The 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. The isotope signatures of the IVAC main rock types: epsilon Sr = -6. . .-10, epsilon Nd = +3. . .+6, epsilon Hf = +4. . .+6, are similar to those for platform ultramafic-alkaline complexes (UACP) and require a depleted mantle source. IVAC miaskites and carbonatites differ by high Sr while carbonatites show slightly depletion in Ba, Nb, Ti, Zr, Hf relatively to UACP rocks. The obtained age data specify the magmatic intrusion of miaskites (O3 -S) and long-lasted metamorphic reworking during formation of the IVAC at collision (D2-3 , C1 ) and post-collision (P-T) stages of the Urals Belt evolution. Close resemblance of the IVAC and UACP complexes according to isotope and geochemical characteristics are fixed, but IVAC carbonatites have much in common with carbonatites localized in linear post-collision zones. The IVAC origin is supposeded to be connected with a mantle source, but the melting of the UACP rocks or oceanic crust rocks with the same isotope signatures is not ruled out.1 page(s