TRACE ELEMENTS AND Hf ISOTOPE COMPOSITION AS INDICATORS OF ZIRCON GENESIS DUE TO THE EVOLUTION OF ALKALINE-CARBONATITE MAGMATIC SYSTEM (Il’meny-Vishnevogorsky complex, Urals, Russia)
I.L. Nedosekova1, B.V. Belyatsky2, E.A. Belousova3
1Zavaritsky Institute of Geology and Geochemistry, Ural Branch of the Russian Academy of Sciences, Pochtovyi per. 7, Yekaterinburg, 620075, Russia
2A.P. Karpinsky Russian Geological Research Institute, Srednii pr. 74, St. Petersburg, 199106, Russia
3GEMOC ARC National Key Centre, Macquarie University, Sydney, Australia
Keywords: Zircon, rare elements, Hf isotope composition, carbonatites, alkaline rocks
Subsection: PETROLOGY, GEOCHEMISTRY, AND MINERALOGY
Abstract
We present results of investigation of the trace-element (REE, HFSE) and Hf isotope compositions and U-Pb age of single zircons crystallized from alkaline-carbonatite magmas of the Ilmeny-Vishnevogorsky complex (IVC) (Urals, Russia). It has been established that the geochemical characteristics of the early zircon (U-Pb age is 430-410 Ma) from alkaline rocks and carbonatites of this complex are determined mainly by the magmatic evolution of parental fluid-saturated alkaline-carbonatite melts and, to a large extent, are associated with the cocrystallization of zircon and uranium rare-metal minerals (gatchettolite and pyrochlore) at the final stages of the magmatic-system activity. Early IVC zircons have a moderately depleted Hf isotope composition (εHf from +11.3 to +4.7), confirming the mantle nature of the magma source and indicating the participation of DM-like and enriched-source (probably, lower-crust component) substances in the magma generation. The considerable variations in the initial Hf isotope composition of the early zircons testify to the multistage zircon crystallization involving new portions of melts with different isotope compositions related to mixing of substances at their source. Late IVC zircons (250-350 Ma) have strongly disturbed «rejuvenated» isotope systems and a geochemical composition different from that of the magmatic zircons. They formed apparently at the metamorphic stage without a substantial input of additional material.
|
