2013

Hf-isotope and trace element variations as a signature of zircon genesis during evolution of Ilmeny-Vishnevogorsky Alkaline-Carbonatite Complex (IVAC), Urals, Russia

I.L. Nedosekova¹, E.A. Belousova², B.V. Belyatsky³

¹IGG UB RAS, Ekaterinburg, Russia; vladi49@yandex.ru

²GEMOC/CCFS, Macquarie University, Sydney, Australia

³IPGG RAS, St. Petersburg, Russia

It has been widely accepted that Hf isotope composition of zircon is relatively stable and does not underwent by significant alteration due to influence of the many of hypogenic processes [Patchett et al, 1981].We have studied Hf isotope composition coupled with U-Pb-age and trace-elements distribution by the means of LA ICP-MS from different populations of zircon from IVAC carbonatites and miaskites.

The results indicate that the early magmatic zircon I from carbonatites forms concordant age cluster (with individual U-Pb-age varies from 410 to 424 Ma and discordance D<5) and displays moderately depleted isotope Hf composition (εHf=11.3-4.9), a variation of which plausibly reflects primary heterogeneity of the magma sources. Linear covariations for many elements (REE, Hf, Y, U, Th etc.) are also typified for zircon I and mainly determined by magmatic evolution of the parental melt.

Fig. 1: Hf isotope composition calculated for individual zircon ages plotted against U-Pb ages for the IVAC zircons

The outer zones of the zircon I grains are characterized by slightly disturbed U-Pb systems (5<D<20) and lower (¹⁷⁶Hf/¹⁷⁷Hf)_i and εHf values (shifting by 3-4 units in regard to the grain centers). These changes do not correspond to simple Hf isotope evolution model for zircon initial composition or radiogenic Pb loss trends (Fig. 1). Thus we infer that observed differences in Hf isotope composition between central and outer parts of zircon grains have been determined by diffusive REE mobility, including parental Lu, during metamorphic stage of IVAC formation.

The late newforming zircons II dated at 250-320 Ма, but they frequently have highly disturbed U-Pb isotope systems (D>50-90). The zircons II are slightly depleted in radiogenic Hf (εHf₂₅₀ = -5 up to +11) and have low U, Th and REE contents. Coincidence of calculated initial Hf compositions (¹⁷⁶Hf/¹⁷⁷Hf)₄₂₀ between zircons I and II indicates the zircons II were formed under recrystallization of the earlier zircons I without input of additional REE and Hf during the later ca 250 Ma metamorphic event.

The study is supported by Program of UD RAS, projects №12-S-5-1031 and 12-P-5-2015.

1. Patchett P.J., Kouvo O., Hedge C.E., Tatsumoto M. Evolution of continental crust and mantle heterogeneity: evidence from Hf isotopes // Contributions to Mineralogy and Petrology, 1981. № 78. P. 279–297.