Geochronology, isotope geochemistry and mantle sources of the Il'meno-Vishnevogorsky alkaline complex based on the new Lu-Hf, U-Pb, Sm-Nd and Rb-Sr isotope data (Urals, Russia)

*Nedosekova I.L., **Belyatsky B.V., ***Belousova E.A., **** Bayanova T.B.

*Zavaritsky Institute of Geology and Geochemistry, Yekaterinburg, Russia; **VNIIOkeangeologia,St.Petersburg, Russia; ***Macquarie University, Sydney, Australia

 **** Geological Institute, Apatity, Russia

 

vladi49@yandex.ru

 

The Il'meno-Vishnevogorsky complex (IVC) – one of the world's largest miaskite-carbonatite complexes of carbonatite-nepheline-syenite formation. Despite of a detail and long-lasted studying of this complex a number of the important genetic and geochronological questions till now remain debatable. Here we present new U-Pb, Lu-Hf, Sm-Nd and Rb-Sr isotopic data on the main rock types and minerals of IVC (Vishnevogorsky and Il'menogorsky miaskite massifs, Central Alkaline Belt, Buldym ultramafic massif).

U-Pb dating of zircons from miaskites and carbonatites of Vishnevogorsky massif as well as from dolomitic carbonatites of Buldym massif was conducted by local isotope methods (SHRIMP, St.-Petersburg; laser-ablation – La-ICP MS, GEMOC, Macquarie University, Sydney). U-Pb isotopic ages of the pyrochlores from miaskite-pegmatite of Vishnevogorsky massif and dolomite-calcite carbonatites of Buldym massif were studied for the first time by TIMS (VSEGEI, St.-Petersburg). The Lu-Hf isotope data accompanied by U-Pb isotope ages for zircons from Vishnevogorsky massif and dolomitic carbonatites of Buldym massif have been obtained by laser-ablation technique (Macquarie University, GEMOC, Sydney). To characterize the mantle sources for the Il'meno-Vishnevogorsky complex Sm-Nd and Rb-Sr isotopic systems have been analyzed by HR-TIMS at IGG UD (Yekaterinburg) and GI KSC (Apatity).

Zircon of the miaskites and carbonatites of Vishnevogorsky massif is represented by grains of several morphological types with – ill-defined forms, of prismatic and dipyramidal-shape, differing also in colour, transparency and presence of inclusions. The early zircon I, possibly was formed at crystallization of miaskite melt, has irregular form, it is not transparent enough, usually demonstrates darker CL images and spotty BSE images, with relics of growth zoning. Late zircon II forms short-prismatic grains, transparent, with relics of the primary zoning, highly homogeneous in BSE and with distinct CL. But most of zircon grains are represented by the intermediate types illustrating various degrees of early zircon alteration and its replacement by late zircon generation.

The U-Pb age of zircon I formation (of IVC) is 417+7 Ma for miaskite (SHRIMP, St.-Petersburg) and 411+14 Ma for carbonatites (laser-ablation, Sydney). Six-points whole-rock Rb-Sr isochron for the miaskites and carbonatites of the Vishnevogorsky massif corresponds to the IVC age estimation at 440+8 Ma and MSWD =0.62.

Alongside with it, later processes are dated in the same rocks and different minerals of IVC. Thus, U-Pb SHRIMP-dating of late zircon II from miaskites and carbonatites has shown the age 279±10 Ma. Most of zircon grains from miaskites and carbonatites are characterized by intermediate types, which are dated 383±14 Ma and 359+25, accordingly (Krasnobaev et al., 2008; 2009). At the same time, U-Pb ages of pyrochlores from miaskite-pegmatite Vishnevogorsky massif and from calcite-dolomite carbonatites of Buldym massif correspond to 233+3.1и 210+13 Ma. The age of dolomitic carbonatites equals to 254+30 Ma according U-Pb dating of monazite. But, zircon U-Pb age for dolomitic carbonatites of Buldym massif reveals 268+6 Ma and MSWD = 8.2 (n = 11, La-ICP MS).

Sm-Nd isotope data evidence to Precambrian (600 Ma – (V)?) age for ultramafic rocks of the Buldym massif. Alkaline processes within the ultramafic rocks of the Buldym massif is evident at 443+23 (MSWD = 0.46) and 324+7 Ma (MSWD =0.64, n = 5, Sm-Nd). Thus for miaskites and carbonatites of Il'meno-Vishnevogorsky complex Sm-Nd isotope system is a little bit disturbed and does not give meaningful isochrons.

Initial Sr and Nd isotope ratios for IVC carbonatite-miaskite complex correspond to moderately depleted mantle (⁸⁷Sr/⁸⁶Sr)₄₄₀ = 0.7034-0.7038, (¹⁴³Nd/¹⁴⁴Nd)₄₄₀ = 0.512219-0.512372, εNd = +2.9 ÷ +5.9. Sr and Nd isotope parameters for Buldym massif dolomite-calcite carbonatites are (⁸⁷Sr/⁸⁶Sr)₄₄₀= 0.7042-0.7048, (¹⁴³Nd/¹⁴⁴Nd)₄₄₀ = 0.511999-0.511896, εNd = -1.4 ÷ -3.4. While dolomitic carbonatites of Buldym massif have low enough Nd isotope signatures: (¹⁴³Nd/¹⁴⁴Nd)₂₈₂=0.512197-0.512149 and εNd =-1.9 ÷-2.8. Initial Sr and Nd isotope ratios of Buldym massif correspond to isotopic characteristics of enriched mantle of ЕМ1 type. But, at the same time the possibility of the differences in isotope parameters of Vishnevogorsky and Buldym massifs connected with contamination processes couldn’t be ruled out.

Initial Hf isotope ratios for zircons I of Vishnevogorsky massif have insignificant variations – (¹⁷⁶Hf/¹⁷⁷Hf)₄₁₀= 0.282617-0.282678, εHf = +3.5 ÷ +5.7 and also testify to an origin of zircon from moderately depleted source. Zircons from miaskites, miaskite-pegmatites and carbonatites have close values of Hf isotope parameters that evidence common source of their substance. Hf isotopes are also determined in zircon II grains with U-Pb age of 282 Ma. Zircon II considerably differs from early zircon I by high (¹⁷⁶Hf/¹⁷⁷Hf)₂₈₂ = 0.283055 and εHf = +16, corresponding to depleted mantle characteristics that can specify occurrence of a new source of substance participating in transformations of miaskites and carbonatites. However this question demands statistical substantiation and further research.

Hf isotope composition of zircon from Buldym massif of dolomitic carbonatites: (¹⁷⁶Hf/¹⁷⁷Hf)₂₆₈ =0.282525-0.282555, εHf = - 0.2 ÷ -1.8, is close to chondrite and differs from those for zircons from miaskites and carbonatites of Vishnevogorsky massif by lower initial ¹⁷⁶Hf/¹⁷⁷Hf and εHf, that testifies to participation of different material in their formation.

The isotope signatures of early zircons from Il'meno-Vishnevogorsky complex on the diagram εNd - εHf are within the limits of “mantle array” in the field of development of moderately depleted mantle compositions and deep-seated rocks (Patchett et al., 1981; Vervoort et al., 2000). Only one point of late zircon II from carbonatites of Vishnevogorsky massif with 282 m.y. U-Pb age is out of “Terrestrial Array” (Vervoort et al., 1999).

To estimate the time of melts generation for Il'meno-Vishnevogorsky complex we calculated Lu-Hf model age: Т_DM and T_DMC. For calculation of model ages (T_DM) compared to depleted-mantle source, we have adopted a model with (¹⁷⁶Hf/¹⁷⁷Hf)_i = 0.279718 at 4.56 Ga and ¹⁷⁶Lu/¹⁷⁷Hf = 0.0384; this produces a present-day value of ¹⁷⁶Hf/¹⁷⁷Hf (0.28325), similar to that of average MORB. For early zircons I we obtained T_DM = 790-880 Ma. In general, T_DM ages, which are calculated using the measured ¹⁷⁶Lu/¹⁷⁷Hf ratios for studied zircons, as a rule, give only minimum age estimation for the magmatic source from which this zircon was crystallized. Therefore we have also calculated, for each zircon type, a “crustal” two-stage Hf model age (T_DMC) which assumes that its parental magma was produced from a volume of average continental crust (¹⁷⁶Lu/¹⁷⁷Hf = 0.015; Geochemical Earth Reference Model database, http://www.earthref.org/) that was originally derived from a depleted mantle. The age of IVC protolith is T_DMC = 1040-1170 Ma. For late generation of zircon II from Vishnevogorsky massif carbonatites T_DM is 270 Ma and well coincides with U-Pb age of these zircons that can evidence to formation of late zircon generation in the course of collision.

Thus, the Lu-Hf-isotope data obtained for early zircons of Il'meno-Vishnevogorsky complex together with Sr-Nd whole-rock isotope data (Pribavkin, Nedosekova, 2006; Nedosekova et al., 2009) confirm the mantle character of the source for the IVC and specify the participation in IVC magmas generation of moderately depleted mantle (DM) component and enriched source (possibly, lower crust component). The isotopic composition of IVC rocks could be the result of a mixture of the substances from these sources. Model age of IVC protolith corresponds to initial stages of Riphean rifting, during the Riphean-Vendian oceanic crust formation (top structural division of the Sysert-Il'menogorsky block). Thus, it is possible to assume that protolith of Il'meno-Vishnevogorsky complex was represented by Riphean oceanic layers with close Sr-Nd isotope characteristics and model Sm-Nd age of a source (Echtler et al., 1997), and their melting has generated the IVC magmas during paleozoic riftogenesis within the Urals.

 

This study was financially supported by programm of interdisciplinary projects Ural Division, Siberian Branch and DO of the Russian Academy of Science 2009-2011 № 09-С-5-1014.

 

References:

Echtler H.P., Ivanov K.S., Ronkin Y.L. et al. The Tectono-metamorphic evolution of gneiss complexes in the Middle Urals, Russia // Tectonophysics. 1997. Vol. 276. P. 229-251.

Krasnobaev А.А., Rusin A.I., Busharina S.V.et al. // Zirkonologija of miaskites of Il'meny mountain // Year-book - 2007. IGG UD, Russian Academy of Science. 2008. С. 264-268 (in Russian).

Krasnobaev А.А., Nedosekova I.L., Busharina S.V. Zirkonologija of carbonatites of Vishnevogorsky massif (S. Ural) // Year-book - 2008. IGG UD, Russian Academy of Science. 2009. P. 261-263 (in Russian).

 Nedosekova I.L., Vladykin N.V., Pribavkin S.V., BаyanovaT.B. The Ilmeno-Vishnevogorsky Miaskite-Carbonatite Complex: Origin, Ore Resourece Potential, and Sources (Urals Mountains, Russia) // Geology of ore deposits. 2009. Vol. 51. № 2. P. 157-181 (in Russian).

Pribavkin S.V., Nedosekova I.L Carbonatite Sources of the Il'meny-Vishnevye Gory Complex: Evidence from Sr and Nd Isotope Data on Carbonates // DAN. 2006. № 3. Vol. 408. P. 381-385 (in Russian).

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. Vol. 78. P. 279-297.

Vervoort J. D., Patchett J. P., Albarede F. et al. Hf-Nd isotopic evolution of the lower crust // Earth and Planetary Science. 2000. Vol. 181. Р. 115-129.

Vervoort J. D., Patchett J. P., Blichert-Toft et al. Relationships between Lu–Hf and Sm–Nd isotopic systems in the global sedimentary system // Earth and Planetary Science Letters. 1999. Vol. 168. P. 79-99.