2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

U-Pb and Lu-Hf isotopic systems in zircons and isotopic Hf-Nd systematics of the Khartes kimberlites (Subpolar Urals).

 Lokhov K.I.^*, Lukyanova L.I.^**,  Kapitonov I.N.^**, Lepekhina E.N.^**,  Antonov A.V.^**,  Sergeev S.A.^**

* St.-Petersburg State University, geological faculty, St.-Petersburg, Russia;

** VSEGEI, St.Perersburg, Russia.

Kirill_Lokhov@vsegei.ru

On the Subpolar Urals in the Lemvinskaya zone it are known some specific kimberlites and alkaline lamprofires. These rocks were described and studied in (Makhotkin, e.a., 1988). Petrological data indicate that deep source of the rocks was forming continuously during formation of rift system, which means existing of thick and relatively ancient crust in the region. Isotopic data gave no clear evidence to age of kimberlites and geochemical source type: K-Ar age in range 244-261 Ma, Rb-Sr isochrone 257±5 Ma and Sm-Nd isochrone 506±184 Ma.

We carried out investigation of isotopic U-Pb and Lu-Hf systems in zircons and Sm-Nd in kimberlite sample. Investigation of the U-Pb isotopic system in zircons was done by SIMS SHRIMP-II, Lu-Hf isotopic system by laser ablation LAICPMS ThermoFinnigan Neptune/DUV-193 in the same areas of SIMS analysis, Sm-Nd system in rocks by conventional chemistry procedures with measurements by TIMS ThermoFinnigan Tritone at the Center of isotopic research, VSEGEI, St.-Petersburg.

Fig.1. Isotopic Hf-Nd systematics. Lines are defining “terrestrial array” (TA) correlation field for magmatic rocks. Ellipse – field of type-I kimberlites by (Nowell, e.a., 2004)

Fig.2. Correlation of values of Lu/Hf ratio with measured U-Pb age of the zircons.

U-Pb data dot not permit to argue, that all the zircons originated from the same rock and form uniform concordant cluster: formally it can be calculated concordant age 534±3 Ma but with exceptionally low probability of concordance, about 0.3. By uranium contents these zircons cannot be considered as typical kimberlitic, because of high (>30 ppm) contents of uranium, and particular morphology type: idiomorphic relatively small crystals with clear coarse oscillatory zoning in CL.

Hf-Nd isotopic systematics has shown, that all the zircons are not magmatic for the host kimberlite, but captured ones (fig.1). Among zircons with U-Pb age in the range 520-550 Ma one crystal with the age 2680 Ma was found. It demonstrate the highest uranium contents, have clear fine ocillatory zoning, negative εHf(T) value, and can be interpreted as originated in Arhean metamorphic rocks, So it confirms presence of an ancient crust in the region.

The values of Lu/Hf ratios in the zircons clearly indicate, that them are not kimberlitic, in most cases the ratios are elevated and more, than 0.0005. The result is, that the values of Lu/Hf are decreasing from older to younger ones, and for the youngest (520 Ma) fall into kimberlitic range (fig.2). This can indicate, that the zircons were forming in deep source during its evolution over range of  30 Ma, and evolution due to differentiation had direction to the “kimberlite-type” source. 

Fig.3. Evolution of Hf isotopic composition of the deep source by zircons.

Evolution of hafnium isotopic composition in the deep source (fig.3) can confirm that the source was very close to the typical kimberlitic ones, because the evolution line comes in between of CHUR and DM reservoirs, which is characteristic for kimberlites of the first type(Nowell, e.a., 2004, Lokhov, e.a., 2010). From other hand, the initial ¹⁷⁶Hf/¹⁷⁷Hf ratio of two zircons with U-Pb age 528 and 546 Ma correspond to the DM evolution line, and probably indicate some admixing of some matter from the depleted mantle reservoir to kimberlitic-type deep source. Notably figurative points fro these two zircons on the Hf-Nd plot (fig.1) within error fit to the TA area.

Geochronologic data by zircons can allow to give the lower time level of rock formation: T<520 Ma, thus Sm-Nd isochrone data from (Makhotkin, e.a., 1988) seems to be robust, despite of the high error in age estimation.

References:

Lokhov K.I., Saltykova Т.Е., Kapitonov I.N., e.a. Correct interpretation of U-Pb age by zircons basing on isotopic geochemistry of hafnium and neodymium (on example of some magmatic complexes of basement of East-European platform). Regionalnaya geologiya I metallogeniya 38, 2009b, p.62-72 (in Russ).

Lokhov K.I., Lukyanova L.I., Shokalsky S.P., e.a. U-Pb and Lu-Hf isotopic systems in zircons from some kimberlites of the Siberian platform and from Ebeliakh alluvial deposit: age and geochemic peculiarities of the source rocks. Abstr. Conf. on geochemistry of magmatic rocks, Moskow-Koktebel, 2010, http://alkaline2010.web.ru/

Nowell D.M, Pearson D.G., Bell D.R., e.a. Hf isotope systematics of Kimberlites and their Megacrysts: New constrains on their Source regions.// Journal of Petrology, 2004, aug 1, p. 1-30.

Makhotkin I.L., Zhuravlev D.Z., Lebedev I.A., e,a. Age and ganesis of mantle sources of kibmerlites and potassic lamprofires of SubPolar Urals// Abstr. VI Urals petrographic conference, Ecaterinburg, 1997, p. 27-29.