2011

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Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

   

U-Pb and Lu-Hf  isotopic systems in captured zircons from basic dykes as indicators of endogenic development of the Earths crust (on example of Aginsk megazone in the Eastern Transbaikalia).

Polyanskiy R.V.* , Lokhov K.I*., Kapitonov I.N.*, Kurilenko A.V.**

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

**Chitageolsemka, Chita, Russia

Polyanskiy___Rom@mail.ru

 

Investigation of endogenic development of particular tectonic regions (terreins) is an important problem of modern geology. This information can be obtained by isotopic studies of minerals from terrigenic sedimentary rocks, including zircons. Terrigenic sedimentary rocks contain a lot of zircons from different magmatic complexes, which were denuded at different times. To get information by direct investigation of zircons from sediments it is needed to investigate statistically sufficient number of zircons, which is a long task. Even so, one has no assurance, that all sediments, including ones unoutcroped, are characterized.  

We offer to apply another mode of sampling: to get zircons not from sediments, but from some basic dykes, which usually contain large amounts of captured zircons, even delivering zirons from deep and not outcropped complexes of Earths crust.

We carried out investigation of zircons from basic dyke, which cuts sedimentary rocks in Aginsk megazone in the Eastern Transbaikalia.

Isotopic U-Pb and Lu-Hf systems in the zircons from the dyke were investigated both with Sm-Nd system in the whole rock 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.

The zircon genesis was estimated basing main idea from (Lokhov, e.a., 2009), correlation of Lu-Hf and U-Pb isotopic data in zircons for genetic considerations analogous to the method Terreinchrone (Belousova, e.a., 2009).

UPb isotopic data allow to define at least five age clusters (fig.1).

 

. 1. Isotopic U-Pb system in zircons.

 

The isotopic Hf-Nd systematics exhibits, that main part of the zircons are captured. Zircons of magmatic genesis have age 172 Ma, and captured 1841, 1749, 1566, 852 335 Ma.

For estimation regularities of Earth crust formation, the dependence of difference of model hafnium age and U-Pb crystallization age (THf(DM)-TUPb) from crystallization time of zircons TUPb was put on a plot (fig.2). The minimum values of THf(DM)-TUPb marks addition of deep juvenile matter to the crust, this difference increases in case of magmatic reworking inside crust of the existing matter.

 

Fig.2. Dependence of difference between hafnium model age and time of crystallization from time of crystallization of the zircons.

 

The most ancient of found zircons (1841 Ma) demonstrates even less model hafnium age, than crystallization time. Such effect is characteristic for hydrothermal and metasomatic zircons (Goltsin, e.a., 2010, Valley, e.a., 2010), hence it is possible to suppose existing of the crustal matter more ancient than 1841 Ma. The graph on the fig.2 demonstrates, that in the time span 1749 1566 Ma was dominant income of juvenile deep matter into the crust, but from 1500 up to 335 Ma sequentially was increasing amount of magmatic rocks, originated inside crust due to its magmatic reworking. In the time range from 335 Ma till crystallization of the dyke at 172 Ma, again it was increased input to the crust of the deep juvenile material.

 

The work was supported by SPbU grants 3.37.81.2011 3.3.86.2011.

 

References:

Goltsin N.A., Lokhov K.I., Kapitonov I.N., e.a. Polystage alteration of high carbon rocks from Ludikovi of the Onega depression. // Regional Geology and Metallogeny, 2010, 41, .66-79. (in Russian)

Lokhov K.I., Saltykova .., 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).

Belousova E.A., Reid A.J., Griffin W.L., e.a. Rejuanation xs. Recycling of Archean crust in the Gawler craton, South Australia: Evidence from U-Pb ad Hf isotopes in detrial zircon. // Lithos, 2009, 113, p. 570-582.

Valley P.M., Fischer C.M., Hanchar J.M., e.a. Hafnium isotopes in zircon: A tracer of fluid-rock interaction during magnetite-apatite (Kiruna type) mineralization // Chemical Geology, 2010, 275, p. 208-220.