Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Postmagmatic zircons in rapakivi granites of the Salmi batolith and high potassium granite massives in its North-West surrounding (Karelia).

Lokhov D.K.^*, Rodionov N.V.^*, Presniakov S.L.^**, Kapitonov I.N.^**, Lokhov K.I.^*

* SPbU, geological faculty, St.Petersburg, Russia;

** VSEGEI, St.Petersburg, Russia.

dklhv@yandex.ru

One of the results of our investigation of pegmatites connected with the Salmi rapakivi granites in its Nort—West contact with host metasedimentary rocks (Hopunvaara, Torpussuo) was finding that the rocks contain as ancient suconcordant zirons (1480-1520 Ma), so some late generations with U-Pb age from 230 to 500 Ma, for which  presence of excess radiogenic hafnium is characteristic [Sundblad, e.a., 2011]. For this young zircons the value of calculated model hafnium age (T DM Hf) in lower, than age by U-Pb isotopic system, thus them have metasomatic origin [Paleoproterozoic…,2011]. In this rocks are present only some new generation, but no “core-rim” structures were identified, thus the question are these zircon crystals from the given rocks, or it is contamination, is still open. Moreover, contamination of the rocks can be supported by observation, that the rocks have no visible secondary alteration features.

Investigation of the Torpussuo alaskite topaz bearing granites (a little massive near contact of the Salmi massive with Ladoga gneisses), have shown, that in this case there are zircon crystals with clear bright in CL cores and powerful dark rims with thickness about 30-50 microns. Investigation of isotopic U-Pb system in these zircons revealed, that the inner cores have subconcordant age 1576±82 Ma, and the rims have high uranium content (up to 22000 ppm U) and demonstrate a range of concordant U-Pb ages from 70 to 560 Ma.

  To study this fenomena in details we investigated zircons from the rocks of I intrusive phase of the Salmi batolith itself and from small Nietjarvi intrusion, believed to be analogous to the II intrusive phase of the Salmi rapakivi granites. These rocks also do not reveal any traces of secondary postmagmatic alteration, and we specially checked zircon grains for outer rims, and have got the follows results:

1). Zircons from Nietjarvi massive have not visible rims, and concordant U-Pb age 1512±12 Ma was obtained.

2). In the Salmi grannies the zircons as a rule exhibit structure “low-uranium core (30-50 ppm) – relatively high uranium crust (150-600 ppm)”. The measurements by SIMS SHRIMP-II have shown, that all them are subconcordant, contain less than 0.7% of common ²⁰⁶Pb, and have characteristic for magmatic rocks Th/U=0.4-0.8. The age estimation is 1541±12 Ma (weightered average ²⁰⁷Pb/²⁰⁶Pb), which is in concordance with ID TIMS data [Amelin, e.a., 1997]; In some zircon crystals were found very thin (<10 microns), dark in CL rims. Their investigation revealed that them are high uranium (4500-9500 ppm), strongly discordant, contain from 0.7 to 40% of common ²⁰⁶Pb, and show low Th/U=0.09-0.15. Obtained U-Pb isotopic data are forming a Discordia with intercepts  444±15 and 1540±15 Ma (Fig.1). Lower intercept from our point of view corresponds to some real geological event, which was not fixed by ID TIMS, probably due to discrimination of zircons before the analysis. High uranium thin rims probably were totally recrystalyzed at thermal event of the Caledonian time.

High uranium thin rims could originate during postmagmatic stage of fluid-autometasomatic alteration of the rocks. This mean migration of high strength field elements zirconium and hafnium, fro which a water-halogene fluid required. Presence of thik high uranium rims in the topaz Torpussuo Li-F granites confirm it. Circulation of such fluids outside the massives of granites must lead to input of radiogenic hafnium from ancient host rocks, which comes to the fluid phase due to recrystallization of minerals with high Lu/Hf ratio (apatite, garnet). Investigation of Lu-Hf isotopic system in zircons do confirm this model (Fig.2). All outer high-uranium rims of the zircons in Salmi and Torpussuo granites exhibit some excess radiogenic hafnium, proofing their metasomatic genesis. The inner “normal magmatic” cores the Salmi and Torpussuo zircons as well as zircons from Nietjarvi within the errors are belonging to the correlation field for magmatic rocks (TA), hence them really correspond to crystallization time of the granites.

Fig.1. U-Pb system in zircons of the Salmi batoloth rapakivi rock. Bold lines difine error ellipses for outer high uranium rims of the zircons.

Fig.2. Isotopic Hf-Nd systematics of the granites. Dotted lines define correlation area for magmatic rocks “terrestrial array” (ТА).

This work was supported by SPbU grants 3.37.81.2011 and  3.37.86.2011

Literature:

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Sundblad K., Lokhov D.K., Lokhov K.I., e.a. U-Pb and Lu-Hf isotopic systems in zircons from pegmatitoids and granite stocks connected with the Salmi Rapakivi Batholith, Ladoga area, Russia. Материалы XVIII Международной конференции «Рудный потенциал Щелочного, кимберлитового и карбонатитового магматизма», Москва-Минск 9-16 сентября 2011, Минск «Право и экономика», 2011, с 121-123.

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