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Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

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Ore potential of alkaline, kimberlite

and carbonatite magmatism

Phanerozoic time alteration of the Precambrian rocks from the Onega structure (Karelia).

Lokhov K.I.*,  Kapitonov I.N.**, Antonov A.V.**, Sergeev S.A.**

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

** VSEGEI, St.Petersburg, Russia.



At investigation of volcanics and high carbon (shungite bearing) metasedimants from Ludikovi sequence of the Onega structure it was found, that them contain numerous zircons with Phanerozoic ages (Goltsin, e.a., 2010). Investigation of Precambrian rocks from the Onega parametric drill hole (OPD) have shown, that as Paleoproterozoic volcanics, so Neoarchean granitoids of the basement, suffered polystage endogenic alterations. Such late processes are known for some parts of the Baltic shield [Larson, Tullborg, 1998, Sergeev, e.a., 2008 and others].

Investigation of zircons from plagioclase-microcline granites of the basement in OPD revealed that even in the less altered rocks the zircons exhibit as some partial loss of radiogenic lead, so crystallization of the new generations (structures “core-rim”) at, at least two stages of the rock alteration, and the last one corresponds to Phanerozoic event(s). The Ludikovi volcanics contain: a). some discordant zircons, by which lead loss at numerous events of Phanerozoic time van be estimated; b). new generations of zircon crystals, and the given precision of the SIMS SHRIMP-II do not allow estimate if there are existing some groups of concordant zircons with ages 148, 165, 200, 300 and 650 Ma, or them belong to unique discordia with parameters 154±6 and 619±54 Ma. Investigation of Lu-Hf isotopic system in these zircons show, that there exist several age groups of zircons, which are containing excess and sequently more radiogenic hafnium. Such zircons could be originated at low-temperature fluid-metasomatic processes, during which, some minerals from sedimentary rocks with high Lu/Hf ratio, about 0.15, were recrystallized. These minerals could be phosphates from terrigenic rocks of Ludicovi, formed near 2000 Ma ago. Presence of powerful sequence of salts (halite) in the basement of Paleoproterozoic rocks from OPD [Paleoproterozoic…,2011] defined circulation in the rocks of water-halogene fluids, which transported high strength field elements (Zr, Hf) effectively [Korzhinskaya, Ivanov, 1988, Aja, e.a., 1995 and others], thus one can expect elevated concentration of hafnium in the metasomatic zircons. Zircons with excess radiogenic hafnium, for which calculated model age by hafnium (T DM Hf) is less, than calculated age by U-Pb isotopic system, are marking Phanerozoic time events of rock alteration in the Onega structure. Notably in these zircons really are extremely high hafnium contents: from 1.5 to 7%.

Presence of salt sequence in the basement of Paleoproterozoic sedimentary-volcanic rocks in the Onega structure, which are responsible for water-halogene fluid circulation, define, that in this structure some young processes are fixed more contrast as in comparison with Archean granitoids and volcanics, in which these events are fixed in rear cases and in from of some partial loss of radiogenic lead from zircons, leading to their discordance. One can expect, that in the rock reach by halogens, for example in lithum-fluorine granites, analogous effects also are widely represented.


This work was supported by SPbU grants and



Goltsin N.A., Lokhov K.I., Kapitonov I.N., e.a. Polystage alterations of high carbon sedimentary rocks of Ludikovi of the Onega structure. // Regional Geology and Metallogeny, 2010, 41, p.66-79.

Korzhinskaya V.S., Ivanov I.P. Experimental study incoengruent dissolution of zircon in the system  ZrO2-SiO2-H2O-HCl at T=400-600oC and P=1 kbar. // DAN, 1988, v.299, N4, p. 970-973.

Paleoproterozoic Onega structure Ed. By L.V.Glushanin, N.V.Sharov, V.V.Shiptsov, Petrozavodsk, Karelian sci. center RAS, 2011 432p.

Sergeev S.A., Lobach-Zhuchenko S.B., Arestova N.A., e.a. Age and geochemistry of zircons from oldiest granitoids of the Vyg river, South-East Karelia. // Geokhimiya, 2008, №6, p.647-659.

Aja S.U., Wood S.A., Williams-Jones A.E. The aqueous geochemistry of Zr and the solubility of some Zr-bearing minerals. Applied geochemistry, 1995, v.10, Iss 6, p. 603-620.

Larson S.A., Tullborg E-L. Why Baltic shield zircons yield late Paleozoic lower-intercept ages on U-Pb Concordia. Geology, 1998, v.26, p. 919-922.