U-Pb dating of zircons from paleoweathering profiles

Alfimova N.A, Matrenichev V.A., Zinger T.F.

Institute of Precambrian Geology and Geochronology RAS, Saint-Petersburg, Russia

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Last years zircon is used as a main geochronometer, because is believed to be very resistant in secondary processes. In this work we investigated zircons from Paleoproterozoic weathering profile, developed on Archean granitoids from N.Karelia (l. Kiskinlampi, N.Karelia, Russia). The basement here is composed of medium granular pink granite-gneiss intruded by veins of red microcline granite with a thickness of up to 3 m. The weathering profile developed on granitoids of the basement has a thickness of ~10 m and includes a zone of physical disintegration of the substrate, a zone of eluvial breccia with terrigenous–carbonate cement, which composes the main part of the hypergene profile, and a clay zone [1].

Zircons for investigation were extracted from the zone of physical disintegration of the ancient weathering profile of granite gneiss (Sample 205e) and bright red microcline granite (Sample 1405). Zircon crystals from all rock varieties are characterized by a corroded surface with traces of dissolving and closed fractures along the prism facets and by a complex internal structure. There are three types of zones, which may be either combined in the same crystal or occur separately: 1 - newly formed rims on the prisms facets, 2 - relicts of magmatic zircon preserved as zones and areas with thin oscillatory zonation, 3- zones with cavernous, “spongy” structure developed after magmatic zircon and replacing it. Described processes affect chemical composition of zircons as well [2].

U-Pb system of these zircons had been studied at VSEGEI by SHRIMP II. On the figure one can see two Discordia: 1 – sample 205e with upper intersection 2749±15 Ma and lower 425±38 Ma, 2 – sample 1405 with upper intersection 2726±32 Ma and lower 944±130 Ma. Zircon grains with the relicts of oscillatory zoning, characterizing magmatic zircons, are mostly placed near Concordia in both samples, whereas zircons with the “sponge” structures are highly discordant. This tendency in discordancy is greatly correlated with the amount of LREE and U in zircons [2].

Thus, there is a controversy between geological and isotopic data. Geological studies indicate that red granites had intruded granitoides of basement before forming the weathering profile. There are no signs of thermal geological processes, have affected rocks afterwards. U-Pb isotopes indicate four processes – upper intersections can be identified as a time of rock forming (2749 Ma for granite-gneiss and 2726 Ma for intrusion of red granites) and, lower intersections, representing young events have no geological evidences and, according to the figure, should have been affecting rocks separately (425 Ma for granite-gneiss, and 944 Ma for red granite), thus supposing separate geological history for these rocks after 1000 Ma at least. No isotopic signature seen of paleoproterozoic weathering process (2300-2100Ma) from the first glance, clearly red from the geology and geochemistry of the object.

Fig. U-Pb data for studied samples.

All data can be explained, if we assume, that U-Pb isotope system in all zircons had been changed in two weathering episodes – Paleoproterozoic and Phanerozoic. Numerical model, based on time-limits for these two events had been build [3]. For defining time-limits we used isotopic data for both samples – most ancient and the youngest data-points. According to the measured U-Pb ratios, we calculated ages of two episodes: 2210-2162 Ma for the ancient and 402-322 Ma for Phanerozoic. Further calculations proved that all the measured data can be formed by extracting Pb from magmatic zircon of each sample in these two episodes.

Conclusions. Putting together geological and U-Pb isotopic data for zircons from Paleoproterozoic weathering profile we can define, that first episode of changing U-Pb system occurred 2210-2162 Ma, and the second 402-322 Ma. First episode should be identified as paleoweathering, clearly proved by geological data. The second episode is likely to be hypergene in origin, because there are now geological evidences of Phanerozoic thermal events in the region. Lower intersections of Discordia and Concordia, seen on figure, have no geological meaning, but had been formed as a superposition of previously described events.

References:

1. Alfimova, N.A. (2010) Geology and chemical composition of the Palaeoproterozoic weathering profiles of Karelia. Book of IPGG RAS young scientists’ achievements. In Russian, Polytechnic University publishing, Saint-Petersburg, p. 183–212.

2. Alfimova, N.A., Matrenichev, V.A., Zinger, T.F., Skublov, S.G. (2011) Geochemistry of zircons from the proterozoic weathering profile of granitoids in N.Karelia. Doklady of Earth Sciences RAS, Pleiades Publishing, V.438. №1. P. 101-104.

3. Alfimova, N.A., Matrenichev, V.A (2011). Hypergenous effect of U-Pb system in zircon from Archean granites of the Fennoscandinavian shield – Fanerozoic evidences, Abstracts of III Russian conference of geology-geodynamic problems of Precambrian. p. 8-11.

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