New data for the Sr and Nd isotopic composition of Paleozoic igneous rocks of Belarus

Mikhailov N.D.*, Laptsevich A.G*., Vladykin N.V.**

*Republican Unitary Enterprise «Belarusian Research Geological Exploration Institute», Minsk, Belarus, **A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Academy of Sciences, Irkutsk, Russia

The Paleozoic alkali igneous formation of Belarus is described by the magmatism evidences manifested within two geotectonic areas – Pripyat paleograben, which is the northwestern end of the Pripyat-Donets Paleorift, and North-Pripyat igneous region, which is a zone of junction of the largest platform geological structures and the Pripyat Trough. The prospects for discovering diamond and ore occurrences within the Paleozoic igneous complex of Belarus are associated with the geological objects of the North-Pripyat igneous region, therefore, the question if the primitive mantle was unique for all the alkali igneous rocks of the Pripyat Trough and North-Pripyat region still remains open over the last 15 years.

New data obtained for the Nd and Sr geochemistry of alkali rocks from all the regions of the territory of Belarus, where the magmatism evidences were determined, together with the comprehensive analysis and some recalculations of their previous values (Wilson, Lyashkevich, 1996; Markwick et al., 2001; Mikhailov et al., 2002; Pervov et al., 2004) that are presented in the Table, considerably improved the available knowledge necessary to solve this problem. The Late-Devonian age of all these igneous rocks was constrained by their occurrence between Upper-Devonian sedimentary strata stratified in detail (to stages and horizons) and well dated by paleontological methods (Kruchek, Obukhovskaya, 1996; 1997) and was confirmed by isotope datings of rocks at ca 370 Ma (Wilson, Lyashkevich, 1996). The Nd and Sr isotope ratio is presented in the diagram, where some data for the model sources and several evidences of the similar age alkali magmatism in the Arkhangelsk province and East Azov sea region are given for comparison purposes.

The Sr-Nd isotope distribution in the studied samples of Palaeozoic alkali igneous rocks of Belarus form in general a linear field constrained between the isotope data of the primitive mantle (PREMA) and those of the enriched mantle source EM 1, some representative points of the isotopic composition falling within the EM 1 region (see Figure). As is seen from the diagram of the Sr-Nd isotopic composition and the Table, with rather similar ⁸⁷Sr/⁸⁶Sr values (0.7030-0.7050) the studied rocks fall into two groups different in the ¹⁴³Nd/¹⁴⁴Nd values that range from 0.51211 to 0.51275. The εNd value varies in these groups from 5.2 to -4.5. The first (positive) group is formed therewith by rocks of the Pripyat graben, a paleovolcano of the North-Pripyat zone of steps and, partly, the southern margin of the Zhlobin Saddle. The second group involves alkali-ultramafic rocks of the Zhlobin Saddle, alkali basaltoids of the Gomel structural dam that are close to the EM 1 region and are sometimes richer in the radiogenic Sr isotope as against the rocks of the Pripyat graben.

The simulated Sm-Nd age of sources of igneous rocks in Belarus (see Table) conforms to the age interval T_DM=0.65-1.23, T_DM2=0.78-1.46 Ga and represents the age of metasomatic enrichment of the mantle source of these rocks. The older age datings were obtained for the most part of rocks of the North-Pripyat region 1.15-1.51 Ga, the maximum values being recorded in the rocks of the Gomel structural dam (1.20-1.51 Ga). The second age group is formed by igneous rocks of the Pripyat Graben, the southern part of the Zhlobin Saddle and a palaeovolcano in the North-Pripyat zone of steps 0.70-0.98 Ga. It should be noted that the rocks of these two simulated age groups correspond to different εNd types: the first one to εNd(-) , and the second one to the εNd(+).

This study was performed under the financial baking of the BRFFI, grant X08P-087.

References:

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Figure. The Nd and Sr isotope ratio in Paleozoic igneous rocks of Belarus.

Rocks analyzed: 1-2 – Zhlobin diatreme field (1- from authors’ data, 2- after Markwick et al., 2001;after Pervov et al., 2004); 3 – North- Pripyat zone of steps; 4 – Gomel structural dam; 5-6 – Pripyat Graben (5- from authors’ data, 6- Wilson, Lyashkevich, 1996); 7 – alkali basalt of the Dnieper-Donets depression after Wilson, Lyashkevich, 1996; 8 – kimberlite field of the Arkhangelsk diamondiferous province after Parsadanjan et al., 1996; 9 – kimberlite field of the East Azov Sea Region after Yutkina et al., 2004.