L.М. Stepanyik 1, S.I. Kurylo 1, I.М. Kotvytska 1, O.V. Hrinchenko 2
1 — Institute of Geochemistry, Mineralogy and Ore Formation, NAS of Ukraine
2 — Kyiv National Taras Shevchenko University, Institute of Geology
Language: English
Geochemistry and ore formation 2019, 40: 58-63
https://doi.org/10.15407/gof.2019.40.058
Potassium-bearing, mostly two-bifeldspar granites, are widely widespread within Ukrainian Shield. Early published papers have discussed the problem of potassium source for two-feldspar granites which indicate final stages or evolved continental crust formation, with granites of Middle Dnipro and granulites of Dniester-Bug megablocks being taken as typical examples. The conclusion was made that biotites might be a possible source of potassium, which occurrence in the upper layers of lithosphere marks the formation of evolved continental crust. These potassium-rich melts results in formation of two-feldspar granites as well as intensive potassic metasomatism occurred in granulite-gneiss regions. The biotite would be destroyed within instability zone (zone of transition from amphibolite to granulite facies) and, as a result of selective melting of source rocks it (together with rubidium and radiogenic isotope 87Sr) can migrate into melt or fluid, at PT-parameter that do not reach melting conditions. Subduction might be treated as condition (setting) favorable for entering of potassium into granulite rock associations, because it provides sinking of biotite- bearing rocks below granulitic blocks. Granulites are characterterised by low contnets of biotite that is not enough for production (release) of large amounts of potassium. This paper discusses calculation results of mass balance on potassium and strontium, with Middle Dnipro megablock as an example. It is established that for granite selective melts with K2O > 4% and primary isotope ratio values of two-feldspar granites more than 0,750 to be formed the proportion ratio as 8/1 is enough to be reached between volumes of substratum and two-feldspar granites (parental melt). This value is much less than that established for rocks of Middle Dnipro megablock. Сriteria for classification (systematization) of tectonic blocks of Ukrainian Shield are offered.
Keywords. Two-feldspar granites, rubidium-strontium isotope system, mass balance, tectonic block, Ukrainian Shield.
References
1. Сatalogue of isotope datings of Ukrainian Shield rocks. (1978) Edt. Scherbak N.P., Zlobenko G.V et al., Kyiv, Naukova Dumka, 224 p. [in Russian].
2. Correlative chronostratigraphic scheme of Early Precambrian of Ukrainian Shield (2003). [in Ukrainian].
3. Ponomarenko O.M., Stepanyuk L.M., Kryvdik S.G., Sinytsyn V.О. (2012). Radiogeochronology of metasomatic processes in crystalline rocks of Ukrainian Shield. Geochemistry and ore formation. 31/32, pp. 24-29 [in Ukrainian]. https://doi.org/10.15407/gof.2012.31.024
4. Scherbak N.P., Bartnitsky E.N., Lugovaya I.P. (1981), Kyiv, Naukova Dumka, 248 [in Russian].
5. Scherbakov I.B. (2005) Petrology of Ukrainian shield, Lviv: Zuktz, 366 [in Russian].
6. Stepanyuk L.M. (1997). Metasomatic nature of biotite and biotite-garnet gneisses of Middle Bug area. Reports of NAS of Ukraine, 1, pp. 133-136 [in Ukrainian].
7. Stepanyuk L.M., Ponomarenko O.M., Petritchenko K.V., Kurylo S.I., Dovbush T.I., Sergeev S.A., Rodionov M.V. (2015) Uranium-lead isotopic geochronology of Berdychiv type granitoids of the Bug area (Ukrainian shield). Mineralogical Journal (Ukraine), 37 (3), pp. 51-66 [in Ukrainian]. https://doi.org/10.15407/mineraljournal.37.03.051
8. Stepanyuk L.M., Kurylo S.І., Kotvitska І.М. (2016). Possible sources of primary magmas of two-feldspar granites and conditions of magma origins (Middle Dnipro megablock of Ukrainian Shield as an example). Mineralogical Journal (Ukraine), 38 (3), pp. 81-90. https://doi.org/10.15407/mineraljournal.38.03.081
9. Stepanyuk L.M., Dovbush T.I.., Kurylo S.І., Lisna І.М. (2016). Final stage of granite magmatism in Dnister-Bug megablock of Ukrainian Shield. Geochemistry and Ore Formation, 36, pp. 72-81. https://doi.org/10.15407/gof.2016.36.072
10. Stepanyuk L.M., Kotvitska I.M., Andreev О.V., Kurylo S.І., Hrinchenko О.V. (2018) Mature continental crust and source of potassium. Mineralogical Journal (Ukraine), 40 (4), pp. 79-88 [in Ukrainian]. https://doi.org/10.15407/mineraljournal.40.04.079
11. Yesypchuk K.E., Orsa V.I., Scherbakov I.B., Sheremet Е.М., Skobelev V.М., Riabokon V.V., Galetsky L.S., Panov B.S. (1993) Granitoids of Ukrainian Shield: petrochemistry, geochemistry, ore content Кyiv: Naukova Dumka, 232 p. [in Russian].
