L.M. Stepanyuk, I.M. Kotvitska, A.V. Andreev, S.I. Kurylo, O.V. Grinchenko. AN EVOLVED CONTINENTAL CRUST AND A SOURCE OF POTASSIUM
UDC 552.4 + 550.93
L.M. Stepanyuk 1, I.M. Kotvitska 1, A.V. Andreev 2, S.I. Kurylo 1, O.V. Grinchenko 2
1 M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of the NAS of Ukraine
34, Acad. Palladin Ave., Kyiv, Ukraine, 03142
2 Kyiv Taras Shevchenko National University. Educational-scientific institute "Institute of Geology"
90, Vasylkivska Str., Kyiv, Ukraine, 03022
Mineralogical journal 2018, 40 (4): 79-88
AN EVOLVED CONTINENTAL CRUST AND A SOURCE OF POTASSIUM
Abstract: The early rock associations of Earth are characterized by a large predominance of sodium over potassium. The oldest rocks of the Dniester-Bug megablock are pyroxene plagiogneisses (enderbite gneisses) of the Palaeoarchean age. Rocks of the Na-K series (charnockites, biotitic and garnet-biotitic gneisses), related to Zelenolevadivka strata and present as well in other stratigraphic units of the Bug area, together with two-feldspar granitoids (Pobuzhzhya and Berdychiv complexes) form about 40 % of the megablock’s volume. According to the results of U-Pb isotope monazite dating, they were formed at 2.06-1.96 Ga. So, potassium appeared in the megablock mainly in the Palaeoproterozoic. Previously, the petrological model was proposed to explain a possible source of potassium in two-feldspar granitoids of the Middle Dnieper domain. According to the model, biotite of the parental (substrate) rocks is envisaged as a possible source of radiogenic 87Sr, as well as Rb and K, in which these granitoids are enriched. At PT-conditions of amphibolite to granulite facies transition, biotite becomes unstable and is replaced by hypersthene, whereas mentioned elements and water expelled into the fluid phase. To recognize the possible source of potassium in supracrustal rocks (gneisses, granulites) and granitoids (antiperthite enderbites, charnockites, two-feldspar granites) of the Dniester-Bug megablock, the Rb-Sr isotope systematics of plagioclases and apatites are studied. The 87Sr/86Srin ratio in protholith rocks (plagiogneisses, enderbite gneisses) of the Middle Bug area varies within 0.7013-0.7132. At the same time, granulites, biotite gneisses, charnockitoides, biotite granites and their aplite-pegmatite varieties formed at 2.06-2.03 Ga, have much higher initial (primary) 87Sr/86Srin values (0.719-0.817). As the oldest rocks of this megablock contain biotite as accessory mineral, we suppose that, similarly to two-feldspar granitoids of the Middle Dnieper domain, biotite was the source of potassium during the formation of two-feldspar granitoids. However, unlike granitoids of the Middle Dnieper domain, this biotite was present in much younger, possibly Neoarchean or (and) Paleoproterozoic rock associations, that were buried at depths (PT-conditions) of granulite facies metamorphism. Such a subsidence could have place occur as a result of subduction at collision geodynamic conditions.
Keywords: continental crust, Archean, Proterozoic, potassium, radiogenic strontium.
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