POSSIBLE SOURCES OF ORIGINAL MAGMAS OF TWO-FELDSPAR GRANITES AND CONDITIONS OF MAGMA GENERATION (ON THE EXAMPLE OF THE MIDDLE DNIEPER MEGABLOCK OF USH)

UDC 550.93

https://doi.org/10.15407/mineraljournal.38.03.081

Stepanyuk L.M., Kurylo S.I., Kotvitska I.M.
M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of the NAS of Ukraine
34, Acad. Palladina Pr., Kyiv-142, Ukraine, 03680
E-mail: stepaniuk@nas.gov.ua; kurylo.sergiy@yandex.ru
POSSIBLE SOURCES OF ORIGINAL MAGMAS OF TWO-FELDSPAR GRANITES AND CONDITIONS OF MAGMA GENERATION (ON THE EXAMPLE OF THE MIDDLE DNIEPER MEGABLOCK OF USH)
Language: Ukrainian
Mineralogical journal 2016, 38 (3): 81-90

Abstract: The rubidium-strontium isotope systems of plagioclase and isotopic composition (87Sr/86Sr) of apatites of two-feldspar granites from Middle Dnieper have been studied. It is proved that relatively high, core typical values (0.712—0.720) of primary isotope relations (87Sr/86Sr) in are noted along with extremely high values (up to 0.86). That cannot be explained by simply selective melting of core substrate, because the maximum (supercrystal rocks of Aul suite and plagiogranitoids of Dnipropetrovsk complex, to a less extent of the Sura complex) values of primary isotope relations (87Sr/86Sr) in — 0.712 and relation Rb/Sr (0.5) in the substrate rocks even during 500 Ma (the maximum time of gap between the formation of the Aul suite rocks (3.2 bln. years ago)) and of two-feldspar granites (2.7 bln. years). The primary ratio in the substrate rocks may increase only to 0.725. The increase of the primary isotope ratio (87Sr/86Sr) in , compared with the calculated one, by the bringing of strontium with a higher isotopic ratio 87Sr/86Sr (higher than 0.86), as a result of imposition of metasomatic processes is unlikely. That is because there are no strontium sources with high isotopic ratios except for rubidium minerals concentrators (biotite and K-feldspar), in which as the result of radioactive decay of 87Rb radiogenic isotope 87Srrad is accumulated. To explain this phenomenon we have proposed a mechanism of granite forming. Biotite is the main mineral which contains rubidium in the gneisses of the Aul suit and plagiogranitoids, at the same time it does not almost contain strontium. It accumulates a certain quantity of pore radiogenic isotope 87Srrad for 500 Ma. In conditions of a higher core, where partial pressure of water in the fluid is low (PT-conditions of granulite facies), biotite is an unstable mineral and will be destroyed with formation of K-feldspar and hypersthene. This process is the result of solid-phase recrystallization. All components of the reaction, including 87Srrad , will more or less enrich the fluid. The appearance of water and potassium in the system will reduce the melting point. As a result of this process the quarts-feldspar eutectic, enriched with potassium, rubidium and radiogenic isotope 87Srrad , will be smelt. Thus the high-potassium granite melts which are enriched with rubidium can be formed. They will contain strontium with quite high 87Sr/86Sr isotopic ratio because radiogenic strontium in biotite is practically completely transferrred into melt.

Keywords: Ukrainian Shield, Middle Dnieper megablock, isotope geochemistry, petrology, two-feldspar granitoids.

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