Uraniumlead Age of Monazites from Biotitic Gneisses of the Middle Bug Area Of the Ukrainian Shield

UDC 550.93
https://doi.org/10.15407/mineraljournal.39.02.046

L.M. Stepanyuk, T.I. Dovbush, S.I. Kurylo, O.V. Zyultsle, T.B. Yaskevich
M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of the NAS of Ukraine
34, Acad. Palladin Av., Kyiv-142, Ukraine, 03680
E-mail: stepaniuk@nas.gov.ua, kurylo.sergiy@gmail.com
Language: Ukrainian
Mineralogical journal 2017, 39 (2): 46-56
Abstract: Biotite and garnetbiotite gneisses are widely distributed in the area of South Bug river valley, stretching from hayvoron to Pervomaisk towns. they are described in Bug Group, tyvrovka and Zelena Levada sections of the Dniester-Bug groups where they are treated as stratotype formations. According to our study, most biotite and garnetbiotite gneisses are formed as a result of structural and metamorphic transformations of pyroxene plagiogneisses and enderbite gneisses, which are accompanied by potassicsilica metasomatism. Monazites of biotite gneiss (metasomatite) are dated by classical uraniumlead isotopic method. Gneisses as layerlike body in enderbite-gneisses occur at the northern margin of the east kosharo-oleksandrіvka open pit. Age of monazite is 2026.5 ± 2.6 Ma. In the right riverside of South Bug river, near the Zelena Levada village, potassic and migmatized biotite plagiogneisses are widespread to various extent. They are commonly hypersthene-bearing and include some small bodies of granites found here. the monazite found in weakly potassic plagiogneiss is formed at 2031.5 ± 6.7 Ma ago, practically simultaneously with monazite formation in granite dated by 2029.3 ± 2.1 Ma. Age of plagiogneiss sampled from interval 203—349 m of borehole 6546, within Maysky deposit, is determined by "precious" zircon and monazite. Crystals of "precious" zircon are crystallized 2918 ± 7 Ma ago during structural and metamorphic alterations occurred under PT-conditions of granulitic facies. The monazite is crystallized much later than "precious" zircon and is dated by 1979.4 ± 1.7 Ma. Thus, biotite gneisses of the Middle Bug area, that are now erroneously treated as stratified formations, were formed in Paleoproterozoic — within the age interval of 2.03—1.98 billion years. They are formed as a result of structural and metamorphic transformations of plagiogneisses (enderbitegneisses) with subsequent potas-sicsilica metasomatic alterations.
Keywords: biotitic gneiss, isotopic age, monazite, metasomatism, Middle Bug area.
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