Isotopic Age of Zircon from Gabbroids of Gorodische Massif (Korsun-Novomyrgorod Pluton, Ingul Megablock, Ukrainian Shield)

UDC 550.93 (477)

https://doi.org/10.15407/mineraljournal.37.02.063

O.E. Shestopalova (1), L.M. Stepanyuk (2), T.I. Dovbush (2), I.M. Kotvitskaya (2)
(1) S.I. Subbotin Institute of Geophysics of the NAS of Ukraine
32, Acad. Palladina Pr., Kyiv-142, Ukraine, 03680
(2) 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: stepanyuk@igmof.gov.ua
Isotopic Age of Zircon from Gabbroids of Gorodische Massif (Korsun-Novomyrgorod Pluton, Ingul Megablock, Ukrainian Shield)
Language: Russian
Mineralogical journal 2015, 37 (2): 63-75

Abstract: Korsun-Novomyrgorod pluton is situated in the north-western part of Ingul megablock. It is composed by intrusive formations of gabbro-anorthosite-rapakivi granite complex of rocks. Gabbro-anorthosite in the composition of the pluton forms four massifs: Mezhyrich, Gorodische, Smila and Novomyrgorod. Gorodische massif (about 200 km2) is located in the western part of Korsun-Novomyrgorod pluton. About 80 % of the area of this massif is composed by coarse-grained anorthosite and norite-anorthosite. Norites and anorthosites form the tabular bodies, prolonged in the north-western direction. U/Pb isotopic method was used to determine the age of gabbroids massifs by zircon. Samples for the isotopic dating were collected in natural and man-made outcrops in the surrounding populated localities Khlystunivka and Voronivka. Complex structure of zircon crystals and the presence of relict centers (nuclei) in some of them were determined by optical and electronic microscope methods. Zircon complex crystals as well as the wide variety of their types are characteristic of anorthosite differences of gabbroids that is likely caused by more prolonged existence of anorthosite melts and by more difficult history of their formation. U/Pb isotopic method was used to determine the age of homogeneous crystals of zircons of norite and anorthosite differences of gabbroids of 1740—1770 Ma. 207Pb/206Pb isotopic ratio was obtained in the interval 1774.6—1750.4 Ma (discordance 4.0—8.5 %) for olivine gabbro-norite-anorthosite zircons of the eastern part of Gorodische. Baddeleyite age is determined as 1758.2 Ma (by the isotopic ratio 207Pb/ 206Pb). Age of zircons of olivine gabbro-noriteanorthosite is calculated by the upper crossing of concordia with discordia and they are dated at 1738.8 ± 3.6 Ma. Weighted mean of the age by the 207Pb/206Pb isotopic ratio is 1739.4 ± 3.4 Ma. The age of porphyritic norite zircons (Khlystunivka vil.) is 1749 ± 0.5 Ma. For olivine norite-troctolite zircons (Voronivka vil.) the age is determined by light rose crystals of zircons as 1750.1 ± 1.2 Ma. We suppose probable contamination of the initial melts by the enclosed rocks, or the incomplete melting of substratum rocks. It is confirmed by high values of initial isotopic ratio Sr87/Sr86 — 0.7053—0.7069, which were calculated for plagioclases and apatites from dated samples of gabbroids.

Keywords: gabbroids, gabbro-norite-anorthosite, zircons, relict centers (nuclei), geochronological dating, age, Korsun-Novomyrgorod pluton.

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