Features of U-Pb Isotopic Systems of Zircon and Monazite From Granite — "Xenolith" Rock Association

UDC 550.93
https://doi.org/10.15407/mineraljournal.39.01.063

STEPANYUK L.M., KURYLO S.I., SYOMKA V.O., BONDARENKO S.M., KOVALENKO O.O., DOVBUSH T.I., VYSOTSKIY O.B.
M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of the NA S 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 (1): 63-74
Abstract:
Based on U-Pb dating of zircon from enderbitic gneisses, including Palaeoproterozoic granite’s analectic melting and bodies of mafic granulites, which are common northwards the Zavallya village; it has been found that zircon from mafic rock is of a less age, more than 30 Ma, than one from granite. Similar situation is observed for rock association of the Upper Bug area. Two concepts are proposed for explaining the fact. According to the first one, the zircons of mafic rocks and granitoids were crystallized simultaneously, but the older age dating is due to ancient radiogenic lead, which is located in zircon as relic zircon’s nucleus, or due to anomalous isotopic composition of "common" lead. According to the second one, the zircons of mafic rocks were crystallized under the influence of fluids released from magma chamber on the final crystallization stage of granite melting, i. e. later than the zircons of granitoids. To solve the problem the U-Pb systems of monazites from granitoids and their xenoliths have been studied. Monazites are usually absent in the rocks of substrate, thus the monazite’s age fits best their crystallization age. Moreover, uranium content in monazites is many times richer than in zircon, the impact of contaminated radiogenic lead, if it might be hosted by monazite, is therefore not sufficient, as for zircon. Main host rocks of the Zhezhelivka open pit are biotite-garnetiferous Berdychiv "granites"; there are hypersthen-biotitic crystalline schists, seldom two-piroxen-biotitic crystalline schists biotitic and plagiogneisses, as xenoliths among these host rocks. The age of monazite from hypersthen-biotitic crystalline schists is 2023.1 ± 3.3 Ma. The monazite age of Berdychiv "granite" is 2042.9 ± 3.2 Ma. The age of monazite from leucocratic part of the granite substrate is 2040.9 ± 2.6 Ma. It is an unambiguous proof that the monazite from xenolithe of basic composition was crystallized later than that from Berdychiv "granites" and another one from leucocratic part of the granite substrate. Similar result has been obtained for monazites of the Novgorodka open pit rock association, where porphyritic granites of the Kirovograd type are widespread. There are numerous xenoliths there, essentially biotitic plagiogneisses. Porphyritic granites and often xenoliths are intersected by veins of medium- and coarse-grained granites. Based on results of U-Pb dating the age of monazite from porphyritic granite is 2039.9 ± 3.3 Ma, 2034.8 ± 1.2 Ma is for veined granite, 2022.8 ± 4.3 Ma for monazites from xenoliths of biotitic plagiogneisses. Thus, the monazites in xenoliths of basic and acid composition were crystallized 20 Ma later than in granites which host the xenoliths. Crystallization of the monazite in xenoliths of both basic and acid composition proceeded apparently under the influence of fluids released at the final stage of granitic melting. Granite magma affected slightly the rocks it intruded. In any case, there is no monazite in xenoliths of basic composition, or in more acid ones, as biotitic plagiogneiss.
Keywords: U-Pb dating, granite, xenolithe, monazite, zircon.
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