GEOCHRONOLOGY OF GRANITOIDS OF THE DOBROPIL MASSIF OF THE AZOV REGION (UKRAINIAN SHIELD)

Abstract

The Dobropil granitoid massif is confined to the junction of the Gulyaipil and Remiv blocks of the Azov region. The granitoids of the massif intrude the Kosivtsiv greenstone structure. The massif is represented by a fairly wide range of rocks connected by gradual transitions (quartz diorites, granodiorites, quartz monzonites, monzo-diorites, tonalites, plagiogranites and granites). A characteristic feature of the granitoids of the massif is the presence in them of various amounts of small xenoliths of rocks of different composition (amphibololites/metapyroxenites, amphibolites, quartz diorites and granitoids of the normal series). According to geological data, the formation of the massif took place in two stages, which correspond to the formation of two corresponding phases of granitoids. The article presents the results of uranium-lead dating of various generations of accessory zircon from tonalites of the second phase of the massif intrusion using the SHRIMP-II ion-ion microprobe. It is shown that zircon crystals are composed of three generations. Zircon of the first generation is represented by heterogeneous cores on which magmatogenic zircon actually grows — the second generation. Zircon of the third forms rather thin shells on the first two, its crystallization is due to the processes of post-magmatic kalishpatization, which took place, most likely, at the pneumatolite stage of the evolution of the silicate melt. According to the results of uranium-lead ion-ion dating, it was found that among the zircon of the first generation, a fairly wide range of numerical age values (according to the 207Pb/206Pb ratio) is noted, from 3.6 to 2.8 billion years. The age of 2078 ± 20 million years ago was obtained for the upper intersection of the concordia with the discordia, constructed on the basis of analytical data obtained for thin-zoned shells (zircon of the 2nd generation) and zircon shells of the third generation, which corresponds to the time of rooting of the tonalites of the second phase of the intrusion.