Granitoids play a key role in the geological structure of the Ros-Tikych megablock. Supercrustal rocks of the Ros-Tikych series have been preserved in the granitoids only in the form of isolated fragments such as elongated remains, small skialites and even smaller "melted" xenoliths. In particular, in the Ostrivsky quarry, located on the right bank of the Ros River east of Bila Tserkva, granitoids are found (even-grained, porphyry-like granites) among which, as a rule, small bodies of granodiorites, plagiogranites and amphibolites occur. In order to determine the source of the parent magmas of rocks the properties of zircon crystals and the isotopic composition (87Sr/86Sr ratio) of apatite were studied. An analysis of the zircon crystals of the crystalline rocks exposed at the Ostrivsky quarry allows us to propose that the and plagio- and difeldspar granites were formed from one protolith. This is because they contain similar virtually identical zircon relics as nucleus. In addition, none of the granitoids contain zircon crystals whose internal structure is similar to zircon crystals found in amphibolite. This suggests that the granitoids were not derived by melting of amphibolites. Most likely, amphibolites are relicts of the protolith that were not assimilated during granite formation. The occurrence of heterogeneous zircon crystals (relic zircon cores of the protolith) in the protolith of the various studied granitoids indicates that they formed from volcanic-sedimentary rocks. Apatites in plagiogranitoids and porphyry granite contain strontium of similar isotopic composition. Their 87Sr/86Sr isotopic ratio is 0.70680 in apatite granodiorite and 0.70822 in granite. A high ratio of 87Sr/86Sr = 0.77940 was measured for apatite from monazite-bearing granite, thus indicating a different source for its parent magma.
The uranium-lead isotope method is used to date titanites from granitoids of the Ros'-Tikych megablock of the Ukrainian Shield (middle reaches of the Ros' river, below Bila Tserkva). Their isotopic behavior is analyzed along with published results, of other titanites found in crystalline rocks of the Ukrainian Shield. Titanites of granitoids from the Ros'-Tikych megablock contain impurity lead and this leads to an abnormal isotopic composition. The overestimation in age values calculated by methods that do not require correction for ordinary lead (method normalized, agreed differences and the slope of the regression line on the graph in the coordinates 207Pb/204Pb — 206Pb/204Pb) is up to several tens of millions of years, compared to, calculated on the model of Ahrens-Wetherill. The influence of impurity lead of anomalous isotopic composition on the distortion of the calculated values of the isotopic age, in the ratio 207Pb/206Pb is modeled. It was found that the entry into the isotope system, 2.0 billion years old, of ordinary lead with a modern isotopic composition, even with a ratio of impurity lead to radiogenic lead 1:1, will reduce the age by 207Pb/206Pb by only 4 million years. Significantly greater distortions (overestimations) of the isotopic ages is will be caused by impurity lead, in the isotopic composition of which there is an ancient (for example, 500 million older than the age of crystallization of titanite) radiogenic lead. Thus, when the ratio share of impurity lead is 20%, the overestimation of age is more than 50 million years, and it reaches 235 million years forat a ratio of 1:1 (50%). The numerical values of the isotopic age will be even more inflated at the ratio of 207Pb/206Pb in cases when impurity lead contains radiogenic lead of older age, even with a smaller share of it in the composition of impurity lead.
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.
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