CC BY
6
УДК 549.514.81+550.93(470.5)
http://doi.org/10.21440/2307-2091-2022-2-26-32
Chemical composition and age of the monazite from granitic pegmatites of the Beloyarskaya vein (Zenkovsky massif, Middle Urals)
Vera Vital'evna KHILLER*
The Zavaritsky Institute of Geology and Geochemistry of the Ural Branch of RAS, Ekaterinburg, Russia Abstract
The relevance of the work is due to the need to improve the method of chemical dating as applied to high-thorium accessory minerals, which are difficult to date by isotope research methods.
The purpose of the work is to study the chemical composition of the accessory monazite from granitic pegmatites of the Beloyarskaya vein (Zenkovsky massif, Middle Urals) and to determine its age.
Research methodology. Quantitative analysis of the monazite chemical composition was performed on a CAMECA SX 100 X-ray electron probe microanalyzer. Measurement conditions: accelerating voltage 15 kV, current 250 nA, electron beam diameter 2 ^m. The pressure in the sample chamber is 2 • 10-4 Pa. The spectra were obtained on tilted wave spectrometers, the intensity was measured using analytical lines: Th Ma, U Mb, Pb Ma, Y La, Si Ka, Ca Ka, P Ka, Ce La, La La, Pr Lb, Nd La, Sm Lb, Dy La, Gd Lb. The calculation of age was carried out according to well-known methods of foreign authors in addition to the authors' own developments.
Results. The monazite chemical composition allows it to be attributed to the cerium variety, the content of radiogenic components varies greatly (in wt. %): ThO2 - 7.88-12.43; UO2 - 0.12-0.44; PbO - 0.09-0.15. Of the impurities, significant concentrations of SiO2 (up to 2.4 wt. %), Y2O3 (up to 2.8 wt. %) and CaO (up to 0.9 wt. %) are noted. It turns out that huttonite (Th4+(U4+) + Si4+ REE3+ + P5+) and cheralite (Th4+(U4+) + Ca2+(Sr2+, Ba2+, Pb2+) 2REE3+) types of isomorphism are realized in the monazite. A decent lead content and quite good crystallinity of cerium phosphate make it possible to use this mineral as a geochronometer mineral.
Conclusions. New data on the chemical composition of the monazite-(Ce) have been obtained, and the Late Permian age of granitic pegmatites of the Beloyarskaya vein has been established by microprobe dating. The U-Th-Pb point ages of the monazite together give a weighted average age of 255 ± 12 Ma (MSWD = 0.10) and an isochrone of 257 ± 24 Ma (MSWD = 0.45). The obtained age values for the monazite from the Beloyarskaya vein are in good agreement with the dating of granitic pegmatites of the Adui granite massif, and it is quite possible that the vein bodies of the Zenkovsky massif may belong to the Adui pegmatite field.
Keywords: monazite, chemical dating, granitic pegmatites, Beloyarskaya vein, Zenkovsky massif, Middle Urals.
Introduction
The Zenkovsky massif (or stock) belongs to the ore-bearing granite massifs (granites of the granite-leucogranite series). Its granites are similar to the granites of the Malyshevsky massif, with which the molybdenum mineralization is associated (the well-known Yuzhno-Shameyskoye deposit). In terms of geological and geochemical characteristics, these granites are comparable with the geochemical type of plumasite rare-metal leuco-granites [1].
Granite pegmatites of the Zenkovsky massif are poorly represented in the open literature. The most famous and largest is the Beregovaya vein, located 8 km northwest of the city of Zarechny on the eastern shore of the Beloyarsk reservoir. Fergusonite-(Y), samarskite-(Y), ferro- and manganocolum-bite, monazite-(Ce), ilmenorutile, struverite, polycrase-(Y), uranium pyrochlore, betafite, uraninite, thorite, brockite, and zircon-xenotime intergrowths were found in this vein [2-5]. Moreover, fergusonite crystals reached an impressive 5 cm in length (they were discovered by V. A. Gubin). In addition, within the Zenkovsky massif, the Beloyarskaya and Rezhikska-ya pegmatite veins are mentioned, which are currently practi-
cally unexplored, and the latter has already been flooded in the quarry. Unfortunately, the age of the pegmatite veins, as well as the massif itself, has not yet been determined.
Geological position of the Zenkovsky massif and pegmatites. The Zenkovsky massif is located approximately 5-7 km northwest of the city of Zarechny (Sverdlovsk region) on the eastern shore of the Beloyarsk reservoir. The granites of the Zenkovsky massif cut through the granodiorites of the Kamensky massif in its southwestern part. On the geological map, the granites of the Zenkovsky massif are mapped in the form of two bodies (fig. 1), in the open literature [6, 7] referred to as the Zenkovsky and Kvartsevogorsky massifs. Geologists of the Ural geological survey expedition led by V. A. Rybalko attributed them to the Malyshevsky complex. Granites of the Zenkovsky massif belong to the group of young granites of the granite-leucogranite series, which complete the granitoid magmatism of the Urals. Granites of the Zenkovsky massif are formed in two phases: 1st phase - leucocratic medium-grained granites, 2nd phase - leucocratic granites with smoky quartz and molybdenite. Even-grained, medium-grained leucocrat-
EDhilvervit@mail.ru
https://orcid.org/0000-0001 -8491 -4958
Figure 1. Geological map of the Kamensky granodiorite-adamellite complex (C12k) with bodies of the Zenkovsky massif (P2z). Fragment of sheet 0-41-XXVI, Sredneuralskaya series (according to data from the Adui GMP, ОАО UGSE, 2001). Scale: bottom of themap(20km)
Рисунок 1. Геологическая карта Каменского гранодиорит-адамеллитового комплекса (C12k) с телами Зенковского массива (P2z). Фрагмент листа О-41-XXVI, серия Среднеуральская (по данным Адуйской ГСП, ОАО "УГСЭ", 2001). Масштаб: низ карты (20 км)
ic granites of the first phase were observed in two quarries. In appearance, these granites are somewhat similar to the Malyshevsky granites, but the granites of the Malyshevsky massif have a pronounced porphyry appearance. Leucocrat-ic granites of the second phase contain disseminated molybdenite-scheelite mineralization [8].
As already mentioned, the age of the Zenkovsky massif was not determined, but due to its similarity with the Malyshevsky massif, the age of the latter is usually taken. According to the results of measuring the parameters of the Rb-Sr system in bulk samples and monofractions of minerals, the isotope age of the Malyshevsky leucogranites is 277 Ma [9]. Similar dates are confirmed by the Re-Os age of molybdenites (273 and 282 Ma) from the Shameyskoe deposit [10].
The Beloyarskaya vein is located 2 km north-northwest of the Beregovaya vein, also on the eastern coast of the Beloyarsk reservoir (fig. 2). The pegmati e body has not been opened and is block debris. Rock fragments show that the graphic zone of the vein is composed of quartz, two feldspars with muscovite and garnet, and the block zone contains rare earth-rare metal mineralization. In add tion, there is a quartz core. The
Figure 2. Location of the Beloyarskaya (circle) and Beregovaya (diamond) pegmatite veins on the eastern shore of the Beloyarsk reservoir. Sea le: sideof the s qua re (2 km)
Ри сунок 2. Местонахождение пегматитовых жил Белоярская (круг) и Береговая (ромб) на восточномберегу Белоярского водохранилищ) l Масштаб: сторона квадрата (2 км)
following minerals have been identified: potassium feldspar, plagioclase, quartz, muscovite, spessartine, ferro- and manga-nocolumbite, monazite-(Ce), samarskite-(Y), polycrase-(Y), pyrite,and illite[3, 5].
Research methodology
Quantitative analysis of the chemical composition of the monazite was performed on a CAMECA SX 100 electron probe microanalyzer (IGG Ural Branch of the Russian Academy of Sciences, Ekaterinburg). A polished section was made from two fragments of a monazite crystal up to 0.5 cm in size, then the specimen was sprayed with a thin layer of carbon. Measurement conditions: accelerating voltage 15 kV, current strength 250 nA, electron beam diameter 2 |im. The pressure in the sample chamber is 2 • 10-4 Pa. The spectra were obtained on tilted wave spectrometers, the intensity was measured using analytical lines: Th Ma, U Mb, Pb Ma, Y La, Si Ka, Ca Ka, P Ka, Ce La, La La, Pr Lb, Nd La, Sm Lb, Dy La, Gd Lb. Standard samples used for calibration: thorium and uranium oxides, diopside, synthetic rare-earth phosphates, Pb2P2O7. When performing a quantitative analysis, the intensity measurement time at the peak (400 s for thorium, 400 s for lead, 400 s for uranium, 20 s for silicon, yttrium, and calcium, and 10 s for other elements) was two times longer than the background measurement time. For the monazite, the following detection limits have been achieved: Th - 118, U - 72 and Pb - 65 ppm. The oxygen content was determined under the assumption of stoichiometry of the composition. The theoretical and practical substantiation of the method of chemical dating using X-ray spectral microprobe analysis is given in numerous publications on this topic [11, 12], including the authors' team [13, 14]. The main condition of this method is that the mineral did not lose radiogenic lead during evolution (i. e., the Th-U-Pb system was closed), all lead in the mineral was formed due to the decay of thorium and uranium.
В. В. Хиллер /Известия УГГУ. 2022. Вып. 2(66). С. 26-32 Age, Ma
310
290
270
250
230
210
Tav = 255 ±12 Ma MSWD = 0.098
Figure 3. image of monazite crystal fragments from the Beloyarskaya vein. BSE-photo, CAMECA SX 100. Photo made by pastingsmallerpicturestogether
Рисунок 3. Изображение фрагментов кристалла монацита из Белоярской жилы. BSE-фото, CAMECASX 100. Фото с/делано путем склеивания более мелких картинок
Obtained results and discuss on
For study and dating, we used a relatively large and opaque monazite crystal, or rather, two of its fragments from the block zone, fig. 3. It was a flattened twin of rotation along the [100] plane, which looks like a "dovetail" up to 0.5 cm in length. The mineral is characterized by a dark brown color and does not associate with any other accessory minerals.
The chemical composition of the studied monazite is quite homogeneous (table 1) and is in go d agreement with the previously obtained data [3]. Unfortunately, only two analyzes were made. Based on the data of a l 18 analyzes presented, it can be concluded that the monazit in the block zone is represented by a cerium variety (since cerium significantly predominates over other light rare earths) with a fairly high content of thorium (ThO2 up to 12.4 wt. %). Among other impurities, the presence of significant co centrations of silica (SiO2 up to 2.4 wt. %), yttrium (Y2O3 up to 2.8 wt. %) and calcium (CaO up to 0.9 wt. %) can be noted. The content of radiogenic components varies quite strongly, wt. %: ThO2 - 7.88-12.43; UO2 - 0.12-0.44; PbO - 0.09-0.15. The sums of analyzes close to 100% indicate that this monazite did not experience secondary changes.
It is known that for thorium and uranium impurities in the monazite, the huttonite (Th4+(U4+) + Si4+ REE3+ + P5+) and (or) cheralite (Th4+(U4+) + Ca2+(Sr2+, Ba2+, Pb2+) 2REE3+) type of isomorphism is realized. From the available analyzes of the chemical composition of the monazite, it can be argued that both types of isomorphism are realized in the phosphate matrix.
In the well-known work [15], the parameter ^ = (Si + Ca)/ (Th + U + Pb), characterizing the degree of charge compensation of Th4+(U4+) impurities, was considered as an indicator of the closedness of the Th-U-Pb system of the monazite: at its close to unity, the system is considered closed. For the monazite from the Beloyarskaya mine, the parameter ^ = 1.01-1.09, which indicates the closed nature of the system and, therefore, the possibility of a correct assessment of the age of the mineral.
6 8 10 12 14 Point number on grains
16 18
Figure 4. Weighted average Th-U-Pb age of the Beloyarsk monazite according to microprobe analyzes (18 analyzes in total) Рисунок 4. Средневзвешенный Th-U-Pb-возраст белоярского монацита по данным микрозондовых анализов (всего сделано 18 определений)
0,18
0,15
0,12
0,09
0,06
0,03
0,00
PbO wt.%
Monazite from the Beloyarsk mine, Zenkovsky massif
T = 257 ± 24 Ma MSWD = 0.45
15
wt.%
Figure 5. Th02*-Pb0 data for the monazite from the Beloyarskaya vein. The ellipses correspond to the error values 1o, the dash-dotted line is the regression line with two symmetrical hyperbolas fixing the errors
Рисунок 5. ThO^-PbO-данные для монацита из Белоярской жилы. Эллипсы соответствуют значениям погрешности 1ст, штрихпунктир - линия регрессии с двумя симметричными гиперболами, фиксирующими погрешности
A closed system, sensitive lead contents and quite a decent dispersion of thorium contents make it possible to use this accessory rare earth phosphate as a geochronometer mineral. The U-Th-Pb point ages of the monazite lie in a wide range of 240-270 Ma and together give a weighted average age of 255 ± 12 Ma with MSWD = 0.10 (fig. 4).
When constructing the dependence ThO2* - PbO, based on the totality of the results of the analysis, the points lie on one straight line (or isochrone [16]), which indicates their si-
multaneous formation. Here ThO2* = (ThO2 + UO2eq), where UO2eq is the uranium content recalculated to the equivalent thorium content capable of producing the same amount of lead during the lifetime of the system if the U-Pb and Th-Pb age values are equal. Calculation of the age from the slope of the isochron gives a dating of 257 ± 24 Ma, MSWD = 0.45 (fig. 5). The content of non-radiogenic lead in the monazite, calculated from the intersection of the Th*-Pb isochrone with the PbO axis, is less than 0.005 wt. %, which is comparable to the limit
of its detection in the mineral and, therefore, had no effect on the age calculation.
The obtained age values for the monazite from granitic pegmatites of the Beloyarskaya mine are difficult to estimate, since no dating for the Zenkovsky massif has yet been given. The age of the host Kamensky granodiorite-adamellite complex is currently determined within 298-309 Ma [17]. The bodies of the Zenkovsky massif are cut through by the granitoids of the Kamensky complex, and the Late Permian age of the granitic
Table 1. Chemical composition of the monazite from granite pegmatites of the Beloyarskaya vein, wt. % Таблица 1. Химический состав монацита из гранитных пегматитов Белоярской жилы, мас. %
Oxides Numbers of analyzes
1 2 3 4 5 6 7 8 9
PA 26,02 25,72 25,64 26,14 24,40 25,02 25,10 25,66 26,86
ThO2 8,34 10,34 9,92 10,67 11,92 12,02 12,30 9,80 7,88
uo2 0,26 0,36 0,30 0,29 0,12 0,44 0,37 0,29 0,20
SiO2 1,46 1,87 1,61 1,74 2,23 2,32 2,06 1,62 1,22
^2O3 11,09 10,47 10,27 9,91 10,95 10,38 9,91 10,19 10,13
Ce2O3 25,42 24,18 24,27 24,02 25,02 23,51 23,31 24,12 24,96
Рфз 3,12 3,13 3,05 3,19 3,27 2,99 3,23 3,14 3,30
Nd2O3 12,78 12,59 13,38 13,21 13,97 12,23 12,73 12,85 13,73
Sm2O3 3,79 3,75 4,00 3,97 3,72 3,60 3,73 3,99 4,23
E"2°3 - - 0,04 0,08 0,07 - - 0,04 0,01
Gd2O3 2,45 2,30 2,48 2,63 1,93 2,42 2,33 2,46 2,78
Tb2O3 0,10 0,10 0,13 0,21 0,04 0,12 0,06 0,07 0,16
Dy2O3 0,58 0,56 0,56 0,58 0,17 0,65 0,65 0,70 0,67
H°2O3 0,07 - 0,02 - 0,03 - 0,13 - -
E^2O3 - 0,01 - - - - - 0,02 0,09
Y2O3 2,28 2,55 2,47 2,38 1,02 2,51 2,49 2,62 2,75
PbO 0,10 0,12 0,12 0,12 0,14 0,15 0,15 0,12 0,09
CaO 0,67 0,77 0,68 0,69 0,46 0,79 0,76 0,70 0,63
Total 98,54 98,85 98,94 99,83 99,47 99,16 99,30 98,40 99,66
Oxides Numbers of analyzes
10 11 12 13 14 15 16 17 18
P O 2 5 26,36 25,69 25,39 24,48 25,88 26,05 26,58 25,48 25,70
ThO2 8,74 10,56 11,92 12,43 9,34 9,91 9,09 11,64 11,21
UO2 0,27 0,35 0,34 0,13 0,28 0,30 0,25 0,16 0,41
SiO2 1,53 1,93 2,04 2,39 1,63 1,61 1,48 2,07 2,12
La2O3 10,53 10,85 10,12 10,56 11,09 10,14 10,21 10,55 10,38
Ce2O3 24,89 24,46 23,47 24,92 25,23 24,18 24,50 25,32 23,89
Pr*2O3 3,04 3,07 3,08 3,32 3,24 3,27 3,29 3,24 3,06
Nd2O3 13,25 12,84 13,04 13,70 12,72 13,34 13,41 13,37 12,29
Sm2O3 3,94 3,72 3,58 3,62 3,76 4,11 3,99 3,52 3,63
^2O3 0,03 0,05 - 0,06 - - 0,06 0,01 -
Gd2O3 2,40 2,31 2,50 1,88 2,33 2,56 2,57 1,74 2,46
Tb2O3 0,10 0,06 0,14 - 0,10 0,16 0,14 - 0,11
Dy2O3 0,57 0,57 0,58 0,15 0,54 0,66 0,64 0,29 0,60
H°2O3 - - - 0,11 - 0,09 0,02 0,03 -
Er2O3 - - 0,01 - - 0,02 - - 0,05
Y2O3 2,44 2,38 2,43 1,01 2,21 2,59 2,46 1,49 2,46
PbO 0,10 0,13 0,14 0,14 0,12 0,11 0,12 0,13 0,13
CaO 0,73 0,77 0,72 0,49 0,73 0,70 0,66 0,58 0,90
Total 98,91 99,73 99,49 99,39 99,20 99,80 99,46 99,61 99,41
Note: IGG Ural Branch of the Russian Academy of Sciences, CAMECA SX 100 microanalyzer, analyst V. V. Khiller. Примечание: ИГГ УрО РАН, микроанализатор CAMECA SX 100, аналитик В. В. Хиллер.
pegmatites of the Beloyarskaya vein is quite consistent with this geological fact. Does it mean that the age of the leucogran-ites of the Zenkovsky massif is Late Permian? Not at all, since pegmatite processes are often separated in time from the host rocks. Given the proximity of the Kamensky and Adui massifs, it can be assumed that the vein bodies of the Zenkovsky massif can directly relate to the Adui pegmatite field, since the pegmatites of this field are also characterized by rare earth-rare metal mineralization and very close age relationships. Thus, the formation time of pegmatites of the Adui granite massif was determined as 256 ± 0.6 Ma (according to the monazite [18]) and 255-241 Ma (according to micas [19]). In granite pegmatites near the village Ozerny (the northern part of the massif), chemical dating was used to study accessory zircon, whose age was calculated within the range of 255 ± 7 Ma [20]. In adjacent veins, quite reliable Th-U-Pb dating was obtained for accessory monazites, 254 ± 15 Ma [16], and in the western part
of the massif, for the monazite from the Semeninskaya mine, 256 ± 21 Ma [21]. It is interesting that no molybdenite was found in the granite pegmatites of the Beregovaya, Beloyarskaya, and Rezhikskaya veins [3], which is an indirect sign of the foreignness of these pegmatites in relation to the Zenkovsky massif.
Conclusions
Thus, we have studied the accessory monazite from granitic pegmatites of the Beloyarskaya vein located in the little-studied Zenkovsky massif. According to microprobe analysis, the monazite belongs to the cerium variety and is characterized by a high content of thorium (ThO2 up to 12.4 wt. %). An isochron age of 255 ± 12 Ma was calculated for it. The obtained age values for the monazite from the Beloyarskaya vein are in good agreement with the dating of pegmatites of the Adui granite massif, and it is quite possible that the vein bodies of the Zenkovsky massif may belong to the Adui pegmatite field.
The work was carried out within the framework of the state task of the IGG, Ural Branch of the Russian Academy of Sciences, no. reg. AAAA-A18-118052590032-6.
REFERENCES
1. Morozova A. V. 2006, Geochemistry of granites of the Middle and Southern Urals, productive on W-Mo and Be-W-Mo mineralization. Litosfera [Lithosphere], no. 3, pp. 171-177. (In Russ.)
2. Popova V. I. 2009, Radiation-epigenetic changes in polycrase in granitic pegmatites of the Adui and Zenkovsky massifs in the Middle Urals. Litosfera [Lithosphere], no. 6, pp. 90-94. (In Russ.)
3. Popova V. I., Gubin V. A. 2008, Mineralogy of the granitic ceramic pegmatites in the Aduy, Sokolovsky, and Senkovsky massifs from middle ural. Ural'skiy mineralogicheskiy sbornik [Ural mineralogical collection], no 15, pp. 61-74. (In Russ.) URL: https://elibrary.ru/item.asp?id=23876011
4. Popova V. I., Gubin V. A., Popov V. A., Bazhenova L. F. 1988, On the characterization of some accessory minerals of pegmatites of the Zenkovsky and Adui granite massifs. Materials for the mineralogy of the ore regions of the Urals. Sverdlovsk, pp. 67-73. (In Russ.)
5. Popova V. I., Churin E. I., Blinov I. A., Gubin V. A. 2012, Fergusonite-(Y) and products of its alteration in granitic pegmatite of the Beregovaya vein of the Zenkovsky massif in the Urals. Novyye dannyye o mineralakh [New data on minerals], issue 47, pp. 47-55. (In Russ.)
6. Grabezhev A. I. 1981, Metasomatism, ore formation and granite magmatism. Moscow, 292 p. (In Russ.)
7. 1987, Geochemical criteria for rare-metal ore-bearing of granites (using the example of the Urals). A. I. Grabezhev [et al.]. Sverdlovsk, 128 p. (In Russ.)
8. Morozova A. V. 2006, Petrographic and geochemical characteristics of the granites of the Zenkovsky massif (Middle Urals). Trudy Instituta geologii i geokhimii UrO RAN [Proceedings of the Institute of Geology and Geochemistry, Ural Branch of the Russian Academy of Sciences], issue 153, pp. 179-182. (In Russ.)
9. Smirnov V. N., Ivanov K. S., Ronkin Yu. L., Levin V. Ya., Bushlyakov I. N., Lepikhina O. P., Popova O. Yu. 2005, Results of K-Ar- and Rb-Sr-dating of the Malyshevsky leucogranite massif (eastern slope of the Middle Urals). Doklady RAN [Reports of the Russian Academy of Sciences], vol. 405, no. 1, pp. 108-112. (In Russ.)
10. Mao J., Du A., Seltmann R., Yu J. 2003, Re-Os ages for the Shameika porphyry Mo deposit and the Lipovy Log rare metal pegmatite, central Urals, Russia. Mineralium Deposita, vol. 38, pp. 251-257. https://doi.org/10.1007/s00126-002-0331-2
11. Montel J.-M., Foret S., Veschambre M., Nicollet C., Provost A. 1996, Electron microprobe dating of monazite. Chemical Geology, vol. 131, issues 1-4, pp. 37-53. https://doi.org/10.1016/0009-2541(96)00024-1
12. Suzuki K., Adachi M. 1991, Precambrian provenance and Silurian metamorphism of the Tsubonosawa paragneiss in the South Kitakami terrain, Northeast Japan, revealed by the chemical Th-U-total Pb isochron ages of monazite, zircon and xenotime. Geochemical Journal, vol. 25, pp. 357-376. https://doi.org/10.2343/geochemj.25.357
13. Popova V. I., Khiller V. V., Erokhin Yu. V., Popov V. A. 2010, Monazites of late granitic pegmatites of the Ilmensky Mountains: chemical dating of the age of zonal-sectoral crystals. Novyye dannyye o mineralakh [New data on minerals], issue 45, pp. 72-78. (In Russ.)
14. Khiller V. V., Erokhin Yu. V., Zakharov A. V., Ivanov K. S. 2014, Th-U-Pb dating of granite pegmatites from the Lipovskoe ore field (Urals) for three minerals. Doklady akademii nauk [Reports of the Academy of Sciences], vol. 455, no. 2, pp. 216-219. (In Russ.) http://dx.doi.org/10.7868/ S0869565214080210
15. Suzuki K., Kato T. 2008, CHIME dating of monazite, xenotime, zircon and polycrase: Protocol, pitfalls and chemical criterion of possibly discordant age data. Gondwana Research, vol. 14, issue 4, pp. 569-586. https://doi.org/10.1016/j.gr.2008.01.005
16. Votyakov S. L., Khiller V. V., Shchapova Y. V. 2012, Composition and chemical microprobe dating of U-Th-bearing minerals. Part I. Monazites from the Urals and Siberia. Zapiski rMo [Proceedings of the Russian mineralogical society], vol. 141, no. 1, pp. 45-60. (In Russ.)
17. Vishnyakova M. D., Borodina N. S., Fershtater G. B., Soloshenko N. G., Streletskaya M. V. 2018, The migmatization and granite formation processes within the western exocontact of the Aduyskiy massif. Middle Urals. Vestnik Permskogo universiteta. Geologiya [Bulletin of Perm university. Geology], vol. 17, no. 3, pp. 208-227. (In Russ.)
18. Fershtater G. B., Gerdes A., Smirnov V. N. 2003, Age and history of the formation of the Adui granite massif. Trudy Instituta geologii i geokhimii UrO RAN [Proceedings of the Institute of Geology and Geochemistry, Ural Branch of the Russian Academy of Sciences], issue 150, pp. 146-150. (In Russ.)
19. Smirnov V. N., Ivanov K. S., Krasnobaev A. A., Bushlyakov I. N., Kaleganov B. A. 2006, Results of K-Ar dating of the Adui granite massif (eastern slope of the Middle Urals). Litosfera [Lithosphere], no. 2, pp. 148-156. (In Russ.)
20. Khiller V. V., Erokhin Yu. V. 2018, The chemical composition and dating of accessory zircon from granitic pegmatites in the north-eastern part of the Aduisky massif. Izvestiya UGGU [News of the Ural State Mining University], issue 4 (52), pp. 26-32. (In Russ.) https://doi.org/10.21440/2307-2091-2018-4-26-32
21. Khiller V. V., Erokhin Yu. V. 2021, Chemical composition and age of monazite from granite pegmatites of the Semeninskaya mine (Adui massif, Middle Urals). Izvestiya UGGU [News of the Ural State Mining University], issue 4 (64), pp. 7-15. (In Russ.) http://doi.org/10.21440/2307-2091-2021-4-7-15
The article was received on January 14, 2022
УДК 549.514.81+550.93(470.5) http://doi.org/10.21440/2307-2091-2022-2-26-32
Химический состав и возраст монацита из гранитных пегматитов Белоярской жилы (Зенковский массив, Средний Урал)
Вера Витальевна ХИЛЛЕР*
Институт геологии и геохимии им. А. Н. Заварицкого УрО РАН, Екатеринбург, Россия Аннотация
Актуальность работы обусловлена необходимостью совершенствования метода химического датирования в применении к высокоториевым акцессорным минералам, которые сложно датировать изотопными методами исследования.
Цель работы: исследование химического состава акцессорного монацита из гранитных пегматитов Белоярской жилы (Зенковский массив, Средний Урал) и определение его возраста.
Методология исследования. Количественный анализ химического состава монацита выполнен на рентгеноспектральном электронно-зондовом микроанализаторе CAMECA SX 100. Условия измерения: ускоряющее напряжение 15 кВ, сила тока 250 нА, диаметр пучка электронов 2 мкм. Давление в камере образцов 2 • 10-4 Па. Спектры получены на наклонных волновых спектрометрах, измерение интенсивности проводились по аналитическим линиям: Th Ma, U Mb, Pb Ma, Y La, Si Ka, Ca Ka, P Ka, Ce La, La La, Pr Lb, Nd La, Sm Lb, Dy La, Gd Lb. Расчет возраста проводился по известным методикам зарубежных авторов в дополнении к собственным наработкам.
Результаты. Химический состав монацита позволяет относить его к цериевой разновидности, содержание радиогенных компонентов сильно варьирует, мас. %: ThO2 - 7,88-12,43; UO2 - 0,12-0,44; PbO - 0,09-0,15. Из примесей отмечаются существенные концентрации SiO2 (до 2,4 мас.%), Y2O3 (до 2,8 мас.%) и CaO (до 0,9 мас.%). Получается, что в монаците реализуются хаттонитовый (Th4+(U4+)+Si4+ ■ REE3+ + P5+) и чералитовый (Th4+(U4+) + Ca2+(Sr2+, Ba2+, Pb2+) ■ 2REE3+) типы изоморфизма. Приличное содержание свинца и вполне хорошая кристалличность фосфата церия позволяют использовать данный минерал в качестве минерала-геохронометра.
Выводы. Получены новые данные по химическому составу монацита-(Се) и методом микрозондового датирования установлен позднепермский возраст гранитных пегматитов Белоярской жилы. Значения точечных U-Th-Pb-возрастов монацита в совокупности дают средневзвешенный возраст 255 ± 12 млн лет (СКВО = 0,10) и изохрону в 257 ± 24 млн лет (СКВО = 0,45). Полученные значения возраста для монацита из Белоярской жилы хорошо согласуются с датировками гранитных пегматитов Адуйского гранитного массива, и вполне возможно, что жильные тела Зенковского массива могут относиться к Адуйскому пегматитовому полю.
Ключевые слова: монацит, химическое датирование, гранитные пегматиты, Белоярская жила, Зенковский массив, Средний Урал.
ЛИТЕРАТУРА
1. Морозова А. В. Геохимия гранитов Среднего и Южного Урала, продуктивных на W-Mo и Be-W-Mo оруденение // Литосфера. 2006. № 3. С. 171-177.
2. Попова В. И. Радиационно-эпигенетические изменения поликраза в гранитных пегматитах Адуйского и Зенковского массивов на Среднем Урале // Литосфера. 2009. № 6. С. 90-94.
3. Попова В. И., Губин В. А. Минералогия гранитных керамических пегматитов Адуйского, Соколовского и Зенковского массивов на Среднем Урале // Уральский минералогический сборник. 2008. № 15. С. 61-74. URL: https://elibrary.ru/item.asp?id=23876011
4. Попова В. И., Губин В. А., Попов В. А., Баженова Л. Ф. К характеристике некоторых акцессорных минералов пегматитов Зенковского и Адуйского гранитных массивов // Материалы к минералогии рудных районов Урала. Свердловск: УрО АН СССР, 1988. С. 67-73.
5. Попова В. И., Чурин Е. И., Блинов И. А., Губин В. А. Фергусонит-^) и продукты его изменения в гранитном пегматите жилы Береговой Зенковского массива на Урале // Новые данные о минералах. 2012. Вып. 47. С. 47-55.
6. Грабежев А. И. Метасоматизм, рудообразование и гранитный магматизм. М.: Наука, 1981. 292 с.
7. Геохимические критерии редкометальной рудоносности гранитов (на примере Урала) / А. И. Грабежев [и др.]. Свердловск: УНЦ АН СССР, 1987. 128 с.
8. Морозова А. В. Петрографическая и геохимическая характеристика гранитов Зенковского массива (Средний Урал) // Труды Института геологии и геохимии УрО РАН (Ежегодник-2005). 2006. Вып. 153. С. 179-182.
9. Смирнов В. Н., Иванов К. С., Ронкин Ю. Л., Левин В. Я., Бушляков И. Н., Лепихина О. П., Попова О. Ю. Результаты K-Ar- и Rb-Sr-датирования Малышевского лейкогранитного массива (восточный склон Среднего Урала) // Доклады РАН. 2005. Т. 405. № 1. С. 108-112.
10. Mao J., Du A., Seltmann R., Yu J. Re-Os ages for the Shameika porphyry Mo deposit and the Lipovy Log rare metal pegmatite, central Urals, Russia // Mineralium Deposita. 2003. Vol. 38. P. 251-257. https://doi.org/10.1007/s00126-002-0331-2
11. Montel J.-M., Foret S., Veschambre M., Nicollet C., Provost A. Electron microprobe dating of monazite // Chemical Geology. 1996. Vol. 131. Issues 1-4. P. 37-53. https://doi.org/10.1016/0009-2541(96)00024-1
EDhiivervit@mail.ru
https://orcid.org/0000-0001 -8491 -4958
12. Suzuki K., Adachi M. Precambrian provenance and Silurian metamorphism of the Tsubonosawa paragneiss in the South Kitakami terrain, Northeast Japan, revealed by the chemical Th-U-total Pb isochron ages of monazite, zircon and xenotime // Geochemical Journal. 1991. Vol. 25. P. 357-376. https://doi.org/10.2343/geochemj.25.357
13. Попова В. И., Хиллер В. В., Ерохин Ю. В., Попов В. А. Монациты поздних гранитных пегматитов Ильменских гор: химическое датирование возраста зонально-секториальных кристаллов // Новые данные о минералах. 2010. Вып. 45. С. 72-78.
14. Хиллер В. В., Ерохин Ю. В., Захаров А. В., Иванов К. С. Th-U-Pb-датирование гранитных пегматитов Липовского рудного поля (Урал) по трем минералам // Доклады АН. 2014. Т. 455. № 2. С. 216-219. http://dx.doi.org/10.7868/S0869565214080210
15. Suzuki K., Kato T. CHIME dating of monazite, xenotime, zircon and polycrase: Protocol, pitfalls and chemical criterion of possibly discordant age data // Gondwana Research. 2008. Vol. 14. Issue 4. P. 569-586. https://doi.org/10.1016/j.gr.2008.01.005
16. Вотяков С. Л., Хиллер В. В., Щапова Ю. В. Особенности состава и химическое микрозондовое датирование U-Th-содержащих минералов. Ч. 1. Монациты ряда геологических объектов Урала и Сибири // Записки РМО. 2012. Ч. 141. № 1. С. 45-60.
17. Вишнякова М. Д., Бородина Н. С., Ферштатер Г. Б., Солошенко Н. Г., Стрелецкая М. В. Процессы мигматизации и гранитообразования в западном экзоконтакте Адуйского массива (Средний Урал) // Вестник Пермского университета. Геология. 2018. Т. 17. № 3. С. 208-227.
18. Ферштатер Г. Б., Гердес А., Смирнов В. Н. Возраст и история формирования Адуйского гранитного массива // Труды Института геологии и геохимии УрО РАН (Ежегодник-2002). 2003. Вып. 150. С. 146-150.
19. Смирнов В. Н., Иванов К. С., Краснобаев А. А., Бушляков И. Н., Калеганов Б. А. Результаты K-Ar датирования Адуйского гранитного массива (восточный склон Среднего Урала) // Литосфера. 2006. № 2. С. 148-156.
20. Khiller V. V., Erokhin Yu. V. The chemical composition and dating of accessory zircon from granitic pegmatites in the north-eastern part of the Aduisky massif // Известия УГГУ. 2018. Вып. 4 (52). С. 26-32. https://doi.org/10.21440/2307-2091-2018-4-26-32
21. Khiller V. V., Erokhin Yu. V. Chemical composition and age of monazite from granite pegmatites of the Semeninskaya mine (Aduy massif, Middle Urals) // Известия УГГУ. 2021. Вып. 4 (64). С. 7-15. http://doi.org/10.21440/2307-2091-2021-4-7-15
Статья поступила в редакцию 14 января 2022 года
