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254. Ковдерко В. Э. Алогизмы и патовые ситуации в естествознании. /В. Э. Ковдерко. -Гомель: ГГТУ им. П. О. Сухого, 2015. - 99 с. : ил. - ISBN 978-985-535-242
2020. 02. 17
INVESTIGATION MAGNETIC FIELD AND ROCK MAGNETISM DIVERSITY OF THE MELNYKOVTCI PLOT OF THE UKRAINIAN SHIELD
ReshetnykM.1, Starokadomsky D.2, Popov C.3, Khomenko R.4
1 - PhD geology, senior researcher, geological department, The National Museum of Natural History at the National Academy of Sciences of Ukraine 2 - PhD chemistry, senior researcher, M.P. Semenenko Institute of geochemistry, mineralogy and ore formation of the National Academy of sciences of Ukraine and acad.Chuiko Institute of Surface Chemistry of the National Academy of sciences of Ukraine
3 - PhD geology, directory of geophysical laboratory, Educational and Scientific Institute "Institute of Geology",
Taras Shevchenko National University of Kyiv
4 - PhD geology, engineer of geophysical laboratory, Educational and Scientific Institute "Institute of Geology",
Taras Shevchenko National University of Kyiv .
Аннотация. Методом магнитного сканирования проведено исследование обнажающегося участка Украинского Щита в его юго-западной части (Побужье). Определены значения индукции Т магнитного поля (49750+100 nT, без пересчёта на глобальное поле), и показатели магнитной восприимчивости MS "in-situ". Установлено что значения MS изменяются в пределах от 010-3 u.SI до 5010-3 u.SI, с присутсвием одиночных областей более 5010-3 u.SI. Не обнаружено регулярных прямых корреляций и закономерностей изменения показателей MS и Т. Сделана попытка выявить корреляции между колебаниями магнитного поля и перепадами MS. Полученные результаты интерпретированы с позиций локальной тектоник. Образцы, отобранные с обнажения, были изучены на плотнеость, гистерезис термомагнитных свойств, ориентацию остаточной намагничеснности.
Abstarct. The magnetic scanning method was used to study the exposed section of the Ukrainian Shield in its southwestern part (Pobuzhie region). The values of induction T of the magnetic field (49750+-100 nT, not counting on the global field), and the magnetic susceptibility MS indices "in situ" were determined. It was found that the MS values vary in 0 - 50 • 10-3 u.SI, with the presence of single regions of more than 50 • 10-3 u.SI. Not found regular direct correlations and patterns of changes in the indicators MS and T. An attempt has been made to reveal correlations between fluctuations of the magnetic field and MS-differences. The results are interpreted from the standpoint of local tectonics. Samples taken from exposure, were studied for density, hysteresis of thermomagnetic properties, orientation of the residual magnetization.
Introduction
Ukraine has a favorable position for development of geophysical methods for solving the problems of mapping the Precambrian basement. A wide network of rivers and a large number of quarries open the Ukrainian Shield (USh). In fact, geologists are in a situation where "through the cracks in the fence, we need to see a complete picture". Geophysical methods and (in the first place - Magnetic Exploration) serve to assist the restoration of the geological structure of the USh. In [5] no homogeneity orientation natural and remanent magnetization rocks of USh is explained for local tectonic. A great advantage in the study of USh is the climate, which allows half a year to work in the field and the availability of roads everywhere. And most importantly, a lot of exploration results have been accumulated in Ukraine, which is practically absent in the world [2]. In addition, the USh is one of the oldest structures in the world, the Precambrian foundation which lies on the surface [13]. It is the "key to understanding" the formation of the primary Earth's crust. Today's development of science/technology makes a detailed study and "re-mapping" of even the most complex geological structures of USh profitable [3,4]. However, now this is not a priority for the policy of our country. At the same time, Western or Russian research teams are technically unable to organize field expeditions to USh. All this makes new information from local geological groups about USh very valuable all over the world.
Our geophysical team (scientists of NMNH Geological Department, Taras Shevchenko NUK Institute of Geology and institute of Geochemistry) has 20 years of fieldwork experience at USh. We are one of the few Ukrainian geological groups that are able to do further field geological and geophysical studies of the USh and scientific analysis of the data that have been obtained. Our work prompted the creation of repair reinforcing materials and modification of devices, led to the creation of a magnetic scanning method [1, 14, 15].
In the geological mapping of the Precambrian basement, field geological work is usually done separately from field magnetometric, and laboratory studies [6]. The results obtained are artificially combined, all of which lead to significant errors in the interpretation of magnetic data. This work demonstrates the results of the
implementation of a comprehensive and detailed method of magnetic scanning in the practice of geological and geophysical works.
The term "Magnetic Scanning" is borrowed from the English-language geological literature [12] and is understood as sequential reading of magnetic information (magnetic field induction T, magnetic susceptibility MS) by scanners (magnetometer, kappametr) step by step from the surface of the object [7, 10]. The basic algorithm of the Magnetic Scanning method is described in [10] consists in a detailed analysis and combination of the data of performed geological\geophysical surveys. Magnetic Scanning method includes the taking of new data, the selection of oriented samples of rocks and their laboratory studies, the subsequent interpretation using comparative magnetic analysis [11]. Precambrian mesostructures it is legitimate to state that similar geological formations correspond to such ultradetal T curves.
The research site is located within the Raigorodskiy stream between the villages Slobodka and Melnykovtci (Nemyriv district, Vinnitsa oblast). In this area, there is a sub-latitudinal orientation of the isolines of the anomalous magnetic field, and several high-intensity positive anomalies are distinguished. When compared with the geological map, the magnetic field map is poorly matched (Fig. 1). The nature of these local and high-intensity positive magnetic field anomalies remains unclear (see Fig.1). It is well known that competent interpretation of maps of anomalous magnetic field allows to decipher the geological structure [3].
Fig.1. Fragment of geological map and maps of anomalous gravimagnetic fields (plot Melnykovtci). Symbols: 1 - granites, 2 - plagiogranites, 3 - diorites, 4 - faults, 5,6,7 - isolines of high-, zero-, and low-values of anomalous magnetic field (nT), 8 - isolines ofpositive Bouguer anomalies (0.2 mGal).
By black squares indicate areas with high magnetite content.
Analysis of the ultradetal curve T, that was obtained by magnetic scanning along the Raigorodskiy stream in south of Melnykovtci village showed that Taverage = 49750+100 nT (fig.2). The first 150 m in curve T is characterized by generally lower Taver values of the field and a high frequency of change T. While the second sector (100 m) fragment AB on the curve has higher values of T. Areas of low values, unlike high, have different appearance. While the next region (200 m) is exact average values of the obtained T. The fragment CD is higher values of T like AB. Last area unlike other (20 m), is the lowest and with a two deep minimum (<49300 nTl).
It is possible to pick out two high velue regions AB and CD. The T curve in the regions AB and CD exceed value 5000 nT and reach high value 51300 nT. These fragments T curve with high value is a "conglomerate" of several maximums (the gap in fragment AB on the curve T may be is little maximum). The fragment AB and CD is so similar (fig.3). According comparative magnetic analysis give a chance to expect what these anomalies created by two similar sources. That situation is another plot described in [8, 9]. The profile crosses two magnetic field magnetic field anomalies elongated in the north-east direction (see fig. 1) these anomalies crossed on the AB and CD fragments on the ultradetail curve T.
MS studies in-situ have been performed fragmentarily on the outcrops, in particular in the field of small field values. From fig.4 it can see - the different areas within the studied area are characterized by a noticeable difference in the amplitudes of the oscillations of the curve MS.
Studies show that while T is oscilated near T=49750 nTl, 90% of MS-measurements vary from 0.01 to 10-103 uSI (non-magnetic or weak magnetic) but with inclusions MS = 50-100 10-3 uSI. Areas of small values (4910049300 nT) and are corresponding to the lowest values of MS - less than 110-3 uSI. That is, in this case we have good consistency between the measured values of MS and T. On last plots in fig.4 there are two types of rocks with a predominance of MS<10 10-3 u.SI (90% of measurements) and above, and with MS<1 • 10-3 u.SI.
Where the field values are usually higher than 50000 nT, the values of MS are correspondly higher. Indeed, about 40% of the values MS vary in 0-1010-3 u.SI, and another 40% - MS= 10-2010-3 u.SI, there are already higher values of MS - up to 4010-3 u.SI (almost 20% of measurements). The same distribution of magnetic susceptibility values is observed on 3-510-3 u.SI investigated areas (20% of measurements for MS 0-510-3 u.SI, 20% - for 5-10-10-3 u.SI, 20% - 10-15 10-3 u.SI, 20% - 15-2010-3 u.SI), where field values are still increasing (up to 50300 nT). Here, we see some (and more complicated) correlation between the values of T and MS: T grows with growing of MS.
This may indicate a certain heterogeneity of rocks in the study area, with values MS everywhere being low (with rare exceptions in Fig.4).
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Fig.4. MS of the studied on the outcrops along the Raygorodsky stream.
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The distribution of MS on the vertical surface of the quarry is shown in Figure 5. The quarry is a combination of rocks with different magnetic properties. On it, you can select areas of increased MS (A, B) and low MS (G). The quarry is represented in its upper part by pink granites, and in the lower part by dark gray, with areas of increased MS (30-50 • 10-3 u.SI). A more detailed picture is given by the detailed surface capacitance. So, fig.5 shows that non-magnetic and low-magnetic rocks are extremely unevenly distributed, and form vast arrays surrounded by magnetic or comparatively-magnetic rocks. Figure 5 shows three such arrays. Among the nonmagnetic and comparatively magnetic ones, single absolutely nonmagnetic inclusions (MS<110-3 u.SI) are constantly encountered, as indicated by the analysis of the samples.
Fig.5. Magnetic susceptibility distribution along the wall of the Forest quarry. The value ofMS10-3 u.SI change: 0-3 - white, 3-10 - raw, 10-30 - pink, 30-100 - purple, >100 - bleak crosses.
According to analysis of samples of rocks selected from these sites, their similarity is also observed. For samples in the interval AB, varies on 30-65 10-3 u.SI, for samples with a AB fragment of the T curve MSab=20-60 10-3 u.SI. MSab>MScd may be that the deviations are less upturned here (tab.1). Factor In varies from 0.7 to 1.2 A/m and density of 2.65 g/cm3 - for samples at both intervals. Such density challenges the thesis that plagiogranites and granites are deposited here, or perhaps they are highly migmatized. The rocks at the studied intervals include a small number of xenoliths with high MS 70-100 -10-3 u.SI.
Fig.6. Xenoliths in granites have higher MS.
According to detailed surface cappametry (fig.5), high-magnetic rocks in the section quarry "Forest" (tab.1) occur rarely and are located single or compact in small areas. Comparison of the detailed cappametry data with the data of tab. 1 indicates the need to combine these methods to obtain complete information. Really, even the selection of a large number of oriented samples does not allow one to know the exact distribution of MS on the surface.
Despite the practical absence of high magnetic inclusions, a significant proportion (16%) of the quarry samples are magnetic 45<MS<8510"3 u.SI. This distinguishes them from other distractions in the area, in particular from the section outcrops along Rajgorodsky stream (tab.1), where there are relatively many highly magnetic xenoliths but few magnetic samples. Since in the other groups (non-magnetic, low-magnetic, relatively magnetic) concentrates by 22-25% of the samples, it is difficult to distinguish the modal value (it has a large error, fig.4 & Tab. 1), and the proportion of these samples (22%) is smaller than usual (25-40%).
Analysis of the samples shows that non-magnetic rocks (except array of four samples) are single inclusions, usually in contact with weak and comparatively magnetic samples. The analysis of the samples indicates the dominance of rocks with medium and (especially) relatively high values of In (tab.1) - in sum, these two groups make up 62% of the samples. The maximum values of In are many times smaller, and the high values of In are smaller here than on the adjacent array of outcrops along Rajgorodsky stream sites (rich in magnetic xenoliths). Only relatively low and low In samples are present in a relatively large number, amounting to 30% of the samples. Unlike the outcrops along Rajgorodsky stream sites, there is a much smaller discrepancy between In values, in particular the Im / Imodal ratio is only 2.2. This indicates greater uniformity of the rocks. The high value of the Imach / Imin ratio = 1.3 says confirms the surface capacitance data (which detects highly magnetic and nonmagnetic arrays in the quarry (fig.5). A very weak correlation of In /Ii = Q can be noted, unlike other plots. It is manifested in some cases by the Qmach-Qmin correspondences, but the Qmin-Qmas correspondences are not noticeable. Almost half of the samples have Q 1 0.2, and most of the other samples are down to only two Q values.
Table 1.
All obtained properties of the simples selected at the points of the studied area.
Simples selected in the quarry "Forester"
Mark MS-10-3, uSI In, A/m Q J, o D, o Is, A/m J Is, o D Is, o p, r/CM3
1 1.21.07.08 28,7 1,12 1,1 58 280 2,03 66 317 2,70
2 2.21.07.08 54,2 1,63 0,9 24 241 2,70 62 280 2,64
3 3.21.07.08 32,4 1,18 1,1 64 33 2,34 64 17 2,69
4 5.21.07.08 39,5 1,22 0,9 69 248 2,48 75 326 2,61
5 1.22.07.08 11,7 0,79 2,0 41 275 1,10 54 294 2,66
6 2.22.07.08 9,3 0,55 1,7 48 280 0,82 59 303 2,57
7 3.22.07.08 47,6 1,24 0,8 47 272 2,69 64 317 2,66
8 4.22.07.08 22,1 1,10 1,5 49 253 1,67 66 285 2,61
9 5.22.07.08 12,9 1,07 2,4 59 280 1,46 65 301 2,49
10 6.22.07.08 19,6 1,30 1,9 41 49 1,93 50 37 2,66
11 7.22.07.08 29,3 1,53 1,5 21 295 2,25 41 311 2,76
12 8.22.07.08 25,4 1,04 1,2 67 325 1,94 66 345 2,77
13 9.22.07.08 33,7 0,80 0,7 23 86 1,71 54 50 2,62
14 10.22.07.08 4,9 0,27 1,6 54 87 0,42 63 60 2,60
15 12.22.07.08 14,1 0,76 1,6 18 293 1,08 39 309 2,63
16 13.22.07.08 6,3 0,15 0,7 56 313 0,37 61 341 2,53
17 14.22.07.08 40,2 0,79 0,6 56 337 2,23 60 353 2,60
18 15.22.07.08 3,0 0,06 0,5 -71 5 0,08 32 4 2,43
19 18.22.07.08 16,7 0,28 0,5 27 207 0,67 80 306 2,61
20 19.22.07.08 18,8 0,33 0,5 43 205 0,84 81 322 2,57
21 19.22.07.08(2) 37,8 0,78 0,6 23 200 1,53 82 266 2,61
22 20.22.07.08 28,0 0,90 0,9 67 348 1,91 64 357 2,75
23 21.22.07.08 0,1 0,03 8,5 15 260 0,03 22 264 2,52
24 25.22.07.08 12,9 0,45 1,0 63 43 0,91 64 22 2,63
25 27.22.07.08 32,9 0,71 0,6 42 278 1,73 62 323 2,61
26 28.22.07.08 0,7 0,02 0,8 62 290 0,04 66 333 2,40
27 29.22.07.08 33,5 1,17 1,0 79 169 2,24 80 11 2,67
28 30.22.07.08 38,7 1,10 0,8 75 269 2,40 73 340 2,64
29 31.22.07.08 2,4 0,07 0,8 63 258 0,14 72 321 2,59
30 33.22.07.08 3,3 0,18 1,6 57 229 0,27 74 264 2,43
31 36.22.07.08 21,1 1,13 1,6 2 57 1,56 27 45 2,83
32 36'.22.07.08 20,0 1,21 1,8 -1 42 1,62 22 34 2,92
33 37.22.07.08 10,9 0,58 1,6 38 220 0,78 65 240 2,80
34 39.22.07.08 4,7 1,29 7,9 40 246 1,37 46 250 2,83
35 40.22.07.08 51,1 1,55 0,9 64 303 3,32 66 337 2,66
36 41.22.07.08 29,5 1,15 1,1 45 244 1,89 68 282 2,71
37 42.22.07.08 62,6 2,65 1,2 78 317 4,84 72 348 2,65
38 43.22.07.08 14,1 2,02 4,2 26 256 2,20 37 263 2,79
39 44.22.07.08 23,2 0,83 1,1 54 194 1,41 85 232 2,71
40 45.22.07.08 83,8 2,00 0,7 67 238 4,66 75 332 2,72
41 46.22.07.08 54,3 1,97 1,1 22 25 3,67 42 18 2,74
42 47.22.07.08 78,1 2,39 0,9 72 349 5,19 66 358 2,78
43 50.22.07.08 58,8 2,13 1,1 80 121 4,08 77 24 2,69
44 51.22.07.08 17,6 0,61 1,0 46 63 1,19 57 38 2,62
45 55.22.07.08 12,5 0,42 1,0 52 236 0,76 74 289 2,60
46 56.22.07.08 2,4 0,07 0,9 42 240 0,13 69 288 2,58
47 57.22.07.08 7,3 0,06 0,3 16 354 0,31 53 0 2,58
48 58.22.07.08 0,8 0,08 2,8 21 246 0,09 38 256 2,54
49 59.22.07.08 1,4 0,02 0,5 30 11 0,07 52 7 2,49
50 61.22.07.08 23,9 0,54 0,7 80 284 1,36 71 352 2,54
Simples selected in the outcrops along Rajgorodsky stream in-south of village Melnikovtsi
51 1.23.07.08 0,4 0,04 3,6 28 270 0,05 40 279 2,56
52 2.23.07.08 0,2 0,05 7,0 53 97 0,06 57 90 2,63
53 3.23.07.08 159,7 2,46 0,5 58 270 7,77 67 337 2,88
54 3.23.07.08 153,3 2,53 0,5 48 279 7,54 64 333 2,88
55 4.23.07.08 188,1 2,07 0,3 58 357 8,90 60 2 2,75
56 6.23.07.08 35,2 0,90 0,7 20 260 1,71 57 300 2,60
57 7.23.07.08 44,4 0,94 0,6 48 115 2,26 69 50 2,54
58 9.23.07.08 56,5 2,14 1,1 38 258 3,57 61 292 2,57
59 11.23.07.08 1,6 0,24 4,4 23 224 0,24 36 229 2,41
60 12.23.07.08 53,9 12,09 6,6 65 190 13,34 71 191 2,56
61 12a.23.07.08 80,8 15,76 5,7 83 46 18,47 81 28 2,47
62 13.23.07.08 98,5 1,36 0,4 29 271 4,32 62 328 2,90
63 14.23.07.08 37,4 0,94 0,7 75 311 2,26 68 350 2,46
64 15.23.07.08 36,5 1,27 1,0 59 12 2,59 60 7 2,55
65 16.23.07.08 33,8 1,14 1,0 65 31 2,35 64 16 2,50
66 17.23.07.08 21,2 0,83 1,1 47 352 1,58 54 356 2,84
67 19.23.07.08 6,9 0,15 0,6 54 350 0,40 59 358 2,90
According to the thermomagnetic analysis of the samples, the main ferromagnet is magnetite, and possibly maghemite, since the heating curve has a slight increase, which decreases at 300 ° C (fig. 7). There are no kinks in the cooling curve, which means that indeed the increase in the heating curve showed that maghemite turned into hematite.
Fig. 7. Heating-cooling hysteresis curves for samples of rocks studied.
We can say that this site is a typical representative of the highly metamorphosed rocks of USh. The physical data obtained may be useful for understanding the situation in other areas of the USh.
Conclusions
1. New data on the study of the granites of the plot Melnykovtci of the Ukraine shield were obtained. It is established that the rocks deposited in the studied area are composed of non or low-magnetic light granites, and middle-magnetic dark rocks.
2. It is shown that, within the investigated areas, the magnetic susceptibility indices are generally not high (within 1 - 6010-3 u. SI), with absolutely nonmagnetic (MS<110-3 u. SI). The rocks at the outcrops plot Melnykovtci include a small number of xenoliths with high MS 70-100-10-3 u.SI.
3. Termomagnetic analysis showed a presents of magnetic minerals - magnetite and maghemite.
4. Therefore, we can assume that there is a fragments own geological body in the area studied. This assumption completely changes the geological mesostructure previously depicted on the geological map. This demonstrates the effectiveness of the Magnetic Scanning method for geological mapping purposes.
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