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The U-Pb geochronology of the Mukhal alkaline massif (western Transbaikalia)
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Russian Geology and Geophysics 53 (2012) 169–174 www.elsevier.com/locate/rgg
The U-Pb geochronology of the Mukhal alkaline massif (western Transbaikalia) A.G. Doroshkevich a, G.S. Ripp a,*, S.A. Sergeev b, D.L. Konopel’ko c a
b
Geological Institute, Siberian Branch of the Russian Academy of Sciences, ul. Sakh’yanovoi 6a, Ulan Ude, 670047, Russia A.P. Karpinsky All-Russian Research Geological Institute, Center of Isotope Studies, Srednii pr. 74, St. Petersburg, 199106, Russia c St. Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg, 199034, Russia Received 2 June 2010; accepted 9 November 2010
Abstract We present results of U-Pb (SHRIMP II) geochronological study of the rocks of the Mukhal alkaline massif in the Vitim alkaline province, western Transbaikalia. The available K-Ar and Rb-Sr dates for the alkaline rocks (Saizhen complex) of the Vitim province, including the Mukhal massif, vary over a broad range of values. The obtained age of crystallization of the Mukhal urtites refines the time when the regional alkaline magmatism began. The age of zircons and magmatic processes within the Barguzin area (315–275 Ma) is close to the peak of main events, which occurred between 295 and 275 Ma. These processes took place at the early stage of evolution of the Late Paleozoic rift system in Central Asia, whose activity was associated with the activity of mantle superplume. © 2012, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: geochronology; alkaline magmatism; Mukhal massif
Introduction In the Late Paleozoic, the Central Asian Fold Belt (CAFB) was involved in global intraplate processes, which formed a rift system stretching along the southern margin of the North Asian continent for more than 3000 km and being up to 600 km in width (Kovalenko et al., 2002; Yarmolyuk, 2009; Yarmolyuk et al., 2000). The system is composed of rift zones of different ages including large zonal magmatic areas: Barguzin, South Mongolian, Hangayn, and Henteyn. At the early stage of its evolution, the Barguzin magmatic area formed, which includes the Angara–Vitim batholith more than 150,000 km2 in area and the Vitim alkaline-magmatism zone. These zones abound in massifs of alkaline ultrabasic and basic rocks and alkali and nepheline syenites. The Vitim province unites more than 20 massifs of alkaline rocks (Saizhen complex) in a band of NE strike, >450 km long and ~50 km wide (Fig. 1). The massifs were assigned to an alkali-gabbroid association (Konev, 1982), based on the coexistence of gabbro-pyroxenites, nepheline-pyroxene rocks of the urthite-jacupirangite series, and nepheline and alkali syenites. By the set of rocks, these massifs can be assigned
* Corresponding author. E-mail address: [email protected] (G.S. Ripp)
to an alkali-ultrabasic association according to the modern classification (Petrographic Code, 2009). The results of geological mapping evidence that alkaline rocks intrude Precambrian and Cambrian strata and are overlain by Upper Mesozoic–Cenozoic deposits. The available geochronological data on these rocks (Andreev and Sharakshinov, 1967; Andreeva, 1982; Konev, 1982; Konev et al., 1975; Shaposhnikov et al., 1991; Zaguzin et al., 1976) vary widely even within a massif, which might be due to the study of isotope systems poorly resistant to superposed processes. The ages of the rocks of the Saizhen complex do not ensure unambiguous conclusions about the time of alkaline magmatism and its duration in the region. Therefore, to determine the more accurate time of the massif formation, it is necessary to use other, more precise methods for rock dating. We chose the Mukhal massif for study and performed a U-Pb SIMS SHRIMP-II dating of accessory zircon from urthites.
Investigation technique For U-Pb dating, we separated zircon from urthites (core from BH-13, depth ranges 167.8–168.2 and 355.5–362.5 m). The isotope studies were carried out at the Center of Isotope Studies of the All-Russian Research Geological Institute,
1068-7971/$ - see front matter D 201 2, V . S. S o bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2011.12+.013
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Fig. 2. Schematic geologic structure of the Mukhal massif, with its assumed outlines beneath the basaltic cover, after Sharakshinov (1984). 1, crystalline limestones; 2, skarning zones; 3, ijolites; 4, urthites; 5, lines of disjunctions: a, proved, b, assumed; 6, erosion “window” outlines; 7, boundaries of facies transitions; 8, localities of sampling for geochronological studies.
Fig. 1. Schematic occurrence of alkaline massifs in the Vitim province, after Yarmolyuk et al. (1997). Igneous complexes (1–4): 1, Barguzin, 2, Zaza, 3, Chivyrkui, 4, massifs of alkaline basites and syenites of the Saizhen complex (asterisked is the Mukhal massif); structure-facies zones (5–11): 5, Kotera– Udokan, 6, Baikal–Muya, 7, Mama–Bodaibo, 8, Dzhida–Vitim, 9, Selenga– Stanovoi, 10, Chara–Udokan, 11, Siberian Platform, 12, boundaries of structureformational zones.
St. Petersburg. The manually sampled crystals were implanted into epoxy resin together with grains of the TEMORA and 91500 International geochronological zircon standards. The dot of local U-Pb dating was chosen, using optical, BSE, and cathodoluminescent images reflecting the internal structure and zoning of zircon crystals. The U/Pb ratios were measured following the technique described by Williams (1998). The intensity of a primary O2 beam was 4 nA, the diameter of the sampling spot (crater) was 25 µm, and the depth of sampling reached 5 µm. The data obtained were processed using the SQUID program (Ludwig, 2000). The U/Pb ratios were normalized to the value of the TEMORA standard zircon, 0.0668, which corresponds to an age of 416.75 Ma (Black et al., 2003). The errors of singular analyses (determination of U/Pb ratios and ages) are at the 1σ level, and the errors of the calculated concordant ages are at the 2σ level. The plots with a concordia were constructed using the ISOPLOT/EX program (Ludwig, 1999).
Geological characteristics of the Mukhal massif The massif is one of the largest in the Vitim province and is of practical importance as a source of nepheline raw materials. Its rocks expose in an “erosion window” 0.5 ×
1.5 km in size among the overlying Cenozoic basalts (Fig. 2). The total area of the massif, determined by drilling, is more than 5 km2. Riphean limestones with dolomite intercalates are host rocks. The massif has steep, close to vertical, contacts with carbonate rocks. It is composed mainly of urthites grading in the northern direction into ijolites and, then, melteigites. Dikes of nepheline and alkali syenites are subordinate. The massif formed in three stages (Vrublevskaya, 1988): (1) ijolites, (2) urthites and melteigites, and (3) syenites. The urthites, ijolites, and melteigites are of fine- to mediumgrained texture and massive-banded and trachytoid structure. They have variable contents of nepheline, aegirine-augite, hastingsite, riebeckite, and calcite. Secondary and accessory minerals are garnet, titanite, apatite, magnetite, cancrinite, biotite, and zircon. The syenites consist mainly of microcline, oligoclase, and nepheline with accessory apatite, magnetite, and titanite. Magnesian and calcareous skarns are confined to the endocontacts of ijolites with limestones and their xenoliths, at which brucite marbles are developed. According to Konev (1982), this points to a shallow depth of the intrusion formation and the high temperature of the parental magma. Metasomatic and postmagmatic processes are of different intensities; these are garnetization, albitization, microclinization, and nephelinization. The rocks show great variations in chemical composition, with a gradual increase in SiO2 (39–63 wt.%) and, partly, alkali (6–16 wt.%) contents and a decrease in Fe (10– 1.2 wt.%) and Ca (16–1.2 wt.%) contents in passing from early to late rocks formed at the final stage of the massif formation. All rock varieties have low contents of Mg (0.1–3.9 wt.%) and Ti (≤1.2 wt.%) and high contents of Al (up to 28 wt.%); Na2O/K2O = 2–3. The Nb/U ratios in the ijolites and urthites vary from 100 to 175, and those in the syenites, from 22 to 34. The Zr/Nb
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Table 1. The age of alkaline rocks of the Mukhal massif, Vitim province Method
Rock
Material under study
Age, Ma
Source
K-Ar
Melteigite
Nepheline
370
(Zaguzin et al., 1976)
K-Ar
Melteigite
Aegirine-augite
294
(Zaguzin et al., 1976)
K-Ar
Melteigite
Amphibole
341
(Zaguzin et al., 1976)
K-Ar
Urthite
Rock
337
(Sharakshinov, 1984)
K-Ar
Melteigite
Nepheline
247
(Zaguzin et al., 1976)
Rb-Sr
Ijolite-urthites
Rock, K-feldspar
279 ± 16
(Sharakshinov et al., 1991)
K-Ar
Congressite
Rock
312.5 ± 3.5
(Konev et al., 1975)
ratios also fall in two ranges: high values for the ijolites and urthites (4–7) and low values for the syenites (~0.5). The La/Yb ratios are also higher in the syenites (23–24) as compared with the ijolite-urthites (11–16). All rock varieties have high contents of REE (120–285 ppm), Sr (up to 1400 ppm), and Ba (up to 1500 ppm). The K-Ar and Rb-Sr dating (Andreev and Sharakshinov, 1967; Andreeva, 1982; Konev, 1982; Konev et al., 1975; Sharakshinov et al., 1991; Zaguzin et al., 1976) yielded an Early Triassic-Late Devonian age of the massif rocks (Table 1), 247–370 Ma.
Results of geochronological studies Zircon from the urthites has large (200–600 µm) transparent crystals of short-prismatic habit. On the cathodoluminescent
images, most of the grains show genetically different crystal domains (Fig. 3, dot 5.1). All grains have thin light secondary overgrowth or recrystallization rims (Fig. 3, dots 4.2 and 12.1). Some crystals (Fig. 3, dots 5.2 and 9.1) have complex inner domains with irregular luminescence and abundant inclusions. In general, the zircons have high contents of U and Th (Table 2), which is expressed as the suppression of cathodoluminescence (Fig. 3, dots 4.1, 6.1, and 8.1). The crystals with the best expressed signs of magmatic origin (morphology, geochemistry, internal structure) and without mechanical defects (cracks, inclusions) contain, on the average, 1830 ppm U (Th/U = 0.4). The overgrowth/recrystallization rims contain ~200 ppm U (Th/U = 0.1), and the heterogeneous domains have abnormal Th/U ratios (up to 2.8) because of the high Th content (U = 1200–1800 ppm). The results of isotope analyses are listed in Table 2 and in Fig. 4, a, b. A total of 14 analyses of 11 grains were made.
Fig. 3. Cathodoluminescent images of zircons extracted from the urthites of the Mukhal massif. White circles show the location of the sampling crater within the crystal.
172
Note. Errors are at the level 1σ; Pbc and Pb*, common and radiogenic lead, respectively. The error of the standard calibration was 0.70% (ignored in the above errors but taken into account on comparison of results from different samples). (1), correction for common lead was made by measured 204Pb.
0.170 2.1 0.0531 12 12 0.0516 2.1 18.8 5 0.0812 2 18.11 334 ± 6.9 3.67 U1.9.1
1161
1413
1.26
55.1
18.72 323 ± 5.9 3.75 U1.5.2
1758
4747
2.79
80.6
19.78
1.8
0.0852
1.6
19.45
1.9
6.8
0.378
0.816
0.268 1.9 0.05135 7
0.622
0.0551
0.39
1.8
1.9 0.04929
0.0505 2.2
3.1 0.353
0.05228
0.364
2.4
1.3
0.0519 1.9
1.8 19.8
20.29 2
1.1 0.05313
0.0534 20.25
205.0
318 ± 5.6
0.99
0.16 716
1329
0.11 U1.7.1
1391 0.19 U1.2.1
4714
59.0
310 ± 5.8
1.8
1.3 1.8 20.32 1.2 0.05244 1.8 20.31 0.04 U1.1.1
3490
373
0.11
148.0
310 ± 5.4
1.9
0.158
0.815 1.8 0.04922 2.2
0.530
0.05209
0.3535
6.5
1.9 0.04693
0.0474 41
3.5
41
0.63
3
0.097 6.5 21.1
21.31 1.8
6.9 0.157
0.05405
19.39
1.9 21.25 296 ± 5.4 55.9 0.07
0.04 1
100
7.89 U1.4.2
32
0.30 U1.6.1
1384
1.4
299 ± 19
4
1.9
0.0517
0.334
0.473
0.436 1.9
1.9 0.04653
0.04669 4.3
3.9 0.334
0.323
3.5
3.9
0.052
0.0501
1.9
1.9 21.41
21.49 2.1
1.8 0.05367
0.0561
21.32
1.9 21.38 81.7
293 ± 5.3
0.61
0.82 1614
781 U1.5.1
1320
0.50
0.44
U1.10.2
2033
53.2
294 ± 5.4
1.9
0.430
0.726 1.8
1.9 0.04622
0.0462 2.5
4.5 0.343
1.7
4.1
0.05139 1.8 21.65
21.63 2.2
1.2 0.05288
0.0569 1.9
21.61
21.55 36.3
291 ± 5.5
0.12
0.68 596
431
0.39 U1.13.1
3743 0.19 U1.4.1
910
149.0
291 ± 5.2
1.8
1.9
0.0538
0.3273
0.724
0.404 1.9
1.8 0.04469
0.04615 4.7
2.5 0.3236
0.321
1.7
4.3
0.05252
0.0504
1.8
1.9 21.67
22.38 1.6
1.7 0.05424
0.05356 1.8
1.9
22.35
21.56
282 ± 5
291 ± 5.3 64.0
140.0 1.33
0.07 115
4673 3644
1606
0.13
0.47 U1.8.1
2.1 22.27 282 ± 6 14.6 0.21 76 378 0.54 U1.12.1
ppm %
U1.10.1
0.341 2.2 0.04465 0.334 6 0.0542 0.0585
3.5
22.39
2.2
6.4
(1) ±% 207 Pb*/235U Pb/206Pb ±% 207
U/206Pb ±% 238
Age, Ma (1) 206 Pb/238U Pb*, ppm
206
Th/238U 232
Th U Pbc,
206
Dot
Table 2. Results of U-Pb studies of zircons from the urthites of the Mukhal massif, western Transbaikalia
±% (1) 238 206 U/ Pb*
(1) ±% 207 Pb/206Pb*
(1) ±% 206 Pb*/238U
Correlation of errors
A.G. Doroshkevich et al. / Russian Geology and Geophysics 53 (2012) 169–174
We have not revealed any significant difference in the age of genetically different crystal domains (of magmatic origin and rims). The diagram with a concordia (Fig. 4, a) constructed from the analytical data on 11 points yields an age of 294.5 ± 4.7 Ma. Three concordant values (Fig. 4, b) can be united into a cluster with an age of 324 ± 7 Ma. These dates correspond to the inner parts of intricate crystals; they have no exact identification and can be regarded as seeding cores of xenogenic nature. The heterogeneous internal structure and chemical composition of zircons might be due to the partial metasomatism of the mineral. This agrees with Vrublevskaya’s (1988) data on autometasomatic processes in the massif, which accompanied the melt crystallization. The obtained Early Permian (Sakmarian) age (294.5 Ma), probably, corresponds to the time of the urthite crystallization. The zircons, which are of Carboniferous age (324 ± 7 Ma), were trapped from the host (most likely, basic) rocks.
Discussion The earlier obtained K-Ar and Rb-Sr dates for the alkaline rocks of the Vitim province show wide variations, especially for the earliest rocks—pyroxenites, urthites, and ijolites: Saizhen massif—260–595 Ma, Verkhne-Burul’zai—205– 394 Ma, and Nizhne-Burul’zai—261–510 Ma. Moreover, different minerals are of different ages even in the same rock. For example, in the Saizhen massif, the age of ijolite-urthite developed after nepheline and after augite is 294 and 595 Ma, respectively (Andreeva, 1982); in the Mukhal massif, melteigite developed after pyroxene is dated at 294 Ma, and that developed after nepheline, at 370 Ma (Zaguzin et al., 1976). The ages of minerals developed after alkali and nepheline syenites are in a more narrow range of values. In accordance with the obtained geochronological data, Konev (1982) separated two main groups of dates: Carbonaceous (290–350 Ma) and Triassic (170–230 Ma), whereas Sharakshinov et al. (1991) assigned the syenites to the Early Carboniferous (325–352 Ma) and Early Permian–Early Triassic (241–265 Ma) and interpreted the Triassic–Jurassic dates (229–167 Ma) as the time of the occurrence of Na-metasomatism. The close ages of syenites from different massifs of the Vitim province might reflect the time of the completion of alkaline magmatism, whereas their variations in the earlier rocks (pyroxenites, urthites, and ijolites) of the Saizhen complex hamper an unambiguous conclusion about the beginning of alkaline magmatism and its duration in the region. Based on the radiological age, some researchers, e.g., Andreeva (1982), concluded that the complex formed for a long time, from 100 to 200 Myr. In contrast to the available K-Ar and Rb-Sr data on the early rocks of the Saizhen complex, the obtained age of the urthites from the Mukhal massif refines the time of alkaline magmatism in the region. Surely, a more exact determination of the time and duration of alkaline magmatism in the Vitim province calls for additional geochronological studies.
A.G. Doroshkevich et al. / Russian Geology and Geophysics 53 (2012) 169–174
173
Fig. 4. The Tera–Wasserburg diagram for zircons from the urthites of the Mukhal massif.
The U-Pb age of zircons matches the age range (310– 275 Ma) of magmatic processes in the zonal Barguzin area (Yarmolyuk, 2009). In the central part of the latter, the world’s largest Angara–Vitim granite batholith formed at 305–275 Ma (Tsygankov et al., 2007; Yarmolyuk et al., 1997). The alkaline rocks of the Synnyr complex, framing the northwestern part of the batholith, also have close ages (287–330 Ma) (Kostyuk et al., 1990; Pokrovskii and Zhidkov, 1993). In the period 302–284 Ma, abundant massifs of alkali granitoids formed in southern Mongolia, as well as bimodal basalt-trachyrhyolitecomendite volcanic associations (Yarmolyuk et al., 2008). Westward, these zones are changed by picrodolerite and picrite intrusions with an age of 292–275 Ma and the Tarim trap area formed at 275–287 Ma (Zhang et al., 2010). The formation of these rocks marks the early stage of evolution of the Late Paleozoic rift system in Central Asia (Kovalenko et al., 2002; Yarmolyuk et al., 2000; Zhang et al., 2010), which was related to the activity of a mantle superplume and took place in the settings of rifting and active continental margin. This work was supported by grant 08-05-98028 from the Russian Foundation for Basic Research and Programs 10.2 and MK-2873.2010.5 from the Geosciences Department.
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Editorial responsibility: A.E. Izokh
Russian Geology and Geophysics 53 (2012) 169–174 www.elsevier.com/locate/rgg
The U-Pb geochronology of the Mukhal alkaline massif (western Transbaikalia) A.G. Doroshkevich a, G.S. Ripp a,*, S.A. Sergeev b, D.L. Konopel’ko c a
b
Geological Institute, Siberian Branch of the Russian Academy of Sciences, ul. Sakh’yanovoi 6a, Ulan Ude, 670047, Russia A.P. Karpinsky All-Russian Research Geological Institute, Center of Isotope Studies, Srednii pr. 74, St. Petersburg, 199106, Russia c St. Petersburg State University, Universitetskaya nab. 7/9, St. Petersburg, 199034, Russia Received 2 June 2010; accepted 9 November 2010
Abstract We present results of U-Pb (SHRIMP II) geochronological study of the rocks of the Mukhal alkaline massif in the Vitim alkaline province, western Transbaikalia. The available K-Ar and Rb-Sr dates for the alkaline rocks (Saizhen complex) of the Vitim province, including the Mukhal massif, vary over a broad range of values. The obtained age of crystallization of the Mukhal urtites refines the time when the regional alkaline magmatism began. The age of zircons and magmatic processes within the Barguzin area (315–275 Ma) is close to the peak of main events, which occurred between 295 and 275 Ma. These processes took place at the early stage of evolution of the Late Paleozoic rift system in Central Asia, whose activity was associated with the activity of mantle superplume. © 2012, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: geochronology; alkaline magmatism; Mukhal massif
Introduction In the Late Paleozoic, the Central Asian Fold Belt (CAFB) was involved in global intraplate processes, which formed a rift system stretching along the southern margin of the North Asian continent for more than 3000 km and being up to 600 km in width (Kovalenko et al., 2002; Yarmolyuk, 2009; Yarmolyuk et al., 2000). The system is composed of rift zones of different ages including large zonal magmatic areas: Barguzin, South Mongolian, Hangayn, and Henteyn. At the early stage of its evolution, the Barguzin magmatic area formed, which includes the Angara–Vitim batholith more than 150,000 km2 in area and the Vitim alkaline-magmatism zone. These zones abound in massifs of alkaline ultrabasic and basic rocks and alkali and nepheline syenites. The Vitim province unites more than 20 massifs of alkaline rocks (Saizhen complex) in a band of NE strike, >450 km long and ~50 km wide (Fig. 1). The massifs were assigned to an alkali-gabbroid association (Konev, 1982), based on the coexistence of gabbro-pyroxenites, nepheline-pyroxene rocks of the urthite-jacupirangite series, and nepheline and alkali syenites. By the set of rocks, these massifs can be assigned
* Corresponding author. E-mail address: [email protected] (G.S. Ripp)
to an alkali-ultrabasic association according to the modern classification (Petrographic Code, 2009). The results of geological mapping evidence that alkaline rocks intrude Precambrian and Cambrian strata and are overlain by Upper Mesozoic–Cenozoic deposits. The available geochronological data on these rocks (Andreev and Sharakshinov, 1967; Andreeva, 1982; Konev, 1982; Konev et al., 1975; Shaposhnikov et al., 1991; Zaguzin et al., 1976) vary widely even within a massif, which might be due to the study of isotope systems poorly resistant to superposed processes. The ages of the rocks of the Saizhen complex do not ensure unambiguous conclusions about the time of alkaline magmatism and its duration in the region. Therefore, to determine the more accurate time of the massif formation, it is necessary to use other, more precise methods for rock dating. We chose the Mukhal massif for study and performed a U-Pb SIMS SHRIMP-II dating of accessory zircon from urthites.
Investigation technique For U-Pb dating, we separated zircon from urthites (core from BH-13, depth ranges 167.8–168.2 and 355.5–362.5 m). The isotope studies were carried out at the Center of Isotope Studies of the All-Russian Research Geological Institute,
1068-7971/$ - see front matter D 201 2, V . S. S o bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2011.12+.013
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Fig. 2. Schematic geologic structure of the Mukhal massif, with its assumed outlines beneath the basaltic cover, after Sharakshinov (1984). 1, crystalline limestones; 2, skarning zones; 3, ijolites; 4, urthites; 5, lines of disjunctions: a, proved, b, assumed; 6, erosion “window” outlines; 7, boundaries of facies transitions; 8, localities of sampling for geochronological studies.
Fig. 1. Schematic occurrence of alkaline massifs in the Vitim province, after Yarmolyuk et al. (1997). Igneous complexes (1–4): 1, Barguzin, 2, Zaza, 3, Chivyrkui, 4, massifs of alkaline basites and syenites of the Saizhen complex (asterisked is the Mukhal massif); structure-facies zones (5–11): 5, Kotera– Udokan, 6, Baikal–Muya, 7, Mama–Bodaibo, 8, Dzhida–Vitim, 9, Selenga– Stanovoi, 10, Chara–Udokan, 11, Siberian Platform, 12, boundaries of structureformational zones.
St. Petersburg. The manually sampled crystals were implanted into epoxy resin together with grains of the TEMORA and 91500 International geochronological zircon standards. The dot of local U-Pb dating was chosen, using optical, BSE, and cathodoluminescent images reflecting the internal structure and zoning of zircon crystals. The U/Pb ratios were measured following the technique described by Williams (1998). The intensity of a primary O2 beam was 4 nA, the diameter of the sampling spot (crater) was 25 µm, and the depth of sampling reached 5 µm. The data obtained were processed using the SQUID program (Ludwig, 2000). The U/Pb ratios were normalized to the value of the TEMORA standard zircon, 0.0668, which corresponds to an age of 416.75 Ma (Black et al., 2003). The errors of singular analyses (determination of U/Pb ratios and ages) are at the 1σ level, and the errors of the calculated concordant ages are at the 2σ level. The plots with a concordia were constructed using the ISOPLOT/EX program (Ludwig, 1999).
Geological characteristics of the Mukhal massif The massif is one of the largest in the Vitim province and is of practical importance as a source of nepheline raw materials. Its rocks expose in an “erosion window” 0.5 ×
1.5 km in size among the overlying Cenozoic basalts (Fig. 2). The total area of the massif, determined by drilling, is more than 5 km2. Riphean limestones with dolomite intercalates are host rocks. The massif has steep, close to vertical, contacts with carbonate rocks. It is composed mainly of urthites grading in the northern direction into ijolites and, then, melteigites. Dikes of nepheline and alkali syenites are subordinate. The massif formed in three stages (Vrublevskaya, 1988): (1) ijolites, (2) urthites and melteigites, and (3) syenites. The urthites, ijolites, and melteigites are of fine- to mediumgrained texture and massive-banded and trachytoid structure. They have variable contents of nepheline, aegirine-augite, hastingsite, riebeckite, and calcite. Secondary and accessory minerals are garnet, titanite, apatite, magnetite, cancrinite, biotite, and zircon. The syenites consist mainly of microcline, oligoclase, and nepheline with accessory apatite, magnetite, and titanite. Magnesian and calcareous skarns are confined to the endocontacts of ijolites with limestones and their xenoliths, at which brucite marbles are developed. According to Konev (1982), this points to a shallow depth of the intrusion formation and the high temperature of the parental magma. Metasomatic and postmagmatic processes are of different intensities; these are garnetization, albitization, microclinization, and nephelinization. The rocks show great variations in chemical composition, with a gradual increase in SiO2 (39–63 wt.%) and, partly, alkali (6–16 wt.%) contents and a decrease in Fe (10– 1.2 wt.%) and Ca (16–1.2 wt.%) contents in passing from early to late rocks formed at the final stage of the massif formation. All rock varieties have low contents of Mg (0.1–3.9 wt.%) and Ti (≤1.2 wt.%) and high contents of Al (up to 28 wt.%); Na2O/K2O = 2–3. The Nb/U ratios in the ijolites and urthites vary from 100 to 175, and those in the syenites, from 22 to 34. The Zr/Nb
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Table 1. The age of alkaline rocks of the Mukhal massif, Vitim province Method
Rock
Material under study
Age, Ma
Source
K-Ar
Melteigite
Nepheline
370
(Zaguzin et al., 1976)
K-Ar
Melteigite
Aegirine-augite
294
(Zaguzin et al., 1976)
K-Ar
Melteigite
Amphibole
341
(Zaguzin et al., 1976)
K-Ar
Urthite
Rock
337
(Sharakshinov, 1984)
K-Ar
Melteigite
Nepheline
247
(Zaguzin et al., 1976)
Rb-Sr
Ijolite-urthites
Rock, K-feldspar
279 ± 16
(Sharakshinov et al., 1991)
K-Ar
Congressite
Rock
312.5 ± 3.5
(Konev et al., 1975)
ratios also fall in two ranges: high values for the ijolites and urthites (4–7) and low values for the syenites (~0.5). The La/Yb ratios are also higher in the syenites (23–24) as compared with the ijolite-urthites (11–16). All rock varieties have high contents of REE (120–285 ppm), Sr (up to 1400 ppm), and Ba (up to 1500 ppm). The K-Ar and Rb-Sr dating (Andreev and Sharakshinov, 1967; Andreeva, 1982; Konev, 1982; Konev et al., 1975; Sharakshinov et al., 1991; Zaguzin et al., 1976) yielded an Early Triassic-Late Devonian age of the massif rocks (Table 1), 247–370 Ma.
Results of geochronological studies Zircon from the urthites has large (200–600 µm) transparent crystals of short-prismatic habit. On the cathodoluminescent
images, most of the grains show genetically different crystal domains (Fig. 3, dot 5.1). All grains have thin light secondary overgrowth or recrystallization rims (Fig. 3, dots 4.2 and 12.1). Some crystals (Fig. 3, dots 5.2 and 9.1) have complex inner domains with irregular luminescence and abundant inclusions. In general, the zircons have high contents of U and Th (Table 2), which is expressed as the suppression of cathodoluminescence (Fig. 3, dots 4.1, 6.1, and 8.1). The crystals with the best expressed signs of magmatic origin (morphology, geochemistry, internal structure) and without mechanical defects (cracks, inclusions) contain, on the average, 1830 ppm U (Th/U = 0.4). The overgrowth/recrystallization rims contain ~200 ppm U (Th/U = 0.1), and the heterogeneous domains have abnormal Th/U ratios (up to 2.8) because of the high Th content (U = 1200–1800 ppm). The results of isotope analyses are listed in Table 2 and in Fig. 4, a, b. A total of 14 analyses of 11 grains were made.
Fig. 3. Cathodoluminescent images of zircons extracted from the urthites of the Mukhal massif. White circles show the location of the sampling crater within the crystal.
172
Note. Errors are at the level 1σ; Pbc and Pb*, common and radiogenic lead, respectively. The error of the standard calibration was 0.70% (ignored in the above errors but taken into account on comparison of results from different samples). (1), correction for common lead was made by measured 204Pb.
0.170 2.1 0.0531 12 12 0.0516 2.1 18.8 5 0.0812 2 18.11 334 ± 6.9 3.67 U1.9.1
1161
1413
1.26
55.1
18.72 323 ± 5.9 3.75 U1.5.2
1758
4747
2.79
80.6
19.78
1.8
0.0852
1.6
19.45
1.9
6.8
0.378
0.816
0.268 1.9 0.05135 7
0.622
0.0551
0.39
1.8
1.9 0.04929
0.0505 2.2
3.1 0.353
0.05228
0.364
2.4
1.3
0.0519 1.9
1.8 19.8
20.29 2
1.1 0.05313
0.0534 20.25
205.0
318 ± 5.6
0.99
0.16 716
1329
0.11 U1.7.1
1391 0.19 U1.2.1
4714
59.0
310 ± 5.8
1.8
1.3 1.8 20.32 1.2 0.05244 1.8 20.31 0.04 U1.1.1
3490
373
0.11
148.0
310 ± 5.4
1.9
0.158
0.815 1.8 0.04922 2.2
0.530
0.05209
0.3535
6.5
1.9 0.04693
0.0474 41
3.5
41
0.63
3
0.097 6.5 21.1
21.31 1.8
6.9 0.157
0.05405
19.39
1.9 21.25 296 ± 5.4 55.9 0.07
0.04 1
100
7.89 U1.4.2
32
0.30 U1.6.1
1384
1.4
299 ± 19
4
1.9
0.0517
0.334
0.473
0.436 1.9
1.9 0.04653
0.04669 4.3
3.9 0.334
0.323
3.5
3.9
0.052
0.0501
1.9
1.9 21.41
21.49 2.1
1.8 0.05367
0.0561
21.32
1.9 21.38 81.7
293 ± 5.3
0.61
0.82 1614
781 U1.5.1
1320
0.50
0.44
U1.10.2
2033
53.2
294 ± 5.4
1.9
0.430
0.726 1.8
1.9 0.04622
0.0462 2.5
4.5 0.343
1.7
4.1
0.05139 1.8 21.65
21.63 2.2
1.2 0.05288
0.0569 1.9
21.61
21.55 36.3
291 ± 5.5
0.12
0.68 596
431
0.39 U1.13.1
3743 0.19 U1.4.1
910
149.0
291 ± 5.2
1.8
1.9
0.0538
0.3273
0.724
0.404 1.9
1.8 0.04469
0.04615 4.7
2.5 0.3236
0.321
1.7
4.3
0.05252
0.0504
1.8
1.9 21.67
22.38 1.6
1.7 0.05424
0.05356 1.8
1.9
22.35
21.56
282 ± 5
291 ± 5.3 64.0
140.0 1.33
0.07 115
4673 3644
1606
0.13
0.47 U1.8.1
2.1 22.27 282 ± 6 14.6 0.21 76 378 0.54 U1.12.1
ppm %
U1.10.1
0.341 2.2 0.04465 0.334 6 0.0542 0.0585
3.5
22.39
2.2
6.4
(1) ±% 207 Pb*/235U Pb/206Pb ±% 207
U/206Pb ±% 238
Age, Ma (1) 206 Pb/238U Pb*, ppm
206
Th/238U 232
Th U Pbc,
206
Dot
Table 2. Results of U-Pb studies of zircons from the urthites of the Mukhal massif, western Transbaikalia
±% (1) 238 206 U/ Pb*
(1) ±% 207 Pb/206Pb*
(1) ±% 206 Pb*/238U
Correlation of errors
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We have not revealed any significant difference in the age of genetically different crystal domains (of magmatic origin and rims). The diagram with a concordia (Fig. 4, a) constructed from the analytical data on 11 points yields an age of 294.5 ± 4.7 Ma. Three concordant values (Fig. 4, b) can be united into a cluster with an age of 324 ± 7 Ma. These dates correspond to the inner parts of intricate crystals; they have no exact identification and can be regarded as seeding cores of xenogenic nature. The heterogeneous internal structure and chemical composition of zircons might be due to the partial metasomatism of the mineral. This agrees with Vrublevskaya’s (1988) data on autometasomatic processes in the massif, which accompanied the melt crystallization. The obtained Early Permian (Sakmarian) age (294.5 Ma), probably, corresponds to the time of the urthite crystallization. The zircons, which are of Carboniferous age (324 ± 7 Ma), were trapped from the host (most likely, basic) rocks.
Discussion The earlier obtained K-Ar and Rb-Sr dates for the alkaline rocks of the Vitim province show wide variations, especially for the earliest rocks—pyroxenites, urthites, and ijolites: Saizhen massif—260–595 Ma, Verkhne-Burul’zai—205– 394 Ma, and Nizhne-Burul’zai—261–510 Ma. Moreover, different minerals are of different ages even in the same rock. For example, in the Saizhen massif, the age of ijolite-urthite developed after nepheline and after augite is 294 and 595 Ma, respectively (Andreeva, 1982); in the Mukhal massif, melteigite developed after pyroxene is dated at 294 Ma, and that developed after nepheline, at 370 Ma (Zaguzin et al., 1976). The ages of minerals developed after alkali and nepheline syenites are in a more narrow range of values. In accordance with the obtained geochronological data, Konev (1982) separated two main groups of dates: Carbonaceous (290–350 Ma) and Triassic (170–230 Ma), whereas Sharakshinov et al. (1991) assigned the syenites to the Early Carboniferous (325–352 Ma) and Early Permian–Early Triassic (241–265 Ma) and interpreted the Triassic–Jurassic dates (229–167 Ma) as the time of the occurrence of Na-metasomatism. The close ages of syenites from different massifs of the Vitim province might reflect the time of the completion of alkaline magmatism, whereas their variations in the earlier rocks (pyroxenites, urthites, and ijolites) of the Saizhen complex hamper an unambiguous conclusion about the beginning of alkaline magmatism and its duration in the region. Based on the radiological age, some researchers, e.g., Andreeva (1982), concluded that the complex formed for a long time, from 100 to 200 Myr. In contrast to the available K-Ar and Rb-Sr data on the early rocks of the Saizhen complex, the obtained age of the urthites from the Mukhal massif refines the time of alkaline magmatism in the region. Surely, a more exact determination of the time and duration of alkaline magmatism in the Vitim province calls for additional geochronological studies.
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173
Fig. 4. The Tera–Wasserburg diagram for zircons from the urthites of the Mukhal massif.
The U-Pb age of zircons matches the age range (310– 275 Ma) of magmatic processes in the zonal Barguzin area (Yarmolyuk, 2009). In the central part of the latter, the world’s largest Angara–Vitim granite batholith formed at 305–275 Ma (Tsygankov et al., 2007; Yarmolyuk et al., 1997). The alkaline rocks of the Synnyr complex, framing the northwestern part of the batholith, also have close ages (287–330 Ma) (Kostyuk et al., 1990; Pokrovskii and Zhidkov, 1993). In the period 302–284 Ma, abundant massifs of alkali granitoids formed in southern Mongolia, as well as bimodal basalt-trachyrhyolitecomendite volcanic associations (Yarmolyuk et al., 2008). Westward, these zones are changed by picrodolerite and picrite intrusions with an age of 292–275 Ma and the Tarim trap area formed at 275–287 Ma (Zhang et al., 2010). The formation of these rocks marks the early stage of evolution of the Late Paleozoic rift system in Central Asia (Kovalenko et al., 2002; Yarmolyuk et al., 2000; Zhang et al., 2010), which was related to the activity of a mantle superplume and took place in the settings of rifting and active continental margin. This work was supported by grant 08-05-98028 from the Russian Foundation for Basic Research and Programs 10.2 and MK-2873.2010.5 from the Geosciences Department.
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