AbstractâThis study characterizes some issues of the Paleozoic and Mesozoic tectonomagmatic evolution of Precambrian structures from the southwestern ...
ISSN 1028334X, Doklady Earth Sciences, 2010, Vol. 432, Part 1, pp. 547–552. © Pleiades Publishing, Ltd., 2010. Original Russian Text © V.A. Vernikovsky, A.E. Vernikovskaya, N.Yu. Matushkin, I.V. Romanova, N.G. Berezhnaya, A.N. Larionov, A.V. Travin, 2010, published in Doklady Akademii Nauk, 2010, Vol. 432, No. 1, pp. 82–88.
GEOLOGY
Paleozoic and Early Mesozoic Magmatism Manifestations in the Early Precambrian Structure of the South Yenisei Ridge Corresponding Member of the RAS V. A. Vernikovskya, b, A. E. Vernikovskayaa, N. Yu. Matushkina, b,
I. V. Romanovaa, N. G. Berezhnayac, A. N. Larionovc, and A. V. Travind Received November 24, 2009
Abstract—This study characterizes some issues of the Paleozoic and Mesozoic tectonomagmatic evolution of Precambrian structures from the southwestern margin of the Siberian craton. The relationship between the Devonian and Triassic magmatic events is demonstrated from the example of the Severnaya riftrelated struc ture, South Yenisei Ridge. U–Pb SHRIMP dating yielded ages of 387 ± 5 Ma for leucogranites and 240 ± 3 Ma for the overlying alkaline trachytes. These ages show good agreement with Ar–Ar geochronological data (392–387 Ma) obtained for micas from paragneisses and leucogranite dykes of the Yenisei suture zone, the extension of which is superimposed by the studied riftrelated structure. The previous geological evidence and the Devonian age estimate first obtained for magmatic rocks of the Yenisei Ridge allow us to interpret the studied leucogranites as products of Devonian continental rifting, similar to volcanic and intrusive rocks of the North Minusa depression and Agul graben. Like other localities within the western margin of Siberian craton, the formation of Triassic alkaline rocks may be related to the Siberian superplume activity. Key words: western margin of the Siberian craton, Yenisei Ridge, rifting, alkaline trachytes and syenites, A– type leucogranites, U–Pb and Ar–Ar dating. DOI: 10.1134/S1028334X10050016
The Yenisei Ridge is a Precambrian accretionary– collision orogen, which is part of the western framing of the Siberian craton. The geological history of this structure is traced back to the Early Proterozoic. The oldest rocks of the study area are identified in its
southern part, within the Angara–Kan terrane where the granulite and amphibolite facies of metamorphic rocks cut by granitoids were formed at 1900–1840 Ma [1]. It was established later in [2, 3] that after a period of 1 Ga and more, during the Late Neoproterozoic,
Fig. 1. Geological and structural sketch map showing preJurassic complexes in the southwestern framing of the Siberian craton. (1) Volcanic and volcanosedimentary complexes of the Agul graben (D); (2) terrigenous complexes of the Rybinsk depression (D); (3) carbonateterrigenous complexes of the sedimentary cover (Є); (4) island arc and ophiolite complexes of the Yenisei belt and Eastern Sayan (NP2–3); (5) trap complex (T1); (6) alkaline and nepheline syenites, alkaline trachytes, urtites, jacupirangites, ijolites, and carbonatites (T1); (7, 8) syenites and granitoids of the Agul graben: (7) undifferentiated alkaline and subalkaline syen ites and granites (D), (8) granites, subalkaline leucogranites (O); (9) Posol’naya syenites and granites (511 Ma) and Nizhnekan diorites, granodiorites, granites, leucogranites (455 Ma); (10) granites, leucogranites, alkaline and nepheline syenites, ultrama fites, ijolites, urtites, trachytes, trachydolerites, and carbonatites of the Tatarka complex (711–630 Ma); (11) Tatarka–Ishimba suture zone; (12) terranes; (13) boundaries of regional geostructures (a), terranes (b); (14) marginal suture of the Siberian craton (during NP3): proven (a), inferred (b); (15) faults proven (a), inferred (b). Numbers in circles refer to terranes as follows: 1, East Angara, terrigenouscarbonate rock of passive continental margin (NP); 2, Central Angara, metamorphosed flyschoid and car bonate rocks (MP?NP), Rybnaya–Panimba ophiolite belt (MP), collisional granitoids of the Teya (880–865 Ma), Ayakhta and Glushikha (760–720 Ma) complexes; 3, AngaraKan, granuliteamphibolite complexes (PP); 4, 5, respectively, Isakovka and Predivinsk island arc and ophiolite complexes (700–630 Ma); 6, Biryusa: granuliteamphibole complexes (AR?–PP); 7, Derbin, metaterrigenouscarbonate complexes (MP–NP); 8, Kan, metasedimentary and metavolcanic complexes (PP–NP); 9, Tuman shet, metamorphosed volcanic and volcanosedimentary complexes (PP). The inset shows the geological sketch of the Severnaya volcanoplutonic rift structure modified after largescale mapping data by V.V. Semenyako and A.A. Serednev. 1, gneisses, schists (PP); 2, Taraka complex granitoids (PP3?); Porozhnaya massif: 3, rhyolites, eruptive breccias, tuff breccias, ignibrites (D); 4, leucogranites (D2); 5, alkaline granites (T?); 6, alkaline syenites, alkaline trachytes (T1–2); 7, mylonitization zones; 8, faults proven (a) and inferred (b).
a Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia b
Novosibirsk State University, Novosibirsk, Russia Karpinskii AllRussia Scientific and Research Geological Institute (VSEGEI), St. Petersburg, Russia d Sobolev Institute Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia c
547
548
VERNIKOVSKY et al. 94°03′ E
90° E
4
94°09′
ka
Yenisei R.
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92° Ankinov fault
re Ka
56°27′
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94° Siberian craton 90° 60° N 60° 2
387 ± 5 Ma U–Pb R.
1 Sev erna ya
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92° 58°
ara
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West Siberian plate
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240 ± 3 Ma U–Pb
58°
2 100 km
3
Yenisei fault
4 Porozhnaya massif 5
5
Siberian craton 3
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7
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Main Sayan Fault
the Predivinsk island arc terrane accreted to the afore mentioned Early Proterozoic terrane comprised metarhyolites and plagiogranites with ages of 637 ± 5.7 and 628 ± 3 Ma, respectively. Subsequent studies have shown that the tectonomagmatic evolution of this Precambrian structure continued at least into Paleo
zoic times. For example, the Posol’naya and Nizhne kan massifs previously regarded by convention as being of Precambrian age were interpreted [4] to have formed, respectively, in the Middle Cambrian and Late Ordovician times, i.e., 511 and 455 Ma, in response to the early Caledonian accretion–collision DOKLADY EARTH SCIENCES
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PALEOZOIC AND EARLY MESOZOIC MAGMATISM MANIFESTATIONS
events, which were widely manifested in the Central Asian fold belt [5–7], including in the Altai–Sayan folded area. The occurrence of the youngest magmatic events in the Yenisei Ridge is related to the Siberian plume activity. The rocks of the trap formation formed at the Permo–Triassic boundary were identified in the TransAngarian part of the study area, mostly on its eastern boundary running along the Ankinov fault (Fig. 1). They also occur as small exposures in the Vorogovka graben located in the nothern part of the Tatarka–Ishimba suture zone [8]. Some other records of the aforementioned mantle plume event might be represented by the formation of nepheline syenites, alkaline syenites, urtites, jacupirangites, and ijolites of the Kiya massif in the middle part of the Yenisei suture zone, which yield the Rb–Sr isochron age of 251 ± 12 Ma [9]. These small bodies of the above rocks and associated carbonatites cut across flat lying terrige nouscarbonate red beds and conglomerates of the Chernaya Rechka graben, which are interpreted to be Lower Paleozoic. At the same time, some issues of the nature and time of formation of Phanerozoic mag matic rocks, including their alkaline varieties, and their relation to continental rifting processes that took place in the southwestern framing of the Siberian cra ton are still debated. The Severnaya volcanoplutonic riftrelated struc ture filled by alkaline syenites, trachytes, granites, and rhyolites of the Porozhnaya massif is one of basic keys to deciphering the tectonic and magmatic evolution of the South Yenisei Ridge. Magmatic rocks from this massif were previously thought to be Neoproterozoic [10] to Middle Proterozoic in age [11]. However, the volcanics of this structure were later attributed to the Devonian Byskar Series, which is seen to represent the lower structural level within large intracontinental depressions of the Altai–Sayan folded area and the Agul graben. The studied magmatic rocks of the Severnaya vol canoplutonic riftrelated structure (some 36 km2 in area) are located in the interfluve of the Karen’ka and Severnaya rivers, two right tributaries of the Nemkina River (Fig. 1). This structure is superimposed on the northern faults splaying off the Main Sayan Fault, the southward extension of the Yenisei suture zone. The latter represents a complicated fault system of over thrust–upthrust kinematics along which the Isakov and Predivinsk terranes (Neoproterozoic) were obducted onto the craton margin. The reconstruction of structural trends within the northwestern part of the Predivinsk terrane indicates incremental deformation in the Neoproterozoic island arc and ophiolite com plexes when moving towards the Yenisei fault zone [12]. It was reported in the same study that Neoprot erozoic deformed rocks of the Yenisei fault zone are intruded by a network of twomica leucogranite dykes. The base of the Severnaya structure consists of dif ferent Early Proterozoic rocks of the AngaraKan ter DOKLADY EARTH SCIENCES
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rane, such as migmatized gneisses, schists, and pla giogneisses, as well as granitoids of the Taraka mag matic complex. As a result of largescale mapping completed in the 1990s, the Porozhnaya massif was counted the same as a single volcanic edifice. The caldera is filled with volcanic products (alkaline tra chytes, rhyolites), eruptive breccias, tuff breccias, and ignimbrites, which locally contain small bodies of fel sic to intermediate intrusive rocks including alkaline varieties as well. Magmatic rocks outcrop as oval shaped bodies confined to the fault zone. The prevail ing rocks are alkaline trachytes that occupy about half the caldera area and overlie granitoids. Alkaline tra chytes are crosscut by alkaline syenites that form stocklike bodies. This study focuses on leucogranites, alkaline tra chytes, and alkaline syenites from the Porozhnaya massif. Leucogranites are sphenebearing rocks, con sisting mostly of pelitized K–feldspar and quartz with a subordinate amount of plagioclase (up to 10 vol %). The rocks have weakly peraluminous compositions, and belong to calcalkalic to alkalicalcic magmatic series, have moderate LaN/YbN ratios (13.1–17.9), and negative Eu (Eu/Eu* = 0.10–0.12), Sr, P, and Ti anomalies. These rocks have a high FeOtot/(FeOtot + MgO) ratio, high REE (ΣREE up to 1250 ppm) and Rb, Th, Hf, Zr, Ta, Nb, Y abundances, thus exhibiting geochemical characteristics typical of Atype leucog ranites. The alkaline trachytes and syenites mostly consist of orthoclase with minor albite, alkaline pyroxene, Ca–Na amphibole, and lepidomelane; common opaque and accessory minerals are ilmenite, titano magnetite, apatite, and zircon. Some of the alkaline trachytes contain augite whereas alkaline amphiboles are present in alkaline syenites. These rocks have total alkalis ranging from 11.71 to 12.41 wt % and an excess of Na2O over К2О (Na2O/К2О = 1.06–1.23). The LaN/YbN ratios are similar to those in leucogranites, whereas the Eu/Eu* ratio increases to 0.30–0.76. At the same time, their intrusive analogs display more dif ferentiated element patterns, with wellpronounced negative anomalies for Ba and Eu, as well as P and Ti, implying a dominant role for K–feldspar, along with plagioclase, apatite, and sphene, during the fraction ation processes. Like the leucogranites, they also exhibit significant enrichment in REE (ΣREE up to 400 ppm), Ta, Nb, Hf, and Zr, which indicates the hypothetical presence of enriched mantle components in the magmatic sources of the studied rocks. U–Pb isotope dating was performed with the SHRIMP II ion microprobe at the VSEGEI Center for Isotopic Research, and the argon isotopic compo sition was measured on a Noble Gas 5400 mass spec trometer (Novosibirsk) using standard procedures described in [3, 8]. For isotope analyses, 11 and 12 zircon grains were recovered from, respectively, leucogranite (sample P071) and alkaline trachyte (sample P0712) of the
550
VERNIKOVSKY et al.
Pb/206Pb 0.08
tions. Due to the older ages (Silurian and Neoprotero zoic) yielded by two zircons in each sample, and with regard to their morphology, these grains are presum ably xenogenic. Excluding data on xenocrysts showing significant age dispersion and therefore a rejuvenation of apparent isotopic ages due to partial Pb loss results in a leucogranite (sample P071) crystallization age of 387 ± 5 Ma (n = 7, MSWD = 0.91) (Fig. 2a), whereas the alkaline trachyte (sample P0712) yields an age of 240 ± 3 Ma (n = 10, MSWD = 0.60) (Fig. 2b).
207
(a) T(mean 206Pb/238U) = 387 ± 5 Ma MSWD = 0.91
0.07 900 800
700
0.06
600 500 400
0.05 P071 9.1
100 µm
0.04 6
8
10
12 (b)
14
16
18
20
T(mean 206Pb/238U) = 240 ± 3 Ma MSWD = 0.60
0.09 0.08 0.07 500
0.06
420
380
460
340
300
260
220
P07126.1
0.05 0.04 100 µm 0.03 12
16
20
24
28 238
32 U/206Pb
Fig. 2. Concordia plots for zircons from leucogranites (sample P071) (a) and alkaline trachyte (sample P0712) (b) of the Porozhnaya massif and CL images of typical zir cons. Analysis point error ellipses (SHRIMP II) (see Table 1) are 2σ.
Porozhnaya massif. All analytical data are given in Table 1. Zircon is present in both samples as idiomorphic, poorly colored transparent crystals of typical zircon habit with an elongation ratio of 1.5–4. In addition, the rounded xenocrysts are also present in minor amounts in the granite and alkaline trachyte as well. The presence of melt inclusions and distinct cathod oluminescence zoning in most prevailing zircons points to their magmatic origin. The rounded xenoc rysts may have been inherited by magmas from either a crustal source or country rocks. The majority of analyses for both samples form a chronologically consistent cluster of age determina
The Ar/Ar stepwise heating ages were obtained for two biotites (samples PR0726, PR0745) recovered from quartz–plagioclase–biotite–sillimanite–almand ine paragneisses of the Yenisei Series in the western Angara–Kan terrane and for muscovite (sample PR 0736) recovered from a leucogranite dyke crosscut ting the Neoproterozoic island arc complex in the northwestern Predivinsk terrane within the Yenisei suture zone. The obtained age spectra for all micas revealed a distinct age plateau and ages within 386.6 ± 4.0, 392.7 ± 4.5, and 387.1 ± 6.6 Ma, which all agree within the error limits (Fig. 3). On the basis of the above results, felsic and inter mediate magmatic rocks from the Porozhnaya massif of the South Yenisei Ridge, despite being formed within the same riftrelated structure, were attributed to two different complexes. The U–Pb data on zircons from this massif yielded a leucogranite age of 387 ± 5 Ma, whereas the age of the overlying alkaline trachytes is 240 ± 3 Ma. This age is very close to the ages of the traps from the Siberian platform, as well as nepheline syenites and related magmatic rocks in the Yenisei suture zone, in the Transangarian part of the Yenisei Ridge [9]. Similar rocks of the same age were reported from northwestern Taimyr [13]. Therefore, anoro genic granite–syenite massifs and related volcanics, including alkaline varieties, were formed during the Early Triassic along the western margin of the Siberian craton as a result of superplume activity. A Devonian age of 392–387 Ma of the latest tec tonothermal events in the Yenisei suture zone syn chronous with Devonian continental rifting widely manifested in the Altai–Sayan folded area was first inferred from Ar–Ar mica ages. Geological evidence indicates that the formation of the Agul graben in the Sayan area probably occurred at that time along the Main Sayan Fault. The age of the intrusive and volca nic rocks filling this graben can be correlated with the inception age of the North Minusa depression (407.5 ± 0.2 Ma, according to U–Pb ages for zircons from trachyrhyodacite [14]), which is consistent with paleontological data [15]. Therefore, the proposed geological explanation and the first U–Pb ages obtained for magmatic rocks of the South Yenisei Ridge allowed us to interpret the studied leucogranites of the DOKLADY EARTH SCIENCES
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1489
718
823
5.1
6.1
7.1
Note:
1078
2.1
437
90
3070
1936
1744
1241
2072
2706
2786
1478
1395
1252
46
135
174
123
210
69
89
45.2
37.5
49.5
35.7
35.1
30.1
38.8
43.4
48.2
27.2
30
26.5
29
18.2
22.9
10.4
24.2
20.9
18.5
5.56
19
15
26.6
206Pb*
(1)
0.0537 ± 5.9
0.057 ± 2.4
0.0617 ± 15
0.0568 ± 13
0.0555 ± 12
0.057 ± 9.4
0.0573 ± 12
0.057 ± 15
0.0566 ± 14
0.0554 ± 13
0.0562 ± 12
0.0484 ± 15
0.0637 ± 2.4
0.0558 ± 5.1
0.0537 ± 5.1
0.053 ± 8.6
0.0581 ± 3.7
0.0543 ± 6.1
0.0547 ± 5
0.0563 ± 11
0.052 ± 4.6
0.0564 ± 6.8
0.0541 ± 5.3
(±%)
%
232Th/238U
(1) (±%)
207Pb*/235U
0.63
–
0.92
1.27
1.40
1.58
–
4.75
–
3.37
0.62
0.17
0.45
0.46
0.67
0.46
0.60
0.42
0.69
0.25
0.68
0.15
1.042 ± 3.1
0.532 ± 6
0.507 ± 6.1
0.463 ± 9.6
0.505 ± 4.6
0.469 ± 7.2
0.466 ± 5.9
0.478 ± 13
0.435 ± 5.3
0.467 ± 7.9
0.437 ± 5.7
S a m p l e P071, l e u c o g r a n i t e
206Pb , с
1.63
1.66
1.06
0.30
–
–
–
0.57
1.84
0.72
2.45
–
0.55
0.13
2.13
1.86
1.69
1.39
1.80
2.08
1.96
1.81
1.56
1.57
0.473 ± 7.5
0.476 ± 3.1
0.329 ± 16
0.303 ± 14
0.293 ± 13
0.3 ± 11
0.301 ± 14
0.298 ± 17
0.295 ± 15
0.286 ± 15
0.288 ± 13
0.247 ± 16
(1)
0.1187 ± 1.5
0.0691 ± 1.7
0.0684 ± 1.7
0.0633 ± 1.8
0.063 ± 1.8
0.0626 ± 1.7
0.06179 ± 1.6
0.0616 ± 2.2
0.06068 ± 1.6
0.0601 ± 1.7
0.05861 ± 1.5
(±%)
206Pb*/238U
0.0639 ± 2.4
0.06067 ± 1.4
0.03865 ± 2.2
0.03864 ± 2.1
0.03833 ± 2
0.03816 ± 1.9
0.03815 ± 2.1
0.03794 ± 2.2
0.03781 ± 2.1
0.03737 ± 2.1
0.03722 ± 2
0.03699 ± 2.1
S a m p l e P0712, a l k a l i n e t r a c h y t e
207Pb*/206Pb*
Isotopic ratios
0.77
0.63
0.87
0.87
0.86
0.83
0.86
0.88
0.89
0.87
0.85
0.82
0.646
0.634
0.660
0.647
0.643
0.681
0.623
0.649
0.567
0.684
0.424
Rho
399.2 ± 9.2
379.7 ± 5.3
244.5 ± 5.4
244.4 ± 5
242.5 ± 4.8
241.4 ± 4.5
241.3 ± 4.9
240 ± 5.3
239.3 ± 5
236.5 ± 5
235.6 ± 4.7
234.1 ± 4.7
723 ± 10
430.7 ± 7.1
426.8 ± 6.9
395.4 ± 7.1
393.9 ± 6.9
391.6 ± 6.6
386.5 ± 6.1
385.1 ± 8.2
379.8 ± 6
376.5 ± 6.4
367.2 ± 5.3
358 ± 21
492 ± 12
664 ± 100
484 ± 63
432 ± 52
492 ± 46
503 ± 60
492 ± 74
476 ± 67
428 ± 56
460 ± 55
119 ± 18
732 ± 18
444 ± 23
358 ± 18
329 ± 28
534 ± 20
384 ± 23
400 ± 20
464 ± 51
285 ± 13
468 ± 32
375 ± 20
(1) 207Pb/206Pb
(1) 206Pb/238U
Age, Ma
–10
29
171
98
78
104
108
105
99
81
95
–49
1
3
–16
–17
35
–2
3
21
–25
24
2
D, %
All errors are 1σ values; 206Pbc and 206Pb* are common and radiogenic lead, respectively; (1) the correction for common lead is made assuming concordance between 206Pb/238U and 208Pb/232Th ages; discordance (D, %); error correlation coefficient (Rho) for U/Pb ratios. An empty string means no data.
923
1.1
1064
1191
12.1
8.1
1342
9.1
11.1
845
1471
3.1
925
10.1
388
11.1
824
191
2.1
4.1
467
285
388
1.1
6.1
308
349
9.1
4.1
225
104
10.1
7.1
142
363
8.1
74
289
191
526
Th
5.1
U
3.1
Analysis point
Content, ppm
U–Th–Pb zircon data from rocks of the Porozhnaya massif
PALEOZOIC AND EARLY MESOZOIC MAGMATISM MANIFESTATIONS 551
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VERNIKOVSKY et al. Age, Ma 800 PR0726 biotite PR0736 muscovite PR0745 biotite 386.6 ± 4.0 Ma
600
400 387.1 ± 6.6 Ma 200
0
392.7 ± 4.5 Ma
20
40 60 Released 39Ar, %
80
100
Fig. 3. Ar–Ar age spectra for micas from the Yenisei suture zone: biotites from paragneisses in the western part of the Angara– Kan terrane (samples PR0726 and PR0745) and muscovite from lecugranites (sample PR0736) in the northwestern part of the Predivinsk terrane.
Severnaya structure as products of Devonian continen tal rifting, similar to volcanic and intrusive rocks of the North Minusa depression and Agul graben. ACKNOWLEDGMENTS This work was supported by the Russian Founda tion for Basic Research (project nos. 070500703 and 080500733), Integration Project no. 44 of the Sibe rian Branch, Russian Academy of Sciences, and the Division of Earth Sciences, Russian Academy of Sci ences (program ONZ10). REFERENCES 1. E. V. Bibikova, T. V. Gracheva, V. A. Makarov, et al., Stratigr. Geol. Korrelyatsiya 1, 35–41 (1993). 2. V. A. Vernikovsky, A. E. Vernikovskaya, E. B. Sal’nik ova, et al., Geol. Geofiz. 40, 255–259 (1999). 3. V. A. Vernikovsky, A. E. Vernikovskaya, A. B. Kotov, et al., Tectonophysics 375, 147–168 (2003). 4. A. E. Vernikovskaya, V. A. Vernikovsky, V. M. Datsenko, et al., Dokl. Akad. Nauk 397, 374–379 (2004) [Dokl. Earth Sci. 397, 747 (2004)]. 5. I. V. Gordienko, Paleozoic Magmatism and Geodynam ics of the Central Asian Fold Belt (Nauka, Moscow, 1987) [in Russian].
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