ISSN 1028-334X, Doklady Earth Sciences, 2017, Vol. 477, Part 1, pp. 1265–1269. © Pleiades Publishing, Ltd., 2017. Original Russian Text © V.A. Lebedev, O.Z. Dudauri, Yu.V. Gol’tsman, 2017, published in Doklady Akademii Nauk, 2017, Vol. 477, No. 2, pp. 199–203.
GEOLOGY
Early Pleistocene Magmatism in the Central Part of the Greater Caucasus V. A. Lebedeva,*, O. Z. Dudaurib†, and Yu. V. Gol’tsmana Presented by Academician V.V. Yarmolyuk July 6, 2015
Received June 17, 2015
Abstract—An isotope-geochronological study of young magmatism in the central part of the Greater Caucasus (Kazbek neovolcanic area) on the territory of Russia and Georgia has been carried out. It was proved for the first time that, in the Early Pleistocene, there was a separate impulse of magmatic activity in this area. The area of endogenic activity for the period identified was contoured on the basis of the integrated isotope-geochronological, petrological-geochemical, and geological data. It has been shown that the Early Pleistocene volcanism inherits the area of Neogene volcanism in the Kazbek region and, therefore, presents the final impulse of the second (Pliocene) stage of the Late Cenozoic magmatism. Thus, Early Pleistocene volcanism was not a precursor of Late Quaternary magmatism as the latter has other spatial patterns of the location of volcanic centers. DOI: 10.1134/S1028334X17110149
The Greater Caucasus is the only area in the European part of Russia, where intense magmatic activity occurred in the Quaternary. The Caucasus is a heavily populated region, which is potentially unsafe in terms of the resumption of a catastrophic eruption [1]. Therefore, further complex geological study of the regional Neogene–Quaternary volcanic formations is an important task for the region. The isotope-geochronological study allowed us to create a novel geochronological scale of young magmatism for the Greater Caucasus, having integrated data for three volcanic areas of this region: Elbrus, Central-Georgian, and Kazbek [1–4]. The main part of the work to define the position of the young magmatic formations in the scale was finished for Elbrus and Central Georgia. However, for the Kazbek area, which involves the central part of the Greater Caucasus mountain country, many questions concerning the volcanic stratigraphy remain open for consideration. † Deceased.
aInstitute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Moscow, 119017 Russia bJanelidze Institute of Geology, Javakhishvili Tbilisi State University, Georgia *e-mail:
[email protected]
In particular, Early Pleistocene volcanism was not established in the Kazbek area: the period from 2.0– 1.8 Ma to 0.45–0.40 Ma was usually considered amagmatic [1, 4]. This implied an interruption for 1.5 my in the development of the Neogene—Quaternary magmatic activity, which is unusual for the Elbrus area situated further to the west. However, three estimated K–Ar dates were obtained in the 20th century for a number of young, moderately acid volcanites from the central part of the Greater Caucasus: Kalkva Massif, 1.6 Ma; a neck in the upper course of the Tanadon River, 0.7 Ma; a dyke in granodiorite of the Tepli Massif, 0.85 Ma [5]. These dates, falling in the indicated timeframe, called into question the presumed lack of Early Pleistocene activity in the Kazbek area. Within the scope of this paper, we carried out isotopic dating for several manifestations of young magmatism in this part of the Caucasus, for which Early Quaternary age could be assumed on the basis of the K–Ar dates mentioned. The Kalkva hypabyssal massif of dacite is the largest among the objects studied. It is situated in the watershed of the Greater Caucasus Mountain Range in Georgia, near the source of the Khevsureti Aragvi River (Fig. 1). Known as Kalko, it was first described at the beginning of the 20th century [6]. The massif is remarkable in that it traces the easternmost boundary of the zone of distribution of the young magmatic formations within the entire Greater Caucasus. The mas-
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45°
Nalchik
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Elbrus volcano
G
r
43° N
e
U
S
S
I
A
Beslan
a
t
e
Magas
r Vladikavkaz
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a
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Tanadon
Zopkhito
c
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s u
Tepli
s Kazbek volcano Stepantsminda Kalkva
M
o
u
n
t
a
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n
R
a
n
g
e
Khoni
Fig. 1. The orographic map of the central part of the Greater Caucasus. The Quaternary volcanic centers studied are indicated. The crosshatched area is the region of the Rukhs-Dzuar suite distribution.
sif appears to be a hypabyssal body (2.5 × 1.5 km) composed of foliate dacite, which intrudes into the Early Jurassic shale formations south of the Main Caucasian Fault. Dacites are massive, porphyry with phenocrysts of zonal plagioclase (andesine), fused quartz, and biotite. The groundmass is quartz–feldspar with small mica flakes, microcrystals of apatite, and ore minerals. Early Quaternary volcanism can also be assumed in the mountain part of North Ossetia (Iraf Gorge) in the zone of the Greater Caucasus Mountain Range (basin of the Tanadon River, a right-bank tributary of the Urukh River). In this area, the outcrops of young magmatic rocks (dacite, andesite) were described for the first time in [7, 8]; more detailed information was given in [9]. It was noted that the three dacite necks (Eldyrdon, Verkhnii Eldyrdon, and Taimazi) from 100 × 150 m to 600 × 700 m in size, which cut the Paleozoic granitoids of the Scythian Plate, were revealed in the upper course of the Tanadon River (a valley of the right-bank tributary—the Eldyrdon River and the edge of the Taimazi Glacier). The necks are attributed to the area where the NW–SE striking Tanadon Fault is intersected by a series of transverse submeridional dislocations. These necks likely trace the remains of preexisting volcanic vents. We studied samples from the Eldyrdon (sample Ur-7, 42°5245.6 N, 43°3552.8 E) and Taimazi necks (sample Ur-10, 42°5234.7 N, 43°3522.4 E; sample Ur-11, 42°5232.3 N, 43°3525.4 E). Rocks are represented by massive porphyry dacite with phenocrysts of zonal plagioclase (andesine–labrador), quartz, biotite, and,
sparsely, of opacitized amphibole. The groundmass is composed of laths of anorthoclase and plagioclase, biotite flakes, and microcrystals of quartz, apatite and ore minerals. We carried out the K–Ar and Rb–Sr isotopic dating of the series of dacite samples collected from the Kalkva Massif and from the necks in the upper course of the Tanadon River. Tables 1 and 2 show the results. The main characteristics of the methods used are given in [3]. The chemical composition of the vulcanites studied was defined by X-ray fluorescence analysis (Table 3). The K–Ar dating of the Kalkva Massif and two necks in the upper course of the Tanadon River points to their Early Pleistocene age (Calabrian, 1.45– 1.35 million years ago). The dating coincidence within the analytical uncertainty for all objects studied suggests their synchronous formation, in spite of the significant distance from each other (more than 100 km). Moreover, the same (within the uncertainty) K–Ar dates, which were obtained on biotite and the groundmass of RK-21 sample (Kalkva Massif), prove the reliability of the age range, during which the rocks were formed. Therefore, the isotopic data indicate the Early Pleistocene impulse of magmatism in the central part of the Greater Caucasus and the broad area of Calabrian volcanic activity. The Rb–Sr dating was carried out for sample RK-21 of dacite (Kalkva Massif) on the separates of plagioclase, biotite, and groundmass (Table 2). The data point to differences in the initial isotopic composition of Sr in the rock constituents, which hinder the plot-
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Table 1. Results of K–Ar isotopic dating of rocks of the Greater Caucasus Sample
Material
40
K, % ±σ
Arrad, ng/g ±σ
40
Arair, % in sample
Age, Ma ± 2σ
Kalkva Massif of dacite biotite 7.01 ± 0.07 0.676 ± 0.008 81.8 biotite 6.87 ± 0.07 0.619 ± 0.023 83.9 groundmass 2.50 ± 0.03 0.228 ± 0.008 69.8 Necks of dacite in the upper courses of the Tanadon River groundmass 2.10 ± 0.03 0.210 ± 0.003 75.0 '' 2.77 ± 0.03 0.278 ± 0.004 58.3 '' 2.69 ± 0.03 0.246 ± 0.005 38.9
K-605 RK-21
Ur-7 Ur-10 Ur-11
1.39 ± 0.04 1.30 ± 0.10 1.32 ± 0.10 1.44 ± 0.06 1.45 ± 0.05 1.32 ± 0.06
Table 2. Results of Rb–Sr isotopic dating of dacite of the Kalkva Massif (sample RK-21) Material Biotite Groundmass Plagioclase
Rb, g/t
Sr, g/t
420 102
26 297
7.5
87Rb/86Sr
± 2σ
46.17 ± 15 0.9998 ± 29 0.0215 ± 3
1010
87Sr/86Sr
Rb-Sr age
± 2σ
0.706136 ± 10 0.705197 ± 10 0.704990 ± 9
(biotite–groundmass) ± 2σ = 1.46 ± 0.02 Ma (
87Sr/86Sr)
0
= 0.70518
Table 3. Chemical composition (% wt) of young magmatic rocks of the Greater Caucasus Sample
SiO2
TiO2
Ur-7 Ur-10 Ur-11
67.74 69.64 69.73
0.54 0.56 0.58
68/65*
68.12
0.47
51** 53** 335** 337** 471** 346** 348**
69.02 70.10 69.42 72.63 68.00 70.83 69.02
0.38 0.37 0.14 0.25 0.24 0.31 0.27
Al2O3
Fe2O3
MnO
MgO
CaO
Necks of dacite in the upper courses of the Tanadon River 16.42 3.95 0.05 0.54 3.72 16.05 2.82 0.04 0.45 3.06 16.71 2.06 0.02 0.41 2.93 The Kalkva Massif of dacite 16.32 2.91 0.03 1.17 3.10 Necks and dykes of dacite at Zopkhito Glacier 14.88 3.41 0.07 1.59 2.92 14.93 2.97 0.07 1.71 2.24 15.47 2.85 0.07 0.72 2.76 15.48 3.75 0.11 0.52 1.82 15.11 5.59 0.01 1.38 3.92 14.75 4.12 0.08 0.81 3.17 15.04 4.80 0.08 1.34 3.77
Na2O
K 2O
P2O5
4.74 4.28 4.39
2.13 2.90 3.00
0.17 0.19 0.18
4.93
2.70
0.24
4.29 3.82 2.77 3.33 4.38 4.27 3.57
3.26 3.62 5.56 1.81 1.16 1.10 3.27
0.18 0.17 0.24 0.30 0.21 0.56 0.33
Results of analyses are normalized to 100%. *, data [6]; **, data [12].
ting of the isochrone line (Fig. 2a). The Rb–Sr age calculated for the biotite–groundmass pair is 1.46 ± 0.02 Ma. Within the uncertainty, this age coincides with K–Ar dates for the Kalkva Massif showing the equilibrium of the Rb–Sr system in this pair, and consequently, the compliance of this time estimate with a real geological event. However, the Rb–Sr date on the biotite–plagioclase pair is older and the initial isotopic composition of Sr in feldspar is less radiogenic compared to the groundmass ([87Sr/86Sr]i, 0.7050 and 0.7052, respectively). The imbalance of the Rb–Sr DOKLADY EARTH SCIENCES
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system in the indicated mineral pair is likely associated with the zonal structure and, as a consequence, with the isotope heterogeneity of plagioclase phenocrysts. The petrographic study of dacite of the Kalkva Massif shows that plagioclase has a more mafic composition in crystal cores, where, most likely, crystalization from magma began in the deep seated volcanic chamber. Later, as a result of assimilation combined with fractional crystallization (AFC) processes, the Sr isotopic composition in the melt of the near-surface chamber changed towards enrichment in radiogenic 87Sr, which
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Sr/86Sr 0.7062
(Na2O + K2O), %
(a)
(b)
Biotite 15
Phonolite
0.7060 Trachyte Biotite-groundmass T = 1.46 ± 0.02 Ma
0.7058
RukhsDzuar suite
Phoidite 10
0.7056 Biotite-plagioclase T = 1.75 ± 0.02 Ma
0.7054 0.7052
5
Groundmass
Basalt 0.7050 0.7048
Rhyolite
Trachyandesite
Andesite
Dacite
60
70
Plagioclase 0
10
20
30
40
50 87 Rb/86Sr 1
0
2
40 3
50
80 SiO2, %
4
Fig. 2. (a) Rb–Sr isotopic diagram for the RK-21 sample (dacite of the Kalkva Massif). (b) TAS diagram for the Quaternary magmatic rocks of the Greater Caucasus: (1) dacite of the Kalkva Massif, (2) dacite from the necks in the upper courses of the Tanadon River, (3) dacite from the neck above Zopkhito Glacier, (4) dacite from dykes near Zopkhito Glacier. The field of the composition of volcanite fragments from the Rukhs-Dzuar suite [13] is shown.
was assimilated from the host crust formations. Therefore, plagioclase of the phenocryst rims will be in equilibrium in relation to the groundmass (melt) and biotite, which was formed at the later stages of evolution of the magmatic system; plagioclase of the inner zones will be characterized by smaller values of the (87Sr/86Sr)i ratios. In this regard, calculation of Rb–Sr age for the biotite–zonal plagioclase pair, where the second constituent is heterogenous in relation to the Sr isotopic composition, makes no sense. The initial isotopic composition Sr of the Kalkva Massif dacite (87Sr/86Sr = 0.705) is generally close to the isotopic composition of the other rocks with the mantle–crust origin in the Kazbek region [10]. It is important that coeval volcanites of the Kalkva Massif and necks in the upper course of the Tanadon River are characterized by a close chemical composition (Table 3). The total alkali-silica diagram (TAS) (Fig. 2b) shows that their points generate a compact cluster in the field of dacite (rhyodacite) of normal alkalinity. All rocks fall into the calc-alkali petrochemical series; according to the K2O/SiO2 ratio, they are moderate- or high potassium and characterized by low magnesian (Mg# = 0.21–0.44). According to [11], there are outcrops of neck and numerous dacite dykes in a distance of 13 km west of the magmatic locus located in the upper courses of the Tanadon River, in the crest of the Greater Caucasus
Mountain Range, above Zopkhito Glacier (Fig. 1). These bodies intruded the Paleozoic granitoids and shales or the Jurassic sedimentary rocks, and their rocks have similar geochemical characteristics and geological position like we have studied for Calabrian dacites in North Ossetia (Table 3, Fig. 2b). It makes the basis to establish an Early Pleistocene age for magmatic rocks of Zopkhito Glacier. Moreover, as was noted, the K–Ar dating of the dacite intrusion [5], which entered the Pliocene granodiorite of the Tepli Massif in North Ossetia, allows us to assume a close time of this hypabyssal body. Therefore, the results form the basis to distinguish the specific Early Pleistocene (Calabrian) impulse of the youngest dacite magmatism in the Kazbek region, which occurred 1.45–1.35 million years ago. Moreover, it is possible to contour the area of volcanic activity for this time (Fig. 1). To the west, this area involves the crest of the Greater Caucasus Mountain Range between Zemo Racha (Geogia) and Digoria (North Ossetia), from Zopkhito Glacier to Mamison Pass; further to the east, it runs along the Bokovoi Range, crossing the Tepli Massif and the Kazbek volcanic center, to the upper courses of the Assa and Khevsureti Aragvi rivers, where, again, the Kalkva Massive is situated within the zone of the Greater Caucasus Mountain Range. Therefore, the area of Calabrian magmatism is a NW–SE striking narrow lane 125 km long in
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the highest part of the Greater Caucasus. It is important that there is a chain of Pliocene (3.5–1.8 Ma) intrusions of granitoids (Songutidon, Tepli, Dzhimara, and others) within the same area [1]. Thus, the Early Pleistocene endogenic activity of the Kazbek region inherits the area of Neogene activity manifested in the region and can be considered as the final impulse of the second (Pliocene) stage of young magmatism of the Caucasus. At the same time, Calabrian activity was not a precursor of Late Quaternary magmatism, which has other spatial patterns of the location of volcanic centers [1, 4]. Due to the heavy uplift and surface erosion, the Early Pleistocene volcanic cones of the Greater Caucasus was destroyed. Therefore, it is not possible to set out clearly the scope of the activity impulse and the volumes of eruptive material. However, the Ossetia basin and the neighboring basins, which are situated on the northern slopes of the Greater Caucasus, are built of the Rukhs-Dzuar volcanic-clastic series (Late Pliocene–Early Pleistocene) presented by redeposited fragments of disintegrated rocks of the Greater Caucasus Mountain Range and the Bokovoi Range (Fig. 1). For several sections, the volume of volcanic material exceeds 60% in the suite composition; rock fragments are mainly presented by dacite [12], which is similar to the dacite studied in this work (Fig. 2b). We consider that debris of the Pliocene and Early Pleistocene magmatic formations of the Greater Caucasus participated in the formation of the Rukhs-Dzuar series. The similar chemistry of the rock fragments composing the series and the studied volcanic centers indicate the domination of the Early Pleistocene volcanic material in the composition of the Rukhs-Dzuar sequences. By taking into account the vast volumes of the volcanogenic part of deposits of the Rukhs-Dzuar series, this fact points to the intensive development of Calabrian magmatism of the Kazbek region, the traces of which were destroyed to a large extent by hypergene processes.
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ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research, project no. 14-05-00071. REFERENCES 1. V. A. Lebedev, I. V. Chernyshev, and E. V. Sharkov, Dokl. Earth Sci. 441 (2), 1656–1660 (2011). 2. V. A. Lebedev, I. V. Chernyshev, A. V. Chugaev, et al., Dokl. Earth Sci. 408 (4), 657–661 (2006). 3. V. A. Lebedev, I. V. Chernyshev, A. V. Chugaev, et al., Geochem. Int. 48 (1), 41–67 (2010). 4. V. A. Lebedev, A. V. Parfenov, G. T. Vashakidze, et al., Dokl. Earth Sci. 458 (1), 1092–1099 (2014). 5. A. M. Borsuk, Mesozoic and Cainozoic Magmatic Formations of Greater Caucasus (Nauka, Moscow, 1979) [in Russian]. 6. D. S. Belyankin, Izv. Mineral. Lab. St.-Peterb. Politekh. Inst. Imp. Petra Velikogo, Otd. Tekh., Estestvozn. Mat. 18, 21–43 (1912). 7. L. A. Vardanyants, Probl. Sov. Geol., No. 7, 519–531 (1937). 8. D. S. Belyankin, V. P. Eremeev, and V. P. Petrov, Tr. Inst. Geol. Nauk Akad. Nauk SSSR, Petrogr. Ser., Issue 4, No. 3, 1–21 (1938). 9. M. M. Konstantinov, Kh. Kh. Laipanov, V. A. Danil’chenko, et al., Razved. Okhr. Nedr, Nos. 2–3, 2–10 (2005). 10. V. A. Lebedev and G. T. Vashakidze, J. Volcanol. Seismol. 8 (2), 93–107 (2014). 11. M. G. Togonidze and O. Z. Dudauri, Tr. - Geol. Inst. im. A. I. Dzhanelidze, Akad. Nauk Gruz., Nov. Ser., No. 124, 232–237 (2008). 12. N. V. Koronovskii and L. I. Demina, Priroda (Moscow, Russ. Fed.), No. 10, 37–43 (2003).
Translated by V. Krutikova
