ISSN 1028334X, Doklady Earth Sciences, 2011, Vol. 436, Part 1, pp. 32–38. © Pleiades Publishing, Ltd., 2011. Original Russian Text © V.A. Lebedev, S.N. Bubnov, A.I. Yakushev, 2011, published in Doklady Akademii Nauk, 2011, Vol. 436, No. 1, pp. 79–85.
GEOCHEMISTRY
Magmatic Activity within the Northern Caucasus in the Early Neopleistocene: Active Volcanoes of the Elbrus Center, Chronology, and Character of Eruptions V. A. Lebedev, S. N. Bubnov, and A. I. Yakushev Presented by Academician Yu.G. Leonov May 11, 2010 Received May 17, 2010
DOI: 10.1134/S1028334X11010065
(PaleoElbrus: ignimbrite of the BiitikTebe River head, K–Ar, 880 ± 70 and 810 ± 90 ka [1], Rb–Sr, 910 ± 15 ka [1]; subvolcanic body of the Kyukyurtlyu Wall, Ar–Ar, 620 000 ± 33 ka [6]; lavas of the Ullukam River head, U–Pb, 667 000 ± 40 ka [6]; ignimbrite of the Kyukyurtlyu Wall, U–Pb, 722 000 ± 15 ka [6]; Chuch khur; ignimbrite: K–Ar, 790 000 ± 70 ka, U–Pb, 689 ± 30 ka [6]), as well as estimative K–Ar dating for tra chyandesite of the lava remnant in the Khudes River mouth (850 ± 250 ka), a conclusion about the Early Neoplestocene age of this magmatic phase in the Elbrus Center was made [1–3]. Later on, youngest volcanism of the Elbrus Center was renewed only ~250–200 ka and proceeded during the three phases up to the end of Late Neopleistocene and possibly Holocene [1–3, 8]. Only the doublecone Elbrus volcano, which erupted dacite lava flows, was active at that time. The results of the isotopic–geochronological study for the most of known volcanic edifices within the western sector of the Elbrus Center, which allowed us to determine the position of their activity in the geo chronological scale of Late Cenozoic magmatism in the Greater Caucasus, are given and discussed in this paper. Based on the data obtained, we specified the age limits and main regularities in the evolution of young est volcanic activity of the center in the Early Neo pleistocene. The results of K–Ar dating are given in Table 1; the main characteristics of K–Ar dating tech nique for young magmatic formations applied at the Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences were described in [9]. The PaleoElbrus most likely was the largest vol cano active in the Early Pleistocene [1]; its remnants are observed in heads of the Kyukyurtlyu and Ullu Kam glaciers, to the west from the modern Elbrus summit (Fig. 1). According to the available geological data and K–Ar datings obtained (Table 1), the initial stage of its eruptions (~800 ka) most likely of the fis
Forecast of possible scenarios of eruptive activity for volcanic centers and individual volcanoes as well as the type and character of their future eruptions is an important aspect of investigations of modern (Quater nary) magmatism of the Earth. Prognostic reconstruc tions of this type are impossible without analysis of tendencies in the development of volcanic centers and determination of the main time characteristics of magmatic activity occurred inside them. The North ern Caucasus is the only region in European Russia where modern magmatism was manifested, although only to a small extent. Magmatic activity in the Anthro pogene occurred there exclusively within the Elbrus neovolcanic center in the southern part of which the Elbrus, one of the largest Quaternary volcanoes of the Northern Eurasia, was formed lesser than 250 ka [1–3]. Quaternary volcanism of the Elbrus center started ~950–900 ka [4] with eruptions of three lava volca noes (Tyzyl, Tashlysyrt, and Syltran) located within the linear submeridional zone at the Baksan and Malka interfluves. The regular decrease of SiO2 con tent in young effusive rocks along the linear zone from the south to the north with transition from calcalka line to K–Nasubalkaline rock varieties is an impor tant peculiarity of the first (Eopleistocene) phase of modern magmatic activity of the center mentioned in [4]. This is most likely explained by a decrease in the role of the crustal component in petrogenesis of parental magmas in the northern part of the region. The next phase of Quaternary magmatism pro ceeded in the western sector of the Elbrus Center (Fig. 1), where several individual Quaternary volca noes (PaleoElbrus, Chuchkhur, Chomartkol, and TashTebe) are known [1, 5]. Based on a small number of isotopic datings for two volcanoes of this group
Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Moscow email:
[email protected] 32
E
Kuban
Khudes flow 2141/0 2151/0
33
42°22′30′′
MAGMATIC ACTIVITY WITHIN THE NORTHERN CAUCASUS ka al M 43°35′ N
Kh es ud
Elbrusskii settl.
Khudes 991
TashTebe
Sh
Chuchkhur
l ko au
Chuchkhur1
Tokhana
Chu chk hur
2171/0
Kub
Bt2/97
Bt12/97 Bt13/97
lyu urt uky Ky
Khurzuk settl.
M alk a
Chomartkol
UlluKhurzuk
Bt8/97
an
209/0 211/0 2121/0
Elbrus
1
PaleoElbrus 0
2
4
Ku
6 km
2102/0
ban
2
ik Ir
1 а
b
2 3
4 Elbrus 337
5 Georgia
Terskol
Baksan
Fig. 1. Geological map of the western part of the Elbrus volcanic center. (1) Late Quaternary dacite lavas of the Elbrus Volcano; (2) Early Neopleistocene lavas and pyroclastic formations (a), remnants of Early Neopleistocene volcanoes (b); (3) ignimbrites and tuffs of the northeastern Elbrus slope; (4) Quaternary volcanoes; (5) places of sampling and sample numbers. Numbers in circles: (1) Kyukyurtlyu Wall, (2) UlluKam Wall.
sure type was controlled by the formation of covers of rhyodacitic ignimbrite and welded tuff, which formed volcanoclastic sequences with a thickness up to 120– 140 m at the Kyukyurtlyu, BiitikTebe, and Kyzylkol river heads (Fig. 2). Currently covers of pyroclastic formations in the BiitikTebe River valley are mainly preserved in its head beneath Late Quaternary lavas of the Elbrus Volcano and downstream ignimbrites are observed only as individual remnants on the right bank terraces up to the modern mouth of the river. The remnant preserved near the interflow of the Biitik Tebe and Kyukyurtlyu rivers, the most distant from the eruption center is located ~15 km to the northwest DOKLADY EARTH SCIENCES
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from the PaleoElbrus Volcano (Fig. 1). Ignimbrite horison in the Kyukyurtlyu River head is outcropped on the right side of the river valley, where it is overlain by thick sequences of PaleoElbrus dacitic lavas com posing the Kyukyurtlyu and UlluKam Walls. The ign imbrite remnant is also known on the northern slope of the modern Elbrus edifice in the area of the Ullukol glacier, where these rocks overlie on rhyolitic pyroclas tic formations and rocks of the crystalline basement. Rocks of PaleoElbrus volcanic activity usually have rhyodacitic composition, and crystalloclasts in them are represented by plagioclase (from oligoclase to oli goclase–andesine), biotite, quartz, and rarely hyper
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Results of K–Ar dating of volcanic rocks from the western part of the Elbrus area Sample
Bt2/97 Bt8/97 Bt12/97
Bt13/97
209/0 2171/0
Place of sampling
Ignimbrite cover in the Biitik Tebe River mouth BiitikTebe River head, welded tuff horizon on the left bank BiitikTebe River head, ignim brite remnant on the right bank, upper part of the section BiitikTebe River head, ignim brite remnant on the right bank, lower part of the section Kyukyurtlyu Glacier, ignimbrite
Analyzed material
K, % ± σ
PaleoElbrus Volcano Glass 3.33 ± 0.04 Biotite 7.10 ± 0.08 Groundmass 3.50 ± 0.04
40Ar , rad
ppb ± σ
40Ar
atm, % Age, ka ± 2σ (in sample)
0.2 ± 0.1 0.56 ± 0.04 0.25 ± 0.06
99.8 90.2 99.0
1000 ± 1000 1100 ± 200 1000 ± 400
4.60 ± 0.09 7.04 ± 0.08
0.291 ± 0.008 0.382 ± 0.005
91.1 81.6
910 ± 60 780 ± 30
Groundmass 3.67 ± 0.04 Biotite 6.83 ± 0.07
0.206 ± 0.015 0.376 ± 0.010
97.2 87.1
810 ± 90 790 ± 40
Groundmass 3.32 ± 0.04 Не обнаружен Biotite 6.93 ± 0.07 0.411 ± 0.018 Groundmass 3.37 ± 0.04 Не обнаружен
>99.9 94.4 >99.9
– 850 ± 80 –
0.155 ± 0.004 0.254 ± 0.006 0.122 ± 0.002
45.6 61.0 55.8
690 ± 40 750 ± 40 500 ± 25
0.162 ± 0.005
89.8
680 ± 40
0.167 ± 0.006
93.3
700 ± 50
0.177 ± 0.003
48.2
830 ± 30
0.128 ± 0.004
92.9
690 ± 50
0.148 ± 0.004
79.0
750 ± 40
Glass Biotite
Upper ignimbrite cover beneath the Ullukol Glacier 211/0 Dacite lavas of the Kyukyurtlyu Groundmass 3.26 ± 0.04 Wall Biotite 4.89 ± 0.05 2121/0 Subvolcanic dacite massif of the Groundmass 3.54 ± 0.04 Kyukyurtlyu Wall Neck above the Kyukyurtlyu Glacier 2102/0 Daciteporphyrite Groundmass 3.46 ± 0.04 Chuchkhur Volcano Chuchkhur1 Ignimbrite Groundmass 3.45 ± 0.04 TashTebe Volcano 991* Andesite lava flow in the Tokhana Groundmass 3.06 ± 0.04 River valley The same 2.71 ± 0.03 2141/0 Trachyandesite lava remnant in the Khudes River mouth, lower part of the section 2151/0 Trachyandesite lava remnant “ 2.82 ± 0.03 in the Khudes River mouth, up per part of the section
* Sample from the collection of V.Yu. Gerasimov (Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences).
sthene. The ignimbrite groundmass often composed of quartz–feldspar aggregate or fluidal glass contains for mations of “fiamme” type, usually anisotropic with a transfibrous texture. The rocks contain 67.8–69.8% SiO2, 6.8–7.9% K2O + Na2O with 3.4–3.7% K2O and are related to the calcalkaline petrochemical series. K–Ar datings obtained for six biotite–groundmass pairs from ignimbrite samples are in a good agreement allowing us to estimate the time of explosive activity of the PaleoElbrus Volcano (800 ± 50 ka). Note that a slightly “younger” dating (722 ± 15 ka) obtained by the U–Pb method (SHRIMP) was given in [6] for ignim brite of the Kyukyurtlyu glacier. The second stage of PaleoElbrus activity was con trolled eruptions of most likely central type and lava covers of moderate acidity; their stratigraphic section
with a thickness of several hundreds meters is observed in scarps of the Kyukyurtlyu and UlluKam Walls. The composition of volcanic rocks corresponds to dacite or rhyodacite. Phenocrysts (up to 20%) are represented by plagioclase (from oligoclase–andesine to labra dorite), orthopyroxene, quartz, and biotite; single amphibole crystals are observed in some varieties as well. The rocks contain 67.2–70.1% SiO2, 7.0–7.6% K2O + Na2O with 2.9–3.6% K2O and are basically related to the calcalkaline petrochemical series. K–Ar dating obtained for the biotite–groundmass pair from dacite of the Kyukyurtlyu Wall allows us to determine the time of their formation (720 ± 30 ka) (Table 1). A U–Pb isotopic age similar to our dating was previ ously reported for these rocks (667 ± 40 ka) [6]. DOKLADY EARTH SCIENCES
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Kuban
42°22′30′′
MAGMATIC ACTIVITY WITHIN THE NORTHERN CAUCASUS
M
ka al
43°35′
hu
es ud Kh
K
de sf
lo
Khudes
w
TashTebe l ko au Sh
Chu chk hur
Chuchkhur lkol Kyzy
n
lyu urt uky Ky
a Kub
Ma lka
Chomartkol BiitikT ebe
UlluKhurzuk
PaleoElbrus 0
2
4
6 km
Ku
ban
ik Ir
1 2 3 MAIN CAUCASUS RIDGE
Baksan
Fig. 2. Expansion of Early Neopleistocene volcanism in the Elbrus area. (1) Early Neopleistocene lavas and pyroclastic forma tions; (2) volcanoes; (3) position of modern volcanic cones of the Elbrus Volcano.
Modern intense tectonic processes in the Elbrus area resulted in the destruction of most of the Paleo Elbrus Volcano edifice and revealed its internal struc ture on the day surface in the sections of the UlluKam and Kyukyurtlyu Walls. According to the data of [6], outcrops of the subvolcanic massive intruding the ear lier lava covers and most likely forming as a result of melt consolidation in one of the nearsurface volcanic chambers are observed in the central part of the Kyukyurtlyu Wall. The set of phenocrysts typical for rocks of the massif comprises plagioclase, quartz, and biotite; some varieties contain sanidine intergrown with plagioclase [6] and rarely orthopyroxene. The rocks correspond to two petrochemical series, calc DOKLADY EARTH SCIENCES
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alkaline and K–Nasubalkaline, at approximately equal proportions and contain 67.8–69.2% SiO2, 7.3– 8.4% K2O + Na2O with 3.7–4.1% K2O. Our K–Ar dating for the groundmass of dacite (500 ± 25 ka, Table 1) is significantly “younger” than the age of ign imbrites and lavas of the PaleoElbrus. Previously [6] the Ar–Ar dating obtained by sanidine megacrysts (620 ± 33 ka) was reported for rocks of the “Kyukyurt lyu Wall extrusion.” All these data provide evidence for the fact that the real age of melt intrusion and forma tion of subvolcanic body rocks is most likely close to the age of dacitic lava eruption (720 ± 30 ka), whereas the isotopic datings obtained correspond to the time of massif cooling to the temperatures of the K–Ar system
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closing in the hightemperature feldspar (~500°C) and groundmass (800°C during its intrusion [10] to 1000°C [11] and others), but by the specific mechanism of their flow as well. Note that the last among the known lava rem nants of the TashTebe Volcano in the Khudes River gorge is located 20 km to the west from the cone, in the Chuchkhur River mouth area, on the upper terrace of the valley at a height of ~300 m above the level of its modern downcutting (Fig. 1). Currently there is still no data on lava remnants in the Khudes River valley banks from the place of the Chuchkhur River fall to the mouth on a distance of 10 km. This may be explained by thick forests in the gorge impeding detailed geological mapping. Another, most probable cause is that the middle segment of the Khudes flow was almost completely destructed by erosion and was not preserved up to now. To a definite degree this could be favored by a narrow Vlike shape of the valley with quite steep slopes, as well as by the quite long period (800 000–700 000 years) after eruption of the Tash Tebe Volcano and the formation of the lava river. The assumed high rate of erosion of volcanic rocks from the upper and middle segments of the Khudes flow, which we basically explain by the most probable spe cific mechanism of the lava motion, is very important as well. The geological study of the remnant in the left bank of the Khudes River mouth demonstrated the sym metric structure of the volcanic section with a quite clearly reflected axial zone formed by volcanic rocks with gentle slab parting. The lower and upper parts of the section are composed by massive, rarely porous, usually fluidal lavas with elements of subvertical columnar jointing. The surface of the Khudes flow in the Khudes River mouth is often represented by a con glomeration of lava blocks and plates sometimes standing on edges. The morphology and peculiarities of Khudes flow structure are practically analogous to those registered for the Quaternary valley lava rivers of the Southern Baikal volcanic area [12 and others]. The motion of melts for long distances (for exam ple, the length of lava river in the Malyi Yenisei valley exceeds 175 km [12]) was mainly provided by their flow inside lava tunnels formed as a result of the for DOKLADY EARTH SCIENCES
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mation of the flow crust preventing the melt from heat loss. As this took place, the axial zone of lava river sec tions formed by volcanic rocks with slab parting corre sponds to the areas of differentiated horizontal lava flow. In our case we may assume the high rate of ero sion (including washout) of volcanic rocks of the upper and middle segments of the Khudes flow because of natural morphological peculiarities of lava tunnels (absence of continuous layering of lava mate rial, presence of lava pits and wells, lava bars, domes, swells, and like that). The lower part of the lava river preserved up to now as an isolated remnant near the Khudes and Kuban’ river interflow most likely had a higher thickness in comparison with the upper and middle segments of the lava river. Andesite flows had mass and energy sufficient for reaching the mouth part of the paleovalley of the Khudes River, where they met the barrier of basement rocks. This is directly evident from steeply dipping (up to subvertical) contacts between young lavas and Variscian formations, as well as the existence of folds of gravitational flattening in the contact zones of the flow with the formation of fantype structures. Because of the lava damming, the mouth area of the paleovalley was most likely filled to a height of not less than 100 m. Note that the definite petrochemical difference between lavas of the upper and lower segments of the Khudesskii lava river is a quite abundant phenomenon for such volcanic forma tions (see, for example, [12]) and explained by their formation at various stages of the TashTebe Volcano activity. Thus, the isotope–geochronological data obtained allow us to determine the age of the second (Early Neopleistocene) phase of magmatism for the Elbrus center (800–700 ka). All volcanoes active at that time period are located in its western part, within the linear submeridional zone with a length of up to 20–25 km (Figs. 1, 2). Similarly to the previously distinguished zone of Eopleistocene volcanic activity in the eastern part of the region [4], the products of Early Neopleis tocene magmatism have a tendency of their SiO2 decrease from rhyodacite to andesite and trachyandes ite, from the south to the north. This confirms our pre vious assumption [4] that contamination or mixing of primary mantle subalkaline basaltic melts with crustal material was less intense within the whole northern part of the Elbrus center in comparison with its south ern part. The Early Neopleistocene (800–700 ka) age of ign imbrites and welded tuffs from the western part of the Elbrus provides evidence for the fact that they are not the products of activity of the Late Neopleistocene Elbrus Volcano, which was formed ~250–200 ka [1–3, and others], as this was suggested by some researchers (for example, [13]). It is clear that pyroclastic rocks erupted by three separate, relatively small volcanoes (PaleoElbrus, Chomartkol, Chuchkhur) are not material evidence of the existence of the hypothetical caldera stage in the evolution of the Elbrus center,
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which, according to the studies [6, 13, and others], demonstrated basement caving in the area of the mod ern Elbrus Volcano (Elbrus Caldera) accompanied by the extensive tuff outbursts and ignimbrite eruptions. The data obtained allow us to state that the caldera stage in the Elbrus history most likely was not passed yet and remain a high probability for such a scenario of magmatism in the Elbrus area in the future. ACKNOWLEDGMENTS This study was supported by the Russian Founda tion for Basic Research (project no. 090591220ST). REFERENCES 1. I. V. Chernyshev, V. A. Lebedev, S. N. Bubnov, et al., Dokl. Akad. Nauk 380 (3), 384–389 (2001) [Dokl. Earth Sci. 380, 848 (2001)]. 2. V. A. Lebedev, I. V. Chernyshev, S. N. Bubnov, et al., Dokl. Akad. Nauk 405 (6), 389–394 (2005) [Dokl. Earth Sci. 405A, 1321 (2005)]. 3. V. A. Lebedev, I. V. Chernyshev, A. V. Chugaev, et al., Geokhimiya, No. 1, 45–73 (2010) [Geochem. Int. 48, 41 (2010)]. 4. V. A. Lebedev, V. G. Sakhno, and A. I. Yakushev, Dokl. Akad. Nauk 430 (2), 232–238 (2010) [Dokl. Earth Sci. 430 (1), 80 (2010)].
5. E. E. Milanovskii and N. V. Koronovskii, Orogenic Vol canism and Tectonics of the Alpine Belt of Eurasia (Nedra, Moscow, 1973) [in Russian]. 6. V. M. Gazeev, Petrology and Ore Potential of the Elbrus Volcanic Center (Northern Caucasus) Candidate’s Dis sertation in Geology and Mineralogy (Moscow, 2003) [in Russian]. 7. A. M. Borsuk, Mesozoic and Cenozoic Magmatic Forma tions of Greater Caucasus (Nauka, Moscow, 1979) [in Russian]. 8. O. A. Bogatikov, I. V. Melekestsev, A. G. Gurbanov, et al., Dokl. Akad. Nauk 363 (2), 219–221 (1998) [Dokl. Earth Sci. 363, 1093 (1998)]. 9. I. V. Chernyshev, V. A. Lebedev, and M. M. Arake lyants, Petrologiya 14 (1), 69–89 (2006) [Petrology 14, 62 (2006)]. 10. V. M. Gazeev, A. A. Nosova, L. V. Sazonova, et al., Vul kanol. Seismol., No. 1, 1–22 (2004). 11. M. L. Tolstykh, V. B. Naumov, A. G. Gurbanov, et al., Geokhimiya, No. 4, 441–448 (2001) [Geochem. Int. 39, 391 (2001)]. 12. V. V. Yarmolyuk, A. M. Kozlovskii, E. A. Kudryashova, et al., Vulkanol. Seismol., No. 4, 3–20 (2004). 13. O. A. Bogatikov, I. V. Melekestsev, A. G. Gurbanov, et al., Dokl. Akad. Nauk 363 (4), 515–517 (1998) [Dokl. Earth Sci. 363A, 1202 (1998)].
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