The Potentially Active Porak Neovolcanic Center (Lesser Caucasus, Armenia): The Composition of Lavas and Melt Sources

Abstract

In this paper we report the first results of petrological and isotope-geochemical studies of young lavas of the Porak neovolcanic center within the eastern part of the potentially active Vardenis volcanic area of the Lesser Caucasus (Armenia).
According to a number of material characteristics, the newest lavas of the Porak neovolcanic center are close to the continental intraplate formations. These characteristics are consistent with the ideas on the regional deep mantle reservoir of the OIB type as a source of primary melts for lavas of the center and provide evidence for the significant role of assimilation by the crustal material in their petrogenesis.

Full text

ISSN 1028334X, Doklady Earth Sciences, 2014, Vol. 459, Part 1, pp. 1365–1370. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © S.N. Bubnov, V.A. Lebedev, I.V. Chernyshev, A.K. Sagatelyan, A.Ya. Dokuchaev, Yu.V. Gol’tsman, T.I. Oleinikova, I.G. Griboedova, 2014, published in
Doklady Akademii Nauk, 2014, Vol. 459, No. 1, pp. 73–79.
1365
The prediction of likely consequences of volcanic
activity renewal is one of the main aims of the study of
magmatism of the modern active areas of the Earth.
Study of the character, probable change of the regime,
and type of eruptions for concrete territories in the
future (e.g., from lava eruptions to catastrophic explo
sions) is impossible without knowledge of the regional
peculiarities of petrogenesis of young magmatic rocks,
the sources of matter for parental melts, and the main
regularities of their geochemical evolution in time.
The studies related to prediction of consequences of
future eruptions for natural ecosystems and social
infrastructure are of special importance for densely
populated areas, in which renewal of endogenic activ
ity is possible. The Caucasus region is undoubtedly
related to those areas.
In this paper we report the first results of petrolog
ical and isotope–geochemical studies of young lavas of
the Porak neovolcanic center within the eastern part of
the potentially active Vardenis volcanic area of the
Lesser Caucasus (Armenia).
The Vardenis neovolcanic area occupies the terri
tory of the highland of the same name, which is sepa
rated by Sevan Lake in the north and the Arpa River
valley in the south; it is connected with the Geghama
highland in the west and with the Syunik highland in
the southeast.
The Porak neovolcanic center is located on the
watershed ridge in the eastern part of this area, to the
southeast from Sevan Lake. This center includes the
Porak Volcano (AkharBakhar, “gutted belly” in Azer
baijan) and numerous secondary cones and centers of
fissure lava eruptions (Fig. 1). In this area the volca
noes are mostly controlled by the NW–SE zone of the
deep Syunik fault, which, in the opinion of some
researchers, is a typical pull apart structure ([1] and
others). Rocks of the basement are represented by vol
canogenic (porphyrite, tuff breccia, tuff sandstone,
tuff conglomerate, tuffite) and sedimentary (lime
stone, marl, sandstone) Late Cretaceous and Eocene
series, which are intruded by Paleogene granitoids in
some places. These rocks are related to the subduc
tion–accretion complex of the Late Mesozoic and
Early Cenozoic formed as a result of convergence and
subsequent joining of the Zangezur (South Armenian)
Paleozoic terrane to the Eurasian Plate.
Four generations of the newest lavas of the center
with different ages, geological settings, and morpho
structural peculiarities are distinguished (Fig. 1). The
earliest lavas (the first generation) composing the
Porak Volcano are traditionally considered as Late
Pleistocene, whereas the three younger generations of
lava flows that erupted from the secondary cones and
from the centers of fissure eruptions are Holocene ([1]
and others). The Holocene age from the latter is evi
dent from the reported
14
C data (~6500 ka) [1] as well.
We confirmed the Late Neopleistocene–Holocene
age of the products of the Porak Volcano activity by the
K–Ar dating for lavas of the third generation (
15
±
15
ka
at a K content of
2.50
±
0.03
wt %,
40
Ar
rad
= 0.0031
±
0.0019
ppb, and
40
Ar
air
(in sample) = 99.6%) obtained
at the Institute of Geology of Ore Deposits, Petrogra
phy, Mineralogy, and Geochemistry, Russian Acad
emy of Sciences. Some researchers believe that there is
archeological and written evidence for the activity of
this neovolcanic center in historical time. For exam
ple, A.S. Karakhanyan et al. [1] found the facts in
favor of the historical activity of the Porak Volcano in
the wedge writing of the Urart King Argishti I (787–
766 BC) and his son Sardur II (765–733 BC) and
interpret petroglyphs found to the east from the main
volcanic cone and dated as the end of the fifth century
BC as an image of a volcanic eruption. If these
The Potentially Active Porak Neovolcanic
Center (Lesser Caucasus, Armenia):
The Composition of Lavas and Melt Sources
S. N. Bubnov
a
, V. A. Lebedev
a
,
Academician
I. V. Chernyshev
a
, A. K. Sagatelyan
b
, A. Ya. Dokuchaev
a
,
Yu. V. Gol’tsman
a
, T. I. Oleinikova
a
, and I. G. Griboedova
a
Received June 4, 2014
DOI:
10.1134/S1028334X14110014
a
Institute of Geology of Ore Deposits, Petrography,
Mineralogy, and Geochemistry, Russian Academy of Sciences,
Moscow, Russia
b
Center of Ecological–Noosphere Studies,
National Academy of Sciences, Yerevan, Armenia
email: bubnov@igem.ru
GEOCHEMISTRY

1366
DOKLADY EARTH SCIENCES Vol. 459 Part 1 2014
BUBNOV
et al.
Sevan LakeSevan LakeSevan Lake
TsovakTsovakTsovak
KarchakhpyurKarchakhpyurKarchakhpyur
LchavanLchavanLchavan
KhonarassarKhonarassarKhonarassar
KarmirdarKarmirdarKarmirdar MakenisMakenisMakenis
GegakarGegakarGegakar
VanevanVanevanVanevan TorfavanTorfavanTorfavan
VardenisVardenisVardenis
LusakunkLusakunkLusakunk
KhachagbyurKhachagbyurKhachagbyur
AkunkAkunkAkunk
222222
999
101010
888
111111
131313
777
666
555
Lesser Alagel LakeLesser Alagel LakeLesser Alagel Lake
Greater AlagelGreater AlagelGreater Alagel
LakeLakeLake
121212
PorakPorakPorak
MaralsarMaralsarMaralsar
SandukhkasarSandukhkasarSandukhkasar
SarigagatSarigagatSarigagat
333
181818
LyultagLyultagLyultag
KhoraporKhoraporKhorapor
AkhpradzorAkhpradzorAkhpradzor
Kyumbez Kyumbez Kyumbez
MechitdarMechitdarMechitdar
111
777
222
888
333
999
444
101010
555
111111
666
121212
AkkayadagAkkayadagAkkayadag
2 km
45
°
45
′
40
°
10
′

DOKLADY EARTH SCIENCES Vol. 459 Part 1 2014
THE POTENTIALLY ACTIVE PORAK NEOVOLCANIC CENTER 1367
assumptions are correct, the Porak volcanic center
may be considered as an active one (category A [2]).
Products of the Porak volcanic center activity have
a close petrographic composition and mostly include
olivine basaltic trachyandesite and trachyandesite. In
addition to olivine, phenocrysts in rocks are usually
represented by augite and mainly zoned plagioclase,
rarely quartz and hypersthene. The dominating tex
ture of the groundmass is pilotaxitic, sometimes tran
siting to microlitic.
Lavas of the center contain 54.5–59.0 wt %
SiO
2
,
6
.8–7.3 wt %
Na
2
O + K
2
O
at 2.4–3.0 wt %
K
2
O
;
according to the chemical classification TAS [3], the
rocks correspond to shoshonite (basaltic trachyandesite)
and latite (trachyandesite). The highest
K
2
O/Na
2
O
ratios (up to 0.71) are usually typical of Holocene lavas
of the last generation. All rocks are characterized by
low concentrations of
TiO
2
(0.89–1.12 wt %), rela
tively low Mg# (0.49–0.51), low Ti/Y (213–301), and
enrichment in Sr (up to 980 ppm).
As a whole, the geochemical features of volcanic
rocks correspond to those of K–Na subalkaline basic
and intermediate rocks of the continental rifts and hot
spots [4]. The similarity of the petrogeochemical
parameters of the studied volcanic rocks and basic
rocks of intraplate geodynamic environments is evi
dent from the compositions of the newest lavas plotted
on the known petrogenetic diagrams. On the diagram
Zr–Zr/Y [5] (Fig. 2a), the points plot in the field of
intraplate basalt, and on the diagram Zr/4–Y–2 ×Nb
[6] they plot in the field of intraplate basalts of high
alkalinity (Fig. 2b).
In analyzing the OIBnormalized [7] multiele
ment spectra of rocks, first of all, we should emphasize
the clear similarity of the newest lavas from the Porak
volcanic center to OIB by a number of geochemical
parameters (Fig. 2c). The studied volcanic rocks are
characterized by concentrations of phosphorus (
P
n
=
0.71–1.02), strontium (
Sr
n
= 1.18–1.48), yttrium
(
Y
n
= 0.66–0.93), cerium (
Ce
n
= 1.14–1.24), neody
mium (
Nd
n
= 0.86–0.99), ytterbium (
Yb
n
= 0.88–
1.02), lutetium (
Lu
n
= 0.93–1.10), and others, which
are similar to those of OIB. The multielement spectra
of the newest volcanic rocks show strong positive
anomalies of Th, U and Pb, as well as negative anom
alies of Nb and Ti. We assume that in our case such
anomalies are the geochemical signs of assimilation of
the crustal material by primarily basic mantle melts of
the OIB type.
The chondritenormalized [8] REE distribution is
characterized by strong fractionation of LREEs in
relation to HREEs (
La/Yb
n
= 16.8–18.8) and moder
ate fractionation of LREEs in relation to MREEs
(
La/Sm
n
= 5.6–5.9); the spectrum is flat in the HREE
area, which is reflected by the low
Dy/Yb
n
= 1.2–1.3
(Fig. 2d). The REE spectra of the newest lavas of the
Porak Volcano show a poor negative Eu anomaly with
Eu/Eu* = 0.87–0.92. We should point to some simi
larity of the REE spider diagrams for the studied vol
canic rocks and OIBs [7].
The erupted lavas are characterized by a close
strontium isotope composition: the values of the initial
isotope ratios
87
Sr/
86
Sr
match within the analytical
errors for all studied samples and range within
≈
0.70436–0.70438 (table). An analogous situation is
Fig. 1.
Geological map of the northeastern part of the Vardenis highland, compiled by V.A. Lebedev on the basis of field observa
tions and the results of satellite image processing. (
1
) Quaternary sedimentary deposits; (
2
–
5
) Porak Volcano lavas: (
2
) the fourth
phase, (
3
) the third phase, (
4
) the second phase, (
5
) the first phase; (
6
) Middle and Early Quaternary lavas of the basic and inter
mediate compositions; (
7
) Pliocene acid and midacid volcanic rocks; (
8
) Paleogene and Cretaceous volcanogenic and sedimen
tary formations; (
9
) Quaternary slag cones; (
10
) secondary cones of the Porak Volcano; (
11
) modern active tectonic dislocations;
(
12
) places of sampling and sample numbers. Coordinates of the places of sampling (WGS 84), original sample numbers, and rock
names: (5)
39
°
59
′
47.8
″
N,
045
°
43
′
34.3
″
E, M05/12, latite; (6)
40
°
00
′
06.3
″
N,
045
°
43
′
43.8
″
E, M06/12, latite; (7)
40
°
00
′
17.3
″
N,
045
°
43
′
46.5
″
E, M07/12, latite; (8)
40
°
01
′
21.2
″
N,
045
°
44
′
11.1
″
E, M08/12, shoshonite; (9)
40
°
01
′
44.5
″
N,
045
°
44
′
21.4
″
E,
M09/12, latite; (10)
40
°
01
′
39.9
″
N,
045
°
44
′
12.5
″
E, M10/12, latite; (11)
40
°
01
′
25.5
″
N,
045
°
44
′
03.5
″
E, M11/12, shoshonite;
(12)
40
°
00
′
30.4
″
N,
045
°
42
′
25.0
″
E, M12/12, shoshonite; (13)
40
°
00' 59.4
″
N,
045
°
44' 11.3
″
E, M13/12, shoshonite;
(18)
40
°
03
′
24.3
″
N,
045
39
′
47.6
″
E, M18/12, latite; (22)
40
°
03
′
24.3
″
N,
045
°
39
′
47.6
″
E, M22/12, latite.
Sr and Nd isotope composition of the newest lavas of the Porak Volcano group
Sample Rb Sr
87
Rb/
86
Sr
±
2
σ
87
Sr/
86
S
±
2
σ
Sm Nd
147
Sm/
144
Nd
±
2
σ
143
Nd/
144
Nd
±
2
σε
Nd
ppm ppm
М05/12 79 840 0.271
±
10.704377
±
10 31 5.0 0.0990
±
10.512820
±
73.6
М08/12 48 1028 0.1352
±
60.704356
±
10 37 5.5 0.0896
±
10.512818
±
73.5
М09/12 54 965 0.1631
±
60.704356
±
10 34 5.7 0.0997
±
10.512820
±
73.6
М18/12 73 880 0.2389
±
80.704363
±
10 33 5.9 0.1072
±
10.512817
±
73.5
М22/12 74 926 0.2313
±
80.704364
±
10 34 5.5 0.0988
±
10.512808
±
73.3
Places of sampling are shown in Fig. 1. The bulk rock samples were analyzed. The error of ε
Nd
estimation is ±0.1.

1368
DOKLADY EARTH SCIENCES Vol. 459 Part 1 2014
BUBNOV
et al.
20
10
5
2
1
Intraplate
basalt
Island
arc
basalt
MORB
10 20 50 100 200 500 1000
Zr
Zr/Y
(a)
Zr/4 Y
2
×
Nb
AI
AII
C
B
D
100
10
1
0.1
Rock/OIB
Cs
Rb
Ba
Th
U
Nb
K
La
Ce
Pb Sr PNd
Zr
Sm
EuTi
DyYYb
Lu
Pr
1000
Rock/Chondrite
100
10
1
La Сe Pr NdPmSmEuGdTb DyHo ErTmYb Lu
(b)
(c) (d)
87
Sr/
86
Sr
10
8
6
4
2
0
−
2
−
4
0.702 0.703 0.704 0.705 0.706 0.707
ε
Nd
DMM
MORB
COMMON
OIB
Kazbek area
Quaternary lavas of the Geghama area
Central Georgian area
Elbrus area
Youngest lavas of the Porak
Javakheti area
“Caucasus”
(e)
Fig. 2.
Variations of the geochemical and isotope compositions of lavas from t
he Porak ne
ovolcanic center. The fields on the
Zr/4–Y–Nb
×
2 diagram [6] (Fig. 2b): (
AI
) intraplate alkaline basalt; (
AII
) intraplate alkaline basalt and tholeiite; (
B
) Etype
MORB; (
C
) intraplate tholeiite and island arc basalt; (
D
) Ntype MORB and island arc basalt. Figure 2c shows the compositional
field (gray color) of the studied lavas. The data on the young volcanic rocks of the Caucasian region for the isotope–correlation
Sr–Nd diagram (Fig. 2e) are taken from [9, 10] and others. The light triangles in Figs. 2a, 2b, and 2d indicate the composition of
OIBtype basalt [7].
neovolcanic center

DOKLADY EARTH SCIENCES Vol. 459 Part 1 2014
THE POTENTIALLY ACTIVE PORAK NEOVOLCANIC CENTER 1369
observed for the neodymium isotope composition: the
values of the initial isotope ratios
143
Nd/
144
Nd
are
≈
0.51281–0.51282 (
ε
Nd
3.4
±
0.2
). The compositions
of the newest lavas of the Porak volcanoes plot com
pactly in the field of OIBs on the isotope Sr–Nd dia
gram (Fig. 2e).
The isotope–geochemical data, particularly the
identity of the Sr–Nd isotope characteristics of the
newest volcanic rocks of all phases of activity of the
Porak neovolcanic center, provide evidence for a high
homogeneity of the source, which generated parental
melts in the Late Neopleistocene–Holocene, at least
within the eastern part of the Vardenis neovolcanic
area of the Lesser Caucasus. As a whole, the Sr–Nd
isotope parameters of the newest lavas of Porak Vol
cano are close to those for the young magmatic forma
tions from the other neovolcanic areas of the Cauca
sian region, particularly for Quaternary lavas of the
Geghama highland, Pliocene and Quaternary lavas of
various compositions from the Javakheti highland
within the Lesser Caucasus, midalkaline Miocene
basalt from the Central Georgian area, and the most
basic rock varieties from the Kazbek neovolcanic area
of the Greater Caucasus ([9, 10] and others, Fig. 2e).
The data on volcanic rocks from the Porak neovol
canic center in the eastern part of the Vardenis neovol
canic area in the Lesser Caucasus and products of
young volcanism in the other areas of the Caucasian
region are in agreement with the existence of a single
regional deep mantle source of the OIB type called
“Caucasus” ([9] and others), which is responsible for
the formation of parental magmas for the Neogene–
Quaternary igneous rocks in this part of the Alpine
foldbelt.
Thus, the geochemical and isotope–geochemical
data for the newest lavas of the Porak volcanic center
allow us to suggest the fundamental contribution of
the mantle component of the OIB type to their petro
genesis. However, the abovementioned geochemical
data provide clear evidence for the remarkable contri
bution of the crustal component in petrogenesis of the
newest lavas. These data may be interpreted in terms of
two models: (1) the contribution of the subduction
component at the expense of the latent source in the
mantle enriched during the previous subduction
events and/or (2) crustal assimilation (mixing of melts
and/or contamination of parental melts by crustal
material) in comparison with hightemperature man
tle melts. We opt for the second model. Actually, the
high degree of LREE fractionation in the newest lavas
is not accompanied by a remarkable increase of the
level of HREE fractionation (Fig. 2d). The upper
crustal nature of contaminated material is the most
natural explanation in this case.
The crustal assimilation of mantle melts as the pro
cess participating in petrogenesis of the studied rocks
is evident from the relationships between some indic
ative trace elements in them as well. For example, the
compositions of the newest lavas on the known dia
gram Ta/Yb–Th/Yb [11] plot in the area of the average
composition of the upper continental crust. Analysis
of the obtained geochemical and isotope data allowed
us to conclude that the role of the crustal assimilates in
our case was most likely played by volcanogenic for
mations of the Meso–Cenozoic basement with Sr–
Nd isotope characteristics similar to those of the new
est lavas of Porak. The possibility of assimilation of
young primarily mantle magmas by the material of
volcanogenic formations of the MZ–CZ continental
paleomargin was discussed in [12] on the basis of the
petrological and isotope–geochemical data on the
Pliocene–Quaternary volcanic rocks of the southern
(Armenian) part of the Javakheti highland. The cited
paper contains isotope–geochemical markers of the
MZ–CZ volcanogenic formations (
87
Sr/
86
Sr =
0.7051–0.7042;
143
Nd/
144
Nd
= 0.51266–0.51285,
ε
Nd
from +0.5 to +4.2). Together with the basement rocks
of the same age from the Vardenis neovolcanic area of
the Lesser Caucasus, they compose a single geological
structure, namely the Rhodopian–Pontic–Lesser
Caucasus continental maleomargin, which was active
in the Late Mesozoic–Early Cenozoic.
The petrographic data provided visual confirma
tion of the hybrid origin of the newest lavas of Porak. It
was established that, as a whole, almost all studied
rock varieties contained a nonequilibrium set of phe
nocrysts: olivine + quartz + clinopyroxene + orthopy
roxene + sodic plagioclase + calcic plagioclase. Prac
tically all these minerals may occur in one rock sam
ple. A detailed petrographic and mostly microprobe
study of the newest volcanic rocks allowed us to sepa
rate phenocryst minerals into several parageneses:
(1) olivine of type I + calcic plagioclase + clinopyrox
ene of type I
±
orthopyroxene; (2) quartz + sodic pla
gioclase; (3) olivine of type II + intermediate plagio
clase + clinopyroxene of type II.
The paragenesis (1) is typical of olivine basalt, most
likely of the hypersthene series. Mg# of olivine from
this paragenesis ranges from 0.85 to 0.86. The mineral
is usually resorbed and has poor direct zoning and in
the outer zones, poor inverse zoning. According to the
experimental data [13], olivine with Mg# >0.82 crys
tallizes from basaltic melt at
≈
1250
°
C
(at an oxygen
fugacity of
10
–7
atm). Mg# of clinopyroxene from this
paragenesis varies from 0.82 to 0.83. The central parts
of clinopyroxene crystals often contain orthopyroxene
(hypersthene) relics with
Mg
#
= 0.81–0.82. The com
position of plagioclase phenocrysts from the paragen
esis (1) corresponds to labradorite (
An
52–58
). Thus, the
partially crystallized basaltic melt undoubtedly partic
ipated in the formation of the newest lavas of Porak.
The paragenesis (2) belongs to the partially crystal
lized magma of acid composition. Resorbed plagio
clase crystals of this paragenesis, usually with reticu
late texture, have an acid composition corresponding
to oligoclase or sodiumrich andesine (
An
21–33
).
According to the experimental data [14], the reticulate
texture is formed when relatively sodiumrich plagio

1370
DOKLADY EARTH SCIENCES Vol. 459 Part 1 2014
BUBNOV
et al.
clase is partially dissolved in the more acid melt. Pla
gioclase from this paragenesis usually has direct zon
ing, which may be changed by the poor inverse zoning
in the outer parts. Phenocrysts of quartz from this
paragenesis are resorbed and fringed by clinopyroxene
corona formed upon the interaction of quartz dia
crysts with the basic melt. Mg# of clinopyroxene from
rims ranges from 0.76 to 0.79. Note that the presence
of quartz diacrysts surrounded by the clinopyroxene
rim is an indicator of a number of young hybrid igne
ous rocks of intermediate composition in the Cauca
sian region ([15] and others).
The paragenesis (3) includes a “hybrid” associa
tion of phenocrysts, the minerals of which were
formed either as a result of the interaction between
diacrysts and nonequilibrium melts (association 3a) or
from the newly formed hybrid melt after the processes
of mixing of the acid and basic melts (association 3b).
With a high probability, some phenocrysts of “iron
rich” clinopyroxene of type II with
Mg
#
= 0.77–0.80
may be attributed to the association (3a). These phe
nocrysts were most likely formed at the place of quartz
diacrysts as a result of their interaction with the non
equilibrium primarily basic or hybrid intermediate
melt. This association mostly includes minerals of
pyroxene aggregates. As a whole, the composition of
pyroxene of this type is close to that of clinopyroxene
from coronas around quartz of the “acid” mineral
paragenesis. The association (3b), which is in equilib
rium with the newly formed subalkaline intermediate
hybrid melt, includes “ironrich” olivine of type II
(
Mg
#
= 0.83–0.79) with a clear direct zoning, inter
mediate plagioclase (
An
33–48
) usually with direct zon
ing, and some clinopyroxene crystals of type II
(Mg# = 0.80–0.82) with clear crystallographic contours.
Clear signs of the processes of hybridism were
revealed in the study of groundmass in the volcanic
rocks of the center. The groundmass of the studied
rocks, as a whole, corresponding to the composition of
trachyandesite (54.4–59.8 wt %
SiO
2
at 6.1–9.1 wt %
K
2
O + Na
2
O
) often contains inclusions of acid glass of
a high alkalinity (up to 69.4 wt %
SiO
2
at 8.0 wt %
K
2
O+ Na
2
O
).
The presence of two nonequilibrium parageneses of
phenocrysts (“basic” and “acid”) in rocks and inclu
sions of trachyrhyodacite material in the groundmass
of the trachyandesite and basaltic trachyandesite stud
ied shows that these rocks are hybrid and formed as a
result of mixing of partially crystallized basic magma
with acid, most likely the crustal melt, which has
already contained solid phases, such as sodic plagio
clase and quartz.
Thus, according to a number of material character
istics, the newest lavas of the Porak neovolcanic center
are close to the continental intraplate formations.
These characteristics are consistent with the ideas on
the regional deep mantle reservoir of the OIB type as a
source of primary melts for lavas of the center and pro
vide evidence for the significant role of assimilation by
the crustal material in their petrogenesis.
ACKNOWLEDGMENTS
This study was supported by the Russian Founda
tion for Basic Research (project nos. 130590612
Arm and 140500728) and by the Presidium of the
Russian Academy of Sciences (program no. 4).
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