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The Armenian Ophiolite: insights for Jurassic back-arc formation,
Lower Cretaceous hot spot magmatism and Upper Cretaceous
obduction over the South Armenian Block
Y. ROLLAND1*, G. GALOYAN1,2, M. SOSSON1, R. MELKONYAN2 & A. AVAGYAN2
1
Université de Nice-Sophia Antipolis, OCA, UMR GéoAzur, CNRS, Parc Valrose,
06108 Nice cedex 2, France
2
Institute of Geological Sciences, National Academy of Sciences of Armenia,
24a Baghramian avenue, Yerevan, 375019, Armenia
*Corresponding author (e-mail: [email protected])
Abstract: Similar geological, petrological, geochemical and age features are found in various
Armenian ophiolitic massifs (Sevan, Stepanavan and Vedi). These data argue for the presence
of a single large ophiolite unit obducted on the South Armenian Block (SAB). Lherzolite Ophiolite
type rock assemblages evidence a Lower–Middle Jurassic slow-spreading rate. The lavas and
gabbros have a hybrid geochemical composition intermediate between arc and Mid Ocean
Ridge Basalt (MORB) signatures which suggest they were probably formed in a back-arc basin.
This oceanic sequence is overlain by pillowed alkaline lavas emplaced in marine conditions.
Their geochemical composition is similar to plateau-lavas. Finally, this thickened oceanic crust
is overlain by Upper Cretaceous calc-alkaline lavas likely formed in a supra-subduction zone
environment. The age of the ophiolite is constrained by 40Ar/39Ar dating experiments provided
a magmatic crystallization age of 178.7+2.6 Ma, and further evidence of greenschist facies crystallization during hydrothermal alteration until c. 155 Ma. Thus, top-to-the-south obduction likely
initiated along the margin of the back-arc domain, directly south of the Vedi oceanic crust, and was
transported as a whole on the SAB in the Coniacian times (88–87 Ma). Final closure of the basin is
Late Cretaceous in age (73–71 Ma) as dated by metamorphic rocks.
The history of central and northern Neotethys can be
inferred from the study of oceanic crust domains
obducted in the Armenian Lesser Caucasus. It is
important to depict the geodynamic evolution of
oceanic domains that were formed in the NeoTethyan domain, as they provide key time and
palaeogeographic data in the Middle East Basin
Evolution (e.g. Sengör & Yilmaz 1981; Tirrul
et al. 1983; Ricou et al. 1985; Dercourt et al.
1986; Ricou 1994; Okay & Tüysüz 1999; Stampfli
& Borel 2001; Barrier & Vrielynck 2008). Furthermore, they provide constraints on the timing of
oceanic closure and obduction, by the study of metamorphic rocks associated to the ophiolites. Finally,
their geometry is also important to infer the preservation potential of oil resources that could be contained underneath.
During the Mesozoic, the Southern margin of the
Eurasian continent has been featured by closure of
the Palaeo-Tethys and opening Neo-Tethys Ocean
(Fig. 1). Later on, subductions, obductions and
micro-plate accretions, ranging mostly from the
Cretaceous to the Eocene and finally continent–
continent collision have occurred between Eurasia
and Arabia. The study of Armenian ophiolites
allows unravelling part of this complex history.
Previous geological, petrological and geochemical
works undertaken on those date back to late 1970s
and 1980s, and have never been undertaken at the
scale of the Armenian ophiolites. This work is particularly difficult due to the very large number of
tectonic and volcanic events that have occurred
after ophiolite obduction. The polyphased tectonic
history of the Lesser Caucasus region includes arccontinent accretion and subduction-exhumation in
or above accretionary prisms followed by continent –continent collision (e.g. Okay & Tüysüz
1999; Rolland et al. 2007). The tectonic events
have dissected the ophiolites (Avagyan et al.
2005), which were further overlain by very thick
(.1 km) sequences of volcanic rocks.
In this paper, we propose a synthesis of the
research undertaken on Armenian ophiolites,
based on recently published papers of individual
ophiolite zones and new geological, petrological,
geochemical and 40Ar/39Ar geochronological data
obtained on these different zones. Three ophiolites
have been studied, located in NW Armenia
From: Sosson, M., Kaymakci, N., Stephenson, R. A., Bergerat, F. & Starostenko, V. (eds) Sedimentary Basin
Tectonics from the Black Sea and Caucasus to the Arabian Platform. Geological Society, London, Special Publications,
340, 353–382. DOI: 10.1144/SP340.15 0305-8719/10/$15.00 # The Geological Society of London 2010.
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354
Y. ROLLAND ET AL.
Black
Sea
LC
LC
Albroz CCB
i
i
i
i
i
i
i
i
LC: Lesser Caucasus
Albroz CCB: Albroz CarboniferousCimmerian belt
Fig. 1. Tectonic map of the Middle East –Caucasus area, with main blocks and suture zones, after Avagyan et al.
(2005), modified.
(Stepanavan), north Armenia (Sevan) and central
Armenia (Vedi; Fig. 2). In the three zones, we
show the presence of three superposed levels of
lavas corresponding to three distinct environments,
from bottom to top: (1) back-arc; (2) ‘Ocean
Island Basalt’ (‘OIB’)-like; and (3) arc. Moreover,
we demonstrate that these ophiolite windows
should correlate with each other and be part of a
unique obducted nappe above the South Armenian
Block (or SAB, Knipper & Khain 1980; Zakariadze
et al. 1983). We propose that tectonic transport of
this nappe onto the SAB occurred directly after
the OIB ‘plateau’ event, in the Early Upper Cretaceous, and shortly preceded the final oceanic closure
along the Eurasian margin in the latest Cretaceous
(c. 73 –71 Ma).
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ARMENIAN OPHIOLITES
355
Fig. 2. Sketch geological map of Armenia, with location of the studied areas: 1, Stepanavan area; 2, Sevan area; 3, Vedi
area. 4 is the location of Zangezur ophiolites, located along a NNW–SSE striking fault.
Geological setting
The ophiolites are located in the northern part of
the Lesser Caucasus region (Fig. 1). They are situated in three geographic zones (Figs 2–5).
(1)
(2)
The Sevan– Akera zone at the northern rim
of the SAB and at the southern edge of the
European active continental margin (Knipper
1975; Knipper & Khain 1980; Adamia et al.
1980). In the present paper, we present
detailed mapping of the Stepanavan (NW
Armenia, Fig. 3) and of the Sevan (north
Armenia, Fig. 4) ophiolites.
The Vedi zone (Fig. 5), disposed in a more
southerly position, above the SAB (Knipper
(3)
& Sokolov 1977; Knipper & Khain 1980;
Zakariadze et al. 1983), or within a suture
zone eventually correlating with Central Iran
or Alborz ophiolites (Sokolov 1977; Adamia
et al. 1981).
The Zangezur zone situated along the Zangezur fault (Aslanyan & Satian 1977, 1982),
between the two domains, interpreted as an
ophiolite suture by Knipper & Khain (1980)
and Adamia et al. (1981).
A companion paper written on the geology of the
Sevan ophiolite has already put up in detail the
lithologies and radiometric age of this ophiolite
(Galoyan et al. 2009). Main features are summarized below. The lithological assemblages found
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356
Y. ROLLAND ET AL.
Fig. 3. Sketch geological map of the Stepanavan ophiolite (NW Armenia).
agree with a Lherzolite Ophiolite Type (LOT;
Nicolas 1989); these include the following.
(i) A high level of fractional crystallization in the
series, with cumulate olivine gabbros and two
pyroxene gabbros overlain and intruded by
generally amphibole-bearing gabbros, and frequent differentiated melts (diorites to plagiogranites). These most differentiated melts are
generally emplaced in ductile extensive shear
zones cross-cutting the gabbros. This complete
differentiation series suggests low partial
melting levels and long-lived cooling as is proposed in LOT settings (Lagabrielle et al. 1984;
Lagabrielle & Cannat 1990). Absolute radiometric datings indicate oceanic crust emplacement in the Middle Jurassic, constrained at
165– 160 Ma by zircon U –Pb age of one
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ARMENIAN OPHIOLITES
357
Fig. 4. Sketch geological map of the Sevan ophiolite (north Armenia), after Galoyan et al. (2009), modified.
tonalite (160 +4 Ma; Zakariadze et al. 1990)
and by 40Ar/39Ar amphibole age on gabbro
(165.3 +1.7 Ma; Galoyan et al. 2009).
(ii) Rare pillow lavas, with compositions ranging
from tholeiitic basalts to andesites. The
feeding dyke swarm is reduced, as rare dolerite
dykes have been found crosscutting the series.
The calc-alkaline affinity is also evidenced by
Nb –Ta negative anomalies, which agree with
some interaction with slab-derived component.
These support a slow spreading rate in a
back-arc setting.
(iii) Peridotites are frequent and often exhumed at
sea-floor level. They are generally serpentinized, and witness further hydrothermal alteration when exhumed at sea-floor level
(‘listwenites’). The lherzolitic nature of the
mantle-derived ultramafic rocks is then difficult to assess. The previous petrographical
investigations on the serpentinized ultramafics
suggest that the protoliths was mantle-derived
with various compositions ranging from
lherzolites to harzburgites and dunites (e.g.
Melikyan et al. 1967; Harutyunyan 1967;
Palandjyan 1971; Abovyan 1981; Ghazaryan
1987; Zakariadze et al. 1990). Undeformed
ultramafics have a magmatic cumulative origin,
shown by the poikilitic texture of olivine
inclusions within large enstatite crystals (up
to 10– 15 mm; Palandjyan 1971). We observe
similar textures, together with layers, contained in magmatic pods cross-cutting the serpentinites in the Stepanavan area (Galoyan
et al. 2007). These latter serpentinites are
strongly deformed and altered, thus it was
difficult to unravel their origin. However, the
ductile character of deformation is in agreement with a mantle origin for these rocks.
(iv) Radiolarites are found as interlayers or as
unconformably overlying the various above
lithologies. The fact that they overlie gabbros,
plagiogranites and serpentinites shows that
these rocks were uplifted and exhumed by
normal faults. Radiolarite datings undertaken
in the different ophiolites all agree with
oceanic accretion in the Middle– Upper Jurassic (Danelian et al. 2007, 2008).
The ophiolitic sequences are weakly deformed
with anchizonal metamorphism. Only some outcrops show evidence of small shear zones ascribed
to the ophiolite obduction in the Coniacian (Zakariadze et al. 1983). High pressure (HP) metamorphism is described in the Stepanavan region (Figs 2
& 3), where blueschists (Aghamalyan 1981, 1998)
outcrop in small km2 size tectonic windows below
the ophiolite. Timing of metamorphism from radiometric 40Ar/39Ar phengite datings indicates HP
metamorphic peak at c. 95 Ma, and medium
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358
Y. ROLLAND ET AL.
Fig. 5. Sketch geological map of the Vedi ophiolite (Central Armenia).
pressure-medium temperature (MP-MT) retrogression at 73–71 Ma (Rolland et al. 2007).
The ophiolite series are locally overlain from
bottom to top by (1) alkaline lavas, which have
age ranges from 114 to 95 Ma (Baghdasaryan
et al. 1988; Satian & Sarkisyan 2006) recently confirmed by 40Ar/39Ar dating of 117.3+0.9 Ma
(Rolland et al. 2009); and (2) Upper Cretaceous
andesites and detrital series (Dali valley; Stepanavan; Galoyan et al. 2007). The alkaline lavas are
alternatively interpreted as (1) intra-continental
rifting (Satian et al. 2005) in the Vedi area; and
(2) plume-derived magmatism above the oceanic
crust before the obduction (Galoyan et al. 2007).
The calc-alkaline series should be related to
intra-oceanic arc emplacement above this oceanic
crust sequence and implies the presence of a subduction zone between the ophiolite and the SAB, featured by the Stepanavan blueschists (Rolland et al.
2007). These two magmatic sequences closely
predate the ophiolite obduction onto the SAB
during the Coniacian (Sokolov 1977).
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ARMENIAN OPHIOLITES
Analytical methods
Mineral compositions were determined by electron
probe microanalysis (EPMA). The analyses are presented in Figure 6. They were carried out using a
Cameca Camebax SX100 electron microprobe at
15 kV and 1 nA beam current, at the Blaise Pascal
University (Clermont-Ferrand, France). Natural
samples were used as standards.
Thirty-seven samples of magmatic rocks from
the Sevan, Stepanavan and Vedi ophiolites have
been analysed for major, trace and Rare Earth
elements (REE) (Table 1). Samples were analysed
at the C.R.P.G. (Nancy, France). Analytical procedures and analyses of standards can be found
on the following website (http://www.crpg.cnrsnancy.fr/SARM).
Amphiboles were separated from the gabbro
sample AR-05-110, from the Vedi ophiolite. Geochronology of amphiboles was undertaken by laser
40
Ar/39Ar dating. The results are presented in
Table 2. Amphibole crystals were separated under
a binocular microscope. The samples were then irradiated in the nuclear reactor at McMaster University
in Hamilton (Canada), in position 5c, along with
Hb3gr hornblende neutron fluence monitor, for
which an age of 1072 Ma is adopted (Turner et al.
1971). The total neutron flux density during
irradiation was 9.0 1018 neuton cm22. The estimated error bar on the corresponding 40Ar*/39ArK
ratio is +0.2% (1s) in the volume where
the samples were set. Three amphibole grains
(c. 500 mm in diameter) were chosen for analysis
on a laser UV spectrometer of Nice (Géosciences
Azur). Analyses were undertaken by step heating
with a 50 W CO2 Synrad 48-5 continuous laser
beam. Measurement of isotopic ratios was done
with a VG3600 mass spectrometer, equipped with
a Daly detector system; see detailed procedures in
Jourdan et al. (2004). The typical blank values for
extraction and purification of the laser system are
in the range 4.2 –8.75, 1.2–3.9 and 2– 6 cc STP
for masses 40, 39 and 36, respectively. The massdiscrimination was monitored by regularly analysing air pipette volume. Decay constants are those
of Steiger & Jäger (1977). Uncertainties on apparent
ages in Table 2 are given at the 1s level and do not
include the error on the 40Ar*/39Ark ratio of the
monitor. Uncertainties on plateau ages in Figure 7
are given at the 2s level and do not include the
error on the age of the monitor.
Results
Field relationships
Synthetic logs are drawn on Figure 8, showing
the lithological associations and the structural
359
relationships in each zone. In Stepanavan (Fig. 8a,
b), ophiolite sections exhibit abundant serpentinites,
cross-cut by normal fault and shear zones in which
gabbronorites, gabbros and plagiogranites are intrusive and deformed. Laterally, thick layers of pillow
basalts are observed which interlayer with radiolarites. At the top of the ophiolite section, a thin layer
of alkaline lava flows is found. Above, these lavas
are uncomformably overlain by Upper Cretaceous
conglomerates and limestones, and calc-alkaline
pillow basalts or graywackes. The ophiolite
sequence is thrust over a blueschist facies metamorphic sole, which outcrops in two km2 sized
tectonic windows.
In the Sevan area, sections are extremely variable laterally (Fig. 8c–e). Pillow lavas are rare,
and serpentinites are frequently found at sea-floor
level. Intense hydrothermal alteration (‘listwenites’)
has transformed the uppermost part of exhumed
serpentinites. Rare dolerites are observed. Large
intrusive pods of amphibole-bearing gabbros and
plagiogranites are also exhumed and overlain by
radiolarites. Normal faults are observed, and are
interpreted as the cause of such lateral variations,
by vertical uplifting of footwall sections, and local
infilling of axial rift valleys, in agreement with
the LOT ophiolite model (e.g. Lagabrielle et al.
1984; Lagabrielle & Cannat 1990). Locally, thick
sequences of alkaline pillow lavas are observed.
The ophiolite is locally eroded, and uncomformably
overlain by conglomerates and soils, overlain by an
Upper Cretaceous section of reef-limestones with
graywackes interlayers.
In the Vedi area, the ophiolite section is much
thinner (Fig. 8f–h). The basal tectonic contact is
exposed, exhibiting top-to-the-south sense of shear.
At the base, the ophiolite rests on a serpentinite
layer. The ophiolite is intensely sheared above the
basal contact with boudins of tholeiitic basalts
(Fig. 8h). Laterally, the ophiolite consists mainly
of gabbros (Fig. 8g) or serpentinites, which suggests
similar lithological features as in the Sevan area.
However, the different parts of the ophiolite are
dismembered and displaced from each other as a
result of obduction deformation. Above the ophiolites, layers of radiolarites are found below a very
thick section of alkaline pillow lavas (Fig. 8h).
These alkaline lavas are amphibole-bearing. Pillows
are larger (metre-scale) than the ophiolite ones
(several decimetre scale), and interlayer with thin
pink limestones. At the front of the obduction, an
olistolith formation with conglomerates and slided
blocks in a muddy matrix is present (Fig. 8f). The
age of the olistostrom is Coniacian –Santonian
(see Sosson et al. 2010), it connects laterally to
Lower Coniacian series below the ophiolite, and
with Santonian reef limestones above the obduction.
Therefore, the obduction age can be bracketed
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360
Y. ROLLAND ET AL.
0.10
Sevan
0.09
G150 olivine gabbro
AR-05-86 olivine gabbro
AR-03-10 hornblende gabbro
AR-03-02 diabase
0.08
0.07
Stepanavan
AR-04-02 wehrlite
AR-04-03 websterite
AR-04-04 websterite
AR-04-51 websterite
AR-04-53 anorthosite
AR-04-45D hornblende gabbro
AR-04-31 basaltic andesite
Ti
0.06
0.05
0.04
Alkaline
Tholeiitic
0.03
Vedi
0.02
AR-05-112A hornblende gabbro
AR-05-113 hornblende gabbro
AR-05-103 gabbro-diabase
0.01
0.00
0.5
0.6
0.7
0.8
0.9
1.0
Na + Ca
Fig. 6. Chemical compositions of studied clinopyroxenes plotted in the Ti v. (Na þ Ca) diagram of Letterier et al.
(1982). Note that a majority of data plot in the Alkaline compositional field, and a minority is in the Tholeiitic part.
to the Coniacian –Santonian (88 –83 Ma). Laterally,
the upper part of the ophiolite is made of kilometrescale slided blocks, mainly comprised of alkaline
pillow basalts and calc-schists. These blocks slide
on a greenish mudstone rock, probably originated
from the alteration of the ophiolite itself. The
Upper coniacian uncomformity is variably marked
by conglomerates, marls and reef limestones.
As emphasized in Galoyan et al. (2009) in the
Sevan area, and by Galoyan et al. (2007) in
the Stepanavan area, the lithologies found in all
the exposed Armenian ophiolites are in good agreement with the hypothesis of a slow expansion rate
spreading centre, as described for the western Alps
ophiolites (LOT; Nicolas & Jackson 1972; Nicolas
1989). These LOT features include the following,
as already stressed by Galoyan et al. (2009).
(1)
(2)
(3)
A high degree of fractional crystallization
shown by coarse amphibole-bearing gabbros
and widespread plagiogranites.
Both plutonic rock types are often strained in
ductile-to brittle conditions of amphibolite to
lower greenschist metamorphic conditions
and often exhumed at sea-floor level, as for
serpentinites. These tectonic features are in
agreement with a slow-spreading system in
which the overall morphology is dominated
by normal faults.
The scarcity of basalt lava flows and the
restricted dolerite dyke swarm are also in
agreement with a slow spreading rate
system, featured by low partial melting levels.
The similar lithological and age features found in
the several Armenian ophiolites suggest that they
were part of the same oceanic crust section. This
has to be confirmed by the comparison of geochemical data from each zone. The presence of three magmatic series: ophiolite sensu stricto (tholeiitic),
‘OIB’ (alkaline) and arc (calc-alkaline) in the same
structural position (from bottom to top, respectively) has been evidenced in the three zones.
Petrography and mineral chemistry
The field and microscopic analyses of the Armenian
ophiolite magmatic rocks have evidenced a continuous magmatic succession from ultramafic cumulates
(wehrlites, websterites) to gabbros and plagiogranites, exhumed at sea-floor level and overlain by
pillow-basalts.
Ophiolite plutonic rocks. Wehrlites are found in
the Stepanavan ophiolite and have a poikilitic
texture showing numerous clinopyroxene crystals
with diopside composition (Wo45-47En48-50Fs2-4),
included in large olivine Fo87-88 (.60 –65%) porphyric grains. Spinel is less than 5%, and is of
magnetite composition.
Gabbros are the most abundant rocks in the
crustal complex. Their petrography evolves from
cumulate-banded olivine gabbros in their lower
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ARMENIAN OPHIOLITES
part towards more leucocratic plagioclase-rich
gabbros in the upper part (Abovyan 1981; Ghazaryan 1987, 1994). The Cumulative banded olivine
gabbros and websterites are described only in
Sevan and Stepanavan areas, while more leucocratic
gabbros are widespread.
Olivine gabbros are fresh, massive, and fine- to
medium-grained (0.5–1 mm and 1.2–2 mm correspondingly). They have cumulate, ophitic textures
and consist of plagioclase (c. 60– 65%; An68-74,
An80-89), olivine (c. 5–10%; Fo72-76) and clinopyroxene (c. 25– 35%). Clinopyroxene is of augite
(Wo39-44En45-48Fs11-13) and diopside (Wo45En44
Fs11) types. Some enstatitic orthopyroxenes (Wo2
En75Fs23) are also found rimming olivine
porphyrocrysts.
Websterites have a granular texture with large
2–8 mm porphyrocrysts of orthopyroxene (30 –
70%), clinopyroxene (70–30%) and olivine grains
(0–35%; Fig. 9a). Orthopyroxenes are enstatite-rich
(Wo1-5En59-84Fs11-37) and clinopyroxenes are
augites (Wo35-42En36-40Fs15-19) and olivine is relatively rich in forsterite (Fo84-88). Gabbronorites
have a gabbroic texture, with plagioclase
(10–60%, 1– 3 mm), clino- and ortho-pyroxene.
Plagioclase is of bytownite type (An80-85), while
orthopyroxenes (1–4 mm) are enstatites (Wo2-5
En59-61Fs34-37), and clinopyroxenes are augites
(Wo35-42En36-40Fs15-19). Spinel is less than 5% and
is of magnetite composition.
Mesocratic to leucocratic gabbros of the upper
section are massive, fine- to medium-grained and
have gabbroic (or gabbro-ophitic), subautomorphe
to xenomorphe granular texture (0.5 –4 mm), with
plagioclase (c. 40– 65%; An50-75, An72-93), clinopyroxene (8– 45%; augite) and hornblende (0–40%)
and lack olivine. Accessory minerals (1–10%) are
apatite, titanomagnetite, ilmenite and rarely quartz.
The hornblende-rich gabbros have coarse granular
textures (Fig. 9b), with c. 50 –65% euhedral to subhedral plagioclase (An54-58) and (c. 35– 50%) anhedral to subhedral amphibole (Fig. 10a –d). Brown
Ti-rich euhedral hornblende is presumed to be a
primary mineral as it appears in the centre of phenocrysts; while a Ti-poor subhedral to xenomorphic
green magnesio-hornblende (Leake et al. 1997) is
thought to be a secondary phase related to
hydrothermal alteration (Fig. 10d). The augite
(Wo40-42 En39-47Fs11-14), diopside (Wo45-48
En40-44Fs8-15), and enstatite (Wo2En57Fs41) relicts
(5–10%) are found in the magnesio-hornblendes
crystals replacing the pyroxenes. However, it is
not related to shear zones and fractures, and
should also be a late magmatic mineral. In leucocratic gabbros, the clinopyroxene (augite Wo40-41
En33-35Fs18-19) content does not exceed 25%.
Normal zoning is observed in plagioclase (from
An85 to An60), which are frequently altered.
361
Clinopyroxenes have alkaline to slightly tholeiitic
compositions (0.8 , Na þ Ca , 0.9; Leterrier
et al. 1982; Fig. 6). Pegmatitic gabbros crosscutting
the plutonic sequence (Vedi zone) are composed
of plagioclase and hornblende, and are mainly
altered in chlorite, carbonate, sericite, albite,
quartz, actinolite, and so on.
Diorites (well-known in Sevan and Vedi zone)
which occur as small, apparently intrusive bodies
within the gabbro units (Palandjyan 1971;
Abovyan 1981; Ghazaryan 1987, 1994), have a
porphyritic to subhedral granular (1– 4 mm) texture
and have relatively similar hornblende contents
(5– 30%) as in the gabbros. Plagioclase (c. 65 –
70%) is albite-rich (An34-38) and accessory minerals
(quartz, opaque oxides) are rare. Amphibole grains
are magnesio-hornblendes in which actinolite and
sometimes epidote aggregates are present. Laterally
and towards upper sections (Sevan zone) diorites
grade into quartz-diorite (quartz 5–10%).
Plagiogranites (Fig. 10e, f) appear to be the most
differentiated component of the gabbro-dioritic
intrusives. They form diffused segregations or
discontinuous networks of veins with local coarse
pegmatic, or hypidiomorphic to xenomorph granular (0.5–4 mm) texture within and around gabbrodiorite intrusives. They are formed by 40–65%
plagioclase (An15-30), 25–45% quartz, minor
biotite (,5%), ortho-amphibole (,5%; Stepanavan), K-feldspar (0 –10%, microcline; Vedi zone;
Fig. 9c) and accessory phases (titanomagnetite,
hematite, sphene and apatite). Amphiboles are
rarely preserved, commonly replaced by chloriteand epidote-group minerals.
Ophiolite volcanic and subvolcanic rocks. Diabases
are present in several locations (Sevan and Stepanavan areas) as isolated dikes, crosscutting the
layered gabbros. They are generally altered (chlorite, epidote, carbonates) and have the subdoleritic
texture composed of plagioclase (60– 65%;
An65-75) and two clinopyroxenes (augite Wo41-44
En44-47Fs11-13 and diopside Wo45En37Fs18).
The volcanic rocks of the studied Armenian
ophiolites are present as pillowed and massive lava
flows and pillowed breccias. In general they are
relatively altered due to hydrothermalism but still
relict igneous textures are preserved. The basalts
and basaltic andesites are vacuolar (1– 5 mm, vesicles are filled with carbonate-calcite, chlorite and
quartz) and largely aphyric (intersertal, spilitic,
microdoleritic and variolitic, up to 1.5– 2 mm in
diameter), composed mainly of albitized plagioclase and/or plagioclase-clinopyroxene microlites,
Ti-magnetite and hematite microlites, in a devitrified (calcite þ chlorite) groundmass (Fig. 9d).
Alkaline lavas. The alkaline basalts are found
in several locations of the three ophiolite zones.
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362
Y. ROLLAND ET AL.
Table 1. Representative whole-rock analyses of samples from the Sevan, Stepanavan and Vedi areas. Major
Groups no.
Sevan ophiolitic series
Flaser
gabbro
Sample
SiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
TiO2
P2O5
LOI
Total
Mg#
Rb
Sr
Y
Zr
Nb
Ba
Hf
Ta
Pb
Th
U
V
Cr
Co
Ni
Cu
Zn
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Eu/Eu*
(La/Sm)N
(La/Yb)N
Olivine
gabbro
AR-03-25 AR-05-86
45.36
13.32
14.91
0.14
7.44
10.89
3.15
0.22
2.55
0.31
1.46
99.8
52.1
2.67
231.7
61.81
175.9
4.51
36.77
4.25
0.35
–
0.08
0.11
440.7
35.43
26.81
77.98
18.16
58.57
14.89
34.75
5.057
24.09
7.38
3.11
9.32
1.60
10.47
2.22
6.37
0.94
6.28
0.95
1.15
1.27
1.60
48.09
16.72
5.94
0.11
10.51
14.1
1.65
–
0.27
0.02
2.8
100.2
79.3
0.48
102
6.41
5.28
0.08
4.1
0.22
0.01
–
–
–
138
802
40.2
188
102
28.3
0.33
0.94
0.18
1.23
0.58
0.34
0.87
0.16
1.14
0.24
0.65
0.10
0.65
0.09
1.45
0.36
0.34
Olivine
gabbro
Gabbro
G150
AR-03-39
AR-03-24
AR-03-10
48.39
15.63
6.44
0.12
10.48
16.65
1.16
–
0.29
0.04
0.65
99.8
77.9
–
101
7.24
4.85
–
–
0.21
–
–
–
–
190
412
41.7
131
111
29.6
0.36
1.02
0.19
1.28
0.61
0.32
0.98
0.18
1.24
0.26
0.74
0.11
0.72
0.11
1.28
0.37
0.34
49.49
14.11
11.59
0.18
6.79
9.38
3.52
0.29
1.32
0.14
2.92
99.7
56.1
3.05
189.5
30.05
75.20
1.55
120.8
2.12
0.11
–
0.29
0.09
319.90
94.50
34.77
32.35
52.59
86.18
3.39
9.57
1.57
8.27
2.93
1.07
4.01
0.71
4.89
1.03
3.02
0.46
3.03
0.48
0.95
0.73
0.76
50.60
7.2
7.77
0.17
15.29
17.65
0.48
–
0.20
0.06
0.79
100
81.1
–
58.2
7.84
5.24
–
14.14
0.20
–
–
–
–
195.9
810.3
41.88
136.2
142.3
47.98
0.28
0.99
0.22
1.40
0.67
0.28
1.07
0.20
1.36
0.29
0.83
0.13
0.86
0.13
1.01
0.26
0.22
50.68
18.17
9.09
0.16
6.75
9.26
3.21
0.15
0.36
0.05
1.21
99.1
61.6
0.84
303.7
11.93
22.89
0.50
34.36
0.81
0.04
–
0.14
0.07
222.2
104.3
33.71
29.23
44.81
71.99
1.60
4.51
0.74
3.80
1.34
0.48
1.70
0.30
1.92
0.41
1.22
0.19
1.32
0.21
0.97
0.75
0.82
Their structural position is as large massive pillowlavas flows on top of the ophiolite section or as
dykes of diabase cross-cutting it (Stepanavan
area), but their relationships with the ophiolite
Gabbro-norite Hornblende
gabbro
Diorite
Diorite
AR-04-218 AR-03-23
55.09
13.45
8.51
0.15
9.84
8.76
2.59
0.17
0.24
0.04
1.87
100.7
71.6
1.42
207.4
7.82
21.08
0.32
34.9
0.76
0.03
–
0.07
0.04
158.1
562.7
39.6
148.1
15.63
73.35
1.44
3.73
0.57
2.87
0.94
0.35
1.13
0.20
1.31
0.28
0.82
0.13
0.92
0.15
1.03
0.97
1.06
57.41
14.10
8.84
0.14
2.24
4.92
6.36
0.12
0.87
0.16
4.33
99.5
35.6
1.81
212.5
25.22
75.07
1.83
55.59
2.27
0.14
2.62
0.75
0.25
122.6
136.9
15.76
10.09
21.02
50.45
4.62
11.73
1.82
9.05
2.91
0.97
3.68
0.69
4.60
0.97
2.95
0.47
3.25
0.52
0.91
1.00
0.96
(sensu stricto) pillow lavas remain often unclear.
The first group of alkaline rocks displays large
vacuoles (0.5– 3 mm), filled with carbonates and
rarely chlorites, and have both phyric and aphyric
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on September 19, 2016
ARMENIAN OPHIOLITES
363
oxides are in wt%, and trace elements and REE in ppm
Sevan alkaline series
Plagiogranite
Diabase
Trachy-andesite
Basaltic
trachyandesite
Andesite
Basanite Trachybasalt Trachyandesite
AR-03-19
AR-03-02
G154
AR-03-17
AR-03-34
G142
AR-05-80
AR-03-33
74.91
12.32
3.58
0.03
0.42
3.05
4.20
0.31
0.21
0.04
0.57
99.6
20.2
2.22
145.2
27.65
71.64
2.15
65.83
2.48
0.10
1.32
1.09
0.62
50.8
1464
5.52
37.19
6.91
9.98
5.38
12.66
1.78
8.36
2.68
0.67
3.50
0.64
4.33
0.94
2.94
0.46
3.30
0.53
0.67
1.26
1.10
46.02
16.29
8.39
0.13
7.73
10.68
3.53
0.37
1.26
0.17
4.61
99.2
66.6
13.97
630.9
23.93
127.4
2.95
285.3
2.91
0.24
1.51
1.01
0.29
179.3
277.0
38.26
54.43
58.20
66.89
7.07
18.32
2.71
12.68
3.53
1.36
3.95
0.67
4.21
0.84
2.45
0.36
2.39
0.37
1.12
1.26
2.00
53.70
14.09
11.35
0.15
4.52
3.64
6.07
–
1.36
0.12
4.85
99.9
46.2
–
28.76
27.82
81.43
1.77
16.09
2.36
0.14
1.61
0.55
0.35
334.9
–
29.48
8.95
102.8
67.20
4.25
11.14
1.67
8.76
2.93
1.14
4.04
0.71
4.78
1.01
2.99
0.46
3.16
0.51
1.01
0.91
0.91
54.27
15.16
12.36
0.19
3.74
4.38
6.63
–
1.33
0.15
1.78
100
39.5
–
49.82
29.88
85.60
1.44
19.01
2.38
0.12
–
0.37
0.27
321.8
–
24.33
5.14
15.61
80.0
3.91
10.93
1.81
9.35
3.26
1.15
4.18
0.75
4.99
1.06
3.11
0.47
3.22
0.51
0.95
0.75
0.82
55.48
14.13
12.45
0.13
4.07
5.49
3.96
0.61
1.17
0.11
2.18
99.8
41.6
5.05
102.5
27.49
54.39
1.69
16.65
1.63
0.12
–
0.33
0.14
347.4
251.7
27.09
16.09
5.23
19.68
2.69
7.17
1.15
6.17
2.32
0.79
3.36
0.62
4.34
0.95
2.82
0.44
2.98
0.47
0.86
0.73
0.61
40.63
14.40
11.70
0.29
4.15
11.40
3.64
1.42
2.06
0.48
10.01
100.2
43.4
31.89
147.0
25.58
153.4
40.63
166.7
3.57
2.99
4.98
4.13
1.43
257.4
33.86
38.26
34.29
61.86
100.1
32.41
64.63
7.64
29.89
6.02
1.97
5.58
0.82
4.69
0.90
2.48
0.35
2.33
0.36
1.04
3.39
9.39
43.80
17.58
9.46
0.11
6.70
4.95
4.09
2.24
1.68
0.44
8.89
99.9
60.5
44.83
330.8
17.37
131.4
17.95
299.0
2.86
1.19
4.63
4.06
0.94
271.7
21.68
31.44
28.76
54.48
91.55
29.46
59.03
6.92
27.11
5.21
1.63
4.45
0.62
3.42
0.61
1.64
0.22
1.44
0.22
1.03
3.56
13.83
51.57
14.34
6.06
0.11
0.77
9.78
6.34
0.56
1.98
1.08
6.65
99.3
21.7
7.93
341.5
52.91
411.2
49.22
168.7
9.03
3.80
4.45
5.15
4.61
286.1
162.0
24.06
13.83
18.34
92.74
48.54
107.1
13.38
56.55
12.98
4.14
12.43
1.84
10.27
1.87
4.80
0.64
4.05
0.60
1.0
2.35
8.09
(Continued )
(Fig. 9e) intersertal textures, with plagioclase megacrysts (c. 5%; 0.5–2 mm), microliths and opaque
minerals (3–10%), surrounded by a calcite-chlorite
mesostase. The second group (Vedi and Stepanavan
zones) have doleritic (Fig. 9f ) to ophitic (gabbroophitic in central parts of lavas flows) textures, and
are mainly composed of plagioclase (c. 40– 55%;
1– 3 mm), clinopyroxene (10–30%; 1– 4 mm),
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364
Y. ROLLAND ET AL.
Table 1. Continued
Groups no.
Sample
SiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
TiO2
P2O5
LOI
Total
Mg#
Rb
Sr
Y
Zr
Nb
Ba
Hf
Ta
Pb
Th
U
V
Cr
Co
Ni
Cu
Zn
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Eu/Eu*
(La/Sm)N
(La/Yb)N
Stepanavan ophiolitic series
Websterite
Hornblende
gabbro
Hornblende
gabbro
Plagiogranite
Basaltic
trachy-andesite
Basaltic
trachy-andesite
AR-04-03
AR-04-16
AR-04-45D
AR-04-44
AR-04-20
AR-04-30
53.24
1.03
6.02
0.15
23.18
16.52
0.12
–
0.05
0.03
0.54
100.9
90.0
–
11.79
1.05
–
–
3.71
–
–
–
–
–
135.1
2804
55.82
361.3
340.3
25.68
–
0.15
0.02
0.15
0.08
0.03
0.13
0.03
0.18
0.04
0.12
0.02
0.12
0.02
0.85
0.0
0.0
47.30
14.39
12.90
0.21
9.11
10.14
2.93
0.19
1.18
0.07
1.76
100.2
60.4
1.12
125.4
20.91
42.74
1.01
32.71
1.21
0.08
–
0.18
0.05
324.7
236.4
51.46
78.0
–
60.28
2.40
6.37
1.08
5.76
2.09
0.96
2.98
0.53
3.53
0.74
2.15
0.32
2.10
0.32
1.17
0.72
0.77
53.77
14.00
8.92
0.15
7.81
6.98
3.34
2.42
0.16
0.05
2.44
100.1
65.6
30.17
213.4
5.79
21.26
2.14
228.1
0.63
0.21
3.61
1.27
0.43
94.47
324.2
31.5
101.7
189.9
60.33
3.06
6.28
0.65
2.30
0.49
0.19
0.55
0.10
0.81
0.19
0.64
0.12
0.91
0.17
1.12
3.94
2.27
75.35
12.20
2.71
0.03
0.77
2.05
5.03
–
0.11
0.02
1.07
99.3
38.1
0.58
91.04
1.23
6.48
0.35
20.58
0.15
–
–
0.02
0.01
30.74
421.5
7.71
24.86
189.5
23.13
2.42
3.90
0.42
1.58
0.29
0.35
0.24
0.04
0.22
0.05
0.15
0.03
0.21
0.04
4.05
5.25
7.92
51.53
14.69
14.81
0.23
4.15
4.86
5.74
0.18
1.62
0.13
1.89
99.8
37.7
1.4
61.01
35.91
86.0
1.62
19.58
2.54
0.13
2.29
0.43
0.12
459.8
99.23
38.85
22.76
64.58
130.6
4.23
11.08
1.88
9.88
3.44
1.28
4.69
0.87
5.93
1.28
3.79
0.58
3.89
0.61
0.98
0.77
0.73
48.55
13.29
8.67
0.15
6.86
10.49
4.74
0.24
1.08
0.11
6.01
100.2
63.1
7.94
95.7
26.52
68.43
1.9
21.73
1.83
0.15
1.93
0.19
0.09
305.4
316.7
42.98
109.1
132.8
81.13
2.53
7.37
1.31
7.04
2.55
0.99
3.49
0.65
4.34
0.93
2.75
0.42
2.84
0.44
1.02
0.63
0.60
amphibole (c. 25%; 1–3 mm) and accessory
Ti-magnetite (.5– 10%), apatite (c. 3%; prismatic,
acicular, 0.5 –1.5 mm) and rarely biotite. Apatites
are frequent. Vitreous interstices are filled by
carbonates or carbonate-chlorite assemblages.
The tabular plagioclase laths show a transitional
zoning with bytownite to labrador (An72-60) or
labrador to andesine (An55-32) compositions. Thin
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on September 19, 2016
ARMENIAN OPHIOLITES
Stepanavan alkaline series
365
Stepanavan calk-alkaline series
Basaltic
trachy-andesite
Basaltic
trachy-andesite
Diabase
Olivine
basalt
Basaltic
trachy-andesite
Basaltic
trachy-andesite
AR-06-02
AR-03-53
AR-04-05
AR-04-32
AR-04-40A
AR-04-31
45.37
14.27
13.52
0.32
6.22
4.09
1.53
5.37
3.12
1.31
5.11
100.2
49.8
45.3
157
40.6
268
52.8
608
5.97
3.97
7.04
5.39
1.34
172
–
26.3
6.5
20.4
177
50.3
96.1
12.1
53.3
11.2
3.87
10.5
1.50
8.11
1.49
3.75
0.51
3.18
0.45
1.09
2.83
10.68
48.54
15.01
12.65
0.27
4.25
5.33
3.93
2.69
2.64
1.08
3.16
99.5
42.3
33.17
322.6
44.41
294.4
57.95
578.3
6.51
4.20
2.54
5.98
1.46
94.35
25.77
16.75
–
9.48
137.1
50.59
107.0
12.97
53.32
11.26
4.08
10.41
1.52
8.69
1.57
4.16
0.57
3.64
0.56
1.15
2.83
9.38
50.19
13.91
13.73
0.24
3.27
5.85
5.11
0.42
3.39
0.67
2.94
99.7
34.0
7.62
198.8
51.24
373.5
42.33
156.6
8.01
3.24
2.24
4.65
1.20
201.6
–
31.93
–
14.94
152.7
40.02
85.12
10.85
45.27
10.35
3.39
10.3
1.63
9.50
1.80
4.93
0.70
4.51
0.69
1.0
2.43
5.99
49.15
18.53
10.19
0.16
5.25
8.25
4.36
0.52
0.86
0.14
3.12
100.5
52.7
9.61
520.3
16.0
44.4
2.14
133.9
1.25
0.17
7.22
0.72
0.19
241.5
21.05
29.51
15.55
12.6
150.9
4.93
11.46
1.74
8.35
2.36
0.94
2.60
0.44
2.80
0.57
1.62
0.24
1.60
0.25
1.15
1.31
2.07
49.79
15.80
8.82
0.15
3.54
9.12
3.54
1.24
1.07
0.20
7.27
100.6
48.4
18.12
303.8
24.4
99.18
2.29
239.1
2.69
0.18
3.42
1.46
0.67
279.1
31.91
29.05
22.38
188.1
86.53
7.87
18.51
2.68
12.58
3.47
1.13
3.89
0.65
4.09
0.85
2.46
0.38
2.54
0.39
0.94
1.43
2.09
52.20
17.05
9.28
0.16
3.59
6.56
4.61
1.00
0.94
0.18
5.36
100.9
44.5
18.45
282.3
24.19
95.3
3.32
213.6
2.61
0.26
5.66
1.67
0.58
263.7
73.83
27.36
18.95
170.9
100.0
8.69
18.05
2.53
11.65
3.15
1.04
3.57
0.61
3.96
0.83
2.46
0.38
2.54
0.40
0.95
1.74
2.31
(Continued )
rims of pure albite (Ab – 98%) are also present.
The clinopyroxenes are generally chloritized,
but its relics are still recognizable and belong to
diopside (Wo49En35Fs16). The amphibole is a
kaersutite (Leake et al. 1997), with zonation from
kaersutite to ferro-kaersutite from core to rim,
respectively. Some samples show abundant calcitefilled veins and pockets.
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on September 19, 2016
366
Y. ROLLAND ET AL.
Table 1. Continued
Groups no.
Vedi ophiolitic series
Hornblende
gabbro
Sample
SiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
TiO2
P2O5
LOI
Total
Mg#
Rb
Sr
Y
Zr
Nb
Ba
Hf
Ta
Pb
Th
U
V
Cr
Co
Ni
Cu
Zn
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Eu/Eu*
(La/Sm)N
(La/Yb)N
Diorite
Plagiogranite
Basalt
Vedi alkaline series
Basaltic
andesite
Basalt
Trachybasalt
Basaltic
trachyandesite
Trachydacite
AR-05-113 AR-05-110 AR-05-111 AR-05-114 AR-05-106 AR-05-78 AR-05-104 AR-05-102 AR-04-75
45.09
21.24
4.32
0.07
7.88
14.29
2.12
0.17
0.16
–
5.08
100.4
79.7
2.41
492.7
3.70
4.30
0.08
131.6
0.16
–
–
–
–
89.5
785.2
30.98
130.7
92.4
24.06
0.231
0.71
0.14
0.78
0.36
0.24
0.53
0.10
0.64
0.14
0.38
0.06
0.35
0.06
1.64
0.40
0.44
58.57
16.03
6.66
0.11
5.18
7.08
3.94
0.47
0.33
0.04
2.08
100.5
62.7
4.12
254
10.1
42.3
0.49
57.5
1.3
0.04
1.1
0.42
0.12
185
123
22.8
42.4
18.3
50.7
2.09
4.84
0.72
3.74
1.20
0.42
1.46
0.25
1.65
0.34
1.01
0.16
1.14
0.18
0.97
1.10
1.24
70.45
14.69
4.44
0.08
1.06
3.87
4.47
0.2
0.43
0.09
1.04
100.8
34
1.11
161
13.1
86.5
0.59
33.5
2.35
0.05
1.12
0.48
0.13
44.5
9.1
6.6
6.4
–
39
2.91
6.15
0.81
4.48
1.45
0.69
1.81
0.32
2.11
0.46
1.35
0.21
1.52
0.25
1.31
1.26
1.29
47.5
16.17
8.76
0.15
8.46
8.57
3.99
0.72
0.93
0.09
4.78
100.1
67.6
3.51
134.6
20.78
54.9
0.69
141.7
1.48
0.07
–
0.15
0.06
211.3
416.7
42.44
195.5
14.05
65.52
1.96
6.25
1.09
5.9
2.15
0.9
3.0
0.53
3.48
0.74
2.08
0.32
2.15
0.33
1.08
0.57
0.62
48.3
15.03
10.14
0.16
5.97
8.01
3.96
0.18
1.2
0.12
7.14
100.2
56.0
4.14
110.1
26.91
73.98
2.46
16.2
2.04
0.20
–
0.22
0.10
238.3
324.7
49.08
122.6
81.78
94.28
3.07
8.51
1.47
7.97
2.88
1.12
4.02
0.70
4.67
0.97
2.78
0.42
2.83
0.45
1.0
0.67
0.73
44.58
12.52
9.36
0.12
2.63
15.53
3.83
–
2.35
0.33
9.17
100.4
37.8
0.63
153.5
24.94
160.8
23.22
1097
3.91
1.76
1.53
2.085
0.643
240.7
50.2
28.9
26.28
28.41
106.8
18.44
39.11
5.02
21.85
5.5
1.93
5.72
0.85
4.85
0.88
2.30
0.31
1.95
0.29
1.05
2.11
6.39
44.64
15.41
11.99
0.14
4.85
7.85
4.24
0.96
3.67
0.85
5.04
99.6
46.6
10.48
926
36.26
318.8
67.52
444.9
6.81
4.88
1.25
4.60
1.18
219.8
4.22
31.4
21.81
50.82
146.8
49.35
109.7
14.15
59.18
12.52
4.15
10.96
1.48
7.84
1.32
3.26
0.42
2.49
0.35
1.08
2.48
13.39
50.39
16.2
7.76
0.13
5.05
7.16
3.24
1.96
2.36
0.64
5.07
99.9
58.4
27.1
643
22.4
260
43.3
659
6.03
3.29
6.71
8.39
1.8
135
136
64.3
126
42.2
134
50.7
91.8
9.83
42.9
8.47
2.67
7.36
0.99
5.01
0.81
1.92
0.25
1.46
0.22
1.03
3.77
23.44
59.61
17.48
7.64
0.12
1.11
1.89
6.37
2.4
0.72
0.25
2.14
99.7
23.16
64.89
260.4
58.18
680.5
82.24
422
14.76
5.99
4.30
12.28
2.78
5.26
66.9
4.98
–
9.93
167.9
74.8
142.6
15.84
59.79
12.64
3.68
11.69
1.877
10.97
2.08
5.77
0.86
5.84
0.88
0.93
3.73
8.64
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ARMENIAN OPHIOLITES
Table 2.
Step
40
367
Ar/39Ar dating results of single grain AR-05-110 gabbro amphiboles from the Vedi ophiolite
Laser
power
(mW)
Contamin.
atmosph.
(%)
AR-05-110(A) (K103)
1
500
99.437
2
620
73.967
3
690
134.382
4
850
93.450
5
1050
38.568
6
1150
19.355
7
1200
24.516
8
1300
17.488
9
2500
17.374
AR-05-110(B) (K119)
1
500
82.734
2
585
83.526
3
670
29.902
4
760
17.905
5
806
15.376
6
860
20.078
7
1800
13.653
AR05 110(C) (K132)
1
484
81.313
2
520
70.539
3
600
30.721
4
660
22.277
5
710
32.287
6
785
25.007
7
862
33.799
8
2000
18.441
39
Ar (%)
37
ArCa/39ArK
40
Ar*/39ArK
Age (Ma +1s)
1.03
0.58
0.54
0.91
11.19
39.90
16.76
5.37
23.73
16.751
5.709
7.676
17.798
34.521
40.983
44.907
35.012
37.852
0.237
3.169
2
0.599
3.440
3.920
3.601
4.116
4.009
9.792+116.946
126.879+159.432
2+ 2
24.696+125.797
137.322+10.256
155.682+5.710
143.488+6.732
163.105+17.008
159.066+4.931
1.70
1.36
9.58
27.30
23.98
7.09
28.99
13.380
18.310
50.630
48.774
43.418
42.179
40.579
30.304
15.052
30.333
29.942
30.465
29.367
30.589
178.902+22.943
91.078+30.863
179.068+4.364
176.868+2.342
179.810+2.604
173.631+5.472
180.506+2.321
1.51
1.35
8.14
17.99
12.26
18.02
11.28
29.44
12.110
11.216
45.281
38.256
46.619
52.601
62.023
34.698
6.266
3.010
4.824
4.168
3.678
4.278
5.175
4.251
243.436+50.894
121.065+52.838
190.263+10.923
165.539+4.834
146.840+8.284
169.727+10.053
203.359+6.919
168.686+2.481
A few dacitic dyke-like bodies crop out among
the basaltic pillow flows in the Vedi valley. As in
the pillow basalts, plagioclase is the main mineral
phase and Fe-oxides (c. 5 –10%) are the accessory
minerals (Fig. 9g). Some plagioclase unzoned
phenocrysts (1–2 mm, oligoclase-andesine?) are
distributed in the fine-grained (,0.2–0.5 mm)
hyalopilitic or micro-cryptocrystalline devitrified
groundmass made of albitic plagioclase, opaque
microlites, and carbonate-quartz-chlorite aggregates. There are also thin hydrothermal veins made
of calcite.
Calc-alkaline lavas of Stepanavan zone.
These consist of large pillow-lavas of basaltic
and basaltic andesitic compositions with microcryptocrystalline (Fig. 9h) to intersertal textures of
large andesine-oligoclase plagioclase phenocrysts
(2–7 mm) and microliths, and minor augite
(Wo36-38En42-43Fs13-15) clinopyroxenes. These
lavas overlie Upper Cretaceous limestones, uncomformably lying on the ophiolite s.s.
Major– trace – REE geochemistry
The geochemical analyses of the ophiolitic rocks
from the Sevan ophiolite are of relatively alkaline
composition in comparison to MORB. Major
element data of pillow- lavas, diabase and gabbros
show that they have predominantly basalt to
trachybasalt compositions.
Major elements. Major element analysis of plutonic
rocks ranges from gabbros to granites ( plagiogranites) with intermediate dioritic compositions
(Fig. 11a). These magmatic rocks appear to define
a large trend (Le Maitre et al. 1989). Similarly, in
the alkali v. iron and magnesium (AFM) diagram
(Fig. 11b) most rocks lie close to the limit
between the tholeiitic and calc-alkaline fields.
† Overall, the rocks of the ophiolitic suite are
enriched in MgO and more depleted in TiO2,
K2O and P2O5 relative to the alkaline suite
(Figs 11 & 12; Table 1). The volcanic rocks
from the different studied areas plot in the
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368
Y. ROLLAND ET AL.
Fig. 7. 40Ar/39Ar age spectra and isochrons undertaken on single amphibole (hornblende) grains of a gabbro sample
(AR-05-110) from Vedi ophiolite.
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ARMENIAN OPHIOLITES
Fig. 8. Representative geological logs of the Stepanavan, Sevan and Vedi ophiolites.
369
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370
Fig. 9.
Y. ROLLAND ET AL.
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ARMENIAN OPHIOLITES
same compositional range (from basalts to andesites and trachyandesites), and are relatively
richer in Na2O than the plutonic rocks of the
same series.
† The studied alkaline lavas from different zones
plot in the same range, varying compositionally
from basanite-trachybasalt to basaltic trachyandesite and trachyandesite, and are clearly in
the calc-alkaline/alkaline domain of the AFM
diagram (Fig. 11a, b). One of the most significant
features of the alkaline lavas is their higher TiO2,
K2O and P2O5 contents.
† The arc-type calc-alkaline lavas, having trachybasalt and basaltic trachyandesite compositions in TAS diagram (Fig. 11), plot essentially
in a transitional position between ophiolitic
and alkaline domains in Harker’s diagrams
(Fig. 12), except lower TiO2 and higher Al2O3,
depend on the abundance of plagioclase in
such rocks.
Regarding now the spread of compositional variations in major elements within series, it appears
that SiO2, Al2O3, MgO, correlate relatively well
with variations in TiO2, an element considered as
more immobile during alteration processes (e.g.
Staudigel et al. 1996), (Fig. 12). Other elements,
and particularily Na have relatively scattered
compositions, even in individual magmatic suites,
which could be ascribed to alteration or relatively
complex magmatic processes. This is also supported
by thin section observations and previous studies of
the Armenian ophiolites (e.g. Palandjyan 1971;
Abovyan 1981; Ghazaryan 1994), which indicate
that the whole magmatic sequence has been affected
by oceanic low-temperature hydrothermal alteration events. These processes induced modification
of the most lithophile elements, as revealed by the
increase of LOI (loss on ignition) in whole-rock
chemistry (Table 1).
Trace elements. High field-strength elements
(HFSE) are not mobilized during alteration and
371
their contents reflect, without ambiguity, those
of their parental magma (Staudigel et al. 1996).
Trace elements contents confirm the presence of
three clearly distinct magmatic suites, as defined
in previous section.
† Basalts and gabbros of the ophiolite suite show
strong enrichments in LILE (Large Ion Lithophile Elements: Ba, Rb, K and Th) are close
and up to ten times MORB values and bear negative anomalies in Nb-Ta and Ti (Fig. 13a, b),
which is generally indicative of volcanic island
arc environments (Taylor & McLennan 1985;
Plank & Langmuir 1998).
† Overall, the concentrations of each element in
the alkaline basalts exceed those in the basalts
from ophiolitic series (Fig. 13c). Moreover, alkaline series basalts are characterized by high
abundances of LILE, high field strength
elements (Nb, Ta, Zr and Ti), and light rare-earth
elements (LREE).
† The calc-alkaline suite rocks show strong
depletions in Nb and Ta, relative to Th and La,
and slight Ti negative anomaly (Fig. 13d).
They globally show slightly stronger enrichments in LREE and LILE relative to the ophiolite suite rocks.
These differences in normalized element patterns
support that these basalts are not petrogenetically
related and most likely derived from melts formed
in different tectonic settings: (1) N-type MORB
(and/or Back-arc basin type); (2) Ocean-island
and/or within-plate alkali basalts; and (3) volcanic
island arc.
REE geochemistry. In the chondrite-normalized rare
earth element (REE) diagrams (Fig. 13), analysed
ophiolite basalts and gabbros have flat and parallel
REE spectra in chondrite-normalized plots ((La/
Yb)N ¼ 0.6–0.9), showing some slight depletions
in LREE and a slight enrichment in MREE
(Fig. 13e, f ). No extensive Eu anomalies were
Fig. 9. (Continued) Microphotographs of representative magmatic rock types from different ophiolite complex.
Plutonic and volcanic ophiolite series: (a) subautomorph granular texture of a cumulate banded websterite (sample
AR-04-36, Stepanavan area, Cheqnagh valley); (b) coarse-grained hornblende gabbro with normally zoned plagioclases
(sample AR-05-110, Vedi area, massif of Qarakert); (c) xenomorph granular texture of a microcline (Mc) bearing
plagioclase rich leucogranite (sample AR-05-109, in the same massif ); (d) aphyric, intersertal (spilitic) and variolitic
basalt composed of mainly albitized plagioclase, Ti-magnetite and hematite microlites, in a devitrified groundmass
(sample AR-05-106, Vedi area, Khosrov valley). Alkaline series: (e) aphyric, intersertal basalt, totally devoid of
phenocrysts, and composed of carbonatized plagioclase microlites and opaque minerals (c. 5%) in a chlorite-carbonate
groundmass (sample AR-05-80, Sevan area, Tsapatagh valley); (f ) doleritic texture in a trachybasalt composed of
plagioclase, chloritized clinopyroxene, kaersutite (Krs), Ti-magnetite and apatite (sample AR-05-104, Vedi area);
(g) phyric trachydacite with a hyalopilitic to cryptocrystalline texture (sample AR-04-75, Vedi valley). Calc-alkaline
series: (h) olivine-bearing, plagioclase phyric (15– 40%) basalt with a microcrystalline (plagioclase, quartz, opaque
minerals) texture from pillow lavas suite (sample AR-04-32, Stepanavan area, Herher valley), in which the olivine
phenocrysts are entirely pseudomorphosed to quartz and rims of iron oxides. From a–c under crossed nichols, and d –h
under parallel nichols.
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372
Y. ROLLAND ET AL.
Fig. 10. Microphotographs of plutonic rocks from the Vedi ophiolite. (a) Gabbro (sample AR-05-112A) composed of
euhedral to subhedral plagioclase (Pl) with normal zoning with green euhedral amphibole (Amph), containing
clinopyroxene inclusions (Cpx). (b) In the same sample, subhedral plagioclase coated by anhedral amphibole. (c) In
gabbro sample AR-05-110, detailed back-scattered image showing zoning in large amphibole crystals, from
magnesio-hornblende (1), to edenite (2) and pargasite (3) toward the rim. (d) Subhedral amphibole crystallized over a
previous anhedral amphibole core (in sample AR-05-110), with some interstitial quartz (Q). (e) Plagiogranite (sample
AR-05-111) formed by subhedral plagioclase, de anhedral quartz and little proportion of chloritized þ epidotized (Ep)
amphibole. (f ) Leucocratic plagiogranite (sample AR-05-109), showing anhedral granular structure of plagioclase,
K-feldspar (with microcline twining, Mcc) and quartz. Observations are under parallel (//) or crossed () nicols all at
the same scale.
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ARMENIAN OPHIOLITES
(a)
373
(b)
Gabbros
Diorites
Plagiogranite
Ophiolitic lavas
Alkaline lavas
Gabbros
Plagiogranite
Ophiolitic lavas
Alkaline lavas
Arc-type lavas
Gabbros
Diorites
Plagiogranite
Ophiolitic lavas
Alkaline lavas
Trachydacite
Fig. 11. Plots of magmatic rocks (ophiolite, ‘OIB’ and arc series) in the (a) (Na2O þ K2O) v. SiO2 (Le Maitre et al.
1989) and (b) AFM (Irvine & Baragar 1971) diagrams.
Gabbros
Diorites
Plagiogranite
Ophiolitic lavas
Alkaline lavas
Gabbros
Diorites
Plagiogranite
Ophiolitic lavas
Alkaline lavas
Trachydacite
Gabbros
Plagiogranite
Ophiolitic lavas
Alkaline lavas
Arc-type lavas
Fig. 12. Harker variation diagrams showing the compositions of the three (ophiolitic, alkaline and calc-alkaline) series.
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on September 19, 2016
374
Y. ROLLAND ET AL.
(a)
(e)
(b)
(f)
(c)
(g)
(d)
(h)
Fig. 13. Trace and REE plots of the three studied magmatic suites. The multi-element spiderdiagrams are normalized to
the N-MORB values of Sun & McDonough (1989), and REE plots are normalized to the Chondrite values of Evensen
et al. (1978). Patterns for the studied magmatic rocks: ophiolitic volcanic (a, e) and plutonic (b, f ) series; OIB type
alkaline series (c, g), and arc type calk-alkaline series (d, h).
observed (Eu/Eu* ¼ 0.95–1.15), which show that
almost no plagioclase fractionation has occurred.
Thus, plagioclase likely remained stagnant and
was enriched in the final liquid. The concentration
of REE varies from 8 to 30 times chondrite compositions in volcanic rocks, and from 1 to 15 times in
the gabbros. Only one gabbro sample (sample
AR-03-25) shows an extreme 60 times-chondrite
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ARMENIAN OPHIOLITES
REE concentration, which may be explained by
fluid alteration as it displays a flaser –structure.
These features are interpreted as a result of
extreme crystal fractionation involving plagioclase,
clinopyroxene, orthopyroxene and, to a lesser
extent, olivine accumulation (Pallister & Knight
1981).
The websterite and gabbronorite have the lowest
concentrations of REE (0.1– 0.9 and 1–5 times
chondrite respectively) with patterns characterized
by depletion in LREE (Fig. 13f). One hornblende
gabbro (sample AR-04-45D from Stepanavan
ophiolite) is characterized by LREE enrichment
((La/Yb)N ¼ 2.27) and some depletion in MREE
(a convex downward pattern) with smaller positive
Eu anomalies (Eu/Eu* ¼ 1.12).
The REE patterns of the diorites (6-20 times
chondrite) and plagiogranites are parallel to those
of the gabbros, with smaller enrichment in LREE
((La/Yb)N ¼ 1.1). While the plagiogranite from
Stepanavan (sample AR-04-44) is characterized by
more depletion in the middle to heavy REE compared to other plagiogranites, and strongly positive
Eu anomalies (Eu/Eu* ¼ 4.05) due to its cumulative nature that ascribed to high plagioclase contents.
In contrast, chondrite-normalized REE patterns
of alkaline lavas (Fig. 13g) show huge LREE enrichments and HREE depletion ((La/Yb)N ¼ 6–14),
being characteristic of intraplate continental
basalts, as compared to ophiolite lavas. Meanwhile,
no extensive Eu anomalies were observed (Eu/
Eu* ¼ 0.95–1.15). The pattern of a trachydacites
(sample AR-04-75) is parallel to those of the
basanite-trachyandesite series having the highest
overall REE concentration.
Chondrite-normalized REE patterns of calcalkaline lavas are strongly parallel and form a
narrow domain (Fig. 13h). They have similar
HREE contents as volcanics of previous series
with significantly more depleted LREE contents
than alkaline series rocks ((La/Yb)N ¼ 2.1–2.3).
These differences of trace elements behaviour
between the three studied series further support
that these basalts are petrogenetically unrelated
and, most likely derived from melts formed in
different tectonic settings.
40
Ar/39Ar dating
Three analyses have been done on amphibole single
grains from one gabbro sample (AR-05-110) from
the Vedi ophiolite, which is described in the ‘Petrography and mineral chemistry’ section. These are
presented in Table 2 and Figure 14.
In the first dating (k103), a plateau age is defined
by the four last steps at 154.7+6.9 Ma (2s) comprising 86% of 39Ar. Further, using all the steps, an
375
isochron age of 154.4+8.1 Ma (MSWD ¼ 1.4) is
obtained. The initial 40Ar/36Ar ratio is of 264+7
shows a slight shift from the air value which shall
be ascribed to the large error bars in the low
temperature steps.
In the second experiment (k119) a wellconstrained plateau of 178.7+2.6 Ma (2s) is
obtained, with 97% of 39Ar. The average 37ArCa/
39
ArK ratio is high as for the latter experiment,
c. 40 in low temperature steps, decreasing steadily
to c. 30 in high-temperature steps. An isochron is
obtained using the five steps used in the estimate
of the plateau age, with an initial 40Ar/36Ar ratio
close to the atmospheric value. Even with all of
the steps, including the lower temperature ones,
except step 2 (which has a large error and represents
only 1.4% of degased 39Ar) we calculate a similar
within-error isochron age of 177.6+2.6 Ma.
The third experiment (k132) provides a more
disturbed Ar spectra, which does not provide any
plateau due to one high-temperature step, featured
by a higher age. However, we obtain a weighted
average age of 172+6 Ma, using steps 4–8, or a
pseudo-plateau age of 167.3+6.6 when excluding
step 7. The calculated isochron age including all
steps and that obtained only with the HT steps
provide a similar within error age of about 160 Ma
(see Fig. 7). The initial 40Ar/36Ar ratio varies
from 346+7 with all steps to 318+3 with the
high temperature ones, which is slightly lower
than the air value, and might be ascribed to some
disturbance of the Ar system.
Age variations within samples are ascribed to
the contribution of finely inter-fingered mineral
components within the dated amphiboles, following the works done by Villa et al. (2000). In the
three datings, 37ArCa/39ArK ratios are close the
obtained electron micro probe (EMP) values of the
amphiboles, with some slight variations. The clearest case is the experiment K119, which also shows
the flatter age pattern. In this sample, the slight
decrease from higher 37ArCa/39ArK in low temperature steps towards lower 37ArCa/39ArK in
higher temperature steps is ascribed to core-rim
variations in amphibole. Indeed, the petrographic
analyses show the presence of several mineral generation (see Fig. 10c, d) and EMP analyses show
higher Ca/K ratios in the rim (actinolite) v. the in
the (hornblende) core of minerals. Therefore, it is
likely that the amphibole rim contributes more to
the low temperature steps, while the core contributes more to the high temperature steps, as featured
in Villa et al. (2000) and Rolland et al. (2006). Such
process of amphibole recrystallization may explain
the large range in obtained ages within the same
sample. Plateau ages are always similar (within
error) to isochron ages, with initial 40Ar/36Ar
ratios very close to the air value. Thus, all the ages
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376
Y. ROLLAND ET AL.
obtained here have a geological meaning. However,
the slight disturbances observed in initial 40Ar/36Ar
ratio of the most disturbed Ar spectra (K132) is a
good argument to explain part of the age spread
by some fluid-rock interaction process. The flatter
Ar spectrum (k119) also provided the older age
(178.7+2.6 Ma). It is therefore probable that this
age is very close to the initial magmatic crystallization age. This age is also in agreement with
other geological data, comprising palaeontological age of the pelagic limestones (Sokolov 1977)
and radiolarian interbedded in the pillow lavas
(Danelian et al. 2008) all attributed to the Middle
Jurassic period. Therefore, the perturbed (k 132)
and c. 155 Ma (k103) amphiboles agree for a
later or long-lasting alteration process, related to
the crystallization of greenschist to epidote amphibolite minerals within very heterogeneous and
discrete zones.
In conclusion this is the first Ar-dating undertaken on the plutonic part of the Vedi ophiolite.
The initial crystallization age of the gabbro is at
the limit between the Lower and Middle Jurassic
(Toarcian-Aalenian), and predates shortly the deposition of radiolarites and limestones. This age is also
slightly older than similar Ar-dates obtained on one
gabbro from the Sevan ophiolite (Galoyan et al.
2009), and other evidence for the formation of the
Sevan ophiolite in the Middle Jurassic (Zakariadze
et al. 1990).
Discussion
Ophiolites of the Lesser Caucasus region of
Armenia are generally separated into three distinct
zones: (1) the Sevan–Akera zone in the North
(Knipper 1975; Adamia et al. 1980); (2) the Zangezur zone in the SE (Aslanyan & Satian 1977;
Knipper & Khain 1980; Adamia et al. 1981) of
Armenia, respectively; and (3) the Vedi zone to
the south (Knipper & Sokolov 1977; Zakariadze
et al. 1983).
Due to the importance of Cenozoic volcanism
that covered most of the surface of Armenia
(Fig. 2), it is still difficult to conclude if whether
the different ophiolites correlate with each other,
or if they represent various suture zones delimitating
several continental micro-blocks. As emphasized in
the following discussion, we will show further that
these ophiolites show some similarities and differences in their age, structure, lithological successions
and geochemical features; but these features remain
compatible with an origin from a sole oceanic
domain. This domain opened in the Lower–
Middle Jurassic and has registered several phases
of magmatic emplacement, evidenced in each of
the different investigated geographic zones. These
correlations provide insights into the evolution of
the Tethyan domain, and in particular allow us to
propose a geodynamical model for the obduction
of the ophiolite over the Armenian block.
In the following discussion, we will evaluate the
following points.
† Petrographically and geochemically, the Armenian ophiolites are similar to island-arc tholeiites. Such geochemical features are typical for
oceanic crust, formed on a back-arc setting
with the melting of a shallow asthenospheric
source contaminated by slab-derived fluids
(Tarney et al. 1981; Saunders & Tarney 1984).
Such a hypothesis has already been proposed
for ophiolitic gabbros from Turkey (Kocak
et al. 2005), but has to be further evaluated
considering isotopic compositions and partial
melting rates constraints.
† Lower Cretaceous Alkaline lavas of variable
thicknesses overlain this ophiolitic sequence.
Their origin has to be considered. (i) Do they
also derive from the same ophiolitic series?
(ii) Did they formed in an island-arc setting or
in an oceanic plateau environment? The occurrence of alkaline magmatism prior to obduction
in the the Late Lower Cretaceous may be of
significant importance for the obduction model
of the ophiolite crustal sequence.
† Finally, the calc-alkaline lavas are Upper Cretaceous in age. Then, these volcanic arc-related
series may be formed during closure of the
Neo-Tethys Ocean. Their geochemical feature
will be considered to evaluate this hypothesis.
Significance of Armenian ophiolites:
MOR or back-arc setting?
We could not find any outcrops of the Zangezur
ophiolites. These might be very thin stretched
rocks, which we believe mark the base of a major
thrust in SE Armenia. Petrographically and geochemically, the Armenian ophiolite rocks from
the different studied localities (Stepanavan, Sevan
and Vedi) share the same lithological and petrological features. They have geochemical features intermediate between MOR basalts and island arc
tholeiites. However, as shown by field relationships
in the ‘Geological setting’ section, it is clear that
these ophiolites were emplaced at oceanic spreading
centres. The association of exhumed serpentinites,
gabbros and plagiogranites at sea-floor levels, overlain by radiolarites is the result of intra-oceanic tectonics (Lagabrielle et al. 1984), typical of LOT
ophiolites (Nicolas & Jackson 1972; Nicolas 1989;
Lagabrielle & Cannat 1990). In this context, the
paucity of sea-floor lava spreading explains that
deep sections of the oceanic crust are exhumed by
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ARMENIAN OPHIOLITES
extensional faults. Faults and shear zones have
guided magmatic infiltration, which explains the
intense hydrothermal activity observed in the serpentinites (‘listwenites’) and the pervasive alteration of lavas and diabase dykes. In addition,
Armenian ophiolitic series are shown to be tholeiites
with slight calc-alkaline character, ranging from
basalts to basaltic andesites and basaltic trachyandesites. Spider diagrams show clear Nb–Ta negative
anomalies (Fig. 13a, b), LILE enrichments and flat
to slightly LREE-enriched spectra. These observations do not support a geochemical ‘normal’
ophiolitic crust and are more probably in agreement
with typical volcanic arc settings, in which enrichments in LILE, LREE result from slab fluids/
melts contamination (Pearce et al. 1984). For
these reasons it appears most likely that the Armenian ophiolites were emplaced in a back-arc
setting with contamination provided by slab fluids.
We did not find any evidence of the associated
Upper Jurassic volcanic arc system, which we
suggest might have been subducted or eroded
during the obduction.
The very strong (greenschist to epidote amphibolite facies) and long-lasting (.20 Ma) hydrothermal imprint which is evidenced in the 40Ar/39Ar
dating experiments (Fig. 7) is also in agreement
with a LOT environment. In such context, the time
for magmatic crystallization and hydrothermalism
is longer due to slow-spreading rate. In the present
case, a time of .23 Ma for greenschist facies hydrothermalism may suggest accretion rates ,1 cm a21.
The time of formation of the Vedi (178.7 +2.6 Ma,
this work) and Sevan (165.3+1.7 Ma; Galoyan
et al. 2009) ophiolites, constrained by 40Ar/39Ar on
gabbro amphibole, show that the age of oceanic
crust is older in the southern leading edge of the
obducted sequence than on its northern side. This
age difference is also seen in the age of radiolarians
that are younger in Sevan and Stepanavan than in
Vedi (Danelian et al. 2007, 2010). Such difference
is in agreement with the Vedi ophiolite being very
close to the rim of the former back-arc basin, the
obduction may then have been triggered along the
margin of the back-arc domain, directly south of
the Vedi oceanic crust.
Origin of alkaline lavas
Mineral chemistry and geochemistry of the alkaline
volcanic series of Sevan, Stepanavan and Vedi
ophiolites is similar to that of OIBs. As shown in
the Mineral Chemistry section, pyroxenes bear an
alkaline composition. The alkaline lava samples
show strong enrichments in incompatible elements
(up to 100 times chondrite values). In the Vedi
area, Satian et al. (2005) already pointed out the
alkaline character of the lava series, which they
377
interpreted as intra-continental rifting. However,
these lavas were emplaced above, and formed
after the ophiolites. An age of 117.3+0.9 Ma was
recently obtained with the 40Ar/39Ar dating
method undertaken on single-grain amphibole by
laser step-heating (Rolland et al. 2009). Moreover,
they are interstratified and overlain by reef limestone, which suggests a shallow marine environment just after emplacement. Thus, we interpret
these series as resulting from a plume event that
occurred in an intra-oceanic setting. This plume
was sufficiently large to overlain the various ophiolite zones present in Armenia, over a surface of
.5000 km2, thus this plume event might have
formed a plateau, which by itself may explain the
obduction (Rolland et al. 2009).
Such alkaline magmatism is widely documented
in the Middle-East region, along the Arabian and
Indian platforms, in relationship with the formation
of the Neo-Tethys ocean (Lapierre et al. 2004).
Similar Cretaceous alkaline series are found above
the Iranian ophiolite (Ghazi & Hassanipak 1999),
and in Turkey (Tüysüz et al. 1995; Tankut et al.
1998). However, it is still difficult to relate these
alkaline events due to their geographical and temporal distance, the paucity of radio-chronological
and Sr, Nd, Pb isotopic data.
Reconstruction of the ‘ophiolite’ history
From all the available geological data, we can
propose the following model for the evolution of
the Armenian Ophiolite (Fig. 14).
† The SAB is of Gondwanian origin according to
lithological associations found in central and
SE Armenia (Knipper & Khain 1980; Kazmin
et al. 1987; Aghamalyan 2004). Therefore, it is
likely that the Sevan oceanic basin opened as a
response to the north-dipping subduction of
Neo-Tethys (Fig. 14). Emplacement of the
ophiolite occurred in the Early to Middle Jurassic (Galoyan et al. 2009) in an intra-oceanic
back-arc setting between the Armenian block
and the active Eurasian margin. The older age
of the Vedi ophiolite, with respect to that of
Sevan implies that it should be at the rim of the
back-arc system.
† Emplacement of an Oceanic Plateau above the
back-arc oceanic crust during the late Lower
Cretaceous (114 –102 Ma).
† The calc-alkaline lavas unconformably overlain
the ophiolite and related alkaline series
(Galoyan et al. 2007). These lavas have similar
geochemical features as volcanic arc series.
Their emplacement is bracketed in the Upper
Cretaceous, as for the high pressure metamorphism constrained in the Stepanavan area (Meliksetyan et al. 1984 and references therein),
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378
Y. ROLLAND ET AL.
Intra-oceanic subduction
Eurasian Active Margin
Arc volcanism
S
South Armenian Block
+
+
+
(1)
N
Vedi
+
Sevan/
Stepanavan
+
+
+
?
?
+
+
+
+
+
+
+
Back-arc extension
and decompressional
melting
Lower–Upper Jurassic
Slab fluid
metasomatism
Eurasian Active Margin
Intra-oceanic subduction
S
Arc volcanism (reduced)
South Armenian Block
N
Oceanic plateau
+
+
+
(2)
+
+
+
+
+
?
Sevan/
Stepanavan
Vedi
+
+
+
+
+
Hot spot
Alkaline
magmatism
Crustal
thickening
+
Slab retreat
Late Lower Cretaceous
(Albian-Aptian, 120–115 Ma)
Eurasian Active Margin
S
South
Armenian
Block
Volcanic arc
subduction
N
Erosion
Vedi
+
+
+
(3)
+
?
Subduction of
the arc series
(Stepanavan Blueschists)
c. 95–90 Ma
Slab retreat
Arc volcanism
Frontal flysch
sequence
Obduction
Erosion
Vedi
Erosion
South
Armenian
+
Block+
Eurasian Margin
Shallow to pelagic
carbonated sedimentation
+grawackes
+
+
N
Sevan/
Stepanav
an
+
+
+
+
+
+
+
+
+
+
Turonian - Lower Coniacian
95–88 Ma
Frontal molasse
sequence
Slab retreat
Obduction
Stopped
S
Upper Coniacian
Uncomformity
Erosion
Erosion-slumps
South
Armenian
Block +
Blocking of the subduction
at c. 73–71 Ma
Eurasian Margin
N
Vedi
+
+
+
+
+
Santonien
83 Ma
Fig. 14.
+
+
+
+
S
(5)
+
+
+
+
Cenomanien
Upper Cretaceous
c. 102–95 Ma
(4)
Sevan/
Stepanavan
+
anavan
+
+
+
Sevan/S
tep
+
+
+
+
+
+
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ARMENIAN OPHIOLITES
constrained at c. 95 –90 Ma (Rolland et al.
2007). Therefore this magmatic event can be
related to the N-dipping subduction of the NeoTethys ocean below the Sevan–Akera back-arc
prior to the obduction of the Armenian Ophiolites onto the SAB.
† Then, the SAB enters the subduction zone in
the Cenomanian (102 –95 Ma), which triggers
a ‘collision’ with the thickened plateau. During
this process, the volcanic arc has probably
been subducted below the Oceanic plateau
and metamorphosed in the blueschist facies
(Rolland et al. 2007). The large variety of
lithologies comprising metabasites, marls and
conglomerates in a pelitic matrix, within the
Stepanavan blueschists, is in agreement with
such a scenario.
† The obduction of the ‘ophiolite’ section over
the SAB is further constrained by the Lower
Coniacian frontal flysch sequences, found
below and in front of the Vedi obducted
sequence. The calc-alkaline series found above
the Stepanavan ophiolite show that a volcanic
arc was active during this time above the
obducted sequence.
† The end of the obduction is constrained by Upper
Coniacian fauna in sediments unconformably
overlying the ophiolite. The subduction below
the Eurasian margin may stop at 73– 71 Ma, as
shown by 40Ar/39Ar age of MT-LP metamorphism in the Stepanavan blueschists and the
general tectonic uplifting of the region, witnessed by erosion and absence of sedimentary
record during the Late Cretaceous-Palaeocene
(Rolland et al. 2007). This 73– 71 Ma event is
thus interpreted as the insight of ‘collision’.
Conclusions: geodynamic significance of
Armenian ophiolites
(1)
(2)
The Armenian ophiolites show evidence for
the obduction of a single oceanic crust
sequence above the SAB. Similar geological,
petrological, geochemical and age features
are found in the studied Armenian ophiolitic
massifs (Sevan, Stepanavan and Vedi).
The age of the ophiolite is constrained by
40
Ar/39Ar dating experiments undertaken on
gabbro amphibole in the Vedi area, which
(3)
(4)
(5)
379
provided a magmatic crystallization age of
178.7+2.6 Ma, and further evidence of
greenschist facies crystallization during
hydrothermal alteration until c. 155 Ma. As
compared to the Sevan ophiolite, where
oceanic crust formation is dated at
165.3+1.7 Ma with the same method, the
oceanic crust sequence in the Vedi area is significantly older, suggesting that the initial
lateral oceanic age relationships are
preserved.
The oceanic crust sensu stricto corresponds to
a Lherzolite Ophiolite Type (LOT), formed in
the Early-Middle Jurassic by slow-spreading
accretion. The long-time span of alteration
(.20 Ma) recorded in 40Ar/39Ar ages of
dated amphiboles is also suggestive of such
a slow spreading environment. In addition,
the hybrid arc-MORB geochemical signature
of the ophiolite rocks strongly suggests they
formed in a back-arc basin by melting of an
asthenosphere source contaminated by subducted slab-derived products.
Alkaline volcanic series with OIB-type
geochemical features are found above the
ophiolite sequence in each of the studied
areas. Hot-spot related magmatism may have
led to the formation of Oceanic island(s) or
even Oceanic plateau(s), with significant
crustal thickening dated to the Albian –Cenomanian (114 –95 Ma) like in many other parts
of the world.
These alkaline series are also locally overlain
by calc-alkaline volcanic series, which were
likely formed in a supra-subduction zone
environment. Further evidence of this subduction is provided by blueschists series dated at
95–90 Ma. Therefore Plateau formation and
volcanic arc formation shortly pre-dated the
obduction, which occurred in the Coniacian –Santonian (88–83 Ma). The obduction
was followed by final collision of the SAB
with the Eurasian margin at c. 73 –71 Ma.
This work was supported by the Middle East Basins
Evolution project jointly supported by a consortium
including oil companies and the CNRS. Many thanks to
the MEBE programme coordinators Eric Barrier and
Maurizio Gaetani for their support and encouragements.
Special thanks to Marie-Françoise Brunet for her
Fig. 14. (Continued) Geodynamic reconstruction of the ophiolite formation from the Lower Jurassic to the Upper
Cretaceous periods. (1) Formation of the ophiolite in a back-arc setting between the SAB and the Active Eurasian
margin; (2) Formation of OIB-type series above a hot-spot in the late Early Cretaceous; (3) Obduction of the ophiolite
initiated directly after hot-spot magmatism in the early Late Cretaceous leading (4) to its emplacement above the SAB
from the Early Coniacian to Santonian times (5). The final blocking of the north-dipping subduction below the Eurasian
margin, and insight of the collision at 73– 71 Ma is constrained by the study of blueschists in the Stepanavan area.
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on September 19, 2016
380
Y. ROLLAND ET AL.
inalterable patience and Analytical data were acquired
with the help of the Geosciences Azur Laboratory, in
which we thank M. Fornari and G. Féraud for their involvement during data acquisition. We also thank the support
of the French Embassy at Yerevan for the MAE PhD grant
granted to G. Galoyan. This paper was improved by
detailed reviews undertaken by R. Hebert and P. Agard.
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